[MMUSIC] SDP attribute level for disaggregated media [was Re: 4566bis outstanding issues]

Marc Petit-Huguenin <petithug@acm.org> Sun, 15 June 2014 21:11 UTC

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Date: Sun, 15 Jun 2014 15:11:10 -0600
From: Marc Petit-Huguenin <petithug@acm.org>
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Subject: [MMUSIC] SDP attribute level for disaggregated media [was Re: 4566bis outstanding issues]
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On 06/06/2014 03:19 PM, Ali C. Begen (abegen) wrote:
> Hi everyone


> These are the issues I was able to identify. There is one more thing
> related to ICE and I sent an email about this to Marc Petit-Huguenin. As
> soon as he responds and depending on his answer, I might add one more
> issue.

Let me first explain the problem as I perceive it:

The SDP was designed so that different endpoints can be part of an RTP
session, i.e. each m= section can used a different IP address.  For example
one can imagine that during an IETF meeting each microphone has its own IP
address and stream audio directly to some destination (in that example the IP
address in the SDP is only use for RTCP).  If this use case was not possible,
then it would be only possible to use c= at the session level.
draft-loreto-splices-disaggregated-media was, I think, the first document to
call this "media disaggregation".

Now if we expand this to ICE, it means that each ICE agent declared in a SDP
has to be independent of each other, i.e. each microphone in our example has
it's own ICE agent, and they do not even to be from the same vendor.  That
means that all SDP attributes needs to be at least media-level attributes, and
eventually both media- and session- level attributes.  IANA fixed the problem
for ice-mismatch, but the problem still exists for ice-options, and fixing it
is part of rfc5245bis.

Now my suggestion was that for future SDP attributes, people should really
think about the disaggregated case when they choose between a session-level
only attribute or an attribute that works at both session- and media-level,
because using a session-level only attribute prevents media disaggregation.  I
could not find this problem in any existing attribute at the time I did the
analysis but in ICE attributes, but note that this is not a problem specific
to ICE.

So I would suggest adding some text after this paragraph in section 8.2.4:

'Submitters of registrations should ensure that the specification is
 in the spirit of SDP attributes, most notably that the attribute is
 platform independent in the sense that it makes no implicit
 assumptions about operating systems and does not name specific pieces
 of software in a manner that might inhibit interoperability.'

Something like this:

'Submitters of registrations should also carefully choose the type of
attribute. They should not choose a session-level only type when the attribute
can have different values when media is disaggregated, i.e. when each m=
section has its own IP address on a different endpoint.  In that case the
attribute type chosen should be "both".'


- -- 
Marc Petit-Huguenin
Email: marc@petit-huguenin.org
Blog: http://blog.marc.petit-huguenin.org
Profile: http://www.linkedin.com/in/petithug
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