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Re: Last Call: draft-ietf-dccp-rtp (RTP and the DatagramCongestion Control Protocol (DCCP)) to Proposed Standard

2007-06-11 08:26:13
Dan,

On 8 Jun 2007, at 18:38, Dan Wing wrote:
The draft is easy to read and understand.

1. Section 5.2, "Service Codes", refers to DCCP service codes. Several are defined in this specification (SC:RTPA, SC:RTPV, SC:RTPT, and SC:RTPO). The draft needs to say something about the relationship between the SDP media type field (the "audio" and "video" of m=audio and m=video) and the DCCP
service code -- I expect they SHOULD match.

Yes - I'll clarify.

2. I wonder if an optimization would be useful, where
"a=dccp-service-code:SC:RTPV" is assumed if the associated media type is "video" (m=video); a similar optimization seems reasonable for audio (RTPA
is assumed when the media type is audio (m=audio)).

I tend to prefer explicit to implicit, so while this would reduce the size of the SDP slightly, I'm not sure it's worthwhile.

3. I believe the draft needs to define a value for its IANA- registered DCCP
service codes, as service codes appear to be sent in the DCCP packets
themselves according to RFC4340.

The service codes sent in the DCCP packets are the 4-character ASCII strings listed (e.g. RTPV).

4. I'm surprised a=rtcp (RFC3605) lacked normative language in section 5.1. I know for UDP this is widely considered best practice. I'm not confident that we should expect DCCP will be able to avoid that quagmire. I suggest
adding something like:

   If RTCP is to be sent on a separate DCCP connection
   to RTP, the RTCP connection SHOULD use the next higher destination
   port number, unless an alternative DCCP port is signalled using the
   "a=rtcp:" attribute [13].  For improved interoperability, "a=rtcp"
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   SHOULD be used whenever an alternative DCCP port is used.
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Makes sense - I'll add this.

Cheers,
--
Colin Perkins
http://csperkins.org/



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