[OpenAFS-devel] Question about RX & congestion

[Rainer Toebbicke]

Still on speeding up RX:

What's a decent way to detect congestion, and what's the best way to 
handle it?

The current RX way of three nacks in a row is hardly adequate - UDP 
packets can and do get reordered!

Also, running at wire speed does not prevent the odd TCP packet sneak 
in and cause a drop at the uplink (as seen from the receiver) - here 
an optimistic retransmit (for example in the second half of the xmit 
window) without reducing window sizes or waiting for acks helps a lot.

But in a chronic congestion scenario (e.g. two streams ending up on 
the same receiver) this does not help: even the re-transmitted packet 
has a reasonable chance of getting dropped and the protocol is running 
  at retransmit-timeout speed again. (it's actually worse: 
retransmitting wildly, even if insured only once per packet, almost 
doubles the traffic -worst case- and increases the chances for drops). 
Here slowing down at the sender helps keeping the timeouts out of the 
game.

In the above scenario the sender actually knows a lot upon ack - how 
fast the receiver is digesting packets and what holes he's got. What 
would you conclude if you received an ACK looking like

----++++++++++++++++----++++--------+++++----++++++++++++ ?

Common practice suggests keeping sender & receive at the same pace is 
best, hence reducing the window would be the answer. But the price is 
high as the pace can change and the sender would know only late 
because of latency! And since memory is cheap, perhaps always filling 
big windows with lots (200? 500?) of packets assuming efficient 
queueing and dealing with drops is better, even under congestion?

(I loosely followed discussions on TCP window sizes for trans-atlantic 
fibres in the late 90s... is this summarized somewhere)?

-- 
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Rainer Toebbicke
European Laboratory for Particle Physics(CERN) - Geneva, Switzerland
Phone: +41 22 767 8985       Fax: +41 22 767 7155