RFC 8373
Negotiating Human Language in Real-Time Communications
RFC 8373: Negotiating Human Language in Real-Time Communications 🎀
Okay bestie, this is RFC 8373 from the IETF (Internet Engineering Task Force) 🌈
- Author: R. Gellens, Core Technology Consulting 💅
- Category: Standards Track ✨
- Date: May 2018
- ISSN: 2070-1721
- Obsoletes / Updates: the title block doesn't list any, so this one doesn't obsolete or update anything 🧚
Abstract ✨
Okay so people have all kinds of human (aka natural) language needs, skills, and preferences when it comes to spoken, written, and signed languages 🫶 This doc defines new Session Description Protocol (SDP) media-level attributes so that when two people set up a live call, the caller's language and media needs can get matched up with what the called party can actually do. The way that works is they negotiate, which means they communicate and match 💖
This is especially huge for emergency calls, because a call can be handled by a call taker who can actually talk to the user, or a translator or relay operator can get bridged in during setup 😭 But it also works for non-emergency calls, like calling a company call center 📞
This doc describes the need and a solution that uses the new SDP media attributes 🦋
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 got public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more info on Internet Standards, check out Section 2 of RFC 7841 👑
For the current status, any errata, and how to give feedback on this doc, go to https://www.rfc-editor.org/info/rfc8373 🔥
Copyright Notice 💜
Copyright (c) 2018 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please read these carefully, because they describe your rights and restrictions for this doc 🙈 Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they come without warranty as described in the Simplified BSD License.
Table of Contents 🍓
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Desired Semantics . . . . . . . . . . . . . . . . . . . . . . 5
4. The Existing 'lang' Attribute . . . . . . . . . . . . . . . . 5
5. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. The 'hlang-send' and 'hlang-recv' Attributes . . . . . . 5
5.2. No Language in Common . . . . . . . . . . . . . . . . . . 7
5.3. Usage Notes . . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. att-field Subregistry of SDP Parameters . . . . . . . . . 10
6.2. Warning Codes Subregistry of SIP Parameters . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 131. Introduction 🌷
A language everyone can understand is so helpful for talking to each other, honestly 💖 This doc is about negotiating human language and media modality (spoken, signed, or written) in real-time communications. There's a companion doc, [RFC8255], that handles language selection in email 📧
Unless the caller and callee already know each other, or there's context or out-of-band info that tells you the language(s) and media modalities, spoken, signed, or written languages need to get negotiated based on what the caller needs and what the callee can do 🫶 This applies to both emergency and non-emergency calls. An example of a non-emergency call is a caller contacting a company call center. An emergency call usually means a caller reaching a Public Safety Answering Point (PSAP) 🚨 In either case, it's super helpful for the caller to say which signed, written, and/or spoken languages they prefer, and for the callee to say what it's capable of. That way the call can go ahead using the languages and media forms that both sides support ✨
Okay so there are a bunch of reasons it makes sense to use a per-stream negotiation mechanism called the Session Description Protocol (SDP), including being able to set up multiple streams with different media (voice, text, and/or video) 🦋 Using SDP [RFC4566] means the solution in this doc can apply to all interactive communications negotiated with SDP, for both emergency and non-emergency situations 💅
Treating language as another SDP attribute that gets negotiated alongside the other parts of a media stream means you can accommodate a range of users' needs and the called party's facilities 🧚 For example, some users can speak several languages but still have a preference. Some called parties support some of those languages in-house but need a translation service for others, or they might only have a few call takers who can use certain languages. Another example is a user who can speak but is deaf or hard of hearing, and who wants a voice stream to send spoken language plus a text stream to receive written language 🙈 Making language a media attribute lets standard session negotiation handle all of this, by giving the endpoints the info and mechanism they need to make good decisions 👑
We use the term "negotiation" here instead of "indication" because human language (spoken/written/signed) can be negotiated the same way as media (audio/text/video) and codecs 🎀 Say a user is calling an airline reservation center. The user might be able to use a set of languages, maybe with preferences for one or a few, while the reservation center supports a fixed set of languages. Negotiation should select the user's most preferred language that the call center supports. Both sides should also know which language got negotiated 💜
In the offer/answer model used here, the offer contains a set of languages per media (and direction) that the offerer can use, and the answer contains one language per media (and direction) that the answerer will support 🌈 Supporting languages and/or modalities can take extra steps, like bridging in external translation or relay resources, or having the call handled by an agent who speaks a requested language and/or can use a requested modality ⭐ The answer shows the media and languages the answerer is committing to support (possibly after those extra steps have been taken). This model also gives both ends the knowledge of what got negotiated 🦋 Heads up: the additional steps needed to support the indicated languages or modalities may or may not be in place in time for any early media 😭
Since this is a protocol mechanism, the user equipment (UE) client needs to know the user's preferred languages. This doc doesn't cover how clients figure that out, but reasonable techniques could include a configuration mechanism with a default of the language of the user interface 💅 In some cases, a UE client could tie language and media preferences together, like a preference for a video stream using a signed language and/or a text or audio stream using a written/spoken language 🫶
This doc doesn't address user interface (UI) issues, like if or how a UE client tells a user the result of the language and media negotiation 🙈
1.1. Applicability 🌸
In this doc, it's assumed that the negotiating endpoints have already been determined, so a per-stream negotiation based on SDP can go ahead 💖
When setting up interactive communication sessions, signaling messages need to be routed to the appropriate endpoint(s). This doc doesn't tackle the problem of language-based routing 🧚
2. Terminology 💜
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 🎀
3. Desired Semantics ✨
Okay so the dream solution is a media attribute (preferably per direction) that can be used in an offer to indicate the preferred language(s) of each (direction of a) media stream, and in an answer to indicate the accepted language. When an offer includes multiple languages for a media stream, they get listed in order of preference, most preferred first 👑
Heads up: negotiating multiple simultaneous languages within one media stream is out of scope for this doc 🙈
4. The Existing 'lang' Attribute 🦋
RFC 4566 [RFC4566] specifies an attribute 'lang' that's kind of similar to what's needed here, but it isn't specific or flexible enough for this doc's needs 😭 Also, 'lang' isn't mentioned in [RFC3264], and there are no known implementations in SIP. Plus, it's useful to be able to specify language per direction (sending and receiving). So this document defines two new attributes 💅
5. Solution 🌈
An SDP attribute (per direction) seems like the natural choice for negotiating the human language of an interactive media stream, using the language tags of [BCP47] 🧚
To be continued in part 2 💖
5.1. The hlang-send and hlang-recv Attributes 💖
Okay bestie, so this doc gives us two media-level attributes: hlang-send and hlang-recv (they're registered over in Section 6 🎀). Both start with hlang, short for "human language" 🌸. They're how two parties agree on which human language gets used in each direction of each interactive media stream. Not every stream has to get used, btw ✨. They can show up on media streams in both offers and answers.
In an offer, the hlang-send value is a list of one or more language(s) the offerer is willing to use when sending on that media, and the hlang-recv value is a list of one or more language(s) the offerer is willing to use when receiving on it. The list goes in preference order, so the first one is the most preferred 👑.
When a media is meant for interactive communication in only one direction, either hlang-send or hlang-recv MAY be omitted. The example is a user in France who has trouble speaking but can hear, and who wants to receive French over audio and send French over text 🇫🇷. The media can still be useful in both directions, though. When a media isn't really about language at all (like a video or audio stream that's just for background), both SHOULD be omitted. Otherwise, both SHOULD have the same value.
Heads up: specifying different languages for each direction (instead of the same, or basically the same, language in different modalities) can make it really hard to finish the call 😭. Like, asking to send audio in Hungarian and receive audio in Portuguese? Good luck with that.
In an answer, hlang-send is the language the answerer will send if using the media for language (which in most cases is one of the languages in the offer's hlang-recv). hlang-recv is the language the answerer expects to receive if using the media for language (which in most cases is one of the languages in the offer's hlang-send).
Now the syntax rules, which are important 🧐. In an offer, each value MUST be a list of one or more language tags per [BCP47], separated by white space. In an answer, each value MUST be one language tag per [BCP47]. [BCP47] describes the mechanisms for matching language tags. Also, Section 4.1 of RFC 5646 [BCP47] advises to "tag content wisely" and not include unnecessary subtags. Be wise, be minimal 🫶.
When placing an emergency call, and in any other case where the language can't be inferred from context, each OFFERed media stream primarily intended for human language communication SHOULD specify the hlang-send and/or hlang-recv attributes for the direction(s) intended for interactive communication 🚨.
Clients acting on behalf of end users are expected to set one or both of the hlang-send and hlang-recv attributes on each OFFERed media stream primarily intended for human communication when placing an outgoing session. When receiving incoming calls, they're expected to either ignore the attributes or take them into consideration, based on local configuration and capabilities. Systems acting on behalf of call centers and PSAPs are expected to take the attributes into account when processing inbound calls 📞.
One more thing: media and language negotiation might end up accepting more media streams than the users actually need (for example, if more preferred and less preferred combinations of media and language all get accepted). This is not a problem 💅.
5.2. No Language in Common 🙅♀️
A question that comes up when negotiating language is whether the call proceeds or fails if the callee doesn't support any of the languages the caller asked for. This document does not mandate either behavior, so that's up to the implementation.
When a call is rejected because there's no language in common, the SIP response has SIP response code 488 (Not Acceptable Here) or 606 (Not Acceptable) [RFC3261]. It also has a Warning header field [RFC3261] with a warning code of 308, and warning text saying that there are no mutually supported languages. The warning text SHOULD also list the supported languages and media, so the caller knows what would've worked 🥺.
Example:
Warning: 308 proxy.example.com "Incompatible language
specification: Requested languages not supported. Supported
languages are: es, en; supported media are: audio, text."5.3. Usage Notes 📝
A sign-language tag with a video media stream is read as an indication for sign language in the video stream. A non-sign-language tag with a text media stream is read as an indication for written language in the text stream. A non-sign-language tag with an audio media stream is read as an indication for spoken language in the audio stream. Easy peasy 🦋.
This document does not define any other use for language tags in video media (such as how to indicate visible captions in the video stream).
This document does not define the use of sign-language tags in text or audio media.
In the IANA registry for language subtags per [BCP47], a language subtag with a Type field of "extlang" combined with a Prefix field value of "sgn" indicates a sign-language tag. If that "sgn" prefix is absent, it's a non-sign-language tag 🤟.
This document does not define the use of language tags in media other than interactive streams of audio, video, and text (such as "message" or "application"). Such use could be supported by future work or by application agreement 🔮.
5.4. Examples 🎀
Here are some examples. For clarity, only the most directly relevant parts of the SDP block are shown.
An offer or answer indicating spoken English both ways:
m=audio 49170 RTP/AVP 0
a=hlang-send:en
a=hlang-recv:enAn offer indicating American Sign Language both ways:
m=video 51372 RTP/AVP 31 32
a=hlang-send:ase
a=hlang-recv:aseAn offer requesting spoken Spanish both ways (most preferred), spoken Basque both ways (second preference), or spoken English both ways (third preference):
m=audio 49250 RTP/AVP 20
a=hlang-send:es eu en
a=hlang-recv:es eu enAn answer to the above offer indicating spoken Spanish both ways:
m=audio 49250 RTP/AVP 20
a=hlang-send:es
a=hlang-recv:esAn alternative answer to the above offer indicating spoken Italian both ways (as the callee does not support any of the requested languages but chose to proceed with the call):
m=audio 49250 RTP/AVP 20
a=hlang-send:it
a=hlang-recv:itAn offer or answer indicating written Greek both ways:
m=text 45020 RTP/AVP 103 104
a=hlang-send:gr
a=hlang-recv:grAn offer requesting the following media streams: video for the caller to send using Argentine Sign Language, text for the caller to send using written Spanish (most preferred) or written Portuguese, and audio for the caller to receive spoken Spanish (most preferred) or spoken Portuguese:
m=video 51372 RTP/AVP 31 32
a=hlang-send:aed
m=text 45020 RTP/AVP 103 104
a=hlang-send:sp pt
m=audio 49250 RTP/AVP 20
a=hlang-recv:sp ptAn answer for the above offer, indicating text in which the callee will receive written Spanish and audio in which the callee will send spoken Spanish. (The answering party has no video capability):
m=video 0 RTP/AVP 31 32
m=text 45020 RTP/AVP 103 104
a=hlang-recv:sp
m=audio 49250 RTP/AVP 20
a=hlang-send:spAn offer requesting the following media streams: text for the caller to send using written English (most preferred) or written Spanish, audio for the caller to receive spoken English (most preferred) or spoken Spanish, and supplemental video:
m=text 45020 RTP/AVP 103 104
a=hlang-send:en sp
m=audio 49250 RTP/AVP 20
a=hlang-recv:en sp
m=video 51372 RTP/AVP 31 32An answer for the above offer, indicating text in which the callee will receive written Spanish, audio in which the callee will send spoken Spanish, and supplemental video:
m=text 45020 RTP/AVP 103 104
a=hlang-recv:sp
m=audio 49250 RTP/AVP 20
a=hlang-send:sp
m=video 51372 RTP/AVP 31 32Note that, even though the examples show the same (or essentially the same) language being used in both directions (even when the modality differs), there is no requirement that this be the case. However, in practice, doing so is likely to increase the chances of successful matching 💜.
6. IANA Considerations 🌈
6.1. att-field Subregistry of SDP Parameters
The syntax in this section uses ABNF per RFC 5234 [RFC5234].
IANA has added two entries to the "att-field (media level only)" subregistry of the "Session Description Protocol (SDP) Parameters" registry.
The first entry is for hlang-recv:
Attribute Name: hlang-recv
Long-Form English Name: human language receive
Contact Name: Randall Gellens
Contact Email Address: rg+ietf@coretechnologyconsulting.com
Attribute Value: hlang-value
Attribute Syntax:
hlang-value = hlang-offv / hlang-ansv
; hlang-offv used in offers
; hlang-ansv used in answers
hlang-offv = Language-Tag *( SP Language-Tag )
; Language-Tag as defined in [BCP47]
SP = 1*" " ; one or more space (%x20) characters
hlang-ansv = Language-Tag
Attribute Semantics: Described in Section 5.1 of RFC 8373
Usage Level: media
Mux Category: NORMAL
Charset Dependent: No
Purpose: See Section 5.1 of RFC 8373
O/A Procedures: See Section 5.1 of RFC 8373
Reference: RFC 8373The second entry is for hlang-send:
Attribute Name: hlang-send
Long-Form English Name: human language send
Contact Name: Randall Gellens
Contact Email Address: rg+ietf@coretechnologyconsulting.com
Attribute Value: hlang-value
Attribute Syntax:
hlang-value = hlang-offv / hlang-ansv
Attribute Semantics: Described in Section 5.1 of RFC 8373
Usage Level: media
Mux Category: NORMAL
Charset Dependent: No
Purpose: See Section 5.1 of RFC 8373
O/A Procedures: See Section 5.1 of RFC 8373
Reference: RFC 83736.2. Warning Codes Subregistry of SIP Parameters
IANA has added the value 308 to the "Warning Codes (warn-codes)" subregistry of the "Session Initiation Protocol (SIP) Parameters" registry. (The value lies within the range allocated for indicating problems with keywords in the session description.) The reference is to this document. The warn text is:
"Incompatible language specification: Requested languages not supported. Supported languages are [list of supported languages]; supported media are: [list of supported media]."7. Security Considerations 🚨
The Security Considerations of [BCP47] apply here. An attacker with the ability to modify signaling could prevent a call from succeeding by altering any of several crucial elements, including the hlang-send or hlang-recv values. RFC 5069 [RFC5069] discusses such threats. Use of TLS or IPsec can protect against such threats 🔐.
Emergency calls are of particular concern. RFC 6881 [RFC6881], which is specific to emergency calls, mandates use of TLS or IPsec (in ED-57/SP-30). This is not a drill, bestie 😱.
8. Privacy Considerations 🙈
Language and media information can suggest a user's nationality, background, abilities, disabilities, etc. So be mindful of what you're sharing 🫣.
9. References 📚
9.1. Normative References 📚
Okay bestie, these are the docs the spec leans on, so they're required reading 🤓💖
[BCP47] Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
2006.
Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.9.2. Informative References 🧠
These ones are the extra-credit reading, the "ooh tell me more" stuff 🦋
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
[RFC5069] Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
Shanmugam, "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069,
DOI 10.17487/RFC5069, January 2008,
<https://www.rfc-editor.org/info/rfc5069>.
[RFC6881] Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in Support of Emergency Calling",
BCP 181, RFC 6881, DOI 10.17487/RFC6881, March 2013,
<https://www.rfc-editor.org/info/rfc6881>.
[RFC8255] Tomkinson, N. and N. Borenstein, "Multiple Language
Content Type", RFC 8255, DOI 10.17487/RFC8255, October
2017, <https://www.rfc-editor.org/info/rfc8255>.Acknowledgements 🫶
Huge, huge thank-you hugs to everyone who showed up with reviews, corrections, suggestions, and both email and in-person discussions 💅✨ Bernard Aboba, Harald Alvestrand, Flemming Andreasen, Francois Audet, Eric Burger, Keith Drage, Doug Ewell, Christian Groves, Andrew Hutton, Hadriel Kaplan, Ari Keranen, John Klensin, Mirja Kuhlewind, Paul Kyzivat, John Levine, Alexey Melnikov, Addison Phillips, James Polk, Eric Rescorla, Pete Resnick, Alvaro Retana, Natasha Rooney, Brian Rosen, Peter Saint-Andre, and Dale Worley, you are all absolutely iconic 👑🌈
Contributors 🎀
Gunnar Hellstrom deserves a special shoutout 📣 for his reviews and for helping out along the way 💜 literally a legend.
Author's Address 🦋
Randall Gellens
Core Technology Consulting
Email: rg+ietf@coretechnologyconsulting.com
URI: http://www.coretechnologyconsulting.com📦 where this page comes from
- ietf: rfc8373 2018-05
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms