RFC 5188
RTP Payload Format for the Enhanced Variable Rate Wideband Codec (EVRC-WB) and the Media Subtype Updates for EVRC-B Codec
RFC 5188: RTP Payload Format for the Enhanced Variable Rate Wideband Codec (EVRC-WB) and the Media Subtype Updates for EVRC-B Codec ✨
Okay bestie, gather round 🎀 this is RFC 5188 and it's giving main character energy 💅. It's a Standards Track document from February 2008, written by H. Desineni of Qualcomm and Q. Xie of Motorola. It updates RFC 4788 and doesn't obsolete anything, so the EVRC-B media type registrations are getting a glow-up here 🦋.
Status of This Memo
This document is an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🫶. If you want the current standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 💜.
Abstract
So what's the tea? ☕ This document defines Real-time Transport Protocol (RTP) payload formats for the Enhanced Variable Rate Wideband Codec (EVRC-WB), and it updates the media type registrations for the EVRC-B codec 💖. It registers several media types for EVRC-WB RTP payload formats. On top of that, it specifies a file format for moving EVRC-WB speech data around in storage mode applications, like email 📧✨.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. EVRC-WB Codec . . . . . . . . . . . . . . . . . . . . . . . . 3
5. RTP Header Usage . . . . . . . . . . . . . . . . . . . . . . . 4
6. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Congestion Control Considerations . . . . . . . . . . . . . . 5
8. Storage Format for the EVRC-WB Codec . . . . . . . . . . . . . 5
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
9.1. Media Type Registrations . . . . . . . . . . . . . . . . . 5
9.1.1. Registration of Media Type audio/EVRCWB . . . . . . . 6
9.1.2. Registration of Media Type audio/EVRCWB0 . . . . . . . 8
9.1.3. Registration of Media Type audio/EVRCWB1 . . . . . . . 9
9.1.4. Updated Registration of Media Type audio/EVRCB . . . . 11
9.1.5. Updated Registration of Media Type audio/EVRCB0 . . . 13
10. SDP Mode Attributes for EVRC-WB and EVRC-B . . . . . . . . . . 15
11. EVRC-B Interoperability with Legacy Implementations (RFC 4788) 15
12. Mapping EVRC-WB Media Type Parameters into SDP . . . . . . . . 16
13. Mapping EVRC-B Media Type Parameters into SDP . . . . . . . . 16
14. Offer-Answer Model Considerations for EVRC-WB . . . . . . . . 16
15. Offer-Answer Model Considerations for EVRC-B . . . . . . . . . 18
16. Declarative SDP Considerations . . . . . . . . . . . . . . . . 18
17. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
18. Security Considerations . . . . . . . . . . . . . . . . . . . 22
19. Changes to RFC 4788 . . . . . . . . . . . . . . . . . . . . . 22
20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
20.1. Normative References . . . . . . . . . . . . . . . . . . . 22
20.2. Informative References . . . . . . . . . . . . . . . . . . 231. Introduction 🌸
Here's the vibe. This document specifies how to packetize EVRC-WB encoded speech into RTP 🎧. It supports the header-free, interleaved/bundled, and compact bundle packet formats for EVRC-WB, and it adds discontinuous transmission (DTX) support for EVRC-WB speech sent over RTP 🦋. EVRC-WB has better speech quality than EVRC and EVRC-B, and it belongs to the EVRC family of codecs 👑. This document also updates the media type registrations for the EVRC-B codec. Iconic behavior, honestly ✨.
2. Conventions 📏
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 [1]. Those words are doing serious work, so read them carefully 🫶.
3. Background 🍓
EVRC-WB is a wideband extension of the EVRC-B [4] speech codec. It was developed in the Third Generation Partnership Project 2 (3GPP2) and it supports discontinuous transmission (DTX) 💖. The big upgrade is enhanced (wideband) voice quality, and that's a whole mood 🎀.
The EVRC-WB codec operates on 20-ms frames, and its default sampling rate is 16 kHz. It also supports input and output at an 8-kHz sampling rate 🌈. It can operate in three modes (0, 4, and 7) defined in [5]. EVRC-WB modes 4 and 7 are interoperable with EVRC-B 🫶. Mode 4 uses full-rate, 1/2-rate, and 1/8-rate frames, while mode 7 uses only 1/2-rate and 1/8-rate frames. A mode change changes the codec output bit-rate, but it doesn't cause any decoding problems at the receiver. For successful decoding, the decoder doesn't need to know the encoder's current mode of operation. Lowkey genius design 🧚.
EVRC-WB gives you a standardized solution for packetized voice applications that switch between narrowband and wideband telephony 📞. The most important service it addresses is IP telephony. Target devices include IP phones, Voice over IP (VoIP) handsets, media gateways, voice messaging servers, and so on 💅.
4. EVRC-WB Codec 💜
The EVRC-WB codec operates on 20-ms frames. It produces output frames in one of three sizes: 171 bits, 80 bits, or 16 bits. There are also two zero-bit codec frame types: blank (null) frames and erasure frames 🙈. The default sampling rate is 16 kHz, and input and output at 8 kHz are supported too ✨.
The frame type values and the sizes of the matching codec data frames are in the table below 🍓:
Value Rate Total codec data frame size in bytes (and in bits)
--------------------------------------------------------------------
0 Blank 0 (0 bit)
1 1/8 2 (16 bits)
2 1/4 5 (40 bits)
3 1/2 10 (80 bits)
4 1 22 (171 bits; 5 bits padded at the end)
5 Erasure 0 (SHOULD NOT be transmitted by sender)5. RTP Header Usage 🦋
The RTP header format is specified in RFC 3550 [6]. The EVRC-WB payload formats (Section 6) use the RTP header fields in a way that's consistent with RFC 3550 [6] 🎀.
EVRC-WB can also operate with 8-kHz sampled input and output signals 🌸. The decoder doesn't need a priori knowledge of the sampling rate of the original signal at the encoder input. The decoder output can be 8 kHz or 16 kHz, no matter what sampling rate the encoder used. So depending on the implementation and the electro-acoustic audio capabilities of the devices, the encoder input and/or the decoder output can be configured at 8 kHz. Still, a 16-kHz RTP clock rate MUST always be used, no exceptions 💅. The RTP timestamp increases by 320 for each 20 milliseconds ⏰.
The RTP header marker bit (M) SHALL be set to 1 if the first frame in the packet contains a speech frame that starts a talkspurt 🎙️. For all other packets, the marker bit SHALL be set to zero (M=0) 🫶.
6. Payload Format 👑
Three RTP packet formats are supported for EVRC-WB: the interleaved/bundled packet format, the header-free packet format, and the compact bundled packet format 🧚. For all of these formats, the operational details and capabilities of EVRC-WB are exactly the same as EVRC-B, as defined in [3]. Things like the Table of Contents (ToC), interleaving, DTX, and bundling all match. The one exception is the mode change request field in the ToC, which MUST be interpreted according to the definition of the RATE_REDUC parameter in EVRC-WB [5]. Plot twist, but it's a good one 🔥.
The media type audio/EVRCWB maps to the interleaved/bundled packet format. audio/EVRCWB0 maps to the header-free packet format, and audio/EVRCWB1 maps to the compact bundled packet format 💖.
7. Congestion Control Considerations 🌈
Congestion control for RTP SHALL be used in accordance with RFC 3550 [6], and with any applicable RTP profile, for example RFC 3551 [11] 🫶.
Here's the bandwidth tea ☕. Because of header overhead, the number of frames packed into each RTP packet affects the overall bandwidth of the RTP stream. Packing more frames into each packet can reduce the number of packets sent, and so reduce the header overhead. The catch is increased delay and reduced error robustness 💀.
8. Storage Format for the EVRC-WB Codec 🎧
The storage format is used to store EVRC-WB encoded speech frames, for example as a file or an email attachment 📧.
The file begins with a magic number that identifies the vocoder being used. The magic number for EVRC-WB corresponds to the ASCII character string "#!EVCWB\n", which is 0x23 0x21 0x45 0x56 0x43 0x57 0x42 0x0A 🪄.
The codec data frames are stored in consecutive order. Each one is prefixed by a single ToC entry field, extended to one octet. The ToC field is extended to one octet by setting the four most significant bits of the octet to zero. For example, a ToC value of 4 (a full-rate frame) is stored as 0x04. See Section 4 for the mapping from frame type to ToC value 🍓.
Speech frames lost in transmission, and frames that were never received, MUST be stored as erasure frames (ToC value of 5) so they stay in sync with the original media 🙈. That's a MUST, bestie 🫶.
9. IANA Considerations 🎀
This document updates the audio/EVRCB and audio/EVRCB0 media types defined in RFC 4788 [3], and it adds new EVRC-WB 'audio' media subtypes 💅.
9.1. Media Type Registrations 💖
Following the guidelines in RFC 4855 [9] and RFC 4288 [10], this section registers new 'audio' media subtypes for EVRC-WB. It also updates the audio/EVRCB and audio/EVRCB0 media type registrations contained in RFC 4788 [3] 🦋.
9.1.1. Registration of Media Type audio/EVRCWB 💖
Type name: audio
Subtype name: EVRCWB
Required parameters: None
Optional parameters:
These parameters apply to RTP transfer only. 🎀
mode-set-recv: A subset of EVRC-WB modes. Possible values are a comma-separated list of modes from the set {0,4,7} (see Table 2.5.1.2-1 in 3GPP2 C.S0014-C). A decoder can use this attribute to inform an encoder of its preference to operate in a specified subset of modes. Absence of this parameter signals the mode set {0,4,7}.sendmode: A mode of the EVRC-WB codec. An encoder can use this to signal its current mode of operation. Possible values are 0,4,7 (see Table 2.5.1.2-1 in 3GPP2 C.S0014-C). Absence of this parameter signals mode 0.ptime: See RFC 4566.maxptime: See RFC 4566.maxinterleave: Maximum number for interleaving length (field LLL in the Interleaving Octet)[0..7]. The interleaving lengths used in the entire session MUST NOT exceed this maximum value. If not signaled, the maxinterleave length MUST be 5. 🧚silencesupp: See Section 6.1 in RFC 4788.dtxmax: See Section 6.1 in RFC 4788.dtxmin: See Section 6.1 in RFC 4788.hangover: See Section 6.1 in RFC 4788.
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and is defined for transfer of EVRC-WB encoded data via RTP using the interleaved/bundled packet format specified in RFC 3558. 🌸
Security considerations: See Section 18 of RFC 5188.
Interoperability considerations: None
Published specification:
The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C. The transfer method with the interleaved/bundled packet format via RTP is specified in RFC 3558 and RFC 5188.
3GPP2 C.S0050-B, 3GPP2 File Formats for Multimedia Services.
3GPP2 specifications are publicly accessible at http://www.3gpp2.org
Applications that use this media type:
It is expected that many VoIP applications (as well as mobile applications) will use this type. 📞
Additional information:
The following applies to stored-file transfer methods:
- Magic number:
#!EVCWB\n(see Section 8 of RFC 5188) - File extensions: evw, EVW
- Macintosh file type code: None
- Object identifier or OID: None
EVRC-WB speech frames may also be stored in the file format "3g2" defined in 3GPP2 C.S0050-B, which is identified using the media types "audio/3gpp2" or "video/3gpp2" registered by RFC 4393. 🦋
Person & email address to contact for further information:
Harikishan Desineni <hd@qualcomm.com>
Intended usage: COMMON
Restrictions on usage:
When this media type is used in the context of transfer over RTP, the RTP payload format specified in Section 4.1 of RFC 3558 SHALL be used. In all other contexts, the file format defined in Section 8 of RFC 5188 SHALL be used. ✨
Author:
Harikishan Desineni
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
9.1.2. Registration of Media Type audio/EVRCWB0 🌈
Type name: audio
Subtype name: EVRCWB0
Required parameters: None
Optional parameters:
These parameters apply to RTP transfer only. 🎀
mode-set-recv: A subset of EVRC-WB modes. Possible values are a comma-separated list of modes from the set {0,4,7} (see Table 2.5.1.2-1 in 3GPP2 C.S0014-C). A decoder can use this attribute to inform an encoder of its preference to operate in a specified subset of modes. Absence of this parameter signals the mode set {0,4,7}.sendmode: A mode of the EVRC-WB codec. An encoder can use this to signal its current mode of operation. Possible values are 0,4,7 (see Table 2.5.1.2-1 in 3GPP2 C.S0014-C). Absence of this parameter signals mode 0.ptime: See RFC 4566.silencesupp: See Section 6.1 in RFC 4788.dtxmax: See Section 6.1 in RFC 4788.dtxmin: See Section 6.1 in RFC 4788.hangover: See Section 6.1 in RFC 4788.
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and is defined for transfer of EVRC-WB encoded data via RTP using the header-free packet format specified in RFC 3558. 🍓
Security considerations: See Section 18 of RFC 5188.
Interoperability considerations: None
Published specification:
The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C. The transfer method with the header-free packet format via RTP is specified in RFC 3558 and RFC 5188.
3GPP2 C.S0050-B, 3GPP2 File Formats for Multimedia Services.
3GPP2 specifications are publicly accessible at http://www.3gpp2.org
Applications that use this media type:
It is expected that many VoIP applications (as well as mobile applications) will use this type. 📞
Additional information: None
Person & email address to contact for further information:
Harikishan Desineni <hd@qualcomm.com>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing and hence is only defined for transfer via RTP [6]; the RTP payload format specified in Section 4.2 of RFC 3558 SHALL be used. This media type SHALL NOT be used for storage or file transfer using the file format defined in Section 8 of RFC 5188; instead, audio/EVRCWB SHALL be used. 🙈
Author:
Harikishan Desineni
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
9.1.3. Registration of Media Type audio/EVRCWB1 🦋
Type name: audio
Subtype name: EVRCWB1
Required parameters: None
Optional parameters:
These parameters apply to RTP transfer only. 🎀
mode-set-recv: A subset of EVRC-WB modes. Possible values are a comma-separated list of modes from the set {0,4,7} (see Table 2.5.1.2-1 in 3GPP2 C.S0014-C). A decoder can use this attribute to inform an encoder of its preference to operate in a specified subset of modes. A value of 0 signals the support for wideband fixed rate (full or half rate, depending on the value of thefixedrateparameter). A value of 4 signals narrowband fixed full rate. A value of 7 signals narrowband fixed half rate. Absence of this parameter signals mode 0.sendmode: A mode of the EVRC-WB codec. An encoder can use this to signal its current mode of operation. Possible values are 0,4,7 (see Table 2.5.1.2-1 in 3GPP2 C.S0014-C).sendmodewith value 0 signals wideband fixed-rate operation (full or half rate, depending on the value of thefixedrateparameter).sendmodewith value 4 signals narrowband fixed full-rate operation.sendmodewith value 7 signals narrowband fixed half-rate operation. Thefixedrateparameter MUST NOT be present when thesendmodevalue is 4 or 7. Absence of this parameter signals mode 0.ptime: See RFC 4566.maxptime: See RFC 4566.fixedrate: Indicates the EVRC-WB rate of the session while in single-rate operation. Valid values include 0.5 and 1, where a value of 0.5 indicates the 1/2 rate while a value of 1 indicates the full rate. If this parameter is not present, 1/2 rate is assumed.silencesupp: See Section 6.1 in RFC 4788.dtxmax: See Section 6.1 in RFC 4788.dtxmin: See Section 6.1 in RFC 4788.hangover: See Section 6.1 in RFC 4788.
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and is defined for transfer of EVRC-WB encoded data via RTP using the compact bundle packet format specified in RFC 4788. 🫶
Security considerations: See Section 18 of RFC 5188.
Interoperability considerations: None
Published specification:
The EVRC-WB vocoder is specified in 3GPP2 C.S0014-C. The transfer method with the compact bundled packet format via RTP is specified in RFC 4788 and RFC 5188.
3GPP2 C.S0050-B, 3GPP2 File Formats for Multimedia Services.
3GPP2 specifications are publicly accessible at http://www.3gpp2.org
Applications that use this media type:
It is expected that many VoIP applications (as well as mobile applications) will use this type. 📞
Additional information: None
Person & email address to contact for further information:
Harikishan Desineni <hd@qualcomm.com>
Intended usage: COMMON
Restrictions on usage:
This media type depends on RTP framing and hence is only defined for transfer via RTP [6]; the RTP payload format specified in Section 4 of RFC 4788 SHALL be used. This media type SHALL NOT be used for storage or file transfer using the file format defined in Section 8 of RFC 5188; instead, audio/EVRCWB SHALL be used. 💅
Author:
Harikishan Desineni
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
9.1.4. Updated Registration of Media Type audio/EVRCB 👑
Type name: audio
Subtype name: EVRCB
Required parameters: None
Optional parameters:
These parameters apply to RTP transfer only. 🎀
recvmode: A mode of the EVRC-B codec. A decoder can use this attribute to inform an encoder of its preference to operate in a specified mode. Possible values are 0..7 (see the encoder operating point column in Table 2-6 of 3GPP2 C.S0014-B).sendmode: A mode of the EVRC-B codec. An encoder can use this to signal its current mode of operation. Possible values are 0..7 (see encoder operating point column in Table 2-6 of 3GPP2 C.S0014-B).ptime: See RFC 4566.maxptime: See RFC 4566.maxinterleave: Maximum number for interleaving length (field LLL in the Interleaving Octet). The interleaving lengths used in the entire session MUST NOT exceed this maximum value. If not signaled, the maxinterleave length MUST be 5. 🌷silencesupp: See Section 6.1 of RFC 4788 for a definition. If this parameter is not present, the default value 1 MUST be assumed.dtxmax: See Section 6.1 of RFC 4788.dtxmin: See Section 6.1 of RFC 4788.hangover: See Section 6.1 of RFC 4788.
Encoding considerations:
This media type is framed binary data (see RFC 4288, Section 4.8) and is defined for transfer of EVRC-B encoded data via RTP using the interleaved/bundled packet format specified in RFC 3558. 🧁
Security considerations: See Section 9 of RFC 4788.
Interoperability considerations: None
Published specification:
The EVRC-B vocoder is specified in 3GPP2 C.S0014-B. The transfer method with the interleaved/bundled packet format via RTP is specified in RFC 3558, RFC 4788, and RFC 5188.
Applications that use this media type:
It is expected that many VoIP applications (as well as mobile applications) will use this type. 📞
Additional information: The following information applies for the storage format only.
- Magic number:
#!EVRC-B\n(see Section 5 of RFC 4788) - File extensions: evb, EVB
- Macintosh file type code: None
- Object identifier or OID: None
Person & email address to contact for further information:
Harikishan Desineni <hd@qualcomm.com>
Intended usage: COMMON
Restrictions on usage:
When this media type is used in the context of transfer over RTP, the RTP payload format specified in Section 4.1 of RFC 3558 SHALL be used. In all other contexts, the file format defined in Section 5 of RFC 4788 SHALL be used. 🌟
Author:
Qiaobing Xie / Harikishan Desineni
Change controller:
IETF Audio/Video Transport working group delegated from the IESG.
9.1.5. Updated Registration of Media Type audio/EVRCB0 🦋
Okay bestie, here's the updated registration for audio/EVRCB0 💖
Type name: audio
Subtype name: EVRCB0
Required parameters: None
Optional parameters: These only apply to RTP transfer 🎀
recvmode: A mode of the EVRC-B codec. A decoder can use this attribute to tell an encoder which mode it would prefer to operate in. Possible values are 0..7 (see the encoder operating point column in Table 2-6 of 3GPP2 C.S0014-B).sendmode: A mode of the EVRC-B codec. An encoder can use this to signal the mode it's currently operating in. Possible values are 0..7 (see the encoder operating point column in Table 2-6 of 3GPP2 C.S0014-B).silencesupp: See Section 6.1 of RFC 4788 for the definition. If this parameter is not present, the default value 1 MUST be assumed 💅dtxmax: See Section 6.1 of RFC 4788.dtxmin: See Section 6.1 of RFC 4788.hangover: See Section 6.1 of RFC 4788.
Encoding considerations: This media type is framed binary data (see RFC 4288, Section 4.8). It's defined for transferring EVRC-B encoded data via RTP using the header-free packet format specified in RFC 3558 ✨
Security considerations: See Section 9 of RFC 4788. 🔐
Interoperability considerations: None
Published specification: The EVRC-B vocoder is specified in 3GPP2 C.S0014-B. The transfer method with the header-free packet format via RTP is specified in RFC 3558, RFC 4788, and RFC 5188 🌸
Applications that use this media type: It's expected that lots of VoIP applications (as well as mobile applications) will use this type 📱
Additional information: None
Person & email address to contact for further information: Harikishan Desineni <hd@qualcomm.com>
Intended usage: COMMON
Restrictions on usage: When this media type is used in the context of transfer over RTP, the RTP payload format specified in Section 4.2 of RFC 3558 SHALL be used.
This media type depends on RTP framing, so it's only defined for transfer via RTP [6]. The RTP payload format specified in Section 4.2 of RFC 3558 SHALL be used. This media type SHALL NOT be used for storage or file transfer using the file format defined in Section 5 of RFC 4788. Instead, audio/EVRCB SHALL be used. No mixing it up, okay? 🙈
Author: Qiaobing Xie / Harikishan Desineni
Change controller: IETF Audio/Video Transport working group delegated from the IESG.
10. SDP Mode Attributes for EVRC-WB and EVRC-B ✨
sendmode can be used by a sender (EVRC-WB or EVRC-B) to announce the mode its encoder is currently operating in. A sender can change its mode anytime, and this does not cause any decoding problems at the receiver 🦋
recvmode is defined for use with EVRC-B. A decoder can use this attribute to tell an encoder which mode it would prefer to operate in. The receiver will keep decoding properly even if the sender doesn't operate in the preferred mode 💜
mode-set-recv is defined for use with EVRC-WB. A decoder can use this attribute to tell an encoder which subset of modes it prefers to operate in. The receiver will keep decoding properly even if the sender doesn't operate in one of the preferred modes. A set has been defined so that several modes can be expressed as a preference in one go. For instance, the set {4,7} signals that the receiver prefers the sender to operate in the narrowband modes of EVRC-WB 🎀
11. EVRC-B Interoperability with Legacy Implementations (RFC 4788) 💅
This document adds the new optional parameters recvmode and sendmode to the original EVRC-B media types audio/EVRCB and audio/EVRCB0, which were defined in RFC 4788 [3]. Existing RFC 4788 [3] implementations won't send these parameters in the Session Description Protocol (SDP), and they'll ignore them if they're received. This allows interoperability between RFC 4788 [3] and RFC 5188 implementations of EVRC-B 🤝 For an example offer-and-answer exchange, see Section 17.
12. Mapping EVRC-WB 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) [8], which is commonly used to describe RTP sessions. When SDP is used to specify sessions employing EVRC-WB encoded speech, the mapping goes like this:
- The media type (
audio) goes in SDPm=as the media name. - The media subtype (
EVRCWB,EVRCWB0, orEVRCWB1) goes in SDPa=rtpmapas the encoding name. - The optional parameters
ptimeandmaxptime(for subtypesEVRCWBandEVRCWB1) go in the SDPa=ptimeanda=maxptimeattributes, respectively. - Any remaining parameters (for subtypes
EVRCWB,EVRCWB0, andEVRCWB1) go in the SDPa=fmtpattribute. You copy them from the media type string as a semicolon-separated list of parameter=value pairs.
13. Mapping EVRC-B Media Type Parameters into SDP 🌈
The new optional parameters recvmode and sendmode (for the audio subtypes EVRCB and EVRCB0) go in the SDP a=fmtp attribute. You copy them directly from the media type string.
For all other media type parameters, the specification in Section 6.7 of RFC 4788 [3] still applies 📝
14. Offer-Answer Model Considerations for EVRC-WB 🫶
These considerations apply when you're using the SDP offer-answer procedures of RFC 3264 [7] to negotiate the use of an EVRC-WB payload in RTP:
- Since EVRC-WB is an extension of EVRC-B, the offerer SHOULD announce EVRC-B support in its
m=audioline, with EVRC-WB as the preferred codec. This allows interoperability with an answerer that only supports EVRC-B 🌸
Here's an example of such an offer:
m=audio 55954 RTP/AVP 98 99
a=rtpmap:98 EVRCWB0/16000
a=rtpmap:99 EVRCB0/8000
a=fmtp:98 mode-set-recv=0,4;sendmode=0
a=fmtp:99 recvmode=0 sendmode=4If the answerer supports EVRC-WB, it can keep payload type 98 in its answer, and the conversation can happen using EVRC-WB 🎉 Otherwise, if the answerer only supports EVRC-B, it leaves only payload type 99 in its answer, and the conversation happens using EVRC-B.
An example answer for the above offer is this:
m=audio 55954 RTP/AVP 98
a=rtpmap:98 EVRCWB0/16000
a=fmtp:98 mode-set-recv=4;sendmode=4mode-set-recvis a unidirectional receive-only parameter.sendmodeis a unidirectional send-only parameter.- Using
sendmode, a sender can signal its current mode of operation. A receiver may receive RTP media well before the arrival of SDP with a (first-time, or updated)sendmodeparameter, so keep that in mind 👀 - An offerer can use
mode-set-recvto request that the remote sender's encoder be limited to the list of modes signaled inmode-set-recv. A remote sender MAY ignoremode-set-recvrequests 🙃 - The parameters
maxptimeandptimewill in most cases not affect interoperability. However, setting them can affect the performance of the application. The SDP offer-answer handling of theptimeparameter is described in RFC 3264 [7], and themaxptimeparameter MUST be handled in the same way 🍓 - For a sendonly stream, the
mode-set-recvparameter isn't useful, so it SHOULD NOT be used. - For a recvonly stream, the
sendmodeparameter isn't useful, so it SHOULD NOT be used. - When using EVRCWB1, the entire session MUST use the same fixed rate and mode (0-Wideband or 4,7-Narrowband) ⭐
- For additional rules that MUST be followed while negotiating DTX parameters, see Section 6.8 in [3].
- Any unknown parameter in an SDP offer MUST be ignored by the receiver, and it MUST NOT be included in the SDP answer 🚫
15. Offer-Answer Model Considerations for EVRC-B 💜
See Section 6.8 of [3] for offer-answer usage of EVRC-B. Here are a few extra considerations for EVRC-B:
recvmodeis a unidirectional receive-only parameter.sendmodeis a unidirectional send-only parameter.- Using
recvmode, a receiver can signal the remote sender to operate its encoder in the specified mode. A remote sender MAY ignorerecvmoderequests. - Using
sendmode, a sender can signal its current mode of operation. A receiver may receive RTP media well before the arrival of SDP with a (first-time, or updated)sendmodeparameter. - For a sendonly stream, the
recvmodeparameter isn't useful, so it SHOULD NOT be used. - For a recvonly stream, the
sendmodeparameter isn't useful, so it SHOULD NOT be used.
16. Declarative SDP Considerations 🎀
For declarative use of SDP in the Session Announcement Protocol (SAP) [12] and the Real Time Streaming Protocol (RTSP) [13], the following considerations apply:
- Any
maxptimeandptimevalues should be selected with care, so the session's participants can get reasonable performance 🧐 - The payload format configuration parameters are all declarative, and a participant MUST use the configuration(s) provided for the session. More than one configuration may be provided if necessary by declaring multiple RTP payload types. However, the number of types should be kept small. For declarative examples, see Section 17 ✨
17. Examples ✨
okay so this is where it all clicks 🧚 here are some SDP session descriptions using EVRC-WB and EVRC-B encodings! heads up: the long a=fmtp lines are folded so they fit the column width of this doc. when you see a backslash ("") at the end of a line and the carriage return after it, just ignore both 💅 also, media subtype names are case-insensitive, and parameter names are case-insensitive both in media types and in the mapping to the SDP a=fmtp attribute 🎀
Example usage of EVRCWB:
m=audio 49120 RTP/AVP 97 98
a=rtpmap:97 EVRCWB/16000
a=rtpmap:98 EVRCB0/8000
a=fmtp:97 mode-set-recv=0,4;sendmode=0
a=fmtp:98 recvmode=0 sendmode=0
a=maxptime:120Example usage of EVRCWB0:
m=audio 49120 RTP/AVP 97 98
a=rtpmap:97 EVRCWB0/16000
a=rtpmap:98 EVRCB0/8000
a=fmtp:97 mode-set-recv=0,4;sendmode=0
a=fmtp:98 recvmode=0 sendmode=0Example SDP answer from a media gateway requesting a terminal to limit its encoder operation to EVRC-WB mode 4:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCWB0/16000
a=fmtp:97 mode-set-recv=4;sendmode=4Example usage of EVRCWB1:
m=audio 49120 RTP/AVP 97 98
a=rtpmap:97 EVRCWB1/16000
a=fmtp:97 mode-set-recv=4;sendmode=4
a=maxptime:100Example usage of EVRCWB with DTX with silencesupp=1:
m=audio 49120 RTP/AVP 97 98
a=rtpmap:97 EVRCWB/16000
a=rtpmap:98 EVRCB0/8000
a=fmtp:97 silencesupp=1;dtxmax=32;dtxmin=12;hangover=1 \
mode-set-recv=0,4; sendmode=0
a=fmtp:98 recvmode=0 sendmode=0
a=maxptime:120Example usage of EVRCWB with DTX with silencesupp=0:
m=audio 49120 RTP/AVP 97 98
a=rtpmap:97 EVRCWB/16000
a=rtpmap:98 EVRCB0/8000
a=fmtp:97 silencesupp=0;dtxmax=32;dtxmin=12;hangover=1 \
mode-set-recv=0,4;sendmode=0
a=fmtp:98 recvmode=0 sendmode=0
a=maxptime:120Example usage of EVRCB:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB/8000
a=fmtp:97 recvmode=0 sendmode=4
a=maxptime:120Example usage of EVRCB0:
m=audio 49120 RTP/AVP 97
a=rtpmap:97 EVRCB0/8000
a=fmtp:97 recvmode=0 sendmode=4Example offer-answer exchange between EVRC-WB and legacy EVRC-B (RFC 4788) 🦋
Offer:
m=audio 55954 RTP/AVP 98 99
a=rtpmap:98 EVRCWB0/16000
a=rtpmap:99 EVRCB0/8000
a=fmtp:98 mode-set-recv=0,4;sendmode=0
a=fmtp:99 recvmode=0 sendmode=0Answer:
m=audio 55954 RTP/AVP 99
a=rtpmap:99 EVRCB0/8000Example offer-answer exchange between EVRC-WB and updated EVRC-B (RFC 5188) 💜
Offer:
m=audio 55954 RTP/AVP 98 99
a=rtpmap:98 EVRCWB0/16000
a=rtpmap:99 EVRCB0/8000
a=fmtp:98 mode-set-recv=0,4; sendmode=0
a=fmtp:99 recvmode=0 sendmode=0Answer:
m=audio 55954 RTP/AVP 99
a=rtpmap:99 EVRCB0/8000
a=fmtp:99 recvmode=0 sendmode=4In the above example, the answerer picked sendmode 4 even though the offerer was happy to receive in mode 0. so recvmode is the receiver's preference, but the sender can totally send in a different mode 🫶
Example offer-answer exchanges for interoperability between legacy (RFC 4788) and updated EVRC-B (RFC 5188) implementations 🌈
Offer from an offerer that supports the updated EVRC-B (RFC 5188) implementation:
m=audio 55954 RTP/AVP 99
a=rtpmap:99 EVRCB0/8000
a=fmtp:99 recvmode=0 sendmode=4Answer from an answerer that supports only the legacy EVRC-B (RFC 4788) implementation:
m=audio 55954 RTP/AVP 99
a=rtpmap:99 EVRCB0/8000Offer from an offerer that supports only the legacy EVRC-B (RFC 4788) implementation:
m=audio 55954 RTP/AVP 99
a=rtpmap:99 EVRCB0/8000Answer from an answerer that supports the updated EVRC-B (RFC 5188) implementation:
m=audio 55954 RTP/AVP 99
a=rtpmap:99 EVRCB0/8000
a=fmtp:99 recvmode=0 sendmode=418. Security Considerations 🔒
okay this part is seriously important, bestie 😭 compression gets applied to the payload formats end-to-end, and the encodings don't show any big non-uniformity. because of that, anything that implements this spec is subject to ALL the security considerations in RFC 3558 [2]. and if you're using the payload defined here, you also have to think about the security stuff in RFC 3558 [2], RFC 3550 [6], and whatever profile applies (like RFC 3551 [11]). no skipping the homework, periodt 💀
19. Changes to RFC 4788 🦋
this one's the tea ☕ this document updates RFC 4788 [3], and here's the rundown:
- added a new media type attribute "sendmode" to the media subtypes EVRCB and EVRCB0. it can be used to signal the EVRC-B encoder's current mode of operation 🎀
- added a new media type attribute "recvmode" to the media subtypes EVRCB and EVRCB0. it can be used to signal the EVRC-B decoder's preferred operating mode to a remote sender 💖
20. References 📚
20.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Li, A., "RTP Payload Format for Enhanced Variable Rate Codecs (EVRC) and Selectable Mode Vocoders (SMV)", RFC 3558, July 2003.
[3] Xie, Q. and R. Kapoor, "Enhancements to RTP Payload Formats for EVRC Family Codecs", RFC 4788, January 2007.
[4] "Enhanced Variable Rate Codec, Speech Service Option 3 and 68 for Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014-B v1.0 , May 2006.
[5] "Enhanced Variable Rate Codec, Speech Service Option 3,68 and 70 for Wideband Spread Spectrum Digital Systems", 3GPP2 C.S0014-C v1.0 , October 2006.
[6] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, March 1997.
[7] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.
[8] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[9] Casner, S., "Media Type Specifications and Registration Procedures", RFC 4855, February 2007.
[10] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005.
20.2. Informative References
[11] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[12] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000.
[13] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998.
Authors' Addresses 🎀
Harikishan Desineni
Qualcomm
5775 Morehouse Drive
San Diego, CA 92126
USA
Phone: +1 858 845 8996
EMail: hd@qualcomm.com
URI: http://www.qualcomm.com
Qiaobing Xie
Motorola
1501 W. Shure Drive, 2-F9
Arlington Heights, IL 60004
USA
Phone: +1-847-372-8481
EMail: Qiaobing.Xie@Gmail.com
URI: http://www.motorola.comFull Copyright Statement 💜
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.
This document and the information contained herein 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 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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copies of IPR disclosures made to the IETF Secretariat, any assurances of licenses to be made available, or the result of an attempt to get a general license or permission for implementers or users of this spec can be found in the IETF on-line IPR repository at http://www.ietf.org/ipr.
the IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology required to implement this standard. send that info to the IETF at ietf-ipr@ietf.org 🫶
📦 where this page comes from
- ietf: rfc5188 2008-02
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms