Paul Kyzivat <> Mon, 27 May 2013 17:42 UTC

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Date: Mon, 27 May 2013 13:42:25 -0400
From: Paul Kyzivat <>
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On 5/27/13 1:16 PM, Christer Holmberg wrote:
> Hi,
>>>>> At the interim yesterday, we agreed to use the SDP BUNDLE group
>>>>> attribute to indicate on which port bundled media will be sent. In an
>>>>> SDP offer, the order of the attribute mid values can be used by the
>>>>> offerer to indicate a "wish", but the first attribute mid value in the
>>>>> answer indicates where the answerer will eventually send bundled media.
>>>> This could be interpreted to mean the first a=mid value in the SDP.
>>>> I'm sure you mean "the first mid value in the a=group:bundle" line.
>>> Yes.
>>>> Or perhaps "the first mid value in the bundle group" - assuming there is a definition that "bundle group" means an a=group:bundle line.
>>> In the draft "bundle group" means (or, at least is supposed to mean :) the bundle grouped m- lines, and in general I think it is better to explicitly mention the line/attribute.
>>>> This can be "future proofed" by saying that the first mid value in the bundle group in the answer that references an m-line with non-zero
>>>> port value. I realize this adds nothing unless 5888 is revised to permit m-lines with zero port in a bundle group. But at least it will be ready for that, and harms nothing.
>>> I agree, but suggest a little different wording: by saying the first mid value in the group attribute MUST NOT represent a port zero m- line.
>> I just want to be sure we're clear here.
>> Are you saying that even if we relax 5888 so that we may have m-lines
>> with port zero in a bundle group, that the *first* mid in the bundle
>> group can't designate a zero port???
> Yes, for BUNDLE. It doesn't have to be a generic 5888 rule.
>> I guess that should always be possible as long as there is at least one
>> with a non-zero port.
>> Do we have a possibility, with ICE, that all the m-lines in the bundle
>> will have zero port? (Because we are waiting for ICE to figure out the
>> actual port.) Or are we going to use something else for that purpose?
>> As long as there is no possibility of having a bundle where all the
>> ports are zero then I'm ok with what you propose.
> I hear what you're saying.
> But, in that case saying "first non-zero mid" does not work either, does it? :)

Yep, you're right!

> Another option is to also allow mids for zero-port m- lines to be first. A first mid for a port zero m- line would mean that there is no port for bundled media. Maybe there could be a use-case for that.

That sounds like a bad choice if one of the other m-lines in the bundle 
has a non-zero port.

So maybe your rule is best, if it works: first mid in bundle must 
identify an m-line in the bundle with non-zero port if there is one. If 
there are non with non-zero port then it can refer to one that is zero.

But we can't take this too far without deciding if we want to allow 
rejected m-lines in bundles, and if so, what that means.