Re: [rtcweb] New topic: Model for multi-unicast support
Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com> Sat, 03 September 2011 05:30 UTC
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From: Stefan Håkansson LK <stefan.lk.hakansson@ericsson.com>
To: Harald Alvestrand <harald@alvestrand.no>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Date: Sat, 03 Sep 2011 07:31:36 +0200
Thread-Topic: [rtcweb] New topic: Model for multi-unicast support
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Subject: Re: [rtcweb] New topic: Model for multi-unicast support
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Harald wrote: >In the thread about draft-perkins-rtcweb-usage, the following exchange >took place: >>> > Speaking for some implementors of the WEBRTC API, it's also clear that there's a significant cost to implementing the multiway RTP session concept - both in code >complexity and API complexity. I think this warrants more discussion, where all 3 outcomes should be on the table: >>> > >>> > - We recommend doing multi-unicast with a shared RTP session only >>> > - We recommend supporting both shared and split RTP sessions for multi-unicast >>> > - We recommend doing multi-unicast with split RTP sessions only >>> > >>> > We should probably do this in a new thread. >> Indeed; I'd be interested to know what complexity you're seeing. I would have expected the shared RTP session model to decrease complexity, since the RTP layer can be >>oblivious to the detail of the underlying transport connections, and wouldn't have to worry about tracking SSRC and payload type mapping separately for each session. >I think this is a topic that deserves its own thread. > >The context is use case 4.2.7, "Multiparty video communication", the one >without a central server; each party sends its video and audio streams >to all participants in the meeting directly. > >One can imagine supporting this with a single RTP session, where all >video streams are equal and sent with the same SSRC to all participants, >all participants send receiver reports (RRs) about all the SSRCs they >see to all other participants, and all senders send their SR reports to >all participants. To me an RTP session was connected to a port/IP address, so I don't understand how this could be one session (the destination port/IP would be different for each stream). (I expect to be corrected!) Is multicast assumed? > >Now, this has implications for both API design, service design and >protocol operation. > >API: The model presented is no longer "this object is your connection to >A; that object is your connection to B". There has to be a >representation of your "cloud" as a multidestination object, with >procedures for adding to and subtracting from the list of participants. >Error reporting must include which participant the error relates to. > >Service: The requirement for perfect equality means that you cannot >allow any party to adapt its presence to what it can receive; for >instance, as I understand it, nobody can drop the video and only be an >audio participant, because that would break the model of equality, >rendering sender reports meaningless to some participants. > >Protocol: The requirement for a single agreed RTCP bandwidth means that >this bandwidth has to be computed for the session as a whole - with the >simplistic answer being 5% of the lowest common denominator. > >In contrast, if one chooses the multiple-RTP-session approach: > >- the API retains its "one object, one destination" orientation. > >- the degree to which service is equal to all parties is a matter for >the application, not the protocol > >- rate adaption, choice of RTP bandwidht and so on can be negotiated on >each link individually > >I think this forum needs to pick one of the options in my initial note, >and design its protocols and APIs accordingly. >Thoughts? This was to some extent discussed on the webrtc list earlier. My view is the same as then: we should go for the '"one object, one destination" and multiple RTP-session' approach. Stefan Harald _______________________________________________ rtcweb mailing list rtcweb@ietf.org https://www.ietf.org/mailman/listinfo/rtcweb
- [rtcweb] New topic: Model for multi-unicast suppo… Harald Alvestrand
- Re: [rtcweb] New topic: Model for multi-unicast s… Stefan Håkansson LK
- Re: [rtcweb] New topic: Model for multi-unicast s… Magnus Westerlund
- Re: [rtcweb] New topic: Model for multi-unicast s… Magnus Westerlund
- Re: [rtcweb] New topic: Model for multi-unicast s… Harald Alvestrand
- Re: [rtcweb] New topic: Model for multi-unicast s… Harald Alvestrand
- Re: [rtcweb] New topic: Model for multi-unicast s… Colin Perkins