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

Paul Kyzivat <pkyzivat@alum.mit.edu> Mon, 20 May 2013 15:09 UTC

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Date: Mon, 20 May 2013 11:09:03 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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To: Emil Ivov <emcho@jitsi.org>
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Cc: mmusic <mmusic@ietf.org>, Christer Holmberg <christer.holmberg@ericsson.com>
Subject: Re: [MMUSIC] Bundle offer with different ports - where to expect media?
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On 5/20/13 10:51 AM, Emil Ivov wrote:
> Hey Christer,
>
> On 20.05.13, 17:16, Christer Holmberg wrote:
>> Hi,
>>
>>>>> What happens when the offerer knows the answerer has bundle
>>>>> support, sends all m-lines with the same port, then the
>>>>> answerer splits the first line away from the bundle? Would the
>>>>> answerer still send everything to the same port?
>>>>
>>>> We discussed this week,
>>>>
>>> Yes, sorry, I didn't follow this closely.
>>>
>>>> and the outcome (at least my read of it :) was that the answerer
>>>> is not allowed to split any m- lines away from the bundle in this
>>>> case. Instead the answerer will have to send a new offer for the
>>>> split, allowing new ports to be negotiated at both ends.
>>>
>>> OK, so shouldn't the same thing happen in the case with different
>>> ports?
>>
>> I suggested that it should never be allowed to split an m- line from
>> a bundle group in an answer, but others had other opinions.
>
> I don't see how we could allow it in one case and disallow it in the
> other. The only difference between the two cases is how informed the
> offerer is about the answerers bundle support capabilities and I don't
> really understand why this would influence the decision to allow
> splitting bundles one way or the other.

It is a different case because the same port *can't* both be used as the 
bundle port and a port for an unbundled m-line.

>> HOWEVER, it would still not help in the case where the 1st m- line is
>> rejected.
>
> Well, how about looking at it this way: the offerer specifies a bundle
> port in the first m=line. This also happens to be the port for the first
> media line but the two are different things and just happen to have the
> same value for reasons related to syntax and convenience.
>
> A bundle supporting answerer should understand this. After receiving the
> offer that answerer has learned the bundle port number. Rejecting the
> first m=line in the answer does not change this.

There are many reasons that an answerer may reject an m-line.
It is *possible* that it is rejecting it because it has a problem with 
the address (c=) for the m-line in the offer. If so, then if you insist 
on using it as the bundle address, even if the m-line is refused, then 
there is no way for the answerer to refuse it.

(*Why* it would have a problem is an open question. Maybe its IPv6 and 
the answerer can't use it, or maybe its an FQDN and it can't be 
resolved. I realize this is unlikely. But making the assumption that the 
address must be acceptable to the answerer is IMO not a good idea.)

> Of course the answer would come with the first m=line having a 0 port
> but the offerer would then just learn the bundle port number at the
> first m=line with a non-zero port.

I presume you must have a motivation for wanting to go this way.
Are you thinking this will simplify the implementation? What do you 
think you can avoid doing my making this assumption?

	Thanks,
	Paul

> Subsequent offers and answers would all use that same m=line for the
> bundle port.
>
> Does this make sense?
>
> Emil
>
>
>>
>> Regards,
>>
>> Christer
>>
>>
>