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Audio/Video Transport Working Group
Internet Draft                                                                               SPIRIT DSP
Intended status: Informational                                                              February 10, 2009

RTP Payload Format for SPIRIT IP-MR Speech Codec Software draft-ietf-avt-rtp-ipmr-01.txt

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Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 1]

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 payload, introduced redundancy for robustness against packet loss, and payload format extension for future versions compatibility.

Table of Contents

1. Introduction 2
2. IP-MR RTP Payload Formats    2
2.1. Standard Payload Format    3
2.1.1. Payload Format Structure 3
2.1.2. Payload Header   3
2.1.3. Speech Table of Contents 4
2.1.4. Speech Data      4
2.1.5. Redundancy Header        5
2.1.6. Redundancy Table of Contents     5
2.1.7. Redundancy Data  6
2.2. Payload Examples   6
2.2.1. Standard Payload Carrying a Single Frame 6
2.2.2. Standard Payload Carrying Multiple Frames with Redundancy        7
2.2.3. Extended Payload Carrying a Single Frame 8
3. Media Type Registration      8
3.1. Registration of MIME media type audio/ip-mr_v2.5   8
3.2. Mapping Media Type Parameters into SDP     9
4. Security Considerations      10
6. Normative References 10
Author's Addresses      10
Expiration date 10
Legal Terms     10

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 payload, introduced redundancy for robustness against packet loss, and payload format extension for future versions compatibility.

2. IP-MR RTP Payload Formats
The payload has two formats: standard optimized for current use-cases and extended for future versions compatibility. The payload format is defined by first bit of header. Both of these formats will be described bellow.





Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 2]

2.1. Standard Payload Format

2.1.1. Payload Format Structure

The standard payload consists of a payload header with general information about packet, a speech table of contents (TOC), and speech data. An optional redundancy section follows after speech data. The redundancy section consists of redundancy header, redundancy TOC and redundancy data payload.

  The following diagram shows the standard payload format layout:
  +---------+--------+--------+- - - - - - +- - - - - - +- - - - - - +
  | payload | speech | speech | redundancy | redundancy | redundancy |
  | header  | TOC    | data   | header     | TOC        | data       |
  +---------+--------+--------+- - - - - - +- - - - - - +- - - - - - +
2.1.2. Payload Header

  The payload header has the following format:
   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 compatibility with future extensions. Should be set  to 0.

  o  CR (3 bits): coding rate of frame(s) in this packet, as per the following table:
               +-------+--------------+
               |  CR   | 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    |
               +-------+--------------+
  Table 1 Coding rates of IP-MR codec

 The CR value 7 (NO_DATA) indicates that there is no speech data (and speech TOC accordingly) in the payload. This MAY be used to transmit redundancy data only. The value 6 is reserved. If receiving this value the packet SHOULD be discarded.





Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 3]

  o  BR (3 bits): base rate for core layer of frame(s) in this packet. Values in the range 0-5 indicate bitrates for core layer, same as     for CR. Values 6 and 7 are reserved. If one of these values is     received the packet SHOULD be discarded. The base rate is the     lowest rate for scalability, so speech payload can be scaled down     not lower than BR value. If a received packet has BR > CR then     during decoding it will be assumed that BR = CR.

  o  D (1 bit): indicates if the DTX mode is allowed or not.

  o  A (1 bit): byte-aligned payload. If A=1 then all speech frames     MUST be byte-aligned. This mode speeds up speech data access. The A=0 value specifies bandwidth-efficient mode with no byte     alignment (including end of header).

  o  GR (2 bits): number of frames in packet (grouping size). Actual     grouping size is GR + 1, thus maximum grouping supported is 4. If greater grouping size is required the extended payload format  (sec. 2.2) MAY be used.

  o  R (1 bit): redundancy presence bit. If R=1 then the packet contains redundancy information for lost packets recovery. In this case after speech TOC redundancy flags and TOC sections are present. If R=0 then speech TOC is the last section of payload header.

2.1.3. Speech Table of Contents

  The speech TOC contains entries for each frame in packet (grouping size in total). Each entry contains a single field:

   0
  +-+
  |E|
  +-+

  o  E (1 bit): frame existence indicator. If set to 0, this indicates     the corresponding frame is absent and the receiver should set the teRxFrType to LOST_FRAME. This can be followed by the lost frame     itself or by empty frames generated by the encoder during silence intervals in DTX mode.

  Note that if CR field from coding flags is 7 (NO_DATA) then speech  TOC is empty.

2.1.4. Speech Data

Speech data of a payload contains one or more speech frames or comfort noise frames, as specified in the speech TOC of the payload.

Each speech frame represents 20 ms of speech encoded with the rate indicated in the CR and base rate indicated in BR field of the payload header. The length of the speech frame is variable due to the nature of the codec and can be calculated after decoding or by using GetFrameInfo function detailed in [1].


Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 4]

2.1.5. Redundancy Header

  If a packet contains redundancy (R field of payload header is 1) the speech data is followed by redundancy header:

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

Redundancy header consists of two fields. Each field contains class specifier for redundancy partly taken from the preceding packet (CL1) and pre-preceding packet (CL2), e.g. distant from the current packet by 1 and 2 packets accordingly. The values are listed in the table below:

            +-------+-------------------+
            |  CL   | amount redundancy |
            +-------+-------------------+
            |   0   |       NONE        |
            |   1   |      CLASS A      |
            |   2   |      CLASS B      |
            |   3   |      CLASS C      |
            |   4   |      CLASS D      |
            |   5   |      CLASS E      |
            |   6   |      CLASS F      |
            |   7   |     (reserved)    |
            +-------+-------------------+

Each specifier takes 3 bits, thus the total redundancy header size is 6 bits. In case of redundancy usage followed by preceding (or pre- preceding) packet loss the receiver sets the special flag for decoder with CL class specifier.

2.1.6. Redundancy Table of Contents

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Pkt1 Entries| Pkt2 Entries|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The redundancy TOC contains entries for redundancy frames from preceding and pre-preceding packets. Each entry takes 1 bit like speech TOC entry (2.1.3):
   0
  +-+
  |E|
  +-+

  o  E (1 bit): frame existence indicator. If set to 0, this indicates the corresponding frame is absent.




Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 5]

  o  For each preceding and pre-preceding packet the number of entries is equal to the grouping size of the current packet. E.g. maximum number of entries is 4*2 = 8.

  o  If class specifier in the redundancy header is CL=0 (NO_DATA) then  there is no entries for corresponding packet redundancy.

2.1.7. Redundancy Data

  Redundancy data of a payload contains redundancy information for one or more speech frames or comfort noise frames that may be lost during transition, as specified in the redundancy TOC of the payload.  Actually redundancy is the most important part of preceding frames representing 20 ms of speech. The length of redundancy frame is variable and can be calculated after decoding or by using  GetFrameInfo function detailed in [1].

2.2. Payload Examples

2.2.1. Standard Payload Carrying a Single Frame

  The following diagram shows a standard IP-MR payload carrying a
  single speech frame without redundancy:

   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 |0|0|0 0|0|1|sp(0)                                |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      sp(193)|P|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In the payload the speech frame is not damaged at the IP origin (E=1), the coding rate is 9.7 kbps (CR=1), the base rate is 7.8 kbps (BR=0), and the DTX mode is off. There is no byte alignment (A=0) and no redundancy (R=0). The encoded speech bits - s(0) to s(193) - are placed immediately after TOC. Finally, one zero bit is added at the end as padding to make the payload byte aligned.


Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 6]

2.2.2. Standard Payload Carrying Multiple Frames with Redundancy

The following diagram shows a payload that contains three frames, one of them with no speech data.  The coding rate is 7.7 kbps (CR=0), the base rate is 7.7 kbps (BR=0), and the DTX mode is on. The speech frames are byte aligned (A=1), so 1 zero bit is added at the end of the header. Besides the speech frames the payload contains six redundancy frames (three per each delayed packet).

The first speech frame consists of bits sp1(0) to sp1(92). After that 3 bits are added for byte alignment. The second frame does not contain any speech information that is represented in the payload by its TOC entry. The third frame consists of bits sp3(0) to sp3(171).

The redundancy header follows after speech data. The one-packet- delayed redundancy contains class A+B bits (CL1=2), and two-packet- delayed redundancy contains class A bits (Cl2=1). The one-packet- delayed redundancy contains three frames with 20, 39 and 35 bits respectively. The first frame of two-packet-delayed redundancy is absent, it is represented in its TOC entry, and two other frames have sizes 15 and 19 bits.

Note that all speech frames are padded with zero bits for byte alignment.

   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=2 |BR=1 |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(0)                    red1_1(19)|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |red1_2(0)
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   red1_2(38)|red1_3(0)                                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |         red1_3(34)|red2_2(0)          red2_2(14)|red2_3(0)    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             red2_3(18)|P|P|P|P|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Berlizova, E                 SPIRIT IP-MR Speech Codec Software          [Page 7]

2.2.3. Extended Payload Carrying a Single Frame

The following diagram shows an extended IP-MR payload carrying a single speech frame without redundancy:

   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |1|CR=1 |BR=0 |0|0|0 0|0|1|P|P|P|0 1 1|0| header extension data |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               |sp(0)                                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            sp(193)| optional payload extenson ...
  +-+-+-+-+-+-+-+-+-+-+ - - - - - - - - - - - - - - -

The standard header is the same as in example 2.3.1 except for the first bit that is set to 1 to reflect extended payload type. The standard header is padded with zeros to achieve byte alignment. After that the size of header extension follows (HESZ=3). Then the header extension data is placed. In 3 bytes (HESZ) from header extension beginning, the standard speech payload starts. After that, the optional payload extension MAY be added.

3. Media Type Registration

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

3.1. Registration of MIME media type audio/ip-mr_v2.5

Type name: audio

Subtype name:  ip-mr_v2.5

Required parameters:  none

Optional parameters:

Berlizova, E                     SPIRIT IP-MR Speech Codec Software                     [Page 8]

  o  ptime: Gives the length of time in milliseconds represented by the media in a packet. Allowed values are: 20, 40, 60 and 80.

Encoding considerations:

This media type is framed binary data (see RFC 4288, Section 4.8).

Security considerations: See RFC 3550

Applications that use this media type:

Audio and video streaming and conferencing tools.

Additional information: none

Intended usage: COMMON

Restrictions on usage:

This media type depends on RTP framing, and hence is only defined for transfer via RTP (RFC 3550).

3.2. 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:

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

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

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

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.








Berlizova, E                     SPIRIT IP-MR Speech Codec Software                     [Page 9]

4. 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 any appropriate RTP profile.  This implies that confidentiality of the media streams is achieved by encryption.  Encryption may be performed after compression so there is no conflict between the two operations.

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.

6. Normative References

  [1]  SPIRIT IP-MR v2.5 User Guide, website http://spiritdsp.com
  [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
  [3]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.
  [4]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
Author's Addresses

SPIRIT DSP
Russia
109004 B.Kommunisticheskaya st. 27
Tel: +7 495 661-2178
Fax: +7 495 912-6786
Email: Elena Berlizova berlizova@spiritdsp.com

Expiration date

This Internet-Draft will expire on August 09, 2009.
Legal Terms
All IETF Documents and the information contained therein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.





Berlizova, E                 SPIRIT IP-MR Speech Codec Software                         [Page 10]

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Berlizova, E                 SPIRIT IP-MR Speech Codec Software                         [Page 11]
Internet-Draft    RTP Payload Format for SPIRIT IP-MR           February 2009


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