[e2e] Does congestion control belong to The TransportLayer?

David G. Andersen dga at lcs.mit.edu
Mon Apr 26 08:24:30 PDT 2004


On Mon, Apr 26, 2004 at 10:58:59AM -0400, Stephen Suryaputra scribed:
> Ethernet uses hop-by-hop flow control (pause frames) for congestion control. This is
> for full duplex. For half-duplex, it sends carrier on the congested input port (arbitrates
> access to the medium, like you said).

  Ethernets can do this signalling in a reliable manner.  No
amount of congestion in the reverse direction will prevent the
carrier or pause frame from getting through.  Source quench,
on the other hand, is an unreliable congestion signal.  In
extreme congestion, the signal to go slower can get lost - 
think "positive feedback loop".  Source quench could conceivably
provide benefits similar to those of ECN, but the fundamental
"back off on drop" behavior provides a safety guarantee that
source quench never will.

  -Dave

-- 
work: dga at lcs.mit.edu                          me:  dga at pobox.com
      MIT Laboratory for Computer Science           http://www.angio.net/


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