Adam s m line musings
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Adam’s m = line Musings. draft-roach-mmusic-mlines-00 RTCWEB Interim Boston, MA February 5 th , 2013. Three Proposed Approaches. Approach 1a: One m= section per media type, regardless of how many streams of that type in the session Approach 1b: One m= section per session (MMT)

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Adam s m line musings

Adam’s m= line Musings

draft-roach-mmusic-mlines-00

RTCWEB Interim

Boston, MA

February 5th, 2013


Three proposed approaches
Three Proposed Approaches

  • Approach 1a: One m= section per media type, regardless of how many streams of that type in the session

  • Approach 1b: One m= section per session (MMT)

  • Approach 2: One m= section per media stream


Codec selection
Codec Selection

  • Handling of preferences in RFC3264 currently assumes one-stream-per-m-section model

  • This requires fix-up for approaches 1a and 1b (or ambiguity and loss of control)

  • Favors approach 2


Port number handling
Port Number Handling

  • SDP syntax does not natively support bundling more than one m= section into a single session (port # is mandatory).

  • For solutions 1a and 2, we need to choose “the lesser of two evils”:

    • Indicating the same port in all lines trips up legacy use

    • Indicating different ports (some bogus) in each line trips up some SBCs

  • Favors approach 1b


Attribute handling
Attribute Handling

  • Since attributes can roughly apply to either sessions or to streams, creating a distinction requires updates to attribute handling

  • Approaches 1a and 1b can use RFC5576 for stream-level attributes

  • Approaches 1a and 2 can indicate transport-level attributes in all relevant m= sections

  • Because 1a must address both situations, it is more complicated than the either two.


Unknown unknowns
Unknown Unknowns

  • Problems such as the codec selection problem are yet to be identified for the general case.

  • Addressing these issues in the future is easiest if we have the most granular control surface possible.

    • This also makes future extensions more compatible with non-RTCWEB SDP use

  • This supports selection of the most granular control surface, which is alternative 2.


Problems with all solutions
Problems with all solutions

  • Offerer and answerer needing different number of streams

  • Attribute bidirectionality

  • Implementation complexity