Re: [aqm] ECN Manifesto iD - now uploaded

Gorry Fairhurst <> Wed, 25 March 2015 18:29 UTC

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From: Gorry Fairhurst <>
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Subject: Re: [aqm] ECN Manifesto iD - now uploaded
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I agree that we can improve the language we use to describe ECN. In particular saying what we mean by congestion, as you note below. 

What I spoke about in the AQM meeting at the IETF in Dallas was the idea of trying to harmonise language - where possible - between the various AQM drafts, using the language we agreed in RFC 2309.bis as the starting point. Some people thought this was a good idea, and I was encouraged to try to align this in the draft you commented upon.

I also promised to look at the recommendations in the evaluation guidelines from this perspective,

Expect a new rev soon!


> On 24 Mar 2015, at 08:26, John Leslie <> wrote:
> Gorry Fairhurst <> wrote:
>   I have been reviewing an intermediate version. I doubt I'll find time
> this week for a review of the latest. Sorry about that...
>   Generally, I think the document could be very significantly improved
> by being more careful about the word "congestion".
>   In the IETF, we (long ago) observed buffer-overflow and started
> calling it "congestion". There are, of course, other meanings that
> "congestion" has historically indicated.
>   As we have introduced Active Queue Management, we have started to
> introduce "congestion" signals other than buffer overflow: thus we now
> have at least two meanings attached to the word "congestion". This
> document, as of 18 March, continues to spread the confusion between
> the two (and perhaps other meanings as well).
>   In particular, I stumbled badly over the phrase "before there is
> significant congestion".
>   What do we actually mean to say here?
>   What I think we mean to say is that an AQM "congestion" signal can
> be received and acted upon before the buffer at that node actually
> overflows.
>   (Of course, this may or may not be true. Sometimes a node's buffer
> will go from empty to full in less than one RTT.)
>   Myself, I find it helpful to think in terms of "congestion" meaning
> that packets are arriving faster than they can be forwarded -- but I
> don't ask anyone else to agree with me.
>   I could suggest terms for the different meanings we want to talk
> about; but that doesn't seem helpful right now.
>   But I do think we should separate the "buffer overflow" idea from
> the "latency is rising" idea; and I think the document should actually
> mention the problem of feedback-that-can-be-acted-upon lagging the
> actual status of the forwarding node in question. I could supply some
> text about that...
>   The overall point I think we want to make is that ECN gives an
> unambiguous signal of whatever-it-signals that can be acted upon in
> one RTT -- which is in many cases before there is danger of buffer
> overflow; and that acting faster upon this signal can significantly
> reduce the latency while packets sit in the buffer at that node.
> --
> John Leslie <>