Subject: [AVT] Header formats for RTP profile for TFRC

From: Ladan Gharai <[email-masked]>
Date: Tue, 2 Aug 2005 07:40:28 -0400

Dear AVT'ers:

Following the discussion of the RTP profile for TFRC today, I would  
be interested in your feedback on the following two possible header  
formats for AVPCC?

Header I is the current header in the draft, whereas header II  
incorporates some of the comments made in the AVT session today.

Header I (current):
-------------------------
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |V=2|P|X|  CC   |M|R|    PT             |             sequence  
number            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                   
timestamp                                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      synchronization source (SSRC)  
identifier                     |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                                              send  
timestamp                                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                         
RTT                                                     |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

pros:
   - RTP header is extended by 4 octets and occasionally by 8 (if the R
         bit is set and RTT field in present).
   - the RTT timestamp field is a full 32 bit field

cons:
    - the RTT cannot be used for other time measurements
    - header compression is not fully optimum
    - payload type is a 6 bit fields

Header II (possible alternative):
--------------------------------------------
      In this alternative a 30 bit general "time" field is added,  
where the
      first two bits, "tt", indicate the type of time measurement   
i.e, RTT, decode
      timestamp, etc.,
     Therefore when the TFRC algorithm is not conveying RTT measurements
      it is possible to use this field for conveying a different type  
of time
      measurement.

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |V=2|P|X|  CC   |M|  PT             |             sequence  
number                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                   
timestamp                                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      synchronization source (SSRC)  
identifier                     |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                                              send  
timestamp                                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   tt  |                                                    
time                                                  |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

pros:
    - payload field remains at 7 bits
    - works seamlessly with header compression
    - allows the RTT field to be used for other types of time  
measurements

cons:
    - RTP header is always extended by 8 octets
     - the extra time field is a 30 bit field

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