[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-ramalho-payload-g7110) 00 01 02 03 04 05 06 RFC 7655

Network Working Group                                    M. Ramalho, Ed.
Internet-Draft                                                  P. Jones
Intended status: Standards Track                           Cisco Systems
Expires: December 22, 2013                                     N. Harada
                                                                     NTT
                                                              M. Perumal
                                                           Cisco Systems
                                                                 L. Miao
                                                     Huawei Technologies
                                                           June 20, 2013


                     RTP Payload Format for G.711.0
                      draft-ietf-payload-g7110-00

Abstract

   This document specifies the Real-Time Transport Protocol (RTP)
   payload format for ITU-T Recommendation G.711.0.  ITU-T Rec. G.711.0
   defines a lossless and stateless compression for G.711 packet
   payloads typically used in IP networks.  This document also defines a
   storage mode format for G.711.0 and a media type registration for the
   G.711.0 RTP payload format.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 22, 2013.

Copyright Notice

   Copyright (c) 2013 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



Ramalho, et al.         Expires December 22, 2013               [Page 1]


Internet-Draft           G.711.0 Payload Format                June 2013


   (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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  G.711.0 Codec Background  . . . . . . . . . . . . . . . . . .   3
     3.1.  General Information and Use of the ITU-T G.711.0 Codec  .   3
     3.2.  Key Properties of G.711.0 Design  . . . . . . . . . . . .   4
     3.3.  G.711 Input Frames to G.711.0 Output Frames . . . . . . .   6
   4.  RTP Header and Payload  . . . . . . . . . . . . . . . . . . .   8
     4.1.  G.711.0 RTP Header  . . . . . . . . . . . . . . . . . . .   8
     4.2.  G.711.0 RTP Payload . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Single G.711.0 Frame per RTP Payload Example  . . . .   9
       4.2.2.  Multiple G.711.0 Frames per RTP Payload Example . . .  10
       4.2.3.  G.711.0 RTP Payload Decoding Process  . . . . . . . .  12
       4.2.4.  G.711.0 RTP Payload for Multiple Channels . . . . . .  13
   5.  Payload Format Parameters . . . . . . . . . . . . . . . . . .  15
     5.1.  Media Type Registration . . . . . . . . . . . . . . . . .  16
     5.2.  Mapping to SDP Parameters . . . . . . . . . . . . . . . .  17
     5.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  18
     5.4.  SDP Examples  . . . . . . . . . . . . . . . . . . . . . .  18
       5.4.1.  SDP Example 1 . . . . . . . . . . . . . . . . . . . .  18
       5.4.2.  SDP Example 2 . . . . . . . . . . . . . . . . . . . .  19
   6.  G.711.0 Storage Mode Conventions and Definition . . . . . . .  19
     6.1.  G.711.0 PLC Frame . . . . . . . . . . . . . . . . . . . .  20
     6.2.  G.711.0 Erasure Frame . . . . . . . . . . . . . . . . . .  20
     6.3.  G.711.0 Storage Mode Definition . . . . . . . . . . . . .  21
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  23
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     11.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26









Ramalho, et al.         Expires December 22, 2013               [Page 2]


Internet-Draft           G.711.0 Payload Format                June 2013


1.  Introduction

   The International Telecommunication Union (ITU-T) Recommendation
   G.711.0 [G.711.0] specifies a stateless and lossless compression for
   G.711 packet payloads typically used in Voice over IP (VoIP)
   networks.  This document specifies the Real-Time Transport Protocol
   (RTP) RFC 3550 [RFC3550] payload format and storage modes for this
   compression.

2.  Requirements Language

   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].

3.  G.711.0 Codec Background

   ITU-T Recommendation G.711.0 [G.711.0] is a lossless and stateless
   compression mechanism for ITU-T Recommendation G.711 [G.711] and thus
   is not a "codec" in the sense of "lossy" codecs typically carried by
   RTP.  When negotiated end-to-end ITU-T Rec. G.711.0 is negotiated as
   if it were a codec, with the understanding that ITU-T Rec. G.711.0
   losslessly encoded the underlying (lossy) G.711 pulse code modulation
   (PCM) sample representation of an audio signal.  For this reason
   ITU-T Rec. G.711.0 will be interchangeably referred to in this
   document as a "lossless data compression algorithm" or a "codec",
   depending on context.  Within this document, individual G.711 PCM
   samples will be referred to as "G.711 symbols" or just "symbols" for
   brevity.

   This section describes the ITU-T Recommendation G.711 [G.711] codec,
   its properties, typical uses cases and its key design properties.

3.1.  General Information and Use of the ITU-T G.711.0 Codec

   ITU-T Recommendation G.711 is the benchmark standard for narrowband
   telephony.  It has been successful for many decades because of its
   proven voice quality, ubiquity and utility.  A new ITU-T
   recommendation, G.711.0, has been established for defining a
   stateless and lossless compression for G.711 packet payloads
   typically used in VoIP networks.  ITU-T Rec. G.711.0 is also known as
   ITU-T Rec. G.711 Annex A [G.711-A1], as ITU-T Rec. G.711 Annex A is
   effectively a pointer ITU-T Rec. G.711.0.  Henceforth in this
   document, ITU-T Rec. G.711.0 will simply be referred to as "G.711.0"
   and ITU-T Rec. G.711 simply as "G.711".

   G.711.0 may be employed end-to-end; in which case the RTP payload
   format specification and use is nearly identical to the G.711 RTP



Ramalho, et al.         Expires December 22, 2013               [Page 3]


Internet-Draft           G.711.0 Payload Format                June 2013


   specification found in RFC 3550 [RFC3550].  The only significant
   difference for G.711.0 is the use of a dynamic payload type (the
   static PT of 0 or 8 are virtually always used with G.711) and the
   recommendation not to use Voice Activity Detection (see Section 4.1).

   G.711.0, being both lossless and stateless, may also be employed as a
   lossless compression mechanism anywhere in between end systems which
   have negotiated use of G.711.  Because the only significance between
   the G.711 RTP payload format header and the G.711.0 payload format
   header is the payload type, a G.711 RTP packet can be losslessly
   converted to a G.711.0 RTP packet simply by compressing the G.711
   payload (thus creating a G.711.0 payload), changing the payload type
   to the dynamic value desired and copying all the remaining G.711 RTP
   header fields into the corresponding G.711.0 RTP header.  Conversely,
   the corresponding decompression of a G.711.0 RTP packet back to the
   original source G.711 RTP packet can be accomplished by losslessly
   decompressing the G.711.0 payload back to the original source G.711
   payload, changing the payload type back to the payload type of the
   original G.711 RTP packet and copying all the remaining G.711.0 RTP
   header fields into the corresponding G.711 RTP header.

   It is special to note that G.711.0, being both lossless and
   stateless, can be employed multiple times (e.g., on multiple,
   individual hops or series of hops) of a given flow with no
   degradation of quality relative to end-to-end G.711.  Stated another
   way, multiple "lossless transcodes" from/to G.711.0/G.711 do not
   affect voice quality as typically occurs with lossy transcodes to/
   from dissimilar codecs.

   Lastly, it is expected that G.711.0 will be used as an archival
   format for recorded G.711 streams.  Therefore, a G.711.0 Storage Mode
   Format is also included in this document.

3.2.  Key Properties of G.711.0 Design

   The fundamental design of G.711.0 resulted from the desire to
   losslessly encode and compress frames of G.711 symbols independent of
   what types of signals those G.711 frames contained.  The primary
   G.711.0 use case is for G.711 encoded, zero-mean, acoustic signals
   (such as speech and music).

   G.711.0 attributes are below:

   A1  Compression for zero-mean acoustic signals: G.711.0 was designed
         as its primary use case for the compression of G.711 payloads
         which contained "speech" or other zero-mean acoustic signals.
         G.711.0 obtains greater than 50% average compression in service
         provider environments [ICASSP].



Ramalho, et al.         Expires December 22, 2013               [Page 4]


Internet-Draft           G.711.0 Payload Format                June 2013


   A2  Lossless for any G.711 payload: G.711.0 was designed to be
         lossless for any valid G.711 payload - even if the payload
         consisted of apparently random G.711 symbols (e.g., a modem or
         FAX payload).  G.711.0 could be used for "aggregate 64 kbps
         G.711 channels" carried over IP without explicit concern if a
         subset of these channels happened to be carrying something
         other than voice or general audio.  To the extent that a
         particular channel carried something other than voice or
         general audio, G.711.0 ensured that it was carried losslessly,
         if not significantly compressed.

   A3  Stateless: Compression of a frame of G.711 symbols was only to be
         dependent on that frame and not on any prior frame.  Although
         greater compression is usually available by observing a longer
         history of past G.711 symbols, it was decided that the
         compression design would be stateless to completely eliminate
         error propagation common in many lossy codec designs (e.g.,
         ITU-T Rec. G.729 [G.729], ITU-T Rec. G.722 [G.722]).  That is,
         the decoding process need not be concerned about lost prior
         packets because the decompression of a given G.711.0 frame is
         not dependent on potentially lost prior G.711.0 frames.  Owing
         to this stateless property, the frames input to the G.711.0
         encoder may be changed "on-the-fly" (a 5 ms encoding could be
         followed by a 20 ms encoding).

   A4  Self-describing: This property is defined as the ability to
         determine how many source G.711 samples are contained within
         the G.711.0 frame solely by information contained within the
         G.711.0 frame.  Generally, the number of source G.711 symbols
         can be determined by decoding the initial octets of the
         compressed G.711.0 frame (these octets are called "prefix
         codes" in the standard) [ICASSP].  A G.711.0 decoder need not
         know what ptime is, as it is able to decompress the G.711.0
         frame presented to it without signaling knowledge.

   A5  Accommodate G.711 payload sizes typically used in IP: G.711 input
         frames of length typically found in VoIP applications represent
         SDP ptimes (see RFC 4566 [RFC4566]) of 5 ms, 10 ms, 20 ms, 30
         ms or 40 ms.  Since the dominant sampling frequency for G.711
         is 8000 samples per second, G.711.0 was designed to compress
         G.711 input frames of 40, 80, 160, 240 or 320 samples.

   A6  Bounded expansion: Since attribute A2 above requires G.711.0 to
         be lossless for any payload, by definition there exists at
         least one potential G.711 payload which must be
         "uncompressible".  Since the quantum of compression is an
         octet, the minimum expansion of such an uncompressible payload
         was designed to be the minimum possible of one octet.  Thus



Ramalho, et al.         Expires December 22, 2013               [Page 5]


Internet-Draft           G.711.0 Payload Format                June 2013


         G.711.0 "compressed" frames can be of length one octet to X+1
         octets, where X is the size of the input G.711 frame in octets.
         G.711.0 can therefore be viewed as a Variable Bit Rate (VBR)
         encoding in which the size of the G.711.0 output frame is a
         function of the G.711 symbols input to it.

   A7  Algorithmic delay: G.711.0 was designed to have the algorithmic
         delay equal to the time represented by the number of samples in
         the G.711 input frame (i.e., no "look-ahead").

   A8  Low Complexity: Less than 1.0 WMOPS average and low memory
         footprint (~5k octets RAM, ~5.7k octets ROM and ~3.6 basic
         operations) [ICASSP] [G.711.0].

   A9  Both A-law and Mu-law supported: G.711 has two operating laws,
         A-law and Mu-law.  These two laws are also known as PCMA and
         PCMU in RTP applicaitons RFC 3550 [RFC3550].

   These attributes generally make it trivial to compress a G.711 input
   frame consisting of 40, 80, 160, 240 or 320 samples.  After the input
   frame is presented to a G.711.0 encoder, a G.711.0 "self-describing"
   output frame is produced.  The number of samples contained within
   this frame is easily determined at the G.711.0 decoder by virtue of
   attribute A4.  The G.711.0 decoder can decode the G.711.0 frame back
   to a G.711 frame by using only data within the G.711.0 frame.

   Lastly we note that losing a G.711.0 encoded packet is identical in
   effect of losing a G.711 packet (when using RTP); this is because a
   G.711.0 payload, like the corresponding G.711 payload, is stateless.
   Thus, it is anticipated that existing G.711 PLC mechanisms will be
   employed when a G.711.0 packet is lost and an identical MOS
   degradation relative to G.711 loss will be achieved.

3.3.  G.711 Input Frames to G.711.0 Output Frames

   G.711.0 is a lossless and stateless compression of G.711 frames.  The
   following figure depicts this where "A" is the process of G.711.0
   encoding and "B" is the process of G.711.0 decoding.

        1:1 Mapping from G.711 Input Frame to G.711.0 Output Frame











Ramalho, et al.         Expires December 22, 2013               [Page 6]


Internet-Draft           G.711.0 Payload Format                June 2013


    |--------------------------|  A   |------------------------------|
    |    G.711 Input Frame     |----->|     G.711.0 Output Frame     |
    |       of X Octets        |      |  containing 1 to X+1 Octets  |
    | (where X MUST be 40, 80, |      | (precise value dependent on  |
    | 160, 240 or 320 octets)  |<-----| G.711.0 ability to compress) |
    |__________________________|  B   |______________________________|


                                 Figure 1

   Note that the mapping is 1:1 (lossless) in both directions, subject
   to two constraints.  The first constraint is that the input frame
   provided to the G.711.0 encoder (process "A") has a specific number
   of input G.711 symbols consistent with attribute A5 (40, 80, 160, 240
   or 320 octets).  The second constraint is that the compression law
   used to create the G.711 input frame (A-law or Mu-law) must be known,
   consistent with attribute A9.

   Subject to these two constraints, the input G.711 frame is processed
   by the G.711.0 encoder ("A") and produces a "self-describing" G.711.0
   output frame, consistent with attribute A4.  Depending on the source
   G.711 symbols, the G.711.0 output frame can contain anywhere from 1
   to X+1 octets, where X is the number of input G.711 symbols.
   Compression results for virtually every zero-mean acoustic signal
   encoded by G.711.0.

   Since the G.711.0 output frame is "self-describing", a G.711.0
   decoder (process "B") can losslessly reproduce the original G.711
   input frame with only the knowledge of which companding law was used
   (A-law or Mu-law).  The G.711.0 frame, being "self-describing",
   allows for the G.711.0 decoder ("B") to know precisely how many G.711
   symbols to create.

   Since G.711.0 was designed with typical G.711 payload lengths as a
   design constraint (attribute A5), this lossless encoding can be
   performed only with knowledge of the companding law being used.  This
   information is anticipated to be signaled in SDP and will be
   described later in this document.

   If the original inputs were known to be from a zero-mean acoustic
   signal coded by G.711, an intelligent G.711.0 encoder could infer the
   G.711 companding law in use (via G.711 input signal amplitude
   histogram statistics).  Likewise, an intelligent G.711.0 decoder
   producing G.711 from the G.711.0 frames could also infer which
   encoding law in use.  Thus G.711.0 could be designed for use in
   applications that have limited stream signaling between the G.711
   endpoints (i.e., they only know "G.711 at 8k sampling is being used",
   but nothing more).  Such usage is not further described in this



Ramalho, et al.         Expires December 22, 2013               [Page 7]


Internet-Draft           G.711.0 Payload Format                June 2013


   document.  Additionally, if the original inputs were known to come
   from zero-mean acoustic signals, an intelligent G.711.0 encoder could
   tell if the G.711.0 payload had been encrypted - as the symbols would
   not have the distribution expected in either companding law and would
   appear random.  Such determination is also not further discussed in
   this document.

   It is easily seen that this process is 1:1 and that G.711.0 based
   lossless compression can be employed multiple times, as the original
   G.711 input symbols are always reproduced with 100% fidelity.

   G.711.0 frames containing more source G.711 symbols from a given
   channel will typically result in higher compression as a general
   rule, but there are exceptions.  For example, an intelligent G.711.0
   encoder may choose to encode 20 ms of G.711 as two individual 10 ms
   G.711.0 frames if a higher overall compression will result (this
   might occur if the first 10 ms was "silence" and two, 10 ms G.711.0
   frames contained fewer octets than one 20 ms G.711.0 frame).  For
   this reason, we will explicitly allow multiple G.711.0 encoded frames
   in the G.711.0 RTP payload in Section 4.2.2 below even though the
   usual case is anticipated to be only one G.711.0 frame per RTP
   payload.

4.  RTP Header and Payload

   In this section we describe the precise format for G.711.0 frames
   carried via RTP.  We begin with RTP header description relative to
   G.711, then provide two G.711.0 payload examples.

4.1.  G.711.0 RTP Header

   Relative to G.711 RTP headers, the utilization of G.711.0 does not
   create any special requirements with respect to the contents of the
   RTP packet header.  The only significant difference is that the
   payload type (PT) RTP header field will have a value corresponding to
   the dynamic payload type assigned to the flow (whereas G.711 PCMU
   typically has a static PT = 0 and G.711 PCMA typically has a static
   PT = 8 [RFC3551]).

   Voice Activity Detection (VAD) SHOULD NOT be used when G.711.0 is
   negotiated because G.711.0 obtains high compression during "VAD
   silence intervals" and one of the advantages of G.711.0 over G.711
   with VAD is the lack of any VAD-inducing artifacts in the received
   signal.  However, if VAD is employed, the Marker bit (M) MUST be set
   in the first packet of a talkspurt (the first packet after a silence
   period in which packets have not been transmitted contiguously as per
   rules specified in [RFC3550] for G.711 payloads).  This definition,
   being consistent with the G.711 RTP VAD use, further allows lossless



Ramalho, et al.         Expires December 22, 2013               [Page 8]


Internet-Draft           G.711.0 Payload Format                June 2013


   transcoding between G.711 RTP packets and G.711.0 RTP packets as
   described in Section 3.1.

   With this introduction, the RTP packet header fields are defined as
   follows:

      V - As per [RFC3550]

      P - As per [RFC3550]

      X - As per [RFC3550]

      CC - As per [RFC3550]

      M - As per [RFC3550]

      PT- Dynamic PT assigned, consistent with MIME allocation for
      G711.0 defined in Media Type Definition (Section 5.1).

      SN - As per [RFC3550]

      timestamp - As per [RFC3550]

      SSRC - As per [RFC3550]

      CSRC - As per [RFC3550]

   Where V (version bits), P (padding bit), X (extension bit), CC (CSRC
   count), M (marker bit), PT (payload type), SN (sequence number),
   timestamp, SSRC (synchronizing source) and CSRC (contributing
   sources) are as defined in [RFC3550] and as typically used with
   G.711.  PT (payload type) is as defined in [RFC3550].

4.2.  G.711.0 RTP Payload

   In this section we provide two examples for carrying G.711.0 frames
   in RTP payloads.  The first example is used when it is desired to
   carry only one G.711.0 frame in the RTP payload.  This example is a
   subset of the second and shown separately for clarity.

4.2.1.  Single G.711.0 Frame per RTP Payload Example










Ramalho, et al.         Expires December 22, 2013               [Page 9]


Internet-Draft           G.711.0 Payload Format                June 2013


   This example depicts a single G.711.0 frame in the RTP payload.  This
   is expected to be the dominant RTP payload case for G.711.0, as the
   G.711.0 encoding process supports the SDP packet times (ptime and
   maxptime, see [RFC4566]) commonly used when G.711 is transported in
   RTP.  Additionally, as mentioned previously, larger G.711.0 frames
   generally compress more effectively than a multiplicity of smaller
   G.711.0 frames.

   The following Figure illustrates the single G.711.0 frame per RTP
   payload case.

                 Single G.711.0 Frame in RTP Payload Case


                 |-------------------|-------------------|
                 | One G.711.0 Frame | Zero or more 0x00 |
                 |                   |   Padding Octets  |
                 |___________________|___________________|


                                 Figure 2

   Encoding Process: A single G.711.0 frame is inserted into the RTP
   payload.  The amount of time represented by the G.711 symbols
   compressed in the G.711.0 frame MUST correspond to the ptime signaled
   for applications using SDP.  Although generally not desired, padding
   desired in the RTP payload after the G.711.0 frame MAY be created by
   placing one or more 0x00 octets after the G.711.0 frame.  Such
   padding may be desired based on security considerations (see
   Section 10).

   Decoding Process: Passing the entire RTP payload to the G.711.0
   decoder is sufficient for the G.711.0 decoder to create the source
   G.711 symbols.  Any padding inserted after the G.711.0 frame (i.e.,
   the 0x00 octets) present in the RTP payload is silently ignored by
   the G.711.0 decoding process.  The decoding process is fully
   described in Section 4.2.3 below.

4.2.2.  Multiple G.711.0 Frames per RTP Payload Example

   This example depicts the case where multiple G.711.0 frames are
   desired in the RTP payload.









Ramalho, et al.         Expires December 22, 2013              [Page 10]


Internet-Draft           G.711.0 Payload Format                June 2013


   As described in Section 3.3, an "intelligent G.711.0 encoder" can
   decide to encode, let's say, 20 ms of G.711 symbols as two, 10 ms
   G.711.0 frames because a greater compression is attained for that
   particular 20 ms segment.  The "smart encoding" of such inputs is
   accommodated by the ability to have multiple G.711.0 frames in the
   RTP payload.

   Note that since each G.711.0 frame is self-describing (see Attribute
   A4 in Section 3.2), the individual G.711.0 frames in the RTP payload
   need not represent the same duration of time (i.e., a 5 ms G.711.0
   frame could be followed by a 20 ms G.711.0 frame).  Owing to this,
   the amount of time represented in the RTP payload MAY be any integer
   multiple of 5 ms (as 5 ms is the smallest interval of time that can
   be represented in a G.711.0 frame).

   The following Figure illustrates the multiple G.711.0 frame per RTP
   payload case where the number of G.711.0 frames placed in the RTP
   payload is N.

                Multiple G.711.0 Frames in RTP Payload Case


       |----------|---------|----------|---------|----------------|
       | First    | Second  |          | Nth     | Zero or more   |
       | G.711.0  | G.711.0 |   ...    | G.711.0 |     0x00       |
       | Frame    | Frame   |          | Frame   | Padding Octets |
       |__________|_________|__________|_________|________________|


                                 Figure 3

   We note here that the individual G.711.0 frames can be, and generally
   are, of different lengths.  The decoding process in the following
   section is used to determine the frame boundaries.

   Encoding Process: One or more G.711.0 frames are placed in the RTP
   payload simply by concatenating the G.711.0 frames together.  The
   amount of time represented by the G.711 symbols compressed in all the
   G.711.0 frames in the RTP payload MUST correspond to the ptime
   signaled for applications using SDP.  Although not generally desired,
   padding in the RTP payload SHOULD be placed after the last G.711.0
   frame in the payload and MAY be created by placing one or more 0x00
   octets after the last G.711.0 frame.  Such padding may be desired
   based on security considerations (see Section 10).

   Decoding Process: As G.711.0 frames can be of varying length, the
   payload decoding process described in the following section is used
   to determine where the individual G.711.0 frame boundaries are.



Ramalho, et al.         Expires December 22, 2013              [Page 11]


Internet-Draft           G.711.0 Payload Format                June 2013


4.2.3.  G.711.0 RTP Payload Decoding Process

   The G.711.0 decoding process is a standard part of G.711.0 bit stream
   decoding and is implemented in the ITU-T Rec. G.711.0 reference code.
   The decoding process heuristic described in this section is a slight
   enhancement of the ITU-T reference code to explicitly accommodate RTP
   padding (as described above).

   Before describing the decoding, we note here that the largest
   possible G.711.0 frame is created whenever the largest number of
   G.711 symbols is encoded (320 from Section 3.2, property A5) and
   these 320 symbols are "uncompressible" by the G.711.0 encoder.  In
   this case (via property A6 in Section 3.2) the G.711.0 output frame
   will be 321 octets long.  We also note that the value 0x00 chosen for
   the optional padding cannot be the first octet of a valid ITU-T Rec.
   G.711.0 frame (see [G.711.0]).  We also note that whenever more than
   one G.711.0 frame is contained in the RTP payload, the decoding of
   the individual G.711.0 frames will occur multiple times.

   For the decoding heuristic below, let N be the number of octets in
   the RTP payload (i.e., excluding any RTP padding, but including any
   RTP payload padding), let P equal the number of RTP payload octets
   processed by the G.711.0 decoding process, let K be the number of
   G.711 symbols presently in the output buffer, let Q be the number of
   octets contained in the G.711.0 frame being processed and let "!="
   represent not equal to.  The keyword "STOP" is used below to indicate
   the end of the processing of G.711.0 frames in the RTP payload.  The
   heuristic below assumes an output buffer for the decoded G.711 source
   symbols of length sufficient to accommodate the expected number of
   G.711 symbols and an input buffer of length 321 octets.

   G.711.0 RTP Payload Decoding Heuristic:

   H1  Initialization: Initialize the number of processed octets to zero
         (P = 0).  Initialize the counter for how many G.711 symbols are
         in the output buffer to zero (K = 0).  Initialize N to the
         number of octets in the RTP payload.  Go to H2.

   H2  Read internal buffer: Read min{320+1, (N-P)} octets into the
         internal buffer from the (P+1) octet of the RTP payload.  We
         note at this point, N-P octets have yet to be processed and
         that 320+1 octets is the largest possible G.711.0 frame.  Go to
         H3.

   H3  Analyze the first octet in the internal buffer: If this octet
         0x00 (a padding octet) go to H4, otherwise go to H5 (process a
         G.711.0 frame).




Ramalho, et al.         Expires December 22, 2013              [Page 12]


Internet-Draft           G.711.0 Payload Format                June 2013


   H4  Process padding octet (no G.711 symbols generated): Increment the
         processed packets counter by one (set P = P + 1).  If the
         result of this increment results in P >= N then STOP (as all
         RTP Payload octets have been processed), otherwise go to H2.

   H5  Process an individual G.711.0 frame (produce G.711 samples in the
         output frame): Pass the internal buffer to the G.711.0 decoder.
         The G.711.0 decoder will read the first octet (called the
         "prefix code" octet in ITU-T Rec. G.711.0 [G.711.0]) to
         determine the number of source G.711 samples M are contained in
         this G.711.0 frame.  The G.711.0 decoder will produce exactly M
         G.711 source symbols.  If K = 0, these M symbols will be the
         first in the output buffer and are placed at the beginning of
         the output buffer.  If K != 0, concatenate these M symbols with
         the prior symbols in the output buffer (there are K prior
         symbols in the buffer).  Set K = K + M (as there are now this
         many G.711 source symbols in the output buffer).  The G.711.0
         decoder will have consumed some number of packets, Q, in the
         internal buffer to produce the M G.711 symbols.  Increment the
         number of payload octet processed counter by this quantity (set
         P = P + Q).  If the result of this increment results in P >= N
         then STOP (as all RTP Payload octets have been processed),
         otherwise go to H2.

   At this point, the output buffer will contain precisely K G.711
   source symbols which should correspond to the ptime signaled if SDP
   was used and the encoding process was without error.

   We also note, as an aside, that the heuristic above (and the ITU-T
   G.711.0 reference code) accommodates padding octets (0x00) placed
   anywhere in between G.711.0 frames in the RTP payload as well as
   prior to or after any or all G.711.0 frames.  The ITU-T G.711.0
   reference code does not have Step H3 and H4 as separate steps (i.e.,
   Step H5 immediately follows H2) at the added computational cost of
   some additional buffer passing to/from the G.711.0 frame decoder
   functions.  That is the G.711.0 decoder in the reference code
   "silently ignores" 0x00 padding octets at the beginning of what it
   believes to be a G.711.0 encoded frame boundary.  Thus Step H3 and
   Step H4 above are an optimization over the reference code shown for
   clarity.

   If the decoder is at a playout endpoint location, this G.711 buffer
   SHOULD be used in the same manner as a received G.711 RTP payload
   would have been used (passed to a playout buffer, to a PLC
   implementation, etc.).

4.2.4.  G.711.0 RTP Payload for Multiple Channels




Ramalho, et al.         Expires December 22, 2013              [Page 13]


Internet-Draft           G.711.0 Payload Format                June 2013


   In this section we describe the use of multiple "channels" of G.711
   data encoded by G.711.0 compression.

   The dominant use of G.711 in RTP transport has been for single
   channel use cases.  For this case, the above G.711.0 encoding and
   decoding process is used.  However, the multiple channel case for
   G.711.0 (a frame-based compression) is different from G.711 (a
   sample-based encoding) and is described separately here.

   RFC 3551 [RFC3551] provides guidelines for encoding audio channels
   (Section 4) and for the ordering of the channels within the RTP
   payload (Section 4.1).  The ordering guidelines in RFC 3551,
   Section 4.1 SHOULD be used unless an application-specific channel
   ordering is more appropriate.

   An implicit assumption in RFC 3551 is that all the channel data
   multiplexed into a RTP payload MUST represent the same physical time
   span.  The case for G.711.0 is no different; the underlying G.711
   data for all channels in a G.711.0 RTP payload MUST span the same
   interval in time (e.g., the same "ptime" for a SDP-specified codec
   negotiation).

   RFC 3551 provides guidelines for sample-based encodings such as G.711
   in Section 4.2.  This guidance is tantamount to interleaving the
   individual samples in that they SHOULD be packed in consecutive
   octets.

   RFC 3551 provides guidelines for frame-based encodings in which the
   frames are interleaved.  However, this guidance stems from the
   assumption that "the frame size for frame-oriented codecs is a
   given".  However, this assumption is not valid for G.711.0 in that
   individual consecutive G.711.0 frames (as per Section 4.2.2) can:

      1) represent different time spans (e.g., two 5 ms G.711.0 frames
      in lieu of one 10 ms G.711.0 frame), and

      2) be of different lengths in octets (and typically are).

   Therefore a different, but also simple, concatenation-based approach
   is specified in this RFC.

   For the multiple channel G.711.0 case, each G.711 channel is
   independently encoded into one or more G.711.0 frames defined here as
   a "G.711.0 channel superframe".  Each one of these superframes is
   identical to the multiple G.711.0 frame case illustrated in Figure 3
   of Section 4.2.2 in which each superframe can have one or more
   individual G.711.0 frames within it.  Then each G.711.0 channel
   superframe is concatenated - in channel order - into a G.711.0 RTP



Ramalho, et al.         Expires December 22, 2013              [Page 14]


Internet-Draft           G.711.0 Payload Format                June 2013


   payload.  Then, if optional G.711.0 padding octets (0x00) are
   desired, it is RECOMMENDED that these octets are placed after the
   last G.711.0 channel superframe.  As per above, such padding may be
   desired based on security considerations (see Section 10).  This is
   depicted in the following Figure 4 below.

            Multiple G.711.0 Channel Superframes in RTP Payload


           |----------|---------|----------|---------|---------|
           | First    | Second  |          | Nth     | Zero    |
           | G.711.0  | G.711.0 |   ...    | G.711.0 | or more |
           | Channel  | Channel |          | Channel | 0x00    |
           | Super-   | Super-  |          | Super   | Padding |
           | Frame    | Frame   |          | Frame   | Octets  |
           |__________|_________|__________|_________|_________|


                                 Figure 4

   The G.711.0 decoder at the receiving end simply decodes the entire
   G.711.0 (multiple channel) payload into individual G.711 symbols.  If
   M such G.711 symbols result and there were N channels, then the first
   M/N G.711 samples would be from the first channel, the second M/N
   G.711 samples would be from the second channel, and so on until the
   Nth set of G.711 samples are found.  Similarly, if the number of
   channels was not known, but the payload "ptime" was known, one could
   infer (knowing the sampling rate) how many G.711 symbols each channel
   contained; then with this knowledge determine how many channels of
   data were contained in the payload.  When SDP is used, the number of
   channels is known because the optional parameter is a MUST when there
   is more than one channel negotiated (see Section 5.1).  Additionally,
   when SDP is used the parameter ptime is a RECOMMENDED optional
   parameter.  We note that if both parameters channels and ptime are
   known that one could provide a check for the other and the converse.

   Lastly we note that although any padding for the multiple channel
   G.711.0 payload is RECOMMENDED to be placed at the end of the
   payload, the G.711.0 decoding heuristic described in Section 4.2.3
   will successfully decode the payload in Figure 4 if the 0x00 padding
   octet is placed anywhere before or after any individual G.711.0 frame
   in the RTP payload.  The number of padding octets introduced at any
   G.711.0 frame boundary therefore does not affect the number M of the
   source G.711 symbols produced.  Thus the decision for padding MAY be
   made on a per-superframe basis.

5.  Payload Format Parameters




Ramalho, et al.         Expires December 22, 2013              [Page 15]


Internet-Draft           G.711.0 Payload Format                June 2013


   This section defines the parameters that may be used to configure
   optional features in the G.711.0 RTP transmission.

   The parameters defined here as a part of the media subtype
   registration for the G.711.0 codec.  Mapping of the parameters into
   Session Description Protocol (SDP) RFC 4566 [RFC4566] is also
   provided for those applications that use SDP.

5.1.  Media Type Registration

   Type name: audio

   Subtype name: G7110

   Required Parameters:

      rate: The RTP timestamp clock rate, which is equal to the sampling
      rate.  The typical rate used with G.711 encoding is 8000, but
      other rates may be specified.  The default rate is 8000.

      complaw: Indicates the companding law (A-law or mu-law) employed.
      The case-insensitive values are "al" or "mu" for A-law and mu-law,
      respectively.

   Optional parameters:

      channels: See RFC 4566 [RFC4566] for definition.  Specifies how
      many audio streams are represented in the G.711.0 payload and MUST
      be present if the number of channels is greater than one.  This
      parameter defaults to 1 if not present (as per RFC 4566) an is
      typically a non-zero small-valued positive integer.  It is
      expected that implementations that specify multiple channels will
      also define a mechanism to map the channels appropriately within
      their system design, otherwise the channel order specified in RFC
      3551 [RFC3551] Section 4.1 will be assumed (e.g., left, right,
      center, ... ).

      maxptime: See RFC 4566 [RFC4566] for definition.

      ptime: See RFC 4566 [RFC4566] for definition.  The inclusion of
      "ptime" is RECOMMENDED and SHOULD be in the SDP unless there is an
      application specific reason not to include it (e.g., an
      application that has a variable ptime on a packet-by-packet
      basis).  For constant ptime applications, it is considered good
      form to include "ptime" in the SDP for session diagnostic
      purposes.  For the constant ptime multiple channel case described
      in Section 4.2.2, the inclusion of "ptime" can provide a desirable
      payload check.



Ramalho, et al.         Expires December 22, 2013              [Page 16]


Internet-Draft           G.711.0 Payload Format                June 2013


   Encoding considerations:

      This media type is framed binary data (see Section 4.8 in RFC 4288
      [RFC4288]) compressed as per ITU-T Rec. G.711.0.

   Security considerations:

      This media type does not carry active content.  It does transfer
      compressed data.  See Section 4 of RFC 4856 [RFC4856].

   Interoperability considerations: none

   Published specification:

      ITU-T Rec. G.711.0 and RFC QQQQ.

      [ RFC Editor: please replace QQQQ with a reference to this RFC ]

   Applications that use this media type:

      Audio and video streaming and conferencing tools.

   Additional information: none

   Person & email address to contact for further information:

      Michael Ramalho <mramalho@cisco.com> or <mar42@cornell.edu>

   Intended usage: COMMON

   Restrictions on usage:

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

   Author: Michael Ramalho

   Change controller:

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

5.2.  Mapping to SDP Parameters

   The information carried in the media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP),
   which is commonly used to describe RTP sessions.  When SDP is used to
   specify sessions employing G.711.0, the mapping is as follows:



Ramalho, et al.         Expires December 22, 2013              [Page 17]


Internet-Draft           G.711.0 Payload Format                June 2013


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

   o  The media subtype ("G7110") goes in SDP "a=rtpmap" as the encoding
      name.

   o  The required parameter "rate" also goes in "a=rtpmap" as the clock
      rate.

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

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

5.3.  Offer/Answer Considerations

   The following considerations apply when using the SDP offer/answer
   RFC 3264 [RFC3264] mechanism to negotiate the "channels" attribute.

   o  If the offering endpoint specifies a value for the optional
      channels parameter greater than one and the answering endpoint
      both understands the parameter and cannot support that value
      requested, the answer MUST contain the optional channels parameter
      with the highest value it can support.

   o  If the offering endpoint specifies a value for the optional
      channels parameter the answer MUST contain the optional channels
      parameter unless the only value the answering endpoint can support
      is one, in which case the answer MAY contain the optional channels
      parameter with value of 1.

   o  If the offering endpoint specifies a value for the ptime parameter
      that the answering endpoint cannot support, the answer MUST
      contain the optional ptime parameter.

   o  If the offering endpoint specifies a value for the maxptime
      parameter that the answering endpoint cannot support, the answer
      MUST contain the optional maxptime parameter.

5.4.  SDP Examples

   The following examples illustrate how to signal G.711.0 via SDP.

5.4.1.  SDP Example 1

         m=audio RTP/AVP 98
         a=rtpmap: 98 G7110/8000



Ramalho, et al.         Expires December 22, 2013              [Page 18]


Internet-Draft           G.711.0 Payload Format                June 2013


         a=fmtp:98 complaw = mu

   In the above example the dynamic payload type 98 is mapped to G.711.0
   via the "a=rtpmap" parameter.  The mandatory "complaw" is on the
   "a=fmtp" parameter line.  Note that neither optional parameters
   "ptime" nor "channels" is present; although it is generally good form
   to include "ptime" in the SDP for session diagnostic purposes.

5.4.2.  SDP Example 2

   The following example illustrates an offering endpoint requesting 2
   channels, but the answering endpoint can only support (or render) one
   channel.

   Offer:

         m=audio RTP/AVP 98
         a=rtpmap: 98 G7110/8000/2
         a=ptime: 20
         a=fmtp:98 complaw = al

   Answer:

         m=audio RTP/AVP 98
         a=rtpmap: 98 G7110/8000/1
         a=ptime: 20
         a=fmtp:98 complaw = al

   In this example the offer had an optional channels parameter.  The
   answer must have the optional channels parameter also unless the
   value in the answer is one.  Shown here is when the answer explicitly
   contains the channels parameter (it need not have and it would be
   interpreted as one channel).  As mentioned previously, it is
   considered good form to include "ptime" in the SDP for session
   diagnostic purposes if the session is a contstant ptime session.

6.  G.711.0 Storage Mode Conventions and Definition

   The G.711.0 storage mode definition in this section is similar to
   many other IETF codecs (e.g., iLBC, EVRC-NW) and is essentially a
   concatenation of individual G.711.0 frames.

   We note that something must be stored for any G.711.0 frames that not
   received at the receiving endpoint, no matter what the cause.  In
   this section we describe two mechanisms, a "G.711.0 PLC Frame" and a
   "G.711.0 Erasure Frame".  These G.711.0 PLC and G.711.0 Erasure
   Frames are described prior to the G.711.0 storage mode definition for
   clarity.



Ramalho, et al.         Expires December 22, 2013              [Page 19]


Internet-Draft           G.711.0 Payload Format                June 2013


6.1.  G.711.0 PLC Frame

   When G.711 RTP payloads not received by a rendering endpoint a Packet
   Loss Concealment (PLC) mechanism is typically employed to "fill in"
   the missing G.711 symbols with something that is auditorially
   pleasing and thus the loss may be not noticed by a listener.  Such a
   PLC mechanism for G.711 is specified in ITU-T Rec. G.711 - Appendix 1
   [G.711-AP1].

   An natural extension when creating G.711.0 frames for storage
   environments is to employ such a PLC mechanism to create G.711
   symbols for the span of time in which G.711.0 payloads were not
   received - and then to compress the resulting "G.711 PLC symbols" via
   G.711.0 compression.  The G.711.0 frame(s) created by such a process
   are called "G.711.0 PLC Frames".

   Since PLC mechanisms are designed to render missing audio data with
   the best fidelity and intelligibility, G.711.0 frames created via
   such processing is likely best for most recording situations (such as
   voicemail storage) unless there is a requirement not to fabricate
   (audio) data not actually received.

   After such PLC G.711 symbols have been generated and then encoded by
   a G.711.0 encoder, the resulting frames may be stored in G.711.0
   frame format.  As a result, there is nothing to specify here - the
   G.711.0 PLC Frames are stored as if they were received by the
   receiving endpoint.  In other words, PLC-generated G.711.0 frames
   appear as "normal" or "ordinary" G.711.0 frames in the storage mode
   file.

6.2.  G.711.0 Erasure Frame

   "Erasure Frames", or equivalently "Null Frames", have been designed
   for many frame-based codecs since G.711 was standardized.  These null
   /erasure frames explicitly represent data from incoming audio that
   were either not received by the receiving system or represent data
   that a transmitting system decided not to send.  Transmitting systems
   may choose not to send data for a variety of reasons (e.g., not
   enough wireless link capacity in radio-based systems) and can choose
   to send a "null frame" in lieu of the actual audio.  It is also
   envisioned that erasure frames would be used in storage mode
   applications for specific archival purposes where there is a
   requirement not to fabricate audio data that was not actually
   received.

   Thus, a G.711.0 erasure frame is a representation of the amount of
   time in G.711.0 frames that were not received or not encoded by the
   transmitting system.



Ramalho, et al.         Expires December 22, 2013              [Page 20]


Internet-Draft           G.711.0 Payload Format                June 2013


   Prior to defining a G.711.0 erasure frame it is beneficial to note
   what many G.711 RTP systems send when the endpoint is "muted".  When
   muted, many of these systems will send an entire G.711 payload of
   either 0+ or 0- (i.e., one of the two levels closest to "analog zero"
   in either G.711 companding law).  Next we note that a desirable
   property for a G.711.0 erasure frame is for "non G.711.0 Erasure
   Frame aware" endpoints to be able to playback a G.711.0 erasure frame
   with the existing G.711.0 ITU-T reference code.

   A G.711.0 Erasure Frame is defined as any G.711.0 frame for which the
   corresponding G.711 sample values are either the value 0++ or the
   value 0-- for the entirety of the G.711.0 frame.  The levels of 0++
   and 0-- are defined two levels above or below analog zero,
   respectively.  An entire frame of value 0++ or 0-- is expected to be
   extraordinarily rare when the frame was in fact generated by a
   natural signal (on the order of one in 2^{ptime in samples, minus
   one}), as analog inputs such as speech and music are zero-mean and
   are typically acoustically coupled to digital sampling systems.  Note
   that the playback of a G.711.0 frame characterized as an erasure
   frame is auditorially equivalent to a muted signal (a very low value
   constant).

   These G.711.0 erasure frames can be reasonably characterized as null
   or erasure frames while meeting the desired playback goal of being
   decoded by the G.711.0 ITU-T reference code.  Thus, similarly to
   G.711 PLC frames, the G.711.0 erasure frames appear as "normal" or
   "ordinary" G.711.0 frames in the storage mode format.

6.3.  G.711.0 Storage Mode Definition

   The storage format is used for storing G.711.0 encoded frames.  The
   format for the G.711.0 storage mode file defined by this RFC is shown
   below.

                        G.711.0 Storage Mode Format


          |---------------------------|----------|--------------|
          |       Magic Number        |          |              |
          |                           |  Version | Concatenated |
          | "#!G7110A\n" (for A-law)  |   Octet  |   G.711.0    |
          |            or             |          |    Frames    |
          | "#!G7110M\n" (for Mu-law) |  "0x00"  |              |
          |___________________________|__________|______________|


                                 Figure 5




Ramalho, et al.         Expires December 22, 2013              [Page 21]


Internet-Draft           G.711.0 Payload Format                June 2013


   The storage mode file consists of a magic number and a version octet
   followed by the individual G.711.0 frames concatenated together.

   The magic number for G.711.0 A-law corresponds to the ASCII character
   string "#!G7110A\n", i.e., "0x23 0x21 0x47 0x37 0x31 0x31 0x30 0x41
   0x0A".  Likewise, the magic number for G.711.0 MU-law corresponds to
   the ASCII character string "#!G7110M\n", i.e., "0x23 0x21 0x47 0x37
   0x31 0x31 0x4E 0x4D 0x0A".

   The version number octet allows for the future specification of other
   G.711.0 storage mode formats.  The specification of other storage
   mode formats may be desireable as G.711.0 frames are of variable
   length and a future format may include an indexing methodology that
   would enable playout far into a long G.711.0 recording without the
   necessity of decoding all the G.711.0 frames since the beginning of
   the recording.  Other future format specification may include support
   for multiple channels, metadata and the like.  For these reasons it
   was determined that a versioning strategy was desirable for the
   G.711.0 storage mode definition specified by this RFC.  This RFC only
   specifies Version 0 and thus the value of "0x00" must be used for the
   storage mode defined by this RFC.

   The G.711.0 codec data frames, including any necessary erasure or PLC
   frames, are stored in consecutive order concatenated together as
   shown in Section 4.2.2.

   To decode the individual G.711.0 frames, the heuristic presented in
   Section 4.2.2 may be used to decode the individual G.711.0 frames.
   If the version octet is determined not to be zero, the remainder of
   the payload MUST NOT be passed to the G.711.0 decoder, as the ITU-T
   G.711.0 reference decoder can only decode concatenated G.711.0 frames
   and has not been designed to decode elements in yet to be specified
   future storage mode formats.

7.  Acknowledgements

   There have been many people contributing to G.711.0 in the course of
   its development.  The people listed here deserve special mention:
   Takehiro Moriya, Claude Lamblin, Herve Taddei, Simao Campos, Yusuke
   Hiwasaki, Jacek Stachurski, Lorin Netsch, Paul Coverdale, Patrick
   Luthi, Paul Barrett, Jari Hagqvist, Pengjun (Jeff) Huang, John Gibbs,
   Yutaka Kamamoto, and Csaba Kos.









Ramalho, et al.         Expires December 22, 2013              [Page 22]


Internet-Draft           G.711.0 Payload Format                June 2013


8.  Contributors

   The authors thank everyone who have contributed to this document.
   The people listed here deserve special mention: Ali Begen, Roni Even,
   and Hadriel Kaplan.

9.  IANA Considerations

   One media type (audio/G7110) has been defined and requires IANA
   registration in the media types registry.  See Section 5.1 for
   details.

10.  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 appropriate RTP profile (for
   example RFC 3551 [RFC3551] or [RFC4585].  This implies that
   confidentiality of the media streams is achieved by encryption; for
   example, through the application of SRTP [RFC3711].  Because the data
   compression used with this payload format is applied end-to-end, any
   encryption needs to be performed after compression.

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of RTP packets and their payloads is very dependent on the
   application and on the transport and signaling protocols employed.
   Thus, although SRTP is given as an example above, other possible
   choices exist.

   Note that end-to-end security with either authentication, integrity
   or confidentiality protection will prevent a network element not
   within the security context from performing media-aware operations
   other than discarding complete packets.  To allow any (media-aware)
   intermediate network element to perform its operations, it is
   required to be a trusted entity which is included in the security
   context establishment.

   G.711.0 has no known denial-of-service attacks due to decoding, as
   data posing as a desired G711.0 payload will be decoded into
   something (as per the decoding algorithm) with a finite amount of
   computation.  This is due to the decompression algorithm having a
   finite worst-case processing path (no infinite computational loops
   are possible).

   G.711.0 is a variable bit rate (VBR) audio codec.  There have been
   recent concerns with VBR speech codecs where a passive observer can
   identify phrases from a standard speech corpus by means of the
   lengths produced by the encoder even when the payload is encrypted



Ramalho, et al.         Expires December 22, 2013              [Page 23]


Internet-Draft           G.711.0 Payload Format                June 2013


   [IEEE].  In this paper, it was determined that some code excited
   linear prediction (CELP) codecs would produce discrete packet lengths
   for some phonemes.  And furthermore with the use of appropriately
   designed Hidden Markov Models (HMMs) that such a system could predict
   phrases with unexpected accuracy.  One CELP codec studied, SPEEX, had
   the property that it produced 21 different packet lengths in its
   wideband mode and that these packet lengths probabilistically mapped
   to phonemes that a HMM system could be trained on.  In this paper it
   was determined that a mitigation technique would be to pad the output
   of the encoder with random padding lengths to the effect: 1) that
   more discrete payload sizes would result, and 2) that the
   probabilistic mapping to phonemes would become less clear.  As G.711
   is not a speech model based codec, neither is G.711.0.  A G.711.0
   encoding, during talking periods, produces frames of varying frame
   lengths which are not likely to have a strong mapping to phonemes.
   Thus G.711.0 is not expected to have this same vulnerability.  It
   should be noted that "silence" (only one value of G.711 in the entire
   G.711 input frame)" or "near silence" (only a few G.711 values) is
   easily detectable as G.711.0 frame lengths or one or a few octets.
   If one desires to mitigate for silence/non-silence detection,
   statistically variable padding should be added to G.711.0 frames that
   resulted in very small G.711.0 frames (less than about 20% of the
   symbols of the corresponding G.711 input frame).  Methods of
   introducing padding in the G.711.0 payloads have been provided in the
   G.711.0 RTP payload definitions in Section 4.2.1 and Section 4.2.2.

11.  References

11.1.  Normative References

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

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

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", RFC 4288, December 2005.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, February 2007.

   [RFC4856]  Casner, S., "Media Type Registration of Payload Formats in
              the RTP Profile for Audio and Video Conferences", RFC
              4856, February 2007.






Ramalho, et al.         Expires December 22, 2013              [Page 24]


Internet-Draft           G.711.0 Payload Format                June 2013


   [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.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
              2006.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

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

   [G.711.0]  ITU-T G.711.0, ., "Recommendation ITU-T G.711.0 - Lossless
              Compression of G.711 Pulse Code Modulation", September
              2009.

   [G.711]    ITU-T G.711.0, ., "Recommendation ITU-T G.711: Pulse Code
              Modulation (PCM) of Voice Frequencies", November 1988.

   [G.711-AP1]
              ITU-T G.711 Appendix 1, ., "Recommendation G.711
              Appendix 1: A high quality low-complexity algorithm for
              packet loss concealment with G.711", September 1999.

   [G.711-A1]
              ITU-T G.711 Amendment 1, ., "Recommendation ITU-T G.711
              Amendment 1 - Amendment 1: New Annex A on Lossless
              Encoding of PCM Frames", September 2009.

11.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [G.729]    ITU-T G.729, ., "Recommendation ITU-T G.729 - Coding of
              speech at 8 kbit/s using conjugate-structure algebraic-
              code-excited linear prediction (CS-ACELP)", January 2007.





Ramalho, et al.         Expires December 22, 2013              [Page 25]


Internet-Draft           G.711.0 Payload Format                June 2013


   [G.722]    ITU-T G.722, ., "Recommendation ITU-T G.722 - 7 kHz audio-
              coding within 64 kbit/s", November 1988.

   [ICASSP]   N. Harada, ., Y. Yamamoto, ., T. Moriya, ., Y. Hiwasaki,
              ., M. A. Ramalho, ., L. Netsch, ., Y. Stachurski, ., Miao
              Lei, ., H. Taddei, ., and . Q. Fengyan, "Emerging ITU-T
              Standard G.711.0 - Lossless Compression of G.711 Pulse
              Code Modulation, International Conference on Acoustics
              Speech and Signal Processing (ICASSP), 2010, ISBN
              978-1-4244-4244-4295-9", March 2010.

   [IEEE]     C.V. Wright, ., L. Ballard, ., S.E. Coull, ., F. Monrose,
              ., and . G.M. Masson, "Spot Me if You Can: Uncovering
              Spoken Phrases in Encrypted VoIP Conversations, IEEE
              Symposium on Security and Privacy, 2008, ISBN:
              978-0-7695-3168-7", May 2008.

Authors' Addresses

   Michael A. Ramalho (editor)
   Cisco Systems, Inc.
   8000 Hawkins Road
   Sarasota, FL  34241
   USA

   Phone: +1 919 476 2038
   Email: mramalho@cisco.com


   Paul E. Jones
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC  27709
   USA

   Phone: +1 919 476 2048
   Email: paulej@packetizer.com


   Noboru Harada
   NTT Communications Science Labs.
   3-1 Morinosato-Wakamiya
   Atsugi, Kanagawa  243-0198
   JAPAN

   Phone: +81 46 240 3676
   Email: harada.noboru@lab.ntt.co.jp




Ramalho, et al.         Expires December 22, 2013              [Page 26]


Internet-Draft           G.711.0 Payload Format                June 2013


   Muthu Arul Mozhi Perumal
   Cisco Systems, Inc.
   Cessna Business Park
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Phone: +91 9449288768
   Email: mperumal@cisco.com


   Lei Miao
   Huawei Technologies Co. Ltd
   Q22-2-A15R, Enviroment Protection Park
   No. 156 Beiqing Road
   HaiDian District
   Beijing  100095
   China

   Phone: +86 1059728300
   Email: lei.miao@huawei.com






























Ramalho, et al.         Expires December 22, 2013              [Page 27]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/