Re: [MMUSIC] RTSP 2.0 SDP issue: Directionality attribute

[[e-mail] (Dale R. Worley)]

> From: Magnus Westerlund <[e-mail]>
> 
>    "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.

I know little about RTSP though I've worked with SIP a lot.  I think
you might want to flag this odd inconsistency between RTSP and SIP:

When SDP is used in offer-answer mode, the directionality attributes
indicate directionality *from the point of view of the sender of the
SDP*.  (RFC 3264 section 5.1) So a SIP offer containing "a=recvonly"
indicates that the offerer expects to *receive* media.

When SDP is used in session-announcement mode, the directionality
attributes indicate directionality *from the point of view of the
receiver of the SDP*.  (This is not stated directly in RFC 4566,
rather it says "a=recvonly: This specifies that the tools should be
started in receive-only mode where applicable." but eventually you can
track down that "the tools" means the receiver's device.)  So an RTSP
announcement containing "a=recvonly" indicates that the announcer
expects to *send* media.

This difference is not what I would have expected.  Initially, I
assumed the text in your draft had gotten the directions reversed
relative to reality.

You might want to mention this somewhere, e.g., after the first
paragraph of D.1.5, add the paragraph

    (Note that RTSP uses SDP in session-announcement mode [RFC 4566],
    in which the directionality attributes indicate the direction of
    media flow from the point view of the interpreter of the SDP.
    This contrasts with SDP in offer-answer mode [RFC 3264], in which
    the directionality attributes indicate the direction of media flow
    from the point of view of the generator of the SDP.)

Dale