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Internet Engineering Task Force (IETF)                        R. GellensRequest for Comments: 8373                    Core Technology ConsultingCategory: Standards Track                                       May 2018ISSN: 2070-1721Negotiating Human Language in Real-Time CommunicationsAbstract   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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8373.Gellens                      Standards Track                    [Page 1]

RFC 8373               Negotiating Human Language               May 2018Copyright Notice   Copyright (c) 2018 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .42.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .53.  Desired Semantics . . . . . . . . . . . . . . . . . . . . . .54.  The Existing 'lang' Attribute . . . . . . . . . . . . . . . .55.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .55.1.  The 'hlang-send' and 'hlang-recv' Attributes  . . . . . .55.2.  No Language in Common . . . . . . . . . . . . . . . . . .75.3.  Usage Notes . . . . . . . . . . . . . . . . . . . . . . .75.4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .86.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .106.1.  att-field Subregistry of SDP Parameters . . . . . . . . .106.2.  Warning Codes Subregistry of SIP Parameters . . . . . . .117.  Security Considerations . . . . . . . . . . . . . . . . . . .118.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .119.  References  . . . . . . . . . . . . . . . . . . . . . . . . .129.1.  Normative References  . . . . . . . . . . . . . . . . . .129.2.  Informative References  . . . . . . . . . . . . . . . . .12   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .13   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .13   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .13Gellens                      Standards Track                    [Page 2]

RFC 8373               Negotiating Human Language               May 20181.  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 mayGellens                      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 20182.  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 inBCP 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' AttributeRFC 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 inSection 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 moreGellens                      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 thatSection 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 20185.2.  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 20185.4.  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, andGellens                      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 32Gellens                      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 Considerations6.1.  att-field Subregistry of SDP Parameters   The syntax in this section uses ABNF perRFC 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 inSection 5.1 of RFC 8373   Usage Level:             media   Mux Category:            NORMAL   Charset Dependent:       No   Purpose:                 SeeSection 5.1 of RFC 8373   O/A Procedures:          SeeSection 5.1 of RFC 8373   Reference:RFC 8373Gellens                      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 inSection 5.1 of RFC 8373   Usage Level:             media   Mux Category:            NORMAL   Charset Dependent:       No   Purpose:                 SeeSection 5.1 of RFC 8373   O/A Procedures:          SeeSection 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).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 20189.  References9.1.  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 inRFC2119 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.comGellens                      Standards Track                   [Page 13]