[tcpm] I-D Action:draft-ietf-tcpm-ecnsyn-10.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
	Filename        : draft-ietf-tcpm-ecnsyn-10.txt
	Pages           : 35
	Date            : 2009-05-25

The proposal in this document is experimental.  While it may be
deployed in the current Internet, it does not represent a consensus
that this is the best possible mechanism for the use of ECN in TCP
SYN/ACK packets.

This draft describes an optional, experimental modification to RFC
3168 to allow TCP SYN/ACK packets to be ECN-Capable.  For TCP, RFC
3168 specifies setting an ECN-Capable codepoint on data packets, but
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 retransmission timeout, this document describes 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 retransmission 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
retransmission timer).

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