[re-ECN] Preferential Dropping

John Leslie <john@jlc.net> Mon, 17 May 2010 16:28 UTC

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Date: Mon, 17 May 2010 12:21:10 -0400
From: John Leslie <john@jlc.net>
To: McCann Peter-A001034 <pete.mccann@motorola.com>
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Subject: [re-ECN] Preferential Dropping
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McCann Peter-A001034 <pete.mccann@motorola.com> wrote:
> John Leslie wrote:
>> Backbone routers either do or don't support ECN marking in place of
>> dropping packets.
> It would still be desirable to implement the preferential dropping
> scheme on congested links in the backbone, no?

   This probably deserves some discussion; and I see no reason we
can't discuss it before ConEx is chartered.

   I'm not sure whether preferential dropping belongs in ConEx at all.
It seems like an ECN thing to me.

   The benefit of ECN marking instead of dropping is that a congestion
signal can trigger a slowing of sending rate more quickly if we don't
have to wait for a packet drop to be recognized.

   The downside is that we can't be sure that an ECN mark will actually
make it through and result in a backoff of sending rate. A misguided
operator might scrub ECN markings; an ECN-marked packet might encounter
_other_ congestion and be dropped; or the signal could fail to make it
back to the sender for a number of reasons.

   The issue is whether anything about ConEx markings could signal a
trustworthy assurance that ECN marking is more likely to result in
earlier backoff.

   This doesn't feel particularly in-charter to me (at least not in
the initial charter); but we have until Thursday at least to discuss
things that are _not_ in-charter.


John Leslie <john@jlc.net>