Internet DRAFT - draft-gellens-slim-negotiating-human-language
draft-gellens-slim-negotiating-human-language
Network Working Group R. Gellens
Internet-Draft Qualcomm Technologies Inc.
Intended status: Standards Track November 1, 2015
Expires: May 4, 2016
Negotiating Human Language in Real-Time Communications
draft-gellens-slim-negotiating-human-language-03
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
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This Internet-Draft will expire on May 4, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Expected Use . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Desired Semantics . . . . . . . . . . . . . . . . . . . . . . 5
5. The existing 'lang' attribute . . . . . . . . . . . . . . . . 5
6. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2. New 'humintlang-send' and 'humintlang-recv' attributes . 7
6.3. Advisory vs Required . . . . . . . . . . . . . . . . . . 8
6.4. Silly States . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Changes from Previous Versions . . . . . . . . . . . . . . . 10
9.1. Changes from draft-gellens-slim-...-02 to draft-gellens-
slim-...-03 . . . . . . . . . . . . . . . . . . . . . . . 10
9.2. Changes from draft-gellens-slim-...-01 to draft-gellens-
slim-...-02 . . . . . . . . . . . . . . . . . . . . . . . 10
9.3. Changes from draft-gellens-slim-...-00 to draft-gellens-
slim-...-01 . . . . . . . . . . . . . . . . . . . . . . . 10
9.4. Changes from draft-gellens-mmusic-...-02 to draft-
gellens-slim-...-00 . . . . . . . . . . . . . . . . . . . 10
9.5. Changes from draft-gellens-mmusic-...-01 to -02 . . . . . 11
9.6. Changes from draft-gellens-mmusic-...-00 to -01 . . . . . 11
9.7. Changes from draft-gellens-...-02 to draft-gellens-
mmusic-...-00 . . . . . . . . . . . . . . . . . . . . . . 11
9.8. Changes from draft-gellens-...-01 to -02 . . . . . . . . 12
9.9. Changes from draft-gellens-...-00 to -01 . . . . . . . . 12
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 12
12.2. Informational References . . . . . . . . . . . . . . . . 13
Appendix A. Historic Alternative Proposal: Caller-prefs . . . . 13
A.1. Use of Caller Preferences Without Additions . . . . . . . 14
A.2. Additional Caller Preferences for Asymmetric Needs . . . 16
A.2.1. Caller Preferences for Asymmetric Modality Needs . . 16
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A.2.2. Caller Preferences for Asymmetric Language Tags . . . 17
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 [draft-tomkinson-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
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 using an IMS device. It doesn't
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matter if the call is an emergency call or not (e.g., to an airline
reservation desk). The language information is transparent to the
IMS carrier, 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 can 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 the former case, the language need is ancillary and supplemental.
In the latter 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
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
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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 that may be used within an
offer to indicate the preferred language of each 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.
(While it is true that a conversation among multilingual people often
involves multiple languages, the usefulness of providing a way to
negotiate this as a general facility is outweighed by the complexity
of the desired semantics of the SDP attribute to allow negotiation of
multiple simultaneous languages within an interactive media stream.)
5. The existing 'lang' attribute
RFC 4566 specifies an attribute 'lang' which sounds similar to what
is needed here, the difference being that it specifies that 'a=lang'
is declarative with the semantics of multiple 'lang' attributes being
that all of them are used, while we want a means to negotiate which
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one is used in each stream. This difference means that the existing
'lang' attribute can't be used and we need to define a new attribute.
The text from RFC 4566 [RFC4566] is:
a=lang:<language tag>
This can be a session-level attribute or a media-level attribute.
As a session-level attribute, it specifies the default language
for the session being described. As a media- level attribute, it
specifies the language for that media, overriding any session-
level language specified. Multiple lang attributes can be
provided either at session or media level if the session
description or media use multiple languages, in which case the
order of the attributes indicates the order of importance of the
various languages in the session or media from most important to
least important.
The "lang" attribute value must be a single [RFC3066] language tag
in US-ASCII [RFC3066]. It is not dependent on the charset
attribute. A "lang" attribute SHOULD be specified when a session
is of sufficient scope to cross geographic boundaries where the
language of recipients cannot be assumed, or where the session is
in a different language from the locally assumed norm.
A recent search of RFCs and Internet Drafts turned up only one use of
the 'lang' attribute (in a now-expired draft), and that sole use was
coincidentally in exactly the way we need (erroniously assuming that
the attribute was used for negotiation). The sole use was in an
example in a draft not directly related to language, where the
initial invitation contains two 'a=lang' entries for a media stream
(for English and Italian) and the OK accepts one of them (Italian).
The example serves as evidence of the need for an SDP attribute with
the semantics as described in this document; unfortunately, the
existing 'lang' attribute is not it.
6. Proposed Solution
An SDP attribute 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
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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" to indicate the language used when sending and
receiving media:
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, the 'humintlang-send' values constitute a list in
preference order (first is most preferred) of the languages the
offerer wishes to send using the media, and the 'humintlang-recv'
values constitute a list in preference order of the languages the
offerer wishes to receive using the media. In cases where the user
wishes to use one media for sending and another for receiving (such
as a speech-impaired user who wishes to send using text and receive
using audio), one of the two MAY be unset. In cases where a media is
not primarily intended for language (for example, a video or audio
stream intended for background only) both SHOULD be unset. In other
cases, 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
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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
specify one or both 'humintlang-send' and 'humintlang-recv'
attributes (to avoid ambiguity).
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, but 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.
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.
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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.
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.
7. IANA Considerations
IANA is kindly requested to add two entries to the 'att-field (media
level only)' table of the SDP parameters registry:
+------------------------------+-----------------+-----------------+
| 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
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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. Changes from Previous Versions
9.1. 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
9.2. 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
9.3. Changes from draft-gellens-slim-...-00 to draft-gellens-
slim-...-01
o Revision to keep draft from expiring
9.4. 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 [draft-tomkinson-multilangcontent] which
addresses the non-real-time side.
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9.5. 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.
9.6. 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.
9.7. 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.
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9.8. 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.
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.
9.9. 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
10. Contributors
Gunnar Hellstrom deserves special mention for his reviews,
assistance, and especially for contributing the core text in
Appendix A.
11. 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.
12. References
12.1. 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>.
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[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
12.2. 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.saintandre-sip-xmpp-chat]
Saint-Andre, P., Loreto, S., Gavita, E., and N. Hossain,
"Interworking between the Session Initiation Protocol
(SIP) and the Extensible Messaging and Presence Protocol
(XMPP): One-to-One Text Chat", draft-saintandre-sip-xmpp-
chat-06 (work in progress), June 2013.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", RFC 3066, January 2001.
[draft-tomkinson-multilangcontent]
Tomkinson, N. and N. Borenstein, "Multiple Language
Content Type", draft-tomkinson-multilangcontent (work in
progress), April 2014.
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
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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
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
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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
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
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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.
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
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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
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.
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Author's Address
Randall Gellens
Qualcomm Technologies Inc.
5775 Morehouse Drive
San Diego, CA 92121
US
Email: rg+ietf@randy.pensive.org
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