Subject: [SIP] Correct copying 'tag's between 'To' and 'From' ?...

From: "Piotr S. Kossowski" <[email-masked]>
Date: Sun, 07 Jan 2001 05:49:24 +0100

Dear All,

I have doubts with copying 'tag's in subsequent request.
According to 11.4 of the 2543bis-02:

"Regardless of whether the caller or callee is generating the new request,
the header fields in the request are set as follows. For the desired call 
leg, the To header field is set to the remote address, and  the From 
header field is set to the local address (both including any tags)."

and 6.43:

"A UAS or redirect server MUST add a tag parameter for all final 
responses for all transactions within a call leg."

let's consider:

(Caller - A)  (Callee - B)

1.   ----- INVITE --->    (To: hostB , From: HostA)	
2.   <----- 200 -----     (To: hostB;tag=123 , From: hostA)	

after while:

3.   <---- BYE ------  (To: hostA, From: hostB;tag=123)
4.   ----- 200 ------> (To: hostA;tag=abc , From: hostB;tag=123)

Is this correct ? I mean: 

Are tags added to responses not for INVITE ?

Is this To-From swap correct ?

Would something change if , instead of BYE, Callee sent INVITE 
(actually, re-INVITE) ?

If above is correct, I wonder what next (after re-INVITE in 3.
instead of BYE):

5.   ----- INVITE --->    (To: hostB;tag=123 , From: hostA;tag=abc)	
6.   <----- 200 -----     (To: hostB;tag=123 , From: hostA;tag=abc)

Should Callee reply with 4xx if 5. INVITE did NOT contain 'tag' in 'From'?

If you have any idea, comments are welcome...

Regards,
Piotr

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 8 Jan 2001 02:11:43 -0500

Comments below.

> -----Original Message-----
> From: Piotr S. Kossowski [mailto:[email-masked]]
> Sent: Saturday, January 06, 2001 11:49 PM
> To: SIP discussion list
> Subject: [SIP] Correct copying 'tag's between 'To' and 'From' ?...
>
>
> Dear All,
>
> "A UAS or redirect server MUST add a "tag" parameter for all final

Not if you can avoid it. This issue was discussed at IETF. It is easily
solved by mandating tags in the From field of the initial INVITE. If this is
done, the BYE in 3 already has tags in the To and From, and thus the
response in 4 would not need to add one.

Unfortunately, mandatory From tags are new in rfc2543bis. Thus, an
implementation may need to deal with an older UA that decides to add a tag
in the middle of a call, as above. The conclusion at IETF was not to mandate
that a UA have to handle the addition of a new remote tag mid-call. Rather,
the spec would recommend some kind of reasonable graceful behavior, as a
SHOULD or MAY.

> Is this To-From swap correct ?

Yes.

-Jonathan R.

---
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
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