Re: ECN big picture please

Thanks Carlos for great explenation. I appreciate it.

On 4 January 2011 16:06, Carlos G Mendioroz <tron_at_huapi.ba.ar> wrote:

> Take a look at RFC 3168.
> For the time being, ECN is seldom used AFAIK, because endpoints do not
> support or do not use it.
> Use of ECN is transport protocol dependent. TCP ussage is defined.
>
> ECN bits remain as 00 for non ECN capable endpoints. That way, a router
> knows that it has to drop the packet because forward congestion
> notification via CE (ECN bits 11) is not possible. Either of the other
> two codes (10 os 01) are used as ECT, ECN capable transport.
>
> ECN in IP is a forward congestion notification feature (like FECN in Frame
> or EFCI in ATM. As such, it needs cooperation from the receiving
> endpoint to somehow tell the sender to slow down. This is done via
> ECE bit in TCP header, which is in turn acknowledge by CWR bit (also TCP).
>
> So, back to your Q: if the router decides (because of WRED) to discard
> a packet, then it looks at it to see if it is ECN capable.
> If it is (ECN bits 01 or 10) then it remarks it as 11 (CE) and lets it go.
> If is not (ECN bits 00) then it drops it.
>
> Host2 on seeing CE will ack back with ECE to host1, which will then
> slow down. This is done by the TCP stack implemented by the OS (linux
> or w2k8 e.g.), no need for apache to get involved.
>
> HTH,
> -Carlos
>
> Jack Router @ 02/01/2011 12:48 dixit:
>
>> Hello,
>>
>> Can somone explain how ECN works. Assume following scenario:
>>
>> Host1------f0/0)R1(f0/1)------(f0/1)R2(f0/0)------(f0/0)R3(f0/1)-----Host2
>>
>> Host1 sends data to Host2. This data is cought by a following policy on
>> R2:
>>
>> policy-map PA
>> class CA
>> bandwidth percent 10
>> random-detect
>> random-detect ecn
>>
>> int f0/1
>> service-policy out PA
>>
>> I believe that interesting traffic is maked with ECN bits 11. But what
>> happens next, how host1 is notified to slow down ? And how hosts 1 will
>> know
>> that it has to slow down if it is a refular http server ?
>>
>>
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> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina

Blogs and organic groups at http://www.ccie.net
Received on Wed Jan 05 2011 - 09:12:07 ART