[e2e] A "Railway Model." Re: Codel and Wireless

Detlef Bosau detlef.bosau at web.de
Sat Dec 7 08:17:10 PST 2013

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

Detlef Bosau
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