RFC 7655
RTP Payload Format for G.711.0
Pages: 32
Proposed Standard
Proposed Standard
Part 1 of 2 – Pages 1 to 10
Internet Engineering Task Force (IETF) M. Ramalho, Ed. Request for Comments: 7655 P. Jones Category: Standards Track Cisco Systems ISSN: 2070-1721 N. Harada NTT M. Perumal Ericsson L. Miao Huawei Technologies November 2015 RTP Payload Format for G.711.0Abstract
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 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/rfc7655.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4 3. G.711.0 Codec Background . . . . . . . . . . . . . . . . . . 4 3.1. General Information and Use of the ITU-T G.711.0 Codec . 4 3.2. Key Properties of G.711.0 Design . . . . . . . . . . . . 6 3.3. G.711 Input Frames to G.711.0 Output Frames . . . . . . . 8 3.3.1. Multiple G.711.0 Output Frames per RTP Payload Considerations . . . . . . . . . . . . . . . . . . . 9 4. RTP Header and Payload . . . . . . . . . . . . . . . . . . . 10 4.1. G.711.0 RTP Header . . . . . . . . . . . . . . . . . . . 10 4.2. G.711.0 RTP Payload . . . . . . . . . . . . . . . . . . . 12 4.2.1. Single G.711.0 Frame per RTP Payload Example . . . . 12 4.2.2. G.711.0 RTP Payload Definition . . . . . . . . . . . 13 4.2.2.1. G.711.0 RTP Payload Encoding Process . . . . . . 14 4.2.3. G.711.0 RTP Payload Decoding Process . . . . . . . . 15 4.2.4. G.711.0 RTP Payload for Multiple Channels . . . . . . 17 5. Payload Format Parameters . . . . . . . . . . . . . . . . . . 19 5.1. Media Type Registration . . . . . . . . . . . . . . . . . 20 5.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 22 5.3. Offer/Answer Considerations . . . . . . . . . . . . . . . 22 5.4. SDP Examples . . . . . . . . . . . . . . . . . . . . . . 23 5.4.1. SDP Example 1 . . . . . . . . . . . . . . . . . . . . 23 5.4.2. SDP Example 2 . . . . . . . . . . . . . . . . . . . . 23 6. G.711.0 Storage Mode Conventions and Definition . . . . . . . 24 6.1. G.711.0 PLC Frame . . . . . . . . . . . . . . . . . . . . 24 6.2. G.711.0 Erasure Frame . . . . . . . . . . . . . . . . . . 25 6.3. G.711.0 Storage Mode Definition . . . . . . . . . . . . . 26 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 8. Security Considerations . . . . . . . . . . . . . . . . . . . 27 9. Congestion Control . . . . . . . . . . . . . . . . . . . . . 28 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 29 10.1. Normative References . . . . . . . . . . . . . . . . . . 29 10.2. Informative References . . . . . . . . . . . . . . . . . 30 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
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 specification found in RFC 3551 [RFC3551]. The only significant difference for G.711.0 is the required use of a dynamic payload type (the static PT of 0 or 8 is presently almost always used with G.711 even though dynamic assignment of other payload types is allowed) 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 for G.711 payloads anywhere between end systems that have negotiated use of G.711. Because the only significant difference between the G.711 RTP payload format header and the G.711.0 payload format header defined in this document 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. In a similar manner, the corresponding decompression of the G.711.0 RTP packet thus created 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. As a packet produced by the compression and decompression as described above is indistinguishable in every detail to the source G.711 packet, such compression can be made invisible to the end systems. Specification of how systems on the path between the end systems discover each other and negotiate the use of G.711.0 compression as described in this paragraph is outside the scope of this document. It is informative 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 that contained "speech" or other zero-mean acoustic signals. G.711.0 obtains greater than 50% average compression in service provider environments [ICASSP]. 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] and 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). A G.711.0 decoder need not know how
many symbols are contained in the original G.711 frame (e.g.,
parameter ptime in the Session Description Protocol (SDP)
[RFC4566]), 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 ptime values of 5 ms, 10 ms, 20 ms, 30 ms, or 40 ms.
Because 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 (which could consist of any
combination of octets with each octet spanning the entire space
of 2^8 values), by definition there exists at least one
potential G.711 payload that 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, 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 Weighted Million Operations Per
Second (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 applications [RFC3551].
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 to 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 Packet Loss Concealment (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. Figure 1 depicts this where "A" is the process of G.711.0 encoding and "B" is the process of G.711.0 decoding. |--------------------------| 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: 1:1 Mapping from G.711 Input Frame to G.711.0 Output Frame 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 companding 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 ("process 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 first octet of a G.711.0 frame is called the "Prefix Code" octet; the information within this octet conveys how many G.711 symbols the decoder is to create from a given G.711.0 input frame (i.e., 0, 40, 80, 160, 240, or 320). The Prefix Code value of 0x00 is used to denote zero G.711 source symbols, which allows the use of 0x00 as a payload padding octet (described later in Section 3.3.1).
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 is 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 is 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 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 lossless compression based on G.711.0 can be employed multiple times, as the original G.711 input symbols are always reproduced with 100% fidelity.3.3.1. Multiple G.711.0 Output Frames per RTP Payload Considerations
As a general rule, G.711.0 frames containing more source G.711 symbols (from a given channel) will typically result in higher compression, but there are exceptions to this rule. A G.711.0 encoder may choose to encode 20 ms of input G.711 symbols as: 1) a single 20 ms G.711.0 frame, or 2) as two 10 ms G.711.0 frames, or 3) any other combination of 5 ms or 10 ms G.711.0 frames -- depending on which encoding resulted in fewer bits. As an example, an intelligent encoder might encode 20 ms of G.711 symbols as two 10 ms G.711.0 frames if the first 10 ms was "silence" and two G.711.0 frames took fewer bits than any other possible encoding combination of G.711.0 frame sizes. During the process of G.711.0 standardization, it was recognized that although it is sometimes advantageous to encode integer multiples of 40 G.711 symbols in whatever input symbol format resulted in the most compression (as per above), the simplest choice is to encode the entire ptime's worth of input G.711 symbols into one G.711.0 frame (if the ptime supported it). This is especially so since the larger number of source G.711 symbols typically resulted in the highest
compression anyway and there is added complexity in searching for other possibilities (involving more G.711.0 frames) that were unlikely to produce a more bit efficient result. The design of ITU-T Rec. G.711.0 [G.711.0] foresaw the possibility of multiple G.711.0 input frames in that the decoder was defined to decode what it refers to as an incoming "bit stream". For this specification, the bit stream is the G.711.0 RTP payload itself. Thus, the decoder will take the G.711.0 RTP payload and will produce an output frame containing the original G.711 symbols independent of how many G.711.0 frames were present in it. Additionally, any number of 0x00 padding octets placed between the G.711.0 frames will be silently (and safely) ignored by the G.711.0 decoding process Section 4.2.3). To recap, a G.711.0 encoder may choose to encode incoming G.711 symbols into one or more than one G.711.0 frames and put the resultant frame(s) into the G.711.0 RTP payload. Zero or more 0x00 padding octets may also be included in the G.711.0 RTP payload. The G.711.0 decoder, being insensitive to the number of G.711.0 encoded frames that are contained within it, will decode the G.711.0 RTP payload into the source G.711 symbols. Although examples of single or multiple G.711 frame cases are illustrated in Section 4.2, the multiple G.711.0 frame cases MUST be supported and there is no need for negotiation (SDP or otherwise) required for it.
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