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Versions: (draft-gellens-slim-negotiating-human-language) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 RFC 8373

Network Working Group                                         R. Gellens
Internet-Draft                                Core Technology Consulting
Intended status: Standards Track                           July 21, 2016
Expires: January 22, 2017


         Negotiating Human Language in Real-Time Communications
             draft-ietf-slim-negotiating-human-language-04

Abstract

   Users have various human (natural) language needs, abilities, and
   preferences regarding spoken, written, and signed languages.  When
   establishing interactive communication ("calls") there needs to be a
   way 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 stream
   attributes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 22, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Expected Use  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Desired Semantics . . . . . . . . . . . . . . . . . . . . . .   5
   5.  The existing 'lang' attribute . . . . . . . . . . . . . . . .   6
   6.  Proposed Solution . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  New 'humintlang-send' and 'humintlang-recv' attributes  .   6
     6.3.  Advisory vs Required  . . . . . . . . . . . . . . . . . .   8
     6.4.  Silly States  . . . . . . . . . . . . . . . . . . . . . .   9
     6.5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   10. Changes from Previous Versions  . . . . . . . . . . . . . . .  10
     10.1.  Changes from draft-ietf-slim-...-02 to draft-ietf-
            slim-...-03  . . . . . . . . . . . . . . . . . . . . . .  10
     10.2.  Changes from draft-ietf-slim-...-01 to draft-ietf-
            slim-...-02  . . . . . . . . . . . . . . . . . . . . . .  10
     10.3.  Changes from draft-ietf-slim-...-00 to draft-ietf-
            slim-...-01  . . . . . . . . . . . . . . . . . . . . . .  10
     10.4.  Changes from draft-gellens-slim-...-03 to draft-ietf-
            slim-...-00  . . . . . . . . . . . . . . . . . . . . . .  11
     10.5.  Changes from draft-gellens-slim-...-02 to draft-gellens-
            slim-...-03  . . . . . . . . . . . . . . . . . . . . . .  11
     10.6.  Changes from draft-gellens-slim-...-01 to draft-gellens-
            slim-...-02  . . . . . . . . . . . . . . . . . . . . . .  11
     10.7.  Changes from draft-gellens-slim-...-00 to draft-gellens-
            slim-...-01  . . . . . . . . . . . . . . . . . . . . . .  11
     10.8.  Changes from draft-gellens-mmusic-...-02 to draft-
            gellens-slim-...-00  . . . . . . . . . . . . . . . . . .  11
     10.9.  Changes from draft-gellens-mmusic-...-01 to -02  . . . .  11
     10.10. Changes from draft-gellens-mmusic-...-00 to -01  . . . .  12
     10.11. Changes from draft-gellens-...-02 to draft-gellens-
            mmusic-...-00  . . . . . . . . . . . . . . . . . . . . .  12
     10.12. Changes from draft-gellens-...-01 to -02 . . . . . . . .  12



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     10.13. Changes from draft-gellens-...-00 to -01 . . . . . . . .  13
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informational References . . . . . . . . . . . . . . . .  14
   Appendix A.  Historic Alternative Proposal: Caller-prefs  . . . .  14
     A.1.  Use of Caller Preferences Without Additions . . . . . . .  15
     A.2.  Additional Caller Preferences for Asymmetric Needs  . . .  17
       A.2.1.  Caller Preferences for Asymmetric Modality Needs  . .  17
       A.2.2.  Caller Preferences for Asymmetric Language Tags . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   A mutually comprehensible language is helpful for human
   communication.  This document addresses the real-time, interactive
   side of the issue.  A companion document on language selection in
   email [I-D.ietf-slim-multilangcontent] addresses the non-real-time
   side.

   When setting up interactive communication sessions (using SIP or
   other protocols), human (natural) language and media modality (voice,
   video, text) negotiation may be needed.  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
   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, in this case, SDP.

   This approach has a number of benefits, including that it is generic
   (applies to all interactive communications negotiated using SDP) and
   not limited to emergency calls.  In some cases such a facility isn't
   needed, because the language is known from the context (such as when
   a caller places a call to a sign language relay center, to a friend,
   or colleague).  But it is clearly useful in many other cases.  For
   example, someone calling a company call center or a Public Safety
   Answering Point (PSAP) should be able to indicate if one or more
   specific signed, written, and/or spoken languages are preferred, the
   callee should be able to indicate its capabilities in this area, and
   the call proceed using in-common language(s) and media forms.

   Since this is a protocol mechanism, the user equipment (UE client)
   needs to know the user's preferred languages; a reasonable technique
   could include a configuration mechanism with a default of the



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   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.

   Including the user's human (natural) language preferences in the
   session establishment negotiation is independent of the use of a
   relay service and is transparent to a voice service provider.  For
   example, assume a user within the United States who speaks Spanish
   but not English places a voice call.  The call could be an emergency
   call or perhaps to an airline reservation desk.  The language
   information is transparent to the voice service provider, but is part
   of the session negotiation between the UE and the terminating entity.
   In the case of a call to e.g., an airline, the call could be
   automatically handled by a Spanish-speaking agent.  In the case of an
   emergency call, the Emergency Services IP network (ESInet) and the
   PSAP may choose to take the language and media preferences into
   account when determining how to process the call.

   By treating language as another 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 requires a voice stream plus
   a text stream (known as voice carry over).  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.

   Regarding relay services, in the case of an emergency call requiring
   sign language such as ASL, there are two common approaches: the
   caller initiates the call to a relay center, or the caller places the
   call to emergency services (e.g., 911 in the U.S. or 112 in Europe).
   (In a variant of the second case, the voice service provider invokes
   a relay service as well as emergency services.)  In the former case,
   the language need is ancillary and supplemental.  In the non-variant
   second case, the ESInet and/or PSAP may take the need for sign
   language into account and bridge in a relay center.  In this case,
   the ESInet and PSAP have all the standard information available (such
   as location) but are able to bridge the relay sooner in the call
   processing.

   By making this facility part of the end-to-end negotiation, the
   question of which entity provides or engages the relay service



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   becomes separate from the call processing mechanics; if the caller
   directs the call to a relay service then the human language
   negotiation facility provides extra information to the relay service
   but calls will still function without it; if the caller directs the
   call to emergency services, then the ESInet/PSAP are able to take the
   user's human language needs into account, e.g., by assigning to a
   specific queue or call taker or bridging in a relay service or
   translator.

   The term "negotiation" is used here rather than "indication" because
   human language (spoken/written/signed) is something that can be
   negotiated in the same way as which forms of media (audio/text/video)
   or which codecs.  For example, if we think of non-emergency calls,
   such as 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).

2.  Terminology

   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 [RFC2119].

3.  Expected Use

   This facility may be used by NENA and 3GPP.  NENA has already
   referenced it in NENA 08-01 (i3 Stage 3 version 2) in describing
   attributes of calls presented to an ESInet, and may add further
   details in that or other documents.  3GPP may reference this
   mechanism in general call handling and emergency call handling.  Some
   CRs introduced in SA1 have anticipated this functionality being
   provided within SDP.

4.  Desired Semantics

   The desired solution is a media attribute (preferably per direction)
   that may be used within an offer to indicate the preferred language
   of each (direction of a) media stream, and within an answer to
   indicate the accepted language.  The semantics of including multiple
   values for a media stream within an offer is that the languages are
   listed in order of preference.





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   (Negotiating multiple simultaneous languages within a media stream is
   out of scope, as the complexity of doing so outweighs the
   usefulness.)

5.  The existing 'lang' attribute

   RFC 4566 [RFC4566] specifies an attribute 'lang' which appears
   similar to what is needed here, but is not sufficiently detailed for
   use here.  In addition, it does not seem to be in common use, which
   means there is low risk of conflict or confusion in defining new
   attributes.  Further, there is value in being able to specify
   language per direction (sending and receiving).  This document
   therefore defines two new attributes.

6.  Proposed Solution

   An SDP attribute (per direction) seems the natural choice to
   negotiate human (natural) language of an interactive media stream.
   The attribute value should be a language tag per RFC 5646 [RFC5646]

6.1.  Rationale

   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 video at the SIP level but fail to negotiate any video
   media stream at the SDP layer).

   The mechanism described here for SDP can be adapted to media
   negotiation protocols other than SDP.

6.2.  New 'humintlang-send' and 'humintlang-recv' attributes

   Rather than re-use 'lang' we define two new media-level attributes
   starting with 'humintlang' (short for "human interactive language")
   to negotiate which human language is used in each (interactive) media
   stream.  There are two attributes, one ending in "-send" and the
   other in "-recv":



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      a=humintlang-send:<language tag>
      a=humintlang-recv:<language tag>

   Each can appear multiple times in an offer for a media stream.

   In an offer, 'humintlang-send' indicates the language(s) the offerer
   is willing to use when sending using the media, and 'humintlang-recv'
   indicates the language(s) the offerer is willing to use when
   receiving using the media.  The values constitute a list of languages
   in preference order (first is most preferred).  When a media is
   intended for use in one direction only (such as a speech-impaired
   user sending using text and receiving using audio), either
   humintlang-send or humintlang-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 values in the same order.  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, 'humintlang-send' is the accepted language the answerer
   will send (which in most cases is one of the languages in the offer's
   'humintlang-recv'), and 'humintlang-recv' is the accepted language
   the answerer expects to receive (which in most cases is one of the
   languages in the offer's 'humintlang-send').

   Each value MUST be a language tag per RFC 5646 [RFC5646].  RFC 5646
   describes mechanisms for matching language tags.  While RFC 5646
   provides a mechanism accommodating increasingly fine-grained
   distinctions, in the interest of maximum interoperability for real-
   time interactive communications, each 'humintlang-send' and
   'humintlang-recv' value SHOULD be restricted to the largest
   granularity of language tags; in other words, it is RECOMMENDED to
   specify only a Primary-subtag and NOT to include subtags (e.g., for
   region or dialect) unless the languages might be mutually
   incomprehensible without them.

   In an offer, each language tag value MAY have an asterisk appended as
   the last character (after the registry value).  The asterisk
   indicates a request by the caller to not fail the call if there is no
   language in common.  See Section 6.3 for more information and
   discussion.

   When placing an emergency call, and in any other case where the
   language cannot be assumed from context, each media stream in an
   offer primarily intended for human language communication SHOULD




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   specify both (or in some cases, one of) the 'humintlang-send' and
   'humintlang-recv' attributes.

   Note that while signed language tags are used with a video stream to
   indicate sign language, a spoken language tag for a video stream in
   parallel with an audio stream with the 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
   'humintlang-send' and 'humintlang-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 values
   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).

6.3.  Advisory vs Required

   One important consideration with this mechanism is if the call fails
   if the callee does not support any of the languages requested by the
   caller.

   In order to provide for maximum likelihood of a successful
   communication session, especially in the case of emergency calling,
   the mechanism defined here provides a way for the caller to indicate
   a preference for the call failing or succeeding when there is no
   language in common.  However, the callee is NOT REQUIRED to honor
   this preference.  For example, a PSAP MAY choose to attempt the call
   even with no language in common, while a corporate call center MAY
   choose to fail the call.

   The mechanism for indicating this preference is that, in an offer, if
   the last character of any of the 'humintlang-recv' or 'humintlang-
   send' values is an asterisk, this indicates a request to not fail the
   call (similar to SIP Accept-Language syntax).  Either way, the called
   party MAY ignore this, e.g., for the emergency services use case, a
   PSAP will likely not fail the call.







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6.4.  Silly States

   It is possible to specify a "silly state" where the language
   specified does not make sense for the media type, such as specifying
   a signed language for an audio media stream.

   An offer MUST NOT be created where the language does not make sense
   for the media type.  If such an offer is received, the receiver MAY
   reject the media, ignore the language specified, or attempt to
   interpret the intent (e.g., if American Sign Language is specified
   for an audio media stream, this might be interpreted as a desire to
   use spoken English).

   A spoken language tag for a video stream in conjunction with an audio
   stream with the same language might indicate a request for
   supplemental video to see the speaker.

6.5.  Examples

   Some examples are shown below.  Only the most directly relevant
   portions of the SDP block are shown, for clarity.

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

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

      m=audio 49250 RTP/AVP 20
      a=humintlang-send:es*
      a=humintlang-recv:es*
      a=humintlang-send:eu*
      a=humintlang-recv:eu*
      a=humintlang-send:en*
      a=humintlang-recv:en*

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

7.  IANA Considerations

   IANA is kindly requested to add two entries to the 'att-field (media
   level only)' table of the SDP parameters registry:





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   +------------------------------+-----------------+-----------------+
   |             Type             |       Name      |    Reference    |
   +------------------------------+-----------------+-----------------+
   | att-field (media level only) | humintlang-send | (this document) |
   | att-field (media level only) | humintlang-recv | (this document) |
   +------------------------------+-----------------+-----------------+

              Table 1: att-field (media level only)' entries

8.  Security Considerations

   The Security Considerations of RFC 5646 [RFC5646] apply here (as a
   use of that RFC).  In addition, if the 'humintlang-send' or
   'humintlang-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.

9.  Privacy Considerations

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

10.  Changes from Previous Versions

10.1.  Changes from draft-ietf-slim-...-02 to draft-ietf-slim-...-03

   o  Added Examples
   o  Added Privacy Considerations section
   o  Other editorial changes for clarity

10.2.  Changes from draft-ietf-slim-...-01 to draft-ietf-slim-...-02

   o  Deleted most of Section 5 and replaced with a very short summary
   o  Replaced "wishes to" with "is willing to" in Section 6.2
   o  Reworded description of attribute usage to clarify when to set
      both, only one, or neither
   o  Deleted all uses of "IMS"
   o  Other editorial changes for clarity

10.3.  Changes from draft-ietf-slim-...-00 to draft-ietf-slim-...-01

   o  Editorial changes to wording in Section 5.








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10.4.  Changes from draft-gellens-slim-...-03 to draft-ietf-slim-...-00

   o  Updated title to reflect WG adoption

10.5.  Changes from draft-gellens-slim-...-02 to draft-gellens-
       slim-...-03

   o  Removed Use Cases section, per face-to-face discussion at IETF 93
   o  Removed discussion of routing, per face-to-face discussion at IETF
      93

10.6.  Changes from draft-gellens-slim-...-01 to draft-gellens-
       slim-...-02

   o  Updated NENA usage mention
   o  Removed background text reference to draft-saintandre-sip-xmpp-
      chat-04 since that draft expired

10.7.  Changes from draft-gellens-slim-...-00 to draft-gellens-
       slim-...-01

   o  Revision to keep draft from expiring

10.8.  Changes from draft-gellens-mmusic-...-02 to draft-gellens-
       slim-...-00

   o  Changed name from -mmusic- to -slim- to reflect proposed WG name
   o  As a result of the face-to-face discussion in Toronto, the SDP vs
      SIP issue was resolved by going back to SDP, taking out the SIP
      hint, and converting what had been a set of alternate proposals
      for various ways of doing it within SIP into an informative annex
      section which includes background on why SDP is the proposal
   o  Added mention that enabling a mutually comprehensible language is
      a general problem of which this document addresses the real-time
      side, with reference to [I-D.ietf-slim-multilangcontent] which
      addresses the non-real-time side.

10.9.  Changes from draft-gellens-mmusic-...-01 to -02

   o  Added clarifying text on leaving attributes unset for media not
      primarily intended for human language communication (e.g.,
      background audio or video).
   o  Added new section Appendix A ("Alternative Proposal: Caller-
      prefs") discussing use of SIP-level Caller-prefs instead of SDP-
      level.






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10.10.  Changes from draft-gellens-mmusic-...-00 to -01

   o  Relaxed language on setting -send and -receive to same values;
      added text on leaving on empty to indicate asymmetric usage.
   o  Added text that clients on behalf of end users are expected to set
      the attributes on outgoing calls and ignore on incoming calls
      while systems on behalf of call centers and PSAPs are expected to
      take the attributes into account when processing incoming calls.

10.11.  Changes from draft-gellens-...-02 to draft-gellens-mmusic-...-00

   o  Updated text to refer to RFC 5646 rather than the IANA language
      subtags registry directly.
   o  Moved discussion of existing 'lang' attribute out of "Proposed
      Solution" section and into own section now that it is not part of
      proposal.
   o  Updated text about existing 'lang' attribute.
   o  Added example use cases.
   o  Replaced proposed single 'humintlang' attribute with 'humintlang-
      send' and 'humintlang-recv' per Harald's request/information that
      it was a misuse of SDP to use the same attribute for sending and
      receiving.
   o  Added section describing usage being advisory vs required and text
      in attribute section.
   o  Added section on SIP "hint" header (not yet nailed down between
      new and existing header).
   o  Added text discussing usage in policy-based routing function or
      use of SIP header "hint" if unable to do so.
   o  Added SHOULD that the value of the parameters stick to the largest
      granularity of language tags.
   o  Added text to Introduction to be try and be more clear about
      purpose of document and problem being solved.
   o  Many wording improvements and clarifications throughout the
      document.
   o  Filled in Security Considerations.
   o  Filled in IANA Considerations.
   o  Added to Acknowledgments those who participated in the Orlando ad-
      hoc discussion as well as those who participated in email
      discussion and side one-on-one discussions.

10.12.  Changes from draft-gellens-...-01 to -02

   o  Updated text for (possible) new attribute "humintlang" to
      reference RFC 5646
   o  Added clarifying text for (possible) re-use of existing 'lang'
      attribute saying that the registration would be updated to reflect
      different semantics for multiple values for interactive versus
      non-interactive media.



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   o  Added clarifying text for (possible) new attribute "humintlang" to
      attempt to better describe the role of language tags in media in
      an offer and an answer.

10.13.  Changes from draft-gellens-...-00 to -01

   o  Changed name of (possible) new attribute from 'humlang" to
      "humintlang"
   o  Added discussion of silly state (language not appropriate for
      media type)
   o  Added Voice Carry Over example
   o  Added mention of multilingual people and multiple languages
   o  Minor text clarifications

11.  Contributors

   Gunnar Hellstrom deserves special mention for his reviews,
   assistance, and especially for contributing the core text in
   Appendix A.

12.  Acknowledgments

   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,
   Peter Saint-Andre, and Dale Worley for reviews, corrections,
   suggestions, and participating in in-person and email discussions.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <http://www.rfc-editor.org/info/rfc5646>.






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13.2.  Informational References

   [I-D.iab-privacy-considerations]
              Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", draft-iab-privacy-
              considerations-09 (work in progress), May 2013.

   [I-D.ietf-slim-multilangcontent]
              Tomkinson, N. and N. Borenstein, "Multiple Language
              Content Type", draft-ietf-slim-multilangcontent-02 (work
              in progress), July 2016.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840,
              DOI 10.17487/RFC3840, August 2004,
              <http://www.rfc-editor.org/info/rfc3840>.

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, DOI 10.17487/RFC3841, August 2004,
              <http://www.rfc-editor.org/info/rfc3841>.

Appendix A.  Historic Alternative Proposal: Caller-prefs

   The decision to base the proposal at the media negotiation level, and
   specifically to use SDP, came after significant debate and
   discussion.  It is possible to meet the objectives using a variety of
   mechanisms, but none are perfect.  Using SDP means dealing with the
   complexity of SDP, and leaves out real-time session protocols that do
   not use SDP.  The major alternative proposal was to use SIP.  Using
   SIP leaves out non-SIP session protocols, but more fundamentally,
   would occur at a different layer than the media negotiation.  This
   results in a more fragile solution since the media modality and
   language would be negotiated using SIP, and then the specific media
   formats (which inherently include the modality) would be negotiated
   at a different level (typically SDP, especially in the emergency
   calling cases), making it easier to have mismatches (such as where
   the media modality negotiated in SIP don't match what was negotiated
   using SDP).

   An alternative proposal was to use the SIP-level Caller Preferences
   mechanism from RFC 3840 [RFC3840] and RFC 3841 [RFC3841].

   The Caller-prefs mechanism includes a priority system; this would
   allow different combinations of media and languages to be assigned
   different priorities.  The evaluation and decisions on what to do



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   with the call can be done either by proxies along the call path, or
   by the addressed UA.  Evaluation of alternatives for routing is
   described in RFC 3841 [RFC3841].

A.1.  Use of Caller Preferences Without Additions

   The following would be possible without adding any new registered
   tags:

   Potential callers and recipients MAY include in the Contact field in
   their SIP registrations media and language tags according to the
   joint capabilities of the UA and the human user according to RFC 3840
   [RFC3840].

   The most relevant media capability tags are "video", "text" and
   "audio".  Each tag represents a capability to use the media in two-
   way communication.

   Language capabilities are declared with a comma-separated list of
   languages that can be used in the call as parameters to the tag
   "language=".

   This is an example of how it is used in a SIP REGISTER:



      REGISTER    user@example.net
      Contact:    <sip:user1@example.net> audio; video; text;
                  language="en,es,ase"

   Including this information in SIP REGISTER allows proxies to act on
   the information.  For the problem set addressed by this document, it
   is not anticipated that proxies will do so using registration data.
   Further, there are classes of devices (such as cellular mobile
   phones) that are not anticipated to include this information in their
   registrations.  Hence, use in registration is OPTIONAL.

   In a call, a list of acceptable media and language combinations is
   declared, and a priority assigned to each combination.

   This is done by the Accept-Contact header field, which defines
   different combinations of media and languages and assigns priorities
   for completing the call with the SIP URI represented by that Contact.
   A priority is assigned to each set as a so-called "q-value" which
   ranges from 1 (most preferred) to 0 (least preferred).

   Using the Accept-Contact header field in INVITE requests and
   responses allows these capabilities to be expressed and used during



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   call set-up.  Clients SHOULD include this information in INVITE
   requests and responses.

   Example:



      Accept-Contact:    *; text; language="en"; q=0.2
      Accept-Contact:    *; video; language="ase"; q=0.8

   This example shows the highest preference expressed by the caller is
   to use video with American Sign Language (language code "ase").  As a
   fallback, it is acceptable to get the call connected with only
   English text used for human communication.  Other media may of course
   be connected as well, without expectation that it will be usable by
   the caller for interactive communications (but may still be helpful
   to the caller).

   This system satisfies all the needs described in the previous
   chapters, except that language specifications do not make any
   distinction between spoken and written language, and that the need
   for directionality in the specification cannot be fulfilled.

   To some degree the lack of media specification between speech and
   text in language tags can be compensated by only specifying the
   important medium in the Accept-Contact field.

   Thus, a user who wants to use English mainly for text would specify:



      Accept-Contact:    *;text;language="en";q=1.0

   While a user who wants to use English mainly for speech but accept it
   for text would specify:



      Accept-Contact:    *;audio;language="en";q=0.8
      Accept-Contact:    *;text;language="en";q=0.2

   However, a user who would like to talk, but receive text back has no
   way to do it with the existing specification.








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A.2.  Additional Caller Preferences for Asymmetric Needs

   In order to be able to specify asymmetric preferences, there are two
   possibilities.  Either new language tags in the style of the
   humintlang parameters described above for SDP could be registered, or
   additional media tags describing the asymmetry could be registered.

A.2.1.  Caller Preferences for Asymmetric Modality Needs

   The following new media tags should be defined:

      speech-receive
      speech-send
      text-receive
      text-send
      sign-send
      sign-receive

   A user who prefers to talk and get text in return in English would
   register the following (if including this information in registration
   data):



      REGISTER    user@example.net
      Contact:    <sip:user1@example.net> audio;text;speech-send;text-
                  receive;language="en"

   At call time, a user who prefers to talk and get text in return in
   English would set the Accept-Contact header field to:



      Accept-Contact:    *; audio; text; speech-receive; text-send;
                         language="en";q=0.8
      Accept-Contact:    *; text; language="en"; q=0.2

   Note that the directions specified here are as viewed from the callee
   side to match what the callee has registered.

   A bridge arranged for invoking a relay service specifically arranged
   for captioned telephony would register the following for supporting
   calling users:



      REGISTER    ct@ctrelay.net




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      Contact:    <sip:ct1@ctreley.net> audio; text; speech-receive;
                  text-send; language="en"

   A bridge arranged for invoking a relay service specifically arranged
   for captioned telephony would register the following for supporting
   called users:



      REGISTER    ct@ctrelay.net
      Contact:    <sip:ct2@ctreley.net> audio; text; speech-send; text-
                  receive; language="en"

   At call time, these alternatives are included in the list of possible
   outcome of the call routing by the SIP proxies and the proper relay
   service is invoked.

A.2.2.  Caller Preferences for Asymmetric Language Tags

   An alternative is to register new language tags for the purpose of
   asymmetric language usage.

   Instead of using "language=", six new language tags would be
   registered:

      humintlang-text-recv
      humintlang-text-send
      humintlang-speech-recv
      humintlang-speech-send
      humintlang-sign-recv
      humintlang-sign-send

   These language tags would be used instead of the regular
   bidirectional language tags, and users with bidirectional
   capabilities SHOULD specify values for both directions.  Services
   specifically arranged for supporting users with asymmetric needs
   SHOULD specify only the asymmetry they support.

Author's Address

   Randall Gellens
   Core Technology Consulting

   Email: rg+ietf@randy.pensive.org







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