Re: Pseudorandom Flow Labels

John Leslie <john@jlc.net> Wed, 06 April 2011 00:30 UTC

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Date: Tue, 5 Apr 2011 20:31:52 -0400
From: John Leslie <john@jlc.net>
To: Thomas Narten <narten@us.ibm.com>
Subject: Re: Pseudorandom Flow Labels
Message-ID: <20110406003152.GA24474@verdi>
References: <BD901061-96AC-4915-B7CE-2BC1F70861A5@castlepoint.net> <201104052036.p35KaoHV019253@cichlid.raleigh.ibm.com> <19204E85-5B6E-409C-B450-7E3AC5EF47FA@apple.com> <201104052148.p35LmM9g019765@cichlid.raleigh.ibm.com> <9ED6022F-6863-4267-A268-C73240098539@apple.com> <201104060008.p3608OlC022133@cichlid.raleigh.ibm.com>
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Thomas Narten <narten@us.ibm.com>; wrote:
> 
> What is *required* is that the hash function (or whatever function
> that is used) on the router maps the tuples in a *uniform* way across
> the range of possible outputs.
> 
> If you have 10 links, and all your Flow Labels are clustered around
> low ten values, but in an approximately uniform way, a simple modulo
> hash will get you the kind of distribution you need.
> 
> The range of values of the flow label itself does not need to be
> uniformly distributed.

   I'm strongly in favor of stating the requirement this way (though
perhaps more wordsmithing can make it even better).

   AFAICT, this is our actual concern -- that an Equal Cost Multipath
router can do a simple hash function to distribute flows across multiple
links without endangering in-order delivery of the individual flows
identified in the Flow Label.

   (I'd also like a statement to that effect, or whatever it is we
actually mean to accomplish.)

> If that is a requirement, I'd like to see the justification, not just
> hand waiving.

   Me too.

--
John Leslie <john@jlc.net>;