I am trying to make a list of proposals for using any of the
four remaining bits in the TCP Reserved field, just to understand
how to evaluate the scarcity of space. 


* Nonce Sum:

Robust ECN Signaling with Nonces, draft-ietf-tsvwg-tcp-nonce-00.txt.
 
This is the Nonce Sum needed for the ECN Nonce to be used with TCP.
 

* Xmit flag, Xmit-Echo flag:
 
The Eifel Algorithm for TCP, 
internet-drafts/draft-ietf-tsvwg-tcp-eifel-alg-00.txt.
 
Two TCP bits have been proposed to revolve the retransmission 
ambiguity, to mark retransmits and to mark ACKs for retransmitted
data.  It has also been suggested that this could be done with
just one bit, with the "Xmit-Echo" function using the Xmit flag
in a pure ACK packet.

This could also be done with timestamps instead.
 

* Nonce, Nonce Sum:
 
TCP Congestion Control with a Misbehaving Receiver, Stefan Savage et al.
http://www.cs.ucsd.edu/~savage/pubs.htm
   
The Nonce and Nonce Sum were proposed in this paper to protect the
TCP sender against misbehaving receivers using DUP ACK spoofing or
optimistic ACKing.  For ECN-capable TCP connections, the ECT(0) and
ECT(1) codepoints are used as the nonce instead, so the Nonce bit
would only be needed if it was necessary to provide this protection for 
cooperating receivers from non-ECN-Capable connections.


Last updated: June 7, 2001.