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

Emil Ivov <emcho@jitsi.org> Tue, 19 March 2013 17:03 UTC

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Date: Tue, 19 Mar 2013 18:02:59 +0100
From: Emil Ivov <emcho@jitsi.org>
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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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Hey Mo,

On 19.03.13, 16:28, Mo Zanaty (mzanaty) wrote:
> I understand the RTP session rules, but they are a matter for AVTCORE,
> and several drafts already detail the RTP considerations for
> multiplexing streams, of potentially different media types, over the
> same port (using plain RTP not SHIM). Bundle does not need to solve that
> (solved) problem, it just needs to signal which port(s) to use. If we
> feel we should signal some sort of human-level warning about RTP mux
> rules when signaling ports, the attribute can be named a=rtp-mux:<port>
> instead of a=port <port>.
> 
>  
> 
> I don’t see how any grouping semantics help with any of the rules.
> Actually, I think grouping semantics hurt, not just due to complexity,
> but also due to ambiguity about which attributes are inherited or
> overridden from other m-blocks. Keeping all the m-block attributes
> independent seems simpler and clearer. If an attribute needs to be
> consistent across m-blocks, repeating it seems simpler and clearer than
> inheritance/override rules across m-blocks. (Of course, if all m-blocks
> need the same attribute, it can be defined at session level instead of
> repeating in all media levels.)
> 
>  
> 
> In my simple (perhaps too simple) mind, all we’re trying to do in bundle
> is temporarily lie on the m-line port to avoid confusing
> peers/intermediaries. In the process, I think we have confused ourselves
> about what is truly needed to accomplish this deception. Or maybe I’m
> the only one confused…

In your original mail you seemed to be suggesting something similar to:

http://tools.ietf.org/html/draft-nandakumar-rtcweb-sdp-00

(Sometimes referred to Cundle for Cullen's Bundle)

or this

http://tools.ietf.org/html/draft-alvestrand-one-rtp-01

(Also known as One RTP)

Both would start by offering multiple ports and fall back to a single
port if bundling is supported.

The concern that has been expressed with them is that in case of
bundling, non-use of the discarded RTP ports would cause some SBCs to
believe that there's a problem with the session which may lead to the
SBC tearing down the call.

Hope this helps,
Emil

> 
>  
> 
> Mo
> 
>  
> 
> *From:*Harald Alvestrand [mailto:harald@alvestrand.no]
> *Sent:* Tuesday, March 19, 2013 5:03 AM
> *To:* Mo Zanaty (mzanaty)
> *Cc:* mmusic@ietf.org; rtcweb@ietf.org
> *Subject:* Re: [MMUSIC] Is bundle just a port override?
> 
>  
> 
> On 03/19/2013 05:19 AM, Mo Zanaty (mzanaty) wrote:
> 
>     Does bundle imply anything beyond a simple port override of the
>     m-line port? If not, then why not just directly signal the port
>     override in an attribute without any grouping semantics?
> 
> 
> No, it's not just a port override.
> 
> BUNDLE shoves things into the same RTP session, which means that one has
> to obey the rules for being in the same RTP session:
> - Security done consistently
> - RTCP-mux done consistently
> - No SSRC collision
> - No payload type collision
> So it's a good deal more than just a port override.
> 
> Magnus argued in favour of a solution that inserted a multiplex layer,
> so that one could use the same ports but still have stuff in different
> RTP sessions, but that did not appeal to many.
> 
> 
>  
> 
> 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 40000 RTP/AVP 31
> 
> a=port 40000
> 
> …so audio and video ports are 10000<->40000.
> 
>  
> 
> Answer supports the port override attribute but doesn’t want to demux on
> his end:
> 
> m=audio 40000 RTP/AVP 0
> 
> m=video 40002 RTP/AVP 31
> 
> a=port 40002
> 
> …so audio ports are 10000<->40000 and video ports are 10000<->40002.
> 
>  
> 
> Answer doesn’t support the port override attribute, or doesn’t want
> either end 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.
> 
>  
> 
> 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.
> 
>  
> 
> Has this approach been tried yet? Dismissed?
> 
>  
> 
> Mo
> 
>  
> 
> 
> 
> 
> _______________________________________________
> 
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> 
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> 
> https://www.ietf.org/mailman/listinfo/mmusic
> 
>  
> 
> 
> 
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