Subject: Re: Question about tcp hash function tcp_hashfn()

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 31 May 2006 03:51:24 -0600

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.