Re: Hashing function for PSAMP

[Andy Bierman <abierman@cisco.com>]

At 05:37 AM 1/27/2005, Juergen Quittek wrote:
>Dear all,
>
>Currently, the packet selection document has IPSX mandatory for packet
>selection and CRC32 mandatory for packet digest.
>
>The problem I see with this recommendation is that IPSX is not suitable
>for IPv6.  It does not sound like a good idea to have it mandatory for
>IPv6 systems.
>
>Here are two alternatives:
>
>1. Make IPSX mandatory for IPv4 packet selection and BOB mandatory
>  for IPv6 packet selection.
>  Then, with BOB implemented anyway, we should then replace CRC32
>  with BOB for packet digest, because both perform similarly and
>  there is no good reason for forcing implementors to support also
>  a third hash function.
>
>2. Just make BOB mandatory for packet selection and packet digest.
>  This would simplify implementation, because only a single function
>  is required.  For packet digest this should be OK, see 1.
>  A disadvantage is that BOB is slower than IPSX by factor 7.
>  An advantage is, that BOB is free of IPR, while IPSX is protected
>  by a patent.
>
>Does anybody have a preferences for 1., 2., or the current choice?

I prefer (2) because we can't ignore IPv6 and we are selecting
a hash algorithm for interoperability, so we really only need
one of them.  (Consider the extra expense for HW-assisted implementations.)


>Thanks,
>
>   Juergen

Andy


>-- 
>Juergen Quittek        quittek@netlab.nec.de       Tel: +49 6221 90511-15
>NEC Europe Ltd.,       Network Laboratories        Fax: +49 6221 90511-55
>Kurfuersten-Anlage 36, 69115 Heidelberg, Germany   http://www.netlab.nec.de
>
>
>--On 24.01.2005 16:56 Uhr +0100 Saverio Niccolini wrote:
>
>>Dear all,
>>I would like to ask a simple question to the list.
>>As you have seen in the "Sampling and Filtering Techniques for IP Packet
>>Selection" draft we have tried to give suggestions on which is the best
>>hash function in order to do packet sampling.
>>
>>Hash Functions Suitable for Packet Selection
>>1. IPSX
>>(2. BOB)
>>
>>Hash Functions Suitable for Packet Digesting
>>1. CRC-32
>>(2. BOB)
>>
>>Thinking about it and doing some additional tests it turned out that:
>>1) IPSX can only accepts 16 bytes as input --> it is not useful for IPv6
>>packets.
>>Do we want to stay with IPSX that is 7 times faster than BOB but can not
>>be used with IPv6 packets? What is the list feeling about this?
>>
>>2) BOB is faster than CRC-32 (on software implementation) and achieves
>>as good collision probability as CRC-32.
>>Do we still want to recommend CRC-32 because we believe that hardware
>>implementation of CRC-32 are already out or we just would like to
>>promote BOB to recommended and CRC-32 as optional?
>>
>>I am asking this because we are going to submit a new version of the
>>draft and we would definitely like to fix this issue.
>>
>>Thanks in advance for your comments,
>>Saverio
>>
>>============================================================
>>Dr. Saverio Niccolini
>>Research Staff Member
>>Network Laboratories, NEC Europe Ltd.
>>Kurfuerstenanlage 36, D-69115 Heidelberg
>>Tel.     +49 (0)6221 90511-18
>>Fax:     +49 (0)6221 90511-55
>>e-mail:  saverio.niccolini@netlab.nec.de
>>============================================================
>>
>>
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>
>
>
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