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RTP Payload Format for AC-3 Audio
RFC 4184

Document Type RFC - Proposed Standard (October 2005)
Authors Jason Flaks , Todd Hager , Brian Link
Last updated 2013-03-02
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Allison J. Mankin
Send notices to (None)
RFC 4184
Network Working Group                                            B. Link
Request for Comments: 4184                                      T. Hager
Category: Standards Track                             Dolby Laboratories
                                                                J. Flaks
                                                   Microsoft Corporation
                                                            October 2005

                   RTP Payload Format for AC-3 Audio

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes an RTP payload format for transporting audio
   data using the AC-3 audio compression standard.  AC-3 is a high
   quality, multichannel audio coding system that is used for United
   States HDTV, DVD, cable television, satellite television and other
   media.  The RTP payload format presented in this document includes
   support for data fragmentation.

1.  Introduction

   AC-3 [ATSC] is a high-quality audio codec (audio coding format)
   designed to encode multiple channels of audio into a low bit-rate
   format.  AC-3 achieves its large compression ratios via encoding a
   multiplicity of channels as a single entity.  Dolby Digital, which is
   a branded version of AC-3, encodes up to 5.1 channels of audio.

   AC-3 has been adopted as an audio compression scheme for many
   consumer and professional applications.  It is a mandatory audio
   codec for DVD-video, Advanced Television Standards Committee (ATSC)
   digital terrestrial television and Digital Living Network Alliance
   (DLNA) home networking, as well as an optional multichannel audio
   format for DVD-audio.

   There is a need to stream AC-3 data over IP networks.  The Internet
   Real Time Protocol (RTP) provides a mechanism for stream

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RFC 4184                  RTP Payload for AC-3              October 2005

   synchronization and hence serves as the best transport solution for
   AC-3, which is primarily used in audio-for-video applications.
   Applications for streaming AC-3 include streaming movies from a home
   media server to a display, video on demand, and multichannel Internet

   Section 2 gives a brief overview of the AC-3 algorithm.  Section 3
   specifies values for fields in the RTP header, while Section 4
   specifies the AC-3 payload format.  Section 5 discusses media types
   and SDP usage.  Security considerations are covered in Section 6,
   congestion control in Section 7, and IANA considerations in Section
   8.  References are given in Sections 9 and 10.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Overview of AC-3

   AC-3 can deliver up to 5.1 channels of audio at data rates
   approximately equal to half of one PCM channel [ATSC], [1994AC3],
   [1996AC3].  The ".1" refers to a band-limited, optional, low-
   frequency effects (LFE) channel.  AC-3 was designed for signals
   sampled at rates of 32, 44.1, or 48 kHz.  Data rates can vary between
   32 kbps and 640 kbps, depending on the number of channels and the
   desired quality.

   AC-3 exploits psycho-acoustic phenomena that cause a significant
   fraction of the information contained in a typical audio signal to be
   inaudible.  Substantial data reduction occurs via the removal of
   inaudible information contained in an audio stream.  Source coding
   techniques are further used to reduce the data rate.

   Like most perceptual coders, AC-3 operates in the frequency domain.
   A 512-point TDAC transform is taken with 50% overlap, providing 256
   new frequency samples.  Frequency samples are then converted to
   exponents and mantissas.  Exponents are differentially encoded.
   Mantissas are allocated a varying number of bits depending on the
   audibility of the associated spectral components.  Audibility is
   determined via a masking curve.  Bits for mantissas are allocated
   from a global bit pool.

2.1.  AC-3 Bit Stream

   AC-3 bit streams are organized into synchronization frames.  Each
   AC-3 frame contains a Synchronization Information (SI) field, a Bit
   Stream Information (BSI) field, and 6 audio blocks (ABs) that each
   represent 256 PCM samples for all channels.  The frame ends with an

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   optional auxiliary data field (AUX) and an error correction field
   (CRC).  The entire frame represents the time duration of 1536 PCM
   samples across all coded channels [ATSC].  AC-3 encodes audio sampled
   at 32 kHz, 44.1 kHz, and 48 kHz.  From Annex A, Part 2, of [ATSC],
   the time duration of an AC-3 frame varies with the sampling rate as

      Sampling rate          Frame duration
         48   kHz                32    ms
         44.1 kHz        approx. 34.83 ms
         32   kHz                48    ms

   Figure 1 shows the AC-3 frame format.

   |SI |BSI|  AB0  |  AB1  |  AB2  |  AB3  |  AB4  |  AB5  |AUX|CRC|

                        Figure 1. AC-3 Frame Format

   The Synchronization Information field contains information needed to
   acquire and maintain codec synchronization.  The Bit Stream
   Information field contains parameters that describe the coded audio
   service [ATSC].  Each audio block contains fields that indicate the
   use of various coding tools: block switching, dither, coupling, and
   exponent strategy.  They also contain metadata, optionally used to
   enhance the playback, such as dynamic range control.  Finally, the
   exponents and bit allocation data needed to decode the mantissas into
   audio data, and the mantissas themselves, are included.  The
   placement of these fields in an AC-3 audio block is shown in Figure
   2.  The fields shown in this figure are described in detail in
   [ATSC].  Note that field sizes vary depending on the coded data.

   |  Block  |Dither |Dynamic    |Coupling |Coupling     |Exponent |
   |  Switch |Flags  |Range Ctrl |Strategy |Coordinates  |Strategy |
   |     Exponents       | Bit Allocation  |        Mantissas      |
   |                     | Parameters      |                       |

                     Figure 2. AC-3 Audio Block Format

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3.  RTP AC-3 Header Fields

   o 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).

   o Marker (M) bit: The M bit is set to one to indicate that the RTP
     packet payload contains at least one complete AC-3 frame or
     contains the final fragment of an AC-3 frame.

   o Extension (X) bit: Defined by the RTP profile used.

   o Timestamp: A 32-bit word that corresponds to the sampling instant
     for the first AC-3 frame in the RTP packet.  Packets containing
     fragments of the same frame MUST have the same time stamp.  The
     timestamp of the first RTP packet sent SHOULD be selected at
     random.  Thereafter, the timestamp increases linearly with the
     number of samples included in each frame (i.e., by 1536 for each

4.  RTP AC-3 Payload Format

   This payload format is defined for AC-3, as defined in the main part
   and Annex D of [ATSC].  It is not defined for E-AC-3, as defined in
   Annex E of [ATSC], and it MUST NOT be used to carry E-AC-3.

   According to [RFC2736], RTP payload formats should contain an
   integral number of application data units (ADUs).  The AC-3 frame
   corresponds to an ADU, in the context of this payload format.  Each
   RTP payload MUST start with the two-byte payload header followed by
   an integral number of complete AC-3 frames or by a single fragment of
   an AC-3 frame.

   If an AC-3 frame exceeds the MTU for a network, it SHOULD be
   fragmented for transmission within an RTP packet.  Section 4.2
   provides guidelines for creating frame fragments.

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4.1.  Payload-Specific Header

   There is a two-octet Payload Header at the beginning of each payload.

4.1.1.  Payload Header

   Each AC-3 RTP payload MUST begin with the payload header shown in
   Figure 3.

                  0                   1
                  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                 |    MBZ    | FT|       NF      |

                 Figure 3.  AC-3 RTP Payload Header

   o MBZ (Must Be Zero): Bits marked MBZ SHALL be set to the value zero
     and SHALL be ignored by receivers.  The bits are reserved for
     future extensions.

   o FT (Frame Type): This two-bit field indicates the type of frame(s)
     present in the payload.  It takes the following values:

      0 - One or more complete frames.
      1 - Initial fragment of frame which includes the first 5/8ths of
          the frame.  (See Section 4.2.)
      2 - Initial fragment of frame, which does not include the first
          5/8ths of the frame.
      3 - Fragment of frame other than initial fragment.  (Note that M
          bit in RTP header is set for final fragment).

   o NF (Number of frames/fragments): An 8-bit field whose meaning
     depends on the Frame Type (FT) in this payload.  For complete
     frames (FT of 0), it is used to indicate the number of AC-3 frames
     in the RTP payload.  For frame fragments (FT of 1, 2, or 3), it is
     used to indicate the number fragments (and therefore packets) that
     make up the current frame.  NF MUST be identical for packets
     containing fragments of the same frame.

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   Figure 4 shows the full AC-3 RTP payload format.

         +-+-+-+-+-+-+-+-+-+-+-+-+-+- .. +-+-+-+-+
         | Payload | Frame | Frame |     | Frame |
         | Header  |  (1)  |  (2)  |     |  (n)  |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+- .. +-+-+-+-+

                 Figure 4. Full AC-3 RTP payload

   When receiving AC-3 payloads with FT = 0 and more than a single frame
   (NF > 1), a receiver needs to use the "frmsizecod" field in the
   Synchronization Information (syncinfo) block in each AC-3 frame to
   determine the frame's length.  That way a receiver can determine the
   boundary of the next frame.  Note that the frame length may vary from
   frame to frame.

4.2.  Fragmentation of AC-3 Frames

   The size of an AC-3 frame depends on the sample rate of the audio and
   the data rate used by the encoder (which are indicated in the
   "Synchronization Information" header in the AC-3 frame).  The size of
   a frame, for a given sample rate and data rate, is specified in Table
   5.18 ("Frame Size Code Table") of [ATSC].  This table shows that AC-3
   frames range in size from a minimum of 128 bytes to a maximum of 3840
   bytes.  If the size of an AC-3 frame exceeds the MTU size, the frame
   SHOULD be fragmented at the RTP level.  The fragmentation MAY be
   performed at any byte boundary in the frame.  RTP packets containing
   fragments of the same AC-3 frame SHALL be sent in consecutive order,
   from first to last fragment.  This enables a receiver to assemble the
   fragments in correct order.

   When an AC-3 frame is fragmented, it MAY be fragmented such that at
   least the first 5/8ths of the frame data is in the first fragment.
   This provides greater resilience to packet loss.  This initial
   portion of any frame is guaranteed to contain the data necessary to
   decode the first two blocks of the frame.  Any frame fragments other
   than those containing the first 5/8ths of frame data are only
   decodable once the complete frame is received.  The 5/8ths point of
   the frame is defined in Table 7.34 ("5/8_frame Size Table") of

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5.  Types and Names

5.1.  Media Type Registration

   This registration uses the template defined in [DRAFT-FREED] and
   follows RFC 3555 [RFC3555].

   o Type name:                         audio

   o Subtype name:                      ac3

   o Required parameters:

      rate: The RTP timestamp clock rate that is equal to the audio
         sampling rate.  Permitted rates are 32000, 44100, and 48000.

   o Optional parameters:

      channels: From a sender, the maximum number of channels present in
         the AC3 stream.  From a receiver, the maximum number of output
         channels the receiver will deliver.  This MUST be a number
         between 1 and 6.  The LFE (".1") channel MUST be counted as one
         channel.  Note that the channel order used in AC-3 differs from
         the channel order scheme in [RFC3551].  The AC-3 channel order
         scheme can be found in Table 5.8 of [ATSC].

      ptime: See RFC 2327 [RFC2327].

      maxptime: See RFC 3267 [RFC3267].

   o Encoding considerations:

         This media type is framed (see section 4.8 in [DRAFT-FREED])
         and contains binary data.

   o Security considerations:

         See Section 6 of this document.

   o Interoperability considerations:


   o Published specification:

         This payload format specification and see [ATSC].

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   o Applications that use this media type:

         Multichannel audio compression of audio and audio for video.

   o Additional Information:

         Magic number(s):
                 The first two octets of an AC-3 frame are always the
                 synchronization word, which has the hex value 0x0B77.

   o Person & email address to contact for further information:

         Brian Link <>
         IETF AVT working group.

   o Intended Usage:


   o Restrictions on usage:

         This media type depends on RTP framing, and hence is only
         defined for transfer via RTP [RFC3550].  Transport within other
         framing protocols is not defined at this time.

   Author/Change controller:

         IETF Audio/Video Transport Working Group delegated from the

5.2.  SDP Usage

   The information carried in the MIME media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [RFC2327], which is commonly used to describe RTP sessions.  When SDP
   is used to specify sessions employing AC-3, the mapping is as

   o  The Media type ("audio") goes in SDP "m=" as the media name.

   o  The Media subtype ("ac3") goes in SDP "a=rtpmap" as the encoding

   o  The required parameter "rate" also goes in "a=rtpmap" as the clock
      rate, optionally followed by the parameter "channel".

   o  The optional parameters "ptime" and "maxptime" go in the SDP
      "a=ptime" and "a=maxptime" attributes, respectively.

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   An example of the SDP data for AC-3:

            m=audio 49111 RTP/AVP 100
            a=rtpmap:100 ac3/48000/6

   Certain considerations are needed when SDP is used to perform
   offer/answer exchanges [RFC3264].

      o  The "rate" is a symmetric parameter, and the answer MUST use
         the same value or remove the payload type.

      o  The "channels" parameter is declarative and indicates, for
         recvonly or sendrecv, the desired channel configuration to
         receive, and for sendonly, the intended channel configuration
         to transmit.  All receivers are capable of receiving any of the
         defined channel configurations, and the parameter exchange
         might be used to help optimize the transmission to the number
         of channels the receiver requests.  If the "channels" parameter
         is omitted, a default maximum value of 6 is implied.

      o  The "ptime" and "maxptime" parameters are negotiated as defined
         for "ptime" in RFC 3264 [RFC3264].

6.  Security Considerations

   The payload format described in this document is subject to the
   security considerations defined in the RTP specification [RFC3550]
   and in any applicable RTP profile (e.g., [RFC3551]).  To protect the
   user's privacy and any copyrighted material, confidentiality
   protection would have to be applied.  To also protect against
   modification by intermediate entities and ensure the authenticity of
   the stream, integrity protection and authentication would be
   required.  Confidentiality, integrity protection, and authentication
   have to be provided by a mechanism external to this payload format,
   e.g., SRTP [RFC3711].

   The AC-3 format is designed so that the validity of data frames can
   determined by decoders.  A decoder that encounters a malformed frame
   discards the malformed data and conceals the errors in the audio
   output until a valid frame is detected and decoded.  This is expected
   to prevent crashes and other abnormal decoder behavior in response to
   errors or attacks.

7.  Congestion Control

   The general congestion control considerations for transporting RTP
   data apply to AC-3 audio over RTP as well.  See the RTP specification
   [RFC3550] and any applicable RTP profile (e.g., [RFC3551]).

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   AC-3 encoders may use a range of bit rates to encode audio data, so
   it is possible to adapt network bandwidth by adjusting the encoder
   bit rate in real time or by having multiple copies of content encoded
   at different bit rates.  Additionally, packing more frames in each
   RTP payload can reduce the number of packets sent and hence the
   overhead from IP/UDP/RTP headers, at the expense of increased delay
   and reduced robustness against packet losses.

8.  IANA Considerations

   A new media subtype has been assigned for AC-3; see Section 5.1.

9.  Normative References

   [RFC2119]     Bradner, S., "Key Words for use in RFCs to Indicate
                 Requirement Levels", RFC 2119, March 1997.

   [ATSC]        U.S. Advanced Television Systems Committee (ATSC),
                 "ATSC Standard: Digital Audio Compression (AC-3),
                 Revision B," Doc A/52B, June 2005.

   [RFC2327]     Handley, M. and V. Jacobson, "SDP: Session Description
                 Protocol", RFC 2327, April 1998.

   [RFC3550]     Schulzrinne, H., Casner, S., Frederick, R., and V.
                 Jacobson, "RTP: A Transport Protocol for Real-Time
                 Applications", STD 64, RFC 3550, July 2003.

   [RFC3264]     Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
                 Model with Session Description Protocol (SDP)", RFC
                 3264, June 2002.

   [RFC3267]     Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
                 "Real-Time Transport Protocol (RTP) Payload Format and
                 File Storage Format for the Adaptive Multi-Rate (AMR)
                 and Adaptive Multi-Rate Wideband (AMR-WB) Audio
                 Codecs", RFC 3267, June 2002.

   [RFC3555]     Casner, S. and P. Hoschka, "MIME Type Registration of
                 RTP Payload Formats", RFC 3555, July 2003.

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10.  Informative References

   [RFC2736]     Handley, M. and C. Perkins, "Guidelines for Writers of
                 RTP Payload Format Specifications", BCP 36, RFC 2736,
                 December 1999.

   [RFC3551]     Schulzrinne, H. and S. Casner, "RTP Profile for Audio
                 and Video Conferences with Minimal Control", STD 65,
                 RFC 3551, July 2003.

   [1994AC3]     Todd, C., et al., "AC-3: Flexible Perceptual Coding for
                 Audio Transmission and Storage," Preprint 3796,
                 Presented at the 96th Convention of the Audio
                 Engineering Society, May 1994.

   [1996AC3]     Fielder, L., et al., "AC-2 and AC-3: Low-Complexity
                 Transform-Based Audio Coding," Collected Papers on
                 Digital Audio Bit-Rate Reduction, pp. 54-72, Audio
                 Engineering Society, September 1996.

   [RFC3711]     Baugher, M., et al., "The Secure Real-time Transport
                 Protocol (SRTP)", RFC 3711, March 2004.

   [DRAFT-FREED] Freed, N. and Klensin, J., "Media Type Specifications
                 and Registration Procedures", Work in Progress, April

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Authors' Addresses

   Brian Link
   Dolby Laboratories
   100 Potrero Ave
   San Francisco, CA 94103

   Phone: +1 415 558 0200

   Todd Hager
   Dolby Laboratories
   100 Potrero Ave
   San Francisco, CA 94103

   Phone: +1 415 558 0136

   Jason Flaks
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 722 2543

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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

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