Subject: [SIP] More on tags

From: "Vijay K. Gurbani" <[email-masked]>
Date: Mon, 26 Jun 2000 14:12:44 -0500

Hi:

A collegue asked this question of me, for which I have an answer (see 
below), but would like to validate it.

Question: Why does the RFCbis (June 9, 2000) mandate that the tag be 
"globally unique" (Section 6.23)?  Can't it be unique within the Call-ID 
space it occupies?  The Call-ID already is "a globally unique 
identifier".  Why have 2 globally unique identifiers to identify a 
single call leg?

One reason I can think of is that if multiple UA servers are contacted
for a call, this wording guarantees that each of the UA servers use an
algorithm to generate a globally unique tag; otherwise, if each of them
used, say, the time of day to generate the tag (and they happen to be
in the same time zone etc. etc.), there could be a potential to generate
the same tag at each UA server.

Is this accurate?  Any other reasons?

Thanks,

- vijay
-- 
Vijay K. Gurbani  [email-masked] [email-masked] [email-masked]
IN Architecture and Internet Software Group
Lucent Technologies/Bell Labs Innovations, 263 Shuman Blvd, Rm 1A-418
Naperville, Illinois 60566  Voice: +1 630 224 0216 Fax: +1 630 713 0184

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From: Henning Schulzrinne <[email-masked]>
Date: Fri, 30 Jun 2000 18:41:30 -0400

Generally, it seems that the cost of generating globally unique
identifiers is low, but the cost of not doing it potentially high. Thus,
this seems to be a safe choice.

The tag is used to distinguish several "versions" of the same To/From
identifier, so it doesn't have to chang when the Call-ID changes. There
doesn't seem to be any advantage of making it unique within a call-id,
since you'd have to be sure that all "versions" with the same call-id
choose a different identifier - at that point, you're likely back at a
globally unique identifier, based on (say) the MAC address, CPU
identifier and who knows what else.

Why is this a problem?

> Vijay Gurbani wrote: