Subject: [SIP] SIP - H.323 - INVITE - No media line.

From: "Maddux, Michel" <[email-masked]>
Date: Tue, 14 Nov 2000 15:00:29 -0600

In constructing a gateway connecting Sip to 323, the process of signalling a
call and then negotiating channels, for a Sip caller to 323 callee, is
relatively straightforward. Only one temporizing step is required, and that
is to not provide an RTCP address in opening channels from gateway (actually
Sip) to 323 callee.

The reverse case, of implementing 323 caller to Sip callee, is not so
straightforward. Signalling is tractable, but media channel negotiation is
murky. Two models occur:

1) When 323 calls, wait until it opens first media channel. Then issue the
INVITE, noting only a media line of that media type but with a phony port.
When Sip replies (200 OK presumed), complete the 323 media channel being
negotiated, giving it the Sip's media port. Then negotiate the reverse
channel with 323, saving its port, which is then transmitted to Sip via an
ACK with a message body correcting the 323 media port. If the 323 caller
then opens another media channel, repeat the above process via re-INVITE,
and continue to do so as long as 323 requests media channels.

2) Depend upon language from the 2543 spec, particularly section B.4 and the
ACK section, which state that an INVITE could be sent with NO media lines,
upon receipt of which the Sip callee would respond with a list of its
supported media (AND presumably providing a port for each of these). In this
scenario, an INVITE devoid of media would be issued immediately on receipt
of call from the 323 caller. The resulting 200 OK media list would then be
used to negotiate the media channels with the 323 caller, and would be
returned in final form in the message body of the ACK. [It is possible that
this model would devolve to the first model since it is not known when the
323 caller is through opening channels. Should a mid-call media change be
initiated by the 323 caller, the re-INVITE process would need be used,
anyway.]

We are interested in a recommendation as to how to do this. Are either of
the above models viable, or does a better model exist? What is the spec
status of section B.4?

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From: Vladislav Zubarev <[email-masked]>
Date: Tue, 14 Nov 2000 13:40:48 -0800

The annswers to most of your question in developing such a gateway you
can find in:

http://search.ietf.org/internet-drafts/draft-singh-sip-h323-01.txt

Have fun. :)

--
Vladislav Zubarev        mailto:[email-masked]
Software Engineer
Vovida Networks, Inc.    http://www.vovida.com