Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets
RFC 5562

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Network Working Group                                      A. Kuzmanovic
Request for Comments: 5562                                     A. Mondal
Category: Experimental                           Northwestern University
                                                                S. Floyd
                                                                    ICSI
                                                         K. Ramakrishnan
                                                      AT&T Labs Research
                                                               June 2009

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

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

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Kuzmanovic, et al.            Experimental                      [Page 1]
RFC 5562                ECN and SYN/ACK Packets                June 2009

Abstract

   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 Explicit
   Congestion Notification (ECN) in TCP SYN/ACK packets.

   This document 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).

Kuzmanovic, et al.            Experimental                      [Page 2]
RFC 5562                ECN and SYN/ACK Packets                June 2009

Table of Contents

   1. Introduction ....................................................3
   2. Conventions and Terminology .....................................5
   3. Specification ...................................................6
       3.1. SYN/ACK Packets Dropped in the Network ....................7
       3.2. SYN/ACK Packets ECN-Marked in the Network .................8
       3.3. Management Interface .....................................10
   4. Discussion .....................................................11
       4.1. Flooding Attacks .........................................11
       4.2. The TCP SYN Packet .......................................11
       4.3. SYN/ACK Packets and Packet Size ..........................12
       4.4. Response to ECN-Marking of SYN/ACK Packets ...............12