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

Emil Ivov <emcho@jitsi.org> Tue, 21 May 2013 22:53 UTC

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Date: Wed, 22 May 2013 01:53:50 +0300
From: Emil Ivov <emcho@jitsi.org>
Organization: Jitsi
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To: Justin Uberti <juberti@google.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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On 22.05.13, 01:38, Justin Uberti wrote:
> Lots of discussion here; is this an accurate summary of the discussed
> behavior?

Mostly yes. Just a couple of nitpicks:

> 1) BUNDLE offerers must be able to receive on any port (until they get a
> BUNDLE-compatible answer)

Apparently we are going for: BUNDLE offerers must be able to receive
*bundled* data on any port except for that on the last m= line. They
must also be prepared to received unbundled data on any port.

> 2) Once the answer is received, bundling is activated,

I suppose bundling should be active before the answer is received and it
can be deactivated when that happens.

Emil

> and BUNDLEs use
> the port of the first non-rejected m= line in the offer
> 2a) at this point, the other ports can be discarded
> 3) All m= lines offered in a BUNDLE must be present in the answer,
> unless they are rejected
> 3a) no selective unbundling/BUNDLE splitting is permitted
> 
> 
> On Tue, May 21, 2013 at 10:57 AM, Paul Kyzivat <pkyzivat@alum.mit.edu
> <mailto:pkyzivat@alum.mit.edu>> wrote:
> 
>     On 5/21/13 4:45 AM, Emil Ivov wrote:
> 
>         Hey Paul,
> 
>         On 20.05.13, 18:09, Paul Kyzivat wrote:
> 
>             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.
> 
> 
>         Why not? Obviously the offerer was prepared to demultiplex traffic
>         there. Why wouldn't it be able to continue doing so even if the
>         answerer
>         would prefer to receive it separately?
> 
> 
>     The point of wanting the one line unbundled is so that it can be
>     *received* on a different address/port. Perhaps there is a separate
>     device or process that will be supporting that one, that must be
>     reached at its own addr/port.
> 
>     Unless you expect that this would have a common addr/port on the
>     offering side and different addr/ports on the answering side. But we
>     investigated that approach early in the bundle discussions, and gave
>     it up.
> 
> 
>         Note that I am not defending such an approach. I'd much prefer that
>         splitting is only allowed when re-offering.
> 
>                     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.)
> 
> 
>         Aha! That's a good point.
> 
>         Still, the thing that I don't quite understand is: if the
>         answerer has a
>         problem with the c= line, how could that problem only apply to
>         the first
>         m= line and not to the entire bundle?
> 
> 
>     Presumably this would only happen if the c= for were not all the
>     same in the offer. Certainly that is possible. When allocating
>     "ports" for each m-line one might also end up with different addresses.
> 
> 
>                 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?
> 
> 
>         No. The implementation is going to be slightly more complicated
>         indeed
>         because instead of having one bundle demuxing socket and two regular
>         ones (well ... if rtcp demuxing counts as regular) you would
>         have three
>         of them. I don't think this would be particularly horrifying in
>         terms of
>         complexity though.
> 
>         Still, I think being consistent about the ports where one could get
>         multiplexed data is a good thing, and it could help (humans) when
>         analysing traffic and debugging network issues.
> 
> 
>     Consistency is good. But isn't it just as consistent to say that the
>     first *accepted* m-line defines the bundle addr/port as it is to say
>     that the first offered m-line?
> 
>             Thanks,
>             Paul
> 
> 
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