Subject: [Tsvwg] ECN and congestion avoidance

From: Will Stockdell <[email-masked]>
Date: Fri, 12 Apr 2002 15:55:28 -0400

I am preparing to implement ECN and in reading the RFC (3168) I have
come across something puzzling.  In section 6.1.2 it says:

"The indication of congestion should be treated just as a congestion
loss in  non-ECN-Capable TCP. That is, the TCP source halves the
congestion window "cwnd" and reduces the slow start threshold
"ssthresh"."

According to RFC 2581:
"Furthermore, upon a [retransmit] timeout cwnd MUST be set to no more
than the loss window, LW, which equals 1 full-sized segment
(regardless of the value of IW)."

So what am I missing?  Should the congestion window be halved or set
to one full-size segment when congestion is experienced?  Am I
misreading something?

Thanks,

Will

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From: Neil Spring <[email-masked]>
Date: Fri, 12 Apr 2002 18:44:28 -0700

> to one full-size segment when congestion is experienced?  Am I

Will,

Without looking up the context, I read those passages as:

1) ECN signals are equivalent to packet losses for
congestion: the window is halved in fast retransmit /
fast recovery because data is still flowing.

2) Timeout is handled normally, where the window is reduced
to 1 to reestablish ack pacing.

I think you're assuming 'timeout <==> congestion' when
'timeout -> congestion' (3-dup ack -> congestion too).
Or did I misread your confusion?  I assume the right way
for a Tahoe-TCP (which lacks fast retranmission) to respond
to ECE would be cwnd-halving.

hope it helps,
-neil