I am looking for a way to set the initial slow start threshold to an
appropreate value for the delay and bandwidth of the link.
Tests have shown that setting the initial value for ssthresh to the receiver's
advertised window depends on the receiver setting the correct window size
for the bandwidth and delay.
When the advertised window and thus ssthresh are too large, the congestion
window grows to the link capacity and loss occurs. This frequently results in
a retransmission timeout and ssthresh is halved. Following this quite a long time
is required to reach the full link bandwidth.
The "Ongoing TCP Research Related to Satellites" draft suggests:
"Hoe [Hoe96] proposes using the packet-pai algorithmr [Kes91] to determine a more
appropriate value for ssthresh."
Having consulted these documents it is still not clear to me exactly how
TCP should determine the bandwidth of the link. Could someone explain
the use of the packet-pair algorithm and its application to TCP more
clearly? Is there a document that explains this more clearly?
Would anyone be willing to share code or experience on this?
I am aware that this method for determining link bandwidth has some
drawbacks and is less than perfect, however expecting the receiving
TCP to set an appropreate window size on a per connection basis
is unrealistic. It seems to me that setting an arbitrarily large window
and relying on the sending TCP to determine an appropreate initial
ssthresh for the bandwidth and delay is the best alternative.
Thanks
Craig Buffinton
Senior Software Engineer
FTP Software
978-684-6653
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