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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                         S. Ikonin
Request for Comments: 6262                                    SPIRIT DSP
Category: Standards Track                                    August 2011
ISSN: 2070-1721


               RTP Payload Format for IP-MR Speech Codec

Abstract

   This document specifies the payload format for packetization of
   SPIRIT IP-MR encoded speech signals into the Real-time Transport
   Protocol (RTP).  The payload format supports transmission of multiple
   frames per packet and introduces redundancy for robustness against
   packet loss and bit errors.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6262.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this



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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................2
   2. IP-MR Codec Description .........................................3
   3. Payload Format ..................................................4
      3.1. RTP Header Usage ...........................................4
      3.2. RTP Payload Structure ......................................4
      3.3. Speech Payload Header ......................................5
      3.4. Speech Payload Table of Contents ...........................6
      3.5. Speech Payload Data ........................................6
      3.6. Redundancy Payload Header ..................................7
      3.7. Redundancy Payload Table of Contents .......................8
      3.8. Redundancy Payload Data ....................................8
   4. Payload Examples ................................................9
      4.1. Payload Carrying a Single Frame ............................9
      4.2. Payload Carrying Multiple Frames with Redundancy ..........10
   5. Congestion Control .............................................11
   6. Security Considerations ........................................12
   7. Payload Format Parameters ......................................13
      7.1. Media Type Registration ...................................13
      7.2. Mapping Media Type Parameters into SDP ....................14
   8. IANA Considerations ............................................14
   9. Normative References ...........................................15
   Appendix A. Retrieving Frame Information ..........................16
      A.1. get_frame_info.c ..........................................16

1.  Introduction

   This document specifies the payload format for packetization of
   SPIRIT IP-MR encoded speech signals into the Real-time Transport
   Protocol (RTP).  The payload format supports transmission of multiple
   frames per packet and introduces redundancy for robustness against
   packet loss and bit errors.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].





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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


2.  IP-MR Codec Description

   IP-MR is a wideband speech codec designed by SPIRIT for conferencing
   services over packet-switched networks such as the Internet.

   IP-MR is a scalable codec.  This means that the source not only has
   the ability to change transmission rate on the fly, but the gateway
   is also able to decrease bandwidth at any time without performance
   overhead.  There are 6 coding rates from 7.7 to 34.2 kbps available.

   The codec operates on a frame-by-frame basis with a frame size of 20
   ms at a 16 kHz sampling rate with a total end-to-end delay of 25 ms.
   Each compressed frame is represented as a sequence of layers.  The
   first (base) layer is mandatory while the other (enhancement) layers
   can be safely discarded.  Information about the particular frame
   structure is available from the payload header.  In order to adjust
   outgoing bandwidth, the gateway MUST read the frame(s) structure from
   the payload header, define which enhancement layers to discard, and
   compose a new RTP packet according to this specification.

   In fact, not all bits within a frame are equally tolerant to
   distortion.  IP-MR defines 6 classes ('A'-'F') of sensitivity to bit
   errors.  Any damage of class 'A' bits causes significant
   reconstruction artifacts while the loss in class 'F' may not even be
   perceived by the listener.  Note that only the base layer in a
   bitstream is represented as a set of classes.

   The IP-MR payload format allows frame duplication through the packets
   to improve robustness against packet loss (Section 3.6).  The base
   layer can be retransmitted completely or in several sensitive
   classes.  Enchantment layers are not retransmittable.

   The fine-grained redundancy in conjunction with bitrate scalability
   allows applications to adjust the trade-off between overhead and
   robustness against packet loss.  Note that this approach is supported
   natively within a packet and requires no out-of-band signals or
   session-initialization procedures.

   The main IP-MR features are as follows:

   o  High-quality wideband speech codec.

   o  Bitrate scalable with 6 average rates from 7.7 to 34.2 kbps.

   o  Built-in discontinuous transmission (DTX) and comfort noise
      generation (CNG) support.





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   o  Flexible in-band redundancy control scheme for packet-loss
      protection.

3.  Payload Format

   The payload format consists of the RTP header and the IP-MR payload.

3.1.  RTP Header Usage

   The format of the RTP header is specified in [RFC3550].  This payload
   format uses the fields of the header in a manner consistent with that
   specification.

   The RTP timestamp corresponds to the sampling instant of the first
   sample encoded for the first frame-block in the packet.  The
   timestamp clock frequency SHALL be 16 kHz.  The duration of one frame
   is 20 ms, which corresponds to 320 samples per frame.  Thus, the
   timestamp is increased by 320 for each consecutive frame.  The
   timestamp is also used to recover the correct decoding order of the
   frame-blocks.

   The RTP header marker bit (M) SHALL be set to 1 whenever the first
   frame-block carried in the packet is the first frame-block in a
   talkspurt (see definition of talkspurt in Section 4.1 of [RFC3551]).
   For all other packets, the marker bit SHALL be set to zero (M=0).

   The assignment of an RTP payload type for the format defined in this
   memo is outside the scope of this document.  The RTP profiles in use
   currently mandate binding the payload type dynamically for this
   payload format.  This is basically necessary because the payload type
   expresses the configuration of the payload itself, i.e., basic or
   interleaved mode, and the number of channels carried.

   The remaining RTP header fields are used as specified in [RFC3550].

3.2.  RTP Payload Structure

   The IP-MR payload is composed of two payloads, one for current speech
   and one for redundancy.  Both payloads are represented in this form:
   Header, Table of Contents (TOC), and Data.  Redundancy payload
   carries data for preceding and pre-preceding packets.

     +--------+-----+----------------------+- - - - +- -  +- - - - - +
     | Header | TOC | Data                 | Header | TOC | Data     |
     +--------+-----+----------------------+- - - - +- -  +- - - - - +
     |<- Speech -------------------------->|<- Redundancy (opt) ---->|





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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


3.3.  Speech Payload Header

   This header carries parameters that are common for all frames in the
   packet:

                        0                   1
                        0 1 2 3 4 5 6 7 8 9 0 1
                       +-+-+-+-+-+-+-+-+-+-+-+-+
                       |T| CR  | BR  |D|A|GR |R|
                       +-+-+-+-+-+-+-+-+-+-+-+-+

   o  T (1 bit): Reserved.  MUST always be set to 0.  Receiver MAY
      discard packet if the 'T' bit is not equal to 0.

   o  CR (3 bits): Coding rate index - top enchantment layer available.
      The CR value 7 (NO_DATA) indicates that there is no speech data
      (and thus no speech TOC) in the payload.  This MAY be used to
      transmit redundancy data only.

   o  BR (3 bits): Base rate index - base layer bitrate.  Speech payload
      can be scaled to any rate index between BR and CR.  Packets with
      BR = 6 or BR > CR MUST be discarded.  Redundancy data is also
      considered to have a base rate of BR.

   o  D (1 bit): Reserved.  MUST always be set to 1.  Receiver MAY
      discard packet if the 'D' bit is zero.

   o  A (1 bit): Byte alignment.  The value of 1 specifies that padding
      bits were added to enable each compressed frame (3.5) to start
      with the byte (8-bit) boundary.  The value of 0 specifies
      unaligned frames.  Note that the speech payload is always padded
      to the byte boundary independently on an 'A' bit value.

   o  GR (2 bits): Number of frames in packet (grouping size).  Actual
      grouping size is GR + 1; thus, the maximum grouping supported is
      4.

   o  R (1 bit): Redundancy presence.  Value of 1 indicates redundancy
      payload presence.

   Note that the values of 'T' and 'D' bits are fixed; any other values
   are not allowed by specification.  Padding bits ('P' bits) MUST
   always be set to zero.








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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


   The following table defines the mapping between rate index and rate
   value:

                    +------------+--------------+
                    | rate index | avg. bitrate |
                    +------------+--------------+
                    |      0     |   7.7 kbps   |
                    |      1     |   9.8 kbps   |
                    |      2     |  14.3 kbps   |
                    |      3     |  20.8 kbps   |
                    |      4     |  27.9 kbps   |
                    |      5     |  34.2 kbps   |
                    |      6     |  (reserved)  |
                    |      7     |   NO_DATA    |
                    +------------+--------------+

   The value of 6 is reserved.  If receiving this value, the packet MUST
   be discarded.

3.4.  Speech Payload Table of Contents

   The speech TOC is a bitmask indicating the presence of each frame in
   the packet.  TOC is only available if the 'CR' value is not equal to
   7 (NO_DATA).

                               0 1 2 3
                              +-+-+-+-+
                              |E|E|E|E|
                              +-+-+-+-+
                              |<----->| <-- #(GR+1)

   o  E (1 bit): Frame existence indicator.  The value of 0 indicates
      speech data is not present for the corresponding frame.  The IP-MR
      encoder sets the 'E' flag to 0 for the periods of silence in DTX
      mode.  Applications MUST set this bit to 0 if the frame is known
      to be damaged.

3.5.  Speech Payload Data

   Speech data contains (GR+1) compressed IP-MR frames (20 ms of data).
   A compressed frame has a length of zero if the corresponding TOC flag
   is zero.

   The beginning of each compressed frame is aligned if the 'A' bit is
   nonzero, while the end of the speech payload is always aligned to a
   byte (8-bit) boundary:





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   +- - -+------------+------------+------------+------------+
   | TOC | Frame1     | Frame2     | Frame3     | Frame4     |
   +- - -+------------+------------+------------+------------+   ALWAYS
         |<- aligned  |<- aligned  |<- aligned  |<- aligned  |<- ALIGNED

   Marked regions MUST be padded only if the 'A' bit is set to '1'.

   The compressed frame structure is as follows:

   |<---- sensitive classes ------>|<----- enchantment layers -------->|
   +-------------------------------+----+-----+------+- - - - - +------+
   | L1 (Base Layer)               | L2 | L3  | L4   |          | LN   |
   +-------------------------------+----+-----+------+- - - - - +------+
   |<- A --->|<- B ->| ... |<- F ->|                                   |
   |<- BR rate ------------------->|                                   |
   |<- CR rate ------------------------------------------------------->|

   Appendix A of this document provides a helper routine written in "C"
   that MUST be used to extract sensitivity classes and bounds for the
   enchantment layers from the compressed frame data.

3.6.  Redundancy Payload Header

   The redundancy payload presence is signaled by the 'R' bit of the
   speech payload header.  The redundancy header is composed of two
   fields of 3 bits each:

                               0 1 2 3 4 5
                              +-+-+-+-+-+-+
                              | CL1 | CL2 |
                              +-+-+-+-+-+-+

   The 'CL1' and 'CL2' fields both specify the sensitivity classes
   available for preceding and pre-preceding packets respectively.

                    +-------+--------------------+
                    |  CL   | Redundancy classes |
                    |       |      available     |
                    +-------+--------------------+
                    |   0   |       NONE         |
                    |   1   |        A           |
                    |   2   |        A-B         |
                    |   3   |        A-C         |
                    |   4   |        A-D         |
                    |   5   |        A-E         |
                    |   6   |        A-F         |
                    |   7   |    (reserved)      |
                    +-------+--------------------+



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   A receiver can reconstruct the base layer of preceding packets
   completely (CL=6) or partially (0<CL< 6) based on the sensitivity
   classes delivered.  A decoder MUST discard the redundancy payload if
   'CL' is equal to 0 or 7.

   Note that the index of the base rate and grouping parameter is not
   transmitted for the redundancy payload.  Applications MUST assume
   that 'BR' and 'GR' are the same as for the current packet.

3.7.  Redundancy Payload Table of Contents

   The redundancy TOC is a bitmask indicating the presence of each frame
   in the redundancy payload.  The redundancy TOC is only available if
   the 'CL' value is not equal to 0 or 7.

                 0 1 ...
                +-+-+-+-+-+-+-+-+
                |E|E|E|E|E|E|E|E|
                +-+-+-+-+-+-+-+-+
                |       |<----->| pre-preceding payload #(GR+1)
                |<----->| preceding payload #(GR+1)

   o  E (1 bit): Redundancy frame existence indicator.  The value of 0
      indicates redundancy data is not present for corresponding frame.

3.8.  Redundancy Payload Data

   IP-MR defines 6 classes ('A'-'F') of sensitivity to bit errors.  Any
   damage of class 'A' bits causes significant reconstruction artifacts
   while the loss in class 'F' may not even be perceived by the
   listener.  Note that only the base layer in a bitstream is
   represented as a set of classes.  Together, the sensitivity classes'
   approach and redundancy allow IP-MR duplicate frames through the
   packets to improve robustness against packet loss.

   Redundancy data carries a number of sensitivity classes for preceding
   and pre-preceding packets as indicated by the 'CL1' and 'CL2' fields
   of the redundancy header.  The sensitivity classes' data is available
   individually for each frame only if the corresponding 'E' bit of the
   redundancy TOC is nonzero:

   +---+---+----+----|-----+-----+-----+-----+-----+-----+-----+
   |A-C|A-B|1000|1001|cl_A1|cl_B1|cl_C1|cl_A1|cl_B1|cl_A4|cl_B4|
   +---+---+----+----|-----+-----+-----+-----+-----+-----+-----+
   |<- CL >|<- TOC ->|<- preceding --->|<- pre-preceding ----->|






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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


   Redundancy data is only available if the base rates (BRs) and coding
   rates (CRs) of preceding and pre-preceding packets are the same as
   for the current packet.

   A receiver MAY use redundancy data to compensate for packet loss
   (note that in this case, the 'CL' field MUST also be passed to the
   decoder).  The helper routine provided in Appendix A MUST be used to
   extract sensitivity classes' length for each frame.  The following
   pseudocode describes the sequence of operations:

      int sensitivityBits[numOfRedundancyFrames][6];
      int redundancyBits [numOfRedundancyFrames];
      for(i = 0 ; i < numOfRedundancyFrames; i++) {
          GetFrameInfo(CR, BR, pRedundancyPayloadData, dummy,
                       sensitivityBits[i], dummy);
          redundancyBits[i] = 0;
          for(j = 0; j < CL[i]; j++ ) {
               redundancyBits[i] += sensitivityBits[i][j];
          }
          flushBits(pRedundancyPayloadData, redundancyBits[i]);
      }

4.  Payload Examples

   This section provides detailed examples of the IP-MR payload format.

4.1.  Payload Carrying a Single Frame

   The following diagram shows a typical IP-MR payload carrying one
   (GR=0) non-aligned (A=0) speech frame without redundancy (R=0).  The
   base layer is coded at 7.8 kbps (BR=0) while the coding rate is 9.7
   kbps (CR=1).  The 'E' bit value of 1 signals that compressed frame
   bits s(0) - s(193) are present.  There is a padding bit 'P' to
   maintain speech payload size alignment.

















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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|CR=1 |BR=0 |1|0|0 0|0|1|s(0)                                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       s(193)|P|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2.  Payload Carrying Multiple Frames with Redundancy

   The following diagram shows a payload carrying 3 (GR=2) aligned (A=1)
   speech frames with redundancy (R=1).  The TOC value of '101'
   indicates speech data present for the first (bits sp1(0)-sp1(92)) and
   third frames (bits sp3(0)-sp3(171)).  There are no enchantment layers
   because the base and coding rates are equal (BR=CR=0).  The padding
   bit 'P' is inserted to maintain necessary alignment.

   The redundancy payload present for both preceding and pre-preceding
   payloads (CL1 = A-B, CL2=A), but redundancy data is only available
   for 5 (TOC='111011') of 6 (2*(GR+1)) frames.  There is redundancy
   data of 20, 39, and 35 bits for each of the three frames of the
   preceding packet and 15 and 19 bits for the two frames of the pre-
   preceding packet.


















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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|CR=0 |BR=0 |1|1|1 0|1|1 0 1|P|sp1(0)                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  sp1(92)|P|P|P|sp3(0)                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                               sp3(171)|P|P|P|P|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |CL1=2|CL2=1|1 1 1|0 1 1|red1_1_AB(0)              red1_1_AB(19)|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |red1_2_AB(0)                                                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |red1_2_AB(38)|red1_3_AB(0)                                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      red1_3_AB(34)|red2_2_A(0)      red2_2_A(14)|red2_3_A(0)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           red2_3_A(18)|P|P|P|P|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.  Congestion Control

   The general congestion control considerations for transporting RTP
   data applicable to IP-MR speech over RTP (see RTP [RFC3550] and any
   applicable RTP profile like the Audio-Visual Profile (AVP)
   [RFC3551]).  However, the multi-rate capability of IP-MR speech
   coding provides a mechanism that may help to control congestion,
   since the bandwidth demand can be adjusted by selecting a different
   encoding mode.

   The number of frames encapsulated in each RTP payload highly
   influences the overall bandwidth of the RTP stream due to header
   overhead constraints.  Packetizing more frames in each RTP payload
   can reduce the number of packets sent and thus reduce the overhead
   from IP/UDP/RTP headers, at the expense of increased delay.




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   Due to the scalability nature of the IP_MR codec, the transmission
   rate can be reduced at any transport stage to fit channel bandwidth.
   The minimal rate is specified by the BR field of the payload header
   and can be as low as 7.7 kbps.  It is up to the application to keep
   the balance between coding quality (high BR) and bitstream
   scalability (low BR).  Because coding quality depends on coding rate
   (CR) rather than base rate (BR), it is NOT RECOMMENDED to use high BR
   values for real-time communications.

   Applications MAY utilize bitstream redundancy to combat packet loss.
   However, the gateway is free to chose any option to reduce the
   transmission rate; the coding layer or redundancy bits can be
   dropped.  Due to this fact, it is NOT RECOMMENDED for applications to
   increase the total bitrate when adding redundancy in response to
   packet loss.

6.  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 is achieved through a
   suitable cryptographic integrity-protection mechanism.  Such a
   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
   source authentication at least capable of determining if an RTP
   packet is from a member of the RTP session.

   Note that the appropriate mechanisms to provide security to RTP and
   payloads following this memo may vary.  The security mechanisms are
   dependent on the application, the transport, and the signaling
   protocol employed.  Therefore, a single mechanism is not sufficient;
   although if suitable, usage of the Secure Real-time Transport
   Protocol (SRTP) [RFC3711] is recommended.  Other mechanisms that may
   be used are IPsec [RFC4301] and Transport Layer Security (TLS)
   [RFC5246] (RTP over TCP); other alternatives may exist.

   This payload format does not exhibit any significant non-uniformity
   in the receiver-side computational complexity for packet processing
   and thus is unlikely to pose a denial-of-service threat due to the
   receipt of pathological data.






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7.  Payload Format Parameters

   This section describes the media types and names associated with this
   payload format.

   The IP-MR media subtype is defined as 'ip-mr_v2.5'.  This subtype was
   registered to specify an internal codec version.  Later, this version
   was accepted as final, the bitstream was frozen, and IP-MR v2.5 was
   published under the name of IP-MR.  Currently, the terms 'IP-MR' and
   'IP-MR v2.5' are synonyms.  The subtype name 'ip-mr_v2.5' is being
   used in implementations.

7.1.  Media Type Registration

   Media Type name:     audio

   Media Subtype name:  ip-mr_v2.5

   Required parameters: none

   Optional parameters:
      These parameters apply to RTP transfer only.

      ptime: The media packet length in milliseconds.  Allowed values
      are: 20, 40, 60, and 80.

   Encoding considerations:
      This media type is framed and binary (see RFC 4288, Section 4.8).

   Security considerations:
      See Section 6 of RFC 6262.

   Interoperability considerations:
      none

   Published specification:
      RFC 6262

   Applications that use this media type:
      Real-time audio applications like voice over IP,
      teleconference, and multimedia streaming.

   Additional information:
      none

   Person & email address to contact for further information:
      V. Sviridenko <vladimirs@spiritdsp.com>




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   Intended usage:
      COMMON

   Restrictions on usage:
      This media type depends on RTP framing and thus is only defined
      for transfer via RTP [RFC3550].

   Authors:
      Sergey Ikonin <info@spiritdsp.com>
      Dmitry Yudin <info@spiritdsp.com>

   Change controller:
      IETF Audio/Video Transport working group delegated from the IESG.

7.2.  Mapping Media Type Parameters into SDP

   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [RFC4566], which is commonly used to describe RTP sessions.  When SDP
   is used to specify sessions employing the IP-MR codec, the mapping is
   as follows:

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

   o  The media subtype (payload format name) goes in SDP "a=rtpmap" as
      the encoding name.  The RTP clock rate in "a=rtpmap" MUST be
      16000.

   o  The parameter "ptime" goes in the SDP "a=ptime" attribute.

   Any remaining parameters go in the SDP "a=fmtp" attribute by copying
   them directly from the media type parameter string as a semicolon-
   separated list of parameter=value pairs.

   Note that the payload format (encoding) names are commonly shown in
   uppercase.  Media subtypes are commonly shown in lowercase.  These
   names are case-insensitive in both places.

8.  IANA Considerations

   One media type (ip-mr_v2.5) has been defined and registered in the
   media types registry.









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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.


























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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


Appendix A.  Retrieving Frame Information

   This appendix contains the C code for implementation of the frame-
   parsing function.  This function extracts information about a coded
   frame, including frame size, number of layers, size of each layer,
   and size of perceptual sensitive classes.

A.1.  get_frame_info.c

   /*

   Copyright (c) 2011 IETF Trust and the persons identified as
   authors of the code.  All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:

   - Redistributions of source code must retain the above copyright
     notice, this list of conditions and
     the following disclaimer.

   - Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the
     distribution.

   - Neither the name of Internet Society, IETF or IETF Trust, nor the
     names of specific contributors, may be used to endorse or promote
     products derived from this software without specific prior written
     permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

   */

   /******************************************************************




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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


     get_frame_info.c

     Retrieving frame information for IP-MR Speech Codec

   ******************************************************************/

   #define RATES_NUM       6   // number of codec rates
   #define SENSE_CLASSES   6   // number of sensitivity classes (A..F)

   // frame types
   #define FT_SPEECH       0   // active speech
   #define FT_DTX_SID      1   // silence insertion descriptor

   // get specified bit from coded data
   int GetBit(const unsigned char *buf, int curBit)
   {
       return (buf[curBit>>3]>>(curBit%8))&1;
   }

   // retrieve frame information
   int GetFrameInfo(               // o: frame size in bits
       short rate,                 // i: encoding rate (0..5)
       short base_rate,            // i: base (core) layer rate,
       const unsigned char buf[2], // i: coded bit frame
       int size,                   // i: coded bit frame size in bytes
       short pLayerBits[RATES_NUM],     // o: number of bits in layers
       short pSenseBits[SENSE_CLASSES], // o: number of bits in
                                        //    sensitivity classes
       short *nLayers                   // o: number of layers
   )
   {
       static const short Bits_1[4]    = {  0, 9, 9,15};
       static const short Bits_2[16]   = { 43,50,36,31,46,48,40,44,
                                           47,43,44,45,43,44,47,36};
       static const short Bits_3[2][6] = {{13,11,23,33,36,31},
                                          {25, 0,23,32,36,31},};
       int FrType;
       int i, nBits = 0;

       if (rate < 0 || rate > 5) {
           return 0; // incorrect stream
       }

       // extract frame type bit if required
       FrType = GetBit(buf, nBits++) ? FT_SPEECH : FT_DTX_SID;

       if((FrType != FT_DTX_SID && size < 2) || size < 1) {
           return 0; // not enough input data



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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


       }

       for(i = 0; i < SENSE_CLASSES; i++) {
           pSenseBits[i] = 0;

       }

       {
           int cw_0;
           int b[14];

           // extract meaning bits
           for(i = 0 ; i < 14; i++) {
               b[i] = GetBit(buf, nBits++);
           }

           // parse
           if(FrType == FT_DTX_SID) {
               cw_0 = (b[0]<<0)|(b[1]<<1)|(b[2]<<2)|(b[3]<<3);
               rate = 0;
               pSenseBits[0] = 10 + Bits_2[cw_0];
           } else {

               int i, idx;
               int nFlag_1, nFlag_2, cw_1, cw_2;

               nFlag_1 = b[0] + b[2] + b[4] + b[6];
               cw_1 = (cw_1 << 1) | b[0];
               cw_1 = (cw_1 << 1) | b[2];
               cw_1 = (cw_1 << 1) | b[4];
               cw_1 = (cw_1 << 1) | b[6];

               nFlag_2 = b[1] + b[3] + b[5] + b[7];
               cw_2 = (cw_2 << 1) | b[1];
               cw_2 = (cw_2 << 1) | b[3];
               cw_2 = (cw_2 << 1) | b[5];
               cw_2 = (cw_2 << 1) | b[7];

               cw_0 = (b[10]<<0)|(b[11]<<1)|(b[12]<<2)|(b[13]<<3);
               if (base_rate < 0)    base_rate = 0;
               if (base_rate > rate) base_rate = rate;
               idx = base_rate == 0 ? 0 : 1;

               pSenseBits[0] = 15+Bits_2[cw_0];
               pSenseBits[1] = Bits_1[(cw_1>>0)&0x3] +
                               Bits_1[(cw_1>>2)&0x3];
               pSenseBits[2] = nFlag_1*5;
               pSenseBits[3] = nFlag_2*30;



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RFC 6262       RTP Payload Format for IP-MR Speech Codec     August 2011


               pSenseBits[5] = (4 - nFlag_2)*(Bits_3[idx][0]);

               for (i = 1; i < rate+1; i++) {
                   pLayerBits[i] = 4*Bits_3[idx][i];
               }

           }

           pLayerBits[0] = 0;
           for (i = 0; i < SENSE_CLASSES; i++) {
               pLayerBits[0] += pSenseBits[i];
           }

           *nLayers = rate+1;
       }

       {
           // count total frame size
           int payloadBitCount = 0;
           for (i = 0; i < *nLayers; i++) {
               payloadBitCount += pLayerBits[i];
           }
           return payloadBitCount;
       }
   }

Author's Address

   Sergey Ikonin
   SPIRIT DSP
   Building 27, A. Solzhenitsyna Street
   109004, Moscow
   Russia

   Tel: +7 495 661-2178
   Fax: +7 495 912-6786
   EMail: s.ikonin@gmail.com














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