Document: draft-ietf-avt-rtp-and-rtcp-mux-05.txt
Reviewer: David Black
Review Date: 16 July 2007
IETF LC End Date: 17 July 2007

Summary:
This draft is on the right track, but has open issues,
described in the review.

Comments:

The draft is generally in good shape, although one needs to
be an RTP expert to understand all the details.

The only "open issue" is the missing instructions to IANA
for RTCP packet type registration - from a technical standpoint,
this is a nit, but getting it right is of sufficient importance
to avoidance of future restrictions RTP/RTCP multiplexing that
I've flagged it as an open issue.

Everything else in this review is an editorial comment, although
the absence of the explanation of the mechanics of the RTP/RTCP
conflicts makes that section of the draft difficult to read.

Section 1 talks about NAT (Network Address Translation) as a
motivation, but the real motivation appears to be NAPT (Network
Address Port Translation).  This ought to be discussed, and I
strongly suggest an Informative reference to RFC 3022.  The 
term "NAT pinhole" also needs to be explained here to connect
the problems caused by two UDP ports to NAPT usage, and it
may be useful to mention firewall pinholes as a related issue.

Section 4 should explain the mechanics of the RTP/RTCP conflict:
- The RTCP PT is bits 8-15.
- The RTP PT is bits 9-15.
- RTP uses bit 8 in that word for a M (Marker) bit that
	may be on or off.
The latter item is the cause for needing to check for whether the
RTCP PT conflicts with either the RTP PT or the RTP PT + 128.  

The last 2 paragraphs in Section 4 before the final Note in that
section need attention:
- The paragraph on registration of new RTCP packet types needs to
	instruct IANA on what rules to enforce.  The use of "SHOULD"
	in the current text is not sufficient, instead this needs to
	be restated as instructions to IANA on how to assign RTCP
	packet types and noted in the IANA considerations section.
- In this text in the second paragraph:

   Given these constraints, it is RECOMMENDED to follow the guidelines
   in the RTP/AVP profile [7] for the choice of RTP payload type values,

	Insert the word "dynamic" between "the choice of" and "RTP
	payload type values" for clarity.