From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Mon Jun 19 2006 - 11:10:34 ART
Great, Chris! :)
Okay now, I'll try to summarize all that things as short as possible ;)
1) FECN and BECN are usually set by SP equipment when interfaces get
congested:
<docCD>
FECNs are generated when data is sent out a congested interface; they
indicate to a DTE device that congestion was encountered. Traffic is marked
with BECN if the queue for the opposite direction is deep enough to trigger
FECNs at the current time.
</docCD>
There is some specific way in which FECNs/BECNs are set. Up to some queue
threshold only DE-marked frames are maked with ECN bits. After that every
frame
sent in/out of interfaces is marked.
2) DTE sends BECN bit in response to frames received with FECNs. That
only works if DTE has some data to send back. I dont know exactly how
long it will continue to set BECNs, after last FECN received.
3) DTE can "reflect" FECNs even it has no data to sent back, with special
test
frames. This is useful in scenarios utilizing unidirectional traffic flow.
4) It is the BECN bit that actually signals sender to drop down it's sending
rate.
FECN is used as aux signalling technique, to trigger some sort of "ECN echo"
reply.
-- Petr Lapukhov, CCIE #16379 petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-224-8987 Outside US: 775-826-4344
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