| Network Working Group | R. Gellens |
| Internet-Draft | Core Technology Consulting |
| Intended status: Standards Track | October 13, 2017 |
| Expires: April 16, 2018 |
Negotiating Human Language in Real-Time Communications
draft-ietf-slim-negotiating-human-language-15
Users have various human (natural) language needs, abilities, and preferences regarding spoken, written, and signed languages. This document adds new SDP media-level attributes so that when establishing interactive communication sessions ("calls"), it is possible to negotiate (communicate and match) the caller's language and media needs with the capabilities of the called party. This is especially important with emergency calls, where a call can be handled by a call taker capable of communicating with the user, or a translator or relay operator can be bridged into the call during setup, but this applies to non-emergency calls as well (as an example, when calling a company call center).
This document describes the need and a solution using new SDP media attributes.
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A mutually comprehensible language is helpful for human communication. This document addresses the negotiation of human (natural) language and media modality (spoken, signed, written) in real-time communications. A companion document [RFC8255] addresses language selection in email.
Unless the caller and callee know each other or there is contextual or out-of- band information from which the language(s) and media modalities can be determined, there is a need for spoken, signed, or written languages to be negotiated based on the caller's needs and the callee's capabilities. This need applies to both emergency and non-emergency calls. For example, it is helpful for a caller to a company call center or a Public Safety Answering Point (PSAP) to be able to indicate preferred signed, written, and/or spoken languages, and for the callee to be able to indicate its capabilities in this area, allowing the call to proceed using the language(s) and media forms supported by both.
For various reasons, including the ability to establish multiple streams using different media (e.g., voice, text, video), it makes sense to use a per-stream negotiation mechanism known as the Session Description Protocol (SDP). Utilizing SDP enables the solution described in this document to be applied to all interactive communications negotiated using SDP, in emergency as well as non-emergency scenarios.
By treating language as another SDP attribute that is negotiated along with other aspects of a media stream, it becomes possible to accommodate a range of users' needs and called party facilities. For example, some users may be able to speak several languages, but have a preference. Some called parties may support some of those languages internally but require the use of a translation service for others, or may have a limited number of call takers able to use certain languages. Another example would be a user who is able to speak but is deaf or hard-of-hearing and and desires a voice stream to send spoken language plus a text stream to receive written language. Making language a media attribute allows the standard session negotiation mechanism to handle this by providing the information and mechanism for the endpoints to make appropriate decisions.
The term "negotiation" is used here rather than "indication" because human language (spoken/written/signed) can be negotiated in the same manner as media (audio/text/video) and codecs. For example, if we think of a user calling an airline reservation center, the user may have a set of languages he or she speaks, with perhaps preferences for one or a few, while the airline reservation center will support a fixed set of languages. Negotiation should select the user's most preferred language that is supported by the call center. Both sides should be aware of which language was negotiated. This is conceptually similar to the way other aspects of each media stream are negotiated using SDP (e.g., media type and codecs).
Since this is a protocol mechanism, the user equipment (UE client) needs to know the user's preferred languages; while this document does not address how clients determine this, reasonable techniques could include a configuration mechanism with a default of the language of the user interface; in some cases, a UE could tie language and media preferences, such as a preference for a video stream using a signed language and/or a text or audio stream using a written/spoken language.
Within this document, it is assumed that the negotiating endpoints have already been determined, so that a per-stream negotiation based on the Session Description Protocol (SDP) can proceed.
When setting up interactive communications sessions it is necessary to route signaling messages to the appropriate endpoint(s). This document does not address the problem of language-based routing.
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.
The desired solution is a media attribute (preferably per direction) that may be used within an offer to indicate the preferred language(s) of each (direction of a) media stream, and within an answer to indicate the accepted language. The semantics of including multiple languages for a media stream within an offer is that the languages are listed in order of preference.
(Negotiating multiple simultaneous languages within a media stream is out of scope of this document.)
RFC 4566 [RFC4566] specifies an attribute 'lang' which appears similar to what is needed here, but is not sufficiently specific or flexible for the needs of this document. In addition, 'lang' is not mentioned in [RFC3264] and there are no known implementations in SIP. Further, it is useful to be able to specify language per direction (sending and receiving). This document therefore defines two new attributes.
An SDP attribute (per direction) seems the natural choice to negotiate human (natural) language of an interactive media stream, using the language tags of BCP 47 [RFC5646].
The decision to base the proposal at the media negotiation level, and specifically to use SDP, came after significant debate and discussion. From an engineering standpoint, it is possible to meet the objectives using a variety of mechanisms, but none are perfect. None of the proposed alternatives was clearly better technically in enough ways to win over proponents of the others, and none were clearly so bad technically as to be easily rejected. As is often the case in engineering, choosing the solution is a matter of balancing trade-offs, and ultimately more a matter of taste than technical merit. The two main proposals were to use SDP and SIP. SDP has the advantage that the language is negotiated with the media to which it applies, while SIP has the issue that the languages expressed may not match the SDP media negotiated (for example, a session could negotiate a signed language at the SIP level but fail to negotiate a video media stream at the SDP layer).
The mechanism described here for SDP can be adapted to media negotiation protocols other than SDP.
This document defines two media-level attributes starting with 'hlang' (short for "human interactive language") to negotiate which human language is selected for use in each interactive media stream. There are two attributes, one ending in "-send" and the other in "-recv", registered in Section 6. Each can appear in offers and answers for media streams.
In an offer, the 'hlang-send' value is a list of one or more language(s) the offerer is willing to use when sending using the media, and the 'hlang-recv' value is a list of one or more language(s) the offerer is willing to use when receiving using the media. The list of languages is in preference order (first is most preferred). When a media is intended for interactive communication using a language in one direction only (such as a user sending using text and receiving using audio), either hlang-send or hlang-recv MAY be omitted. When a media is not primarily intended for language (for example, a video or audio stream intended for background only) both SHOULD be omitted. Otherwise, both SHOULD have the same value. The two SHOULD NOT be set to languages which are difficult to match together (e.g., specifying a desire to send audio in Hungarian and receive audio in Portuguese will make it difficult to successfully complete the call).
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'), and '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').
Each value MUST be a list of one or more language tags per BCP 47 [RFC5646], separated by white space. BCP 47 describes mechanisms for matching language tags. Note that [RFC5646] Section 4.1 advises to "tag content wisely" and not include unnecessary subtags.
When placing an emergency call, and in any other case where the language cannot be inferred from context, in an offer each media stream primarily intended for human language communication SHOULD specify both (or for asymmetrical language use, one of) the 'hlang-send' and 'hlang-recv' attributes.
Note that while signed language tags are used with a video stream to indicate sign language, a video stream in parallel with an audio stream, both using the exact same (spoken) language tag, indicates a request for a supplemental video stream to see the speaker.
Clients acting on behalf of end users are expected to set one or both 'hlang-send' and 'hlang-recv' attributes on each media stream primarily intended for human communication in an offer when placing an outgoing session, and either ignore or take into consideration the attributes when receiving incoming calls, based on local configuration and capabilities. Systems acting on behalf of call centers and PSAPs are expected to take into account the attributes when processing inbound calls.
Note that media and language negotiation might result in more media streams being accepted than are needed by the users (e.g., if more preferred and less preferred combinations of media and language are all accepted). This is not a problem.
A consideration with the ability to negotiate language is if the call proceeds or fails if the callee does not support any of the languages requested by the caller. This document does not mandate either behavior.
If the call is rejected due to lack of any languages in common, it is suggested to use SIP response code 488 (Not Acceptable Here) or 606 (Not Acceptable) [RFC3261] and include a Warning header field [RFC3261] in the SIP response. The Warning header field contains a warning code of [TBD: IANA VALUE, e.g., 308] and a warning text indicating that there are no mutually-supported languages; the text SHOULD also contain the supported languages and media.
Example:
With the exception of the case mentioned in Section 5.2 (an audio stream in parallel with a video stream with the exact same (spoken) language tag), the behavior when specifying a non-signed language tag for a video media stream, or a signed language tag for an audio or text media stream, is not defined in this document.
The problem of knowing which language tags are signed and which are not is out of scope of this document.
Some examples are shown below. For clarity, only the most directly relevant portions of the SDP block are shown.
An offer or answer indicating spoken English both ways:
An offer indicating American Sign Language both ways:
An offer requesting spoken Spanish both ways (most preferred), spoken Basque both ways (second preference), or spoken English both ways (third preference):
An answer to the above offer indicating spoken Spanish both ways:
An 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):
An offer or answer indicating written Greek both ways:
An 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, audio for the caller to receive spoken Spanish (most preferred) or spoken Portuguese:
An 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 had no video capability:
An 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, supplemental video:
An 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:
Note 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.
IANA is kindly requested to add two entries to the 'att-field (media level only)' table of the SDP parameters registry:
IANA is requested to add a new value in the warn-codes sub-registry of SIP parameters in the 300 through 329 range that is 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 and media are: [list of supported languages and media]."
The Security Considerations of BCP 47 [RFC5646] apply here. In addition, if the 'hlang-send' or 'hlang-recv' values are altered or deleted en route, the session could fail or languages incomprehensible to the caller could be selected; however, this is also a risk if any SDP parameters are modified en route.
Language and media information can suggest a user's nationality, background, abilities, disabilities, etc.
RFC EDITOR: Please remove this section prior to publication.
Gunnar Hellstrom deserves special mention for his reviews and assistance.
Many thanks to Bernard Aboba, Harald Alvestrand, Flemming Andreasen, Francois Audet, Eric Burger, Keith Drage, Doug Ewell, Christian Groves, Andrew Hutton, Hadriel Kaplan, Ari Keranen, John Klensin, Paul Kyzivat, John Levine, Alexey Melnikov, James Polk, Pete Resnick, Natasha Rooney, Brian Rosen, Peter Saint-Andre, and Dale Worley for reviews, corrections, suggestions, and participating in in-person and email discussions.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [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. |
| [RFC4566] | Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006. |
| [RFC5646] | Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009. |
| [RFC3264] | Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002. |
| [RFC8255] | Tomkinson, N. and N. Borenstein, "Multiple Language Content Type", RFC 8255, DOI 10.17487/RFC8255, October 2017. |