Internet Engineering Task Force Sally Floyd
INTERNET-DRAFT ICIR
draft-ietf-dccp-ccid2-09.txt Eddie Kohler	draft-ietf-dccp-ccid2-08.txt Eddie Kohler
Expires: 7 September 2005 UCLA	Expires: 14 May 2005 UCLA
7 March 2005	14 November 2004


Profile for DCCP Congestion Control ID 2:
TCP-like Congestion Control


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

This document contains the profile for Congestion Control Identifier
2, TCP-like Congestion Control, in the Datagram Congestion Control
Protocol (DCCP). CCID 2 should be used by senders who would like to
take advantage of the available bandwidth in an environment with
rapidly changing conditions, and who are able to adapt to the abrupt
changes in the congestion window typical of TCP's Additive Increase
Multiplicative Decrease (AIMD) congestion control.










































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TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:

Changes from draft-ietf-dccp-ccid2-07.txt:

* Restrict the use of byte-counting to be at most as aggressive
as the current TCP (without byte-counting).

Changes from draft-ietf-dccp-ccid2-06.txt:

* Moved three citations to Informational.

* Added that "The sender SHOULD not attempt Ack Ratio
renegotiations more than once per round-trip time."

* Specified that ssthresh is never less than two, instead of one.

* Added references to RFC 2988 and RFC 2018.

* Specify that the congestion window is only increased for packets
that aren't ECN-marked.

Changes from draft-ietf-dccp-ccid2-05.txt:

* Changes to the discussion about how the sender infers that DCCP-
Ack packets are lost. The sender does not know for sure whether a
missing sequence number is for a dropped ACK packet or a dropped
data packet. Our changes include a new appendix on "The Costs of
Inferring Lost Ack Packets".

* Minor editing for clarity, including some reordering of sections.

* Added a section on response to idle and application-limited
periods.

* Clarifications on changing the Ack Ratio, based on feedback from
Nils-Erik Mattsson.

Changes from draft-ietf-dccp-ccid2-04.txt:

* Minor editing, as follows:
- Added a note that CCID2 implementations MAY check for apps that
are
gaming with regard to the packet size.
- Deleted a statement that the maximum packet size is 1500 bytes.
- Added that the receiver MAY know the round-trip time from its
role as
- Added a note that the initial cwnd is up to four packets.




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* Added Intellectual Property Notice.

Changes from draft-ietf-dccp-ccid2-03.txt:

* Disallow direct tracking of TCP standards.

Changes from draft-ietf-dccp-ccid2-02.txt:

* Added to the section on application requirements.

* Changed the default Ack Ratio to be two, as recommended for TCP.

* Added a paragraph about packet sizes.

Changes from draft-ietf-dccp-ccid2-01.txt:

* Added "Security Considerations" and "IANA Considerations"
sections.

* Refer explicitly to SACK-based TCP, and flesh out Section 3
("Congestion Control on Data Packets").

* When cwnd < ssthresh, increase cwnd by one per newly acknowledged
packet up to some limit, in line with TCP Appropriate Byte Counting.

* Refined definition of quiescence.

Changes from draft-ietf-dccp-ccid2-00.txt:

* Said that the Acknowledgement Number reports the largest sequence
number, not the most recent packet, for consistency with draft-ietf-
dccp-spec.

* Added notes about ECN nonces for acknowledgements, and about
dealing with piggybacked acknowledgements.
















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Table of Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Conventions and Notation. . . . . . . . . . . . . . . . . . . 6
3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Relationship with TCP. . . . . . . . . . . . . . . . . . 7
3.2. Example Half-Connection. . . . . . . . . . . . . . . . . 7
4. Connection Establishment. . . . . . . . . . . . . . . . . . . 9
5. Congestion Control on Data Packets. . . . . . . . . . . . . . 9
5.1. Response to Idle and Application-limited
Periods . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. Response to Data Dropped and Slow Receiver . . . . . . . 12
5.3. Packet Size. . . . . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 13	6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Congestion Control on Acknowledgements . . . . . . . . . 13
6.1.1. Detecting Lost and Marked
Acknowledgements . . . . . . . . . . . . . . . . . . . . . 13
6.1.2. Changing Ack Ratio. . . . . . . . . . . . . . . . . 14
6.2. Acknowledgements of Acknowledgements . . . . . . . . . . 15
6.2.1. Determining Quiescence. . . . . . . . . . . . . . . 15
7. Explicit Congestion Notification. . . . . . . . . . . . . . . 16	7. Explicit Congestion Notification. . . . . . . . . . . . . . . 15
8. Options and Features. . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 16
10.1. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 17	10.1. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 16
10.2. Option Types. . . . . . . . . . . . . . . . . . . . . . 17
10.3. Feature Numbers . . . . . . . . . . . . . . . . . . . . 17
11. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
A. Appendix: Derivation of Ack Ratio Decrease. . . . . . . . . . 18	A. Appendix: Derivation of Ack Ratio Decrease. . . . . . . . . . 17
B. Appendix: Cost of Loss Inference Mistakes to Ack
Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Normative References . . . . . . . . . . . . . . . . . . . . . . 20
Informative References . . . . . . . . . . . . . . . . . . . . . 21	Informative References . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 22	Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 21















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1. Introduction

This document contains the profile for Congestion Control Identifier
2, TCP-like Congestion Control, in the Datagram Congestion Control
Protocol (DCCP) [DCCP]. DCCP uses Congestion Control Identifiers,
or CCIDs, to specify the congestion control mechanism in use on a
half-connection.

The TCP-like Congestion Control CCID sends data using a close
variant of TCP's congestion control mechanisms, incorporating
selective acknowledgements (SACK) [RFC 2018, RFC 3517]. CCID 2 is	selective acknowledgements (SACK) [RFC 2018] [RFC 3517]. CCID 2 is
suitable for senders who can adapt to the abrupt changes in
congestion window typical of TCP's Additive Increase Multiplicative
Decrease (AIMD) congestion control, and particularly useful for
senders who would like to take advantage of the available bandwidth
in an environment with rapidly changing conditions. See Section 3
for more on application requirements.

2. Conventions and Notation

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.

A DCCP half-connection consists of the application data sent by one
endpoint and the corresponding acknowledgements sent by the other
endpoint. The terms "HC-Sender" and "HC-Receiver" denote the
endpoints sending application data and acknowledgements,
respectively. Since CCIDs apply at the level of half-connections,
we abbreviate HC-Sender to "sender" and HC-Receiver to "receiver" in
this document. See [DCCP] for more discussion.

For simplicity, we say that senders send DCCP-Data packets and
receivers send DCCP-Ack packets. Both of these categories are meant
to include DCCP-DataAck packets.

The phrases "ECN-marked" and "marked" refer to packets marked ECN
Congestion Experienced unless otherwise noted.

3. Usage

CCID 2, TCP-like Congestion Control, is appropriate for DCCP flows
that would like to receive as much bandwidth as possible over the
long term, consistent with the use of end-to-end congestion control,
and that can tolerate the large sending rate variations
characteristic of AIMD congestion control, including halving of the
congestion window in response to a congestion event.




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Applications that simply need to transfer as much data as possible
in as short a time as possible should use CCID 2. This contrasts
with CCID 3, TCP-Friendly Rate Control (TFRC) Congestion Control
[CCID 3 PROFILE], which is appropriate for flows that would prefer
to minimize abrupt changes in the sending rate. For example, CCID 2
is recommended over CCID 3 for streaming media applications that
buffer a considerable amount of data at the application receiver
before playback time, insulating the application somewhat from
abrupt changes in the sending rate. Such applications could easily
choose DCCP's CCID 2 over TCP itself, possibly adding some form of
selective reliability at the application layer. CCID 2 is also
recommended over CCID 3 for applications where halving the sending
rate in response to congestion is not likely to interfere with
application-level performance.

An additional advantage of CCID 2 is that its TCP-like congestion
control mechanisms are reasonably well-understood, with traffic
dynamics quite similar to those of TCP. While the network research
community is still learning about the dynamics of TCP after 15 years
of its being the dominant transport protocol in the Internet, some
applications might prefer the more well-known dynamics of TCP-like
congestion control over that of newer congestion control mechanisms,
which haven't yet met the test of widespread Internet deployment.

3.1. Relationship with TCP

The congestion control mechanisms described here closely follow
mechanisms standardized by the IETF for use in SACK-based TCP, and
we rely partially on existing TCP documentation, such as RFC 793,	we rely partially on existing TCP documentation, such as [RFC 793],
RFC 2581, RFC 3465, and RFC 3517. TCP congestion control continues	[RFC 2581], [RFC 3465], and [RFC 3517]. TCP congestion control
to evolve, but CCID 2 implementations SHOULD wait for explicit	continues to evolve, but CCID 2 implementations SHOULD wait for
updates to CCID 2 rather than track TCP's evolution directly. The	explicit updates to CCID 2 rather than track TCP's evolution
differences between CCID 2 and straight TCP include: CCID 2 applies	directly. The differences between CCID 2 and straight TCP include:
congestion control to acknowledgements, a mechanism not currently	CCID 2 applies congestion control to acknowledgements, a mechanism
standardized for use in TCP. DCCP is a datagram protocol, so	not currently standardized for use in TCP. DCCP is a datagram
several parameters whose units are specified in bytes in TCP, such	protocol, so several parameters whose units are specified in bytes
as the congestion window cwnd, have units of packets in DCCP.	in TCP, such as the congestion window cwnd, have units of packets in
Unreliability also leads to differences from TCP: DCCP never	DCCP. Unreliability also leads to differences from TCP: DCCP never
retransmits a packet, so congestion control mechanisms that
distinguish retransmissions from new packets have been redesigned
for the DCCP context.

3.2. Example Half-Connection

This example shows the typical progress of a half-connection using
CCID 2's TCP-like Congestion Control, not including connection
initiation and termination. The example is informative, not
normative.



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1. The sender sends DCCP-Data packets, where the number of packets
sent is governed by a congestion window, cwnd, as in TCP. Each
DCCP-Data packet uses a sequence number. The sender also sends
an Ack Ratio feature option specifying the number of data
packets to be covered by an Ack packet from the receiver; Ack
Ratio defaults to two. The DCCP header's CCVal field is set to
zero.

Assuming that the half-connection is Explicit Congestion
Notification (ECN) capable (the ECN Incapable feature is zero --
the default), each DCCP-Data packet is sent as ECN-Capable with
either the ECT(0) or the ECT(1) codepoint set, as described in
RFC 3540.	[RFC 3540].

2. The receiver sends a DCCP-Ack packet acknowledging the data
packets for every Ack Ratio data packets transmitted by the
sender. Each DCCP-Ack packet uses a sequence number and
contains an Ack Vector. The sequence number acknowledged in a
DCCP-Ack packet is that of the received packet with the highest
sequence number, rather than a TCP-like cumulative
acknowledgement.

The receiver returns the sum of received ECN Nonces via Ack
Vector options, allowing the sender to probabilistically verify
that the receiver is not misbehaving. DCCP-Ack packets from the
receiver are also sent as ECN-Capable, since the sender will
control the acknowledgement rate in a roughly TCP-friendly way
using the Ack Ratio feature. There is little need for the
receiver to verify the nonces of its DCCP-Ack packets, since the
sender cannot get significant benefit from misreporting the ack
mark rate.

3. The sender continues sending DCCP-Data packets as controlled by
the congestion window. Upon receiving DCCP-Ack packets, the
sender examines their Ack Vectors to learn about marked or
dropped data packets, and adjusts its congestion window
accordingly. Because this is unreliable transfer, the sender
does not retransmit dropped packets.

4. Because DCCP-Ack packets use sequence numbers, the sender has
some information about lost or marked DCCP-Ack packets. The
sender responds to lost or marked DCCP-Ack packets by modifying
the Ack Ratio sent to the receiver.

5. The sender acknowledges the receiver's acknowledgements at least
once per congestion window. If both half-connections are
active, the sender's acknowledgement of the receiver's
acknowledgements is included in the sender's acknowledgement of



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the receiver's data packets. If the reverse-path half-
connection is quiescent, the sender sends a DCCP-DataAck packet
that includes an Acknowledgement Number in the header.

6. The sender estimates round-trip times, either through keeping
track of acknowledgement round-trip times as TCP does or through
explicit Timestamp options, and calculates a TimeOut (TO) value
much as the RTO (Retransmit Timeout) is calculated in TCP. The
TO is used to determine when a new DCCP-Data packet can be
transmitted when the sender has been limited by the congestion
window and no feedback has been received from the receiver.

4. Connection Establishment

Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so
the sender MUST send a "Change R(Send Ack Vector, 1)" option to the
receiver as part of connection establishment. The sender SHOULD NOT
send data until it has received the corresponding "Confirm L(Send
Ack Vector, 1)" from the receiver, except possibly for data included
on the initial DCCP-Request packet.

5. Congestion Control on Data Packets

CCID 2's congestion control mechanisms are based on those for SACK-
based TCP [RFC 3517], since the Ack Vector provides all the
information that might be transmitted in SACK options.

A CCID 2 data sender maintains three integer parameters measured in
packets.

1. The congestion window "cwnd", which equals the maximum number of
data packets allowed in the network at any time. ("Data packet"
means any DCCP packet that contains user data: DCCP-Data, DCCP-
DataAck, and occasionally DCCP-Request and DCCP-Response.)

2. The slow-start threshold "ssthresh", which controls adjustments
to cwnd.

3. The pipe value "pipe", which is the sender's estimate of the
number of data packets outstanding in the network.

These parameters are manipulated, and their initial values
determined, according to SACK-based TCP's behavior, except that they
are measured in packets, not bytes. The rest of this section
provides more specific guidance.

The sender MAY send a data packet when pipe < cwnd, but MUST NOT
send a data packet when pipe >= cwnd. Every data packet sent



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increases pipe by 1.

The sender reduces pipe as it infers that data packets have left the
network, either by being received or by being dropped. In
particular:

1. Acked data packets. The sender reduces pipe by 1 for each data
packet newly-acknowledged as received (Ack Vector State 0 or
State 1) by some DCCP-Ack.

2. Dropped data packets. The sender reduces pipe by 1 for each
data packet it can infer as lost due to the DCCP equivalent of
TCP's "duplicate acknowledgements". This depends on the
NUMDUPACK parameter, the number of duplicate acknowledgements
needed to infer a loss. The NUMDUPACK parameter is set to
three, as is currently the case in TCP. A packet P is inferred
to be lost, rather than delayed, when at least NUMDUPACK packets
transmitted after P have been acknowledged as received (Ack
Vector State 0 or 1) by the receiver. Note that the
acknowledged packets following the hole may be DCCP-Acks or
other non-data packets.

3. Transmit timeouts. Finally, the sender needs transmit timeouts,
handled like TCP's retransmission timeouts, in case an entire
window of packets is lost. The sender estimates the round-trip
time at most once per window of data, and uses the TCP
algorithms for maintaining the average round-trip time, mean
deviation, and timeout value [RFC 2988]. (If more than one
measurement per round-trip time was used for these calculations,
then the weights of the averagers would have to be adjusted, so
that the average round-trip time is effectively derived from
measurements over multiple round-trip times.) Because DCCP does
not retransmit data, DCCP does not require TCP's recommended
minimum timeout of one second. The exponential backoff of the
timer is exactly as in TCP. When a transmit timeout occurs, the
sender sets pipe to zero. The adjustments to cwnd and ssthresh
are described below.

The sender MUST NOT decrement pipe more than once per data packet.
True duplicate acknowledgements, for example, MUST NOT affect pipe.
Furthermore, the sender MUST NOT decrement pipe for non-data
packets, such as DCCP-Acks, even though the Ack Vector will contain
information about them.

Congestion events cause CCID 2 to reduce its congestion window. A	Congestion events, namely one or more packets lost or marked from a
congestion event contains at least one lost or marked packet. As in	window of data, cause CCID 2 to reduce its congestion window. For
TCP, two losses or marks are considered to be part of a single	each congestion event, either indicated explicitly as an Ack Vector
congestion event when the second packet was sent before the loss or	State 1 (ECN-marked) acknowledgement or inferred via "duplicate
	acknowledgements", cwnd is halved, then ssthresh is set to the new
	cwnd. Cwnd is never reduced below one packet. After a timeout, the
	slow-start threshold is set to cwnd/2, then cwnd is set to one
Floyd/Kohler Section 5. [Page 10]	packet. When halved, cwnd and ssthresh have their values rounded

INTERNET-DRAFT Expires: 7 September 2005 March 2005	down, except that cwnd is never less than one and ssthresh is never
	less than two.

mark of the first packet was detected. As an approximation, a
sender can consider two losses or marks to be part of a single
congestion event when the packets were sent within one RTT estimate
of one another, using an RTT estimate current at the time the
packets were sent. For each congestion event, either indicated
explicitly as an Ack Vector State 1 (ECN-marked) acknowledgement or
inferred via "duplicate acknowledgements", cwnd is halved, then
ssthresh is set to the new cwnd. Cwnd is never reduced below one
packet. After a timeout, the slow-start threshold is set to cwnd/2,
then cwnd is set to one packet. When halved, cwnd and ssthresh have
their values rounded down, except that cwnd is never less than one
and ssthresh is never less than two.

When cwnd < ssthresh, meaning that the sender is in slow-start, the
congestion window is increased by one packet for every two newly
acknowledged data packets with Ack Vector State 0 (not ECN-marked),
up to a maximum of Ack Ratio/2 packets per acknowledgement. This is
a modified form of Appropriate Byte Counting [RFC 3465] that is
consistent with TCP's current standard (which does not include byte-
counting), but allows CCID 2 to increase as aggressively as TCP when
CCID-2's Ack Ratio is greater than the default value of two. When
cwnd >= ssthresh, the congestion window is increased by one packet
for every window of data acknowledged without lost or marked
packets. The cwnd parameter is initialized to at most four packets
for new connections, following the rules from RFC 3390; the ssthresh
parameter is initialized to an arbitrarily high value.

Senders MAY use a form of rate-based pacing when sending multiple
data packets liberated by a single ack packet, rather than sending
all liberated data packets in a single burst.

5.1. Response to Idle and Application-limited Periods

CCID 2 is designed to follow TCP's congestion control mechanisms to
the extent possible, but TCP does not have complete standardization
for its congestion control response to idle periods (when no data
packets are sent) or to application-limited periods (when the
sending rate is less than that allowed by cwnd). This section is a
brief guide to the standards for TCP in this area.

For idle periods, RFC 2581 recommends that the TCP sender SHOULD
slow-start after an idle period, where an idle period is defined as
a period exceeding the timeout interval. RFC 2861, currently
Experimental, suggests a slightly more moderate mechanism where the
congestion window is halved for every round-trip time that the
sender has remained idle.





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There are currently no standards governing TCP's use of the
congestion window during an application-limited period. In
particular, it is possible for TCP's congestion window to grow quite
large during a long uncongested period when the sender is
application-limited, sending at a low rate. RFC 2861 essentially
suggests that TCP's congestion window not be increased during
application-limited periods, when the congestion window is not being
fully utilized.

5.2. Response to Data Dropped and Slow Receiver

As described in [DCCP], the Data Dropped option lets an endpoint
declare that a packet was dropped at the end host before delivery to
the application -- for instance, because of corruption or receive
buffer overflow. CCID 2 senders respond to these options as
described in [DCCP], with the following further clarifications.

o Drop Code 2 ("receive buffer drop"). The congestion window
"cwnd" is reduced by one for each packet newly acknowledged as
Drop Code 2, except that it is never reduced below one.

o Exiting slow-start. The sender MUST exit slow start whenever it
receives a relevant Data Dropped or Slow Receiver option.

5.3. Packet Size

CCID 2 is optimized for applications that generally use a fixed
packet size, and that vary their sending rate in packets per second
in response to congestion. CCID 2 is not appropriate for
applications that require a fixed interval of time between packets,
and vary their packet size instead of their packet rate in response
to congestion. CCID 2 maintains a congestion window in packets, and
does not increase the congestion window in response to a decrease in
the packet size. However, some attention might be required for
applications using CCID 2 that vary their packet size not in
response to congestion, but in response to other application-level
requirements.

CCID 2 implementations MAY check for applications that appear to be
manipulating the packet size inappropriately. For example, an
application might send small packets for a while, building up a fast
rate, then switch to large packets to take advantage of the fast
rate. (Preliminary simulations indicate that applications may not
be able to increase their overall transfer rates this way, so it is
not clear this manipulation will occur in practice [V03].)






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6. Acknowledgements

CCID 2 acknowledgements are generally paced by the sender's data
packets. Each required acknowledgement MUST contain Ack Vector
options that declare exactly which packets arrived, and whether
those packets were ECN-marked. Acknowledgement data in the Ack
Vector options SHOULD generally cover the receiver's entire
Acknowledgement Window; see [DCCP] (Section 11.4.2).	Acknowledgement Window (Section 11.4.2 of [DCCP]).

CCID 2 senders use DCCP's Ack Ratio feature to influence the rate at
which DCCP-Ack packets are generated, thus controlling reverse-path
congestion. This differs from TCP, which presently has no
congestion control for pure acknowledgement traffic. CCID 2's
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. The default Ack Ratio is two, and CCID 2 with this
Ack Ratio behaves like TCP with delayed acks. [DCCP] (Section 11.3)	Ack Ratio behaves like TCP with delayed acks. Section 11.3 of
describes the Ack Ratio in more detail, including its relationship	[DCCP] describes the Ack Ratio in more detail, including its
to acknowledgement pacing and DCCP-DataAck packets. Section 6.1.1	relationship to acknowledgement pacing and DCCP-DataAck packets.
below describes the sender's detection of lost or marked	Section 6.1.1 below describes the sender's detection of lost or
acknowledgements, and Section 6.1.2 gives the sender's rules for	marked acknowledgements, and Section 6.1.2 gives the sender's rules
changing the Ack Ratio.	for changing the Ack Ratio.

6.1. Congestion Control on Acknowledgements

When Ack Ratio is R, the receiver sends one DCCP-Ack packet per R
data packets, more or less. Since the sender sends cwnd data
packets per round-trip time, the acknowledgement rate equals cwnd/R
DCCP-Acks per round-trip time. The sender keeps the acknowledgement
rate roughly TCP-friendly by monitoring the acknowledgement stream
for lost and marked DCCP-Ack packets, and modifying R accordingly.
For every RTT containing a DCCP-Ack congestion event (that is, a
lost or marked DCCP-Ack), the sender halves the acknowledgement rate
by doubling Ack Ratio; for every RTT containing no DCCP-Ack
congestion event, it additively increases the acknowledgement rate
through gradual decreases in Ack Ratio.

6.1.1. Detecting Lost and Marked Acknowledgements

All packets from the receiver contain sequence numbers, so the
sender can detect both losses and marks on the receiver's packets.
The sender infers receiver packet loss in the same way as it infers
losses of its data packets: a packet from the receiver is considered
lost after at least NUMDUPACK packets with greater sequence numbers
have been received.





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DCCP-Ack packets are generally small, so they might impose less load
on congested network links than DCCP-Data and DCCP-DataAck packets.
For this reason, Ack Ratio depends on losses and marks on the
receiver's non-data packets, not on aggregate losses and marks on
all of the receiver's packets. The non-data packet category
consists of those packet types that cannot carry application data:
DCCP-Ack, DCCP-Close, DCCP-CloseReq, DCCP-Reset, DCCP-Sync, and
DCCP-SyncAck. The sender can easily distinguish non-data marks from
other marks. This is harder for losses, though, since the sender
can't always know whether a lost packet carried data. Unless it has
better information, the sender SHOULD assume, for the purpose of Ack
Ratio calculation, that every lost packet was a non-data packet.
Better information is available via DCCP's NDP Count option, if
necessary. (Appendix B discusses the costs of mistaking data packet
loss for non-data packet loss.)

A receiver that implements its own acknowledgement congestion
control SHOULD NOT reduce its DCCP-Ack acknowledgement rate due to
losses or marks on its data packets.

6.1.2. Changing Ack Ratio

Ack Ratio always meets three constraints: (1) Ack Ratio is an
integer. (2) Ack Ratio does not exceed cwnd/2, rounded up, except
that Ack Ratio 2 is always acceptable. (3) Ack Ratio is two or more
for a congestion window of four or more packets.

The sender changes Ack Ratio within those constraints as follows.
For each congestion window of data with lost or marked DCCP-Ack
packets, Ack Ratio is doubled; and for each cwnd/(R^2 - R)
consecutive congestion windows of data with no lost or marked DCCP-
Ack packets, Ack Ratio is decreased by 1. (See Appendix A for the
derivation.) Changes in Ack Ratio are signalled through feature
negotiation; see [DCCP] (Section 11.3).	negotiation; see Section 11.3 of [DCCP].

For a constant congestion window, this gives an Ack sending rate
that is roughly TCP-friendly. Of course, cwnd usually varies over
time; the dynamics will be rather complex, but roughly TCP-friendly.
We recommend that the sender use the most recent value of cwnd when
determining whether to decrease Ack Ratio by 1.

The sender need not keep Ack Ratio completely up to date. For
instance, it MAY rate-limit Ack Ratio renegotiations to once every
four or five round-trip times, or to once every second or two. The
sender SHOULD NOT attempt to renegotiate the Ack Ratio more than
once per round-trip time. Additionally, it MAY bound Ack Ratio
below by two, or it MAY set Ack Ratio to one for half-connections
with persistent congestion windows of 1 or 2 packets.



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Putting it all together, the receiver always sends at least one
acknowledgement per window of data when cwnd = 1, and at least two
acknowledgements per window of data otherwise. Thus, the receiver
could be sending two ack packets per window of data even in the face
of very heavy congestion on the reverse path. We would note,
however, that if congestion is sufficiently heavy that all of the
ack packets are dropped, then the sender falls back on an
exponentially-backed-off timeout, as in TCP. Thus, if congestion is
sufficiently heavy on the reverse path, then the sender reduces its
sending rate on the forward path, which reduces the rate on the
reverse path as well.

6.2. Acknowledgements of Acknowledgements

An active sender DCCP A MUST occasionally acknowledge its peer DCCP
B's acknowledgements, so that DCCP B can free up Ack Vector state.
When both half-connections are active, A's acknowledgements of B's
acknowledgements are automatically contained in A's acknowledgements
of B's data. If the B-to-A half-connection is quiescent, however,
DCCP A must occasionally send acknowledgements proactively, such as
by sending a DCCP-DataAck packet that includes an Acknowledgement
Number in the header.

An active sender SHOULD acknowledge the receiver's acknowledgements
at least once per congestion window. Of course, the sender's
application might fall silent. This is no problem; when neither
side is sending data, a sender can wait arbitrarily long before
sending an ack.

6.2.1. Determining Quiescence

This section describes how a CCID 2 receiver determines that the	This section refers to quiescence in the DCCP sense (see Section
corresponding sender is not sending any data, and therefore has gone	11.1 of [DCCP]): How does a CCID 2 receiver determine that the
quiescent. See [DCCP] (Section 11.1) for general information on	corresponding sender is not sending any data?
quiescence.

Let T equal the greater of 0.2 seconds and two round-trip times.
(The receiver may know the round-trip time in its role as the sender
for the other half-connection. If it does not, it should use a
default RTT of 0.2 seconds, as described in [DCCP] (Section 3.4).)	default RTT of 0.2 seconds, as described in Section 3.4 of [DCCP].)
Once the sender acknowledges the receiver's Ack Vectors, and the
sender has not sent additional data for at least T seconds, the
receiver can infer that the sender is quiescent. More precisely,
the receiver infers that the sender has gone quiescent when at least
T seconds have passed without receiving any data from the sender,
and the sender has acknowledged receiver Ack Vectors covering all
data packets received at the receiver.




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7. Explicit Congestion Notification

CCID 2 supports Explicit Congestion Notification (ECN) [RFC 3168].
The sender will use the ECN Nonce for data packets, and the receiver
will echo those nonces in its Ack Vectors, as specified in [DCCP]	will echo those nonces in its Ack Vectors, as specified in Section
(Section 12.2). Information about marked packets is also returned	12.2 of [DCCP]. Information about marked packets is also returned
in the Ack Vector. Because the information in the Ack Vector is
reliably transferred, DCCP does not need the TCP flags of ECN-Echo
and Congestion Window Reduced.

For unmarked data packets, the receiver computes the ECN Nonce Echo
as in RFC 3540, and returns it as part of its Ack Vector options.	as in [RFC 3540], and returns it as part of its Ack Vector options.
The sender SHOULD check these ECN Nonce Echoes against the expected
values, thus protecting against the accidental or malicious
concealment of marked packets.

Because CCID 2 acknowledgements are congestion-controlled, ECN may
also be used for its acknowledgements. In this case we do not make
use of the ECN Nonce, because it would not be easy to provide
protection against the concealment of marked ack packets by the
sender, and because the sender does not have much motivation for
lying about the mark rate on acknowledgements.

8. Options and Features

DCCP's Ack Vector option, and its ECN Capable, Ack Ratio, and Send
Ack Vector features, are relevant for CCID 2.

9. Security Considerations

Security considerations for DCCP have been discussed in [DCCP], and
security considerations for TCP have been discussed in RFC 2581.	security considerations for TCP have been discussed in [RFC 2581].
	[RFC 2581] discusses ways that an attacker could impair the
RFC 2581 discusses ways that an attacker could impair the
performance of a TCP connection by dropping packets, or by forging
extra duplicate acknowledgements or acknowledgements for new data.
We are not aware of any new security considerations created by this
document in its use of TCP-like congestion control.

10. IANA Considerations

This specification defines the value 2 in the DCCP CCID namespace
managed by IANA. This assignment is also mentioned in [DCCP].

CCID 2 also introduces three sets of numbers whose values should be
allocated by IANA, namely CCID 2-specific Reset Codes, option types,	allocated by IANA. We refer to allocation policies, such as
and feature numbers. These ranges will prevent any future	Standards Action, outlined in [RFC 2434], and most registries
CCID 2-specific allocations from polluting DCCP's corresponding	reserve some values for experimental and testing use [RFC 3692].



Floyd/Kohler Section 10. [Page 16]

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global namespaces; see [DCCP] (Section 10.3). However, this
document makes no particular allocations from any range, except for
experimental and testing use [RFC 3692]. We refer to the Standards
Action policy outlined in RFC 2434.

10.1. Reset Codes

Each entry in the DCCP CCID 2 Reset Code registry contains a
CCID 2-specific Reset Code, which is a number in the range 128-255;
a short description of the Reset Code; and a reference to the RFC
defining the Reset Code. Reset Codes 184-190 and 248-254 are
permanently reserved for experimental and testing use. The
remaining Reset Codes -- 128-183, 191-247, and 255 -- are currently
reserved, and should be allocated with the Standards Action policy,	reserved, and should be allocated with the IETF Consensus policy,
which requires IESG review and approval and standards-track IETF RFC	which requires RFC publication (not necessarily standards-track).
publication.

10.2. Option Types

Each entry in the DCCP CCID 2 option type registry contains a
CCID 2-specific option type, which is a number in the range 128-255;
the name of the option; and a reference to the RFC defining the
option type. Option types 184-190 and 248-254 are permanently
reserved for experimental and testing use. The remaining option
types -- 128-183, 191-247, and 255 -- are currently reserved, and
should be allocated with the Standards Action policy, which requires	should be allocated with the IETF Consensus policy, which requires
IESG review and approval and standards-track IETF RFC publication.	RFC publication (not necessarily standards-track).

10.3. Feature Numbers

Each entry in the DCCP CCID 2 feature number registry contains a
CCID 2-specific feature number, which is a number in the range
128-255; the name of the feature; and a reference to the RFC
defining the feature number. Feature numbers 184-190 and 248-254
are permanently reserved for experimental and testing use. The
remaining feature numbers -- 128-183, 191-247, and 255 -- are
currently reserved, and should be allocated with the Standards	currently reserved, and should be allocated with the IETF Consensus
Action policy, which requires IESG review and approval and	policy, which requires RFC publication (not necessarily standards-
standards-track IETF RFC publication.	track).

11. Thanks

We thank Mark Handley and Jitendra Padhye for their help in defining
CCID 2. We also thank Mark Allman, Aaron Falk, Nils-Erik Mattsson,
Greg Minshall, Arun Venkataramani, Magnus Westerlund, and members of
the DCCP Working Group for feedback on this document.





Floyd/Kohler Section 11. [Page 17]

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A. Appendix: Derivation of Ack Ratio Decrease

This section justifies the algorithm for increasing and decreasing
the Ack Ratio given in Section 6.1.2.

The congestion avoidance phase of TCP halves the cwnd for every
window with congestion. Similarly, CCID 2 doubles Ack Ratio for
every window with congestion on the return path, roughly halving the
DCCP-Ack sending rate.

The congestion avoidance phase of TCP increases cwnd by one MSS for
every congestion-free window. Applying this congestion avoidance
behavior to acknowledgement traffic, this would correspond to
increasing the number of DCCP-Ack packets per window by one after
every congestion-free window of DCCP-Ack packets. We cannot achieve
this exactly using Ack Ratio, since it is an integer. Instead, we
must decrease Ack Ratio by one after K windows have been sent
without a congestion event on the reverse path, where K is chosen so
that the long-term number of DCCP-Ack packets per congestion window
is roughly TCP-friendly, following AIMD congestion control.

In CCID 2, rough TCP-friendliness for the ack traffic can be
accomplished by setting K to cwnd/(R^2 - R), where R is the current
Ack Ratio.

This result was calculated as follows:

R = Ack Ratio = # data packets / ack packets, and
W = Congestion Window = # data packets / window, so
W/R = # ack packets / window.

Requirement: Increase W/R by 1 per congestion-free window.
Since we can only reduce R by increments of one, we find K
so that, after K congestion-free windows,
W/R + K would equal W/(R-1).

(W/R) + K = W/(R-1), so
K = W/(R-1) - W/R = W/(R^2 - R).


B. Appendix: Cost of Loss Inference Mistakes to Ack Ratio

As discussed in Section 6.1.1, the sender often cannot determine
whether lost packets carried data. This hinders its ability to
separate non-data loss events from other loss events. In the
absence of better information, the sender assumes, for the purpose
of Ack Ratio calculation, that all lost packets were non-data
packets. This may overestimate the non-data loss event rate, which



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can lead to a too-high Ack Ratio, and thus a too-slow
acknowledgement rate. All acknowledgement information will still
get through -- DCCP acknowledgements are reliable -- but
acknowledgement information will arrive in a burstier fashion.
Absent some form of rate-based pacing, this could lead to increased
burstiness for the sender's data traffic.

There are several cases when the problem of an overly-high Ack
Ratio, and the resulting increased burstiness of the data traffic,
will not arise. In particular, call the receiver DCCP B and the
sender DCCP A. Then:

o The problem won't arise unless DCCP B is sending a significant
amount of data itself. When the B-to-A half-connection is
quiescent or low-rate, most packets sent by DCCP B will, in fact,
be pure acknowledgements, and DCCP A's estimate of the DCCP-Ack
loss rate will be reasonably accurate.

o The problem won't arise if DCCP B habitually piggybacks
acknowledgement information on its data packets. The piggybacked
acknowledgements are not limited by Ack Ratio, so they can arrive
frequently enough to prevent burstiness.

o The problem won't arise if DCCP A's sending rate is low, since
burstiness isn't a problem at low rates.

o The problem won't arise if DCCP B's sending rate is high relative
to DCCP A's sending rate, since the B-to-A loss rate must be low
to support DCCP B's sending rate. This bounds the Ack Ratio to
reasonable values even when DCCP A labels every loss as a DCCP-
Ack loss.

o The problem won't arise if DCCP B sends NDP Count options when
appropriate (the Send NDP Count/B feature is true). Then the
sender can use the receiver's NDP Count options to detect, in
most cases, whether lost packets were data packets or DCCP-Acks.

o Finally, the problem won't arise if DCCP A rate-paces its data
packets.

This leaves the case when DCCP B is sending roughly the same amount
of data packets and non-data packets, without NDP Count options, and
with all acknowledgement information in DCCP-Ack packets. We now
quantify the potential cost, in terms of a too-large Ack Ratio, due
to the sender's misclassifying data packet losses as DCCP-Ack
losses. For simplicity, we assume an environment of large-scale
statistical multiplexing, where the packet drop rate is independent
of the sending rate of any individual connection.



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Assume that when DCCP A correctly counts non-data losses, Ack Ratio
is set so that B-to-A data and acknowledgement traffic both have a
sending rate of D packets per second. Then when DCCP A incorrectly
counts data losses as non-data losses, the sending rate for the B-
to-A data traffic is still D pps, but the reduced sending rate for
the B-to-A acknowledgement traffic is f*D pps, with f < 1. Let the
packet loss rate be p. The sender incorrectly estimates the non-
data loss rate as (pD+pfD)/fD, or, equivalently, as p(1 + 1/f).
Because the congestion control mechanism for acknowledgement traffic
is roughly TCP-friendly, and therefore the non-data sending rate and
the data sending rate both grow as 1/sqrt(x) for x the packet drop
rate, we have
fD/D = sqrt(p)/sqrt(p(1 + 1/f)),
so
f^2 = 1/(1 + 1/f).
Solving, we get f = 0.62. If the sender incorrectly counts lost
data packets as non-data in this scenario, the acknowledgement rate
is decreased by a factor of 0.62. This would result in a moderate
increase in burstiness for the A-to-B data traffic, which could be
mitigated by sending NDP Count options or piggybacked
acknowledgements, or by rate-pacing out the data.

Normative References

[DCCP] E. Kohler, M. Handley, and S. Floyd. Datagram Congestion
Control Protocol, draft-ietf-dccp-spec-09.txt, work in progress,
November 2004.

[RFC 793] J. Postel, editor. Transmission Control Protocol.
RFC 793.

[RFC 2018] M. Mathis, J. Mahdavi, A. Floyd, and A. Romanow. TCP
Selective Acknowledgement Options, RFC 2018, October 1996.

[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 2581] M. Allman, V. Paxson, and W. Stevens. TCP Congestion
Control. RFC 2581.

[RFC 2988] V. Paxson and M. Allman, Computing TCP's Retransmission
Timer, RFC 2988, November 2000.

[RFC 3168] K.K. Ramakrishnan, S. Floyd, and D. Black. The Addition
of Explicit Congestion Notification (ECN) to IP. RFC 3168.



Floyd/Kohler [Page 20]

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[RFC 3390] M. Allman, S. Floyd, and C. Partridge. Increasing TCP's
Initial Window. RFC 3390.

[RFC 3517] E. Blanton, M. Allman, K. Fall, and L. Wang. A
Conservative Selective Acknowledgment (SACK)-based Loss Recovery
Algorithm for TCP. RFC 3517.

[RFC 3692] T. Narten. Assigning Experimental and Testing Numbers
Considered Useful. RFC 3692.

Informative References

[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.

[RFC 2861] M. Handley, J. Padhye, and S. Floyd. TCP Congestion
Window Validation. RFC 2861.

[RFC 3465] M. Allman. TCP Congestion Control with Appropriate Byte
Counting (ABC). RFC 3465.

[RFC 3540] N. Spring, D. Wetherall, and D. Ely. Robust Explicit
Congestion Notification (ECN) Signaling with Nonces. RFC 3540.

[V03] Arun Venkataramani, August 2003. Citation for acknowledgement
purposes only.

Authors' Addresses

Sally Floyd <floyd@icir.org>
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA

Eddie Kohler <kohler@cs.ucla.edu>
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
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.




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This document and the information contained herein are provided on
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