RE: Pseudorandom Flow Labels

"Hemant Singh (shemant)" <shemant@cisco.com> Wed, 06 April 2011 01:33 UTC

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Subject: RE: Pseudorandom Flow Labels
Date: Tue, 5 Apr 2011 20:34:53 -0500
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Thread-Topic: Pseudorandom Flow Labels
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From: "Hemant Singh (shemant)" <shemant@cisco.com>
To: "Thomas Narten" <narten@us.ibm.com>, "james woodyatt" <jhw@apple.com>
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-----Original Message-----
From: ipv6-bounces@ietf.org [mailto:ipv6-bounces@ietf.org] On Behalf Of
Thomas Narten
Sent: Tuesday, April 05, 2011 8:08 PM
To: james woodyatt
Cc: 6MAN Working Group
Subject: Re: Pseudorandom Flow Labels


>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.

>That is why incrementing a counter for each new flow provides adequate
>properties. I remain unconvinced that we need to require that they
>exhibit pseudo randomness, or even be uniform.

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

How about something like the text below:

A property of the flow label is that when the flow label is used a key
to a hash function the function produces a uniform distribution of hash
values.  Thus, incrementing a counter for each new flow is one means to
achieve the property described in the previous sentence. 

Hemant