Subject: [SIP] QoS & Third Party Call Control in SIP

From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 04 Jul 2000 10:58:35 +0300

Hi,

The draft "Third Party Call Control in SIP", which can be found at
http://www.cs.columbia.edu/sip/drafts/draft-rosenberg-sip-3pcc-00.txt

outlines a method for establishing a session between two hosts that have
signalling (SIP) relationships with a 3rd host that is not involved in
the session.

The idea is basically to send an INVITE to A-party without SDP and then
send the SDP coming from the B-party in the ACK.

This works fine when no QoS is needed. If preconditions must be used
this method fails. Preconditions cannot be added to the SDP in the ACK,
since ACKs trigger no responses.

Both parties involved in the session have to know both SDP descriptions
in order to perform resource reservation (with RSVP, for instance).
Then, COMET will have to be sent.

Have somebody taken this into account regarding this draft? Any ideas?

Gonzalo
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From: Gonzalo Camarillo <[email-masked]>
Date: Wed, 05 Jul 2000 15:21:28 +0300

Hi,

Some folks have pointed out that this is a similar problem to the one
DCS was trying to solve using the double-stage INVITE. This is actually
true, but I do not like the double-stage INVITE approach.

In the following example C will be the 3rd party, and the media session
will be established between A and B:

C sends an INVITE to A without SDP (issue: how can C state that QoS will
be needed if there is not SDP in the INVITE?)

A responds with 183 with a SDP.

C sends an INVITE to B with the SDP received from A. We can specify QoS
preconditions here.

B responds with 183 with its SDP and begin performing resource
reservation in the path B->A

How can now C send B's SDP to A?? If A does not have B's SDP it cannot
perform resource reservation in the path A->B... Can we re-INVITE A at
this stage?

Thanks,

Gonzalo