Subject: Protocol Action: A More Loss-Tolerant RTP Payload Format for MP3 Audio to Proposed Standard

From: The IESG <[email-masked]>
Date: Thu, 12 Apr 2001 15:00:29 -0400

The IESG has approved the Internet-Draft 'A More Loss-Tolerant RTP
Payload Format for MP3 Audio' <draft-ietf-avt-rtp-mp3-06.txt> as a
Proposed Standard.  This document is the product of the Audio/Video
Transport Working Group.  The IESG contact persons are Allison Mankin
and Scott Bradner.

Technical Summary

While the RTP payload format defined in RFC 2250 [2] is generally
applicable to all forms of MPEG audio or video, it is sub-optimal for
MPEG 1 or 2, layer III audio (commonly known as "MP3").  The reason for
this is that an MP3 frame is not a true "Application Data Unit" - it
contains a back-pointer to data in earlier frames, and so cannot be
decoded independently of these earlier frames.  Because RFC 2250 defines
that packet boundaries coincide with frame boundaries, it handles packet
loss inefficiently when carrying MP3 data.  The loss of an MP3 frame
will render some data in previous (or future) frames useless, even if
they are received without loss.

This document specifies an alternative RTP payload format for MP3
audio.  This format uses a data-preserving rearrangement of the original
MPEG frames, so that packet boundaries now coincide with true MP3
"Application Data Units", which can also (optionally) be rearranged in
an interleaving pattern.  This new format is therefore more
data-efficient than RFC 2250 in the face of packet loss.

This document also registers the mime type audio/mpa-robust.

Working Group Summary

This document is a product of the Audio Video Transport (AVT)
Working Group.  The working group achieved strong consensus for
advancing the document.  During the last call period, the working
group noticed technical concerns with the definition of the
timestamp resolution and use of the RTP M-bit, and the author
agreed with the working group assessment and revised the document
accordingly.

Protocol Quality

The document was reviewed for the IESG by Allison Mankin.
At least one implementation of the payload is known.