This draft specifies a modification to RFC 3168 to allow TCP SYN/ACK
packets to be ECN-Capable. For TCP, RFC 3168 only specified 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 is specifying 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 an ECN
mark by reducing its initial congestion window from two, three, or
four segments to one segment, reducing the subsequent load from that
connection on the network.