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Internet Engineering Task Force                        AVT Working Group
Internet Draft                                              Mark Handley
draft-ietf-avt-rtp-format-guidelines-00.txt                          ISI
December 16, 1997
Expires: June 1998


      Guidelines for writers of RTP payload format specifications

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                                 ABSTRACT


         This document provides general guidelines aimed at
         assisting the authors of RTP Payload Format specifica-
         tions in deciding on good formats. These guidelines
         attempt to capture some of the experience gained with RTP
         as it evolved during its development.

1 Introduction

   This document provides general guidelines aimed at assisting the
   authors of RTP [1] Payload Format specifications in deciding on good
   formats. These guidelines attempt to capture some of the experience
   gained with RTP as it evolved during its development.

2 Background

   RTP was designed around the concept of Application Level Framing



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   (ALF), first described by Clark and Tennenhouse[2]. The key argument
   underlying ALF is that there are many different ways an application
   might be able to cope with misordered or lost packets.  These range
   from ignoring the loss, to resending the missing data (either from a
   buffer or by regenerating it), and to sending new data which super-
   sedes the missing data. The application only has this choice if tran-
   sport protocol is dealing with data in "Application Data Units"
   (ADUs). An ADU contains data that can be processed out-of-order with
   respect to other ADUs. Thus the ADU is the minimum unit of error
   recovery.

   The key property of a transport protocol for ADUs is that each ADU
   contains sufficient information to be processed by the receiver
   immediately. An example is a video stream, wherein the compressed
   video data in an ADU must be capable of being decompressed regardless
   of whether previous ADUs have been received. Additionally the ADU
   must contain "header" information detailing its position in the video
   image and the frame from which it came.

   Although an ADU need not be a packet, there are many applications for
   which a packet is a natural ADU. Such ALF applications have the great
   advantage that all packets that are received can be processed by the
   application immediately.

   RTP was designed around an ALF philosphy. In the context of a stream
   of RTP data, an RTP packet header provides sufficient information to
   be able to identify and decode the packet irrespective of whether it
   was received in order, or whether preceding packets have been lost.
   However, these arguments only hold good if the RTP payload formats
   are also designed using an ALF philosophy.

3 Guidelines

   We identify the following requirements of RTP payload format specifi-
   cations:

        o A payload format should be devised so that in the presence of
         loss, the payload can still be decoded.

        o Ideally all the contents of every packet should be possible to
         be decoded and played out irrespective of whether preceding
         packets have been lost or arrive late.

   The first of these requirements is based on the nature of the inter-
   net. Although it may be possible to engineer parts of the internet to
   produce low loss rates through careful provisioning or the use of
   non-best-effort services, as a rule payload formats should not be
   designed for these special purpose environments. Payload formats



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   should be designed to be used in the public internet with best effort
   service, and thus should expect to see moderate loss rates (for exam-
   ple, a 5% loss rate is not uncommon). Where payload formats do not
   make these assumptions, they should state this explicitly up front,
   and they will be considered special purpose payload formats, unsuit-
   able for use on the public internet without special support from the
   network infrastructure.

   The second of these requirements is more explicit about how RTP
   should cope with loss. If an RTP payload format is properly designed,
   every packet that is actually received should be useful. Typically
   this implies the following guidelines are adhered to:

        o Packet boundaries should coincide with codec frame boundaries.
         Thus a packet should normally consist of one or more complete
         codec frames.

        o A codec's minimum unit of data should never be packetised so
         that it crossed a packet boundary unless it is larger than the
         MTU.

        o If a codecs frame size is larger than the MTU, the payload
         format must not rely on IP fragmentation. Instead it must
         define its own fragmentation mechanism. Such mechanisms may
         involve codec-specific information that allows decoding of
         fragments.  Alternatively they might allow codec-independent
         packet-level forward error correction[3] to be applied that
         cannot be used with IP-level fragmentation.

   In the abstract, a codec frame (i.e., the ADU or the minimum size
   unit that has semantic meaning when handed to the codec) can be of
   arbitrary size. For PCM audio, it is one byte. For GSM audio, a frame
   corresponds to 20ms of audio. For H.261 video, it is a Group of
   Blocks (GOB), or one twelfth of a CIF video frame.

   For PCM, it does not matter how audio is packetised, as the ADU size
   is one byte. For GSM audio, arbitrary packetisation would split a
   20ms frame over two packets, which would mean that is one packet were
   lost, partial frames in packets before and after the loss are mean-
   ingless. This means that not only were the bits in the missing packet
   lost, effectively additional bits in packets that used bottleneck
   bandwidth were also lost because the receiver must throw them away.
   Instead, we would packetise GSM by including several complete GSM
   frames in a packet; typically four GSM frames are included in current
   implementations. Thus every packet received can be decoded because
   even in the presence of loss, no incomplete frames are received.

   The H.261 specification allows GOBs to be up to 3KBytes long,



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   although most of the time they are smaller than this. It might be
   thought that we should insert a group of blocks into a packet when it
   fits, and arbitrarily split the GOB over two or more packets when a
   GOB is large. In the first version of the H.261 payload format, this
   is what was done. However, this still means that there are cir-
   cumstances where H.261 packets arrive at the receiver and must be
   discarded because other packets were lost - a loss multiplier effect
   that we wish to avoid. In fact there are smaller units than GOBs in
   the H.261 bit-stream called macroblocks, but they are not identifi-
   able without parsing from the start of the GOB. However, if we pro-
   vide a little additional information at the start of each packet, we
   can re-instate information that would normally be found by parsing
   from the start of the GOB, and we can packetise H.261 by splitting
   the data stream on macroblock boundaries. This is a less obvious
   packetisation for H.261 than the GOB packetisation, but it does mean
   that a slightly smarter depacketiser at the receiver can reconstruct
   a valid H.261 bitstream from a stream of RTP packets that has experi-
   enced loss, and not have to discard any of the data that arrived.

   An additional guideline concerns codecs that require the decoder
   state machine to keep step with the encoder state machine. Many audio
   codecs such as LPC or GSM are of this form. Typically they are loss
   tolerant, in that after a loss, the predictor coefficients decay, so
   that after a certain amount of time, the predictor error induced by
   the loss will disappear. Most codecs designed for telephony services
   are of this form because they were designed to cope with bit errors
   without the decoder remaining in permanent error. Just packetising
   these formats so that packets consist of integer multiples of codec
   frames may not be optimal, as although the packet received immedi-
   ately after a packet loss can be decoded, the start of the audio
   stream produced will be incorrect (and hence distort the signal)
   because the decoder predictor is now out of step with the encoder. In
   principle, all of the decoder's internal state could be added using a
   header attached to the start of every packet, but for lower bit-rate
   encodings, this state is so substantial that the bit rate is no
   longer low bit rate. However, a compromise can usually be found,
   where a greatly reduced form of decoder state is sent in every
   packet, which does not recreate the encoders predictor precisely, but
   does reduce the magnitude and duration of the distortion produced
   when the previous packet is lost. Such compressed state is by defini-
   tion, very dependent on the codec in question. Thus we recommend:

        o Payload formats for encodings where the decoder contains
         internal data-driven state that attempts to track encoder state
         should normally consider including a small additional header
         that conveys the most critical elements of this state to reduce
         distortion after packet loss.




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4 Summary

   Designing packet formats for RTP is not a trivial task. Typically a
   detailed knowledge of the codec involved is required to be able to
   design a format that is resilient to loss, does not introduce loss
   magnification effects due to inappropriate packetisation, and does
   not introduce unnecessary distortion after a packet loss. We believe
   that considerable effort should be put into designing packet formats
   that are well tailored to the codec in question. Typically this
   requires a very small amount of processing at the sender and
   receiver, but the result can be greatly improved quality when operat-
   ing in typical internet environments.

   It is possible to design generic packetisation formats that do not
   pay attention to the issues described in this document, but such for-
   mats are only suitable for special purpose networks where packet loss
   can be avoided by careful engineering at the network layer, and are
   not suited to current best-effort networks.

5 Bibliography

   [1] H.Schulzrinne, S.Casner, R.Frederick, V. Jacobson, "Real-Time
   Transport Protocol", RFC1899.  [2] D. Clark, D. Tennenhouse, "Archi-
   tectural Considerations for a New Generation of Network Protocols"
   Proc ACM Sigcomm 90.  [3] J. Nonnenmacher, E. Biersack, Don Towsley,
   "Parity-Based Loss Recovery for Reliable Multicast Transmission",
   Proc ACM Sigcomm '97, Canne, France, 1997.
























Mark Handley                                                  [Page 5]


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