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rfc:rfc8373

Internet Engineering Task Force (IETF) R. Gellens Request for Comments: 8373 Core Technology Consulting Category: Standards Track May 2018 ISSN: 2070-1721

       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.

Gellens Standards Track [Page 1] RFC 8373 Negotiating Human Language May 2018

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 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

Gellens Standards Track [Page 2] RFC 8373 Negotiating Human Language May 2018

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

Gellens Standards Track [Page 3] RFC 8373 Negotiating Human Language May 2018

 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.

Gellens Standards Track [Page 4] RFC 8373 Negotiating Human Language May 2018

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

Gellens Standards Track [Page 5] RFC 8373 Negotiating Human Language May 2018

 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.

Gellens Standards Track [Page 6] RFC 8373 Negotiating Human Language May 2018

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.

Gellens Standards Track [Page 7] RFC 8373 Negotiating Human Language May 2018

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

Gellens Standards Track [Page 8] RFC 8373 Negotiating Human Language May 2018

 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

Gellens Standards Track [Page 9] RFC 8373 Negotiating Human Language May 2018

 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

Gellens Standards Track [Page 10] RFC 8373 Negotiating Human Language May 2018

 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.

Gellens Standards Track [Page 11] RFC 8373 Negotiating Human Language May 2018

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>.

Gellens Standards Track [Page 12] RFC 8373 Negotiating Human Language May 2018

 [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

Gellens Standards Track [Page 13]

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