1. Issues from telemedical-callflows
CLUE interim 84.5, 19-sep-2012
Paul Kyzivat
19-sep-2012

2. Major issue from initial discussion on this call flow
When is new O/A needed?
What is relationship of O/A to advertisement and configuration messages?
O/A cover and advertisement and all its configs, so no new O/A needed for a config?
O/A cover a configuration. A new config may require a new O/A.
Multiple O/As might occur for a single config.
O/A precede or follow the CLUE message is applies to?
19-sep-2012

3. Proposal for Relationship of O/A to CLUE messages (1)
What media is sent at any time is determined by the most recently received valid config message, constrained by most recent O/A.
When constraint is bandwidth, sender decides what to drop
A new O/A can make it possible to send more (or less) of the current config.
An O/A should be consistent with the most recent advertisement in each direction.
This gives context to understand why to accept what is offered. (The answerer has no motivation to accept things that aren’t.)
A config message always explicitly refers to an advertisement.
It is invalid, and will be rejected, if it doesn’t refer to the most recently sent advertisement. (This implies there is a message to NACK a config msg.)
Typically the endpoint that sends the config message makes an offer to enable it.
This end is the first to know what is needed to support the config
It is also the end that cares.
Can be before or after the config, or both.
Must accommodate the most recent config received from the other side.
But either side may send an O/A at any time for whatever reason, or may send an offerless INVITE to solicit an offer. (This is basic SIP.)
Require a config message in response to each advertisement.
Ensures no need for continued support of old advertisement
Until a new config is received, sender of the advertisement may cease sending media that aren’t consistent with the new advertisement.
19-sep-2012

4. Proposal for Relationship of O/A to CLUE messages (2)
During call major changes requiring O/A will typically happen independently at each endpoint.
But at start of call there will be an exchange of advertisements and configs.
Want to avoid unnecessary O/As in this case
If receive an advertisement after sending one, before receiving a config:
Should send config before initiating O/A.
If receive offer before sending one, it will typically be sufficient to accommodate your config.
If so, won’t need to initiate another O/A.
There may still be glare at SIP level. Use standard SIP remedies for that.
If so, the O/A that glared may not need to be retried.
First O/A of session occurs before any advertisements or configurations
Must include the CLUE channel
Everything else is speculative until advertisements are exchanged.
May be best guess at what is needed, or placeholder intended to maximize interop with arbitrary devices
Before the first config is received, there should be at most a single capture, chosen by sender, per RTP session.
19-sep-2012

5. Next Steps If proposal is agreeable:
Write it up formally for inclusion in signaling draft
Update the callflow draft to be consistent with this.
19-sep-2012