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Re: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines

Christer Holmberg <christer.holmberg@ericsson.com> Sat, 30 January 2021 19:21 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Justin Uberti <juberti=40google.com@dmarc.ietf.org>
CC: Magnus Westerlund <magnus.westerlund@ericsson.com>, Roman Shpount <roman@telurix.com>, "mmusic@ietf.org" <mmusic@ietf.org>
Thread-Topic: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines
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Date: Sat, 30 Jan 2021 19:21:15 +0000
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Subject: Re: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines
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Hi,

>>> One other thing I would like to point out, that if you are dealing with an end-point that does not support BUNDLE, for the m= lines where the port is set to zero, you would often get something that will copy
>>> all the RTP attributes to the answer from the offer, including the a=bundle-only attribute. Of course, you should not get the session-wide "a=group:BUNDLE" and would be able to detect that the m= line was
>>> rejected that way.
>>>
>>> If the remote endpoint does not support the grouping framework, your actual mileage with the "a=group:BUNDLE" line may wary. There was never an explicit requirement that unsupported attributes must not be copied
>>> from the offer to the answer. RFC 4566 and RFC 8866 only say that the parser must ignore such attributes. Because of this, some endpoints will copy all unknown attributes to the answer, including a=group. Of course, anything
>>> that supports RFC 5888 will not do this.
>>
>> Correct.
>>
>> However, endpoints copying SDP attributes they don't understand can in general cause all kind of issues. It is not a problem unique to the grouping framework, or to  BUNDLE, or to m- lines with port zero.
>>
>> But, at least there is standardized semantics of the meaning of port zero.
>
> Does the BUNDLE doc talk about what to do in this case, I.e., use of bundle-only in m= sections that are not part of an a=group:BUNDLE group? I think this is mostly separate from the issue at hand, but this seems like something endpoints should be robust against.

bundle-only outside a bundle group is unspecified.

   "The usage of the 'bundle-only' attribute is only defined for a
   bundled "m=" section with a zero port value.  Other usage is
   unspecified."

(Section 6 of BUNDLE)

Regards,

Christer

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