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

[Colin Perkins <csp at csperkins.org>]

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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