Subject: Re: [Tsvwg] sctp error check, again

From: Randall Stewart at work <[email-masked]>
Date: Wed, 13 Jun 2001 16:13:54 -0500

Bob:

The one thing that Jonathan seems to most strongly point
out is NOT to use Adler-32... which it seems to me is
not obvious in your forward..

So Jonathan recommends:

1) Moving the check to the end.
2) Picking something DIFFERENT than Adler-32, even
   a 32-bit extension to the tcp sum (which I did test
   in my cpu tests and it does the best... but the down
   side is you are dealing with something completely unknown :-o)

Bottom line is that Jonathan does not vote for our choice
number 1... i.e. don't use adler-32 is plain in his post...

R

Robert Snively wrote:
> 
> Please consider the following e-mail, which correctly states that
> not only is a checksum structure sufficient, but also that a CRC
> is not any better in the real world, as long as both are developed
> with a 32-bit check value.
> 
> Bob
> 
>> Jonathan Stone wrote:
>> 
>> 
>> If there's anything i can recommend to the tsvwg, its to pick even a
>> 32-bit extension of the TCP checksum, rather than Adler32; and to
>> think seriously about moving the error-check bits to the end.  Not to
>> help hardware, but to make whatever error-check you use more resilient
>> against errors in packet-processing engines which (once an error does
>> hit) trash the remainder of the packet.
>> 

-- 
Randall R. Stewart
[email-masked] 815-342-5222 (cell phone)

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---

From: "Adams, Richard Anthony (Richard)" <[email-masked]>
Date: Thu, 14 Jun 2001 11:15:04 +0100

Lyndon,
you wrote:
	"Perhaps we could take another angle on it - what would be the worst
case situation?  Supposing we have some type of short message traffic, such
as signaling, that fits some worst case profile for the Adler-32 - do we
have a plausible example of when this might happen?"

	We're working on this one. But when will we have an answer... ? !!
Ultimately, to demonstrate that a problem does exist, I think that we need
measurements of the ratio of signalling messages containing errors
undetected by Adler to all signalling messages, in a representative sample
of networks. Of course, test networks will usually have very low error
rates, and to introduce artificial errors will open us to arguments that
these are NOT typical ! So, I have a fear that it will be some way down the
road before we know for sure. That is one of the reasons that I am tempted
(but so far I resist!) to go for the optional scheme - if Adler isn't OK
(for some associations and some applications in parts of a network), then
have a management-controlled use of an error scheme that is OK. Then the
"ends" of an association could be updated independently. This principle is
the one behind the "set spare bits to 0 on generation, and don't examine on
reception" statements in SS7 MTP. But it does complicate matters. 

With kind regards,
Richard A. Adams,
[email-masked], tel : +44 1666 83 2503
fax: + 44 1666 83 2712
Lucent Tech.,
Swindon Road,
Malmesbury,
Wiltshire SN16 9NA.