From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Brian F. G. Bidulock" <bidulock@openss7.org>
Subject: Re: Question about tcp hash function tcp_hashfn()
Date: Wed, 31 May 2006 03:51:24 -0600
Message-ID: <20060531035124.B3065@openss7.org>
References: <20060530235525.A30563@openss7.org> <20060531.001027.60486156.davem@davemloft.net> <20060531014540.A1319@openss7.org> <20060531.004953.91760903.davem@davemloft.net> <20060531020009.A1868@openss7.org> <20060531090301.GA26782@2ka.mipt.ru>
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Cc: David Miller <davem@davemloft.net>, draghuram@rocketmail.com,
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To: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
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Evgeniy,

On Wed, 31 May 2006, Evgeniy Polyakov wrote:
> 2. Compared Jenkins hash with XOR hash used in TCP socket selection code.
> http://tservice.net.ru/~s0mbre/blog/2006/05/14#2006_05_14

Two problems with the comparison:

  Port numbers can be collected into a 32 bit register in network
  byte order directly from the TCP packet without taking two 16 bit
  values and shifting and or'ing them.

  Worse: he folded the jenkins algorith result with

   h ^= h >> 16;
   h ^= h >> 8;

  Destroying the coverage of the function.

I, for one, am not suprised that artifacts appeared in the comparison
as a result of this destruction of the coverage of the hashing function.