[MMUSIC] RTSP: Media Properties and aggregated sessions

[Magnus Westerlund <magnus.westerlund at ericsson.com>]

Hi,

I have received some private feedback from people active in 3GPP. They
wonder about if there is any point at all in having this be per media
instead of rather per session. I have spent some time thinking about
this and this is my arguments.

A RTSP session either contains a single media or is an aggregated
session with two or more media. In the first case the control of the
session equals control of the media. In the aggregated case the control
is done on all the media together, thus only the common properties are
what can be addressed. In case the the media in a aggregated session
have different properties there is still a need for the server to
determine what the resulting set of properties becomes for the
aggregate. If the media's properties aren't possible to combine, then
the aggregation should anyway be turned down.

Based on this reasoning there is a simple but necessary clarification
needed for the applicability of the media-properties header. I suggest
that the 2 first pargraphs in Section 16.29 (Media-Properties) is
modified to the following:

This general header is used in SETUP response or PLAY_NOTIFY requests to
indicate the media's properties that currently are applicable to the
RTSP session. PLAY_NOTIFY MAY be used to modify these properties at any
point. However, the client MUST have received the update prior to any
action related to the new media properties take affect. For aggregated
sessions the Media-Properties header will be returned in each SETUP
response and the one that applies is the response to the last request.

The media properties expressed by this header is the one applicable to
all media in the RTSP session. So for aggregated sessions the header
expressed the combined media-properties. As a result aggregation of
media MAY result in a change of the media properties, and thus the
content of the Media-Properties header contained in subsequent SETUP
responses.

The header contains a list of property values that are applicable to the
currently setup media or aggregate of media as indicated by the RTSP URI
in the request. No ordering are enforced within the header. Property
values should be grouped into a single group that handles a particular
orthogonal property. Values or groups that express multiple properties
SHOULD NOT be used. The list of properties that can be expressed MAY be
extended at any time. Unknown property values SHALL be ignored.

Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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Multimedia Technologies, Ericsson Research EAB/TVM
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Ericsson AB                | Phone +46 8 4048287
Färögatan 6                | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: magnus.westerlund at ericsson.com
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