Subject: Congestion avoidance

From: John Leong <[email-masked]>
Date: Thu, 09 Sep 1999 17:37:08 -0700

I wonder if anyone on this list can verify if the following is true
......

Once we are in the congestion avoidance phase (where cwnd is being
increased linearly), it will essentialy stay in that state until
something worse happens .... packet loss as indentified by time out when
we return to the slow start phase ... which will still end up eventually
back in congestion avoidance phase when ssthresh is reached.

There are no other events (such as no packets being sent for a long
time) that will otherwise ends the congestion avoidance phase and let
the cwnd get back to increasing expoentially towards the advertised
window.

Regards,
John Leong

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From: Neal Cardwell <[email-masked]>
Date: Thu, 9 Sep 1999 22:44:46 -0700 (PDT)

I believe that RFC 2581 says that TCP is supposed to reduce cwnd to the
initial cwnd value ("IW") when the connection has been idle for a while
(more than one RTO). Thus idleness is one thing that could end congestion
avoidance and begin a slow start phase. From RFC 2581, section 4.1:

  For the purposes of this standard, we define RW = IW.
  ...
  a TCP SHOULD set cwnd to no more than RW before beginning
  transmission if the TCP has not sent data in an interval exceeding
  the retransmission timeout.

cheers,
neal

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From: John Leong <[email-masked]>
Date: Fri, 10 Sep 1999 12:29:00 -0700

> avoidance and begin a slow start phase. From RFC 2581, section 4.1:

That is somewhat orthogonal to the puzzle I have in mind  for which I
must apologize of being misleading with the bad example (idleness) I
used. ...

>>  the cwnd get back to increasing expoentially towards the advertised

With RFC 2581, after idleness, it will reenter slow start mode after
resetting cwnd back to IW ... i.e. treat it as a fresh start.

Waht I was wondering was whether there is any event that would cause
cwnd to switch back from linear increase to exponential increase (by 1
segment per ACK) without first resetting cwnd ... as in declaring
congestion is over and back to normal.

Regards,
John Leong