Re: [aqm] Updated draft-ietf-aqm-ecn-benefits - comments still welcome

John Leslie <> Thu, 19 March 2015 01:39 UTC

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Date: Wed, 18 Mar 2015 21:39:09 -0400
From: John Leslie <>
To: Dave Taht <>
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Cc: Gorry Fairhurst <>, "" <>
Subject: Re: [aqm] Updated draft-ietf-aqm-ecn-benefits - comments still welcome
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Dave Taht <> wrote:
> section 6 addition. (could use more verbiage)
> 6.3 "An AQM that is ECN aware MUST have overload protection.

   I fear I cannot discern what you mean this to say. :^(

> It is trivial for a malbehaved application/worm/bot to mark all
> its packets with ECN and thus gain priority over other traffic
> not ecn marked.

   This somewhat-paranoid claim rests on several assumptions that I
hope we will recommend against.

- the most obvious is an assumption that a tail-drop node will mark
  _instead_ of dropping ECN-capable packets. This is not actually
  possible, and I hope we will strongly deprecate it. Tail-drop should
  drop packets regardless of ECN bits.

- there is also an assumption that an ECN-capable transport can mark
  its packets as ECN-capable and then never reduce its sending rate.
  I suppose it could; but not-ECN-capable transports can also never
  reduce the sending rate. :^( And the not-ECN-capable transports
  could accomplish the same reduction in "lost" packets by FEC.

   I believe we are going to "suggest" a lower marking threshhold for
ECN-capable packets than the dropping threshhold for not-ECN-capable
packets at AQM-capable nodes. This should reduce the paranoia level,
I hope, since the ECN-capable flows will get congestion signals when
not-ECN-capable packets are _not_ being dropped.

   We should concentrate our efforts on providing useful signals:
that some transports might make poor use of these signals is beyond
our scope. 

> 6.4 Enabling ECN at the application layer requires access to the IP
>     header fields, which are usually abstracted out completely at the
>     tcp layer, and hard to access from udp with multiple non-portable
>     methods to do so.

   Yes, there are TCP stacks which are ECN-unfriendly; but there are
enough _today_ which are friendly to ECN.

>     ECN over UDP in new applications such as webrtc and Quic has
>     great potential for many other applications, however the same
>     care of design that went into ECN on TCP needs to go into
>     future UDP based protocols.

   I wouldn't disagree; but those issues are essentially-solved
problems today.

> Some other section that may end up here?
> ECN marking other sorts of flows (example routing packets) that have a
> higher priority than other flows on link-local packets may be of benefit
> with wider availability of aqm technologies that are ecn aware...

   I suppose there might be _some_ use for ECN on routing packets; but
I doubt this is desirable today. ECN is not-at-all about getting a
higher priority -- it's about getting congestion signals without
packet loss.

John Leslie <>