Subject: [SIP] Re: Tag Values.

From: S_S_Dinesh/India/[email-masked]
Date: Mon, 8 May 2000 14:10:21 +0530

"Fairlie-Cuninghame, Robert" wrote:
>
> I have a couple of questions regarding the tag value added to the {To}
> header.
>
> The SIP RFC says that the tag value should be a UUID and (from a previous
> post) that the UUID should/may be constructed using the standard format.
The
> RFC also says that "The "tag" value MUST be globally unique and
> cryptographically random with at least 32 bits of randomness."
>
> So I take it that the tag can either be a UUID or 32bit random number ?

They are not mutually exclusive. I believe that the timestamp in the
UUID is sufficiently fine grained that a UUID has 32 bits of randomness.

>
> It has puzzled me why many traces have "tag=xxxxxxx" where xxxxxx is a
> decimal number usually much less than 2^31. Why decimal and so small ?
> [Whether the field is hex or decimal or UUID shouldn't matter since a
> case-insensitive string comparison of the field is used, correct?]

32 bit number is not the same thing at all as 32 bits of randomness. 32
bits of randomness means that if you  look at the tags as a random
process, the entropy of the probability mass function is 32 bits. A
simplistic way to view this is that the tags are random numbers between
1 and 2**32, with the probability of picking any particular one being 1
in 2**32. Thus, 1 is a perfectly valid tag and just as likely to show up
as 2**32 -1.

>
> What is the current best practice for adding a tag when there is only one
> Via line present? I think the spec uses an implicit MAY for this case but
> has this been upgraded/clarified to a SHOULD or MUST ? That is, a UAS
> SHOULD/MUST add a tag to final responses if there is one Via header (and
of
> course MUST if there are multiple Via's).

I think rfc2543bis will make tag addition a MUST. Having to special case
this is a big pain, as it turns out. We had it as a MAY in the first
place, since we wanted to keep the direct client to client case simple.

-Jonathan R.
--
Jonathan D. Rosenberg                       200 Executive Drive
Chief Scientist                             Suite 120
dynamicsoft                                 West Orange, NJ 07052
[email-masked]                     FAX:   (732) 741-4778
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

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