Hi
Thank you for the review.
Below some answers.
Aurelien
-----Message d'origine-----
De : Spencer Dawkins [mailto:spencer(_at_)wonderhamster(_dot_)org]
Envoyé : lundi 22 septembre 2008 15:19
À : SOLLAUD Aurelien RD-CORE-LAN
Cc : Cullen Jennings; ietf(_at_)ietf(_dot_)org; General Area Review
Team; Roni Even; Tom Taylor
Objet : Late Last Call Gen-ART review of
draft-ietf-avt-rfc4749-dtx-update-01
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-rfc4749-dtx-update-01
Reviewer: Spencer Dawkins
Review Date: 22-09-2008
IETF LC End Date: 19-09-2008
IESG Telechat date: (if known)
Summary: This document is almost ready for publication as
Proposed Standard.
I have a small number of questions, marked as "(review)".
Comments:
2. Background
G.729.1 supports Discontinuous Transmission (DTX), a.k.a. silence
suppression. It means that the coder includes a Voice Activity
Spencer (clarity): could you put "silence supression" in quotes here?
[AS] I can do it.
Detection (VAD) algorithm, to determine if an audio frame contains
silence or actual audio. During silence periods, the coder may
significantly decrease the transmitted bit rate by sending a small
frame called Silence Insertion Descriptor (SID), and then stop
transmission. The receiver's decoder will generate comfort noise
(CNG) according to the parameters contained in the SID.
This DTX/CNG
scheme is specified in [ITU-G.729.1-C].
3. RTP Header Usage
If DTX is used, the first packet of a talkspurt, that is, the first
packet after a silence period during which packets have not been
transmitted contiguously, SHOULD be distinguished by setting the M
Spencer (review): why not MUST here?
[AS] It is the wording from RFC 3551 (4.1).
It could be a MUST, but I saw no reason to be stronger than the RTP spec.
bit in the RTP data header to one. The M bit in all other packets
MUST be set to zero. The beginning of a talkspurt MAY be used to
adjust the playout delay to reflect changing network delays.
If DTX is not used, the M bit MUST be set to zero in all packets.
4. Payload Format
So the complete payload consists of a payload header of 1
octet (MBS
and FT fields), followed by zero or more consecutive audio
frames at
Spencer (clarity): Could you expand these names, as you
expanded "M" as "Marker" previously?
[AS] The previous sentence is "The payload format is the same as in [RFC4749],
with the option to add a SID at the end."
MBS and FT are defined in RFC 4749, so it is just a recall, to make clear that
it is the same payload header.
I could expand MBS and FT, but it would make a long sentence, and may not help
that much.
the same bit rate, followed by zero or one SID.
To be able to transport a SID alone, that is, without actual audio
frames, we assign the FT value 14 to SID. The actual SID
size (2, 3,
or 6 octets) is inferred from the payload size.
Spencer (clarity): It would be good to say this more
formally, I think (is the size just what's left over after
all other fields are parsed?) I can guess what this means,
but I'm guessing.
[AS] Yes, it can be more explicit.
5.2.1. DTX Offer-Answer Model Considerations
The offer-answer model considerations of [RFC4749] fully apply. In
this section we only define the management of the "dtx" parameter.
The "dtx" parameter concerns both sending and receiving, so both
sides of a bi-directional session MUST have the same DTX
setting. If
one party indicates it does not support DTX, DTX must be
deactivated
both ways. In other words, DTX is actually activated if, and only
if, "dtx=1" in the offer and in the answer.
A special rule apply for multicast: the "dtx" parameter becomes
Spencer (clarity): s/apply/applies/
[AS] OK. Will fix.
declarative and MUST NOT be negotiated. This parameter is
fixed, and
a participant MUST use the configuration that is provided for the
session.
7. Security Considerations
DTX introduces no new security issue.
Spencer (review): This may be obvious, but a sentence on why
you think it's true would be appreciated.
[AS] OK. I could say that DTX is already expected in RTP spec, so nothing new.
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