Subject: Re: [Sip] Basic questions.

From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 08 Jan 2002 13:03:39 -0500

Old email, not sure if it was answered, but the answers might be of 
interest to some.

De Leo Walter wrote:

> Excuse me I am new in this of SIP and have some basic questions whose
> answers I couldnt find in the list digest. If this questions bother you, or
> you think it is not the propper list to post them, please let me know, and
> tell me where should I post them.
> Here the questions:
> 1) In SIPbis (04) the use of tags is mandatory for From headers, it was not
> the case of RFC 2543, what is it required for?. 

To ensure that requests in the reverse direction do not get the tag 
accidentally added in the response.

>(Call legs can be
> distinguished by means of To tags (all other headers being equal). It would
> have sense if URLs are removed (it was a Mr Rosemberg Proposal)from Call leg
> identification but it is not the case yet, is it?)

Since the selection of tag is a local implementation choice, you can 
always choose one uniquely enough to serve as a local dialog (aka call 
leg) identifier. However, other elements cannot count on this uniqueness 
property.

> 2) More basic, what is the minimum length of a tag?

There is none. Whatever is needed for ensure uniqueness.

> 3) I have seen in one of the examples in the draft (the 20.5 forking
> example) that a 200 response to a CANCEL didn't have a To tag. But the 487
> response to the INVITE being cancelled did include a To tag. Why is that. I
> understand that the response to the CANCEL doesn't have a To Tag because the
> call leg does not exist as the callee was not located. But why there is a To
> Tag in the 487, is a short lifed call-leg created between caller and proxy?

I believe that the implication of the new bis is that the CANCEL 
response would indeed have a tag now. We are trying to have a simple and 
uniform rule, which is that when you respond, you add a tag if there 
isn't one already there. This is method independent behavior and thus 
would apply to cancel. The to tag would presumably be the same as the to 
tag in the final response to the INVITE itself.

> 4) Please don't laugh at me but I couldn't find out what is being referenced
> when  the draft sends you to [Hx.x]:(

rfc2616 section x.x.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.            72 Eagle Rock Avenue
Chief Scientist                         First Floor
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[email-masked]                 FAX: (973) 952-5050
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