[Mp4-tech] VOP Start Codes and MPEG-4 over RTP

[Stefan A. Goor]

Hey All,
I'm hoping the answer to this question is going to be very simple.
I am working with MPEG-4 and RTSP so that the content is sent over RTP but
the client I am developing needs to know when the start new VOP is received.
I used the MPEG4IP tools to create a hint track for the content so it can be
streamed from the Darwin Streaming Server.
>From the Standard ISO/IEC 14496-2:2001, I see that the start code for VOPs
is 1b6.  Also having looked at the RFC for RTP, it says the Marker Bit/Flag
should indicate a significant event such as the start of a new frame.
Therefore, I assume that these 2 indicators should coincide.  So I looked at
a number of packets that arrived, and in general this was true, but not
always.  Here is a sample sequence recorded from an RTP Stream:
Pack No:	Marker Bit Set:	Start of Payload:
1		false			100
2		false			1b6
3		false			8e2962c9
4		false			c4582f15
5		true			1e1ef6ad		//Note that here the Marker Bit is set but the start
code is absent
6		true			1b6
7		true			1b6
8		true			1b6
9		true			1b6
.		.			.
.		.			.
.		.			.
So, I am interested to know if anyone can tell me what the Marker Bit being
set but the startcode being absent means?
Also, on a similar note, I am a bit confused regarding the start code for
the very beginning of a video bitstream.  On page 30 of ISO/IEC 14496-2:2001
it says that all start codes should be prefixed by a twenty three bit code
of all 0s except the last bit which is 1.  I have not seen this anywhere in
any of the m4v files I have, including some of the conformance bitstreams.
Is this used at all?  Finally, what is the code or pattern that should be at
the start of every bitstream.
Thanks,
Stefan