[AVT] Q for 3984bis-06

["Ali C. Begen (abegen)" <abegen at cisco.com>]

Page 12 says:

         Informative note: Because H.264 allows the decoding order to 
         be different from the display order, values of RTP timestamps 
         may not be monotonically non-decreasing as a function of RTP 
         sequence numbers.  Furthermore, the value for inter-arrival 

This is not something specific for H.264 content. It applies to almost all codecs. A rewording may be beneficial IMO. 

         jitter reported in the RTCP reports may not be a trustworthy 
         indication of the network performance, as the calculation 
         rules for inter-arrival jitter (section 6.4.1 of RFC 3550) 
         assume that the RTP timestamp of a packet is directly 
         proportional to its transmission time.

What does it mean that RTP timestamp is proportional to its transmission time? By "transmission time" do you mean the instant the packet was sent? By definition, timestamp reflects the sampling instant of the first octet in the RTP packet. The packets belong to a single video frame will most likely have the same timestamp. But that does not mean that they will be transmitted at the same time.

RFC 3550 has a simple rule here and says this rule must be followed. The packets are considered in the order they arrive, not based on their seqnums in the jitter calculation. And the parameters needed to compute the interarrival jitter are clock rate, timestamp and arrival-time timestamp.

I suggest a revision for this informative note, or we can simply remove it since section 6.4.4 of RFC 3550 already provides a more-than-sufficient explanation. 

-acbegen