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AVT Working Group                                             J-M. Valin
Internet-Draft                                     Octasic Semiconductor
Expires: November 9, 2009                                     G. Maxwell
                                                        Juniper Networks
                                                             May 8, 2009

                 RTP Payload Format for the CELT Codec

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 9, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   CELT is an open-source voice codec suitable for use in very low delay
   audio communication applications, including Voice over IP (VoIP).
   This document describes the payload format for CELT generated bit
   streams within an RTP packet.  Also included here are the necessary
   details for the use of CELT with the Session Description Protocol
   (SDP).  At the time of this writing, the CELT bit-stream has NOT been
   finalized yet, and compatibility is usually broken with every new
   release of the codec.

Table of Contents

   1.  Conventions used in this document  . . . . . . . . . . . . . .  3
   2.  Overview of the CELT Codec . . . . . . . . . . . . . . . . . .  4
   3.  RTP payload format for CELT  . . . . . . . . . . . . . . . . .  5
     3.1.  RTP Header . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  CELT payload . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Multiple CELT frames in a RTP packet . . . . . . . . . . .  7
     3.4.  Multiple channels  . . . . . . . . . . . . . . . . . . . .  8
   4.  MIME registration of CELT  . . . . . . . . . . . . . . . . . . 10
   5.  SDP usage of CELT  . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Multichannel Mapping . . . . . . . . . . . . . . . . . . . 14
     5.2.  Low-Overhead Mode  . . . . . . . . . . . . . . . . . . . . 15
   6.  Congestion Control . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

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1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [rfc2119].

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2.  Overview of the CELT Codec

   CELT stands for "Constrained Energy Lapped Transform".  It applies
   some of the CELP principles, but does everything in the frequency
   domain, which removes some of the limitations of CELP.  CELT is
   suitable for both speech and music and currently features:

   o  Ultra-low algorithmic delay (as low as 2 ms)

   o  Full audio bandwidth (up to 20 kHz audio bandwidth)

   o  Support for both voice and music

   o  Stereo support

   o  Packet loss concealment

   o  Constant bitrates from under 32 kbps to 128 kbps and above

   o  Free software/open-source

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3.  RTP payload format for CELT

   For RTP based transportation of CELT encoded audio the standard RTP
   header [rfc3550] is followed by one or more payload data blocks.  An
   optional padding terminator may also be used.

        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
       |                         RTP Header                            |
       |                   one or more frames of CELT ....             |
       |                              ....                             |

3.1.  RTP Header

        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   |M|     PT      |       sequence number         |
       |                           timestamp                           |
       |           synchronization source (SSRC) identifier            |
       |            contributing source (CSRC) identifiers             |
       |                              ...                              |

   The RTP header is defined in the RTP specification [rfc3550].  This
   section defines how fields in the RTP header are used.

   Padding (P): 1 bit

   If the padding bit is set, the packet contains one or more additional
   padding octets at the end which are not part of the payload.  The
   last octet of the padding contains a count of how many padding octets
   should be ignored, including itself.  Padding may be needed by some
   encryption algorithms with fixed block sizes or for carrying several
   RTP packets in a lower-layer protocol data unit.

   Extension (X): 1 bit

   If the extension, X, bit is set, the fixed header MUST be followed by

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   exactly one header extension, with a format defined in Section 5.3.1.
   of [rfc3550].

   Marker (M): 1 bit

   The M bit MUST be set to zero in all packets.  The receiver MUST
   ignore the M bit.

   Payload Type (PT): 7 bits

   Payload Type (PT): The assignment of an RTP payload type for this
   packet format is outside the scope of this document; it is specified
   by the RTP profile under which this payload format is used, or
   signaled dynamically out-of-band (e.g., using SDP).

   Timestamp: 32 bits

   A timestamp representing the sampling time of the first sample of the
   first CELT frame in the RTP payload.  The clock frequency MUST be set
   to the sample rate of the encoded audio data and is conveyed out-of-
   band (e.g., as an SDP parameter).

3.2.  CELT payload

   For the purposes of packetizing the bit stream in RTP, it is only
   necessary to consider the sequence of bits as output by the CELT
   encoder [celt-website], and present the same sequence to the decoder.
   The payload format described here maintains this sequence.

   A typical CELT frame, encoded at a high bitrate, is approx. 128
   octets and the total size of the CELT frames SHOULD be kept below the
   path MTU to prevent fragmentation.  CELT frames MUST NOT be split
   across multiple RTP packets,

   An RTP packet MAY contain CELT frames of the same bit rate or of
   varying bit rates, since the bitrate for the frames is explicitly
   conveyed in band with the signal.  The encoding and decoding
   algorithm can change the bit rate at any frame boundary, with the bit
   rate change notification provided in-band.  No out-of-band
   notification is required for the decoder to process changes in the
   bit rate sent by the encoder.

   It is RECOMMENDED that sampling rates 32000, 44100, or 48000 Hz be
   used for most applications, unless a specific reason exists -- such
   as requirements for a very specific packetization time.  For example,
   51200 Hz sampling may be useful to obtain a 5 ms packetization time
   with 256-sample frames.  For compatibility reasons, the sender and
   receiver MUST support 48000 Hz sampling rate.

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   The CELT codec always produces an integer number of bytes and can
   produce any integer number of bytes, so no padding is ever required.
   Bitrate adjustment SHOULD be used instead of padding.

3.3.  Multiple CELT frames in a RTP packet

   The bitrate used by CELT is implicitly determined by the size of the
   compressed data.  When more than one frame is encoded in the same
   packet, it is not possible to determine the size of each encoded
   frame, so the information MUST be explicitly encoded.  If N frames
   are present in a packet, N compressed frame sizes need to be encoded
   at the beginning of the packet.  Each size that is less than 255
   bytes is encoded in one byte (unsigned 8-bit integer).  For sizes
   greater or equal to 255, a 0xff byte is encoded, followed by the
   size-255.  Multiple 0xff bytes are allowed if there are more than 510
   bytes transmitted.  The length is always the size of the CELT frame
   excluding the length byte itself.  The payload MUST NOT be padded,
   except in accordance with the padding bit definition in the RTP

   Below is an example of two CELT frames contained within one RTP

       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   |M|     PT      |       sequence number         |
      |                           timestamp                           |
      |         synchronization source (SSRC) identifier              |
      |         contributing source (CSRC) identifiers                |
      |                              ...                              |
      | length frame 1| length frame 2|          CELT frame 1...      |
      |         (frame 1)             |        CELT frame 2...        |
      |                          (frame 2)                            |

   The following is an example of C code that interprets the length

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      int i, N, pos;
      int sizes[MAX_FRAMES][channels];
      unsigned int total_size;
      N = 0;
      pos = 0;
      while (total_size < payload_size) {
         for (i=0;i<channels;i++) {
            int s;
            int sum;
            sum = 0;
            do {
               s = payload[pos++];
               sum += s;
               total_size += s+1;
            } while (s == 255);
            sizes[N][i] = sum;

3.4.  Multiple channels

   CELT supports both mono streams and stereo streams.  If more than two
   channels are desired, it is possible to use transmit multiple streams
   in the same packet.  In this case, the number of streams S and the
   pairing must be agreed with out-of-band negotiation such as SDP.
   Each stream can be either mono or stereo, depending on whether the
   channels are assumed to be correlated.  For example, a 5.1 surround
   could have the front-left and front-right channels in a stereo
   stream, the rear-left and rear-right channels in a separate stereo
   stream, while the center and low-frequency channels would be in
   separate mono streams.  In that example, the RTP packet would be:

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       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   |M|     PT      |       sequence number         |
      |                           timestamp                           |
      |         synchronization source (SSRC) identifier              |
      |         contributing source (CSRC) identifiers                |
      |                              ...                              |
      |  Front length |  rear length  | center length |  LFE length   |
      |                        Front stereo                           |
      |                             ...                               |
      |      ...      |        Rear stereo data...                    |
      |                             ...                               |
      |                       Center mono data...                     |
      |                             ...                               |
      |                               |      LFE mono data...         |
      |                             ...                               |

   In the case where streams for multiple channels are used with
   multiple frames of the same streams per packet, then all streams for
   a certain timestamp are encoded before all streams for the following
   timestamp.  In the case of the 5.1 example above with two frames per
   packet, the number of compressed length fields would be S*N = 8.

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4.  MIME registration of CELT

   Full definition of the MIME [rfc2045] type for CELT will be part of
   the Ogg Vorbis MIME type definition application [rfc3534].

   MIME media type name: audio

   MIME subtype: celt

   Optional parameters:

   Required parameters: to be included in the Ogg MIME specification.

   Encoding considerations:

   Security Considerations:

   See Section 6 of RFC 3047.

   Interoperability considerations: none

   Published specification:

   Applications which use this media type:

   Additional information: none

   Person & email address to contact for further information:

      Jean-Marc Valin <jean-marc.valin@octasic.com>

   Intended usage: COMMON

   Author/Change controller:

      Author: Jean-Marc Valin <jean-marc.valin@octasic.com>

      Change controller: Jean-Marc Valin <jean-marc.valin@octasic.com>

      Change controller: IETF AVT Working Group

   This transport type signifies that the content is to be interpreted
   according to this document if the contents are transmitted over RTP.
   Should this transport type appear over a lossless streaming protocol
   such as TCP, the content encapsulation should be interpreted as an
   Ogg Stream in accordance with [rfc3534], with the exception that the
   content of the Ogg Stream may be assumed to be CELT audio and CELT

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   audio only.

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5.  SDP usage of CELT

   When conveying information by SDP [rfc2327], the encoding name MUST
   be set to "CELT".  The sampling frequency is typically between 32000
   and 48000 Hz.  Implementations SHOULD support both 44100 Hz and 48000
   Hz.  The maximum bandwidth permitted for the CELT audio is encoded
   using the "b=AS:" header, as explained in SDP [rfc2327].

   The SDP parameters have the following interpretation with respect to

      b=AS: The maximum bandwidth (in kbit/s) allowed for CELT,
      excluding the header overhead.  The default is 64 kbit/s.

      ptime: The desired packetization time.  The sender SHOULD choose a
      number of frames per packet that corresponds to the smallest
      packetization time greater or equal to the specified ptime for the
      selected frame size.  The default is 20 ms as specified in

      maxptime: The maximum packetization time desired.  If the maximum
      is lower than the smallest packetization time determined from the
      chosen frame size (as described above), then that packtization
      time SHOULD be used despite the maxptime value.  The default is
      "no maximum".

   CELT-specific parameters can be given via the "a=fmtp:" directive.
   Several parameters can be given in a single a=fmtp line provided that
   they are separated by a semi-colon.  The following parameters are
   defined for use in this way:

      frame-size: The frame size is the duration of each frame in
      samples.  If more than one frame size is supported, a comma-
      separated list can be used.  It is possible to use "any" to denote
      that all even frame sizes are supported.  The default is 480.

      mapping: Optional string describing the multi-channel mapping.

   Because the frame-size is not transmitted in-band, an SDP answer MUST
   contain only one frame-size, even if multiple frame sizes were

   The selected frame-size values MUST be even.  They SHOULD be
   divisible by 8 and have a prime factorization which consists only of
   2, 3, or 5 factors.  For example, powers-of-two and values such as
   160, 320, 240, and 480 are recommended.  Implementations MUST support
   receiving and sending the default value of 480, and if the size 480
   is supported it MUST be offered.  Implementations SHOULD also support

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   frame sizes of 256 and 512 since these are the ones that lead to the
   lowest complexity.  When frame sizes that are powers of two are
   supported, they SHOULD be listed first in the offer and chosen over
   non powers of two in the answer.

   Care must be taken when setting the value of ptime: and b=AS: so that
   the RTP packet size does not exceed the path MTU.

   An example of the media representation in SDP for offering a single
   channel of CELT at 48000 samples per second might be:

      m=audio 8088 RTP/AVP 97

      a=rtpmap:97 CELT/48000

   Note that the RTP payload type code of 97 is defined in this media
   definition to be 'mapped' to the CELT codec at a 48kHz sampling
   frequency using the 'a=rtpmap' line.  Any number from 96 to 127 could
   have been chosen (the allowed range for dynamic types).  If there is
   more than one channel being encoded the rtpmap MUST specify the
   channel count.

   The following example illustrates the case where the offerer cannot
   receive more than 64 kbit/s.

      m=audio 8088 RTP/AVP 97


      a=rtmap:97 CELT/48000

   In this case, if the remote party agrees, it should configure its
   CELT encoder so that it does not use modes that produce more than 64
   kbit/s.  Note that the "b=" constraint also applies on all payload
   types that may be proposed in the media line ("m=").

   The following example demonstrates the use of the a=fmtp: parameters:

      m=audio 8008 RTP/AVP 97

      a=ptime: 21

      a=rtpmap:97 CELT/44100

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      a=fmtp:97 frame-size=512;

   This examples illustrate an offerer that wishes to receive a CELT
   stream at 44100 Hz, by packing two 512-sample frames in each packet.

5.1.  Multichannel Mapping

   When more than two channels are used, a mapping parameter MUST be
   provided.  The mapping parameter is defined as comma separated list
   of integers which specify the number of channels contained in each
   CELT stream, OPTIONALLY followed by a '/' and a comma separated list
   of channel identifiers, then OPTIONALLY another '/' and a string
   which provides an application specific elaboration on any speaker-
   feed definitions.  The channels per stream entries MUST be either 1
   or 2.  The total number of channels is indicated by the sum of the
   channels per stream entries.  The sum of the channel counts MUST be
   equal to the total number of channels.

   Channel identifiers are short alphanumeric strings.  Each identifier
   MUST begin with a letter indicating the type of channel.  'A' MUST be
   used to indicate an ambisonic channel, 'S' to indicate a speaker-feed
   channel, or 'O' indicating other usage.

   A channel identifier MAY be repeated, but the meaning of such
   repetition is application specific.  Applications SHOULD attempt to
   utilize channel identifiers such that mixing all identical
   identifiers would produce a reasonable result.

   Non-surround usage such as individual performer tracks, effect send,
   "order wire", or other administrative channels may be given
   application specific identifiers which MUST not conflict with the
   identifiers defined in this draft.  These identifiers SHOULD begin
   with S if it would be sensible to include them in a mono-downmix, or
   O if it would be most sensible to exclude them from a mono-downmix.
   An example usage might be mapping=2,1,2,1,1/

   Ambisonic channels MUST follow the Furse-Malham naming and weighing
   conventions for up to third order spherical[Ambisonic].  Higher order
   ambisonic support is application defined but MUST NOT reuse any of
   WXYZRSTUVKLMNOPQ for higher order components.  For example, second
   order spherical ambisonics SHOULD use the mapping
   "mapping=1,1,1,1,1,1,1,1,1/AW,AX,AY,AZ,AR,AS,AT,AU,AV".  Any set of
   Ambisonic channels MUST contain at least one "AW" channel.

   Speaker-feed identifiers are named based on the intended speaker
   locations.  "L", "R" for the left and right speakers, respectively,

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   in conventional stereo or the front left and right in 4, 5, 5.1, or
   7.1 channel surround.  "LR", "RR" for the left and right rear
   speakers in 4,5 or 5.1 channel surround.  C" is used for a center
   channel, "MLFE" for a low frequency extension channel.  "LS", "RS"
   for the side channels in 7.1 channel surround.  Additional speaker-
   feeds are application specific but should not reuse the prior
   identifiers.  For 5.1 surround in non-ambisonic form the mapping
   SHOULD be "mapping=2,2,1,1/L,R,LR,RR,C,MLFE/ITU-RBS.775-1".  When
   only one or two channels are used, the mapping parameter MAY be
   omitted, in which case the default mapping is used.  For one channel,
   the default is "mapping=1/C", while for two channels, the default is

   For example a stereo configuration might signal:

      m=audio 8008 RTP/AVP 97

      a=ptime: 5

      a=rtpmap:97 CELT/44100/2

      a=fmtp:97 frame-size=256;

   Which specifies a single two-channel CELT stream according to the
   default mapping.

5.2.  Low-Overhead Mode

   A low-overhead mode is defined to make more efficient use of
   bandwidth when transmitting CELT frames.  In that mode none of the
   length values need to be transmitted.  One the a=fmtp: parameter low-
   overhead: is defined and contains a single frame size, followed by a
   '/', followed by the number of frames (per channel) per packet,
   followed by a '/', followed by a comma-separated list of the number
   of bytes per frame for each stream defined in the channel mapping.
   The frame-size: parameter MUST not be specified and SHOULD be ignored
   if encountered in an SDP offer or answer.  The ptime:, maxptime: and
   b=AS: parameters SHOULD also be ignored since the low-overhead:
   parameter makes them redundant.  When the low-overhead: parameter is
   specified, the length of each frame MUST NOT be encoded in the
   payload and the bit-rate MUST NOT be changed during the session.

   For example a low-overhead surround configuration could be signaled

      m=audio 8008 RTP/AVP 97

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      a=ptime: 5

      a=rtpmap:97 CELT/48000/6

      a=fmtp:97 low-overhead=256/1/86,86,43,30;mapping=2,2,1,1/

   In this example, 4 bytes per packet would be saved.  This corresponds
   to a 6 kbit/s reduction in the overhead, although the 60 kbit/s
   overhead of the IP, UDP and RTP headers is still present.

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6.  Congestion Control

   CELT allows for bitrate adjustment in one byte per frame increments
   without any signaling requirement or overhead.  Applications SHOULD
   utilize congestion control to regulate the transmitted bitrate.  In
   some applications it may make sense to increase the packetization
   interval rather than decreasing the codec bitrate.  Congestion
   control implementations should consider the users differential
   tolerance for high latency and low quality.

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7.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [rfc3550], and in any applicable RTP profile.  The main
   security considerations for the RTP packet carrying the RTP payload
   format defined within this memo are confidentiality, integrity and
   source authenticity.  Confidentiality is achieved by encryption of
   the RTP payload.  Integrity of the RTP packets through suitable
   cryptographic integrity protection mechanism.  Cryptographic system
   may also allow the authentication of the source of the payload.  A
   suitable security mechanism for this RTP payload format should
   provide confidentiality, integrity protection and at least source
   authentication capable of determining if an RTP packet is from a
   member of the RTP session or not.

   Note that the appropriate mechanism to provide security to RTP and
   payloads following this memo may vary.  It is dependent on the
   application, the transport, and the signalling protocol employed.
   Therefore a single mechanism is not sufficient, although if suitable
   the usage of SRTP [rfc3711] is recommended.  Other mechanism that may
   be used are IPsec [rfc4301] and TLS [rfc5246] (RTP over TCP), but
   also other alternatives may exist.

   This RTP payload format and its media decoder do not exhibit any
   significant non-uniformity in the receiver-side computational
   complexity for packet processing, and thus are unlikely to pose a
   denial-of-service threat due to the receipt of pathological data.
   Nor does the RTP payload format contain any active content.

   Because this format supports VBR operation small amounts of
   information about the transmitted audio may be leaked by a length
   preserving cryptographic transport.  Accordingly, when CELT is used
   inside a secure transport the sender SHOULD restrict the use of VBR
   to congestion control purposes.

   CELT implementations will typically exhibit tiny content-sensitive
   encoding time variances.  Since transmission is usually triggered by
   an accurate hardware clock and the encoded data is typically
   transmitted as soon as encoding is complete this variance may result
   in a small amount of additional frame to frame jitter which could be
   measured by a third-party.  Encrypted implementations SHOULD transmit
   packets at fixed intervals to avoid the possible information leak.

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8.  Acknowledgments

   The authors would also like to thank the following people for their
   input: Timothy B. Terriberry, Ben Schwartz, Alexander Carot, Thorvald
   Natvig, Brian West, Steve Underwood, and Anthony Minessale.

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9.  References

9.1.  Normative References

   [rfc2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119.

   [rfc3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for real-time
              applications", RFC 3550.

   [rfc2045]  "Multipurpose Internet Mail Extensions (MIME) Part One:
              Format of Internet Message Bodies", RFC 2045,
              November 1998.

   [rfc2327]  Jacobson, V. and M. Handley, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

   [rfc3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control.", RFC 3551,
              July 2003.

   [rfc3534]  Walleij, L., "The application/ogg Media Type", RFC 3534,
              May 2003.

   [rfc3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [rfc4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [rfc5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

9.2.  Informative References

              Xiph.Org Foundation, "The CELT ultra-low delay audio
              codec", CELT website http://www.celt-codec.org/.

              Malham, D., "Higher order Ambisonic systems", Paper http:/
              higher_order_ambisonics.pdf, December 2003.

Valin & Maxwell         Expires November 9, 2009               [Page 20]

Internet-Draft       draft-valin-celt-rtp-profile-00            May 2009

Authors' Addresses

   Jean-Marc Valin
   Octasic Semiconductor
   4101, Molson Street, suite 300
   Montreal, Quebec  H1Y 3L1

   Email: jean-marc.valin@octasic.com

   Gregory Maxwell
   Juniper Networks
   2251 Corporate Park Drive, Suite 100
   Herndon, VA  20171-1817

   Email: gmaxwell@juniper.net

Valin & Maxwell         Expires November 9, 2009               [Page 21]

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