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Re: [babel] ECMP routing

Juliusz Chroboczek <jch@irif.fr> Sun, 14 October 2018 22:06 UTC

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Date: Mon, 15 Oct 2018 00:05:57 +0200
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From: Juliusz Chroboczek <jch@irif.fr>
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Cc: "'STARK, BARBARA H'" <bs7652@att.com>, "'Acee Lindem (acee)'" <acee@cisco.com>, babel@ietf.org
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Subject: Re: [babel] ECMP routing
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>> - ECMP does not improve performance in all topologies; in fact, in many
>> topologies, it decreases performance;

> Can you explain why this is? If you are doing ECMP on a per-flow basis,
> rather than a per-packet basis, and given there is more than one flow,
> it should always improve performance.

ECMP only improves throughput, and in many networks you're not limited by
throughput -- you want to minimise latency, jitter, or packet loss.  And
I'm not sufficiently familiar with the literature to know whether the
effects of per-flow ECMP on metrics other than throughput have been
studied in detail.

I'm not saying ECMP cannot be done right, what I'm saying is that doing
ECMP right in the wide variety of networks that Babel has been designed to
handle with no human intervention is an open research problem, and
therefore one that it would be premature to standardise.

-- Juliusz

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