Subject: [Sip] RFC 3261 From and To header tags: how globally unique?

From: "JOHN J. BARTON" <[email-masked]>
Date: Tue, 03 May 2005 09:34:22 -0700

Hi. I am trying to get a better understanding of SIP design.

RFC 3261, 19.3 Tags says:
    When a tag is generated by a UA for insertion into a request or
    response, it MUST be globally unique and cryptographically random
    with at least 32 bits of randomness.

It also says:
    It serves as a general mechanism to identify a dialog,
    which is the combination of the Call-ID along with two tags, one from
    each participant in the dialog.

Which "globe" is meant here?

Two choices seem likely:
   1) All space and time.
   2) All SIP UAs in a call.
I have seen the second interpretation in SIP UA code, meaning a 32bit
random string is used for the tag.

When discussing Call-ID, RFC3261 calls out a globally unique identifier 
over space and time then points to RFC1750 for cryptographic randomness, 
but the degree of randomness is not specified.

Should both the Call-ID and tag be unique across Internet and time and 
further more contain 32 bits of randomness?

Why both requirements?  If we have a globally unique identifier for the 
call and for each party in it, what does randomness add?  If we are 
counting on randomness to minimize collision, what does uniqueness add?

Thanks!

-- 
John J. Barton              IBM Almaden Research Center
http://www.almaden.ibm.com/cs/people/bartonjj/index.htm

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