David,
Thanks for the review - some comments inline.
On 16 Jul 2007, at 22:35, Black_David@xxxxxxx wrote:
I have been selected as the General Area Review Team (Gen-ART)
reviewer for this draft (for background on Gen-ART, please see
http://www.alvestrand.no/ietf/gen/art/gen-art-FAQ.html).
Please resolve these comments along with any other Last Call
comments you may receive.
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.
We can change "Network Address Translation (NAT)" to "Network Address
Port Translation (NAPT)" in the Introduction, and add a reference to
RFC 3022, sure. Sloppy terminology on my part.
Would changing "since opening multiple NAT pinholes can be costly" to
"since maintaining multiple NAT bindings can be costly" address your
other concern?
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 draft hints at this already in section 4:
This demultiplexing method works because the RTP payload type and
RTCP packet type occupy the same position within the packet.
If you think more is needed, please suggest wording, and I'll add it.
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.
Makes sense. I suggest adding something like the following to the
IANA considerations:
The rules for assignment of RTP RTCP Control Packet Types in the RTP
Parameters registry are updated as follows. When assigning RTP RTCP
Control Packet types, IANA is requested to assign unused values from
the range 200-254 where possible. If that range is fully occupied,
values from the range 194-199 may be used, and then values from the
range 1-191. This improves header validity checking of RTCP packets
compared to RTP packets or other unrelated packets. The values 0 and
255 are avoided for improved validity checking relative to random
packets since all-zeros and all-ones are common values.
- 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.
The guidelines in the RTP/AVP profile apply to both static and
dynamic RTP payload types, so inserting "dynamic" here would be
incorrect.
--
Colin Perkins
http://csperkins.org/
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