Subject: [Tsvwg] Re: RFC 2960bis Issues

From: Ian Rytina <[email-masked]>
Date: Thu, 17 May 2001 15:07:40 +0200

Hi Lyndon,

Yes, it is good that you think we need a "staging post", and try and
summarise where we are at the moment.......I am having a problem
with all this......mainly due to the fact that I am not at all an expert on
the details of checksums.

To help answer questions b) and c) in the Sigtran architecture and
requirments context, is it possible to have a basic summary of exactly
what is wrong with Adler-32 ? I am trying to understand what sorts of
errors the Adler-32 detect or does not detect. Questions that run through
my mind include :-

- what is the length of message where Adler-32 becomes noticiably weaker
compared to say, Fletcher-16 ?

- similarly, what is the throughput of messages where Adler-32 becomes
noticably weaker ?

- what types of errors are we talking about when it is said that Adler-32 is
weaker ? Is it bursty or random ? To me this is a quite an important
distinction when it comes to signalling network design.

- how does this relate to our Sigtran requirements (RFC2719), i.e.
does the Adler-32 fail to meet the "Message Error" requirements in
section 4.2.2 ?

Cheers,

Ian

Lyndon Ong wrote:

<snip>

> We need to revisit these questions:
>
> a) was the original decision correct to use checksum rather than
> CRC?  current view seems to be yes. although ips would prefer CRC, they can
> live with checksum.
>
> b) is there a problem with the original Adler-32?  current view was yes,
> although Doug's email raises some questions.
> (We could, of course, solve the whole bis issue by deciding that Adler-32
> was fine)
>
> c) if not the Adler-32, then what would be better?  we have modified Adler
> and Fletcher under consideration.  It shouldn't be extremely hard to pick
> an alternative.
>
> Cheers,
>
> L. Ong
>
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From: "Randall R. Stewart" <[email-masked]>
Date: Thu, 17 May 2001 10:07:53 -0500

Ian:

Most of your queries are good.. and I would love
the answers to your questions by someone that is
an expert with this... 

However one I am not so sure of.. see below..

> does the Adler-32 fail to meet the "Message Error" requirements in

I have a basic problem with the above in that although SCTP was
developed by sigtran, the fact that it has become a more general
purpose transport may conflict here. For example if it does meet
RFC2719 requirements but in a more general case does NOT meet
requirements for a general purpose transport... we have a conflict...
(Not that this will occur, mind you :>). Also the IPS folks would
like to use SCTP, does this mean we need to go look at their
requirments?

I am not sure of the answer... IPS folks would say YES, we NEED
CRC-32C... 

I don't know the answer and I have the bad feeling that no
one (that is a checksum expert) is going to put forward a 
postion on which one to use... sigh..

R

-- 
Randall R. Stewart
[email-masked] or [email-masked]
815-342-5222 (cell) 815-477-2127 (work)

---

From: Ian Rytina <[email-masked]>
Date: Thu, 17 May 2001 23:26:01 +0200

> postion on which one to use... sigh..

Hi Randy,

I think we have to be pragmatic about this, since SCTP is just one
small part of the "Sigtran stack", and the error loss depends on the
network configuration, so it certainly makes it difficult to relate this
discussion to the requirements in RFC2719. But if anyone has any
practical thoughts on this issue, either related to Sigtran or SCTP as
a general transport, they would be useful to help us form an opinion of
what we should do next, and therefore whether we have rough concensus
to change the checksum in RFC2960.

/Ian.