Re: [rtcweb] [MMUSIC] Is bundle just a port override?

Justin Uberti <juberti@google.com> Thu, 21 March 2013 05:11 UTC

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From: Justin Uberti <juberti@google.com>
Date: Wed, 20 Mar 2013 22:11:12 -0700
Message-ID: <CAOJ7v-2eH9oy8=OWVyoUki5ALWSwaB8x_dwpYUDJ-rY_VJ--kw@mail.gmail.com>
To: "Mo Zanaty (mzanaty)" <mzanaty@cisco.com>
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Cc: "rtcweb@ietf.org" <rtcweb@ietf.org>, "mmusic@ietf.org" <mmusic@ietf.org>
Subject: Re: [rtcweb] [MMUSIC] Is bundle just a port override?
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The point of the different ports is not just to make intermediaries happy,
but to ensure that we're successful when talking to an endpoint who doesn't
support BUNDLE. There are a litany of problems that have been identified
when the same ports are used on the offerer side and the answerer is
BUNDLE-unaware.

On Wed, Mar 20, 2013 at 9:43 PM, Mo Zanaty (mzanaty) <mzanaty@cisco.com>wrote;wrote:

> Hi Dale,
>
> In your draft (and Richard's), I understand your motivation to answer with
> port 0 (or 9 or IP 0), to plug the nostrils of intermediaries that sniff
> ports so they don't smell anything fishy. I won't comment on whether they
> will react better to nostril-plugging than fishy smells, because I don't
> know, but that is orthogonal to what I'm describing here.
>
> Your bundle alternatives (and Richard's) also work fine with a simple
> attribute without complex grouping semantics.
>
> Offer to mux audio and video on the same port:
> m=audio 10000 RTP/AVP 0
> m=video 10002 RTP/AVP 31
> a=port 10000
> i=I really want 10000, but lied on the m-line for fear of confusing you.
> Confused yet?
>
> Answer supports the port override attribute and agrees to mux audio and
> video:
> m=audio 40000 RTP/AVP 0
> m=video 0 RTP/AVP 31         <-- Dale's variant to answer with port 0
> a=port 40000
> ...so audio and video ports are 10000<->40000.
>
> Answer doesn't support the port override attribute, or doesn't want to mux:
> m=audio 40000 RTP/AVP 0
> m=video 40002 RTP/AVP 31
> ...so audio ports are 10000<->40000 and video ports are 10002<->40002.
>
> Cheers,
> Mo
>
>
> -----Original Message-----
> From: Dale R. Worley [mailto:worley@ariadne.com]
> Sent: Tuesday, March 19, 2013 3:37 PM
> To: Mo Zanaty (mzanaty)
> Cc: emcho@jitsi.org; rtcweb@ietf.org; mmusic@ietf.org
> Subject: Re: [rtcweb] [MMUSIC] Is bundle just a port override?
>
> > From: "Mo Zanaty (mzanaty)" <mzanaty@cisco.com>
> >
> > Those drafts use the SDP grouping framework
> > (a=group:BUNDLE/TOGETHER), which I think is unnecessary complexity
> > and ambiguity for something which can be solved in a much simpler
> > and clearer way with a single attribute: a=port <port>. (Or, if we
> > want to warn humans about RTP muxing, then name it a=rtp-mux:<port>,
> > but machines won't care about the name, and their RTP
> > implementations already know how to deal with muxing if they use
> > this attribute, so stronger warnings are unnecessary.)
> >
> > Regarding the concern about non-use of the discarded RTP ports
> > confusing SBCs, see this part of my original message [with
> > clarifications], which is what the current bundle draft also
> > suggests:
> >
> > > Either end can re-offer without lies about ports on the m-line after
> > > confirming the peer is not confused by muxing, if they want to avoid
> > > confusing intermediaries [that may monitor the unused ports].
>
> It's messier than that, unfortunately.
>
> One requirement is that after negotiation is completed, the negotiated
> transport associations (as seen by a legacy intermediary) must match
> the actual media flows.
>
> Another requirement is that two media descriptions shouldn't use the
> same port (because a lot of legacy SBCs can't handle that).  This
> applies to offer/answer updates as well as original offers/answers.
>
> So what we want is a way for a media description to say, "I'm
> reporting port zero so SBCs think this MD is rejected, but I'm
> actually using port 1234 (which is officially reported in another
> MD)."
>
> There are also situations where the we want to signal perferences as
> to which media descriptions share ports with which other media
> descriptions.
>
> Dale
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