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Re: [AVTCORE] What does "same codec+configuration" mean?

Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 22 March 2016 14:55 UTC

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To: Iñaki Baz Castillo <ibc@aliax.net>, "avt@ietf.org" <avt@ietf.org>
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From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [AVTCORE] What does "same codec+configuration" mean?
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Den 2016-03-11 kl. 21:36, skrev Iñaki Baz Castillo:
> Hi,
>
> According to BUNDLE spec, two bundled m= lines can share the same PT
> value only if such a PT value points to the same codec name + codec
> configuration.
>
> What does "codec + configuration" mean? Some codecs have specific
> parameters, some parameters may be optional, and AFAIK there are some
> parameters that don't need to be fully negotiated by both endpoints.
>
> May I get some examples and be pointed to the appropriate spec?
>

So if one reads the bundle specification it says the following:

10.1.2.  Payload Type (PT) Value Reuse

    Multiple bundled "m=" lines might represent RTP based media.  As all
    RTP based media associated with a BUNDLE group belong to the same RTP
    session, in order for a given payload type value to be used inside
    more than one bundled "m=" line, all codecs associated with the
    payload type number MUST share an identical codec configuration.
    This means that the codecs MUST share the same media type, encoding
    name, clock rate and any parameter that can affect the codec
    configuration and packetization.
    [I-D.ietf-mmusic-sdp-mux-attributes] lists SDP attributes, whose
    attribute values must be identical for all codecs that use the same
    payload type value.

So identical a=rtpmap and a=fmtp lines would meet this criteria.

However, two different m= blocks configuring a particular PT for a codec 
could possibly use different values for an optional parameter that can 
be received correctly by either configurations.

Codecs that can produce different encoding bit-rates can in many cases 
be used at different bit-rates and fulfil this requirement.

The underlying issue is that what is identical codec configurations are 
codec specific. Thus hard to write a precise definition of it.

Does this help?

Magnus Westerlund

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