Subject: RE: [Sip] The logic behind the From tag?

From: "Rob Phillips" <[email-masked]>
Date: Tue, 14 Jun 2005 05:02:20 -0500

Actually, I'd say the From tag is there to determine the direction of a call and it's validity.

Consider if you had no From tag: you send out an initial INVITE with no >From and no To tag but with a unique call-ID.  No problem so far.  The callee receives the call and inserts it's To tag in the response.  Again, no problem.

Now the callee wants to put the call on hold.  OK, so it now swaps From and To to generate the request (so you can tell it's generated from the callee and not the caller)...but there's no From tag.  The call-ID is the same, but it looks like a new request: no From tag, no To tag.

The original caller gets this request and ... what is he supposed to do with it?  Is it a re-INVITE?  A separate call?  What if was a non-INVITE request?  You don't know how to handle it because you don't have the To/From tag combination to tell you what is going on.

- rob

--
Rob Phillips, Sr. Software Engineer                 Netrake Corporation
mailto: [email-masked]                           3000 Technology Drive
voice: (214) 291-1096                                         Suite 100
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---

From: Frank Derks <[email-masked]>
Date: Tue, 14 Jun 2005 13:19:45 +0200

Hi Rob,

The fact that the callee uses the same Call-ID is sufficient information 
to 
have the caller decide that this is a request on the same call. If you are
inferring that the Call-ID, although it is supposed to be unique, does not
provide this information, it is equally true that the From tag does not
provide this either. 

However, I do see a point. The fact that a From tag is present, does 
provide
an indication that this is a request on an exisisting dialog. It does, 
however,
raise the issue about the uniqueness of the To tag, the From tag and the 
Call-ID.

What you're saying is that the Call-ID cannot be trusted to be unique and 
that
another, again probably not so unique, parameter is added to determine 
whether 
or not a request belongs to an existing dialog.

Cheers,

Frank

"Rob Phillips" 
2005-06-14 12:02 PM

To
Frank Derks/HVS/BE/PHILIPS@PHILIPS
<[email-masked]>
cc

Subject
RE: [Sip] The logic behind the From tag?
Classification

Actually, I'd say the From tag is there to determine the direction of a 
call and it's validity.

Consider if you had no From tag: you send out an initial INVITE with no 
>From and no To tag but with a unique call-ID.  No problem so far.  The 
callee receives the call and inserts it's To tag in the response.  Again, 
no problem.

Now the callee wants to put the call on hold.  OK, so it now swaps From 
and To to generate the request (so you can tell it's generated from the 
callee and not the caller)...but there's no From tag.  The call-ID is the 
same, but it looks like a new request: no From tag, no To tag.

The original caller gets this request and ... what is he supposed to do 
with it?  Is it a re-INVITE?  A separate call?  What if was a non-INVITE 
request?  You don't know how to handle it because you don't have the 
To/From tag combination to tell you what is going on.

- rob