[tcpm] I-D Action:draft-ietf-tcpm-ecnsyn-07.txt

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Subject: [tcpm] I-D Action:draft-ietf-tcpm-ecnsyn-07.txt
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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-07.txt
	Pages           : 33
	Date            : 2008-11-03

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 the initial TCP SYN/ACK packet 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 TCP responder (the sender of the SYN/ACK
packet) must reply to a report of an ECN-marked SYN/ACK packet by
resending a SYN/ACK packet that is not ECN-Capable.  If the resent
SYN/ACK packet is acknowledged, then the TCP responder reduces its
initial congestion window from two, three, or four segments to one
segment, thereby reducing the subsequent load from that connection on
the network.  If instead the SYN/ACK packet is dropped, or for some
other reason the TCP responder does not receive an acknowledgement in
the specified time, the TCP responder follows TCP standards for a
dropped SYN/ACK packet (setting the retransmit timer).  This document
updates RFC 3168.

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