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Internet Engineering Task Force                                   AVT WG
INTERNET-DRAFT                                          O. Hodson / ICSI
                                                              6 May 2002
                                                  Expires: November 2002

                   RTP Payload for Interleaved Audio

Status of this Document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
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This document is a product of the IETF AVT WG.  Comments should be
addressed to the author, or the WG's mailing list at avt@ietf.org.


     This document describes a payload format for use with the
     Real-time Transport Protocol (RTP) version 2 for interleaving
     encoded audio data.  It is intended for use in audio streaming
     delay tolerant applications operating over best-effort packet
     networks.  The goal of interleaving is to disperse burst
     losses into a series of shorter losses.  The total amount of
     audio lost is not changed by interleaving, but the individual
     loss events are shorter and easier to conceal at the receiver.

Hodson                                                          [Page 1]

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                           Table of Contents

     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   3
     2. Requirements. . . . . . . . . . . . . . . . . . . . . .   3
     3. Interleaver Implementation. . . . . . . . . . . . . . .   4
     4. Payload Format Description. . . . . . . . . . . . . . .   4
     5. Relation to SDP . . . . . . . . . . . . . . . . . . . .   7
     6. Security Considerations . . . . . . . . . . . . . . . .   7
     7. Example Packet. . . . . . . . . . . . . . . . . . . . .   8
     8. Acknowledgements. . . . . . . . . . . . . . . . . . . .   8
     9. Author's Address. . . . . . . . . . . . . . . . . . . .   9
     10. References . . . . . . . . . . . . . . . . . . . . . .   9

Hodson                                                          [Page 2]

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1.  Introduction

     The Real-time Transport Protocol (RTP) [1] is the standardized
method for transporting between end-systems attached to the Internet.
The standard RTP audio profiles [2] allow a number of consecutive audio
frames to be encapsulated within a single packet.  Encapsulating
multiple audio frames within a single packet increases the latency of
communication, but results in fewer packets being transmitted and a
smaller amount of network bandwidth dedicated to IP/UDP/RTP headers.

     When a packet containing multiple audio frames is lost, or a burst
of packet losses occurs, the receiving system experiences a burst of
audio frame losses.  The receiver can apply loss concealment algorithms
to mitigate the frame losses.  However, the performance of receiver
based audio loss concealment schemes varies inversely with the length of
loss [4]. The greater the number of consecutive audio frames lost the
lower the probability of successful concealment.

     Interleaving is a technique for re-arranging the frames from an
audio source.  The technique introduces temporal separation between
adjacent frames for the purposes of transmission.  When burst frame
losses occur in an interleaved stream, they are dispersed into a series
of shorter and easier to conceal losses for the receiver to handle.

     Interleaving is employed in several proprietary audio protocols
used on the Internet and several payloads undergoing standardization
support interleaving in their RTP framing.  The format presented here is
intended to provide interleaving support for audio codecs with fixed
frames and those whose frame size is determinable by inspection of the
payload.  It's anticipated use is in broadcast style applications where
quality is more important than latency.

2.  Requirements

o To provide support for interleavers that re-arrange the ordering of
  audio frames within an RTP audio stream.

o To work with audio codecs that have fixed frame sizes or have self-
  describing frames that allow the frame size to be inferred.

o To support audio streams employing silence suppression as well as
  those that do not.

o To support codec changes mid-stream.

Hodson                                              Section 2.  [Page 3]

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3.  Interleaver Implementation

     For the purpose of clarifying the Payload Format Description we
describe the implementation of a model interleaver.  The description is
intended to be as straightforward as possible.  There are alternative
styles of interleaver implementation, some of which are provably optimal
[5] with regard to latency, however these place constraints on the
configuration parameters.

     Suppose the interleaver module at the sender has two equally sized
buffers: an input buffer and output buffer.  The input buffer holds
audio frames passed from the media encoder.  The output buffer passes
audio frames to the RTP encapsulator.  When a frame is passed to the
input buffer, a frame is removed from the output buffer.  When the input
buffer is full the output buffer is empty and they swap roles.

     We assume throughout this document that frames enter the input
buffer in order and are read from the output buffer out of order.  The
interleaver cycle length is the number of frames that can be stored in
the input buffer.  The interleaver stride length is the separation
between frames originally adjacent in the output buffer.  Consider a
full output buffer with an interleaver cycle length of 12 and a stride
length of 4.  For an input buffer containing audio frames:

                        A B C D E F G H I J K L

the frames leave the output buffer in the order:

                        A E I B F J C G K D H L

     If we denote the interleaver stride length as SL and the
interleaver cycle length as CL, and assume the frames in the output
buffer are labelled 0...CL-1, the buffer index of the n-th frame out of
the interleaver will be:

                 II[n] = n * SL mod CL + (n * SL) / CL

     The payload described in the next section describes how an RTP
interleaver places re-ordered frames within an RTP packet.  The RTP
interleaver may encapsulate any number of frames within a single packet.

4.  Payload Format Description

     Since only a limited set of interleaver stride lengths and cycle
lengths are likely to be of interest for a session, we rely on an
external mechanism, such as the Session Description Protocol [6] , to
communicate payload mappings describing these values.  An SDP format is
proposed in section 5.

Hodson                                              Section 4.  [Page 4]

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     The proposed payload format for interleaved audio is:

                    0                   1
                    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                   |IC |     II      |     PT      |

IC: Interleaver Cycle (2 bits)
     This is a counter that is incremented each time a complete cycle is
     completed at the sender.  A receiver may have multiple decode
     buffers active and this facilitates placing the incoming frames
     into the correct buffer.  The interleaver cycle has a range from 0
     to 3 and is incremented by 1 with the complete transmission of a

II: Interleaver Index (7 bits)
     This is the index of the first audio frame from output buffer,
     which is encapsulated in the current packet.  The interleaver index
     has a range from 0 to the interleaver cycle length - 1.

PT: Audio Payload (7 bits)
     This identifies the type of audio encoding of all the interleaved
     audio frames encapsulated.

     This format allows a sender to interleave the audio frames of
stream and encapsulate one or multiple frames in each packet.  When
multiple frames follow the interleaving header, the offset between each
successive frame is the cycle length CL.  When multiple frames follow
the interleaving header, they should be packed according to the their
default packing rules.  If frames are normally octet aligned, then they
MUST be octet aligned when interleaved.

     The interleaver payload is only intended for codecs with fixed
compressed frame sizes and codecs where the frame boundaries can be
determined by examining the codec data.  For sample based codecs the
number of samples per frame should be the default for the codec
concerned.  In most cases, the number of samples is 160 per frame.  This
differs from the RTP A/V profile [2] which suggests sample based codecs
should have 160 sample per frame, but frames of any length should be
accepted.  This restriction removes the need to specify the length of
each audio frame in an interleaved packet.

     The interleaved audio payload format only supports a single payload
type field.  All of the audio frames following the interleaved MUST be
of the same type.  For ease of implementation packets containing
multiple interleaved frames MUST only contain frames from one

Hodson                                              Section 4.  [Page 5]

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interleaving cycle.  Received packets that do not comply SHOULD be

     An RTP packet carry interleaved audio frames SHALL have a standard
RTP header with a payload indicating interleaved audio.  All fields,
with the exception of the timestamp, should be implemented according the
methods layed out in RTP. The timestamp field merits special
consideration because RTP uses the timestamp field to derive jitter
estimates for reporting and applications may use this value in their
playout calculation.  In the example given in section 3 , frames leave
the interleaver in the order:

                        A E I B F J C G K D H L

     If the encapsulation function only places one or two frames in each
packet there is a potential issue with the timestamp associated with
each packet.  If the timestamp is derived from the sampling time of each
frame then the timestamps will not increase monotonically, e.g. for one
frame per packet the timestamp of the fourth packet is less than the
timestamp of the third packet, ie (t(I) <= t(B)).

     For applications to be able to use interleaving without
modification to their playout calculation we propose the timestamp of
each outgoing packet is the time stamp of the frame that would have been
in the packet if interleaving had not been applied, i.e. for an
interleave with cycle length 12, stride length 4, and a packetizer
encapsulating 2 frames per packet the packets are:

                         AE, IB, FJ, CG, KD, HL

and the timestamps of the outgoing packets are:

                   t(A), t(C), T(E), t(G), t(I), t(K)

which correspond to the timestamps of the packet had interleaving not
been applied:

                         AB, CD, EF, GH, IJ, KL

     This preserves the integrity of existing RTP playout and jitter
calculations and allows interleaving to be implemented without modifying
the RTP processing in existing applications.

     A final point is the interaction with audio codecs using silence
suppression.  At the start of a new talkspurt, the Interleaver should
reset it's cycle counter (IC) and interleaving index (II) to zero.  If
the codec normally sets the marker bit in the RTP header for new
talkspurts, then it should do so when used in conjunction with

Hodson                                              Section 4.  [Page 6]

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5.  Relation to SDP

     The interleaved payload is used an external mapping mechanism may
be required for end-systems to identify a particular RTP payload as
interleaved audio.  A common mechanism for performing this is through
the Session Description Protocol (SDP) [6]. The proposed SDP mapping for
an interleaved audio payload identifier is:

                      m=audio 10000 RTP/AVP 96 14
                      a=rtpmap:96 intl/64/8

This specifies an interleaved audio stream encapsulated in RTP.  The
specified port is 10000 and the payload identifier is 96 (selected from
the dynamic payloads).  The interleaved audio is MPEG-I/II audio (static
payload 14).  The term 'intl' indicates interleaving.  The slash
separated parameters are the interleaving cycle length and the stride
length respectively.  In the example, the interleaver has an
interleaving cycle length of 64 and an interleaving stride length of 8.

6.  Security Considerations

     The security considerations and issues presented in the RTP
protocol definition [1] and the RTP sampling document [3] apply to RTP
streams carrying the interleaved audio payload.

     An additional risk with interleaved stream comes from hostile
senders transmitting an interleaved audio stream with randomly changing
interleaver cycle number and interleaver index fields.  This may cause a
receiver to allocate buffer resources and store a large number of audio
frames.  As a result, implementations SHOULD constrain the number of de-
interleaving buffers at the receiver.

Hodson                                              Section 6.  [Page 7]

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7.  Example Packet

     For an interleaver with a cycle length of 8, stride length 4, and 2
audio frames per packet, the packetized frame sequence is:

                             AE, BF, CG, DH

As an example consider a stream encoded with G.723.1 audio (RTP A/V
payload 4, frame duration 30ms, sample rate 8kHz, channels 1) that uses
this interleaver.  If the timestamp of first frame in an interleaver
sequence is 100 and this is the interleavers first cycle, the second
packet will be:

   0                   1                    2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3  4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |V=2|P|X| CC=0  |M|      PT     |        sequence number        |
   |                          timestamp = 130                      |
   |           synchronization source (SSRC) identifier            |
   | 0 |    II = 1   |   PT = 4    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   |                           G.723.1 Frame B                     |
   |                                                               |
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                           G.723.1 Frame F                     |
   |                                                               |
   |                                                               |
   +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |

8.  Acknowledgements

This document derives from an unsubmitted draft that was markedly
improved by feedback from Colin Perkins and Ross Finlayson.

Hodson                                              Section 8.  [Page 8]

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9.  Author's Address

     Orion Hodson
     International Computer Science Institute
     1947 Center Street (Suite 600)
     Berkeley CA94703 USA

10.  References

[1] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: A
     Transport Protocol for Real-Time Applications", RFC 1889.

[2] H. Schulzrinne, and S. Casner, "RTP Profile for Audio and Video
     Conferences with Minimal Control", Work In Progress, <draft-ietf-
     avt-profile-new-12.txt>, 2001.

[3] J. Rosenberg, and H. Schulzrinne, "Sampling of the Group Membership
     in RTP", RFC 2762.

[4] D.J. Goodman, G.B. Lockhard, O.J. Wasem, and W.-C. Wong, "Waveform
     Substitution Techniques for Recovering Missing Speech Segments in
     Packet Voice Communications", IEEE Transactions on Acoustics,
     Speech, and Signal Processing, pp. 1440-1448, vol. ASSP-34, no. 6,
     December 1986.

[5] J.L. Ramsey, "Realization of Optimium Interleavers", IEEE
     Transactions on Information Theory, pp. 338-345, vol. IT-16, May

[6] M. Handley, and V. Jacobson, "SDP: Session Description Protocol",
     RFC 2327.

Hodson                                             Section 10.  [Page 9]

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