Re: BECN & FECN

It should probably be pointed out that FECN Reflection on the
receiving end and Adaptive Shaping on the sending end are configurable
(not default) behaviors, correct? Also, this is more of a written
detail than a lab detail, but I understand that FR switches can simply
send BECN towards the sending end via "Q.922 test frames" or it can
wait for return traffic from the receiving end towards the sending end
and "reach in" and assert the BECN bit. Having said all of that, FR
isn't exactly my strong suit (none of my clients ever use it), so this
is based on what I've read vs. what I've directly experienced. If you
have v3.0 of the Written Cert Guide, check out page 516 for a nice
summary.

On Sep 7, 2009, at 11:35 , Abraham, Tharak wrote:

> Simply put a FECN is used to describe the reciever that the frame it
> recieved encountered congestion on the way and BECN is used to
> notify the
> sender regarding congestion in the circuit.
>
> A FR switch need not always set the BECN,
> Eg: If a router recieves a FECN, it reflects the FECN by marking a
> BECN in
> the opposite direction towards the sender.
> The router recieving the BECN will reduce the rates therby.
>
> Foresight messages (control packets) also is a good know, used by
> Cisco FR
> switches
> The switch generates it in the opposite direction of the congestion
> on the
> VC, which helps the sender to slow down.
>
>
> Best Regards,
> Tharak Abraham Luke
>
> Sent from Dusseldorf, NW, Germany
>
>
> On Mon, Sep 7, 2009 at 3:17 PM, Molomo <letjedilakopa_at_gmail.com>
> wrote:
>
>> Experts,
>> I have read a number of books on these two topics and I'm yet to find
>> a uniform explanation. Below are my explanations correct me if I'm
>> wrong,
>>
>> BECN- FR switch sets this bit in a frame header as the destination
>> replies to the sender - to inform the sender of congestion.
>>
>> FECN - This bit is set when traffic is unidirectional (eg video
>> stream), but only this is addressed to destination to generate a
>> pseudo packet so that FR switch can set the BECN back to the sender.
>>
>> Am I correct?
>>
>> Thanks in advance.
>>
>> -Molomo
>>
>>
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Received on Mon Sep 07 2009 - 12:00:25 ART