Subject: [tcpm] I-D Action:draft-ietf-tcpm-ecnsyn-06.txt

From: [email-masked]
Date: Fri, 22 Aug 2008 14:30:01 -0700 (PDT)

A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the TCP Maintenance and Minor Extensions Working Group of the IETF.

	Title           : Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets
	Author(s)       : S. Floyd, I. Property
	Filename        : draft-ietf-tcpm-ecnsyn-06.txt
	Pages           : 29
	Date            : 2008-08-22

This draft specifies a modification to RFC 3168 to allow TCP SYN/ACK
packets to be ECN-Capable.  For TCP, RFC 3168 only specifies setting
an ECN-Capable codepoint on data packets, and not on SYN and SYN/ACK
packets.  However, because of the high cost to the TCP transfer of
having a SYN/ACK packet dropped, with the resulting retransmit
timeout, this document specifies the use of ECN for the SYN/ACK
packet itself, when sent in response to a SYN packet with the two ECN
flags set in the TCP header, indicating a willingness to use ECN.
Setting TCP SYN/ACK packets as ECN-Capable can be of great benefit to
the TCP connection, avoiding the severe penalty of a retransmit
timeout for a connection that has not yet started placing a load on
the network.  The sender of the SYN/ACK packet must respond to a
report of an ECN-marked SYN/ACK packet by reducing its initial
congestion window from two, three, or four segments to one segment,
thereby reducing the subsequent load from that connection on the
network.  This document updates RFC 3168.

A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-ietf-tcpm-ecnsyn-06.txt

Internet-Drafts are also available by anonymous FTP at:
ftp://ftp.ietf.org/internet-drafts/

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From: "Adam Langley" <[email-masked]>
Date: Fri, 22 Aug 2008 21:02:15 -0700

>        Filename        : draft-ietf-tcpm-ecnsyn-06.txt

"SYN packets that contain known or unknown IP options [MAF05] (Figure 1).)"

It appears to be figure 5, at least in the version that Google pulled up.

Cheers,

-- 
Adam Langley [email-masked] http://www.imperialviolet.org
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From: "Adam Langley" <[email-masked]>
Date: Sun, 24 Aug 2008 11:56:23 -0700

>        Filename        : draft-ietf-tcpm-ecnsyn-06.txt

This seems like a very worthwhile change for little cost!

Attached is a patch against net-2.6 (Linux development tree) to
implement this. I'd be very grateful if someone could review the
packet traces and verify the behaviour.

All traces are in pcap format. If another format would be more
helpful, please ask.

For the first two traces, the passive open host sends 8192 bytes to
the active side and closes the connection. The active side has a 100ms
delay on incoming frames.

ecn-synack-ecn.pcap shows the connection with the SYNACK marked.
ecn-synack-ce shows the behaviour when the SYNACK is CE marked (with
the cwnd reduced to 1 segment etc).

The third trace shows an example of not ECN marking retransmitted SYNACKs.

Cheers

---

From: Sally Floyd <[email-masked]>
Date: Mon, 25 Aug 2008 12:51:35 -0700

Adam -

>>       Title           : Adding Explicit Congestion Notification  
>> (ECN) Capability to TCP's SYN/ACK Packets

I don't know what version Google gave you, but in the
version on my web page at
   "http://www.icir.org/floyd/papers.html#2005",
and on the ACM CCR archives, it is indeed Figure 1...

- Sally
http://www.icir.org/floyd/

---

From: "Adam Langley" <[email-masked]>
Date: Mon, 25 Aug 2008 12:59:21 -0700

> and on the ACM CCR archives, it is indeed Figure 1...

It is indeed. A Google search for "Measuring the Evolution of
Transport Protocols in the Internet" kicks up
http://www.icir.org/floyd/papers/TCPevolution-May2004.pdf where it's
figure 5. Obviously that's a previous version of the paper.

Cheers

AGL