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Issues from telemedical-callflows

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

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Issues from telemedical-callflows

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  1. Issues from telemedical-callflows CLUE interim 84.5, 19-sep-2012 Paul Kyzivat pkyzivat@alum.mit.edu

  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 configmay 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? pkyzivat@alum.mit.edu

  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 beconsistent 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 maycease sending media that aren’t consistent with the new advertisement. pkyzivat@alum.mit.edu

  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 sendconfigbefore 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. pkyzivat@alum.mit.edu

  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. pkyzivat@alum.mit.edu

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