[e2e] A "Railway Model." Re: Codel and Wireless
Jon.Crowcroft at cl.cam.ac.uk
Sat Dec 7 08:28:46 PST 2013
you have to read the literature - the data center tcp variants that
avoid incast do so by passing l2 feedback (which is part of the
ethernet spec - you can send a control frame which indicates a new
inter-frame gap) through the IP layer and on to TCP sources (actually,
it doesn't HAVE to work that way- you can just have per-flow queues
in the soruce machine and apply the dynamic inter-frame gap/delay
differently to different queues....
so there's no layer 2 congestion +control+ - just queue feedback that
is from l2 and a mechanism to pace packets...
actually, you could do this in DCF in wifi too:)
In missive <52A34A06.6050304 at web.de>, Detlef Bosau typed:
>>Am 07.12.2013 17:04, schrieb Jon Crowcroft:
>>> you might want to read about how people use layer two congestion
>>> signaling from L2 (only) switches to give feedback to TCP which then
>>> uses a distributed scheduler to avoid the incast problem alluded to...
>>> yes, gasp, layer violation - but it works. so engineers like it
>>I don't mind layer violation.
>>However, a L3 resource problem is slightly different from L2 flow control.
>>Only to mention two issues:
>>1.: Passing L2 congestion information to L3 is likely to end up in
>>source quench or the like.
>>Which sources are throttled? To which degree? What happens to traffic
>>which is not associated to any flow?
>>2.: The very first problem met when you want to employ L2 flow control
>>for upper layers is Head of Line Blocking. To my understanding, this is
>>THE very reason why the use of L2 congestion control was silently
>>abandoned in the turn from Cerf's catenet to RFC 791
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