Re: [MMUSIC] AD Evaluation of draft-ietf-mmusic-rfc4566bis-32: Media Description

Christer Holmberg <christer.holmberg@ericsson.com> Mon, 18 February 2019 21:33 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Ben Campbell <ben@nostrum.com>, Paul Kyzivat <pkyzivat@alum.mit.edu>
CC: Magnus Westerlund <magnus.westerlund@ericsson.com>, mmusic WG <mmusic@ietf.org>, "draft-ietf-mmusic-rfc4566bis.all@ietf.org" <draft-ietf-mmusic-rfc4566bis.all@ietf.org>, Colin Perkins <csp@csperkins.org>
Thread-Topic: [MMUSIC] AD Evaluation of draft-ietf-mmusic-rfc4566bis-32: Media Description
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Date: Mon, 18 Feb 2019 21:33:43 +0000
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Subject: Re: [MMUSIC] AD Evaluation of draft-ietf-mmusic-rfc4566bis-32: Media Description
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Hi,

The first thing we need to make clear, because it has caused some confusion in the past, is whether a "media description" only covers the m- line, OR whether it covers the m- line AND each SDP attribute (a- line) associated with it.

BUNDLE says the following:

     "m=" section: SDP bodies contain one or more media descriptions,
     referred to as "m=" sections.  Each "m=" section is represented by
     an SDP "m=" line, and zero or more SDP attributes associated with
     the "m=" line.  A local address:port combination is assigned to
     each "m=" section.

...which doesn't provide any semantics, but does indicate that the attributes are part of the media description.

After a quick scan through BUNDLE, I couldn't find any semantics, other than a media description is used to "describe media".

But, BUNDLE also says that BUNDLE is used for "using a single transport for sending and receiving media associated with SDP multiple media descriptions".

So, while I don't have exact wording at the moment, my suggestion would be to say that an SDP media description describes a TRANSPORT for a media flow, or something like that... 

Regards,

Christer



On 18/02/2019, 23.14, "mmusic on behalf of Ben Campbell" <mmusic-bounces@ietf.org on behalf of ben@nostrum.com> wrote:

    
    
    > On Feb 18, 2019, at 2:23 PM, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
    > 
    > 
    > 
    >> On 2/11/19 10:45 PM, Ben Campbell wrote:
    > 
    >>> §2, “Media Description”: While true, the text here is not a definition of anything but syntax. Please add a sentence or two to say what the media description semantically represents.
    > I'm having trouble finding a suitable semantic definition of Media Description.
    > 
    > My first thought is that it provides the information necessary to establish a media session. But while *Multimedia Session* is described, media Session isn't defined in the document.
    > 
    > I looked at RFC7656 (Taxonomy) for a definition, but it doesn't have one either. It defines RTP Session, and Multimedia Session as one or more RTP Sessions. That is close, but in SDP media sessions need not be RTP.
    > 
    > Is there any suitable existing definition???
    > 
    > If not, then how about:
    > 
    > "A Media Description contains the information needed for one party to establish an application layer network protocol connection to another party." ???
    > 
    > OR
    > 
    > "A Media Description contains the information needed for one party to establish a Media Session with another party."
    > 
    > "A Media Session is an application layer network protocol connection between two parties."
    > 
    > It seems important to get this right! It is essentially a broadening of RFC7656 beyond RTP.
    
    I am okay with either of those. :-)
    
    Ben.