Hi folks,

Could someone please shoot a link or something to a full scale guide on some ECN stuff? the cco one didnt help much.







  • Wikipedia gives nice and short overview. For a
    full-scale (and pretty much scholar) discussion you could ready Sally
    Floyd's papers

    In a few words, ECN allows for explicit signaling of network congestion
    event (defined locally in every router on the path between two entities).
    The idea is that this signaling is useless itself, unless some transit
    system implement a queue management strategy that sets this bit. End hosts
    are supposed to react to the ECN bits (if the TCP/IP stack supports this)
    and respond with some action the triggers sending rate throttling. If you
    compare this to FR FECN/BECN signaling, the difference is that the end
    hosts are responsible for rate-limiting while transit routers are
    responsible for packet marking based on their local settings.


    ----- Original Message -----
    From: "Kami"
    Sent: Sat, June 20, 2009 15:53
    Subject:[CCIE R&S] ECN!

    Hi folks,

    Could someone please shoot a link or something to a full scale guide on
    some ECN
    stuff? the cco one didnt help much.



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    ----- End of original message --
  • Hi Petr,

    Thanks for this. Something that I can't understand is how ECN interprets the phenomenon of "congestion"...I guess it relies on WRED and WRED accordingly has been designed to do the same thing as traditional RED (avg Q depth) but how does it relate to the "actual interface congestion" as one says e.g. 98% of BW is used up thus far so I'd say it's congested therefore the action would be ECN feedback stuff etc.

    How would you define the congestion threshod? Cant find much configuration options for the ECN thing.

    Is it something highly configurable by itself?





  • Hi Kami,


    Here is a link to the Cisco doc.



    Basically it is an extension to WRED.  WRED drops packets based on the queue depth (once the interface becomes congested) that you can configure.  If you add ECN to your QoS config in the WRED section, the router will look at the packets (the 2 rightmost bits of the TOS byte - the ones that everyone says are reserved) to see if the ECN bit has been set by the end hosts.  If it has been set and the interface experiences congestion, the router will set the other bit to signal to the end host to slow it's send rate.  If the hosts do not set the ECN bit (OFF is the default for WinXP [co] - you can change this with a registry change), then the router will drop the packet (via the WRED mechanism).

    Bottom line; configure ECN with WRED (or WRED with ECN [:)].  This will either signal to the end host to slow the send rate (via ECN) or the router will pre-emptively drop the packet when the queue depth is met (via WRED).

    The cool thing is that ECN will help to slow TCP or * UDP * traffic, whereas WRED will only help to slow TCP traffic.

    I hope this helps you.

    -VirtualChris (no number)

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