Subject: Tag Values.

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Tue, 7 Mar 2000 22:19:43 -0800

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 ? 

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?]

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).

Thanks,

Regards,

Robert.

---

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 13 Mar 2000 02:11:20 -0500

> 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.

> 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.

> 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.
-- 
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