[Sipping] Transcoding

[Gonzalo Camarillo <[e-mail]>]

Folks,

I guess it is time to re-start the discussions we had some time ago on 
transcoding invocation. As you know, the transcoding framework defines 
two transcoding models.

http://standards.ericsson.net/gonzalo/papers/draft-ietf-sipping-transc-framework-00.txt

In the first one, referred to as the 3pcc model, the user agent invoking 
the transcoding service has a signalling relationship with the 
transcoder and with the remote user agent. This model has already been 
approved.

http://www.ietf.org/internet-drafts/draft-ietf-sipping-transc-3pcc-02.txt

In the second model, the transcoder receives SIP messages from a user 
agent and forwards them to the other user agent. It is a B2BUA.

The open issue is how to invoke this transcoder. There are currently two 
proposals:

1) Treat the transcoder as a simple conference server and use the 
URI-list service for INVITEs to establish the sessions. That is, the UAC 
sends an INVITE to the transcoder with the URI of the UAS in a body.

INVITE [e-mail]
[SDP]
[[e-mail]]

This approach was documented in the following document:

http://standards.ericsson.net/gonzalo/papers/draft-camarillo-sipping-transc-b2bua-01.txt

2) User Route headers as if the transcoder was a proxy. This is 
basically what SBCs do. The transcoder would rewrite the SDP.

INVITE [e-mail]
Route: [e-mail]
[SDP]


Approach 1 requires user agent to generate one-element XML resource 
lists, and approach 2 has a number of problems associated to it (these 
problems have been discussed in the context of SBCs).

Alternative proposals include defining a new header field similar to 
Route but which is used to include B2BUAs instead of Proxies...

Comments are welcome,

Gonzalo


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