[Spencer Dawkins:]
>I thought the problem with advertising windows that are "too large" is that
>(at least some) TCPs keep trying to probe unsuccessfully into the "too
>large" region.
This is normal sender congestion-control behavior after the slow-start
phase. Whether it's unsuccessful depends on the state of network congestion
of the moment - it varies. Again, my understanding is that this congestion
control was always intended to be the sender's job. If the receiver
advertizes a too-small window, it's inappropriately taking on some of that
function to itself.
>I'm not questioning Peter's math, but it sure sounds like his TCP doesn't
>probe the way I thought TCPs do.
>
Your conclusion is incorrect, but quite reasonable here: it's my fault for
missing a point in my own experiments. I looked again at the flow I
described over the 1.5Mbps/64Kbps ADSL link, and discovered that the
downstream ADSL modem's buffer queues up all the server's packets arriving
at > 1.5Mbps until the server reaches the advertized window (awnd) of 24KB.
After that, since the receiver is getting packets at the downstream link
rate (1000 packets/sec.), it sends ACKs back at just the rate required (500
ACKs/sec.) to clock out packets from the server at just 1000 packets/sec.
Therefore, the queue level ceases to increase at this point, and ACK
clocking takes over. The server is always at the edge of its window from
this point on... it's stuck at the awnd of 24KB, since TCP window =
min(awnd, cwnd). *This* is why there is no probing for bandwidth after that
point in time in my trace.
So it is true, as Phil Karn noted, that the extra packets are getting
stored at the bottleneck link queue until the transfer is over. But if, as
he points out, this is a "problem...with the eexisting TCP congestion
control mechanism", it seems rather kludgy to fix it by reducing the
advertized window - the receiver knows even less about the state of the
network than the sender.
= Peter
Peter G. Warren
Performance Analysis Group
Advanced Systems Laboratory
GTE Laboratories
40 Sylvan Rd., Waltham, MA 02254
(617) 466-4142
This archive was generated by hypermail 2b29 : Mon Feb 14 2000 - 16:14:38 EST