Re: [AVT] Working group last call: JPEG-2000 payload format

[Magnus Westerlund <[e-mail]>]

Colin Perkins skrev:
>>
>> Even if late, I think it looks okay. I am missing some recommendations
>> on not using timestamp rate below 1000.
> 
> That's a good point, and something we discussed previously. Can you
> suggest text?
> 

I would add some text to Section 4.1, on Timestamp, second paragraph:

OLD:
      As for the clock rate, senders and receivers MUST support the
      90kHz RTP timestamp rate, and MAY support other rates.  The clock
      rate MUST be negotiated at the start of the session.  When using
      SDP, it MUST be expressed using the "rtpmap" attributes.  If non-
      90kHz clock rate is to be used, it is RECOMMENDED to present not
      only a preferable clock rate with a 90kHz clock rate with a
      different RTP payload type.


NEW (second sentence):

      As for the clock rate, senders and receivers MUST support the
      90kHz RTP timestamp rate, and MAY support other rates.  RTP
      timestamp rates below 1000 Hz SHOULD NOT be used due to them
      resulting in to insufficient resolution for RTCP measurements
      based on the RTP timestamp, such as the interarrival jitter.  The
      clock rate MUST be negotiated at the start of the session.  When
      using SDP, it MUST be expressed using the "rtpmap" attributes.  If
      non-90kHz clock rate is to be used, it is RECOMMENDED to present
      not only a preferable clock rate with a 90kHz clock rate with a
      different RTP payload type.


I would also like to point out that "rate" (timestamp rate) is  missing
as a REQUIRED parameter in the media type registration. That really
should be fixed.

One proposal could be:

   rate:
        The RTP timestamp clock rate. The default rate is 90000, but
        other rates MAY be specified. Rates below 1000 Hz SHOULD NOT be
        used.


That also leads to a change in Section 8.2 regarding the mapping of the
clock rate in the rtpmap attribute:

OLD:

   o  The clock rate in the "a=rtpmap" line SHOULD be 90000.  Senders
      that wish to use a non-90kHz rate SHOULD also offer the same
      stream using a 90kHz timestamp rate with a different RTP payload
      type allowing graceful fallback to 90kHz for compatibility.

NEW:

   o  The clock rate in the "a=rtpmap" line is set according to the
      "rate" parameter.  Senders that wish to use a non-90kHz rate
      SHOULD also offer the same stream using a 90kHz timestamp rate
      with a different RTP payload type allowing graceful fallback to
      90kHz for compatibility when negotiation is possible.



Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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