Negotiating Human Language in Real-Time Communications

Abstract

Users have various human (i.e., natural) language needs, abilities, and preferences regarding spoken, written, and signed languages. This document defines new Session Description Protocol (SDP) media- level attributes so that when establishing interactive communication sessions ("calls"), it is possible to negotiate (i.e., communicate and match) the caller's language and media needs with the capabilities of the called party. This is especially important for emergency calls, because it allows for a call to be handled by a call taker capable of communicating with the user or for a translator or relay operator to be bridged into the call during setup. However, this also applies to non-emergency calls (for example, calls to a company call center).

This document describes the need as well as a solution that uses 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 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 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8373.

Copyright Notice

Copyright © 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 review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     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  . . . . . . . . . . . . . . . . . . . . . . . .  13

1. Introduction

A mutually comprehensible language is helpful for human communication. This document addresses the negotiation of human language and media modality (spoken, signed, or 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. An example of a non-emergency call is when a caller contacts a company call center; an emergency call typically involves a caller contacting a Public Safety Answering Point (PSAP). In such scenarios, it is helpful for the caller to be able to indicate preferred signed, written, and/or spoken languages and for the callee to be able to indicate its capabilities; this allows 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 (i.e., voice, text, and/or video), it makes sense to use a per-stream negotiation mechanism known as the Session Description Protocol (SDP). Utilizing SDP [RFC4566] 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 they 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 desires a voice stream to send spoken language plus a text stream to receive written language. Making language a media attribute allows standard session negotiation 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 be able to use a set of languages, perhaps with preferences for one or a few, while the airline reservation center may 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.

In the offer/answer model used here, the offer contains a set of languages per media (and direction) that the offerer is capable of using, and the answer contains one language per media (and direction) that the answerer will support. Supporting languages and/or modalities can require taking extra steps, such as bridging external translation or relay resources into the call or having a call handled by an agent who speaks a requested language and/or has the ability to use a requested modality. The answer indicates the media and languages that the answerer is committing to support (possibly after additional steps have been taken). This model also provides knowledge so both ends know what has been negotiated. Note that additional steps required 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; 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 client 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.

This document does not address user interface (UI) issues, such as if or how a UE client informs a user about the result of language and media negotiation.

1.1. Applicability

Within this document, it is assumed that the negotiating endpoints have already been determined so that a per-stream negotiation based on SDP can proceed.

When setting up interactive communication sessions, it is necessary to route signaling messages to the appropriate endpoint(s). This document does not address 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

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. When multiple languages are included for a media stream within an offer, the languages are listed in order of preference (most preferred first).

Note that negotiating multiple simultaneous languages within a media stream is out of scope of this document.

4. The Existing 'lang' Attribute

RFC 4566 [RFC4566] specifies an attribute 'lang' that is 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.

5. Solution

An SDP attribute (per direction) seems the natural choice to negotiate human language of an interactive media stream, using the language tags of [BCP47].

5.1. The 'hlang-send' and 'hlang-recv' Attributes

This document defines two media-level attributes: 'hlang-send' and 'hlang-recv' (registered in Section 6). Both start with 'hlang', short for "human language". These attributes are used to negotiate which human language is selected for use in (each direction of) each interactive media stream. (Note that not all streams will necessarily be used.) Each can appear for media streams in 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 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 in only one direction (e.g., a user in France with difficulty speaking but able to hear who indicates a desire to receive French using audio and send French using text), either 'hlang-send' or 'hlang-recv' MAY be omitted. Note that the media can still be useful in both directions. 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. Note that specifying different languages for each direction (as opposed to the same, or essentially the same, language in different modalities) can make it difficult to complete the call (e.g., specifying a desire to send audio in Hungarian and receive audio in Portuguese).

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').

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 mechanisms for matching language tags. Note that Section 4.1 of RFC 5646 [BCP47] 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, 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, 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 the attributes into account 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.

5.2. No Language in Common

A consideration regarding the ability to negotiate language is whether 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.

When a call is rejected due to lack of any language in common, the SIP response has SIP response code 488 (Not Acceptable Here) or 606 (Not Acceptable) [RFC3261] and a Warning header field [RFC3261] with a warning code of 308 and warning text indicating that there are no mutually supported languages; the warning text SHOULD also contain the supported languages and media.

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 interpreted as an indication for sign language in the video stream. A non-sign- language tag with a text media stream is interpreted as an indication for written language in the text stream. A non-sign-language tag with an audio media stream is interpreted as an indication for spoken language in the audio stream.

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 "extlang" combined with a Prefix field value "sgn" indicates a sign-language tag. The absence of such "sgn" prefix indicates 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

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:

      m=audio 49170 RTP/AVP 0
      a=hlang-send:en
      a=hlang-recv:en

An offer indicating American Sign Language both ways:

      m=video 51372 RTP/AVP 31 32
      a=hlang-send:ase
      a=hlang-recv:ase

An 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 en

An answer to the above offer indicating spoken Spanish both ways:

      m=audio 49250 RTP/AVP 20
      a=hlang-send:es
      a=hlang-recv:es

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):

      m=audio 49250 RTP/AVP 20
      a=hlang-send:it
      a=hlang-recv:it

An offer or answer indicating written Greek both ways:

      m=text 45020 RTP/AVP 103 104
      a=hlang-send:gr
      a=hlang-recv:gr

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, 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 pt

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 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:sp

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, 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 32

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:

      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 32

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.

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 8373

The 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 8373

6.2. 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).

8. Privacy Considerations

Language and media information can suggest a user's nationality, background, abilities, disabilities, etc.

9. References

9.1. Normative References

   [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

   [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

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, 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 for their reviews, corrections, suggestions, and participation in email and in-person discussions.

Contributors

Gunnar Hellstrom deserves special mention for his reviews and assistance.

Author's Address

Randall Gellens
Core Technology Consulting

   Email: rg+ietf@coretechnologyconsulting.com
   URI:   http://www.coretechnologyconsulting.com