Re: [ns] reworded ECN problem in NS

[Sally Floyd <floyd@aciri.org>]

>Now TCP code in NS is supposed 
>to reduce cwnd to 1/2 after receiving ECE Acks. My ECN flow stops after 
>receiving ECE Acks. Is it due to cwnd<1 or any other problem?.

When cwnd is 1, and the CE bit is set on the packet, then
the TCP is required to wait for a retransmit timer to expire when
it receives the ACK notifying it of this ECN.  TCP does not reduce
its congestion window to 1/2 in this case.  The retransmit timers,
and the exponential backoff of the retransmit timers, are a 
fundamental component of TCP congestion control in the highly
congested regime.

This is illustrated in Section 8 of the document 
"ECN Implementations in the NS Simulator"
available from "http://www.aciri.org/floyd/ecn.html", and specified
in the ECN RFC.

- Sally
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http://www.aciri.org/floyd/
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