[re-ECN] RE-ECN without ECN.

[Matt Mathis <[e-mail]>]

Here is a fun little tidbit for people to think about, while others
resolve some of the non-technical issues surrounding launching the
conex.

For all modern reliable protocols over non-ECN networks (where loss is
the primary congestion indicator), the amount of duplicate data is a
direct measure of the down stream congestion.    (e.g. If I observe
both a data packet and its retransmission, then the first transmission
was lost someplace downstream).    This is because for all modern
protocols, the sender almost never sends data that the receiver
already holds.

If you add the assumption that the network itself does not reorder
packets, then the number of out-of-order packets is a direct measure
of the upstream congestion.  (e.g. the first transmission was lost,
and I see the retransmission).

This is sufficient information to implement many of the proposed uses
for RE-ECN marking.

For example, the bottleneck for problematic flows can be moved
upstream so that they are less disruptive to other users, by
throttling the amount of duplicate data permitted through an ingress
router.  In the extreme case, when a particular flow exceeds its quota
for duplicate data, you drop the retransmissions which will
(eventually) cause it to run out of receive window and take a timeout.
 More polite would be to pass the retransmission, but then discard the
next in-order packet on the same flow, normally causing it to slow
down.  There are some details to be worked out...

The point being that for non-encrypted standard transports, many
RE-ECN like things can be implemented unilaterally, without global
deployment of complicated, subtitle and hard to verify new protocol
features.

 Thanks,
--MM--
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Matt Mathis      http://staff.psc.edu/mathis
Work:412.268.3319   Home/Cell:412.654.7529
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Evil is defined by mortals who think they know
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