Steve and AVT/MMUSIC colleagues,
Our position, which has not changed since we brought up this issue to the
notice of the IETF, is similar to Tom's. We would like to see the
packetization of G.726 over RTP be made identical to what is defined for
AAL2 in the ITU I.366.2 recommendation. In this regard, we strongly
recommend that the audio MIME subtypes G726-16, G726-24, G726-32, and
G726-40 be aligned to the ITU I.366.2 recommendation. This alignment
would include the static payload type 2.
If there is a need to interoperate with the "other," non-ITU packing
scheme, we recommend that new MIME types be defined for those non-ITU
schemes rather than for ITU-compatible schemes, which we think should be
associated with the current MIME definitions.
Rajesh Kumar
Cisco Systems
At 08:33 AM 10/15/2002 -0400, Tom-PT Taylor wrote:
For your action if it affects you. There is a clash between the
packetization of G.726 over RTP and AAL2, and a proposal to adopt the AAL2
packetization universally. Comments are requested.
-----Original Message-----
From: Stephen Casner
[<mailto:casner@packetdesign.com>mailto:casner@packetdesign.com]
Sent: Tuesday, October 15, 2002 1:31 AM
To: mmusic@ietf.org
Subject: [MMUSIC] Seeking input from G.726 ADPCM implementers
The IETF Audio/Video Transport working group is seeking input from any
implementers of systems using the G.726 ADPCM audio encoding, in
particular any that use the MIME audio subtypes G726-16, G726-24, G726-32,
and G726-40 or the RTP static paylod type 2 for G726-32.
This notice is being sent to multiple mailing lists to reach as many
interested parties as possible; please reply only to avt@ietf.org.
Background:
The AVT working group is seeking to advance the Real-time Transport
Protocol (RTP) and its associated Profile for A/V Conferences (RFCs 1889
and 1890, respectively) to Draft Standard status. Two drafts have been
prepared to revise these RFCs for advancement:
draft-ietf-avt-rtp-new-11.txt
draft-ietf-avt-profile-new-12.txt
These drafts have been "tentatively approved" for publication by the
IESG. In addition, a new companion draft has been approved for
publication as a Proposed Standard to specify MIME subtype registrations
for all the encoding names defined in the RTP Profile:
draft-ietf-avt-rtp-mime-06.txt
Issue:
The packetization of G.726 audio specified in the RTP Profile packs audio
samples into octets beginning with the least-significant bit of the
octet. This is at odds with the packetization of G.726 audio for ATM AAL2
transport specified in ITU-T Recommendation I.366.2 Annex E, which begins
with the most-significant bit. Implementers of systems that operate with
both transports or gateway between the two have requested that the RTP
packetization be changed to match the I.366.2 packetization to avoid
requiring two different DSP implementations and/or translation between
packings.
Both specifications have existed for some time: I.366.2 has been an
approved standard since 1999, and the packing for the G726-32 rate has
been part of the RTP Profile drafts since 1997. Therefore,
implementations of both packings are likely to exist. Furthermore, since
the RTP Profile did not include packetizations for rates other than 32K
until 2001, some RTP implementations may have used the I.366.2 packings
for those rates. As a consequence, there is no course of action that will
make everyone happy.
Proposal:
After consultation with the IETF Transport Area Directors, it is proposed
that the draft RTP Profile packetization be changed to be consistent with
I.366.2 Annex E before it is published as an RFC. The MIME subtype
registrations for G726-16, G726-24, G726-32, and G726-40 in
draft-ietf-avt-rtp-mime-06, which refer to the specification of the
packetizations in draft-ietf-avt-profile-new-12, would therefore apply to
the changed packetization. In addition, RTP static payload type 2, which
is bound to the G726-32 encoding and packetization by
draft-ietf-avt-profile-new-12, would also change its meaning.
Consequences:
We have already heard from one vendor that has implemented the
packetizations according to the current RTP Profile draft and therefore
objects to the change. Any such systems already in the field would
produce garbled audio when interoperated with RFC-compliant
implementations, and not detect the error. This is a significant
consideration, although draft specifications are not guaranteed to remain
unchanged.
We have also been informed that the format for G.726 audio in the Voice
Profile for Internet Mail (RFC 2421/2) uses the same sample packing as
currently specified in the RTP Profile draft. This is consistent with
ITU-T Recommendation X.420 for X.400 mail. Since the VPIM systems use
MIME type audio/32KADPCM rather than audio/G726-32, there would not be
conflict in meaning if the latter were changed as proposed. However,
voicemail systems that transmit messages over RTP would be forced to
reformat the data.
********************************************************************
* We are seeking statements from interested parties both for and *
* against this proposal, particularly with motivations. *
********************************************************************
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