1. Transcoding Framework
draft-ietf-sipping-transc-framework-01.txt

2. Status Framework defines two models 3pcc: already approved
B2BUA: under discussion

3. Caller’s Invocation using 3pcc
A T B
| | |
| (1) INVITE SDP A---->| |
|<----(2) 200 OK SDP TA+TB---| |
| (3) ACK >| |
| (4) INVITE SDP TA >|
|< (5) 200 OK SDP B |
| (6) ACK >|
| (7) INVITE >| |
|<---(8) 200 OK SDP TA+TB --| |
| (9) INVITE SDP TA >|
|< (10) 200 OK SDP B |
| (11) ACK >|
|------(12) ACK SDP A+B----->| |
| ************************** | ************************** |
|* MEDIA *|* MEDIA *|

4. Motivation for a New Model
A lot of messages over the air
User agents using slow radio links
Many UAs will not support 3pcc
New model needed
Fewer messages over the air
Simpler for UAs

5. Proposals so far Treat the transcoder like a simple conference
Use the URI-list service for INVITE requests mechanism
Address the transcoder as if it was a proxy
Using the Route header field
Use a new mechanism

6. Comparison Conference model INVITE T <URI-list> callee
Route model
INVITE callee
Route: T

7. Comparison (2) Conference model
+ Recipient of the SDP identified by the Request-URI
- Requires UAs to support list bodies
Route model
+ Trivial to support by UAs
- Impossible to tell who is the recipient of the SDP by inspecting a message