This experimental RFC suggest to extend ECN to SYN-ACK packets of TCP three-way handshake as well. The idea is to use initial window of 1 instead of 2, 3, or 4 for congested network. Note, ECN-capable transport (ECT) is not set on SYN packet, but on SYN-ACK packet. This is a precaution that (1) the responder of TCP connection may not be ECN-capable and (2) preventing packet drop in SYN may create a more serious SYN flood attack.

The reason to have ECN in SYN-ACK is to avoid packet drop to SYN-ACK packets. If that happens, by default, the packet is retransmitted after a 3-second timeout (default RTO). If ECN is used, the SYN-ACK packet will have the ECT codepoint if the SYN packet carries the CWR+ECE flag. Such SYN-ACK packet may be marked CE by a congested router. If that happens, the initiator will response with a ECE ACK but without proceed to the ESTABLISHED state. This ECN-echo will make the responder send another SYN-ACK with the same sequence numbers, same ECE flag, but with non-ECN codepoint. Only after the initiator received this SYN-ACK and send its ACK, it can proceed to ESTABLISHED state and send data.

Bibliographic data

   title = "Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK packets",
   author = "A. Kuzmanovic and A. Mondal and S. Floyd and K. Ramakrishnan",
   url = "",
   month = "Jun",
   year = "2009",