Subject: [Sip] a question about the tag in From or To header

From: "Wang, Jin-Gen" <[email-masked]>
Date: Fri, 17 Dec 2004 12:34:09 -0700

We have a debate on how to interpret the following paragraph (RFC 3261, section 19.3):

===========
19.3 Tags
   ...

   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.  A property of this selection
   requirement is that a UA will place a different tag into the From
   header of an INVITE than it would place into the To header of the
   response to the same INVITE.  This is needed in order for a UA to
   invite itself to a session, a common case for "hairpinning" of calls
   in PSTN gateways.  Similarly, two INVITEs for different calls will
   have different From tags, and two responses for different calls will
   have different To tags.

   ...
===========

Interpretation 1: A tag needs to be globally unique across space (aka UAs) but not across time because Call-ID (section 8.1.1.4) is already unique across space and time for calls. So a same UAC can use a same >From tag in different calls it imitates. And a same UAS can use a same To tag to respond to different calls. A same UA uses different tags in >From and To headers. Different UAs use different tags.

Interpretation 2: A tag needs to be different for each call. So a same UAC has to use two different tags in From header when it initiates two different calls. And a same UAS has to use different tags in To headers for different calls. 

We would like to hear from the list which way we should interpret the above paragraph or what is the right interpretation if both interpretations are not accurate.

Thanks.

Jin-Gen Wang
Level3 Communications, Inc.

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---

From: "Nataraju A B" <[email-masked]>
Date: Sat, 18 Dec 2004 14:35:25 +0530

Best Regards,
Nataraju A.B.
-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Wang, Jin-Gen
Sent: Saturday, December 18, 2004 1:04 AM
To: [email-masked]
Subject: [Sip] a question about the tag in From or To header

We have a debate on how to interpret the following paragraph (RFC 3261,
section 19.3):

===========
19.3 Tags
   ...

   ...
===========

Interpretation 1: A tag needs to be globally unique across space (aka
UAs) but not across time because Call-ID (section 8.1.1.4) is already
unique across space and time for calls. So a same UAC can use a same
>From tag in different calls it imitates. And a same UAS can use a same
To tag to respond to different calls. A same UA uses different tags in
>From and To headers. Different UAs use different tags.

[ABN] probably using same from tag for all the outgoing calls from this
entity is OK this eases to some extent for identifying the calling user.
But using same from tag  for each of the outgoing calls as well as same
to tag for answering all the incoming calls may lead serious security
issues...

Interpretation 2: A tag needs to be different for each call. So a same
UAC has to use two different tags in From header when it initiates two
different calls. And a same UAS has to use different tags in To headers
for different calls. 

[ABN] this is a better option....
In this case we may need to have the mapping between the tag and the
user who generated that particular tag. This might add some complexity
in identifying the end user for that call...

If security is not much of an issue then having the same tag for all the
generated (from tag) / answered (to tag) calls would be fine... in this
case the tag to user mapping would be simplified...

Finally it boil down to complexity vs security issue...

We would like to hear from the list which way we should interpret the
above paragraph or what is the right interpretation if both
interpretations are not accurate.

Thanks.

Jin-Gen Wang

---

From: Cullen Jennings <[email-masked]>
Date: Wed, 22 Dec 2004 07:57:16 -0800

It seems that 
 
   Similarly, two INVITEs for different calls will
   have different From tags, and two responses for different calls will
   have different To tags."

is pretty clear that you need to do Interpretation 2. Is there some problem
with this and reason why you want to do 1?

On 12/17/04 11:34 AM, "Wang, Jin-Gen" <[email-masked]> wrote:

> Interpretation 1: A tag needs to be globally unique across space (aka UAs) but
> not across time because Call-ID (section 8.1.1.4) is already unique across
> space and time for calls. So a same UAC can use a same >From tag in different
> calls it imitates. And a same UAS can use a same To tag to respond to
> different calls. A same UA uses different tags in >From and To headers.
> Different UAs use different tags.
> 
> Interpretation 2: A tag needs to be different for each call. So a same UAC has
> to use two different tags in From header when it initiates two different
> calls. And a same UAS has to use different tags in To headers for different
> calls. 
> 
> We would like to hear from the list which way we should interpret the above
> paragraph or what is the right interpretation if both interpretations are not
> accurate.
> 
> Thanks.
> 
> 
> Jin-Gen Wang

---

From: "Nataraju A B" <[email-masked]>
Date: Fri, 24 Dec 2004 10:03:35 +0530

Comments inline...

Best Regards,
Nataraju A.B.
Kodiak Networks Ind Pvt Ltd.,
Phone : +91 80 2222 7960 Xtn 555

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Cullen Jennings
Sent: Wednesday, December 22, 2004 9:27 PM
To: Wang, Jin-Gen; [email-masked]
Subject: Re: [Sip] a question about the tag in From or To header

It seems that 

is pretty clear that you need to do Interpretation 2. Is there some
problem
with this and reason why you want to do 1?

[ABN] the option 1 would be preferred to have simplicity in identifying
the user from the from-tag itself....(that is an implementation issue
only)

> We have a debate on how to interpret the following paragraph (RFC
3261,
UAs) but
> Interpretation 2: A tag needs to be different for each call. So a same
UAC has
> We would like to hear from the list which way we should interpret the
above