Subject: [Sip] Question about SIP bis04

From: "Chen, Xin (Xin)" <[email-masked]>
Date: Thu, 16 Aug 2001 14:57:40 +0100

Could someone please explain to me:

	15.4 If a UAS never receives an ACK for a 2xx response to a
re-INVITE, it SHOULD generate a re-INVITE
	itself in order to synchronize the state of the session.

What does this description mean? If UAS needs a ACK, why it has to generate
a re-INVITE?

Thanks and Regards

Xin Chen

Lucent Technologies
Tel: +44 1793 883137
Mobile: +44 7799 034668

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---

From: "Bert Culpepper" <[email-masked]>
Date: Thu, 16 Aug 2001 15:06:16 -0400

Well, one use of a re-INVITE is to change the media characteristics of
an existing session.  So if a UAS never gets acknowledgement of its
response (times out waiting for the ACK), it can (SHOULD) assume the
transaction failed and continue with the media that existed prior to the
re-INVITE.  But, the other end has already changed presumably to match
its re-INVITE and it may have even received the 200 from the UAS in
which case it did change its media, leading to a "failed call" situation
(UAC using new media and UAS using old media).  The text you ask about
describes a fall-back procedure for the UAS to "recover" the call.  The
UAC is not aware of the problem with the UAS receiving its ACK.

regards,
Bert

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Chen,
Xin (Xin)
Sent: Thursday, August 16, 2001 9:58 AM
To: [email-masked]'
Subject: [Sip] Question about SIP bis04

What does this description mean? If UAS needs a ACK, why it has to
generate
a re-INVITE?

Thanks and Regards

Xin Chen

Lucent Technologies
Tel: +44 1793 883137
Mobile: +44 7799 034668

---

From: Michel de Boer <[email-masked]>
Date: Tue, 21 Aug 2001 09:52:49 +0200

If the UAS never receives the ACK after having retransmitted its 200 OK
several times, it seems that the signalling path is broken. So, I
would think the a re-INVITE transaction started by the UAS in that case
is doomed to fail as well. To me it seems more logical to tear down the
session in this case.

Regards,
Michel

---

From: "Bert Culpepper" <[email-masked]>
Date: Tue, 21 Aug 2001 10:42:04 -0400

Hi,

I agree with you that subsequent signaling transactions are likely to
fail under network conditions that resulted in the ACK never arriving at
the UAS.  However, there may be situations in which a re-INVITE in the
reverse direction may succeed.  There are also other uses of re-INVITE,
such as session-timer, that the procedure described can add value to.
Others more experienced with SIP than myself can better provide the
motivation behind the text asked about.

I suppose the authors feel it is better to attempt (at least once) to
recover the session instead of dropping it right away.  I myself
interpret the text asked about as exhibiting this philosophy, with which
I agree.  I also suspect this is the "best" approach for all uses of
re-INVITE, and is preferable to defining a procedure for each case.

regards,
Bert

> From: [email-masked] [mailto:[email-masked]]On Behalf
> Of Michel
> de Boer
> Sent: Tuesday, August 21, 2001 3:53 AM
> To: Bert Culpepper
> Cc: 'Chen, Xin (Xin)'; [email-masked]
> Subject: Re: [Sip] Question about SIP bis04
>
>