[Codel] sawtooth

Simon Barber simon at superduper.net
Wed Mar 27 19:34:03 EDT 2013

I've been thinking about codel's behaviour with a single TCP stream. TCP 
in steady state exhibits a sawtooth cwin behaviour. cwind rises by 1 
each RTT until the queue drops a packet, then cwind halves, and repeat.

With codel the earliest a packet will be dropped is 100ms after the 
queue size grows to the point the sojourn is 5ms. At this point the cwin 
is halved. For small RTTs or with multiple flows this may ensure good 
utilisation, but for large RTTs and a single flow surely dropping this 
early could result in starvation?


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