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rfc:rfc6262

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

Ikonin Standards Track [Page 1] 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].

Ikonin Standards Track [Page 2] 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.

Ikonin Standards Track [Page 3] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

 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) ---->|

Ikonin Standards Track [Page 4] 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.

Ikonin Standards Track [Page 5] 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:

Ikonin Standards Track [Page 6] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

 +- - -+------------+------------+------------+------------+
 | 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)      |
                  +-------+--------------------+

Ikonin Standards Track [Page 7] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

 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 ----->|

Ikonin Standards Track [Page 8] 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.

Ikonin Standards Track [Page 9] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

     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.

Ikonin Standards Track [Page 10] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

     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.

Ikonin Standards Track [Page 11] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

 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.

Ikonin Standards Track [Page 12] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

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>

Ikonin Standards Track [Page 13] RFC 6262 RTP Payload Format for IP-MR Speech Codec August 2011

 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.

Ikonin Standards Track [Page 14] 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.

Ikonin Standards Track [Page 15] 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:
  1. Redistributions of source code must retain the above copyright

notice, this list of conditions and

   the following disclaimer.
  1. 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.
  1. 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.
  • /
 /******************************************************************

Ikonin Standards Track [Page 16] 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

Ikonin Standards Track [Page 17] 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;

Ikonin Standards Track [Page 18] 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

Ikonin Standards Track [Page 19]

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