Internet Engineering Task Force Eddie Kohler
INTERNET-DRAFT UCLA
draft-ietf-dccp-spec-10.txt Mark Handley	draft-ietf-dccp-spec-09.txt Mark Handley
Expires: 7 September 2005 UCL	Expires: 14 May 2005 UCL
Sally Floyd
ICIR
7 March 2005	14 November 2004


Datagram Congestion Control Protocol (DCCP)


Status of this Memo

This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on 7 September 2005.	This Internet-Draft will expire on 14 May 2005.

Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.






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Abstract

The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that provides bidirectional unicast connections of
congestion-controlled unreliable datagrams. DCCP is suitable for
applications that transfer fairly large amounts of data, but can
benefit from control over the tradeoff between timeliness and
reliability.











































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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

Changes since draft-ietf-dccp-spec-08.txt:

* Added minimum Sequence Window.

* Init Cookie implementation sketch.

* Include reasoning for ignoring options on DCCP-Data.

* More Aggression Penalty explanation.

* More explanation on Ack Vectors that report information on packets
that haven't been sent.

Changes since draft-ietf-dccp-spec-07.txt:

* Many changes, not listed here, for WGLC.

* The more stringent Sequence Number checks on DCCP-Sync and DCCP-
SyncAck packets become SHOULD, not MAY.

Changes since draft-ietf-dccp-spec-06.txt:

* Change extended sequence numbers. Now 48-bit sequence numbers are
MANDATORY, and all packet types except Data, Ack, and DataAck always
use 48-bit sequence numbers. This change improves DCCP's robustness
against blind attacks.

* Removed empty Change options.

* Allow preference list changes during feature negotiations (this
seems easier to implement than the alternative). This required a
new feature negotiation state, UNSTABLE.

* Add Minimum Checksum Coverage feature.

* Add Reset Congestion State option.

* Simplify the implementation of CCID-specific option processing: no
need to check whether the CCID feature is being negotiated.

* Many more minor changes.

Changes since draft-ietf-dccp-spec-05.txt:

* Organization overhaul.




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* Add pseudocode for event processing.

* Remove # NDP; replace with Ack Count.

* Remove Identification, Challenge, ID Regime, and Connection Nonce.

* Data Checksum (formerly Payload Checksum) uses a 32-bit CRC.

* Switch location of non-negotiable features to clarify
presentation; now the feature location controls its value.

* Rename "value type" to "reconciliation rule".

* Rename "Reset Reason" to "Reset Code".

* Mobility ID becomes 128 bits long.

* Add probabilities to Mobility ID discussion.

* Add SyncAck.































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Table of Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 10
2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . . 11
3. Conventions and Terminology . . . . . . . . . . . . . . . . . 12
3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . . 12
3.2. Parts of a Connection. . . . . . . . . . . . . . . . . . 13
3.3. Features . . . . . . . . . . . . . . . . . . . . . . . . 13
3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . . 14
3.5. Security Limitation. . . . . . . . . . . . . . . . . . . 14
3.6. Robustness Principle . . . . . . . . . . . . . . . . . . 14
4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 16
4.3. States . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4. Congestion Control . . . . . . . . . . . . . . . . . . . 18
4.5. Features . . . . . . . . . . . . . . . . . . . . . . . . 19
4.6. Differences From TCP . . . . . . . . . . . . . . . . . . 20
4.7. Example Connection . . . . . . . . . . . . . . . . . . . 21
5. Packet Formats. . . . . . . . . . . . . . . . . . . . . . . . 23
5.1. Generic Header . . . . . . . . . . . . . . . . . . . . . 23
5.2. DCCP-Request Packets . . . . . . . . . . . . . . . . . . 27
5.3. DCCP-Response Packets. . . . . . . . . . . . . . . . . . 28
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets. . . . . . 28
5.5. DCCP-CloseReq and DCCP-Close Packets . . . . . . . . . . 30
5.6. DCCP-Reset Packets . . . . . . . . . . . . . . . . . . . 30
5.7. DCCP-Sync and DCCP-SyncAck Packets . . . . . . . . . . . 33
5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . . 34
5.8.1. Padding Option. . . . . . . . . . . . . . . . . . . 36
5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . . 36
6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . . 37
6.1. Change Options . . . . . . . . . . . . . . . . . . . . . 37
6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . . 38
6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . . 38
6.3.1. Server-Priority . . . . . . . . . . . . . . . . . . 39	6.3.1. Server-Priority . . . . . . . . . . . . . . . . . . 38
6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . . 39
6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . . 39
6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 40
6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . . 42	6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . . 41
6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . . 42
6.6.2. Processing Received Options . . . . . . . . . . . . 43	6.6.2. Processing Received Options . . . . . . . . . . . . 42
6.6.3. Loss and Retransmission . . . . . . . . . . . . . . 45	6.6.3. Loss and Retransmission . . . . . . . . . . . . . . 44
6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . . 46	6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . . 45
6.6.5. Preference Changes. . . . . . . . . . . . . . . . . 47	6.6.5. Preference Changes. . . . . . . . . . . . . . . . . 46
6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . . 47	6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . . 46
6.6.7. Unknown Features. . . . . . . . . . . . . . . . . . 47	6.6.7. Unknown Features. . . . . . . . . . . . . . . . . . 46
6.6.8. Invalid Options . . . . . . . . . . . . . . . . . . 48	6.6.8. Invalid Options . . . . . . . . . . . . . . . . . . 47
6.6.9. Mandatory Feature Negotiation . . . . . . . . . . . 48



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Informative References . . . . . . . . . . . . . . . . . . . . . 125
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 127
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 127
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 128















































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List of Tables

Table 1: DCCP Packet Types . . . . . . . . . . . . . . . . . . . 25
Table 2: DCCP Reset Codes. . . . . . . . . . . . . . . . . . . . 33
Table 3: DCCP Options. . . . . . . . . . . . . . . . . . . . . . 35
Table 4: DCCP Feature Numbers. . . . . . . . . . . . . . . . . . 39
Table 5: DCCP Congestion Control Identifiers . . . . . . . . . . 80	Table 5: DCCP Congestion Control Identifiers . . . . . . . . . . 79
Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . . 90	Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . . 88
Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . . 96	Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . . 95










































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1. Introduction

The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that implements bidirectional, unicast connections of
congestion-controlled, unreliable datagrams. Specifically, DCCP
provides:

o Unreliable flows of datagrams, with acknowledgements.

o Reliable handshakes for connection setup and teardown.

o Reliable negotiation of options, including negotiation of a
suitable congestion control mechanism.

o Mechanisms allowing servers to avoid holding state for
unacknowledged connection attempts and already-finished
connections.

o Congestion control incorporating Explicit Congestion Notification
(ECN) [RFC 3168] and the ECN Nonce [RFC 3540].	(ECN) and the ECN Nonce, as per [RFC 3168] and [RFC 3540].

o Acknowledgement mechanisms communicating packet loss and ECN
information. Acks are transmitted as reliably as the relevant
congestion control mechanism requires, possibly completely
reliably.

o Optional mechanisms that tell the sending application, with high
reliability, which data packets reached the receiver, and whether
those packets were ECN marked, corrupted, or dropped in the
receive buffer.

o Path Maximum Transmission Unit (PMTU) discovery [RFC 1191].	o Path Maximum Transmission Unit (PMTU) discovery, as per [RFC
	1191].
o A choice of modular congestion control mechanisms. Two
mechanisms are currently specified, TCP-like Congestion Control
[CCID 2 PROFILE] and TFRC (TCP-Friendly Rate Control) Congestion
Control [CCID 3 PROFILE], but DCCP is easily extensible to
further forms of unicast congestion control.

DCCP is intended for applications such as streaming media that can
benefit from control over the tradeoffs between delay and reliable
in-order delivery. TCP is not well-suited for these applications,
since reliable in-order delivery and congestion control can cause
arbitrarily long delays. UDP avoids long delays, but UDP
applications that implement congestion control must do so on their
own. DCCP provides built-in congestion control, including ECN
support, for unreliable datagram flows, avoiding the arbitrary
delays associated with TCP. It also implements reliable connection



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setup, teardown, and feature negotiation.

2. Design Rationale

One DCCP design goal was to give most streaming UDP applications
little reason not to switch to DCCP, once it is deployed. To
facilitate this, DCCP was designed to have as little overhead as
possible, both in terms of the packet header size and in terms of
the state and CPU overhead required at end hosts. Only the minimal
necessary functionality was included in DCCP, leaving other
functionality, such as forward error correction (FEC), semi-
reliability, and multiple streams, to be layered on top of DCCP as
desired.

Different forms of conformant congestion control are appropriate for
different applications. For example, on-line games might want to
make quick use of any available bandwidth, while streaming media
might trade off this responsiveness for a steadier, less bursty
rate. (Sudden rate changes can cause unacceptable UI glitches, such
as audible pauses or clicks in the playout stream.) DCCP thus
allows applications to choose from a set of congestion control
mechanisms. One alternative, TCP-like Congestion Control, halves
the congestion window in response to a packet drop or mark, as in
TCP. Applications using this congestion control mechanism will
respond quickly to changes in available bandwidth, but must tolerate
the abrupt changes in congestion window typical of TCP. A second
alternative, TCP-Friendly Rate Control (TFRC) [RFC 3448], a form of	alternative, TCP-Friendly Rate Control (TFRC, [RFC 3448]), a form of
equation-based congestion control, minimizes abrupt changes in the
sending rate while maintaining longer-term fairness with TCP. Other
alternatives can be added as future congestion control mechanisms
are standardized.

DCCP also lets unreliable traffic safely use ECN. A UDP kernel API
might not allow applications to set UDP packets as ECN-capable,
since the API could not guarantee the application would properly
detect or respond to congestion. DCCP kernel APIs will have no such
issues, since DCCP implements congestion control itself.

We chose not to require the use of the Congestion Manager [RFC
3124], which allows multiple concurrent streams between the same
sender and receiver to share congestion control. The current
Congestion Manager can only be used by applications that have their
own end-to-end feedback about packet losses, but this is not the
case for many of the applications currently using UDP. In addition,
the current Congestion Manager does not easily support multiple
congestion control mechanisms, or lend itself to the use of forms of
TFRC where the state about past packet drops or marks is maintained
at the receiver rather than at the sender. DCCP should be able to



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make use of CM where desired by the application, but we do not see
any benefit in making the deployment of DCCP contingent on the
deployment of CM itself.

We intend for DCCP's protocol mechanisms, which are described in
this document, to suit any application desiring unicast congestion-
controlled streams of unreliable datagrams. The congestion control
mechanisms currently approved for use with DCCP, which are described
in separate Congestion Control ID Profiles [CCID 2 PROFILE, CCID 3	in separate Congestion Control ID Profiles [CCID 2 PROFILE] [CCID 3
PROFILE], may, however, cause problems for some applications,
including high-bandwidth interactive video. These applications
should be able to use DCCP once suitable Congestion Control ID
Profiles are standardized.

3. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
document are to be interpreted as described in RFC 2119.	this document are to be interpreted as described in [RFC 2119].

3.1. Numbers and Fields

All multi-byte numerical quantities in DCCP, such as port numbers,
Sequence Numbers, and arguments to options, are transmitted in
network byte order (most significant byte first).

We occasionally refer to the "left" and "right" sides of a bit
field. "Left" means towards the most significant bit, and "right"
means towards the least significant bit.

Random numbers in DCCP are used for their security properties, and
SHOULD be chosen according to the guidelines in RFC 1750.	SHOULD be chosen according to the guidelines in [RFC 1750].

All operations on DCCP sequence numbers, and comparisons such as
"greater" and "greatest", use circular arithmetic modulo 2**48.
This form of arithmetic preserves the relationships between sequence
numbers as they roll over from 2**48 - 1 to 0. Note that the common
technique for implementing circular comparison using two's-
complement arithmetic, whereby A < B using circular arithmetic if
and only if (A - B) < 0 using conventional two's-complement
arithmetic, may be used for DCCP sequence numbers, provided they are	arithmetic, may be used for DCCP sequence numbers, providing they
stored in the most significant 48 bits of 64-bit integers.	are stored in the most significant 48 bits of 64-bit integers.

Reserved bitfields in DCCP packet headers MUST be set to zero by
senders, and MUST be ignored by receivers, unless otherwise
specified. This is to allow for future protocol extensions. In
particular, DCCP processors MUST NOT reset a DCCP connection simply
because a Reserved field has non-zero value [RFC 3360].



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3.2. Parts of a Connection

Each DCCP connection runs between two hosts, which we often name
DCCP A and DCCP B. Each connection is actively initiated by one of
the hosts, which we call the client; the other, initially passive
host is called the server. The term "DCCP endpoint" is used to
refer to either of the two hosts explicitly named by the connection
(the client and the server). The term "DCCP processor" refers more
generally to any host that might need to process a DCCP header; this
includes the endpoints and any middleboxes on the path, such as
firewalls and network address translators.

DCCP connections are bidirectional: data may pass from either
endpoint to the other. This means that data and acknowledgements
may be flowing in both directions simultaneously. Logically,
however, a DCCP connection consists of two separate unidirectional
connections, called half-connections. Each half-connection consists
of the application data sent by one endpoint and the corresponding
acknowledgements sent by the other endpoint. We can illustrate this
as follows:

+--------+ A-to-B half-connection: +--------+
\| \| --> application data --> \| \|
\| \| <-- acknowledgements <-- \| \|
\| DCCP A \| \| DCCP B \|
\| \| B-to-A half-connection: \| \|
\| \| <-- application data <-- \| \|
+--------+ --> acknowledgements --> +--------+

Although they are logically distinct, in practice the half-
connections overlap; a DCCP-DataAck packet, for example, contains
application data relevant to one half-connection and acknowledgement
information relevant to the other.

In the context of a single half-connection, the terms "HC-Sender"
and "HC-Receiver" denote the endpoints sending application data and
acknowledgements, respectively. For example, DCCP A is the HC-
Sender and DCCP B is the HC-Receiver in the A-to-B half-connection.

3.3. Features

A DCCP feature is a connection attribute on whose value the two
endpoints agree. Many properties of a DCCP connection are
controlled by features, including the congestion control mechanisms
in use on the two half-connections. The endpoints achieve agreement
through the exchange of feature negotiation options in DCCP headers.





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DCCP features are identified by a feature number and an endpoint.
The notation "F/X" represents the feature with feature number F
located at DCCP endpoint X. Each valid feature number thus
corresponds to two features, which are negotiated separately and
need not have the same value. The two endpoints know, and agree on,
the value of every valid feature. DCCP A is the "feature location"
for all features F/A, and the "feature remote" for all features F/B.

3.4. Round-Trip Times

DCCP round-trip time measurements are performed by congestion
control mechanisms; different mechanisms may measure round-trip time
in different ways, or not measure it at all. However, the main DCCP
protocol does use round-trip times occasionally, such as in the
initial values for certain timers. Each DCCP implementation thus
defines a default round-trip time for use when no estimate is
available; this parameter should default to not less than
0.2 seconds, a reasonably conservative round-trip time for Internet
TCP connections. Protocol behavior specified in terms of "round-
trip time" values actually refers to "a current round-trip time
estimate taken by some CCID, or, if no estimate is available, the
default round-trip time parameter".

The maximum segment lifetime, or MSL, is the maximum length of time
a packet can survive in the network. The DCCP MSL should equal that
of TCP, which is normally two minutes.

3.5. Security Limitation

DCCP provides no protection against attackers who can snoop on a
connection in progress, or who can guess valid sequence numbers in
other ways. Applications desiring stronger security should use
IPsec [RFC 2401]; depending on the level of security required,
application-level cryptography may also suffice. These issues are
discussed further in Sections 18 and 7.5.5.

3.6. Robustness Principle

DCCP implementations will follow TCP's "general principle of
robustness": "be conservative in what you do, be liberal in what you
accept from others" [RFC 793].

4. Overview

DCCP's high-level connection dynamics echo those of TCP.
Connections progress through three phases: initiation, including a
three-way handshake; data transfer; and termination. Data can flow
both ways over the connection. An acknowledgement framework lets



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senders discover how much data has been lost, and thus avoid
unfairly congesting the network. Of course, DCCP provides
unreliable datagram semantics, not TCP's reliable bytestream
semantics. The application must package its data into explicit
frames, and must retransmit its own data as necessary. It may be
useful to think of DCCP as TCP minus bytestream semantics and
reliability, or as UDP plus congestion control, handshakes, and
acknowledgements.

4.1. Packet Types

Ten packet types implement DCCP's protocol functions. For example,
every new connection attempt begins with a DCCP-Request packet sent
by the client. A DCCP-Request packet thus resembles a TCP SYN; but
DCCP-Request is a packet type, not a flag, so there's no way to send
an unexpected combination such as TCP's SYN+FIN+ACK+RST.

Eight packet types occur during the progress of a typical
connection, shown here. Note the three-way handshakes during
initiation and termination.

Client Server
------ ------
(1) Initiation
DCCP-Request -->
<-- DCCP-Response
DCCP-Ack -->
(2) Data transfer
DCCP-Data, DCCP-Ack, DCCP-DataAck -->
<-- DCCP-Data, DCCP-Ack, DCCP-DataAck
(3) Termination
<-- DCCP-CloseReq
DCCP-Close -->
<-- DCCP-Reset

The two remaining packet types are used to resynchronize after
bursts of loss.

Every DCCP packet starts with a 12-byte generic header. Particular
packet types include additional fixed-size header data; for example,
DCCP-Acks include an Acknowledgement Number. DCCP options and any
application data follow the fixed-size header.

The packet types are as follows:

DCCP-Request
Sent by the client to initiate a connection (the first part of
the three-way initiation handshake).



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DCCP-Response
Sent by the server in response to a DCCP-Request (the second
part of the three-way initiation handshake).

DCCP-Data
Used to transmit application data.

DCCP-Ack
Used to transmit pure acknowledgements.

DCCP-DataAck
Used to transmit application data with piggybacked
acknowledgements.

DCCP-CloseReq
Sent by the server to request that the client close the
connection.

DCCP-Close
Used by the client or the server to close the connection;
elicits a DCCP-Reset in response.

DCCP-Reset
Used to terminate the connection, either normally or abnormally.

DCCP-Sync, DCCP-SyncAck
Used to resynchronize sequence numbers after large bursts of
loss.

4.2. Sequence Numbers

Each DCCP packet carries a sequence number, so that losses can be
detected and reported. Unlike TCP sequence numbers, which are byte-
based, DCCP sequence numbers increment by one per packet. For
example:

DCCP A DCCP B
------ ------
DCCP-Data(seqno 1) -->
DCCP-Data(seqno 2) -->
<-- DCCP-Ack(seqno 10, ackno 2)
DCCP-DataAck(seqno 3, ackno 10) -->
<-- DCCP-Data(seqno 11)

Every DCCP packet increments the sequence number, whether or not it
contains application data. DCCP-Ack pure acknowledgements increment
the sequence number, for instance: DCCP B's second packet above uses
sequence number 11, since sequence number 10 was used for an



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acknowledgement. This lets endpoints detect all packet loss,
including acknowledgement loss. It also means that endpoints can
get out of sync after long bursts of loss; the DCCP-Sync and DCCP-
SyncAck packet types are used to recover (Section 7.5).

Since DCCP provides unreliable semantics, there are no
retransmissions, and it doesn't make sense to have a TCP-style
cumulative acknowledgement field. DCCP's Acknowledgement Number
field equals the greatest sequence number received, rather than the
smallest sequence number not received. Separate options indicate
any intermediate sequence numbers that weren't received.

4.3. States

DCCP endpoints progress through different states during the course
of a connection, corresponding roughly to the three phases of
initiation, data transfer, and termination. The figure below shows
the typical progress through these states for a client and server.

Client Server
------ ------
(0) No connection
CLOSED LISTEN

(1) Initiation
REQUEST DCCP-Request -->
<-- DCCP-Response RESPOND
PARTOPEN DCCP-Ack or DCCP-DataAck -->

(2) Data transfer
OPEN <-- DCCP-Data, Ack, DataAck --> OPEN

(3) Termination
<-- DCCP-CloseReq CLOSEREQ
CLOSING DCCP-Close -->
<-- DCCP-Reset CLOSED
TIMEWAIT
CLOSED

The nine possible states are as follows. They are listed in
increasing order, so that "state >= CLOSEREQ" means the same as
"state = CLOSEREQ or state = CLOSING or state = TIMEWAIT". Section
8 describes the states in more detail.

CLOSED
Represents nonexistent connections.





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LISTEN
Represents server sockets in the passive listening state.
LISTEN and CLOSED are not associated with any particular DCCP
connection.

REQUEST
A client socket enters this state, from CLOSED, after sending a
DCCP-Request packet to try to initiate a connection.

RESPOND
A server socket enters this state, from LISTEN, after receiving
a DCCP-Request from a client.

PARTOPEN
A client socket enters this state, from REQUEST, after receiving
a DCCP-Response from the server. This state represents the
third phase of the three-way handshake. The client may send
application data in this state, but it MUST include an
Acknowledgement Number on all of its packets.

OPEN
The central, data transfer portion of a DCCP connection. Client
and server sockets enter this state from PARTOPEN and RESPOND,
respectively. Sometimes we speak of SERVER-OPEN and CLIENT-OPEN
states, corresponding to the server's OPEN state and the
client's OPEN state.

CLOSEREQ
A server socket enters this state, from SERVER-OPEN, to signal
that the connection is over, but the client must hold TIMEWAIT
state.

CLOSING
Server and client sockets can both enter this state to close the
connection.

TIMEWAIT
A server or client socket remains in this state for 2MSL (4
minutes) after the connection has been torn down, to prevent
mistakes due to the delivery of old packets. Only one of the
endpoints need enter TIMEWAIT state (the other can enter CLOSED
state immediately), and a server can request its client to hold
TIMEWAIT state using the DCCP-CloseReq packet type.

4.4. Congestion Control

DCCP connections are congestion controlled, but unlike in TCP, DCCP
applications have a choice of congestion control mechanism. In



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fact, the two half-connections can be governed by different
mechanisms. Mechanisms are denoted by one-byte congestion control
identifiers, or CCIDs. The endpoints negotiate their CCIDs during
connection initiation. Each CCID describes how the HC-Sender limits
data packet rates, how the HC-Receiver sends congestion feedback via
acknowledgements, and so forth. CCIDs 2 and 3 are currently
defined; CCIDs 0, 1, and 4-255 are reserved. Other CCIDs may be
defined in the future.

CCID 2 provides TCP-like Congestion Control, which is similar to
that of TCP. The sender maintains a congestion window and sends
packets until that window is full. Packets are acknowledged by the
receiver. Dropped packets and ECN [RFC 3168] indicate congestion;
the response to congestion is to halve the congestion window.
Acknowledgements in CCID 2 contain the sequence numbers of all
received packets within some window, similar to a selective
acknowledgement (SACK) [RFC 2018].

CCID 3 provides TFRC Congestion Control, an equation-based form of
congestion control intended to respond to congestion more smoothly
than CCID 2. The sender maintains a transmit rate, which it updates
using the receiver's estimate of the packet loss and mark rate.
CCID 3 behaves somewhat differently from TCP in the short term, it
is designed to operate fairly with TCP over the long term.

Section 10 describes DCCP's CCIDs in more detail. The behaviors of
CCIDs 2 and 3 are fully defined in separate profile documents [CCID
2 PROFILE, CCID 3 PROFILE].	2 PROFILE] [CCID 3 PROFILE].

4.5. Features

DCCP endpoints use Change and Confirm options to negotiate and agree
on feature values. Feature negotiation will almost always happen on
the connection initiation handshake, but it can begin at any time.

There are four feature negotiation options in all: Change L,
Confirm L, Change R, and Confirm R. The "L" options are sent by the
feature location, and the "R" options are sent by the feature
remote. A Change R option says to the feature location, "change
this feature value as follows". The feature location responds with
Confirm L, meaning "I've changed it". Some features allow Change R
options to contain multiple values, sorted in preference order. For
example:








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Client Server
------ ------
Change R(CCID, 2) -->
<-- Confirm L(CCID, 2)
* agreement that CCID/Server = 2 *

Change R(CCID, 3 4) -->
<-- Confirm L(CCID, 4, 4 2)
* agreement that CCID/Server = 4 *

Both exchanges negotiate the CCID/Server feature's value, which is
the CCID in use on the server-to-client half-connection. In the
second exchange, the client requests that the server use either
CCID 3 or CCID 4, with 3 preferred; the server chooses 4 and
supplies its preference list, "4 2".

The Change L and Confirm R options are used for feature negotiations
initiated by the feature location. In the following example, the
server requests that CCID/Server be set to 3 or 2, with 3 preferred,
and the client agrees.

Client Server
------ ------
<-- Change L(CCID, 3 2)
Confirm R(CCID, 3, 3 2) -->
* agreement that CCID/Server = 3 *


Section 6 describes the feature negotiation options further,
including the retransmission strategies that make negotiation
reliable.

4.6. Differences From TCP

Differences between DCCP and TCP apart from those discussed so far
include:

o Copious space for options (up to 1008 bytes or the PMTU).

o Different acknowledgement formats. The CCID for a connection
determines how much acknowledgement information needs to be
transmitted. For example, in CCID 2 (TCP-like), this is about
one ack per 2 packets, and each ack must declare exactly which
packets were received; in CCID 3 (TFRC), it's about one ack per
round-trip time, and acks must declare at minimum just the
lengths of recent loss intervals.





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o Denial-of-service (DoS) protection. Several mechanisms help
limit the amount of state possibly-misbehaving clients can force
DCCP servers to maintain. An Init Cookie option, analogous to
TCP's SYN Cookies [SYNCOOKIES], avoids SYN-flood-like attacks.
Only one connection endpoint need hold TIMEWAIT state; the DCCP-
CloseReq packet, which may only be sent by the server, passes
that state to the client. Various rate limits let servers avoid
attacks that might force extensive computation or packet
generation.

o Distinguishing different kinds of loss. A Data Dropped option
(Section 11.7) lets an endpoint declare that a packet was dropped
because of corruption, because of receive buffer overflow, and so
on. This facilitates research into more appropriate rate-control
responses for these non-network-congestion losses (although
currently such losses will cause a congestion response).

o Acknowledgeability. In TCP, a packet may be acknowledged only
once the data is reliably queued for application delivery. This
does not make sense in DCCP, where an application might, for
example, request a drop-from-front receive buffer. A DCCP packet
may be acknowledged as soon as its header has been successfully
processed. Concretely, a packet becomes acknowledgeable at
Step 8 of Section 8.5's packet processing pseudocode.
Acknowledgeability does not guarantee data delivery, however: the
Data Dropped option may later report that the packet's
application data was discarded.

o No receive window. DCCP is a congestion control protocol, not a
flow control protocol.

o No simultaneous open. Every connection has one client and one
server.

o No half-closed states. DCCP has no states corresponding to TCP's
FINWAIT and CLOSEWAIT, where one half-connection is explicitly
closed while the other is still active. The Data Dropped
option's Drop Code 1, Application Not Listening (Section 11.7),
can achieve a similar effect, however.

4.7. Example Connection

The progress of a typical DCCP connection is as follows. (This
description is informative, not normative.)







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Client Server
------ ------
0. [CLOSED] [LISTEN]
1. DCCP-Request -->
2. <-- DCCP-Response
3. DCCP-Ack -->
4. DCCP-Data, DCCP-Ack, DCCP-DataAck -->
<-- DCCP-Data, DCCP-Ack, DCCP-DataAck
5. <-- DCCP-CloseReq
6. DCCP-Close -->
7. <-- DCCP-Reset
8. [TIMEWAIT]


1. The client sends the server a DCCP-Request packet specifying the
client and server ports, the service being requested, and any
features being negotiated, including the CCID that the client
would like the server to use. The client may optionally
piggyback an application request on the DCCP-Request packet,
which the server may ignore.

2. The server sends the client a DCCP-Response packet indicating
that it is willing to communicate with the client. This
response indicates any features and options that the server
agrees to, begins other feature negotiations as desired, and
optionally includes an Init Cookie that wraps up all this
information and which must be returned by the client for the
connection to complete.

3. The client sends the server a DCCP-Ack packet that acknowledges
the DCCP-Response packet. This acknowledges the server's
initial sequence number and returns the Init Cookie if there was
one in the DCCP-Response. It may also continue feature
negotiation. The client may piggyback an application-level
request on its final ack, producing a DCCP-DataAck packet.

4. The server and client then exchange DCCP-Data packets, DCCP-Ack
packets acknowledging that data, and, optionally, DCCP-DataAck
packets containing data with piggybacked acknowledgements. If
the client has no data to send, then the server will send DCCP-
Data and DCCP-DataAck packets, while the client will send DCCP-
Acks exclusively. (However, the client may not send DCCP-Data
packets before receiving at least one non-DCCP-Response packet
from the server.)

5. The server sends a DCCP-CloseReq packet requesting a close.





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6. The client sends a DCCP-Close packet acknowledging the close.

7. The server sends a DCCP-Reset packet with Reset Code 1,
"Closed", and clears its connection state. DCCP-Resets are part
of normal connection termination; see Section 5.6.

8. The client receives the DCCP-Reset packet and holds state for
two maximum segment lifetimes, or 2MSL, to allow any remaining
packets to clear the network.

An alternative connection closedown sequence is initiated by the
client:

5b. The client sends a DCCP-Close packet closing the connection.

6b. The server sends a DCCP-Reset packet with Reset Code 1,
"Closed", and clears its connection state.

7b. The client receives the DCCP-Reset packet and holds state for
2MSL to allow any remaining packets to clear the network.

5. Packet Formats

The DCCP header can be from 12 to 1020 bytes long. The initial 12
bytes of the header have the same semantics for all currently-
defined packet types. Following this comes any additional fixed-
length fields required by the packet type, and then a variable-
length list of options. The application data area follows the
header. In some packet types, this area contains data for the
application; in other packet types, its contents are ignored.

+---------------------------------------+ -.
\| Generic Header \| \|
+---------------------------------------+ \|
\| Additional Fields (depending on type) \| +- DCCP Header
+---------------------------------------+ \|
\| Options (optional) \| \|
+=======================================+ -'
\| Application Data Area \|
+---------------------------------------+


5.1. Generic Header

The DCCP generic header takes different forms depending on the value
of X, the Extended Sequence Numbers bit. If X is one, the Sequence
Number field is 48 bits long and the generic header takes 16 bytes,
as follows.



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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Source Port \| Dest Port \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Data Offset \| CCVal \| CsCov \| Checksum \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| \| \|X\| \| .
\| Res \| Type \|=\| Reserved \| Sequence Number (high bits) .
\| \| \|1\| \| .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If X is zero, only the low 24 bits of the Sequence Number are
transmitted, and the generic header is 12 bytes long.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Source Port \| Dest Port \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Data Offset \| CCVal \| CsCov \| Checksum \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| \| \|X\| \|
\| Res \| Type \|=\| Sequence Number (low bits) \|
\| \| \|0\| \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


The generic header fields are defined as follows.

Source and Destination Ports: 16 bits each
These fields identify the connection, similar to the
corresponding fields in TCP and UDP. The Source Port represents
the relevant port on the endpoint that sent this packet, the
Destination Port the relevant port on the other endpoint. When
initiating a connection, the client SHOULD choose its Source
Port randomly to reduce the likelihood of attack.

DCCP APIs should treat port numbers similarly to TCP and UDP
port numbers. For example, machines that distinguish between
"privileged" and "unprivileged" ports for TCP and UDP should do
the same for DCCP.

Data Offset: 8 bits
The offset from the start of the packet's DCCP header to the
start of its application data area, in 32-bit words. The



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receiver MUST ignore packets whose Data Offset is smaller than
the minimum-sized header for the given Type, or larger than the
DCCP packet itself.

CCVal: 4 bits
Used by the HC-Sender CCID. For example, the A-to-B CCID's
sender, which is active at DCCP A, MAY send 4 bits of
information per packet to its receiver by encoding that
information in CCVal. The sender MUST set CCVal to zero unless
its HC-Sender CCID specifies otherwise, and the receiver MUST
ignore the CCVal field unless its HC-Receiver CCID specifies
otherwise.

Checksum Coverage (CsCov): 4 bits
Checksum Coverage determines the parts of the packet that are
covered by the Checksum field. This always includes the DCCP
header and options, but some or all of the application data may
be excluded. This can improve performance on noisy links for
applications that can tolerate corruption. See Section 9.

Checksum: 16 bits
The Internet checksum of the packet's DCCP header (including
options), a network-layer pseudoheader, and, depending on
Checksum Coverage, all, some, or none of the application data.
See Section 9.

Reserved (Res): 3 bits
Senders MUST set this field to all zeroes on generated packets,
and receivers MUST ignore its value.

Type: 4 bits
The Type field specifies the type of the packet. The following
values are defined:


















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Type Meaning
---- -------
0 DCCP-Request
1 DCCP-Response
2 DCCP-Data
3 DCCP-Ack
4 DCCP-DataAck
5 DCCP-CloseReq
6 DCCP-Close
7 DCCP-Reset
8 DCCP-Sync
9 DCCP-SyncAck
10-15 Reserved

Table 1: DCCP Packet Types

Receivers MUST ignore any packets with reserved type. That is,
packets with reserved type MUST NOT be processed and they MUST
NOT be acknowledged as received.

Extended Sequence Numbers (X): 1 bit
Set to one to indicate the use of an extended generic header
with 48-bit Sequence and Acknowledgement Numbers. DCCP-Data,
DCCP-DataAck, and DCCP-Ack packets MAY set X to zero or one.
All DCCP-Request, DCCP-Response, DCCP-CloseReq, DCCP-Close,
DCCP-Reset, DCCP-Sync, and DCCP-SyncAck packets MUST set X to
one; endpoints MUST ignore any such packets with X set to zero.
High-rate connections SHOULD set X to one on all packets to gain
increased protection against wrapped sequence numbers and
attacks. See Section 7.6.

Sequence Number: 48 or 24 bits
Identifies the packet uniquely in the sequence of all packets
the source sent on this connection. Sequence Number increases
by one with every packet sent, including packets such as DCCP-
Ack that carry no application data. See Section 7.

All currently defined packet types except DCCP-Request and DCCP-Data
carry an Acknowledgement Number Subheader in the four or eight bytes
immediately following the generic header. When X=1, its format is:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reserved \| Acknowledgement Number .
\| \| (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Acknowledgement Number (low bits) \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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When X=0, only the low 24 bits of the Acknowledgement Number are
transmitted, giving the Acknowledgement Number Subheader this
format:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reserved \| Acknowledgement Number (low bits) \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Reserved: 16 or 8 bits
Senders MUST set this field to all zeroes on generated packets,
and receivers MUST ignore its value.

Acknowledgement Number: 48 or 24 bits
Generally contains GSR, the Greatest Sequence Number Received on
any acknowledgeable packet so far. A packet is acknowledgeable
if and only if its header was successfully processed by the
receiver; Section 7.4 describes this further. Options such as
Ack Vector (Section 11.4) combine with the Acknowledgement
Number to provide precise information about which packets have
arrived.

Acknowledgement Numbers on DCCP-Sync and DCCP-SyncAck packets
need not equal GSR. See Section 5.7.

5.2. DCCP-Request Packets

A client initiates a DCCP connection by sending a DCCP-Request
packet. These packets MAY contain application data, and MUST use
48-bit sequence numbers (X=1).

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=0 (DCCP-Request) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Service Code \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Service Code: 32 bits
Describes the application-level service to which the client
application wants to connect. Service Codes are intended to



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provide information about which application protocol a
connection intends to use, and thus aiding middleboxes and
reducing reliance on globally well-known ports. See Section
8.1.2.

5.3. DCCP-Response Packets

The server responds to valid DCCP-Request packets with DCCP-Response
packets. This is the second phase of the three-way handshake.
DCCP-Response packets MAY contain application data, and MUST use
48-bit sequence numbers (X=1).

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=1 (DCCP-Response) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Service Code \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Acknowledgement Number: 48 bits
Contains GSR. Since DCCP-Responses are only sent during
connection initiation, this will always equal the Sequence
Number on a received DCCP-Request.

Service Code: 32 bits
MUST equal the Service Code on the corresponding DCCP-Request.

5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets

The central data transfer portion of every DCCP connection uses
DCCP-Data, DCCP-Ack, and DCCP-DataAck packets. These packets MAY
use 24-bit sequence numbers, depending on the value of the Allow
Short Sequence Numbers feature (Section 7.6.1). DCCP-Data packets
carry application data without acknowledgements.








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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (16 or 12 bytes) /
/ with Type=2 (DCCP-Data) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DCCP-Ack packets dispense with the data, but contain an
Acknowledgement Number. They are used for pure acknowledgements.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (16 or 12 bytes) /
/ with Type=3 (DCCP-Ack) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DCCP-DataAck packets carry both application data and an
Acknowledgement Number: acknowledgement information is piggybacked
on a data packet.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (16 or 12 bytes) /
/ with Type=4 (DCCP-DataAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A DCCP-Data or DCCP-DataAck packet may have a zero-length
application data area, which indicates that the application sent a
zero-length datagram. This differs from DCCP-Request and DCCP-
Response packets, where an empty application data area indicates the



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absence of application data (not the presence of zero-length
application data). The API SHOULD report any received zero-length
datagrams to the receiving application.

A DCCP-Ack packet MAY have a non-zero-length application data area,
which essentially pads the DCCP-Ack to a desired length. Receivers
MUST ignore the content of the application data area in DCCP-Ack
packets.

DCCP-Ack and DCCP-DataAck packets often include additional
acknowledgement options, such as Ack Vector, as required by the
congestion control mechanism in use.

5.5. DCCP-CloseReq and DCCP-Close Packets

DCCP-CloseReq and DCCP-Close packets begin the handshake that
normally terminates a connection. Either client or server may send
a DCCP-Close packet, which will elicit a DCCP-Reset packet. Only
the server can send a DCCP-CloseReq packet, which indicates that the
server wants to close the connection, but does not want to hold its
TIMEWAIT state. Both packet types MUST use 48-bit sequence numbers
(X=1).

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

As with DCCP-Ack packets, DCCP-CloseReq and DCCP-Close packets MAY
have non-zero-length application data areas, whose contents
receivers MUST ignore.

5.6. DCCP-Reset Packets

DCCP-Reset packets unconditionally shut down a connection.
Connections normally terminate with a DCCP-Reset, but resets may be
sent for other reasons, including bad port numbers, bad option
behavior, incorrect ECN Nonce Echoes, and so forth. DCCP-Resets
MUST use 48-bit sequence numbers (X=1).




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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=7 (DCCP-Reset) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\| Reset Code \| Data 1 \| Data 2 \| Data 3 \|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Error Text) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Reset Code: 8 bits
Represents the reason that the sender reset the DCCP connection.

Data 1, Data 2, and Data 3: 8 bits each
The Data fields provide additional information about why the
sender reset the DCCP connection. The meanings of these fields
depend on the value of Reset Code.

Application Data Area: Error Text
If present, Error Text is a human-readable text string encoded
in Unicode UTF-8, and preferably in English, that describes the
error in more detail. For example, a DCCP-Reset with Reset Code
11, "Aggression Penalty", might contain Error Text such as
"Aggression Penalty: Received 3 bad ECN Nonce Echoes, assuming
misbehavior".

The following Reset Codes are currently defined. Unless otherwise
specified, the Data 1, 2, and 3 fields MUST be set to 0 by the
sender of the DCCP-Reset and ignored by its receiver. Section
references describe concrete situations that will cause each Reset
Code to be generated; they are not meant to be exhaustive.

0, "Unspecified"
Indicates the absence of a meaningful Reset Code. Use of Reset
Code 0 is NOT RECOMMENDED: the sender should choose a Reset Code
that more clearly defines why the connection is being reset.

1, "Closed"
Normal connection close. See Section 8.3.

2, "Aborted"
The sending endpoint gave up on the connection because of lack



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of progress. See Sections 8.1.1 and 8.1.5.

3, "No Connection"
No connection exists. See Section 8.3.1.

4, "Packet Error"
A valid packet arrived with unexpected type. For example, a
DCCP-Data packet with valid header checksum and sequence numbers
arrived at a connection in the REQUEST state. See Section
8.3.1. The Data 1 field equals the offending packet type as an
eight-bit number; thus, an offending packet with Type 2 will
result in a Data 1 value of 2.

5, "Option Error"
An option was erroneous, and the error was serious enough to
warrant resetting the connection. See Sections 6.6.7, 6.6.8,
and 11.4. The Data 1 field equals the offending option type;
Data 2 and Data 3 equal the first two bytes of option data (or
zero if the option had less than two bytes of data).

6, "Mandatory Error"
The sending endpoint could not process an option O that was
immediately preceded by Mandatory. The Data fields report the
option type and data of option O, using the format of Reset Code
5, "Option Error". See Section 5.8.2.

7, "Connection Refused"
The Destination Port didn't correspond to a port open for
listening. Sent only in response to DCCP-Requests. See Section
8.1.3.

8, "Bad Service Code"
The Service Code didn't equal the service code attached to the
Destination Port. Sent only in response to DCCP-Requests. See
Section 8.1.3.

9, "Too Busy"
The server is too busy to accept new connections. Sent only in
response to DCCP-Requests. See Section 8.1.3.

10, "Bad Init Cookie"
The Init Cookie echoed by the client was incorrect or missing.
See Section 8.1.4.

11, "Aggression Penalty"
This endpoint has detected congestion control-related
misbehavior on the part of the other endpoint. See Section
12.3.



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12-127, Reserved
Receivers should treat these codes like Reset Code 0,
"Unspecified".

128-255, CCID-specific codes
Semantics depend on the connection's CCIDs. See Section 10.3.
Receivers should treat unknown CCID-specific Reset Codes like
Reset Code 0, "Unspecified".

The following table summarizes this information.

Reset
Code Name Data 1 Data 2 & 3
----- ---- ------ ----------
0 Unspecified 0 0
1 Closed 0 0
2 Aborted 0 0
3 No Connection 0 0
4 Packet Error pkt type 0
5 Option Error option # option data
6 Mandatory Error option # option data
7 Connection Refused 0 0
8 Bad Service Code 0 0
9 Too Busy 0 0
10 Bad Init Cookie 0 0
11 Aggression Penalty 0 0
12-127 Reserved
128-255 CCID-specific codes

Table 2: DCCP Reset Codes

Options on DCCP-Reset packets are processed before the connection is
shut down. This means that certain combinations of options,
particularly involving Mandatory, may cause an endpoint to respond
to a valid DCCP-Reset with another DCCP-Reset. This cannot lead to
a reset storm; since the first endpoint has already reset the
connection, the second DCCP-Reset will be ignored.

5.7. DCCP-Sync and DCCP-SyncAck Packets

DCCP-Sync packets help DCCP endpoints recover synchronization after
bursts of loss, or recover from half-open connections. Each valid
received DCCP-Sync immediately elicits a DCCP-SyncAck. Both packet
types MUST use 48-bit sequence numbers (X=1).







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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=8 (DCCP-Sync) or 9 (DCCP-SyncAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Acknowledgement Number field has special semantics for DCCP-Sync
and DCCP-SyncAck packets. First, the packet corresponding to a
DCCP-Sync's Acknowledgement Number need not have been
acknowledgeable. Thus, receivers MUST NOT assume that a packet was
processed simply because it appears in the Acknowledgement Number
field of a DCCP-Sync packet. This differs from all other packet
types, where the Acknowledgement Number by definition corresponds to
an acknowledgeable packet. Second, the Acknowledgement Number on
any DCCP-SyncAck packet MUST correspond to the Sequence Number on an
acknowledgeable DCCP-Sync packet. In the presence of reordering,
this might not equal GSR.

As with DCCP-Ack packets, DCCP-Sync and DCCP-SyncAck packets MAY
have non-zero-length application data areas, whose contents
receivers MUST ignore. Padded DCCP-Sync packets may be useful when
performing Path MTU discovery; see Section 14.

5.8. Options

Any DCCP packet may contain options, which occupy space at the end
of the DCCP header. Each option is a multiple of 8 bits in length.
Individual options are not padded to multiples of 32 bits, and any
option may begin on any byte boundary. However, the combination of
all options MUST add up to a multiple of 32 bits; Padding options
MUST be added as necessary to fill out option space to a word
boundary. Any options present are included in the header checksum.

The first byte of an option is the option type. Options with types
0 through 31 are single-byte options. Other options are followed by
a byte indicating the option's length. This length value includes
the two bytes of option-type and option-length as well as any
option-data bytes, and must therefore be greater than or equal to
two.





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Options are processed sequentially, starting at the first option in
the packet header. Options with unknown types MUST be ignored.	the packet header. Options with unknown types, and options with
Also, options with nonsensical lengths (length byte less than two or	invalid lengths (length byte less than two or more than the
more than the remaining space in the options portion of the header)	remaining space in the options portion of the header), MUST be
MUST be ignored, and any option space following an option with	ignored.
nonsensical length MUST likewise be ignored.

The following options are currently defined:

Option DCCP- Section
Type Length Meaning Data? Reference
---- ------ ------- ----- ---------
0 1 Padding Y 5.8.1
1 1 Mandatory N 5.8.2
2 1 Slow Receiver Y 11.6
3-31 1 Reserved
32 variable Change L N 6.1
33 variable Confirm L N 6.2
34 variable Change R N 6.1
35 variable Confirm R N 6.2
36 variable Init Cookie N 8.1.4
37 3-5 NDP Count Y 7.7
38 variable Ack Vector [Nonce 0] N 11.4
39 variable Ack Vector [Nonce 1] N 11.4
40 variable Data Dropped N 11.7
41 6 Timestamp Y 13.1
42 6/8/10 Timestamp Echo Y 13.3
43 4/6 Elapsed Time N 13.2
44 6 Data Checksum Y 9.3
45-127 variable Reserved
128-255 variable CCID-specific options - 10.3

Table 3: DCCP Options

Not all options are suitable for all packet types. For example,
since the Ack Vector option is interpreted relative to the
Acknowledgement Number, it isn't suitable on DCCP-Request and DCCP-
Data packets, which have no Acknowledgement Number. If an option
occurs on an unexpected packet type, it MUST generally be ignored;
any such restrictions are mentioned in each option's description.
The table summarizes the most common restriction: when the DCCP-
Data? column value is N, the corresponding option MUST be ignored
when received on a DCCP-Data packet. (Section 7.5.5 describes why
such options are ignored as opposed to, say, causing a reset.)

Options with invalid values MUST be ignored unless otherwise
specified. For example, any Data Checksum option with option length
4 MUST be ignored, since all valid Data Checksum options have option



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length 6.

This section describes two generic options, Padding and Mandatory.
Other options are described later.

5.8.1. Padding Option

+--------+
\|00000000\|
+--------+
Type=0

Padding is a single-byte "no-operation" option used to pad between
or after options. If the length of a packet's other options is not
a multiple of 32 bits, then Padding options are REQUIRED to pad out	a multiple of 4, then Padding options are REQUIRED to pad out the
the options area to the length implied by Data Offset. Padding may	options area to the length implied by Data Offset. Padding may also
also be used between options -- for example, to align the beginning	be used between options -- for example, to align the beginning of a
of a subsequent option on a 32-bit boundary. There is no guarantee	subsequent option on a word boundary. There is no guarantee that
that senders will use this option, so receivers must be prepared to	senders will use this option, so receivers must be prepared to
process options even if they do not begin on a word boundary.

5.8.2. Mandatory Option

+--------+
\|00000001\|
+--------+
Type=1

Mandatory is a single-byte option that marks the immediately	Mandatory is a single byte option that marks the immediately
following option as mandatory. Say that the immediately following
option is O. Then the Mandatory option has no effect if the
receiving DCCP endpoint understands and processes O. If the
endpoint does not understand or process O, however, then it MUST
reset the connection using Reset Code 6, "Mandatory Failure". For
instance, the endpoint would reset the connection if it did not
understand O's type; if it understood O's type, but not O's data; if
O's data was invalid for O's type; if O was a feature negotiation
option, and the endpoint did not understand the enclosed feature
number; if the endpoint understood O, but chose not to perform the
action O implies; and so forth.

Mandatory options MUST NOT be sent on DCCP-Data packets, and any
Mandatory options received on DCCP-Data packets MUST be ignored.

The connection is in error and should be reset with Reset Code 5,
"Option Error" if option O is absent (Mandatory was the last byte of
the option list), or if option O equals Mandatory. However, the
combination "Mandatory Padding" is valid, and MUST behave like two



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bytes of Padding.

Section 6.6.9 describes the behavior of Mandatory feature
negotiation options in more detail.

6. Feature Negotiation

Four DCCP options, Change L, Confirm L, Change R, and Confirm R, are
used to negotiate feature values. Change options initiate a
negotiation; Confirm options complete that negotiation. The "L"
options are sent by the feature location, and the "R" options are
sent by the feature remote. Change options are retransmitted to
ensure reliability.

All these options have the same format. The first byte of option
data is the feature number, and the second and subsequent data bytes
hold one or more feature values. The exact format of the feature
value area depends on the feature type; see Section 6.3.

+--------+--------+--------+--------+--------
\| Type \| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------

Together, the feature number and the option type ("L" or "R")
uniquely identify the feature to which an option applies. The exact
format of the Value(s) area depends on the feature number.

Feature negotiation options MUST NOT be sent on DCCP-Data packets,
and any feature negotiation options received on DCCP-Data packets
MUST be ignored.

6.1. Change Options

Change L and Change R options initiate feature negotiation. The
option to use depends on the relevant feature's location: To start a
negotiation for feature F/A, DCCP A will send a Change L option; to
start a negotiation for F/B, it will send a Change R option. Change
options are retransmitted until some response is received. They
contain at least one Value, and thus have length at least 4.












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+--------+--------+--------+--------+--------
Change L: \|00100000\| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------
Type=32

+--------+--------+--------+--------+--------
Change R: \|00100010\| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------
Type=34


6.2. Confirm Options

Confirm L and Confirm R options complete feature negotiation, and
are sent in response to Change R and Change L options, respectively.
Confirm options MUST NOT be generated except in response to Change
options. Confirm options need not be retransmitted, since Change
options are retransmitted as necessary. The first byte of the
Confirm option contains the feature number from the corresponding
Change. Following this is the selected Value, and then possibly the
sender's preference list.

+--------+--------+--------+--------+--------
Confirm L: \|00100001\| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------
Type=33

+--------+--------+--------+--------+--------
Confirm R: \|00100011\| Length \|Feature#\| Value(s) ...
+--------+--------+--------+--------+--------
Type=35

If an endpoint receives an invalid Change option -- with an unknown
feature number, or an invalid value -- it will respond with an empty
Confirm option containing the problematic feature number, but no
value. Such options have length 3.

6.3. Reconciliation Rules

Reconciliation rules determine how the two sets of preferences for a
given feature are resolved into a unique result. The reconciliation
rule depends only on the feature number. Each reconciliation rule
must have the property that the result is uniquely determined given
the contents of Change options sent by the two endpoints.

All current DCCP features use one of two reconciliation rules,
server-priority ("SP") and non-negotiable ("NN").




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6.3.1. Server-Priority

The feature value is a fixed-length byte string (length determined
by the feature number). Each Change option contains a list of
values ordered by preference, with the most preferred value coming
first. Each Confirm option contains the confirmed value, followed
by the confirmer's preference list. Thus, the feature's current
value will generally appear twice in Confirm options' data, once as
the current value and once in the confirmer's preference list.

To reconcile the preference lists, select the first entry in the
server's list that also occurs in the client's list. If there is no
shared entry, the feature's value MUST NOT change, and the Confirm
option will confirm the feature's previous value (unless the Change
option was Mandatory; see Section 6.6.9).

6.3.2. Non-Negotiable

The feature value is a byte string. Each option contains exactly
one feature value. The feature location signals a new value by
sending a Change L option. The feature remote MUST accept any valid
value, responding with a Confirm R option containing the new value,
and it MUST send empty Confirm R options in response to invalid
values (unless the Change L option was Mandatory; see Section
6.6.9). Change R and Confirm L options MUST NOT be sent for non-
negotiable features; see Section 6.6.8. Non-negotiable features use
the feature negotiation mechanism to achieve reliability.

6.4. Feature Numbers

This document defines the following feature numbers.




















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Rec'n Initial Section
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
0 Reserved
1 Congestion Control ID (CCID) SP 2 Y 10
2 Allow Short Seqnos SP 1 Y 7.6.1
3 Sequence Window NN 100 Y 7.5.2
4 ECN Incapable SP 0 N 12.1
5 Ack Ratio NN 2 N 11.3
6 Send Ack Vector SP 0 N 11.5
7 Send NDP Count SP 0 N 7.7.2
8 Minimum Checksum Coverage SP 0 N 9.2.1
9 Check Data Checksum SP 0 N 9.3.1
10-127 Reserved
128-255 CCID-specific features 10.3

Table 4: DCCP Feature Numbers


Rec'n Rule The reconciliation rule used for the feature. SP is
server-priority and NN is non-negotiable.

Initial Value The initial value for the feature. Every feature has
a known initial value.

Req'd This column is "Y" if and only if every DCCP
implementation MUST understand the feature. If it is
"N", then the feature behaves like an extension (see
Section 15), and it is safe to respond to Change
options for the feature with empty Confirm options.
Of course, a CCID might require the feature; a DCCP
that implements CCID 2 MUST support Ack Ratio and
Send Ack Vector, for example.

6.5. Examples
Here are three example feature negotiations for features located at
the server, the first two for the Congestion Control ID feature, the
last for the Ack Ratio.

Client Server
------ ------
1. Change R(CCID, 2 3 1) -->
("2 3 1" is client's preference list)
2. <-- Confirm L(CCID, 3, 3 2 1)
(3 is the negotiated value;
"3 2 1" is server's pref list)
* agreement that CCID/Server = 3 *




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1. XXX <-- Change L(CCID, 3 2 1)
2. Retransmission:
<-- Change L(CCID, 3 2 1)
3. Confirm R(CCID, 3, 2 3 1) -->
* agreement that CCID/Server = 3 *


1. <-- Change L(Ack Ratio, 3)
2. Confirm R(Ack Ratio, 3) -->
* agreement that Ack Ratio/Server = 3 *

This example shows a simultaneous negotiation.

Client Server
------ ------
1a. Change R(CCID, 2 3 1) -->
b. <-- Change L(CCID, 3 2 1)
2a. <-- Confirm L(CCID, 3, 3 2 1)
b. Confirm R(CCID, 3, 2 3 1) -->
* agreement that CCID/Server = 3 *

Here are the byte encodings of several Change and Confirm options.
Each option is sent by DCCP A.

Change L(CCID, 2 3) = 32,5,1,2,3
DCCP B should change CCID/A's value (feature number 1, a server-
priority feature); DCCP A's preferred values are 2 and 3, in
that preference order.

Change L(Sequence Window, 1024) = 32,9,3,0,0,0,0,4,0	Change L(Sequence Window, 1024) = 32,6,3,0,4,0
DCCP B should change Sequence Window/A's value (feature number
3, a non-negotiable feature) to the 6-byte string 0,0,0,0,4,0	3, a non-negotiable feature) to the 3-byte string 0,4,0 (the
(the value 1024).	value 1024).

Confirm L(CCID, 2, 2 3) = 33,6,1,2,2,3
DCCP A has changed CCID/A's value to 2; its preferred values are
2 and 3, in that preference order.

Empty Confirm L(126) = 33,3,126
DCCP A doesn't implement feature number 126, or DCCP B's
proposed value for feature 126/A was invalid.

Change R(CCID, 3 2) = 34,5,1,3,2
DCCP B should change CCID/B's value; DCCP A's preferred values
are 3 and 2, in that preference order.

Confirm R(CCID, 2, 3 2) = 35,6,1,2,3,2
DCCP A has changed CCID/B's value to 2; its preferred values



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were 3 and 2, in that preference order.

Confirm R(Sequence Window, 1024) = 35,9,3,0,0,0,0,4,0	Confirm R(Sequence Window, 1024) = 35,6,3,0,4,0
DCCP A has changed Sequence Window/B's value to the 6-byte	DCCP A has changed Sequence Window/B's value to the 3-byte
string 0,0,0,0,4,0 (the value 1024).	string 0,4,0 (the value 1024).

Empty Confirm R(126) = 35,3,126
DCCP A doesn't implement feature number 126, or DCCP B's
proposed value for feature 126/B was invalid.

6.6. Option Exchange

A few basic rules govern feature negotiation option exchange.

1. Every non-reordered Change option gets a Confirm option in
response.

2. Change options are retransmitted until a response for the latest
Change is received.

3. Feature negotiation options are processed in strictly increasing
order by Sequence Number.

The rest of this section describes the consequences of these rules
in more detail.

6.6.1. Normal Exchange

Change options are generated when a DCCP endpoint wants to change
the value of some feature. Generally, this will happen at the
beginning of a connection, although it may happen at any time. We
say the endpoint "generates" or "sends" a Change L or Change R
option, but of course the option must be attached to a packet. The
endpoint may attach the option to a packet it would have generated
anyway (such as a DCCP-Request), or it may create a "feature
negotiation packet", often a DCCP-Ack or DCCP-Sync, just to carry
the option. Feature negotiation packets are controlled by the
relevant congestion control mechanism. For example, DCCP A may send
a DCCP-Ack or DCCP-Sync for feature negotiation only if the B-to-A
CCID would allow sending a DCCP-Ack. In addition, an endpoint
SHOULD generate at most one feature negotiation packet per round-
trip time.

On receiving a Change L or Change R option, a DCCP endpoint examines
the included preference list, reconciles that with its own
preference list, calculates the new value, and sends back a
Confirm R or Confirm L option, respectively, informing its peer of
the new value or that the feature was not understood. Every non-



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reordered Change option MUST result in a corresponding Confirm
option, and any packet including a Confirm option MUST carry an
Acknowledgement Number. (Section 6.6.4 describes how Change	Acknowledgement Number. Generated Confirm options may be attached
reordering is detected and handled.) Generated Confirm options may	to packets that would have been sent anyway (such as DCCP-Response
be attached to packets that would have been sent anyway (such as	or DCCP-SyncAck), or to new feature negotiation packets, as
DCCP-Response or DCCP-SyncAck), or to new feature negotiation	described above.
packets, as described above.

The Change-sending endpoint MUST wait to receive a corresponding
Confirm option before changing its stored feature value. The
Confirm-sending endpoint changes its stored feature value as soon as
it sends the Confirm.

A packet MAY contain more than one feature negotiation option, as
long as no two options refer to the same feature. Note, however,
that a packet is allowed to contain one L option and one R option
with the same feature number, since the two options actually refer
to different features (F/A and F/B).

6.6.2. Processing Received Options

DCCP endpoints exist in one of three states relative to each
feature. STABLE is the normal state, where the endpoint knows the
feature's value and thinks the other endpoint agrees. An endpoint
enters the CHANGING state when it first sends a Change for the
feature, and returns to STABLE once it receives a corresponding
Confirm. The final state, UNSTABLE, indicates that an endpoint in
CHANGING state changed its preference list, but has not yet
transmitted a Change option with the new preference list.

Feature state transitions at a feature location are implemented
according to this diagram. The diagram ignores sequence number and
option validity issues; these are handled explicitly in the
pseudocode that follows.

















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timeout/
rcv Confirm R app/protocol evt : snd Change L rcv non-ack
: ignore +---------------------------------------+ : snd Change L
+----+ \| \| +----+
\| v \| rcv Change R v \| v
+------------+ rcv Confirm R : calc new value, +------------+
\| \| : accept value snd Confirm L \| \|
\| STABLE \|<-----------------------------------\| CHANGING \|
\| \| rcv empty Confirm R \| \|
+------------+ : revert to old value +------------+
\| ^ \| ^
+----+ pref list \| \| snd
rcv Change R changes \| \| Change L
: calc new value, snd Confirm L v \|
+------------+
+---\| \|
rcv Confirm/Change R \| \| UNSTABLE \|
: ignore +-->\| \|
+------------+

Feature locations SHOULD use the following pseudocode, which
corresponds to the state diagram, to react to each feature
negotiation option on each valid packet received. The pseudocode
refers to "P.seqno" and "P.ackno", which are properties of the
packet; "O.type", and "O.len", which are properties of the option;
"FGSR" and "FGSS", which are properties of the connection, and
handle reordering as described in Section 6.6.4; "F.state", which is
the feature's state (STABLE, CHANGING, or UNSTABLE); and "F.value",
which is the feature's value.

First, check for unknown features (Section 6.6.7);
If F is unknown,
If the option was Mandatory, /* Section 6.6.9 */
Reset connection and return
Otherwise, if O.type == Change R,
Send Empty Confirm L on a future packet
Return

Second, check for reordering (Section 6.6.4);
If F.state == UNSTABLE or P.seqno <= FGSR
or (O.type == Confirm R and P.ackno < FGSS),
Ignore option and return

Third, process Change R options;
If O.type == Change R,
If the option's value is valid, /* Section 6.6.8 */
Calculate new value
Send Confirm L on a future packet



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Set F.state := STABLE
Otherwise, if the option was Mandatory,
Reset connection and return
Otherwise,
Send Empty Confirm L on a future packet
/* Remain in existing state. If that's CHANGING, this
endpoint will retransmit its Change L option later. */

Fourth, process Confirm R options (but only in CHANGING state).
If F.state == CHANGING and O.type == Confirm R,
If O.len > 3, /* nonempty */
If the option's value is valid,
Set F.value := new value
Otherwise,
Reset connection and return
Set F.state := STABLE

Versions of this diagram and pseudocode are also used by feature
remotes; simply switch the "L"s and "R"s, so that the relevant
options are Change R and Confirm L.

6.6.3. Loss and Retransmission

Packets containing Change and Confirm options might be lost or
delayed by the network. Therefore, Change options are repeatedly
transmitted to achieve reliability. We refer to this as
"retransmission", although of course there are no packet-level
retransmissions in DCCP: a Change option that is sent again will be
sent on a new packet with a new sequence number.

A CHANGING endpoint transmits another Change option once it realizes
that it has not heard back from the other endpoint. The new Change
option need not contain the same payload as the original; reordering
protection will ensure that agreement is reached based on the most
recently transmitted option.

A CHANGING endpoint MUST continue retransmitting Change options
until it gets some response or the connection terminates.

Endpoints SHOULD use an exponential-backoff timer to decide when to
retransmit Change options. (Endpoints that generate packets
specifically for feature negotiation MUST use such a timer.) The
timer interval is initially set to not less than one round-trip
time, and should back off to not less than 64 seconds. The backoff
protects against delayed agreement due to the reordering protection
algorithms described in the next section. Again, endpoints may
piggyback Change options on packets they would have sent anyway, or
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controlled by the relevant congestion-control mechanism.

Confirm options are never retransmitted, but the Confirm-sending
endpoint MUST generate a Confirm option after every non-reordered
Change.

6.6.4. Reordering

Reordering might cause packets containing Change and Confirm options
to arrive in an unexpected order. Endpoints MUST ignore feature
negotiation options that do not arrive in strictly-increasing order
by Sequence Number. The rest of this section presents two
algorithms that fulfill this requirement.

The first algorithm introduces two sequence number variables that
each endpoint maintains for the connection.

FGSR Feature Greatest Sequence Number Received: The greatest
sequence number received, considering only valid packets
that contained one or more feature negotiation options
(Change and/or Confirm). This value is initialized to
ISR - 1.

FGSS Feature Greatest Sequence Number Sent: The greatest
sequence number sent, considering only packets that
contained one or more non-retransmitted Change options.
(Retransmitted Change options MUST have exactly the same
contents as previously transmitted options, so limited
reordering can safely be tolerated.) This value is
initialized to ISS.

Each endpoint checks two conditions on sequence numbers to decide
whether to process received feature negotiation options.

1. If a packet's Sequence Number is less than or equal to FGSR,
then its Change options MUST be ignored.

2. If a packet's Sequence Number is less than or equal to FGSR, OR
it has no Acknowledgement Number, OR its Acknowledgement Number
is less than FGSS, then its Confirm options MUST be ignored.

Alternatively, an endpoint MAY maintain separate FGSR and FGSS
values for every feature. FGSR(F/X) would equal the greatest
sequence number received, considering only packets that contained
Change or Confirm options applying to feature F/X; FGSS(F/X) would
be defined similarly. This algorithm requires more state, but is
slightly more forgiving to multiple overlapped feature negotiations.
Either algorithm MAY be used; the first algorithm, with connection-



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wide FGSR and FGSS variables, is RECOMMENDED.

One consequence of these rules is that a CHANGING endpoint will
ignore any Confirm option that does not acknowledge the latest
Change option sent. This ensures that agreement, once achieved,
used the most recent available information about the endpoints'
preferences.

6.6.5. Preference Changes

Endpoints are allowed to change their preference lists at any time.
However, an endpoint that changes its preference list while in the
CHANGING state MUST transition to the UNSTABLE state. It will
transition back to CHANGING once it has transmitted a Change option
with the new preference list. This ensures that agreement is based
on active preference lists. Without the UNSTABLE state,
simultaneous negotiation -- where the endpoints began independent
negotiations for the same feature at the same time -- might lead to
the negotiation terminating with the endpoints thinking the feature
had different values.

6.6.6. Simultaneous Negotiation

The two endpoints might simultaneously open negotiation for the same
feature, after which an endpoint in the CHANGING state will receive
a Change option for the same feature. Such received Change options
can act as responses to the original Change options. The CHANGING
endpoint MUST examine the received Change's preference list,
reconcile that with its own preference list (as expressed in its
generated Change options), and generate the corresponding Confirm
option. It can then transition to the STABLE state.

6.6.7. Unknown Features

Endpoints may receive Change options referring to feature numbers
they do not understand -- for instance, when an extended DCCP
converses with a non-extended DCCP. Endpoints MUST respond to
unknown Change options with Empty Confirm options (that is, Confirm
options containing no data), which inform the CHANGING endpoint that
the feature was not understood. However, if the Change option was
Mandatory, the connection MUST be reset; see Section 6.6.9.

On receiving an empty Confirm option for some feature, the CHANGING
endpoint MUST transition back to the STABLE state, leaving the
feature's value unchanged. Section 15 suggests that the default
value for any extension feature should correspond to "extension not
available".




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Some features are required to be understood by all DCCPs (see
Section 6.4). The CHANGING endpoint SHOULD reset the connection
(with Reset Code 5, "Option Error") if it receives an empty Confirm
option for such a feature.

Since Confirm options are generated only in response to Change
options, an endpoint should never receive a Confirm option referring
to a feature number it does not understand. Nevertheless, endpoints
MUST ignore any such options they receive.

6.6.8. Invalid Options

A DCCP endpoint might receive a Change or Confirm option that lists
one or more values that it does not understand. Some, but not all,
such options are invalid, depending on the relevant reconciliation
rule (Section 6.3). For instance:

o All features have length limitations, and options with invalid	o All features have length limitiations, and options with invalid
lengths are invalid. For example, the Ack Ratio feature takes
16-bit values, so valid "Confirm R(Ack Ratio)" options have
option length 5.

o Some non-negotiable features have value limitations. The Ack
Ratio feature takes two-byte, non-zero integer values, so a
"Change L(Ack Ratio, 0)" option is never valid. Note that
server-priority features do not have value limitations, since
unknown values are handled as a matter of course.

o Any Confirm option that selects the wrong value, based on the two
preference lists and the relevant reconciliation rule, is
invalid.

o However, unexpected Confirm options -- that refer to unknown
feature numbers, or that don't appear to be part of a current
negotiation -- are considered valid, although they are ignored by
the receiver.

An endpoint receiving an invalid Change option MUST respond with the
corresponding empty Confirm option. An endpoint receiving an
invalid Confirm option MUST reset the connection, with Reset Code 5,
"Option Error".

6.6.9. Mandatory Feature Negotiation

Change options may be preceded by Mandatory options (Section 5.8.2).
Mandatory Change options are processed like normal Change options,
except that the following failure cases will cause the receiver to
reset the connection with Reset Code 6, "Mandatory Failure", rather



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than send a Confirm option. The connection MUST be reset if:

o The Change option's feature number was not understood;

o The Change option's value was invalid, and the receiver would
normally have sent an empty Confirm option in response; or

o For server-priority features, there was no shared entry in the
two endpoints' preference lists.

There's no reason to mark Confirm options as Mandatory in this
version of DCCP, since Confirm options are sent only in response to
Change options and therefore can't mention potentially-invalid
values or unexpected feature numbers.

7. Sequence Numbers

DCCP uses sequence numbers to arrange packets into sequence, detect
losses and network duplicates, and protect against attackers, half-
open connections, and the delivery of very old packets. Every
packet carries a Sequence Number; most packet types carry an
Acknowledgement Number as well.

DCCP sequence numbers are packet-based. That is, the packets
generated by each endpoint have Sequence Numbers that increase by
one, modulo 2^48, for every packet. Even DCCP-Ack and DCCP-Sync
packets, and other packets that don't carry user data, increment the
Sequence Number. Since DCCP is an unreliable protocol, there are no
true retransmissions; but effective retransmissions, such as
retransmissions of DCCP-Request packets, also increment the Sequence
Number. This lets DCCP implementations detect network duplication,
retransmissions, and acknowledgement loss, and is a significant
departure from TCP practice.

7.1. Variables

DCCP endpoints maintain a set of sequence number variables for each
connection.

ISS The Initial Sequence Number Sent by this endpoint. This
equals the Sequence Number of the first DCCP-Request or
DCCP-Response sent.

ISR The Initial Sequence Number Received from the other
endpoint. This equals the Sequence Number of the first
DCCP-Request or DCCP-Response received.





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GSS The Greatest Sequence Number Sent by this endpoint. Here,
and elsewhere, "greatest" is measured in circular sequence
space.

GSR The Greatest Sequence Number Received from the other
endpoint on an acknowledgeable packet. (Section 7.4 defines
this term.)

GAR The Greatest Acknowledgement Number Received from the other
endpoint on an acknowledgeable packet that was not a DCCP-
Sync.

Some other variables are derived from these primitives.

SWL and SWH
(Sequence Number Window Low and High) The extremes of the
validity window for received packets' Sequence Numbers.

AWL and AWH
(Acknowledgement Number Window Low and High) The extremes
of the validity window for received packets' Acknowledgement
Numbers.

7.2. Initial Sequence Numbers

The endpoints' initial sequence numbers are set by the first DCCP-
Request and DCCP-Response packets sent. Initial sequence numbers
MUST be chosen to avoid two problems:

o Delivery of old packets, where packets lingering in the network
from an old connection are delivered to a new connection with the
same addresses and port numbers.

o Sequence number attacks, where an attacker can guess the sequence
numbers that a future connection would use [M85].

These problems are the same as problems faced by TCP, and DCCP
implementations SHOULD use TCP's strategies to avoid them [RFC 793,	implementations SHOULD use TCP's strategies to avoid them [RFC 793]
RFC 1948]. The rest of this section explains these strategies in	[RFC 1948]. The rest of this section explains these strategies in
more detail.

To address the first problem, an implementation MUST ensure that the
initial sequence number for a given <source address, source port,
destination address, destination port> 4-tuple doesn't overlap with
recent sequence numbers on previous connections with the same
4-tuple. ("Recent" means sent within 2 maximum segment lifetimes,
or 4 minutes.) The implementation MUST additionally ensure that the
lower 24 bits of the initial sequence number don't overlap with the



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lower 24 bits of recent sequence numbers (unless the implementation
plans to avoid short sequence numbers; see Section 7.6). An
implementation that has state for a recent connection with the same
4-tuple can pick a good initial sequence number explicitly.
Otherwise, it could tie initial sequence number selection to some
clock, such as the 4-microsecond clock used by TCP [RFC 793]. Two
separate clocks may be required, one for the upper 24 bits and one
for the lower 24 bits.

To address the second problem, an implementation MUST provide each
4-tuple with an independent initial sequence number space. Then
opening a connection doesn't provide any information about initial
sequence numbers on other connections to the same host. RFC 1948
achieves this by adding a cryptographic hash of the 4-tuple and a
secret to each initial sequence number. For the secret, RFC 1948
recommends a combination of some truly-random data [RFC 1750], an
administratively-installed passphrase, the endpoint's IP address,
and the endpoint's boot time, but truly-random data is sufficient.
Care should be taken when changing the secret; such a change alters
all initial sequence number spaces, which might make an initial
sequence number for some 4-tuple equal a recently sent sequence
number for the same 4-tuple. To avoid this problem, the endpoint
might remember dead connection state for each 4-tuple or stay quiet
for 2 maximum segment lifetimes around such a change.

7.3. Quiet Time

DCCP endpoints, like TCP endpoints, must take care before initiating
connections when they boot. In particular, they MUST NOT send
packets whose sequence numbers are close to the sequence numbers of
packets lingering in the network from before the boot. The simplest
way to enforce this rule is for DCCP endpoints to avoid sending any
packets until one maximum segment lifetime (2 minutes) after boot.
Other enforcement mechanisms include remembering recent sequence
numbers across boots, and reserving the upper 8 or so bits of
initial sequence numbers for a persistent counter that decrements by
two each boot. (The latter mechanism would require disallowing
packets with short sequence numbers; see Section 7.6.1.)

7.4. Acknowledgement Numbers

Cumulative acknowledgements are meaningless in an unreliable
protocol. Therefore, DCCP's Acknowledgement Number field has a
different meaning than TCP's.

A received packet is classified as acknowledgeable if and only if
its header was succesfully processed by the receiving DCCP. In
terms of the pseudocode in Section 8.5, a received packet becomes



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acknowledgeable when the receiving endpoint reaches Step 8. This
means, for example, that all acknowledgeable packets have valid
header checksums and sequence numbers. The Acknowledgement Number
MUST equal GSR, the Greatest Sequence Number Received on an
acknowledgeable packet, for all packet types except DCCP-Sync and
DCCP-SyncAck.

"Acknowledgeable" does not refer to data processing. Even
acknowledgeable packets may have their application data dropped, due
to receive buffer overflow or corruption, for instance. Data
Dropped options report these data losses when necessary, letting
congestion control mechanisms distinguish between network losses and
endpoint losses. This issue is discussed further in Sections 11.4
and 11.7.

DCCP-Sync and DCCP-SyncAck packets' Acknowledgement Numbers differ
as follows: The Acknowledgement Number on a DCCP-Sync packet
corresponds to a received packet, but not necessarily an
acknowledgeable packet; in particular, it might correspond to an
out-of-sync packet whose options were not processed. The
Acknowledgement Number on a DCCP-SyncAck packet always corresponds
to an acknowledgeable DCCP-Sync packet; it might be less than GSR in
the presence of reordering.

7.5. Validity and Synchronization

Any DCCP endpoint might receive packets that are not actually part
of the current connection. For instance, the network might deliver
an old packet, an attacker might attempt to hijack a connection, or
the other endpoint might crash, causing a half-open connection.

DCCP, like TCP, uses sequence number checks to detect these cases.
Packets whose Sequence and/or Acknowledgement Numbers are out of
range are called sequence-invalid, and are not processed normally.

Unlike TCP, DCCP requires a synchronization mechanism to recover
from large bursts of loss. One endpoint might send so many packets
during a burst of loss that when one of its packets finally got
through, the other endpoint would label its Sequence Number as
invalid. A handshake of DCCP-Sync and DCCP-SyncAck packets recovers
from this case.

7.5.1. Sequence and Acknowledgement Number Windows

Each DCCP endpoint defines sequence validity windows that are
subsets of the Sequence and Acknowledgement Number spaces. These
windows correspond to packets the endpoint expects to receive in the
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windows always contain GSR and GSS, respectively. The window widths
are controlled by Sequence Window features for the two half-
connections.

The Sequence Number validity window for packets from DCCP B is [SWL,
SWH]. This window always contains GSR, the Greatest Sequence Number
Received on a sequence-valid packet from DCCP B. It is W packets
wide, where W is the value of the Sequence Window/B feature. One-
fourth of the sequence window, rounded down, is less than or equal
to GSR, and three-fourths is greater than GSR. (This asymmetric
placement assumes that bursts of loss are more common in the network
than significant reordering.)

invalid \| valid Sequence Numbers \| invalid
<---------\|==================================\|*--------->
GSR -\|GSR + 1 - GSR GSR +\|GSR + 1 +
floor(W/4)\|floor(W/4) ceil(3W/4)\|ceil(3W/4)
= SWL = SWH

The Acknowledgement Number validity window for packets from DCCP B
is [AWL, AWH]. The high end of the window, AWH, equals GSS, the
Greatest Sequence Number Sent by DCCP A; the window is W' packets
wide, where W' is the value of the Sequence Window/A feature.

invalid \| valid Acknowledgement Numbers \| invalid
<---------\|===================================\|--------->
GSS - W'\|GSS + 1 - W' GSS\|GSS + 1
= AWL = AWH

SWL and AWL are initially adjusted so that they are not less than
the initial Sequence Numbers received and sent, respectively:
SWL := max(GSR + 1 - floor(W/4), ISR),
AWL := max(GSS - W' + 1, ISS).
These adjustments MUST be applied only at the beginning of the
connection. (Long-lived connections may wrap sequence numbers so
that they appear to be less than ISR or ISS; the adjustments MUST
NOT be applied in that case.)

7.5.2. Sequence Window Feature

The Sequence Window/A feature determines the width of the Sequence
Number validity window used by DCCP B, and the width of the
Acknowledgement Number validity window used by DCCP A. DCCP A sends
a "Change L(Sequence Window, W)" option to notify DCCP B that the
Sequence Window/A value is W.

Sequence Window has feature number 3, and is non-negotiable. It
takes 48-bit (6-byte) integer values, like DCCP sequence numbers.	takes 48-bit (6-byte) integer values, like DCCP sequence numbers,
	but 1- to 5-byte values are also allowed in options; the receiver
	will pad on the left with zero bytes as necessary to total 48 bits.
	Change and Confirm options for Sequence Window are therefore between
Kohler/Handley/Floyd Section 7.5.2. [Page 53]	4 and 9 bytes long. New connections start with Sequence Window 100

INTERNET-DRAFT Expires: 7 September 2005 March 2005	for both endpoints. The minimum valid Sequence Window value is
	Wmin = 32. The maximum valid Sequence Window value is Wmax =
	2^46 - 1 = 70368744177663; circular sequence number comparisons
Change and Confirm options for Sequence Window are therefore 9 bytes	would stop working absent this constraint. Change options
long. New connections start with Sequence Window 100 for both	suggesting Sequence Window values out of this range are invalid and
endpoints. The minimum valid Sequence Window value is Wmin = 32.	MUST be handled accordingly.
The maximum valid Sequence Window value is Wmax = 2^46 - 1 =
70368744177663; circular sequence number comparisons would stop
working absent this constraint. Change options suggesting Sequence
Window values out of this range are invalid and MUST be handled
accordingly.

A proper Sequence Window/A value must reflect the number of packets
DCCP A expects to be in flight. Only DCCP A can anticipate this
number. Values that are too small increase the risk of the
endpoints getting out sync after bursts of loss, and values that are
much too small can prevent productive communication whether or not
there is loss. On the other hand, too-large values increase the
risk of connection hijacking; Section 7.5.5 quantifies this risk.
One good guideline is for each endpoint to set Sequence Window to
about five times the maximum number of packets it expects to send in
a round-trip time. Endpoints SHOULD send Change L(Sequence Window)
options as necessary as the connection progresses. Also, an
endpoint MUST NOT persistently send more than its Sequence Window
number of packets per round-trip time; that is, DCCP A MUST NOT
persistently send more than Sequence Window/A packets per RTT.

7.5.3. Sequence-Validity Rules

Sequence-validity depends on the received packet's type. This table
shows the sequence and acknowledgement number checks applied to each
packet; a packet is sequence-valid if it passes both tests, and
sequence-invalid if it does not. Many of the checks refer to the
sequence and acknowledgement number validity windows [SWL, SWH] and
[AWL, AWH] defined in Section 7.5.1.



















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Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Request SWL <= seqno <= SWH (*) N/A
DCCP-Response SWL <= seqno <= SWH (*) AWL <= ackno <= AWH
DCCP-Data SWL <= seqno <= SWH N/A
DCCP-Ack SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-DataAck SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-CloseReq GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Close GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Reset GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Sync SWL <= seqno AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno AWL <= ackno <= AWH

(*) Check not applied if connection is in LISTEN or REQUEST state.

In general, packets are sequence-valid if their Sequence and
Acknowledgement Numbers lie within the corresponding valid windows,
[SWL, SWH] and [AWL, AWH]. The exceptions to this rule are as
follows:

o Since DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets end a
connection, they cannot have Sequence Numbers less than or equal
to GSR, or Acknowledgement Numbers less than GAR.

o DCCP-Sync and DCCP-SyncAck Sequence Numbers are not strongly
checked. These packet types exist specifically to get the
endpoints back into sync; checking their Sequence Numbers would
eliminate their usefulness.

The lenient checks on DCCP-Sync and DCCP-SyncAck packets allow
continued operation after unusual events, such as endpoint crashes
and large bursts of loss, but there's no need for leniency in the
absence of unusual events -- that is, during ongoing successful
communication. Therefore, DCCP implementations SHOULD use the
following, more stringent checks for active connections, where a
connection is considered active if it has received valid packets
from the other endpoint within the last five round-trip times.

Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Sync SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno <= SWH AWL <= ackno <= AWH

Finally, an endpoint MAY apply the following more stringent checks
to DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets, further
lowering the probability of successful blind attacks using those



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packet types. Since these checks can cause extra synchronization
overhead and delay connection closing when packets are lost, they
should be considered experimental.

Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-CloseReq seqno == GSR + 1 GAR <= ackno <= AWH
DCCP-Close seqno == GSR + 1 GAR <= ackno <= AWH
DCCP-Reset seqno == GSR + 1 GAR <= ackno <= AWH

Note that sequence-validity is only one of the validity checks
applied to received packets.

7.5.4. Handling Sequence-Invalid Packets

Endpoints MUST ignore sequence-invalid DCCP-Sync and DCCP-SyncAck
packets, and MUST respond to other sequence-invalid packets with
(possibly rate-limited) DCCP-Sync packets. Each DCCP-Sync packet
MUST acknowledge the corresponding sequence-invalid packet's
Sequence Number, not GSR. The DCCP-Sync MUST use a new Sequence
Number, and thus will increase GSS; GSR will not change, however,
since the received packet was sequence-invalid.

On receiving a sequence-valid DCCP-Sync packet, the peer endpoint
(say, DCCP B) MUST update its GSR variable and reply with a DCCP-
SyncAck packet. The DCCP-SyncAck packet's Acknowledgement Number
will equal the DCCP-Sync's Sequence Number, not necessarily GSR.
Upon receiving this DCCP-SyncAck, which will be sequence-valid since
it acknowledges the DCCP-Sync, DCCP A will update its GSR variable,
and the endpoints will be back in sync. As an exception, if the
peer endpoint is in the REQUEST state, it MUST respond with a DCCP-
Reset instead of a DCCP-SyncAck. This serves to clean up DCCP A's
half-open connection.

To protect against denial-of-service attacks, DCCP implementations
SHOULD impose a rate limit on DCCP-Syncs sent in response to
sequence-invalid packets, such as not more than eight DCCP-Syncs per
second.

DCCP endpoints MUST NOT process sequence-invalid packets except,
perhaps, by generating a DCCP-Sync. For instance, options MUST NOT
but processed. An endpoint MAY temporarily preserve sequence-
invalid packets in case they become valid later, however; this can
reduce the impact of bursts of loss by delivering more packets to
the application. In particular, an endpoint MAY preserve sequence-
invalid packets for up to 2 round-trip times. If, within that time,
the relevant sequence windows change so that the packets become



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sequence-valid, the endpoint MAY process them again.

Note that sequence-invalid DCCP-Reset packets cause DCCP-Syncs to be
generated. This is because endpoints in an unsynchronized state
(CLOSED, REQUEST, and LISTEN) might not have enough information to
generate a proper DCCP-Reset on the first try. For example, if a
peer endpoint is in CLOSED state and receives a DCCP-Data packet, it
cannot guess the right Sequence Number to use on the DCCP-Reset it
generates (since the DCCP-Data packet has no Acknowledgement
Number). The DCCP-Sync generated in response to this bad reset
serves as a challenge, and contains enough information for the peer
to generate a proper DCCP-Reset. However, the new DCCP-Reset may
carry a different Reset Code than the original DCCP-Reset; probably
the new Reset Code will be 3, "No Connection". The endpoint SHOULD
use information from the original DCCP-Reset when possible.

7.5.5. Sequence Number Attacks

Sequence and Acknowledgement Numbers form DCCP's main line of
defense against attackers. An attacker that cannot guess sequence
numbers cannot easily manipulate or hijack a DCCP connection, and
requirements like careful initial sequence number choice eliminate
the most serious attacks.

An attacker might still send many packets with randomly chosen
Sequence and Acknowledgement Numbers, however. If one of those
probes ends up sequence-valid, it may shut down the connection or
otherwise cause problems. The easiest such attacks to execute are:

o Send DCCP-Data packets with random Sequence Numbers. If one of
these packets hits the valid sequence number window, the attack
packet's application data may be inserted into the data stream.

o Send DCCP-Sync packets with random Sequence and Acknowledgement
Numbers. If one of these packets hits the valid acknowledgement
number window, the receiver will shift its sequence number window
accordingly, getting out of sync with the correct endpoint --
perhaps permanently.

The attacker has to guess both Source and Destination Ports for any
of these attacks to succeed. Additionally, the connection would
have to be inactive for the DCCP-Sync attack to succeed, assuming
the victim implemented the more stringent checks for active
connections recommended in Section 7.5.3.

To quantify the probability of success, let N be the number of
attack packets the attacker is willing to send, W be the relevant
sequence window width, and L be the length of sequence numbers (24



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or 48). The attacker's best strategy is to space the attack packets
evenly over sequence space. Then the probability of hitting one
sequence number window is P = WN/2^L.

The success probability for a DCCP-Data attack using short sequence
numbers thus equals P = WN/2^24. For W = 100, then, the attacker
must send more than 83,000 packets to achieve a 50% chance of
success. For reference, the easiest TCP attack -- sending a SYN
with a random sequence number, which will cause a connection reset
if it falls within the window -- has W = 8760 (a common default) and
L = 32, and requires more than 245,000 packets to achieve a 50%
chance of success.

A fast connection's W will generally be high, increasing the attack
success probability for fixed N. If this probability gets
uncomfortably high with L = 24, the endpoint SHOULD prevent the use
of short sequence numbers by manipulating the Allow Short Sequence
Numbers feature (see Section 7.6.1). The probability limit depends
on the application, however. Some applications, such as those
already designed to handle corruption, are quite resilient to data
injection attacks.

The DCCP-Sync attack has L = 48, since DCCP-Sync packets use long
sequence numbers exclusively; in addition, the success probability
is halved, since only half the Sequence Number space is valid.
Attacks have a correspondingly smaller probability of success. For
a large W of 2000 packets, then, the attacker must send more than
10^11 packets to achieve a 50% chance of success.

Attacks involving DCCP-Ack, DCCP-DataAck, DCCP-CloseReq, DCCP-Close,
and DCCP-Reset packets are more difficult, since Sequence and
Acknowledgement Numbers must both be guessed. The probability of
attack success for these packet types equals P = WXN/2^(2L), where W
is the Sequence Number window, X is the Acknowledgement Number
window, and N and L are as before.

Since DCCP-Data attacks with short sequence numbers are relatively
easy for attackers to execute, DCCP has been engineered to prevent
these attacks from escalating to connection resets or other serious
consequences. In particular, any options whose processing might
cause the connection to be reset are ignored when they appear on
DCCP-Data packets.

7.5.6. Examples

In the following example, DCCP A and DCCP B recover from a large
burst of loss that runs DCCP A's sequence numbers out of DCCP B's
appropriate sequence number window.



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DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
--> DCCP-Data(seq 2) XXX
...
--> DCCP-Data(seq 100) XXX
--> DCCP-Data(seq 101) --> ???
seqno out of range;
send Sync
OK <-- DCCP-Sync(seq 11, ack 101) <--
(GSS=11,GSR=1)
--> DCCP-SyncAck(seq 102, ack 11) --> OK
(GSS=102,GSR=11) (GSS=11,GSR=102)

In the next example, a DCCP connection recovers from a simple blind
attack.

DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
ATTACKER --> DCCP-Data(seq 10^6) --> ???
seqno out of range;
send Sync
??? <-- DCCP-Sync(seq 11, ack 10^6) <--
ackno out of range; ignore
(GSS=1,GSR=10) (GSS=11,GSR=1)

The final example demonstrates recovery from a half-open connection.

DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
(Crash)
CLOSED OPEN
REQUEST --> DCCP-Request(seq 400) --> ???
!! <-- DCCP-Sync(seq 11, ack 400) <-- OPEN
REQUEST --> DCCP-Reset(seq 401, ack 11) --> (Abort)
REQUEST CLOSED
REQUEST --> DCCP-Request(seq 402) --> ...


7.6. Short Sequence Numbers

DCCP sequence numbers are 48 bits long. This large sequence space
protects DCCP connections against some blind attacks, such as the
injection of DCCP-Resets into the connection. However, DCCP-Data,
DCCP-Ack, and DCCP-DataAck packets, which make up the body of any
DCCP connection, may reduce header space by transmitting only the
lower 24 bits of the relevant Sequence and Acknowledgement Numbers.
The receiving endpoint will extend these numbers to 48 bits using
the following pseudocode:



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procedure Extend_Sequence_Number(S, REF)
/* S is a 24-bit sequence number from the packet header.
REF is the relevant 48-bit reference sequence number:
GSS if S is an Acknowledgement Number, and GSR if S is a
Sequence Number. */
Set REF_low := low 24 bits of REF
Set REF_hi := high 24 bits of REF
If REF_low (<) S /* circular comparison mod 2^24 */
&& S \|<\| REF_low, /* conventional, non-circular
comparison */
Return (((REF_hi + 1) mod 2^24) << 24) \| S
Otherwise,
Return (REF_hi << 24) \| S

The two different kinds of comparison in the if statement detect
when the low-order bits of the sequence space have wrapped. (The
circular comparison "REF_low (<) S" returns true if and only if
(S - REF_low), calculated using two's-complement arithmetic and then
represented as an unsigned number, is less than or equal to 2^23
(mod 2^24).) When this happens, the high-order bits are
incremented.

7.6.1. Allow Short Sequence Numbers Feature

Endpoints can require that all packets use long sequence numbers by
setting the Allow Short Sequence Numbers feature to false. This can
reduce the risk that data will be inappropriately injected into the
connection. DCCP A sends a "Change R(Allow Short Seqnos, 0)" option
to ask DCCP B to send only long sequence numbers.

Allow Short Sequence Numbers has feature number 2, and is server-
priority. It takes one-byte Boolean values. DCCP B MUST NOT send
packets with short sequence numbers when Allow Short Seqnos/B is
zero. Values of two or more are reserved. New connections start
with Allow Short Sequence Numbers 1 for both endpoints.

7.6.2. When to Avoid Short Sequence Numbers

Short sequence numbers reduce the rate DCCP connections can safely
achieve, and increase the risks of certain kinds of attacks,
including blind data injection. Very-high-rate DCCP connections,
and connections with large sequence windows (Section 7.5.2), SHOULD
NOT use short sequence numbers on their data packets. The attack
risk issues have been discussed in Section 7.5.5; we discuss the
rate limitation issue here.

The sequence-validity mechanism assumes that the network does not
deliver extremely old data. In particular, it assumes that the



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network must have dropped any packet by the time the connection
wraps around and uses its sequence number again. This constraint
limits the maximum connection rate that can be safely achieved. Let
MSL equal the maximum segment lifetime, P equal the average DCCP
packet size in bits, and L equal the length of sequence numbers (24
or 48 bits). Then the maximum safe rate, in bits per second, is R =
P*(2^L)/2MSL.

For the default MSL of 2 minutes, 1500-byte DCCP packets, and short
sequence numbers, the safe rate is therefore approximately 800 Mb/s.
Although 2 minutes is a very large MSL for any networks that could
sustain that rate with such small packets, long sequence numbers
allow much higher rates under the same constraints: up to
14 petabits a second for 1500-byte packets and the default MSL.

7.7. NDP Count and Detecting Application Loss

DCCP's sequence numbers increment by one on every packet, including
non-data packets (packets that don't carry application data). This
makes DCCP sequence numbers suitable for detecting any network loss,
but not for detecting the loss of application data. The NDP Count
option reports the length of each burst of non-data packets. This
lets the receiving DCCP reliably determine when a burst of loss
included application data.

+--------+--------+-------- ... --------+
\|00100101\| Length \| NDP Count \|
+--------+--------+-------- ... --------+
Type=37 Len=3-5 (1-3 bytes)

If a DCCP endpoint's Send NDP Count feature is one (see below), then
that endpoint MUST send an NDP Count option on every packet whose
immediate predecessor was a non-data packet. Non-data packets
consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
DCCP-Reset, DCCP-Sync, and DCCP-SyncAck. The other packet types,
namely DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck, are
considered data packets, although not all DCCP-Request and DCCP-
Response packets will actually carry application data.

The value stored in NDP Count equals the number of consecutive non-
data packets in the run immediately previous to the current packet.
Packets with no NDP Count option are considered to have NDP Count
zero.

The NDP Count option can carry one to three bytes of data. The
smallest option format that can hold the NDP Count SHOULD be used.





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With NDP Count, the receiver can reliably tell only whether a burst
of loss contained at least one data packet. For example, the
receiver cannot always tell whether a burst of loss contained a non-
data packet.

7.7.1. Usage Notes

Say that K consecutive sequence numbers are missing in some burst of
loss, and the Send NDP Count feature is on. Then some application
data was lost within those sequence numbers unless the packet
following the hole contains an NDP Count option whose value is
greater than or equal to K.

For example, say that an endpoint sent the following sequence of
non-data packets (Nx) and data packets (Dx).

N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13

Those packets would have NDP Counts as follows.

N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13
- 1 2 - 1 - - 1 - - - - 1 2

NDP Count is not useful for applications that include their own
sequence numbers with their packet headers.

7.7.2. Send NDP Count Feature

The Send NDP Count feature lets DCCP endpoints negotiate whether
they should send NDP Count options on their packets. DCCP A sends a
"Change R(Send NDP Count, 1)" option to ask DCCP B to send NDP Count
options.

Send NDP Count has feature number 7, and is server-priority. It
takes one-byte Boolean values. DCCP B MUST send NDP Count options
as described above when Send NDP Count/B is one, although it MAY
send NDP Count options even when Send NDP Count/B is zero. Values
of two or more are reserved. New connections start with Send NDP
Count 0 for both endpoints.

8. Event Processing

This section describes how DCCP connections move between states, and
which packets are sent when. Note that feature negotiation takes
place in parallel with the connection-wide state transitions
described here.





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8.1. Connection Establishment

DCCP connections' initiation phase consists of a three-way
handshake: an initial DCCP-Request packet sent by the client, a
DCCP-Response sent by the server in reply, and finally an
acknowledgement from the client, usually via a DCCP-Ack or DCCP-
DataAck packet. The client moves from the REQUEST state to
PARTOPEN, and finally to OPEN; the server moves from LISTEN to
RESPOND, and finally to OPEN.

Client State Server State
CLOSED LISTEN
1. REQUEST --> Request -->
2. <-- Response <-- RESPOND
3. PARTOPEN --> Ack, DataAck -->
4. <-- Data, Ack, DataAck <-- OPEN
5. OPEN <-> Data, Ack, DataAck <-> OPEN


8.1.1. Client Request

When a client decides to initiate a connection, it enters the
REQUEST state, chooses an initial sequence number (Section 7.2), and
sends a DCCP-Request packet using that sequence number to the
intended server.

DCCP-Request packets will commonly carry feature negotiation options
that open negotiations for various connection parameters, such as
preferred congestion control IDs for each half-connection. They may
also carry application data, but the client should be aware that the
server may not accept such data.

A client in the REQUEST state SHOULD use an exponential-backoff	A client in the REQUEST state SHOULD send use an exponential-backoff
timer to send new DCCP-Request packets if no response is received.
The first retransmission should occur after approximately one
second, backing off to not less than one packet every 64 seconds; or
the endpoint can use whatever retransmission strategy is followed
for retransmitting TCP SYNs. Each new DCCP-Request MUST increment
the Sequence Number by one, and MUST contain the same Service Code
and application data as the original DCCP-Request.

A client MAY give up on its DCCP-Requests after some time
(3 minutes, for example). When it does, it SHOULD send a DCCP-Reset
packet to the server with Reset Code 2, "Aborted", to clean up state
in case one or more of the Requests actually arrived. A client in
REQUEST state has never received an initial sequence number from its
peer, so the DCCP-Reset's Acknowledgement Number MUST be set to
zero.



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The client leaves the REQUEST state for PARTOPEN when it receives a
DCCP-Response from the server.

8.1.2. Service Codes

Each DCCP-Request contains a 32-bit Service Code, which identifies
the application-level service to which the client application is
trying to connect. Service Codes should correspond to application
services and protocols. For example, there might be a Service Code
for SIP control connections and one for RTP audio connections.
Middleboxes, such as firewalls, can use the Service Code to identify
the application running on a nonstandard port (assuming the DCCP
header has not been encrypted).

Endpoints MUST associate a Service Code with every DCCP socket, both
actively and passively opened. The application will generally
supply this Service Code. Each active socket MUST have exactly one
Service Code. Passive sockets MAY, at the implementation's
discretion, be associated with more than one Service Code; this
might let multiple applications, or multiple versions of the same
application, listen on the same port, differentiated by Service
Code. If the DCCP-Request's Service Code doesn't match any of the
server's Service Codes for the given port, the server MUST reject
the request by sending a DCCP-Reset packet with Reset Code 8, "Bad
Service Code". A middlebox MAY also send such a DCCP-Reset in
response to packets whose Service Code is considered unsuitable.

Service Codes are not intended to be DCCP-specific, and are
allocated by IANA. Following the policies outlined in RFC 2434,	allocated by IANA. Following the policies outlined in [RFC 2434],
most Service Codes are allocated First Come First Served, subject to
the following guidelines.

o Service Codes are allocated one at a time, or in small blocks. A
short English description of the intended service is REQUIRED to
obtain a Service Code assignment, but no specification,
standards-track or otherwise, is necessary. IANA maintains an
association of Service Codes to the corresponding phrases.

o Users request specific Service Code values. We suggest that
users request Service Codes that can be interpreted as meaningful
four-byte ASCII strings. Thus, the "Frobodyne Plotz Protocol"
might correspond to "fdpz", or the number 1717858426. The
canonical interpretation of a Service Code field is numeric.

o Service Codes whose bytes each have values in the set {32, 45-57,
65-90} use a Specification Required allocation policy. That is,
these Service Codes are used for international standard or
standards-track specifications, IETF or otherwise. (This set



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consists of the ASCII digits, uppercase letters, and characters
space, '-', '.', and '/'.)

o Service Codes whose high-order byte equals 63 (ASCII '?') are
reserved for Private Use.

o Service Code 0 represents the absence of a meaningful Service
Code, and MUST NOT be allocated.	Code, and MUST never be allocated.

This design for Service Code allocation is based on the allocation
of 4-byte identifiers for Macintosh resources, PNG chunks, and
TrueType and OpenType tables.

8.1.3. Server Response

In the second phase of the three-way handshake, the server moves
from the LISTEN state to RESPOND, and sends a DCCP-Response message
to the client. In this phase, a server will often specify the
features it would like to use, either from among those the client
requested, or in addition to those. Among these options is the
congestion control mechanism the server expects to use.

The server MAY respond to a DCCP-Request packet with a DCCP-Reset
packet to refuse the connection. Relevant Reset Codes for refusing
a connection include 7, "Connection Refused", when the DCCP-
Request's Destination Port did not correspond to a DCCP port open
for listening; 8, "Bad Service Code", when the DCCP-Request's
Service Code did not correspond to the service code registered with
the Destination Port; and 9, "Too Busy", when the server is
currently too busy to respond to requests. The server SHOULD limit
the rate at which it generates these resets, for example to not more
than 1024 per second.

The server SHOULD NOT retransmit DCCP-Response packets; the client
will retransmit the DCCP-Request if necessary. (Note that the
"retransmitted" DCCP-Request will have, at least, a different
sequence number from the "original" DCCP-Request. The server can
thus distinguish true retransmissions from network duplicates.) The
server will detect that the retransmitted DCCP-Request applies to an
existing connection because of its Source and Destination Ports.
Every valid DCCP-Request received while the server is in the RESPOND
state MUST elicit a new DCCP-Response. Each new DCCP-Response MUST
increment the server's Sequence Number by one, and MUST include the
same application data, if any, as the original DCCP-Response.

The server MUST NOT accept more than one piece of DCCP-Request
application data per connection. In particular, the DCCP-Response
sent in reply to a retransmitted DCCP-Request with application data



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SHOULD contain a Data Dropped option, in which the retransmitted
DCCP-Request data is reported with Drop Code 0, Protocol
Constraints. The original DCCP-Request SHOULD also be reported in
the Data Dropped option, either in a Normal Block (if the server
accepted the data, or there was no data), or in a Drop Code 0 Drop
Block (if the server refused the data the first time as well).

The Data Dropped and Init Cookie options are particularly useful for
DCCP-Response packets (Sections 11.7 and 8.1.4).

The server leaves the RESPOND state for OPEN when it receives a
valid DCCP-Ack from the client, completing the three-way handshake.
It MAY also leave the RESPOND state for CLOSED after a timeout of
not less than 4MSL (8 minutes); when doing so, it SHOULD send a
DCCP-Reset with Reset Code 2, "Aborted", to clean up state at the
client.

8.1.4. Init Cookie Option

+--------+--------+--------+--------+--------+--------
\|00100100\| Length \| Init Cookie Value ...
+--------+--------+--------+--------+--------+--------
Type=36


The Init Cookie option lets a DCCP server avoid having to hold any
state until the three-way connection setup handshake has completed,
in a similar fashion as TCP SYN cookies [SYNCOOKIES]. The server
wraps up the Service Code, server port, and any options it cares
about from both the DCCP-Request and DCCP-Response in an opaque
cookie. Typically the cookie will be encrypted using a secret known
only to the server and include a cryptographic checksum or magic
value so that correct decryption can be verified. When the server
receives the cookie back in the response, it can decrypt the cookie
and instantiate all the state it avoided keeping. In the meantime,
it need not move from the LISTEN state.

The Init Cookie option MUST NOT be sent on DCCP-Request or DCCP-Data
packets, and any such options received on DCCP-Request or DCCP-Data
packets MUST be ignored. The server MAY include an Init Cookie
option in its DCCP-Response. If so, then the client MUST echo the
same Init Cookie option in each succeeding DCCP packet until one of
those packets is acknowledged, meaning the three-way handshake has
completed, or the connection is reset. (As a result, the client
MUST NOT use DCCP-Data packets until the three-way handshake
completes or the connection is reset.) The server SHOULD design its
Init Cookie format so that Init Cookies can be checked for
tampering; it SHOULD respond to a tampered Init Cookie option by



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resetting the connection with Reset Code 10, "Bad Init Cookie".

Init Cookie's precise implementation need not be specified here;
since Init Cookies are opaque to the client, there are no
interoperability concerns. An example cookie format might encrypt
(using a secret key) the connection's initial sequence and
acknowledgement numbers, ports, Service Code, any options included
on the DCCP-Request packet and the corresponding DCCP-Reply, a
random salt, and a magic number. On receiving a reflected Init
Cookie, the server would decrypt the cookie, validate it by checking
its magic number, sequence numbers, and ports, and, if valid, create
a corresponding socket using the options.

Init Cookies are limited to at most 253 bytes in length.

8.1.5. Handshake Completion

When the client receives a DCCP-Response from the server, it moves
from the REQUEST state to PARTOPEN and completes the three-way
handshake by sending a DCCP-Ack packet to the server. The client
remains in PARTOPEN until it can be sure that the server has
received some packet the client sent from PARTOPEN (either the
initial DCCP-Ack or a later packet). Clients in the PARTOPEN state
that want to send data MUST do so using DCCP-DataAck packets, not
DCCP-Data packets. This is because DCCP-Data packets lack
Acknowledgement Numbers, so the server can't tell from a DCCP-Data
packet whether the client saw its DCCP-Response. Furthermore, if
the DCCP-Response included an Init Cookie, that Init Cookie MUST be
included on every packet sent in PARTOPEN.

The single DCCP-Ack sent when entering the PARTOPEN state might, of
course, be dropped by the network. The client SHOULD ensure that
some packet gets through eventually. The preferred mechanism would
be a roughly 200-millisecond timer, set every time a packet is
transmitted in PARTOPEN. If this timer goes off and the client is
still in PARTOPEN, the client generates another DCCP-Ack and backs
off the timer. If the client remains in PARTOPEN for more than 4MSL
(8 minutes), it SHOULD reset the connection with Reset Code 2,
"Aborted".

The client leaves the PARTOPEN state for OPEN when it receives a
valid packet other than DCCP-Response, DCCP-Reset, or DCCP-Sync from
the server.

8.2. Data Transfer

In the central data transfer phase of the connection, both server
and client are in the OPEN state.



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DCCP A sends DCCP-Data and DCCP-DataAck packets to DCCP B due to
application events on host A. These packets are congestion-
controlled by the CCID for the A-to-B half-connection. In contrast,
DCCP-Ack packets sent by DCCP A are controlled by the CCID for the
B-to-A half-connection. Generally, DCCP A will piggyback
acknowledgement information on DCCP-Data packets when acceptable,
creating DCCP-DataAck packets. DCCP-Ack packets are used when there
is no data to send from DCCP A to DCCP B, or when the congestion
state of the A-to-B CCID will not allow data to be sent.

DCCP-Sync and DCCP-SyncAck packets may also occur in the data
transfer phase. Some cases causing DCCP-Sync generation are
discussed in Section 7.5. One important distinction between DCCP-
Sync packets and other packet types is that DCCP-Sync elicits an
immediate acknowledgement. On receiving a valid DCCP-Sync packet, a
DCCP endpoint MUST immediately generate and send a DCCP-SyncAck
response (subject to any implementation rate limits); the
Acknowledgement Number on that DCCP-SyncAck MUST equal the Sequence
Number of the DCCP-Sync.

A particular DCCP implementation might decide to initiate feature
negotiation only once the OPEN state was reached, in which case it
might not allow data transfer until some time later. Data received
during that time SHOULD be rejected and reported using a Data
Dropped Drop Block with Drop Code 0, Protocol Constraints (see
Section 11.7).

8.3. Termination

DCCP connection termination uses a handshake consisting of an
optional DCCP-CloseReq packet, a DCCP-Close packet, and a DCCP-Reset
packet. The server moves from the OPEN state, possibly through the
CLOSEREQ state, to CLOSED; the client moves from OPEN through
CLOSING to TIMEWAIT, and after 2MSL wait time (4 minutes), to
CLOSED.

The sequence DCCP-CloseReq, DCCP-Close, DCCP-Reset is used when the
server decides to close the connection, but doesn't want to hold
TIMEWAIT state:

Client State Server State
OPEN OPEN
1. <-- CloseReq <-- CLOSEREQ
2. CLOSING --> Close -->
3. <-- Reset <-- CLOSED (LISTEN)
4. TIMEWAIT
5. CLOSED




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A shorter sequence occurs when the client decides to close the
connection.

Client State Server State
OPEN OPEN
1. CLOSING --> Close -->
2. <-- Reset <-- CLOSED (LISTEN)
3. TIMEWAIT
4. CLOSED

Finally, the server can decide to hold TIMEWAIT state:

Client State Server State
OPEN OPEN
1. <-- Close <-- CLOSING
2. CLOSED --> Reset -->
3. TIMEWAIT
4. CLOSED (LISTEN)


In all cases, the receiver of the DCCP-Reset packet holds TIMEWAIT
state for the connection. As in TCP, TIMEWAIT state, where an
endpoint quietly preserves a socket for 2MSL (4 minutes) after its
connection has closed, ensures that no connection duplicating the
current connection's source and destination addresses and ports can
start up while old packets might remain in the network.

The termination handshake proceeds as follows. The receiver of a
valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet.
The receiver of a valid DCCP-Close packet MUST respond with a DCCP-
Reset packet, with Reset Code 1, "Closed". The receiver of a valid
DCCP-Reset packet -- which is also the sender of the DCCP-Close
packet (and possibly the receiver of the DCCP-CloseReq packet) --
will hold TIMEWAIT state for the connection.

A DCCP-Reset packet completes every DCCP connection, whether the
termination is clean (due to application close; Reset Code 1,
"Closed") or unclean. Unlike TCP, which has two distinct
termination mechanisms (FIN and RST), DCCP ends all connections in a
uniform manner. This is justified because some aspects of
connection termination are the same independent of whether
termination was clean. For instance, the endpoint that receives a
valid DCCP-Reset SHOULD hold TIMEWAIT state for the connection.
Processors that must distinguish between clean and unclean
termination can examine the Reset Code. DCCP-Reset packets MUST NOT
be generated in response to received DCCP-Reset packets. DCCP
implementations generally transition to the CLOSED state after
sending a DCCP-Reset packet.



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Endpoints in the CLOSEREQ and CLOSING states MUST retransmit DCCP-
CloseReq and DCCP-Close packets, respectively, until leaving those
states. The retransmission timer should initially be set to go off
in two round-trip times, and should back off to not less than once
every 64 seconds if no relevant response is received.

Only the server can send a DCCP-CloseReq packet or enter the
CLOSEREQ state. A server receiving a sequence-valid DCCP-CloseReq
packet MUST respond with a DCCP-Sync packet, and otherwise ignore
the DCCP-CloseReq.

DCCP-Data, DCCP-DataAck, and DCCP-Ack packets received in CLOSEREQ
or CLOSE states MAY be either processed or ignored.

8.3.1. Abnormal Termination

DCCP endpoints generate DCCP-Reset packets to terminate connections
abnormally; a DCCP-Reset packet may be generated from any state.
Resets sent in the CLOSED, LISTEN, and TIMEWAIT states use Reset
Code 3, "No Connection", unless otherwise specified. Resets sent in
the REQUEST or RESPOND states use Reset Code 4, "Packet Error",
unless otherwise specified.

DCCP endpoints in CLOSED or LISTEN state may need to generate a
DCCP-Reset packet in response to a packet received from a peer.
Since these states have no associated sequence number variables, the
Sequence and Acknowledgement Numbers on the DCCP-Reset packet R are
taken from the received packet P, as follows.

1. If P.ackno exists, then set R.seqno := P.ackno + 1. Otherwise,
set R.seqno := 0.

2. Set R.ackno := P.seqno.

3. If the packet used short sequence numbers (P.X == 0), then set
the upper 24 bits of R.seqno and R.ackno to 0.

8.4. DCCP State Diagram

The most common state transitions discussed above can be summarized
in the following state diagram. The diagram is illustrative; the
text in Section 8.5 and elsewhere should be considered definitive.
For example, there are arcs (not shown) from every state except
CLOSED to TIMEWAIT, contingent on the receipt of a valid DCCP-Reset.







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+---------------------------+ +---------------------------+
\| v v \|
\| +----------+ \|
\| +-------------+ CLOSED +------------+ \|
\| \| passive +----------+ active \| \|
\| \| open open \| \|
\| \| snd Request \| \|
\| v v \|
\| +----------+ +----------+ \|
\| \| LISTEN \| \| REQUEST \| \|
\| +----+-----+ +----+-----+ \|
\| \| rcv Request rcv Response \| \|
\| \| snd Response snd Ack \| \|
\| v v \|
\| +----------+ +----------+ \|
\| \| RESPOND \| \| PARTOPEN \| \|
\| +----+-----+ +----+-----+ \|
\| \| rcv Ack/DataAck rcv packet \| \|
\| \| \| \|
\| \| +----------+ \| \|
\| +------------>\| OPEN \|<-----------+ \|
\| +--+-+--+--+ \|
\| server active close \| \| \| active close \|
\| snd CloseReq \| \| \| or rcv CloseReq \|
\| \| \| \| snd Close \|
\| \| \| \| \|
\| +----------+ \| \| \| +----------+ \|
\| \| CLOSEREQ \|<---------+ \| +--------->\| CLOSING \| \|
\| +----+-----+ \| +----+-----+ \|
\| \| rcv Close \| rcv Reset \| \|
\| \| snd Reset \| \| \|
\|<---------+ \| v \|
\| \| +----+-----+ \|
\| rcv Close \| \| TIMEWAIT \| \|
\| snd Reset \| +----+-----+ \|
+-----------------------------+ \| \|
+-----------+
2MSL timer expires


8.5. Pseudocode

This section presents an algorithm describing the processing steps a
DCCP endpoint must go through when it receives a packet. A DCCP
implementation need not implement the algorithm as it is described
here, but any implementation MUST generate observable effects
exactly as indicated by this pseudocode, except where allowed
otherwise by another part of this document.



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The received packet is written as P, the socket as S.
Packet variables P.seqno and P.ackno are 48-bit sequence numbers.
Socket variables:
S.SWL - sequence number window low
S.SWH - sequence number window high
S.AWL - acknowledgement number window low
S.AWH - acknowledgement number window high
S.ISS - initial sequence number sent
S.ISR - initial sequence number received
S.OSR - first OPEN sequence number received
S.GSS - greatest sequence number sent
S.GSR - greatest valid sequence number received
S.GAR - greatest valid acknowledgement number received on a
non-Sync; initialized to S.ISS
"Send packet" actions always use, and increment, S.GSS.

Step 1: Check header basics
/* This step checks for malformed packets. Packets that fail
these checks are ignored -- they do not receive Resets in
response */
If the packet is shorter than 12 bytes, drop packet and return
If the packet type is not understood, drop packet and return
If P.Data Offset is too small for packet type, or too large for
packet, drop packet and return
If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
has short sequence numbers), drop packet and return
If the header checksum is incorrect, drop packet and return
If P.CsCov is too large for the packet size, drop packet and
return

Step 2: Check ports and process TIMEWAIT state
Look up flow ID in table and get corresponding socket
If no socket, or S.state == TIMEWAIT,
Generate Reset(No Connection) unless P.type == Reset
Drop packet and return

Step 3: Process LISTEN state
If S.state == LISTEN,
If P.type == Request or P contains a valid Init Cookie option,
/* Must scan the packet's options to check for an Init
Cookie. Only the Init Cookie is processed here,
however; other options are processed in Step 8. This
scan need only be performed if the endpoint uses Init
Cookies */
/* Generate a new socket and switch to that socket */
Set S := new socket for this port pair
S.state = RESPOND
Choose S.ISS (initial seqno) or set from Init Cookie



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Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookie
Continue with S.state == RESPOND
/* A Response packet will be generated in Step 11 */
Otherwise,
Generate Reset(No Connection) unless P.type == Reset
Drop packet and return

Step 4: Prepare sequence numbers in REQUEST
If S.state == REQUEST,
If (P.type == Response or P.type == Reset)
and S.AWL <= P.ackno <= S.AWH,
/* Set sequence number variables corresponding to the
other endpoint, so P will pass the tests in Step 6 */
Set S.GSR, S.ISR, S.SWL, S.SWH
/* Response processing continues in Step 10; Reset
processing continues in Step 9 */
Otherwise,
/* Only Response and Reset are valid in REQUEST state */
Generate Reset(Packet Error)
Drop packet and return

Step 5: Prepare sequence numbers for Sync
If P.type == Sync or P.type == SyncAck,
If S.AWL <= P.ackno <= S.AWH and P.seqno >= S.SWL,
/* P is valid, so update sequence number variables
accordingly. After this update, P will pass the tests
in Step 6. A SyncAck is generated if necessary in
Step 15 */
Update S.GSR, S.SWL, S.SWH
Otherwise,
Drop packet and return

Step 6: Check sequence numbers
Let LSWL = S.SWL and LAWL = S.AWL
If P.type == CloseReq or P.type == Close or P.type == Reset,
LSWL := S.GSR + 1, LAWL := S.GAR
If LSWL <= P.seqno <= S.SWH
and (P.ackno does not exist or LAWL <= P.ackno <= S.AWH),
Update S.GSR, S.SWL, S.SWH
If P.type != Sync,
Update S.GAR
Otherwise,
Send Sync packet acknowledging P.seqno
Drop packet and return

Step 7: Check for unexpected packet types
If (S.is_server and P.type == CloseReq)
or (S.is_server and P.type == Response)



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or (S.is_client and P.type == Request)
or (S.state >= OPEN and P.type == Request
and P.seqno >= S.OSR)
or (S.state >= OPEN and P.type == Response
and P.seqno >= S.OSR)
or (S.state == RESPOND and P.type == Data),
Send Sync packet acknowledging P.seqno
Drop packet and return

Step 8: Process options and mark acknowledgeable
/* Option processing is not specifically described here.
Certain options, such as Mandatory, may cause the connection
to be reset, in which case Steps 9 and on are not executed */
Mark packet as acknowledgeable (in Ack Vector terms, Received
or Received ECN Marked)

Step 9: Process Reset
If P.type == Reset,
Tear down connection
S.state := TIMEWAIT
Set TIMEWAIT timer
Drop packet and return

Step 10: Process REQUEST state (second part)
If S.state == REQUEST,
/* If we get here, P is a valid Response from the server (see
Step 4), and we should move to PARTOPEN state. PARTOPEN
means send an Ack, don't send Data packets, retransmit
Acks periodically, and always include any Init Cookie from
the Response */
S.state := PARTOPEN
Set PARTOPEN timer
Continue with S.state == PARTOPEN
/* Step 12 will send the Ack completing the three-way
handshake */

Step 11: Process RESPOND state
If S.state == RESPOND,
If P.type == Request,
Send Response, possibly containing Init Cookie
If Init Cookie was sent,
Destroy S and return
/* Step 3 will create another socket when the client
completes the three-way handshake */
Otherwise,
S.OSR := P.seqno
S.state := OPEN




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Step 12: Process PARTOPEN state
If S.state == PARTOPEN,
If P.type == Response,
Send Ack
Otherwise, if P.type != Sync,
S.OSR := P.seqno
S.state := OPEN

Step 13: Process CloseReq
If P.type == CloseReq and S.state < CLOSEREQ,
Generate Close
S.state := CLOSING
Set CLOSING timer

Step 14: Process Close
If P.type == Close,
Generate Reset(Closed)
Tear down connection
Drop packet and return

Step 15: Process Sync
If P.type == Sync,
Generate SyncAck

Step 16: Process data
/* At this point any application data on P can be passed to the
application, except that the application MUST NOT receive
data from more than one Request or Response */

9. Checksums

DCCP uses a header checksum to protect its header against
corruption. Generally, this checksum also covers any application
data. DCCP applications can, however, request that the header
checksum cover only part of the application data, or perhaps no
application data at all. Link layers may then reduce their
protection on unprotected parts of DCCP packets. For some noisy
links, and applications that can tolerate corruption, this can
greatly improve delivery rates and perceived performance.

Checksum coverage may eventually impact congestion control
mechanisms as well. A packet with corrupt application data and
complete checksum coverage is treated as lost. This incurs a heavy-
duty loss response from the sender's congestion control mechanism,
which can unfairly penalize connections on links with high
background corruption. The combination of reduced checksum coverage
and Data Checksum options may let endpoints report packets as
corrupt rather than dropped, using Data Dropped options and Drop



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Code 3 (see Section 11.7). This may eventually benefit
applications. However, further research is required to determine an
appropriate response to corruption, which can sometimes correlate
with congestion. Corrupt packets currently incur a loss response.

The Data Checksum option, which contains a strong CRC, lets
endpoints detect application data corruption. An API can then be
used to avoid delivering corrupt data to the application, even if
links deliver corrupt data to the endpoint due to reduced checksum
coverage. However, the use of reduced checksum coverage for
applications that demand correct data is currently considered
experimental. This is because the combined loss-plus-corruption
rate for packets with reduced checksum coverage may be significantly
higher than that for packets with full checksum coverage, although
the loss rate will generally be lower. Actual behavior will depend
on link design; further research and experience is required.

Reduced checksum coverage introduces some security considerations;
see Section 18.1. See Appendix B for further motivation and
discussion. DCCP's implementation of reduced checksum coverage was
inspired by UDP-Lite [RFC 3828].

9.1. Header Checksum Field

DCCP uses the TCP/IP checksum algorithm. The Checksum field in the
DCCP generic header (see Section 5.1) equals the 16 bit one's
complement of the one's complement sum of all 16 bit words in the
DCCP header, DCCP options, a pseudoheader taken from the network-
layer header, and, depending on the value of the Checksum Coverage
field, some or all of the application data. When calculating the
checksum, the Checksum field itself is treated as 0. If a packet
contains an odd number of header and payload bytes to be
checksummed, 8 zero bits are added on the right to form a 16 bit
word for checksum purposes. The pad byte is not transmitted as part
of the packet.

The pseudoheader is calculated as for TCP. For IPv4, it is 96 bits
long, and consists of the IPv4 source and destination addresses, the
IP protocol number for DCCP (padded on the left with 8 zero bits),
and the DCCP length as a 16-bit quantity (the length of the DCCP
header with options, plus the length of any data); see RFC 793	header with options, plus the length of any data); see Section 3.1
(Section 3.1). For IPv6, it is 320 bits long, and consists of the	of [RFC 793]. For IPv6, it is 320 bits long, and consists of the
IPv6 source and destination addresses, the DCCP length as a 32-bit
quantity, and the IP protocol number for DCCP (padded on the left
with 24 zero bits); see RFC 2460 (Section 8.1).	with 24 zero bits); see Section 8.1 of [RFC 2460].

Packets with invalid header checksums MUST be ignored. In
particular, their options MUST NOT be processed.



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9.2. Header Checksum Coverage Field

The Checksum Coverage field in the DCCP generic header (see Section
5.1) specifies what parts of the packet are covered by the Checksum
field, as follows:

CsCov = 0 The Checksum field covers the DCCP header, DCCP
options, network-layer pseudoheader, and all
application data in the packet, possibly padded on
the right with zeros to an even number of bytes.

CsCov = 1-15 The Checksum field covers the DCCP header, DCCP
options, network-layer pseudoheader, and the initial
(CsCov-1)*4 bytes of the packet's application data.

Thus, if CsCov is 1, none of the application data is protected by
the header checksum. The value (CsCov-1)*4 MUST be less than or
equal to the length of the application data. Packets with invalid
CsCov values MUST be ignored; in particular, their options MUST NOT
be processed. The meanings of values other than 0 and 1 should be
considered experimental.

Values other than 0 specify that corruption is acceptable in some or
all of the DCCP packet's application data. In fact, DCCP cannot
even detect corruption in areas not covered by the header checksum,
unless the Data Checksum option is used. Applications should not
make any assumptions about the correctness of received data not
covered by the checksum, and should if necessary introduce their own
validity checks.

A DCCP application interface should let sending applications suggest
a value for CsCov for sent packets, defaulting to 0 (full coverage).
The Minimum Checksum Coverage feature, described below, lets an
endpoint refuse delivery of application data on packets with partial
checksum coverage; by default, only fully-covered application data
is accepted. Lower layers that support partial error detection MAY
use the Checksum Coverage field as a hint of where errors do not
need to be detected. Lower layers MUST use a strong error detection
mechanism to detect at least errors that occur in the sensitive part
of the packet, and discard damaged packets. The sensitive part
consists of the bytes between the first byte of the IP header and
the last byte identified by Checksum Coverage.

For more details on application and lower-layer interface issues
relating to partial checksumming, see [RFC 3828].






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9.2.1. Minimum Checksum Coverage Feature

The Minimum Checksum Coverage feature lets a DCCP endpoint determine
whether its peer is willing to accept packets with reduced Checksum
Coverage. For example, DCCP A sends a "Change R(Minimum Checksum
Coverage, 1)" option to DCCP B to check whether B is willing to
accept packets with Checksum Coverage set to 1.

Minimum Checksum Coverage has feature number 8, and is server-
priority. It takes one-byte integer values between 0 and 15; values
of 16 or more are reserved. Minimum Checksum Coverage/B reflects
values of Checksum Coverage that DCCP B finds unacceptable. Say
that the value of Minimum Checksum Coverage/B is MinCsCov. Then:

o If MinCsCov = 0, then DCCP B only finds packets with CsCov = 0
acceptable.

o If MinCsCov > 0, then DCCP B additionally finds packets with
CsCov >= MinCsCov acceptable.

DCCP B MAY refuse to process application data from packets with
unacceptable Checksum Coverage. Such packets SHOULD be reported
using Data Dropped options (Section 11.7) with Drop Code 0, Protocol
Constraints. New connections start with Minimum Checksum Coverage 0
for both endpoints.

9.3. Data Checksum Option

The Data Checksum option holds a 32-bit CRC-32c cyclic redundancy-
check code of a DCCP packet's application data.

+--------+--------+--------+--------+--------+--------+
\|00101100\|00000110\| CRC-32c \|
+--------+--------+--------+--------+--------+--------+
Type=44 Length=6

The sending DCCP computes the CRC of the bytes comprising the
application data area and stores it in the option data. The CRC-32c
algorithm used for Data Checksum is the same as that used for SCTP
[RFC 3309]; note that the CRC-32c of zero bytes of data equals zero.
The DCCP header checksum will cover the Data Checksum option, so the
data checksum must be computed before the header checksum.

A DCCP endpoint receiving a packet with a Data Checksum option
SHOULD compute the received application data's CRC-32c, using the
same algorithm as the sender, and compare the result with the Data
Checksum value. (The endpoint can indicate its willingness to check
Data Checksums using the Check Data Checksum feature, described



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below.) If the CRCs differ, the endpoint reacts in one of two ways.

o The receiving application may have requested delivery of known-
corrupt data via some optional API. In this case, the packet's
data MUST be delivered to the application, with a note that it is
known to be corrupt. Furthermore, the receiving endpoint MUST
report the packet as delivered corrupt using a Data Dropped
option (Drop Code 7, Delivered Corrupt).

o Otherwise, the receiving endpoint MUST drop the application data,
and report that data as dropped due to corruption using a Data
Dropped option (Drop Code 3, Corrupt).

In either case, the packet is considered acknowledgeable (since its
header was processed), and will therefore be acknowledged using the
equivalent of Ack Vector's Received or Received ECN Marked states.

Although Data Checksum is intended for packets containing
application data, it may be included on other packets, such as DCCP-
Ack, DCCP-Sync, and DCCP-SyncAck. The receiver SHOULD calculate the
application data area's CRC-32c on such packets, just as it does for
DCCP-Data and similar packets; and if the CRCs differ, the packets
similarly MUST be reported using Data Dropped options (Drop Code 3),
although their application data areas would not be delivered to the
application in any case.

9.3.1. Check Data Checksum Feature

The Check Data Checksum feature lets a DCCP endpoint determine
whether its peer will definitely check Data Checksum options.
DCCP A sends a Mandatory "Change R(Check Data Checksum, 1)" option
to DCCP B to require it to check Data Checksum options (the
connection will be reset if it cannot).

Check Data Checksum has feature number 9, and is server-priority.
It takes one-byte Boolean values. DCCP B MUST check any received
Data Checksum options when Check Data Checksum/B is one, although it
MAY check them even when Check Data Checksum/B is zero. Values of
two or more are reserved. New connections start with Check Data
Checksum 0 for both endpoints.

9.3.2. Usage Notes

Internet links must normally apply strong integrity checks to the
packets they transmit [RFC 3828, RFC 3819]. This is the default	packets they transmit [RFC 3828] [RFC 3819]. This is the default
case when the DCCP header's Checksum Coverage value equals zero
(full coverage). However, the DCCP Checksum Coverage value might
not be zero. By setting partial Checksum Coverage, the application



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indicates that it can tolerate corruption in the unprotected part of
the application data. Recognizing this, link layers may reduce
error detection and/or correction strength when transmitting this
unprotected part. This, in turn, can significantly increase the
likelihood of the endpoint receiving corrupt data; Data Checksum
lets the receiver detect that corruption with very high probability.

10. Congestion Control

Each congestion control mechanism supported by DCCP is assigned a
congestion control identifier, or CCID: a number from 0 to 255.
During connection setup, and optionally thereafter, the endpoints
negotiate their congestion control mechanisms by negotiating the
values for their Congestion Control ID features. Congestion Control
ID has feature number 1. The CCID/A value equals the CCID in use
for the A-to-B half-connection. DCCP B sends a "Change R(CCID, K)"
option to ask DCCP A to use CCID K for its data packets.

CCID is a server-priority feature, so CCID negotiation options can
list multiple acceptable CCIDs, sorted in descending order of
priority. For example, the option "Change R(CCID, 2 3 4)" asks the
receiver to use CCID 2 for its packets, although CCIDs 3 and 4 are
also acceptable. (This corresponds to the bytes "35, 6, 1, 2, 3,
4": Change R option (35), option length (6), feature ID (1), CCIDs
(2, 3, 4).) Similarly, "Confirm L(CCID, 1, 2 3 4)" tells the
receiver that the sender is using CCID 2 for its packets, but that
CCIDs 3 and 4 might also be acceptable.

Currently allocated CCIDs are as follows.

CCID Meaning Reference
---- ------- ---------
0-1 Reserved
2 TCP-like Congestion Control [RFC TBA]
3 TFRC Congestion Control [RFC TBA]
4-255 Reserved

Table 5: DCCP Congestion Control Identifiers

New connections start with CCID 2 for both endpoints. If this is
unacceptable for a DCCP endpoint, that endpoint MUST send Mandatory
Change(CCID) options on its first packets.

All CCIDs standardized for use with DCCP will correspond to
congestion control mechanisms previously standardized by the IETF.
We expect that for quite some time, all such mechanisms will be TCP-
friendly, but TCP-friendliness is not an explicit DCCP requirement.




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A DCCP implementation intended for general use, such as an
implementation in a general-purpose operating system kernel, SHOULD
implement at least CCID 2. The intent is to make CCID 2 broadly
available for interoperability, although particular applications
might disallow its use.

10.1. TCP-like Congestion Control

CCID 2, TCP-like Congestion Control, denotes Additive Increase,
Multiplicative Decrease (AIMD) congestion control with behavior
modelled directly on TCP, including congestion window, slow start,
timeouts, and so forth [RFC 2581]. CCID 2 achieves maximum
bandwidth over the long term, consistent with the use of end-to-end
congestion control, but halves its congestion window in response to
each congestion event. This leads to the abrupt rate changes
typical of TCP. Applications should use CCID 2 if they prefer
maximum bandwidth utilization to steadiness of rate. This is often
the case for applications that are not playing their data directly
to the user. For example, a hypothetical application that
transferred files over DCCP, using application-level retransmissions
for lost packets, would prefer CCID 2 to CCID 3. On-line games may
also prefer CCID 2.

CCID 2 is further described in [CCID 2 PROFILE].

10.2. TFRC Congestion Control

CCID 3 denotes TCP-Friendly Rate Control (TFRC), an equation-based
rate-controlled congestion control mechanism. TFRC is designed to
be reasonably fair when competing for bandwidth with TCP-like flows,
where a flow is "reasonably fair" if its sending rate is generally
within a factor of two of the sending rate of a TCP flow under the
same conditions. However, TFRC has a much lower variation of
throughput over time compared with TCP, which makes CCID 3 more
suitable than CCID 2 for applications such streaming media where a
relatively smooth sending rate is of importance.

CCID 3 is further described in [CCID 3 PROFILE]. The TFRC
congestion control algorithms were initially described in RFC 3448.	congestion control algorithms were initially described in [RFC
	3448].
10.3. CCID-Specific Options, Features, and Reset Codes

Half of the option types, feature numbers, and Reset Codes are
reserved for CCID-specific use. CCIDs may often need new options,
for communicating acknowledgement or rate information, for example;
reserved option spaces let CCIDs create options at will without
polluting the global option space. Option 128 might have different
meanings on a half-connection using CCID 4 and a half-connection



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using CCID 8. CCID-specific options and features will never
conflict with global options and features introduced by later
versions of this specification.

Any packet may contain information meant for either half-connection,
so CCID-specific option types, feature numbers, and Reset Codes
explicitly signal the half-connection to which they apply.

o Option numbers 128 through 191 are for options sent from the HC-
Sender to the HC-Receiver; option numbers 192 through 255 are for
options sent from the HC-Receiver to the HC-Sender.

o Reset Codes 128 through 191 indicate that the HC-Sender reset the
connection (most likely because of some problem with
acknowledgements sent by the HC-Receiver); Reset Codes 192
through 255 indicate that the HC-Receiver reset the connection
(most likely because of some problem with data packets sent by
the HC-Sender).

o Finally, feature numbers 128 through 191 are used for features
located at the HC-Sender; feature numbers 192 through 255 are for
features located at the HC-Receiver. Since Change L and
Confirm L options for a feature are sent by the feature location,
we know that any Change L(128) option was sent by the HC-Sender,
while any Change L(192) option was sent by the HC-Receiver.
Similarly, Change R(128) options are sent by the HC-Receiver,
while Change R(192) options are sent by the HC-Sender.

For example, consider a DCCP connection where the A-to-B half-
connection uses CCID 4 and the B-to-A half-connection uses CCID 5.
Here is how a sampling of CCID-specific options are assigned to
half-connections.



















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Relevant Relevant
Packet Option Half-conn. CCID
------ ------ ---------- ----
A > B 128 A-to-B 4
A > B 192 B-to-A 5
A > B Change L(128, ...) A-to-B 4
A > B Change R(192, ...) A-to-B 4
A > B Confirm L(128, ...) A-to-B 4
A > B Confirm R(192, ...) A-to-B 4
A > B Change R(128, ...) B-to-A 5
A > B Change L(192, ...) B-to-A 5
A > B Confirm R(128, ...) B-to-A 5
A > B Confirm L(192, ...) B-to-A 5

B > A 128 B-to-A 5
B > A 192 A-to-B 4
B > A Change L(128, ...) B-to-A 5
B > A Change R(192, ...) B-to-A 5
B > A Confirm L(128, ...) B-to-A 5
B > A Confirm R(192, ...) B-to-A 5
B > A Change R(128, ...) A-to-B 4
B > A Change L(192, ...) A-to-B 4
B > A Confirm R(128, ...) A-to-B 4
B > A Confirm L(192, ...) A-to-B 4

Using CCID-specific options and feature options during a negotiation
for that CCID feature is NOT RECOMMENDED, since it is difficult to
predict the CCID that will be in force when the option is processed.
For example, if a DCCP-Request contains the option sequence
"Change L(CCID, 3), 128", the CCID-specific option "128" may be
processed either by CCID 3 (if the server supports CCID 3) or by the
default CCID 2 (if it does not). However, it is safe to include
CCID-specific options following certain Mandatory Change(CCID)
options. For example, if a DCCP-Request contains the option
sequence "Mandatory, Change L(CCID, 3), 128", then either the "128"
option will be processed by CCID 3 or the connection will be reset.

Servers that do not implement the default CCID 2 might nevertheless
receive CCID 2-specific options on a DCCP-Request packet. (Such a
server MUST send Mandatory Change(CCID) options on its DCCP-
Response, so CCID-specific options on any other packet won't refer
to CCID 2.) The server MUST treat such options as non-understood.
Thus, it will reset the connection on encountering a Mandatory CCID-
specific option, send an empty Confirm for a non-Mandatory Change
option for a CCID-specific feature, and ignore other options.






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10.4. CCID Profile Requirements

Each CCID Profile document MUST address at least the following
requirements:

o The profile MUST include the name and number of the CCID being
described.

o The profile MUST describe the conditions in which it is likely to
be useful. Often the best way to do this is by comparison to
existing CCIDs.

o The profile MUST list and describe any CCID-specific options,
features, and Reset Codes, and SHOULD list those general options
and features described in this document that are especially
relevant to the CCID.

o Any newly defined acknowledgement mechanism MUST include a way to
transmit ECN Nonce Echoes back to the sender.

o The profile MUST describe the format of data packets, including
any options that should be included and the setting of the CCval
header field.

o The profile MUST describe the format of acknowledgement packets,
including any options that should be included.

o The profile MUST define how data packets are congestion
controlled. This includes responses to congestion events, idle
and application-limited periods, and responses to the DCCP Data
Dropped and Slow Receiver options. CCIDs that implement per-
packet congestion control SHOULD discuss how packet size is
factored in to congestion control decisions.

o The profile MUST specify when acknowledgement packets are
generated, and how they are congestion controlled.

o The profile MUST define when a sender using the CCID is
considered quiescent.

o The profile MUST say whether its CCID's acknowledgements ever
need to be acknowledged, and if so, how often.

10.5. Congestion State

Most congestion control algorithms depend on past history to
determine the current allowed sending rate. In CCID 2, this
congestion state includes a congestion window and a measurement of



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the number of packets outstanding in the network; in CCID 3, it
includes the lengths of recent loss intervals; and both CCIDs use an
estimate of the round-trip time. Congestion state depends on the
network path, and is invalidated by path changes. Therefore, DCCP
senders and receivers SHOULD reset their congestion state --
essentially restarting congestion control from "slow start" or
equivalent -- on significant changes in end-to-end path. For
example, an endpoint that sends or receives a Mobile IPv6 Binding
Update message [RFC 3775] SHOULD reset its congestion state for any
corresponding DCCP connections.

A DCCP implementation MAY also reset its congestion state when a
CCID changes (that is, a negotiation for the CCID feature completes
successfully, and the new feature value differs from the old value).
Thus, a connection in a heavily congested environment might evade
end-to-end congestion control by frequently renegotiating a CCID,
just as it could evade end-to-end congestion control by opening new
connections for the same session. This behavior is prohibited. To
prevent it, DCCP implementations MAY limit the rate at which CCID
can be changed -- for instance, by refusing to change a CCID feature
value more than once per minute.

11. Acknowledgements

Congestion control requires receivers to transmit information about
packet losses and ECN marks to senders. DCCP receivers MUST report
all congestion they see, as defined by the relevant CCID profile.
Each CCID says when acknowledgements should be sent, what options
they must use, and so on. DCCP acknowledgements are congestion
controlled, although it is not required that the acknowledgement
stream be more than very roughly TCP-friendly; each CCID defines how
acknowledgements are congestion controlled.

Most acknowledgements use DCCP options. For example, on a half-
connection with CCID 2 (TCP-like), the receiver reports
acknowledgement information using the Ack Vector option. This
section describes common acknowledgement options and shows how acks
using those options will commonly work. Full descriptions of the
ack mechanisms used for each CCID are laid out in the CCID profile
specifications.

Acknowledgement options, such as Ack Vector, generally depend on the
DCCP Acknowledgement Number, and are thus only allowed on packet
types that carry that number (all packets except DCCP-Request and
DCCP-Data). Detailed acknowledgement options are not necessarily
required on every packet that carries an Acknowledgement Number,
however.




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11.1. Acks of Acks and Unidirectional Connections

DCCP was designed to work well for both bidirectional and
unidirectional flows of data, and for connections that transition
between these states. However, acknowledgements required for a
unidirectional connection are very different from those required for
a bidirectional connection. In particular, unidirectional
connections need to worry about acks of acks.

The ack-of-acks problem arises because some acknowledgement
mechanisms are reliable. For example, an HC-Receiver using CCID 2,
TCP-like Congestion Control, sends Ack Vectors containing completely
reliable acknowledgement information. The HC-Sender should
occasionally inform the HC-Receiver that it has received an ack. If
it did not, the HC-Receiver might resend complete Ack Vector
information, going back to the start of the connection, with every
DCCP-Ack packet! However, note that acks-of-acks need not be
reliable themselves: when an ack-of-acks is lost, the HC-Receiver
will simply maintain, and periodically retransmit, old
acknowledgement-related state for a little longer. Therefore, there
is no need for acks-of-acks-of-acks.

When communication is bidirectional, any required acks-of-acks are
automatically contained in normal acknowledgements for data packets.
On a unidirectional connection, however, the receiver DCCP sends no
data, so the sender would not normally send acknowledgements.
Therefore, the CCID in force on that half-connection must explicitly
say whether, when, and how the HC-Sender should generate acks-of-
acks.

For example, consider a bidirectional connection where both half-
connections use the same CCID (either 2 or 3), and where DCCP B goes
"quiescent". This means that the connection becomes unidirectional:
DCCP B stops sending data, and sends only sends DCCP-Ack packets to
DCCP A. For example, in CCID 2, TCP-like Congestion Control, DCCP B
uses Ack Vector to reliably communicate which packets it has
received. As described above, DCCP A must occasionally acknowledge
a pure acknowledgement from DCCP B, so that B can free old Ack
Vector state. For instance, A might send a DCCP-DataAck packet
every now and then, instead of DCCP-Data. In contrast, in CCID 3,
TFRC Congestion Control, DCCP B's acknowledgements generally need
not be reliable, since they contain cumulative loss rates; TFRC
works even if every DCCP-Ack is lost. Therefore, DCCP A need never
acknowledge an acknowledgement.

When communication is unidirectional, a single CCID -- in the
example, the A-to-B CCID -- controls both DCCPs' acknowledgements,
in terms of their content, their frequency, and so forth. For



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bidirectional connections, the A-to-B CCID governs DCCP B's
acknowledgements (including its acks of DCCP A's acks), while the B-
to-A CCID governs DCCP A's acknowledgements.

DCCP A switches its ack pattern from bidirectional to unidirectional
when it notices that DCCP B has gone quiescent. It switches from
unidirectional to bidirectional when it must acknowledge even a
single DCCP-Data or DCCP-DataAck packet from DCCP B.

Each CCID defines how to detect quiescence on that CCID, and how
that CCID handles acks-of-acks on unidirectional connections. The
B-to-A CCID defines when DCCP B has gone quiescent. Usually, this
happens when a period has passed without B sending any data packets;
in CCID 2, for example, this period is the maximum of 0.2 seconds
and two round-trip times. The A-to-B CCID defines how DCCP A
handles acks-of-acks once DCCP B has gone quiescent.

11.2. Ack Piggybacking

Acknowledgements of A-to-B data MAY be piggybacked on data sent by
DCCP B, as long as that does not delay the acknowledgement longer
than the A-to-B CCID would find acceptable. However, data
acknowledgements often require more than 4 bytes to express. A
large set of acknowledgements prepended to a large data packet might
exceed the allowed maximum packet size. In this case, DCCP B SHOULD
send separate DCCP-Data and DCCP-Ack packets, or wait, but not too
long, for a smaller datagram.

Piggybacking is particularly common at DCCP A when the B-to-A half-
connection is quiescent -- that is, when DCCP A is just
acknowledging DCCP B's acknowledgements. There are three reasons to
acknowledge DCCP B's acknowledgements: to allow DCCP B to free up
information about previously acknowledged data packets from A; to
shrink the size of future acknowledgements; and to manipulate the
rate at which future acknowledgements are sent. Since these are
secondary concerns, DCCP A can generally afford to wait indefinitely
for a data packet to piggyback its acknowledgement onto; if DCCP B
wants to elicit an acknowledgement, it can send a DCCP-Sync.

Any restrictions on ack piggybacking are described in the relevant
CCID's profile.

11.3. Ack Ratio Feature

The Ack Ratio feature lets HC-Senders influence the rate at which
HC-Receivers generate DCCP-Ack packets, thus controlling reverse-
path congestion. This differs from TCP, which presently has no
congestion control for pure acknowledgement traffic. Ack Ratio



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reverse-path congestion control does not try to be TCP-friendly. It
just tries to avoid congestion collapse, and to be somewhat better
than TCP in the presence of a high packet loss or mark rate on the
reverse path.

Ack Ratio applies to CCIDs whose HC-Receivers clock acknowledgements
off the receipt of data packets. The value of Ack Ratio/A equals
the rough ratio of data packets sent by DCCP A to DCCP-Ack packets
sent by DCCP B. Higher Ack Ratios correspond to lower DCCP-Ack
rates; the sender raises Ack Ratio when the reverse path is
congested and lowers Ack Ratio when it is not. Each CCID profile
defines how it controls congestion on the acknowledgement path, and,
particularly, whether Ack Ratio is used. CCID 2, for example, uses
Ack Ratio for acknowledgement congestion control, but CCID 3 does
not. However, each Ack Ratio feature has a value whether or not
that value is used by the relevant CCID.

Ack Ratio has feature number 5, and is non-negotiable. It takes
two-byte integer values. An Ack Ratio/A value of four means that
DCCP B will send at least one acknowledgement packet for every four
data packets sent by DCCP A. DCCP A sends a "Change L(Ack Ratio)"
option to notify DCCP B of its ack ratio. An Ack Ratio value of
zero indicates that the relevant half-connection does not use an Ack
Ratio to control its acknowledgement rate. New connections start
with Ack Ratio 2 for both endpoints; this Ack Ratio results in
acknowledgement behavior analogous to TCP's delayed acks.

Ack Ratio should be treated as a guideline rather than a strict
requirement. We intend Ack Ratio-controlled acknowledgement
behavior to resemble TCP's acknowledgement behavior when there is no
reverse-path congestion, and to be somewhat more conservative when
there is reverse-path congestion. Following this intent is more
important than implementing Ack Ratio precisely. In particular:

o Receivers MAY piggyback acknowledgement information on data
packets, creating DCCP-DataAck packets. The Ack Ratio does not
apply to piggybacked acknowledgements. However, if the data
packets are too big to carry acknowledgement information, or the
data sending rate is lower than Ack Ratio would suggest, then
DCCP B SHOULD send enough pure DCCP-Ack packets to maintain the
rate of one acknowledgement per Ack Ratio received data packets.

o Receivers MAY rate-pace their acknowledgements, rather than
sending acknowledgements immediately upon the receipt of data
packets. Receivers that rate-pace acknowledgements SHOULD pick a
rate that approximates the effect of Ack Ratio, and SHOULD
include Elapsed Time options (Section 13.2) to help the sender
calculate round-trip times.



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o Receivers SHOULD implement delayed acknowledgement timers like
TCP's, whereby any packet's acknowledgement is delayed by at most
T seconds. This delay lets the receiver collect additional
packets to acknowledge, and thus reduce the per-packet overhead
of acknowledgements; but if T seconds have passed by and the ack
is still around, it is sent out right away. The default value of
T should be 0.2 seconds, as is common in TCP implementations.
This may lead to sending more acknowledgement packets than Ack
Ratio would suggest.

o Receivers SHOULD send acknowledgements immediately on receiving
packets marked ECN Congestion Experienced, or packets whose out-
of-order sequence numbers potentially indicate loss. However,
there is no need to send such immediate acknowledgements for
marked packets more than once per round-trip time.

o Receivers MAY ignore Ack Ratio if they perform their own
congestion control on acknowledgements. For example, a receiver
that knows the loss and mark rate for its DCCP-Ack packets might
maintain a TCP-friendly acknowledgement rate on its own. Such a
receiver MUST either ensure that it always obtains sufficient
acknowledgement loss and mark information, or fall back to Ack
Ratio when sufficient information is not available, as might
happen during periods when the receiver is quiescent.

11.4. Ack Vector Options

The Ack Vector gives a run-length encoded history of data packets
received at the client. Each byte of the vector gives the state of
that data packet in the loss history, and the number of preceding
packets with the same state. The option's data looks like this:

+--------+--------+--------+--------+--------+--------
\|0010011?\| Length \|SSLLLLLL\|SSLLLLLL\|SSLLLLLL\| ...
+--------+--------+--------+--------+--------+--------
Type=38/39 \___________ Vector ___________...

The two Ack Vector options (option types 38 and 39) differ only in
the values they imply for ECN Nonce Echo. Section 12.2 describes
this further.

The vector itself consists of a series of bytes, each of whose
encoding is:








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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
\|Sta\| Run Length\|
+-+-+-+-+-+-+-+-+

Sta[te] occupies the most significant two bits of each byte, and can
have one of four values, as follows.

State Meaning
----- -------
0 Received
1 Received ECN Marked
2 Reserved
3 Not Yet Received

Table 6: DCCP Ack Vector States

The term "ECN marked" refers to packets with ECN code point 11, CE
(Congestion Experienced); packets received with this ECN code point
MUST be reported using State 1, Received ECN Marked. Packets
received with other ECN code points 00, 01, or 10 (Non-ECT, ECT(0),
or ECT(1), respectively) MUST be reported using State 0, Received.

Run Length, the least significant six bits of each byte, specifies
how many consecutive packets have the given State. Run Length zero
says the corresponding State applies to one packet only; Run Length
63 says it applies to 64 consecutive packets. Run lengths of 65 or
more must be encoded in multiple bytes.

The first byte in the first Ack Vector option refers to the packet
indicated in the Acknowledgement Number; subsequent bytes refer to
older packets. (Ack Vector MUST NOT be sent on DCCP-Data and DCCP-
Request packets, which lack an Acknowledgement Number.) An Ack
Vector containing the decimal values 0,192,3,64,5 and the
Acknowledgement Number is decimal 100 indicates that:

Packet 100 was received (Acknowledgement Number 100, State 0,
Run Length 0).

Packet 99 was lost (State 3, Run Length 0).

Packets 98, 97, 96 and 95 were received (State 0, Run Length 3).

Packet 94 was ECN marked (State 1, Run Length 0).

Packets 93, 92, 91, 90, 89, and 88 were received (State 0, Run
Length 5).




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A single Ack Vector option can acknowledge up to 16192 data packets.
Should more packets need to be acknowledged than can fit in 253
bytes of Ack Vector, then multiple Ack Vector options can be sent;
the second Ack Vector begins where the first left off, and so forth.

Ack Vector states are subject to two general constraints. (These
principles SHOULD also be followed for other acknowledgement
mechanisms; referring to Ack Vector states simplifies their
explanation.)

1. Packets reported as State 0 or State 1 MUST be acknowledgeable:
their options have been processed by the receiving DCCP stack.
Any data on the packet need not have been delivered to the
receiving application; in fact, the data may have been dropped.

2. Packets reported as State 3 MUST NOT be acknowledgeable.
Feature negotiations and options on such packets MUST NOT have
been processed, and the Acknowledgement Number MUST NOT
correspond to such a packet.

Packets dropped in the application's receive buffer MUST be reported
as Received or Received ECN Marked (States 0 and 1), depending on
their ECN state; such packets' ECN Nonces MUST be included in the
Nonce Echo. The Data Dropped option informs the sender that some
packets reported as received actually had their application data
dropped.

One or more Ack Vector options that, together, report the status of
a packet with sequence number less than ISN, the initial sequence
number, SHOULD be considered invalid. The receiving DCCP SHOULD
either ignore the options or reset the connection with Reset Code 5,
"Option Error". No Ack Vector option can refer to a packet that has
not yet been sent, as the Acknowledgement Number checks in Section
7.5.3 ensure, but because of attack, implementation bug, or
misbehavior, an Ack Vector option can claim that a packet was
received before it is actually delivered; Section 12.2 describes how
this is detected and how senders should react. Packets that haven't
been included in any Ack Vector option SHOULD be treated as "not yet
received" (State 3) by the sender.

Appendix A provides a non-normative description of the details of
DCCP acknowledgement handling, in the context of an abstract Ack
Vector implementation.

11.4.1. Ack Vector Consistency

A DCCP sender will commonly receive multiple acknowledgements for
some of its data packets. For instance, an HC-Sender might receive



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two DCCP-Acks with Ack Vectors, both of which contained information
about sequence number 24. (Information about a sequence number is
generally repeated in every ack until the HC-Sender acknowledges an
ack. In this case, perhaps the HC-Receiver is sending acks faster
than the HC-Sender is acknowledging them.) In a perfect world, the
two Ack Vectors would always be consistent. However, there are many
reasons why they might not be. For example:

o The HC-Receiver received packet 24 between sending its acks, so
the first ack said 24 was not received (State 3) and the second
said it was received or ECN marked (State 0 or 1).

o The HC-Receiver received packet 24 between sending its acks, and
the network reordered the acks. In this case, the packet will
appear to transition from State 0 or 1 to State 3.

o The network duplicated packet 24, and one of the duplicates was
ECN marked. This might show up as a transition between States 0
and 1.

To cope with these situations, HC-Sender DCCP implementations SHOULD
combine multiple received Ack Vector states according to this table:

Received State
0 1 3
+---+---+---+
0 \| 0 \|0/1\| 0 \|
Old +---+---+---+
1 \| 1 \| 1 \| 1 \|
State +---+---+---+
3 \| 0 \| 1 \| 3 \|
+---+---+---+

To read the table, choose the row corresponding to the packet's old
state and the column corresponding to the packet's state in the
newly received Ack Vector, then read the packet's new state off the
table. For an old state of 0 (received non-marked) and received
state of 1 (received ECN marked), the packet's new state may be set
to either 0 or 1. The HC-Sender implementation will be indifferent
to ack reordering if it chooses new state 1 for that cell.

The HC-Receiver should collect information about received packets,
which it will eventually report to the HC-Sender on one or more
acknowledgements, according to the following table:







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Received Packet
0 1 3
+---+---+---+
0 \| 0 \|0/1\| 0 \|
Stored +---+---+---+
1 \|0/1\| 1 \| 1 \|
State +---+---+---+
3 \| 0 \| 1 \| 3 \|
+---+---+---+

This table equals the sender's table, except that when the stored
state is 1 and the received state is 0, the receiver is allowed to
switch its stored state to 0.

A HC-Sender MAY choose to throw away old information gleaned from
the HC-Receiver's Ack Vectors, in which case it MUST ignore newly
received acknowledgements from the HC-Receiver for those old
packets. It is often kinder to save recent Ack Vector information
for a while, so that the HC-Sender can undo its reaction to presumed
congestion when a "lost" packet unexpectedly shows up (the
transition from State 3 to State 0).

11.4.2. Ack Vector Coverage

We can divide the packets that have been sent from an HC-Sender to
an HC-Receiver into four roughly contiguous groups. From oldest to
youngest, these are:

1. Packets already acknowledged by the HC-Receiver, where the HC-
Receiver knows that the HC-Sender has definitely received the
acknowledgements.

2. Packets already acknowledged by the HC-Receiver, where the HC-
Receiver cannot be sure that the HC-Sender has received the
acknowledgements.

3. Packets not yet acknowledged by the HC-Receiver.

4. Packets not yet received by the HC-Receiver.

The union of groups 2 and 3 is called the Acknowledgement Window.
Generally, every Ack Vector generated by the HC-Receiver will cover
the whole Acknowledgement Window: Ack Vector acknowledgements are
cumulative. (This simplifies Ack Vector maintenance at the HC-
Receiver; see Appendix A, below.) As packets are received, this
window both grows on the right and shrinks on the left. It grows
because there are more packets, and shrinks because the data
packets' Acknowledgement Numbers will acknowledge previous



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acknowledgements, moving packets from group 2 into group 1.

11.5. Send Ack Vector Feature

The Send Ack Vector feature lets DCCPs negotiate whether they should
use Ack Vector options to report congestion. Ack Vector provides
detailed loss information, and lets senders report back to their
applications whether particular packets were dropped. Send Ack
Vector is mandatory for some CCIDs, and optional for others.

Send Ack Vector has feature number 6, and is server-priority. It
takes one-byte Boolean values. DCCP A MUST send Ack Vector options
on its acknowledgements when Send Ack Vector/A has value one,
although it MAY send Ack Vector options even when Send Ack Vector/A
is zero. Values of two or more are reserved. New connections start
with Send Ack Vector 0 for both endpoints. DCCP B sends a
"Change R(Send Ack Vector, 1)" option to DCCP A to ask A to send Ack
Vector options as part of its acknowledgement traffic.

11.6. Slow Receiver Option

An HC-Receiver sends the Slow Receiver option to its sender to
indicate that it is having trouble keeping up with the sender's
data. The HC-Sender SHOULD NOT increase its sending rate for
approximately one round-trip time after seeing a packet with a Slow
Receiver option. After one round-trip time, the effect of Slow
Receiver disappears and the HC-Sender may again increase its rate,
so the HC-Receiver SHOULD continue to send Slow Receiver options if
it needs to prevent the HC-Sender from going faster in the long
term. The Slow Receiver option does not indicate congestion, and
the HC-Sender need not reduce its sending rate. (If necessary, the
receiver can force the sender to slow down by dropping packets, with
or without Data Dropped, or reporting false ECN marks.) APIs should
let receiver applications set Slow Receiver, and sending
applications determine whether or not their receivers are Slow.

Slow Receiver is a one-byte option.

+--------+
\|00000010\|
+--------+
Type=2

Slow Receiver does not specify why the receiver is having trouble
keeping up with the sender. Possible reasons include lack of buffer
space, CPU overload, and application quotas. A sending application
might react to Slow Receiver by reducing its sending rate, for
example.



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The sending application should not react to Slow Receiver by sending
more data, however. The optimal response to a CPU-bound receiver
might be to increase the sending rate, by switching to a less-
compressed sending format, since a highly-compressed data format
might overwhelm a slow CPU more seriously than the higher memory
requirements of a less-compressed data format. This kind of format
change should be requested at the application level, not via the
Slow Receiver option.

Slow Receiver implements a portion of TCP's receive window
functionality.

11.7. Data Dropped Option

The Data Dropped option indicates that the application data on one
or more received packets did not actually reach the application.
Data Dropped additionally reports why the data was dropped: perhaps
the data was corrupt, or perhaps the receiver cannot keep up with
the sender's current rate and the data was dropped in some receive
buffer. Using Data Dropped, DCCP endpoints can discriminate between
different kinds of loss; this differs from TCP, in which all loss is
reported the same way.

Unless explicitly specified otherwise, DCCP congestion control
mechanisms MUST react as if each Data Dropped packet was marked as
ECN Congestion Experienced by the network. We intend for Data
Dropped to enable research into richer congestion responses to
corrupt and other endpoint-dropped packets, but DCCP CCIDs MUST
react conservatively to Data Dropped until this behavior is
standardized. Section 11.7.2, below, describes congestion responses
for all current Drop Codes.

If a received packet's application data is dropped for one of the
reasons listed below, this SHOULD be reported using a Data Dropped
option. Alternatively, the receiver MAY choose to report as
"received" only those packets whose data were not dropped, subject
to the constraint that packets not reported as received MUST NOT
have had their options processed.

The option's data looks like this:

+--------+--------+--------+--------+--------+--------
\|00101000\| Length \| Block \| Block \| Block \| ...
+--------+--------+--------+--------+--------+--------
Type=40 \___________ Vector ___________ ...

The Vector consists of a series of bytes, called Blocks, each of
whose encoding corresponds to one of two choices:



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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
\|0\| Run Length \| or \|1\|DrpCd\|Run Len\|
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Normal Block Drop Block

The first byte in the first Data Dropped option refers to the packet
indicated in the Acknowledgement Number; subsequent bytes refer to
older packets. (Data Dropped MUST NOT be sent on DCCP-Data or DCCP-
Request packets, which lack an Acknowledgement Number, and any Data
Dropped options received on these packet types MUST be ignored.)
Normal Blocks, which have high bit 0, indicate that any received
packets in the Run Length had their data delivered to the
application. Drop Blocks, which have high bit 1, indicate that
received packets in the Run Len[gth] were not delivered as usual.
The 3-bit Drop Code [DrpCd] field says what happened; generally, no
data from that packet reached the application. Packets reported as
"not yet received" MUST be included in Normal Blocks; packets not
covered by any Data Dropped option are treated as if they were in a
Normal Block. Defined Drop Codes for Drop Blocks are as follows.

Drop Code Meaning
--------- -------
0 Protocol Constraints
1 Application Not Listening
2 Receive Buffer
3 Corrupt
4-6 Reserved
7 Delivered Corrupt

Table 7: DCCP Drop Codes

In more detail:

0 The packet data was dropped due to protocol constraints.
For example, the data was included on a DCCP-Request packet,
but the receiving application does not allow such
piggybacking; or the data was included on a packet with
inappropriately low Checksum Coverage.

1 The packet data was dropped because the application is no
longer listening. See Section 11.7.2.

2 The packet data was dropped in a receive buffer, probably
because of receive buffer overflow. See Section 11.7.2.

3 The packet data was dropped due to corruption. See Section
9.3.



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7 The packet data was corrupted, but delivered to the
application anyway. See Section 9.3.

For example, assume a packet arrives with Acknowledgement Number
100, an Ack Vector reporting all packets as received, and a Data
Dropped option containing the decimal values 0,160,3,162. Then:

Packet 100 was received (Acknowledgement Number 100, Normal
Block, Run Length 0).

Packet 99 was dropped in a receive buffer (Drop Block, Drop Code
2, Run Length 0).

Packets 98, 97, 96, and 95 were received (Normal Block, Run
Length 3).

Packets 95, 94, and 93 were dropped in the receive buffer (Drop
Block, Drop Code 2, Run Length 2).

Run lengths of more than 128 (for Normal Blocks) or 16 (for Drop
Blocks) must be encoded in multiple Blocks. A single Data Dropped
option can acknowledge up to 32384 Normal Block data packets,
although the receiver SHOULD NOT send a Data Dropped option when all
relevant packets fit into Normal Blocks. Should more packets need
to be acknowledged than can fit in 253 bytes of Data Dropped, then
multiple Data Dropped options can be sent. The second option will
begin where the first left off, and so forth.

One or more Data Dropped options that, together, report the status
of more packets than have been sent, or that change the status of a
packet, or that disagree with Ack Vector or equivalent options (by
reporting a "not yet received" packet as "dropped in the receive
buffer", for example), SHOULD be considered invalid. The receiving
DCCP SHOULD either ignore such options, or respond by resetting the	DCCP SHOULD either such options, or respond by resetting the
connection with Reset Code 5, "Option Error".

A DCCP application interface should let receiving applications
specify the Drop Codes corresponding to received packets. For
example, this would let applications calculate their own checksums,
but still report "dropped due to corruption" packets via the Data
Dropped option. The interface SHOULD NOT let applications reduce
the "seriousness" of a packet's Drop Code; for example, the
application should not be able to upgrade a packet from delivered
corrupt (Drop Code 7) to delivered normally (no Drop Code).

Data Dropped information is transmitted reliably. That is,
endpoints SHOULD continue to transmit Data Dropped options until
receiving an acknowledgement indicating that the relevant options



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have been processed. In Ack Vector terms, each acknowledgement
should contain Data Dropped options that cover the whole
Acknowledgement Window (Section 11.4.2), although when every packet
in that window would be placed in a Normal Block no actual option is
required.

11.7.1. Data Dropped and Normal Congestion Response

When deciding on a response to a particular acknowledgement or set
of acknowledgements containing Data Dropped options, a congestion
control mechanism MUST consider dropped packets and ECN Congestion
Experienced marks (including marked packets that are included in
Data Dropped), as well as the packets singled out in Data Dropped.
For window-based mechanisms, the valid response space is defined as
follows.

Assume an old window of W. Independently calculate a new window
W_new1 that assumes no packets were Data Dropped (so W_new1 contains
only the normal congestion response), and a new window W_new2 that
assumes no packets were lost or marked (so W_new2 contains only the
Data Dropped response). We are assuming that Data Dropped
recommended a reduction in congestion window, so W_new2 < W.

Then the actual new window W_new MUST NOT be larger than the minimum
of W_new1 and W_new2; and the sender MAY combine the two responses,
by setting
W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).

The details of how this is accomplished are specified in CCID
profile documents. Non-window-based congestion control mechanisms
MUST behave analogously; again, CCID profiles define how.

11.7.2. Particular Drop Codes

Drop Code 0, Protocol Constraints, does not indicate any kind of
congestion, so the sender's CCID SHOULD react to packets with Drop
Code 0 as if they were received (with or without ECN Congestion
Experienced marks, as appropriate). However, the sending endpoint
SHOULD NOT send data until it believes the protocol constraint no
longer applies.

Drop Code 1, Application Not Listening, means the application
running at the endpoint that sent the option is no longer listening
for data. For example, a server might close its receiving half-
connection to new data after receiving a complete request from the
client. This would limit the amount of state available at the
server for incoming data, and thus reduce the potential damage from
certain denial-of-service attacks. A Data Dropped option containing



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Drop Code 1 SHOULD be sent whenever received data is ignored due to
a non-listening application. Once an endpoint reports Drop Code 1
for a packet, it SHOULD report Drop Code 1 for every succeeding data
packet on that half-connection; once an endpoint receives a Drop
State 1 report, it SHOULD expect that no more data will ever be
delivered to the other endpoint's application, so it SHOULD NOT send
more data.

Drop Code 2, Receive Buffer, indicates congestion inside the
receiving host. For instance, if a drop-from-tail kernel socket
buffer is too full to accept a packet's application data, that
packet should be reported as Drop Code 2. For a drop-from-head or
more complex socket buffer, the dropped packet should be reported as
Drop Code 2. DCCP implementations may also provide an API by which
applications can mark received packets as Drop Code 2, indicating
that the application ran out of space in its user-level receive
buffer. (However, it is not generally useful to report packets as
dropped due to Drop Code 2 after more than a couple round-trip times
have passed. The HC-Sender may have forgotten its acknowledgement
state for the packet by that time, so the Data Dropped report will
have no effect.) Every packet newly acknowledged as Drop Code 2
SHOULD reduce the sender's instantaneous rate by one packet per
round-trip time, unless the sender is already sending one packet per	round-trip time. Each CCID profile defines the CCID-specific
RTT or less. Each CCID profile defines the CCID-specific mechanism	mechanism by which this is accomplished.
by which this is accomplished.

Currently, the other Drop Codes, namely Drop Code 3, Corrupt, Drop
Code 7, Delivered Corrupt, and reserved Drop Codes 4-6, MUST cause
the relevant CCID to behave as if the relevant packets were ECN
marked (ECN Congestion Experienced).

12. Explicit Congestion Notification

The DCCP protocol is fully ECN-aware [RFC 3168]. Each CCID
specifies how its endpoints respond to ECN marks. Furthermore,
DCCP, unlike TCP, allows senders to control the rate at which
acknowledgements are generated (with options like Ack Ratio); since
acknowledgements are congestion-controlled, they also qualify as
ECN-Capable Transport.

A CCID profile describes how that CCID interacts with ECN, both for
data traffic and pure-acknowledgement traffic. A sender SHOULD set
ECN-Capable Transport on its packets' IP headers, unless the
receiver's ECN Incapable feature is on or the relevant CCID
disallows it.

The rest of this section describes the ECN Incapable feature and the
interaction of the ECN Nonce with acknowledgement options such as



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Ack Vector.

12.1. ECN Incapable Feature

DCCP endpoints are ECN-aware by default, but the ECN Incapable
feature lets an endpoint reject the use of Explicit Congestion
Notification. The use of this feature is NOT RECOMMENDED. ECN
incapability both avoids ECN's possible benefits and prevents
senders from using the ECN Nonce to check for receiver misbehavior.
A DCCP stack MAY therefore leave the ECN Incapable feature
unimplemented, acting as if all connections were ECN capable. It is
worth noting that the inappropriate firewall interactions that
dogged TCP's implementation of ECN [RFC 3360] involve TCP header
bits, not the IP header's ECN bits; we know of no middlebox that
would block ECN-capable DCCP packets, but allow ECN-incapable DCCP
packets.

ECN Incapable has feature number 4, and is server-priority. It
takes one-byte Boolean values. DCCP A MUST be able to read ECN bits
from received frames' IP headers when ECN Incapable/A is zero.
(This is independent of whether it can set ECN bits on sent frames.)
DCCP A thus sends a "Change L(ECN Inapable, 1)" option to DCCP B to
inform it that A cannot read ECN bits. If the ECN Incapable/A
feature is one, then all of DCCP B's packets MUST be sent as ECN
incapable. New connections start with ECN Incapable 0 (that is, ECN
capable) for both endpoints. Values of two or more are reserved.

If a DCCP is not ECN capable, it MUST send Mandatory "Change L(ECN
Incapable, 1)" options to the other endpoint until acknowledged (by
"Confirm R(ECN Incapable, 1)") or the connection closes.
Furthermore, it MUST NOT accept any data until the other endpoint
sends "Confirm R(ECN Incapable, 1)". It SHOULD send Data Dropped
options on its acknowledgements, with Drop Code 0 ("protocol
constraints"), if the other endpoint does send data inappropriately.

12.2. ECN Nonces

Congestion avoidance will not occur, and the receiver will sometimes
get its data faster, if the sender isn't told about congestion
events. Thus, the receiver has some incentive to falsify
acknowledgement information, reporting that marked or dropped
packets were actually received unmarked. This problem is more
serious with DCCP than with TCP, since TCP provides reliable
transport: it is more difficult with TCP to lie about lost packets
without breaking the application.

ECN Nonces are a general mechanism to prevent ECN cheating (or loss
cheating). Two values for the two-bit ECN header field indicate



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ECN-Capable Transport, 01 and 10. The second code point, 10, is the
ECN Nonce. In general, a protocol sender chooses between these code
points randomly on its output packets, remembering the sequence it
chose. The protocol receiver reports, on every acknowledgement, the
number of ECN Nonces it has received thus far. This is called the
ECN Nonce Echo. Since ECN marking and packet dropping both destroy
the ECN Nonce, a receiver that lies about an ECN mark or packet drop
has a 50% chance of guessing right and avoiding discipline. The
sender may react punitively to an ECN Nonce mismatch, possibly up to
dropping the connection. The ECN Nonce Echo field need not be an
integer; one bit is enough to catch 50% of infractions, and the
probability of success drops exponentially as more packets are sent	probability of success drops exponentially as more bits are sent
[RFC 3540].

In DCCP, the ECN Nonce Echo field is encoded in acknowledgement
options. For example, the Ack Vector option comes in two forms, Ack
Vector [Nonce 0] (option 38) and Ack Vector [Nonce 1] (option 39),
corresponding to the two values for a one-bit ECN Nonce Echo. The
Nonce Echo for a given Ack Vector equals the one-bit sum (exclusive-
or, or parity) of ECN nonces for packets reported by that Ack Vector
as received and not ECN marked. Thus, only packets marked as State
0 matter for this calculation (that is, valid received packets that
were not ECN marked). Every Ack Vector option is detailed enough
for the sender to determine what the Nonce Echo should have been.
It can check this calculation against the actual Nonce Echo, and
complain if there is a mismatch. (The Ack Vector could conceivably
report every packet's ECN Nonce state, but this would severely limit
its compressibility without providing much extra protection.)

Each DCCP sender SHOULD set ECN Nonces on its packets, and remember
which packets had nonces. When a sender detects an ECN Nonce Echo
mismatch, it behaves as described in the next section. Each DCCP
receiver MUST calculate and use the correct value for ECN Nonce Echo
when sending acknowledgement options.

ECN incapability, as indicated by the ECN Incapable feature, is
handled as follows: An endpoint sending packets to an ECN-incapable
receiver MUST send its packets as ECN incapable, and an ECN-
incapable receiver MUST use the value zero for all ECN Nonce Echoes.

12.3. Aggression Penalties

DCCP endpoints have several mechanisms for detecting congestion-
related misbehavior. For example:

o A sender can detect an ECN Nonce Echo mismatch, indicating
possible receiver misbehavior.




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o A receiver can detect whether the sender is responding to
congestion feedback or Slow Receiver.

o An endpoint may be able to detect that its peer is reporting
inappropriately small Elapsed Time values (Section 13.2).

An endpoint that detects possible congestion-related misbehavior
SHOULD try to verify that its peer is truly misbehaving. For
example, a sending endpoint might send a packet whose ECN header
field is set to Congestion Experienced, 11; a receiver that doesn't
report a corresponding mark is most likely misbehaving.

Upon detecting possible misbehavior, a sender SHOULD respond as if
the receiver had reported one or more recent packets as ECN-marked
(instead of unmarked), while a receiver SHOULD report one or more
recent non-marked packets as ECN-marked. Alternately, a sender
might act as if the receiver had sent a Slow Receiver option, and a
receiver might send Slow Receiver options. Other reactions that
serve to slow the transfer rate are also acceptable. An entity that
detects particularly egregious and ongoing misbehavior MAY also
reset the connection with Reset Code 11, "Aggression Penalty".

However, ECN Nonce mismatches and other warning signs can result
from innocent causes, such as implementation bugs or attack. In
particular, a successful DCCP-Data attack (Section 7.5.5) can cause
the receiver to report an incorrect ECN Nonce Echo. Therefore,
connection reset and other heavyweight mechanisms SHOULD be sent
only as last resorts, after multiple round-trip times of verified
aggression.

13. Timing Options

The Timestamp, Timestamp Echo, and Elapsed Time options help DCCP
endpoints explicitly measure round-trip times.

13.1. Timestamp Option

This option is permitted in any DCCP packet. The length of the
option is 6 bytes.

+--------+--------+--------+--------+--------+--------+
\|00101001\|00000110\| Timestamp Value \|
+--------+--------+--------+--------+--------+--------+
Type=41 Length=6

The four bytes of option data carry the timestamp of this packet.
The timestamp is a 32-bit integer that increases monotonically with
time, at a rate of 1 unit per 10 microseconds. At this rate,



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Timestamp Value will wrap approximately every 11.9 hours. Endpoints
need not measure time at this fine granularity; for example, an
endpoint that preferred to measure time at millisecond granularity
might send Timestamp Values that were all multiples of 100. The
precise time corresponding to Timestamp Value zero is not specified:
Timestamp Values are only meaningful relative to other Timestamp
Values sent on the same connection. A DCCP receiving a Timestamp
option SHOULD respond with a Timestamp Echo option on the next
packet it sends.

13.2. Elapsed Time Option

This option is permitted in any DCCP packet that contains an
Acknowledgement Number (such options received on other packet types
MUST be ignored). It indicates how much time has elapsed, in
hundredths of milliseconds (or, equivalently, multiples of
10 microseconds), since the packet being acknowledged -- the packet
with the given Acknowledgement Number -- was received. The option
may take 4 or 6 bytes, depending on the size of the Elapsed Time
value. Elapsed Time helps correct round-trip time estimates when
the gap between receiving a packet and acknowledging that packet may
be long -- in CCID 3, for example, where acknowledgements are sent
infrequently.

+--------+--------+--------+--------+
\|00101011\|00000100\| Elapsed Time \|
+--------+--------+--------+--------+
Type=43 Len=4

+--------+--------+--------+--------+--------+--------+
\|00101011\|00000110\| Elapsed Time \|
+--------+--------+--------+--------+--------+--------+
Type=43 Len=6

The option data, Elapsed Time, represents an estimated upper bound
on the amount of time elapsed since the packet being acknowledged
was received, with units of hundredths of milliseconds. If Elapsed	was received, with units of tenths of milliseconds. If Elapsed Time
Time is less than a half-second, the first, smaller form of the	is less than a half-second, the first, smaller form of the option
option SHOULD be used. Elapsed Times of more than 0.65535 seconds	SHOULD be used. Elapsed Times of more than 0.65535 seconds MUST be
MUST be sent using the second form of the option. The special	sent using the second form of the option. The special Elapsed Time
Elapsed Time value 4294967295, which corresponds to approximately	value 4294967295, which corresponds to approximately 11.9 hours, is
11.9 hours, is used to represent any Elapsed Time greater than	used to represent any Elapsed Time greater than 42949.67294 seconds.
42949.67294 seconds. DCCP endpoints MUST NOT report Elapsed Times	DCCP endpoints MUST NOT report Elapsed Times that are significantly
that are significantly larger than the true elapsed times. A	larger than the true elapsed times. A connection MAY be reset with
connection MAY be reset with Reset Code 11, "Aggression Penalty", if	Reset Code 11, "Aggression Penalty", if one endpoint determines that
one endpoint determines that the other is reporting a much-too-large	the other is reporting a much-too-large Elapsed Time.
Elapsed Time.




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Elapsed Time is measured in hundredths of milliseconds as a
compromise between two conflicting goals. First, it provides enough
granularity to reduce rounding error when measuring elapsed time
over fast LANs; second, it allows many reasonable elapsed times to
fit into two bytes of data.

13.3. Timestamp Echo Option

This option is permitted in any DCCP packet, as long as at least one
packet carrying the Timestamp option has been received. Generally,
a DCCP endpoint should send one Timestamp Echo option for each
Timestamp option it receives; and it should send that option as soon
as is convenient. The length of the option is between 6 and 10
bytes, depending on whether Elapsed Time is included and how large
it is.

+--------+--------+--------+--------+--------+--------+
\|00101010\|00000110\| Timestamp Echo \|
+--------+--------+--------+--------+--------+--------+
Type=42 Len=6

+--------+--------+------- ... -------+--------+--------+
\|00101010\|00001000\| Timestamp Echo \| Elapsed Time \|
+--------+--------+------- ... -------+--------+--------+
Type=42 Len=8 (4 bytes)

+--------+--------+------- ... -------+------- ... -------+
\|00101010\|00001010\| Timestamp Echo \| Elapsed Time \|
+--------+--------+------- ... -------+------- ... -------+
Type=42 Len=10 (4 bytes) (4 bytes)

The first four bytes of option data, Timestamp Echo, carry a
Timestamp Value taken from a preceding received Timestamp option.
Usually, this will be the last packet that was received -- the
packet indicated by the Acknowledgement Number, if any -- but it
might be a preceding packet. Each Timestamp received will generally
result in exactly one Timestamp Echo transmitted. If an endpoint
has received multiple Timestamp options since the last time it sent
a packet, then it MAY ignore all Timestamp options but the one
included on the packet with the greatest sequence number;
alternatively, it MAY include multiple Timestamp Echo options in its
response, each corresponding to a different Timestamp option.

The Elapsed Time value, similar to that in the Elapsed Time option,
indicates the amount of time elapsed since receiving the packet
whose timestamp is being echoed. This time MUST be in hundredths of
milliseconds. Elapsed Time is meant to help the Timestamp sender
separate the network round-trip time from the Timestamp receiver's



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processing time. This may be particularly important for CCIDs where
acknowledgements are sent infrequently, so that there might be
considerable delay between receiving a Timestamp option and sending
the corresponding Timestamp Echo. A missing Elapsed Time field is
equivalent to an Elapsed Time of zero. The smallest version of the
option SHOULD be used that can hold the relevant Elapsed Time value.

14. Maximum Packet Size

A DCCP implementation MUST maintain the maximum packet size (MPS)
allowed for each active DCCP session. The MPS is influenced by the
maximum packet size allowed by the current congestion control
mechanism (CCMPS), the maximum packet size supported by the path's
links (PMTU, the Path Maximum Transmission Unit) [RFC 1191], and the
lengths of the IP and DCCP headers.

A DCCP application interface SHOULD let the application discover
DCCP's current MPS. Generally, the DCCP implementation will refuse
to send any packet bigger than the MPS, returning an appropriate
error to the application. A DCCP interface MAY allow applications
to request fragmentation for packets larger than PMTU, but not
larger than CCMPS (packets larger than CCMPS MUST be rejected in any
case). Fragmentation SHOULD NOT be the default, since it decreases
robustness: an entire packet is discarded if even one of its
fragments is lost. Applications can usually get better error
tolerance by producing packets smaller than the PMTU.

The MPS reported to the application SHOULD be influenced by the size
expected to be required for DCCP headers and options. If the
application provides data that, when combined with the options the
DCCP implementation would like to include, would exceed the MPS, the
implementation should either send the options on a separate packet
(such as a DCCP-Ack) or lower the MPS, drop the data, and return an
appropriate error to the application.

14.1. Measuring PMTU

Each DCCP endpoint MUST keep track of the current PMTU for each
connection, except that this is not required for IPv4 connections
whose applications have requested fragmentation. The PMTU SHOULD be
initialized from the interface MTU that will be used to send
packets. The MPS will be initialized with the minimum of the PMTU
and the CCMPS, if any.

Classical PMTU discovery uses unfragmentable packets. In IPv4,
these packets have the IP Don't Fragment (DF) bit set; in IPv6, all
packets are unfragmentable once emitted by an end host. As	packets are unfragmentable. As specified in [RFC 1191], when a
specified in RFC 1191, when a router receives a packet with DF set	router receives a packet with DF set that is larger than the next
	link's MTU, it sends an ICMP Destination Unreachable message back to
	the source whose Code indicates that an unfragmentable packet was
	too large to forward (a "Datagram Too Big" message). When a DCCP
Kohler/Handley/Floyd Section 14.1. [Page 105]	implementation receives a Datagram Too Big message, it decreases its

INTERNET-DRAFT Expires: 7 September 2005 March 2005	PMTU to the Next-Hop MTU value given in the ICMP message. If the
	MTU given in the message is zero, the sender chooses a value for
	PMTU using the algorithm described in Section 7 of [RFC 1191]. If
that is larger than the next link's MTU, it sends an ICMP	the MTU given in the message is greater than the current PMTU, the
Destination Unreachable message back to the source whose Code	Datagram Too Big message is ignored, as described in [RFC 1191].
indicates that an unfragmentable packet was too large to forward (a	(We are aware that this may cause problems for DCCP endpoints behind
"Datagram Too Big" message). When a DCCP implementation receives a	certain firewalls.)
Datagram Too Big message, it decreases its PMTU to the Next-Hop MTU
value given in the ICMP message. If the MTU given in the message is
zero, the sender chooses a value for PMTU using the algorithm
described in RFC 1191 (Section 7). If the MTU given in the message
is greater than the current PMTU, the Datagram Too Big message is
ignored, as described in RFC 1191. (We are aware that this may
cause problems for DCCP endpoints behind certain firewalls.)

A DCCP implementation may allow the application to occasionally
request that PMTU discovery be performed again. This will reset the
PMTU to the outgoing interface's MTU. Such requests SHOULD be rate
limited, to one per two seconds, for example.

A DCCP sender MAY treat the reception of an ICMP Datagram Too Big
message as an indication that the packet being reported was not lost
due to congestion, and so for the purposes of congestion control it
MAY ignore the DCCP receiver's indication that this packet did not
arrive. However, if this is done, then the DCCP sender MUST check
the ECN bits of the IP header echoed in the ICMP message, and only
perform this optimization if these ECN bits indicate that the packet
did not experience congestion prior to reaching the router whose
link MTU it exceeded.

A DCCP implementation SHOULD ensure, as far as possible, that ICMP
Datagram Too Big messages were actually generated by routers, so
that attackers cannot drive the PMTU down to a falsely small value.
The simplest way to do this is to verify that the Sequence Number on
the ICMP error's encapsulated header corresponds to a Sequence
Number that the implementation recently sent. (According to current	Number that the implementation recently sent. (Routers are not
specifications, routers should return the full DCCP header and	required to return more than 64 bits of the DCCP header [RFC 792],
payload up to a maximum of 576 bytes [RFC 1812] or the minimum IPv6	but most modern routers will return far more, including the Sequence
MTU [RFC 2463], although they are not required to return more than	Number.) ICMP Datagram Too Big messages with incorrect or missing
64 bits [RFC 792]. Any amount greater than 128 bits will include	Sequence Numbers may be ignored, or the DCCP implementation may
the Sequence Number.) ICMP Datagram Too Big messages with incorrect	lower the PMTU only temporarily in response. If more than three odd
or missing Sequence Numbers may be ignored, or the DCCP	Datagram Too Big messages are received and the other DCCP endpoint
implementation may lower the PMTU only temporarily in response. If	reports more than three lost packets, however, the DCCP
more than three odd Datagram Too Big messages are received and the	implementation SHOULD assume the presence of a confused router, and
other DCCP endpoint reports more than three lost packets, however,	either obey the ICMP messages' PMTU or (on IPv4 networks) switch to
the DCCP implementation SHOULD assume the presence of a confused	allowing fragmentation.
router, and either obey the ICMP messages' PMTU or (on IPv4
networks) switch to allowing fragmentation.

DCCP also allows upward probing of the PMTU [PMTUD], where the DCCP
endpoint begins by sending small packets with DF set, then gradually



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increases the packet size until a packet is lost. This mechanism
does not require any ICMP error processing. DCCP-Sync packets are
the best choice for upward probing, since DCCP-Sync probes do not
risk application data loss. The DCCP implementation inserts
arbitrary data into the DCCP-Sync application area, padding the
packet to the right length; and since every valid DCCP-Sync
generates an immediate DCCP-SyncAck in response, the endpoint will
have a pretty good idea of when a probe is lost.

14.2. Sender Behavior

A DCCP sender SHOULD send every packet as unfragmentable, as
described above, with the following exceptions.

o On IPv4 connections whose applications have requested
fragmentation, the sender SHOULD send packets with the DF bit not
set.

o On IPv6 connections whose applications have requested
fragmentation, the sender SHOULD use fragmentation extension
headers to fragment packets larger than PMTU into suitably-sized
chunks. (Those chunks are, of course, unfragmentable.)

o It is undesirable for PMTU discovery to occur on the initial
connection setup handshake, as the connection setup process may
not be representative of packet sizes used during the connection,
and performing MTU discovery on the initial handshake might
unnecessarily delay connection establishment. Thus, DCCP-Request
and DCCP-Response packets SHOULD be sent as fragmentable. In
addition, DCCP-Reset packets SHOULD be sent as fragmentable,
although typically these would be small enough to not be a
problem. For IPv4 connections, these packets SHOULD be sent with
the DF bit not set; for IPv6 connections, they SHOULD be
preemptively fragmented to a size not larger than the relevant
interface MTU.

If the DCCP implementation has decreased the PMTU, the sending
application has not requested fragmentation, and the sending
application attempts to send a packet larger than the new MPS, the
API MUST refuse to send the packet and return an appropriate error
to the application. The application should then use the API to
query the new value of MPS. The kernel might have some packets
buffered for transmission that are smaller than the old MPS, but
larger than the new MPS. It MAY send these packets as fragmentable,
or it MAY discard these packets; it MUST NOT send them as
unfragmentable.





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15. Forward Compatibility

Future versions of DCCP may add new options and features. A few
simple guidelines will let extended DCCPs interoperate with normal
DCCPs.

o DCCP processors MUST NOT act punitively towards options and
features they do not understand. For example, DCCP processors
MUST NOT reset the connection if some field marked Reserved in
this specification is non-zero; if some unknown option is
present; or if some feature negotiation option mentions an
unknown feature. Instead, DCCP processors MUST ignore these
events. The Mandatory option is the single exception: if
Mandatory precedes some unknown option or feature, the connection
MUST be reset.

o DCCP processors MUST anticipate the possibility of unknown
feature values, which might occur as part of a negotiation for a
known feature. For server-priority features, unknown values are
handled as a matter of course: since the non-extended DCCP's
priority list will not contain unknown values, the result of the
negotiation cannot be an unknown value. A DCCP SHOULD respond
with an empty Confirm option if it is assigned an unacceptable
value for some non-negotiable feature.

o Each DCCP extension SHOULD be controlled by some feature. The
default value of this feature should correspond to "extension not
available". If an extended DCCP wants to use the extension, it
SHOULD attempt to change the feature's value using a Change L or
Change R option. Any non-extended DCCP will ignore the option,
thus leaving the feature value at its default, "extension not
available".

Section 19 lists DCCP assigned numbers reserved for experimental and
testing purposes.

16. Middlebox Considerations

This section describes properties of DCCP that firewalls, network
address translators, and other middleboxes should consider,
including parts of the packet that middleboxes should not change.
The intent is to draw attention to aspects of DCCP that may be
useful, or dangerous, for middleboxes, or that differ significantly
from TCP.

The Service Code field in DCCP-Request packets provides information
that may be useful for stateful middleboxes. With Service Code, a
middlebox can tell what protocol a connection will use without



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relying on port numbers. Middleboxes can disallow connections that
attempt to access unexpected services by sending a DCCP-Reset with
Reset Code 8, "Bad Service Code". Middleboxes should not modify the
Service Code unless they are really changing the service a
connection is accessing.

The Source and Destination Port fields are in the same packet
locations as the corresponding fields in TCP and UDP, which may
simplify some middlebox implementations.

The forward compatibility considerations in Section 15 apply to
middleboxes as well. In particular, middleboxes generally shouldn't
act punitively towards options and features they do not understand.

Modifying DCCP Sequence Numbers and Acknowledgement Numbers is more
tedious and dangerous than modifying TCP sequence numbers. A
middlebox that added packets to, or removed packets from, a DCCP
connection would have to modify acknowledgement options, such as Ack
Vector, and CCID-specific options, such as TFRC's Loss Intervals, at
minimum. On ECN-capable connections, the middlebox would have to
keep track of ECN Nonce information for packets it introduced or
removed, so that the relevant acknowledgement options continued to
have correct ECN Nonce Echoes, or risk the connection being reset
for "Aggression Penalty". We therefore recommend that middleboxes
not modify packet streams by adding or removing packets.

Note that there is less need to modify DCCP's per-packet sequence
numbers than TCP's per-byte sequence numbers; for example, a
middlebox can change the contents of a packet without changing its
sequence number. (In TCP, sequence number modification is required
to support protocols like FTP that carry variable-length addresses
in the data stream. If such an application were deployed over DCCP,
middleboxes would simply grow or shrink the relevant packets as
necessary, without changing their sequence numbers. This might
involve fragmenting the packet.)

Middleboxes may, of course, reset connections in progress. Clearly
this requires inserting a packet into one or both packet streams,
but the difficult issues do not arise.

DCCP is somewhat unfriendly to "connection splicing" [SHHP00], in
which clients' connection attempts are intercepted, but possibly
later "spliced in" to external server connections via sequence
number manipulations. A connection splicer at minimum would have to
ensure that the spliced connections agreed on all relevant feature
values, which might take some renegotiation.





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The contents of this section should not be interpreted as a
wholesale endorsement of stateful middleboxes.

17. Relations to Other Specifications

17.1. RTP

The Real-Time Transport Protocol, RTP [RFC 3550], is currently used
over UDP by many of DCCP's target applications (for instance,
streaming media). Therefore, it is important to examine the
relationship between DCCP and RTP, and in particular, the question
of whether any changes in RTP are necessary or desirable when it is
layered over DCCP instead of UDP.

There are two potential sources of overhead in the RTP-over-DCCP
combination, duplicated acknowledgement information and duplicated
sequence numbers. Together, these sources of overhead add slightly
more than 4 bytes per packet relative to RTP-over-UDP, and that
eliminating the redundancy would not reduce the overhead.

First, consider acknowledgements. Both RTP and DCCP report feedback
about loss rates to data senders, via RTP Control Protocol Sender
and Receiver Reports (RTCP SR/RR packets) and via DCCP
acknowledgement options. These feedback mechanisms are potentially
redundant. However, RTCP SR/RR packets contain information not
present in DCCP acknowledgements, such as "interarrival jitter", and
DCCP's acknowledgements contain information not transmitted by RTCP,
such as the ECN Nonce Echo. Neither feedback mechanism makes the
other redundant.

Sending both types of feedback need not be particularly costly
either. RTCP reports may be sent relatively infrequently: once
every 5 seconds on average, for low-bandwidth flows. In DCCP, some
feedback mechanisms are expensive -- Ack Vector, for example, is
frequent and verbose -- but others are relatively cheap: CCID 3
(TFRC) acknowledgements take between 16 and 32 bytes of options sent
once per round-trip time. (Reporting less frequently than once per
RTT would make congestion control less responsive to loss.) We
therefore conclude that acknowledgement overhead in RTP-over-DCCP
need not be significantly higher than for RTP-over-UDP, at least for
CCID 3.

One clear redundancy can be addressed at the application level. The
verbose packet-by-packet loss reports sent in RTCP Extended Reports
Loss RLE Blocks [RFC 3611] can be derived from DCCP's Ack Vector
options. (The converse is not true, since Loss RLE Blocks contain
no ECN information.) Since DCCP implementations should provide an
API for application access to Ack Vector information, RTP-over-DCCP



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applications might request either DCCP Ack Vectors or RTCP Extended
Report Loss RLE Blocks, but not both.

Now consider sequence number redundancy on data packets. The
embedded RTP header contains a 16-bit RTP sequence number. Most
data packets will use the DCCP-Data type; DCCP-DataAck and DCCP-Ack
packets need not usually be sent. The DCCP-Data header is 12 bytes
long without options, including a 24-bit sequence number. This is 4
bytes more than a UDP header. Any options required on data packets
would add further overhead, although many CCIDs (for instance, CCID
3, TFRC) don't require options on most data packets.

The DCCP sequence number cannot be inferred from the RTP sequence
number since it increments on non-data packets as well as data
packets. The RTP sequence number cannot be inferred from the DCCP
sequence number either [RFC 3550]. Furthermore, removing RTP's
sequence number would not save any header space because of alignment
issues. We therefore recommend that RTP transmitted over DCCP use
the same headers currently defined. The 4 byte header cost is a
reasonable tradeoff for DCCP's congestion control features and
access to ECN. Truly bandwidth-starved endpoints should use some
future header compression scheme.

17.2. Congestion Manager and Multiplexing

Since DCCP doesn't provide reliable, ordered delivery, multiple
application sub-flows may be multiplexed over a single DCCP
connection with no inherent performance penalty. Thus, there is no
need for DCCP to provide built-in, SCTP-style support for multiple
sub-flows.

Some applications might want to share congestion control state among
multiple DCCP flows that share the same source and destination
addresses. This functionality could be provided by the Congestion
Manager [RFC 3124], a generic multiplexing facility. However, the
CM would not fully support DCCP without change; it does not
gracefully handle multiple congestion control mechanisms, for
example.

18. Security Considerations

DCCP does not provide cryptographic security guarantees.
Applications desiring cryptographic security services (integrity,	Applications desiring hard security should use IPsec or end-to-end
authentication, confidentiality, access control, and anti-replay	security of some kind.
protection) should use IPsec or end-to-end security of some kind;
Secure RTP is one candidate protocol [RFC 3711].





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Nevertheless, DCCP is intended to protect against some classes of
attackers: Attackers cannot hijack a DCCP connection (close the
connection unexpectedly, or cause attacker data to be accepted by an
endpoint as if it came from the sender) unless they can guess valid
sequence numbers. Thus, as long as endpoints choose initial
sequence numbers well, a DCCP attacker must snoop on data packets to
get any reasonable probability of success. Sequence number validity
checks provide this guarantee. Section 7.5.5 describes sequence
number security further. This security property only holds assuming	number security further.
that DCCP's random numbers are chosen according to the guidelines in	This security property only holds assuming that DCCP's random
RFC 1750.	numbers are chosen according to the guidelines in [RFC 1750].

DCCP also provides mechanisms to limit the potential impact of some
denial-of-service attacks. These mechanisms include Init Cookie
(Section 8.1.4), the DCCP-CloseReq packet (Section 5.5), the
Application Not Listening Drop Code (Section 11.7.2), limitations on
the processing of options that might cause connection reset (Section
7.5.5), limitations on the processing of some ICMP messages (Section
14.1), and various rate limits, which let servers avoid extensive
computation or packet generation (Sections 7.5.3, 8.1.3, and
others).

DCCP provides no protection against attackers that can snoop on data
packets.

18.1. Security Considerations for Partial Checksums

The partial checksum facility has a separate security impact,
particularly in its interaction with authentication and encryption
mechanisms. The impact is the same in DCCP as in the UDP-Lite
protocol, and what follows was adapted from the corresponding text
in the UDP-Lite specification [RFC 3828].

When a DCCP packet's Checksum Coverage field is not zero, the
uncovered portion of a packet may change in transit. This is
contrary to the idea behind most authentication mechanisms:
authentication succeeds if the packet has not changed in transit.
Unless authentication mechanisms that operate only on the sensitive
part of packets are developed and used, authentication will always
fail for partially-checksummed DCCP packets whose uncovered part has
been damaged.

The IPsec integrity check (Encapsulation Security Protocol, ESP, or
Authentication Header, AH) is applied (at least) to the entire IP
packet payload. Corruption of any bit within that area will then
result in the IP receiver discarding a DCCP packet, even if the
corruption happened in an uncovered part of the DCCP application
data.



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When IPsec is used with ESP payload encryption, a link can not
determine the specific transport protocol of a packet being
forwarded by inspecting the IP packet payload. In this case, the
link MUST provide a standard integrity check covering the entire IP
packet and payload. DCCP partial checksums provide no benefit in
this case.

Encryption (e.g., at the transport or application levels) may be
used. Note that omitting an integrity check can, under certain
circumstances, compromise confidentiality [BEL98].

If a few bits of an encrypted packet are damaged, the decryption
transform will typically spread errors so that the packet becomes
too damaged to be of use. Many encryption transforms today exhibit
this behavior. There exist encryption transforms, stream ciphers,
which do not cause error propagation. Proper use of stream ciphers
can be quite difficult, especially when authentication checking is	can be quite difficult, especially when authentication-checking is
omitted [BB01]. In particular, an attacker can cause predictable
changes to the ultimate plaintext, even without being able to
decrypt the ciphertext.

19. IANA Considerations

DCCP introduces eight sets of numbers whose values should be
allocated by IANA. We refer to allocation policies, such as
Standards Action, outlined in RFC 2434, and most registries reserve	Standards Action, outlined in [RFC 2434], and most registries
some values for experimental and testing use [RFC 3692]. In	reserve some values for experimental and testing use [RFC 3692]. In
addition, DCCP requires a Protocol Number to be added to the
registry of Assigned Internet Protocol Numbers. IANA is requested
to assign IP Protocol Number 33 to DCCP; this number has already
been informally made available for experimental DCCP use.

19.1. Packet Types

Each entry in the DCCP Packet Type registry contains a packet type,
which is a number in the range 0-15; a packet type name, such as
DCCP-Request; and a reference to the RFC defining the packet type.
The registry is initially populated using the values in Table 1
(Section 5.1). This document allocates packet types 0-9, and packet
type 14 is permanently reserved for experimental and testing use.
Packet types 10-13 and 15 are currently reserved, and should be
allocated with the Standards Action policy, which requires IESG	allocated with the Standards Action policy, which requires IETF
review and approval and standards-track IETF RFC publication.	working group review and standards-track RFC publication.

19.2. Reset Codes

Each entry in the DCCP Reset Code registry contains a Reset Code,
which is a number in the range 0-255; a short description of the



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Reset Code, such as "No Connection"; and a reference to the RFC
defining the Reset Code. The registry is initially populated using
the values in Table 2 (Section 5.6). This document allocates Reset
Codes 0-11, and Reset Codes 120-126 are permanently reserved for
experimental and testing use. Reset Codes 12-119 and 127 are
currently reserved, and should be allocated with the IETF Consensus
policy, requiring an IETF RFC publication (standards-track or not)	policy, which requires RFC publication (not necessarily standards-
with IESG review and approval. Reset Codes 128-255 are permanently	track). Reset Codes 128-255 are permanently reserved for CCID-
reserved for CCID-specific registries; each CCID Profile document	specific registries; each CCID Profile document describes how the
describes how the corresponding registry is managed.	corresponding registry is managed.

19.3. Option Types

Each entry in the DCCP option type registry contains an option type,
which is a number in the range 0-255; the name of the option, such
as "Slow Receiver"; and a reference to the RFC defining the option
type. The registry is initially populated using the values in Table
3 (Section 5.8). This document allocates option types 0-2 and
32-44, and option types 31 and 120-126 are permanently reserved for
experimental and testing use. Option types 3-30, 45-119, and 127
are currently reserved, and should be allocated with the IETF
Consensus policy, requiring an IETF RFC publication (standards-track	Consensus policy, which requires RFC publication (not necessarily
or not) with IESG review and approval. Option types 128-255 are	standards-track). Option types 128-255 are permanently reserved for
permanently reserved for CCID-specific registries; each CCID Profile	CCID-specific registries; each CCID Profile document describes how
document describes how the corresponding registry is managed.	the corresponding registry is managed.

19.4. Feature Numbers

Each entry in the DCCP feature number registry contains a feature
number, which is a number in the range 0-255; the name of the
feature, such as "ECN Incapable"; and a reference to the RFC
defining the feature number. The registry is initially populated
using the values in Table 4 (Section 6). This document allocates
feature numbers 0-9, and feature numbers 120-126 are permanently
reserved for experimental and testing use. Feature numbers 10-119
and 127 are currently reserved, and should be allocated with the
IETF Consensus policy, requiring an IETF RFC publication (standards-	IETF Consensus policy, which requires RFC publication (not
track or not) with IESG review and approval. Feature numbers	necessarily standards-track). Feature numbers 128-255 are
128-255 are permanently reserved for CCID-specific registries; each	permanently reserved for CCID-specific registries; each CCID Profile
CCID Profile document describes how the corresponding registry is	document describes how the corresponding registry is managed.
managed.

19.5. Congestion Control Identifiers

Each entry in the DCCP Congestion Control Identifier (CCID) registry
contains a CCID, which is a number in the range 0-255; the name of
the CCID, such as "TCP-like Congestion Control"; and a reference to
the RFC defining the CCID. The registry is initially populated



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using the values in Table 5 (Section 10). CCIDs 2 and 3 are
allocated by concurrently published profiles, and CCIDs 248-254 are
permanently reserved for experimental and testing use. CCIDs 0, 1,
4-247, and 255 are currently reserved, and should be allocated with
the IETF Consensus policy, requiring an IETF RFC publication	the IETF Consensus policy, which requires RFC publication (not
(standards-track or not) with IESG review and approval.	necessarily standards-track).

19.6. Ack Vector States

Each entry in the DCCP Ack Vector State registry contains an Ack
Vector State, which is a number in the range 0-3; the name of the
State, such as "Received ECN Marked"; and a reference to the RFC
defining the State. The registry is initially populated using the
values in Table 6 (Section 11.4). This document allocates States 0,
1, and 3. State 2 is currently reserved, and should be allocated
with the Standards Action policy, which requires IESG review and	with the Standards Action policy, which requires IETF working group
approval and standards-track IETF RFC publication.	review and standards-track RFC publication.

19.7. Drop Codes

Each entry in the DCCP Drop Code registry contains a Data Dropped
Drop Code, which is a number in the range 0-7; the name of the Drop
Code, such as "Application Not Listening"; and a reference to the
RFC defining the Drop Code. The registry is initially populated
using the values in Table 7 (Section 11.7). This document allocates
Drop Codes 0-3 and 7. Drop Codes 4-6 are currently reserved, and
should be allocated with the Standards Action policy, which requires
IESG review and approval and standards-track IETF RFC publication.	IETF working group review and standards-track RFC publication.

19.8. Service Codes

Each entry in the Service Code registry contains a Service Code,
which is a number in the range 0-4294967295; a short English
description of the intended service; and an optional reference to an
RFC or other publicly available specification defining the Service
Code. The registry should list the Service Code's numeric value as
a decimal number, but when each byte of the four-byte Service Code
is in the range 32-127, the registry should also show a four-
character ASCII interpretation of the Service Code. Thus, the
number 1717858426 would additionally appear as "fdpz". Service
Codes are not DCCP-specific. This document does not allocate any
Service Codes, but Service Code 0 is permanently reserved (it
represents the absence of a meaningful Service Code), and Service
Codes 1056964608-1073741823 (high byte ASCII "?") are reserved for
Private Use. Most of the remaining Service Codes are allocated
First Come First Served, with no RFC publication required.
Exceptions are listed in Section 8.1.2.




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20. Thanks

Thanks to Jitendra Padhye for his help with early versions of this
specification.

Thanks to Junwen Lai and Arun Venkataramani, who, as interns at
ICIR, built a prototype DCCP implementation. In particular, Junwen
Lai recommended that the old feature negotiation mechanism be
scrapped and co-designed the current mechanism. Arun	scrapped and codesigned the current mechanism. Arun Venkataramani's
Venkataramani's feedback improved Appendix A.	feedback improved Appendix A.

We thank the staff and interns of ICIR and, formerly, ACIRI, the
members of the End-to-End Research Group, and the members of the
Transport Area Working Group for their feedback on DCCP. We
especially thank the DCCP expert reviewers: Greg Minshall, Eric
Rescorla, and Magnus Westerlund for detailed written comments and
problem spotting, and Rob Austein and Steve Bellovin for verbal
comments and written notes.

We also thank those who provided comments and suggestions via the
DCCP BOF, Working Group, and mailing lists, including Damon
Lanphear, Patrick McManus, Colin Perkins, Sara Karlberg, Kevin Lai,
Bernard Aboba, Youngsoo Choi, Pengfei Di, Dan Duchamp, Gorry
Fairhurst, Derek Fawcus, David Timothy Fleeman, John Loughney,
Ghyslain Pelletier, Tom Phelan, Stanislav Shalunov, David Vos, Yufei
Wang, and Michael Welzl. In particular, Colin Perkins provided
extensive, detailed feedback, Michael Welzl suggested the Data
Checksum option, and Gorry Fairhurst provided extensive feedback on
various checksum issues.

A. Appendix: Ack Vector Implementation Notes

This appendix discusses particulars of DCCP acknowledgement
handling, in the context of an abstract implementation for Ack
Vector. It is informative rather than normative.

The first part of our implementation runs at the HC-Receiver, and
therefore acknowledges data packets. It generates Ack Vector
options. The implementation has the following characteristics:

o At most one byte of state per acknowledged packet.

o O(1) time to update that state when a new packet arrives (normal
case).

o Cumulative acknowledgements.





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o Quick removal of old state.

The basic data structure is a circular buffer containing information
about acknowledged packets. Each byte in this buffer contains a
state and run length; the state can be 0 (packet received), 1
(packet ECN marked), or 3 (packet not yet received). The buffer
grows from right to left. The implementation maintains five
variables, aside from the buffer contents:

o "buf_head" and "buf_tail", which mark the live portion of the
buffer.

o "buf_ackno", the Acknowledgement Number of the most recent packet
acknowledged in the buffer. This corresponds to the "head"
pointer.

o "buf_nonce", the one-bit sum (exclusive-or, or parity) of the ECN
Nonces received on all packets acknowledged by the buffer with
State 0.

We draw acknowledgement buffers like this:

+---------------------------------------------------------------+
\|S,L\|S,L\|S,L\|S,L\| \| \| \| \|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|
+---------------------------------------------------------------+
^ ^
buf_tail buf_head, buf_ackno = A buf_nonce = E

<=== buf_head and buf_tail move this way <===

Each `S,L' represents a State/Run length byte. We will draw these
buffers showing only their live portion, and will add an annotation
showing the Acknowledgement Number for the last live byte in the
buffer. For example:

+-----------------------------------------------+
A \|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\|S,L\| T BN[E]
+-----------------------------------------------+

Here, buf_nonce equals E and buf_ackno equals A.

We will use this buffer as a running example.

+---------------------------+
10 \|0,0\|3,0\|3,0\|3,0\|0,4\|1,0\|0,0\| 0 BN[1] [Example Buffer]
+---------------------------+

In concrete terms, its meaning is as follows:



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Packet 10 was received. (The head of the buffer has sequence
number 10, state 0, and run length 0.)

Packets 9, 8, and 7 have not yet been received. (The three
bytes preceding the head each have state 3 and run length 0.)

Packets 6, 5, 4, 3, and 2 were received.

Packet 1 was ECN marked.

Packet 0 was received.

The one-bit sum of the ECN Nonces on packets 10, 6, 5, 4, 3, 2,
and 0 equals 1.

Additionally, the HC-Receiver must keep some information about the
Ack Vectors it has recently sent. For each packet sent carrying an
Ack Vector, it remembers four variables:

o "ack_seqno", the Sequence Number used for the packet. This is an
HC-Receiver sequence number.

o "ack_ptr", the value of buf_head at the time of acknowledgement.

o "ack_ackno", the Acknowledgement Number used for the packet.
This is an HC-Sender sequence number. Since acknowledgements are
cumulative, this single number completely specifies all necessary
information about the packets acknowledged by this Ack Vector.

o "ack_nonce", the one-bit sum of the ECN Nonces for all State 0
packets in the buffer from buf_head to ack_ackno, inclusive.
Initially, this equals the Nonce Echo of the acknowledgement's
Ack Vector (or, if the ack packet contained more than one Ack
Vector, the exclusive-or of all the acknowledgement's Ack
Vectors). It changes as information about old acknowledgements
is removed (so ack_ptr and buf_head diverge), and as old packets
arrive (so they change from State 3 or State 1 to State 0).

A.1. Packet Arrival

This section describes how the HC-Receiver updates its
acknowledgement buffer as packets arrive from the HC-Sender.

A.1.1. New Packets

When a packet with Sequence Number greater than buf_ackno arrives,
the HC-Receiver updates buf_head (by moving it to the left
appropriately), buf_ackno (which is set to the new packet's Sequence



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Number), and possibly buf_nonce (if the packet arrived unmarked with
ECN Nonce 1), in addition to the buffer itself. For example, if HC-
Sender packet 11 arrived ECN marked, the Example Buffer above would
enter this new state (changes are marked with stars):

+*----------------------------+
11 \|1,0\|0,0\|3,0\|3,0\|3,0\|0,4\|1,0\|0,0\| 0 BN[1]
+*----------------------------+

If the packet's state equals the state at the head of the buffer,
the HC-Receiver may choose to increment its run length (up to the
maximum). For example, if HC-Sender packet 11 arrived without ECN
marking and with ECN Nonce 0, the Example Buffer might enter this
state instead:

** +--*------------------------+
11 \|0,1\|3,0\|3,0\|3,0\|0,4\|1,0\|0,0\| 0 BN[1]
** +--*------------------------+

Of course, the new packet's sequence number might not equal the
expected sequence number. In this case, the HC-Receiver will enter
the intervening packets as State 3. If several packets are missing,
the HC-Receiver may prefer to enter multiple bytes with run length
0, rather than a single byte with a larger run length; this
simplifies table updates if one of the missing packets arrives. For
example, if HC-Sender packet 12 arrived with ECN Nonce 1, the
Example Buffer would enter this state:

+****----------------------------+
12 \|0,0\|3,0\|0,1\|3,0\|3,0\|3,0\|0,4\|1,0\|0,0\| 0 BN[0]
+****----------------------------+

Of course, the circular buffer may overflow, either when the HC-
Sender is sending data at a very high rate, when the HC-Receiver's
acknowledgements are not reaching the HC-Sender, or when the HC-
Sender is forgetting to acknowledge those acks (so the HC-Receiver
is unable to clean up old state). In this case, the HC-Receiver
should either compress the buffer (by increasing run lengths when
possible), transfer its state to a larger buffer, or, as a last
resort, drop all received packets, without processing them
whatsoever, until its buffer shrinks again.

A.1.2. Old Packets

When a packet with Sequence Number S arrives, and S <= buf_ackno,
the HC-Receiver will scan the table for the byte corresponding to S.
(Indexing structures could reduce the complexity of this scan.) If
S was previously lost (State 3), and it was stored in a byte with



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run length 0, the HC-Receiver can simply change the byte's state.
For example, if HC-Sender packet 8 was received with ECN Nonce 0,
the Example Buffer would enter this state:

+--------*------------------+
10 \|0,0\|3,0\|0,0\|3,0\|0,4\|1,0\|0,0\| 0 BN[1]
+--------*------------------+

If S was not marked as lost, or if it was not contained in the
table, the packet is probably a duplicate, and should be ignored.
(The new packet's ECN marking state might differ from the state in
the buffer; Section 11.4.1 describes what is allowed then.) If S's
buffer byte has a non-zero run length, then the buffer might need be
reshuffled to make space for one or two new bytes.

The ack_nonce fields may also need manipulation when old packets
arrive. In particular, when S transitions from State 3 or State 1
to State 0, and S had ECN Nonce 1, then the implementation should
flip the value of ack_nonce for every acknowledgement with ack_ackno
>= S.

It is impossible with this data structure to shift packets from
State 0 to State 1, since the buffer doesn't store individual
packets' ECN Nonces.

A.2. Sending Acknowledgements

Whenever the HC-Receiver needs to generate an acknowledgement, the
buffer's contents can simply be copied into one or more Ack Vector
options. Copied Ack Vectors might not be maximally compressed; for
example, the Example Buffer above contains three adjacent 3,0 bytes
that could be combined into a single 3,2 byte. The HC-Receiver
might, therefore, choose to compress the buffer in place before
sending the option, or to compress the buffer while copying it;
either operation is simple.

Every acknowledgement sent by the HC-Receiver SHOULD include the
entire state of the buffer. That is, acknowledgements are
cumulative.

If the acknowledgement fits in one Ack Vector, that Ack Vector's
Nonce Echo simply equals buf_nonce. For multiple Ack Vectors, more
care is required. The Ack Vectors should be split at points
corresponding to previous acknowledgements, since the stored
ack_nonce fields provide enough information to calculate correct
Nonce Echoes. The implementation should therefore acknowledge data
at least once per 253 bytes of buffer state. (Otherwise, there'd be
no way to calculate a Nonce Echo.)



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For each acknowledgement it sends, the HC-Receiver will add an
acknowledgement record. ack_seqno will equal the HC-Receiver
sequence number it used for the ack packet; ack_ptr will equal
buf_head; ack_ackno will equal buf_ackno; and ack_nonce will equal
buf_nonce.

A.3. Clearing State

Some of the HC-Sender's packets will include acknowledgement
numbers, which ack the HC-Receiver's acknowledgements. When such an
ack is received, the HC-Receiver finds the acknowledgement record R
with the appropriate ack_seqno, then:

o Sets buf_tail to R.ack_ptr + 1.

o If R.ack_nonce is 1, it flips buf_nonce, and the value of
ack_nonce for every later ack record.

o Throws away R and every preceding ack record.

(The HC-Receiver may choose to keep some older information, in case
a lost packet shows up late.) For example, say that the HC-Receiver
storing the Example Buffer had sent two acknowledgements already:

1. ack_seqno = 59, ack_ackno = 3, ack_nonce = 1.

2. ack_seqno = 60, ack_ackno = 10, ack_nonce = 0.

Say the HC-Receiver then received a DCCP-DataAck packet with
Acknowledgement Number 59 from the HC-Sender. This informs the HC-
Receiver that the HC-Sender received, and processed, all the
information in HC-Receiver packet 59. This packet acknowledged HC-
Sender packet 3, so the HC-Sender has now received HC-Receiver's
acknowledgements for packets 0, 1, 2, and 3. The Example Buffer
should enter this state:

+------------------+ *
10 \|0,0\|3,0\|3,0\|3,0\|0,2\| 4 BN[0]
+------------------+ *

The tail byte's run length was adjusted, since packet 3 was in the
middle of that byte. Since R.ack_nonce was 1, the buf_nonce field
was flipped, as were the ack_nonce fields for later acknowledgements
(here, the HC-Receiver Ack 60 record, not shown, has its ack_nonce
flipped to 1). The HC-Receiver can also throw away stored
information about HC-Receiver Ack 59 and any earlier
acknowledgements.




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A careful implementation might try to ensure reasonable robustness
to reordering. Suppose that the Example Buffer is as before, but
that packet 9 now arrives, out of sequence. The buffer would enter
this state:

+----*----------------------+
10 \|0,0\|0,0\|3,0\|3,0\|0,4\|1,0\|0,0\| 0 BN[1]
+----*----------------------+

The danger is that the HC-Sender might acknowledge the HC-Receiver's
previous acknowledgement (with sequence number 60), which says that
Packet 9 was not received, before the HC-Receiver has a chance to
send a new acknowledgement saying that Packet 9 actually was
received. Therefore, when packet 9 arrived, the HC-Receiver might
modify its acknowledgement record to:

1. ack_seqno = 59, ack_ackno = 3, ack_nonce = 1.

2. ack_seqno = 60, ack_ackno = 3, ack_nonce = 1.

That is, Ack 60 is now treated like a duplicate of Ack 59. This
would prevent the Tail pointer from moving past packet 9 until the
HC-Receiver knows that the HC-Sender has seen an Ack Vector
indicating that packet's arrival.

A.4. Processing Acknowledgements

When the HC-Sender receives an acknowledgement, it generally cares
about the number of packets that were dropped and/or ECN marked. It
simply reads this off the Ack Vector. Additionally, it should check
the ECN Nonce for correctness. (As described in Section 11.4.1, it
may want to keep more detailed information about acknowledged
packets in case packets change states between acknowledgements, or
in case the application queries whether a packet arrived.)

The HC-Sender must also acknowledge the HC-Receiver's
acknowledgements so that the HC-Receiver can free old Ack Vector
state. (Since Ack Vector acknowledgements are reliable, the HC-
Receiver must maintain and resend Ack Vector information until it is
sure that the HC-Sender has received that information.) A simple
algorithm suffices: since Ack Vector acknowledgements are
cumulative, a single acknowledgement number tells HC-Receiver how
much ack information has arrived. Assuming that the HC-Receiver
sends no data, the HC-Sender can ensure that at least once a round-
trip time, it sends a DCCP-DataAck packet acknowledging the latest
DCCP-Ack packet it has received. Of course, the HC-Sender only
needs to acknowledge the HC-Receiver's acknowledgements if the HC-
Sender is also sending data. If the HC-Sender is not sending data,



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then the HC-Receiver's Ack Vector state is stable, and there is no
need to shrink it. The HC-Sender must watch for drops and ECN marks
on received DCCP-Ack packets so that it can adjust the HC-Receiver's
ack-sending rate -- for example, with Ack Ratio -- in response to
congestion.

If the other half-connection is not quiescent -- that is, the HC-
Receiver is sending data to the HC-Sender, possibly using another
CCID -- then the acknowledgements on that half-connection are
sufficient for the HC-Receiver to free its state.

B. Appendix: Partial Checksumming Design Motivation

A great deal of discussion has taken place regarding the utility of
allowing a DCCP sender to restrict the checksum so that it does not
cover the complete packet. This section attempts to capture some of
the rationale behind specific details of DCCP design.

Many of the applications that we envisage using DCCP are resilient
to some degree of data loss, or they would typically have chosen a
reliable transport. Some of these applications may also be
resilient to data corruption -- some audio payloads, for example.
These resilient applications might prefer to receive corrupted data
than to have DCCP drop a corrupted packet. This is particularly
because of congestion control: DCCP cannot tell the difference
between packets dropped due to corruption and packets dropped due to
congestion, and so it must reduce the transmission rate accordingly.
This response may cause the connection to receive less bandwidth
than it is due; corruption in some networking technologies is
independent of, or at least not always correlated to, congestion.
Therefore, corrupted packets do not need to cause as strong a
reduction in transmission rate as the congestion response would
dictate (so long as the DCCP header and options are not corrupt).

Thus DCCP allows the checksum to cover all of the packet, just the
DCCP header, or both the DCCP header and some number of bytes from
the application data. If the application cannot tolerate any data
corruption, then the checksum must cover the whole packet. If the
application would prefer to tolerate some corruption rather than
have the packet dropped, then it can set the checksum to cover only
part of the packet (but always the DCCP header). In addition, if
the application wishes to decouple checksumming of the DCCP header
from checksumming of the application data, it may do so by including
the Data Checksum option. This would allow DCCP to discard
corrupted application data, but still not mistake the corruption for
network congestion.





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Thus, from the application point of view, partial checksums seem to
be a desirable feature. However, the usefulness of partial
checksums depends on partially corrupted packets being delivered to
the receiver. If the link-layer CRC always discards corrupted
packets, then this will not happen, and so the usefulness of partial
checksums would be restricted to corruption that occurred in routers
and other places not covered by link CRCs. There does not appear to
be consensus on how likely it is that future network links that
suffer significant corruption will not cover the entire packet with
a single strong CRC. DCCP makes it possible to tailor such links to
the application, but it is difficult to predict if this will be
compelling for future link technologies.

In addition, partial checksums do not co-exist well with IP-level
authentication mechanisms such as IPsec AH, which cover the entire
packet with a cryptographic hash. Thus, if cryptographic
authentication mechanisms are required to co-exist with partial
checksums, the authentication must be carried in the application
data. A possible mode of usage would appear to be similar to that
of Secure RTP. However, such "application-level" authentication
does not protect the DCCP option negotiation and state machine from
forged packets. An alternative would be to use IPsec ESP, and use
encryption to protect the DCCP headers against attack, while using
the DCCP header validity checks to authenticate that the header is
from someone who possessed the correct key. However, while this is
resistant to replay (due to the DCCP sequence number), it is not by
itself resistant to some forms of man-in-the-middle attacks because
the application data is not tightly coupled to the packet header.
Thus an application-level authentication probably needs to be
coupled with IPsec ESP or a similar mechanism to provide a
reasonably complete security solution. The overhead of such a
solution might be unacceptable for some applications that would
otherwise wish to use partial checksums.

On balance, the authors believe that DCCP partial checksums have the
potential to enable some future uses that would otherwise be
difficult. As the cost and complexity of supporting them is small,
it seems worth including them at this time. It remains to be seen
whether they are useful in practice.

Normative References

[RFC 793] J. Postel, editor. Transmission Control Protocol.
RFC 793.

[RFC 1191] J. C. Mogul and S. E. Deering. Path MTU Discovery.
RFC 1191.




Kohler/Handley/Floyd [Page 124]

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[RFC 2119] S. Bradner. Key Words For Use in RFCs to Indicate
Requirement Levels. RFC 2119.

[RFC 2434] T. Narten and H. Alvestrand. Guidelines for Writing an
IANA Considerations Section in RFCs. RFC 2434.

[RFC 2460] S. Deering and R. Hinden. Internet Protocol, Version 6
(IPv6) Specification. RFC 2460.

[RFC 3168] K.K. Ramakrishnan, S. Floyd, and D. Black. The Addition
of Explicit Congestion Notification (ECN) to IP. RFC 3168.

[RFC 3309] J. Stone, R. Stewart, and D. Otis. Stream Control
Transmission Protocol (SCTP) Checksum Change. RFC 3309.

[RFC 3692] T. Narten. Assigning Experimental and Testing Numbers
Considered Useful. RFC 3692.

[RFC 3775] D. Johnson, C. Perkins, and J. Arkko. Mobility Support
in IPv6. RFC 3775.

[RFC 3828] L-A. Larzon, M. Degermark, S. Pink, L-E. Jonsson, editor,
and G. Fairhurst, editor. The Lightweight User Datagram Protocol
(UDP-Lite). RFC 3828.

Informative References

[BB01] S.M. Bellovin and M. Blaze. Cryptographic Modes of Operation
for the Internet. 2nd NIST Workshop on Modes of Operation,
August 2001.

[BEL98] S.M. Bellovin. Cryptography and the Internet. Proc. CRYPTO
'98 (LNCS 1462), pp46-55, August, 1988.

[CCID 2 PROFILE] S. Floyd and E. Kohler. Profile for DCCP
Congestion Control ID 2: TCP-like Congestion Control. draft-
ietf-dccp-ccid2-08.txt, work in progress, November 2004.

[CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye. Profile for
DCCP Congestion Control ID 3: TFRC Congestion Control. draft-
ietf-dccp-ccid3-08.txt, work in progress, November 2004.

[M85] Robert T. Morris. A Weakness in the 4.2BSD Unix TCP/IP
Software. Computer Science Technical Report 117, AT&T Bell
Laboratories, Murray Hill, NJ, February 1985.

[PMTUD] Matt Mathis, John Heffner, and Kevin Lahey. Path MTU
Discovery. draft-ietf-pmtud-method-01.txt, work in progress,



Kohler/Handley/Floyd [Page 125]

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February 2004.

[RFC 792] J. Postel, editor. Internet Control Message Protocol.
RFC 792.

[RFC 1750] D. Eastlake, S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750.

[RFC 1812] F. Baker, editor. Requirements for IP Version 4 Routers.
RFC 1812.

[RFC 1948] S. Bellovin. Defending Against Sequence Number Attacks.
RFC 1948.

[RFC 2018] M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow. TCP
Selective Acknowledgement Options. RFC 2018.

[RFC 2401] S. Kent and R. Atkinson. Security Architecture for the
Internet Protocol. RFC 2401.

[RFC 2463] A. Conta and S. Deering. Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification. RFC 2463.

[RFC 2581] M. Allman, V. Paxson, and W. Stevens. TCP Congestion
Control. RFC 2581.

[RFC 2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.
Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, and V.
Paxson. Stream Control Transmission Protocol. RFC 2960.

[RFC 3124] H. Balakrishnan and S. Seshan. The Congestion Manager.
RFC 3124.

[RFC 3360] S. Floyd. Inappropriate TCP Resets Considered Harmful.
RFC 3360.

[RFC 3448] M. Handley, S. Floyd, J. Padhye, and J. Widmer. TCP
Friendly Rate Control (TFRC): Protocol Specification. RFC 3448.

[RFC 3540] N. Spring, D. Wetherall, and D. Ely. Robust Explicit
Congestion Notification (ECN) Signaling with Nonces. RFC 3540.

[RFC 3550] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson.
RTP: A Transport Protocol for Real-Time Applications. STD 64.
RFC 3550.





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[RFC 3611] T. Friedman, R. Caceres, and A. Clark, editors. RTP
Control Protocol Extended Reports (RTCP XR). RFC 3611.

[RFC 3711] M. Baugher, D. McGrew, M. Naslund, E. Carrara, and K.
Norrman. The Secure Real-time Transport Protocol (SRTP).
RFC 3711.

[RFC 3819] P. Karn, editor, C. Bormann, G. Fairhurst, D. Grossman,
R. Ludwig, J. Mahdavi, G. Montenegro, J. Touch, and L. Wood.
Advice for Internet Subnetwork Designers. RFC 3819.

[SHHP00] Oliver Spatscheck, Jorgen S. Hansen, John H. Hartman, and
Larry L. Peterson. Optimizing TCP Forwarder Performance.
IEEE/ACM Transactions on Networking 8(2):146-157, April 2000.

[SYNCOOKIES] Daniel J. Bernstein. SYN Cookies.
http://cr.yp.to/syncookies.html, as of July 2003.

Authors' Addresses

Eddie Kohler <kohler@cs.ucla.edu>
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
USA

Mark Handley <M.Handley@cs.ucl.ac.uk>
Department of Computer Science
University College London
Gower Street
London WC1E 6BT
UK

Sally Floyd <floyd@icir.org>
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA

Full Copyright Statement

Copyright (C) The Internet Society 2004. This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE



Kohler/Handley/Floyd [Page 127]

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INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.























Kohler/Handley/Floyd [Page 128]

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7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . . 49	7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . . 48
7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . . 49	7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . . 48
7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . . 50	7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . . 49
7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . . 51	7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . . 50
7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . . 51
7.5. Validity and Synchronization . . . . . . . . . . . . . . 52	7.5. Validity and Synchronization . . . . . . . . . . . . . . 51
7.5.1. Sequence and Acknowledgement Number
Windows. . . . . . . . . . . . . . . . . . . . . . . . . . 52
7.5.2. Sequence Window Feature . . . . . . . . . . . . . . 53
7.5.3. Sequence-Validity Rules . . . . . . . . . . . . . . 54	7.5.3. Sequence-Validity Rules . . . . . . . . . . . . . . 53
7.5.4. Handling Sequence-Invalid Packets . . . . . . . . . 56	7.5.4. Handling Sequence-Invalid Packets . . . . . . . . . 55
7.5.5. Sequence Number Attacks . . . . . . . . . . . . . . 57	7.5.5. Sequence Number Attacks . . . . . . . . . . . . . . 56
7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . . 58	7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . . 57
7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . . 59	7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . . 58
7.6.1. Allow Short Sequence Numbers Feature. . . . . . . . 60	7.6.1. Allow Short Sequence Numbers Feature. . . . . . . . 59
7.6.2. When to Avoid Short Sequence Numbers. . . . . . . . 60	7.6.2. When to Avoid Short Sequence Numbers. . . . . . . . 59
7.7. NDP Count and Detecting Application Loss . . . . . . . . 61	7.7. NDP Count and Detecting Application Loss . . . . . . . . 60
7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . . 62	7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . . 61
7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . . 62	7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . . 61
8. Event Processing. . . . . . . . . . . . . . . . . . . . . . . 62	8. Event Processing. . . . . . . . . . . . . . . . . . . . . . . 61
8.1. Connection Establishment . . . . . . . . . . . . . . . . 63	8.1. Connection Establishment . . . . . . . . . . . . . . . . 62
8.1.1. Client Request. . . . . . . . . . . . . . . . . . . 63	8.1.1. Client Request. . . . . . . . . . . . . . . . . . . 62
8.1.2. Service Codes . . . . . . . . . . . . . . . . . . . 64	8.1.2. Service Codes . . . . . . . . . . . . . . . . . . . 63
8.1.3. Server Response . . . . . . . . . . . . . . . . . . 65	8.1.3. Server Response . . . . . . . . . . . . . . . . . . 64
8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . . 66	8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . . 65
8.1.5. Handshake Completion. . . . . . . . . . . . . . . . 67	8.1.5. Handshake Completion. . . . . . . . . . . . . . . . 66
8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . . 67	8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . . 66
8.3. Termination. . . . . . . . . . . . . . . . . . . . . . . 68	8.3. Termination. . . . . . . . . . . . . . . . . . . . . . . 67
8.3.1. Abnormal Termination. . . . . . . . . . . . . . . . 70	8.3.1. Abnormal Termination. . . . . . . . . . . . . . . . 69
8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . . 70	8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . . 69
8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 71	8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . . 70
9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 75	9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 74
9.1. Header Checksum Field. . . . . . . . . . . . . . . . . . 76	9.1. Header Checksum Field. . . . . . . . . . . . . . . . . . 75
9.2. Header Checksum Coverage Field . . . . . . . . . . . . . 77	9.2. Header Checksum Coverage Field . . . . . . . . . . . . . 76
9.2.1. Minimum Checksum Coverage Feature . . . . . . . . . 78	9.2.1. Minimum Checksum Coverage Feature . . . . . . . . . 77
9.3. Data Checksum Option . . . . . . . . . . . . . . . . . . 78	9.3. Data Checksum Option . . . . . . . . . . . . . . . . . . 77
9.3.1. Check Data Checksum Feature . . . . . . . . . . . . 79	9.3.1. Check Data Checksum Feature . . . . . . . . . . . . 78
9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . . 79	9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . . 78
10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 80	10. Congestion Control . . . . . . . . . . . . . . . . . . . . . 79
10.1. TCP-like Congestion Control . . . . . . . . . . . . . . 81	10.1. TCP-like Congestion Control . . . . . . . . . . . . . . 80
10.2. TFRC Congestion Control . . . . . . . . . . . . . . . . 81	10.2. TFRC Congestion Control . . . . . . . . . . . . . . . . 80
10.3. CCID-Specific Options, Features, and Reset
Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81	Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
10.4. CCID Profile Requirements . . . . . . . . . . . . . . . 84	10.4. CCID Profile Requirements . . . . . . . . . . . . . . . 83
10.5. Congestion State. . . . . . . . . . . . . . . . . . . . 84	10.5. Congestion State. . . . . . . . . . . . . . . . . . . . 83
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 85	11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 84
11.1. Acks of Acks and Unidirectional Connections . . . . . . 86	11.1. Acks of Acks and Unidirectional Connections . . . . . . 84
11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . . 87	11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . . 86
	11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . . 86
	11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . . 88
	11.4.1. Ack Vector Consistency . . . . . . . . . . . . . . 90
Kohler/Handley/Floyd [Page 6]	11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . . 92

INTERNET-DRAFT Expires: 7 September 2005 March 2005	11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . . 92
	11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . . 93
	11.7. Data Dropped Option . . . . . . . . . . . . . . . . . . 94
11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . . 87
11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . . 89
11.4.1. Ack Vector Consistency . . . . . . . . . . . . . . 91
11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . . 93
11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . . 94
11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . . 94
11.7. Data Dropped Option . . . . . . . . . . . . . . . . . . 95
11.7.1. Data Dropped and Normal Congestion
Response . . . . . . . . . . . . . . . . . . . . . . . . . 98	Response . . . . . . . . . . . . . . . . . . . . . . . . . 97
11.7.2. Particular Drop Codes. . . . . . . . . . . . . . . 98	11.7.2. Particular Drop Codes. . . . . . . . . . . . . . . 97
12. Explicit Congestion Notification . . . . . . . . . . . . . . 99	12. Explicit Congestion Notification . . . . . . . . . . . . . . 98
12.1. ECN Incapable Feature . . . . . . . . . . . . . . . . . 100	12.1. ECN Incapable Feature . . . . . . . . . . . . . . . . . 98
12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 100	12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . . 99
12.3. Aggression Penalties. . . . . . . . . . . . . . . . . . 101	12.3. Aggression Penalties. . . . . . . . . . . . . . . . . . 100
13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 102	13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 101
13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 102	13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 101
13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 103	13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 102
13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 104	13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 103
14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 105	14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 104
14.1. Measuring PMTU. . . . . . . . . . . . . . . . . . . . . 105	14.1. Measuring PMTU. . . . . . . . . . . . . . . . . . . . . 104
14.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . 107	14.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . 106
15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 108	15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 106
16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 108	16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 107
17. Relations to Other Specifications. . . . . . . . . . . . . . 110	17. Relations to Other Specifications. . . . . . . . . . . . . . 108
17.1. RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 110	17.1. RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 108
17.2. Congestion Manager and Multiplexing . . . . . . . . . . 111	17.2. Congestion Manager and Multiplexing . . . . . . . . . . 110
18. Security Considerations. . . . . . . . . . . . . . . . . . . 111	18. Security Considerations. . . . . . . . . . . . . . . . . . . 110
18.1. Security Considerations for Partial
Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 112	Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 111
19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 113	19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 112
19.1. Packet Types. . . . . . . . . . . . . . . . . . . . . . 113	19.1. Packet Types. . . . . . . . . . . . . . . . . . . . . . 112
19.2. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 113	19.2. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 112
19.3. Option Types. . . . . . . . . . . . . . . . . . . . . . 114	19.3. Option Types. . . . . . . . . . . . . . . . . . . . . . 112
19.4. Feature Numbers . . . . . . . . . . . . . . . . . . . . 114	19.4. Feature Numbers . . . . . . . . . . . . . . . . . . . . 113
19.5. Congestion Control Identifiers. . . . . . . . . . . . . 114	19.5. Congestion Control Identifiers. . . . . . . . . . . . . 113
19.6. Ack Vector States . . . . . . . . . . . . . . . . . . . 115	19.6. Ack Vector States . . . . . . . . . . . . . . . . . . . 113
19.7. Drop Codes. . . . . . . . . . . . . . . . . . . . . . . 115	19.7. Drop Codes. . . . . . . . . . . . . . . . . . . . . . . 113
19.8. Service Codes . . . . . . . . . . . . . . . . . . . . . 115	19.8. Service Codes . . . . . . . . . . . . . . . . . . . . . 114
20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 116	20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 116	A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 115
A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 118	A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 117
A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 118	A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 117
A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 119	A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 118
A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 120	A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 119
A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 121	A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 119
A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 122	A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 121
B. Appendix: Partial Checksumming Design Motivation. . . . . . . 123	B. Appendix: Partial Checksumming Design Motivation. . . . . . . 121
Normative References . . . . . . . . . . . . . . . . . . . . . . 124	Normative References . . . . . . . . . . . . . . . . . . . . . . 123
	Informative References . . . . . . . . . . . . . . . . . . . . . 124
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 125
	Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 126
Kohler/Handley/Floyd [Page 7]	Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 126