Re: [MMUSIC] Bundle offer with different ports - where to expect media?

Justin Uberti <juberti@google.com> Wed, 22 May 2013 22:55 UTC

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From: Justin Uberti <juberti@google.com>
Date: Wed, 22 May 2013 15:55:34 -0700
Message-ID: <CAOJ7v-1tmfy4Fu05ChcpbZT=bO-zr05qtOh9Lnw5bprbUEnoDw@mail.gmail.com>
To: Kevin Dempsey <kevindempsey70@gmail.com>
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Cc: mmusic <mmusic@ietf.org>, Paul Kyzivat <pkyzivat@alum.mit.edu>, Christer Holmberg <christer.holmberg@ericsson.com>
Subject: Re: [MMUSIC] Bundle offer with different ports - where to expect media?
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I think we could do this using the existing syntax, e.g.

an offer of a=group:BUNDLE A B C would indicate that the offerer would
prefer A, B, and C to be muxed onto A, and
an answer of a=group:BUNDLE C A would indicate that the answerer is going
to mux C and A onto C, and send B unbundled.


On Wed, May 22, 2013 at 7:11 AM, Kevin Dempsey <kevindempsey70@gmail.com>wrote;wrote:

> Would it make it simpler if the offer included an attribute in bundled m=
> lines that said where to send the media if bundle is supported by the
> answerer; and for the answer to include an attribute that said where it was
> going to send the media (ie. to the bundled destination or the unbundled
> destination)
>
>
> On 22 May 2013 14:55, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>
>> On 5/22/13 1:00 AM, Justin Uberti wrote:
>>
>>  Taking into account Emil's suggestion as well, this leads to this
>>> restatement of the rules:
>>>
>>> 1.1) An offerer must be prepared to receive unbundled traffic on the
>>> offered ports, as if talking to a legacy endpoint.
>>> 1.2) An offerer must be prepared to receive bundled traffic on any port,
>>> except the port associated with the last m= line.
>>> 2) Once the answer is received, the decision on bundled/unbundled and
>>> which ports to use is finalized.
>>> 2.1) BUNDLEd m= lines use the port of the m= line that appears first in
>>> the bundle.
>>> 2.2) Unused ports can be discarded at this point.
>>> 3.1) m= lines offered with distinct ports may be omitted from the bundle
>>> answer, in line with RFC 5888.
>>> 3.2) m= lines offered with the same port may not be omitted from the
>>> bundle answer, since there is no unbundled port from them to fall back
>>> to.
>>> 3.3) m= lines that are rejected are omitted from the bundle answer, per
>>> RFC 5888.
>>>
>>> Quick check on 2.1: are we using the m= line that appears first in the
>>> SDP, or the m= line that appears first in the a=group:BUNDLE?
>>>
>>
>> IMO it would be best to use the lexically first m-line that has a
>> non-zero port and that appears in the bundle group of the answer.
>>
>> Alternately, it *could* be the m-line with non-zero port that appears
>> first in the bundle group of the answer. But if so, the exception in 1.2
>> would also need to be correspondingly adjusted.
>>
>> AFAICT the only reason to mention that exception in 1.2 is that might
>> provide an opportunity for optimization, by setting up the port with
>> simpler code that doesn't support bundling. I don't see a requirement in
>> 5888 that the bundle group in the answer be ordered the same way as in the
>> offer. So the one m-line guaranteed not to receive bundled traffic would
>> then not be predictable at the time of the offer, so could not be
>> exploited. This problem doesn't exist if the lexical ordering of m-lines is
>> used.
>>
>>         Thanks,
>>         Paul
>>
>>
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>
>