Subject: [SIP] Tags again

From: Shailendra Bhatnagar <[email-masked]>
Date: Sat, 17 Jun 2000 00:55:11 -0400

My understanding is that if initial INVITE is from A to B, B will append
a tag to the To header of the final response ( assuming ofcourse that To
header does not initially have a tag).

Now, for the re-INVITE from B to A, A is not supposed to append a tag 
to the To header of the final response to this re-INVITE (although 
the To header of re-INVITE from B to A does may not have tag). 

My question is that if for some reason, a stateful proxy generates the
final response to this re-INVITE from B to A, should it append a tag to
the To header. 

Thanks,
Shail

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From: Jonathan Rosenberg <[email-masked]>
Date: Sun, 18 Jun 2000 00:34:19 -0400

> the To header of re-INVITE from B to A does may not have tag).

I think it is worth discussing whether or not A would/should append a
tag in the To header of this final response.

I'll note that there is no problem if the initial INVITE from A to B had
a tag in the From field.

The difficulty with not appending the tag in the response to re-INVITE
is a loss of idempotency. This will come and bite you when you have
devices that don't maintain call state (i.e., your proxy), which will
insert the tag since they don't know this is a re-INVITE. It will also
complicate recovery from crash/reboot cycles during a call. The
difficulty in appending the tag to the response is, I believe, purely
one of implementation. Most folks probably use some kind of key, hashed
on various message fields, to index call state. This key would basically
change as a result of this re-INVITE. Kind of ugly.

I have been gradually reaching the conclusion that idempotency is a
really important property, and thus I believe worth the implementation
pain.

Comments?

-Jonathan R.
-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

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From: Shail Bhatnagar <[email-masked]>
Date: Sun, 18 Jun 2000 23:22:14 -0400 (EDT)

Jonathan, Precisely because of the stateful proxy case, I asked the 
question. However, I think on an earlier thread it was suggested that
re-INVITE should not cause any tag to be appended ( I think even
the bis version says this). 
As far as the hash key is concerned, in my humble opinion, lookup
logic should not include tags. Tag comparison should be the state
machine code.

Thanks,
Shail