[MMUSIC] RTSP 2.0 SDP issue: Directionality attribute

[Magnus Westerlund <[e-mail]>]

WG,

Flemming has found an issue in Section D.1.5. This is that this section
actually redefined the default behavior of the a=sendrecv, a=recvonly,
a=sendonly. This is clearly unsuitable and I propose that we change the
current text:

D.1.5.  Directionality of media stream

   The SDP attributes "a=sendrecv", "a=recvonly" and "a=sendonly"
   provide instructions about the direction the media streams flow
   within a session.  When using RTSP the SDP can be delivered to a
   client using either RTSP DESCRIBE or a number of RTSP external
   methods, like HTTP, FTP, and email.  Based on this the SDP applies to
   how the RTSP client will see the complete session.  Thus media
   streams delivered from the RTSP server to the client, would be given
   the "a=recvonly" attribute.

   The direction attributes are not commonly used in SDPs for RTSP, but
   may occur. "a=recvonly" in a SDP provided to the RTSP client MUST
   indicate that media delivery will only occur in the direction from
   the RTSP server to the client.  In SDP provided to the RTSP client
   that lacks any of the directionality attributes (a=recvonly,
   a=sendonly, a=sendrecv) MUST behave as if the "a=recvonly" attribute
   was received.  Note that this overrules the normal default rule
   defined in SDP[RFC4566].  The usage of "a=sendonly" or "a=sendrecv"
   is not defined, nor is the interpretation of SDP by other entities
   than the RTSP client.

To instead become:

D.1.5.  Directionality of media stream

   The SDP attributes "a=sendrecv", "a=recvonly" and "a=sendonly"
   provide instructions about the direction the media streams flow
   within a session.  When using RTSP the SDP can be delivered to a
   client using either RTSP DESCRIBE or a number of RTSP external
   methods, like HTTP, FTP, and email.  Based on this the SDP applies to
   how the RTSP client will see the complete session.  Thus media
   streams delivered from the RTSP server to the client, would be given
   the "a=recvonly" attribute.

   "a=recvonly" in a SDP provided to the RTSP client indicates that
   media delivery will only occur in the direction from the RTSP server
   to the client.  In SDP provided to the RTSP client that lacks any of
   the directionality attributes (a=recvonly, a=sendonly, a=sendrecv)
   would be interpreated as having a=sendrecv.  At the time of writing
   there exist no RTSP mode suitable for media traffic in the direction
   from the RTSP client to the server.  Thus all RTSP SDP SHOULD have
   a=recvonly attribute when using the PLAY mode defined in this
   document.  If future modes are defined for media in client to server
   direction, then usage of a=sendonly, or a=sendrecv may become
   suitable to indicate intended media directions.  The usage of
   "a=sendonly" or "a=sendrecv" SHOULD then be well defined.

This means that we expect SDP files for RTSP 2.0 to do contain the
directionality attribute.

Please provide feedback on this proposal.

Cheers

Magnus Westerlund

----------------------------------------------------------------------
Multimedia Technologies, Ericsson Research EAB/TVM
----------------------------------------------------------------------
Ericsson AB                | Phone  [phone]
Färögatan 6                | Mobile [phone]
SE-164 80 Stockholm, Sweden| mailto: [e-mail]
----------------------------------------------------------------------