Re: TCP hashing function

[andrew may <acmay@acmay.homeip.net>]

On Fri, Mar 29, 2002 at 12:08:18AM -0800, Neil Spring wrote:
> On Thu, Mar 28, 2002 at 08:11:27PM -0500, Marek Zawadzki wrote:
> > Hello,
> 
> >  static __inline__ int dcp_hashfn(__u32 laddr, __u16 lport,
> >                                   __u32 faddr, __u16 fport)
> >  {
> >          int h = ((laddr ^ lport) ^ (faddr ^ fport));
> >          h ^= h>>16;
> >          h ^= h>>8;
> >          /* make it always < size : */
> >          return h & (MY_HTABLE_SIZE - 1);   /* MY_HT... = 128 */
> >  }
> > 
> > Although I am treating it as a blackbox and it works fine for me, my
> > professor pointed the following about this function:
> 
> If you're a student, you should probably try to figure
> this out for yourself; it's the only way to learn.  

Add one to the homework on the list tally.

> > If both IP addresses have the same upper 16 bits (like 155.246.10.5 and
> > 155.246.120.30), then the 1st 4-way XOR will put 16 bits of zero in h.
> > Then "h ^= h>>16" will preserve the upper 16 bits as zero.  Then
> > "h ^= h>>8" will preserve the upper 24 bits!
> > [...]

Look at the size and see why it doesn't matter.