Re: ECN big picture please

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 ?
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
>
>

-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net