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    SubjectRe: ECN: Clearing the air (fwd)
    On Wed, Jan 31, 2001 at 06:02:17PM +0000, Alan Cox wrote:
    > > No. ECN is essential to the continued stability of the Internet. Without
    > > probabilistic queuing (i.e. RED) and ECN the Internet will continue to have
    > > retransmit synchronization and once congested stay congested until people get
    > > frustrated and give it up for a little bit.

    > Arguably so. In theory a vindictive probabilistic queueing is sufficient
    > (do RED but then drop -every- frame from the same route as the packet chosen
    > from the queue)

    THAT actually sounds very similar to what some ATM switches are
    doing when congestion results in lost cells in an IP datagram. Some time
    ago, a buddy up at Sandia National Laboratories was explaining the
    problem with congestion and IP over ATM. Once the congestion level
    reachs a certain point, the probability of the ATM network dropping a
    single cell out of the many cells comprising a complete IP datagram
    exceeds unity. All the transmitted cells, however, are contributing
    to the congestion. The datagram eventually gets retried and adds
    even more to the congestion. Net result is that once you pass this
    congestion threshold, IP throughput completely collapses and retries
    keep it there until higher layers fail.

    The answer for some intelligent ATM switches is to recognize the
    higher layer IP traffic and, when dropping an ATM cell in the middle of
    an IP datagram, to drop ALL the cells in a datagram if any of the cells
    are going to be dropped. That way the remaining cells are not contributing
    to the congestion when the entire IP datagram is going to be retransmitted
    anyways. Purists MIGHT argue that this is a layering violation, but it
    would seem to be a good one. :-/

    You could call it vindictive, or maybe congestion with extreme
    prejudice... :-)

    Mike
    --
    Michael H. Warfield | (770) 985-6132 | mhw@WittsEnd.com
    (The Mad Wizard) | (678) 463-0932 | http://www.wittsend.com/mhw/
    NIC whois: MHW9 | An optimist believes we live in the best of all
    PGP Key: 0xDF1DD471 | possible worlds. A pessimist is sure of it!

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