Internet DRAFT - draft-ietf-slim-negotiating-human-language
draft-ietf-slim-negotiating-human-language
Network Working Group R. Gellens
Internet-Draft Core Technology Consulting
Intended status: Standards Track February 20, 2018
Expires: August 24, 2018
Negotiating Human Language in Real-Time Communications
draft-ietf-slim-negotiating-human-language-24
Abstract
Users have various human (natural) language needs, abilities, and
preferences regarding spoken, written, and signed languages. This
document adds new SDP media-level attributes so that when
establishing interactive communication sessions ("calls"), it is
possible 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 Session
Description Protocol (SDP) media 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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 August 24, 2018.
Copyright Notice
Copyright (c) 2018 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Desired Semantics . . . . . . . . . . . . . . . . . . . . . . 5
4. The existing 'lang' attribute . . . . . . . . . . . . . . . . 5
5. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. The 'hlang-send' and 'hlang-recv' attributes . . . . . . 5
5.2. No Language in Common . . . . . . . . . . . . . . . . . . 7
5.3. Usage Notes . . . . . . . . . . . . . . . . . . . . . . . 7
5.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. att-field Table in SDP Parameters . . . . . . . . . . . . 10
6.2. Warn-Codes Sub-Registry of SIP Parameters . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
9. Changes from Previous Versions . . . . . . . . . . . . . . . 11
9.1. Changes from draft-ietf-slim-...-04 to draft-ietf-
slim-...-06 . . . . . . . . . . . . . . . . . . . . . . . 12
9.2. Changes from draft-ietf-slim-...-02 to draft-ietf-
slim-...-03 . . . . . . . . . . . . . . . . . . . . . . . 12
9.3. Changes from draft-ietf-slim-...-01 to draft-ietf-
slim-...-02 . . . . . . . . . . . . . . . . . . . . . . . 12
9.4. Changes from draft-ietf-slim-...-00 to draft-ietf-
slim-...-01 . . . . . . . . . . . . . . . . . . . . . . . 12
9.5. Changes from draft-gellens-slim-...-03 to draft-ietf-
slim-...-00 . . . . . . . . . . . . . . . . . . . . . . . 13
9.6. Changes from draft-gellens-slim-...-02 to draft-gellens-
slim-...-03 . . . . . . . . . . . . . . . . . . . . . . . 13
9.7. Changes from draft-gellens-slim-...-01 to draft-gellens-
slim-...-02 . . . . . . . . . . . . . . . . . . . . . . . 13
9.8. Changes from draft-gellens-slim-...-00 to draft-gellens-
slim-...-01 . . . . . . . . . . . . . . . . . . . . . . . 13
9.9. Changes from draft-gellens-mmusic-...-02 to draft-
gellens-slim-...-00 . . . . . . . . . . . . . . . . . . . 13
9.10. Changes from draft-gellens-mmusic-...-01 to -02 . . . . . 13
9.11. Changes from draft-gellens-mmusic-...-00 to -01 . . . . . 14
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9.12. Changes from draft-gellens-...-02 to draft-gellens-
mmusic-...-00 . . . . . . . . . . . . . . . . . . . . . . 14
9.13. Changes from draft-gellens-...-01 to -02 . . . . . . . . 15
9.14. Changes from draft-gellens-...-00 to -01 . . . . . . . . 15
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 15
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informational References . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
A mutually comprehensible language is helpful for human
communication. This document addresses the negotiation of human
(natural) language and media modality (spoken, signed, 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. For example, it is helpful for a caller to a
company call center or a Public Safety Answering Point (PSAP) to be
able to indicate preferred signed, written, and/or spoken languages,
and for the callee to be able to indicate its capabilities in this
area, allowing 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 (e.g., voice, text, video), it makes
sense to use a per-stream negotiation mechanism known as the Session
Description Protocol (SDP). Utilizing Session Description Protocol
(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 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 and desires a voice stream
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to send spoken language plus a text stream to receive written
language. 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.
The term "negotiation" is used here rather than "indication" because
human language (spoken/written/signed) can be negotiated in the same
manner as media (audio/text/video) and codecs. For example, if we
think of a user calling an airline reservation center, the user may
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.
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 having a call
handled by an agent who speaks a requested language and/or with the
ability to use a requested modality, or bridging external translation
or relay resources into the call, etc. 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 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 the Session Description Protocol (SDP) can proceed.
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When setting up interactive communications sessions it is necessary
to route signaling messages to the appropriate endpoint(s). This
document does not address the problem of language-based routing.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 RFC 8174 [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. The semantics of including
multiple languages for a media stream within an offer is that the
languages are listed in order of preference.
(Negotiating multiple simultaneous languages within a media stream is
out of scope of this document.)
4. The existing 'lang' attribute
RFC 4566 [RFC4566] specifies an attribute 'lang' which appears
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 (natural) language of an interactive media stream,
using the language tags of BCP 47 [RFC5646].
5.1. The 'hlang-send' and 'hlang-recv' attributes
This document defines two media-level attributes starting with
'hlang' (short for "human language") to negotiate which human
language is selected for use in each interactive media stream. (Note
that not all streams will necessarily be used.) There are two
attributes, one ending in "-send" and the other in "-recv",
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registered in Section 6. Each can appear in offers and answers for
media streams.
In an offer, the 'hlang-send' value is a list of one or more
language(s) the offerer is willing to use when sending using the
media, and the 'hlang-recv' value is a list of one or more
language(s) the offerer is willing to use when receiving using the
media. The list of languages is in preference order (first is most
preferred). When a media is intended for interactive communication
using a language in one direction only (e.g., a user with difficulty
speaking but able to hear who indicates a desire to send using text
and receive using audio), 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 BCP 47 [RFC5646], separated by white space. In an answer, each
value MUST be one language tag per BCP 47. BCP 47 describes
mechanisms for matching language tags. Note that [RFC5646]
Section 4.1 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, in an offer each 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
'hlang-send' and 'hlang-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
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centers and PSAPs are expected to take into account the attributes
when processing inbound calls.
Note that media and language negotiation might result in more media
streams being accepted than are needed by the users (e.g., if more
preferred and less preferred combinations of media and language are
all accepted). This is not a problem.
5.2. No Language in Common
A consideration with the ability to negotiate language is if 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 languages 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 [TBD: IANA VALUE, e.g., 308] and a warning text
indicating that there are no mutually-supported languages; the
warning text SHOULD also contain the supported languages and media.
Example:
Warning: [TBD: IANA VALUE, e.g., 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).
In the IANA registry of language subtags per BCP 47 [RFC5646], 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.
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This document does not define the use of sign-language tags in text
or audio media.
This document does not define the use of language tags in media other
than interactive streams of audio, video, and text (such as "message"
or "application"). Such use could be supported by future work or by
application agreement.
5.4. Examples
Some examples are shown below. For clarity, only the most directly
relevant portions of the SDP block are shown.
An offer or answer indicating spoken English both ways:
m=audio 49170 RTP/AVP 0
a=hlang-send:en
a=hlang-recv:en
An offer indicating American Sign Language both ways:
m=video 51372 RTP/AVP 31 32
a=hlang-send:ase
a=hlang-recv:ase
An offer requesting spoken Spanish both ways (most preferred), spoken
Basque both ways (second preference), or spoken English both ways
(third preference):
m=audio 49250 RTP/AVP 20
a=hlang-send:es eu en
a=hlang-recv:es eu en
An answer to the above offer indicating spoken Spanish both ways:
m=audio 49250 RTP/AVP 20
a=hlang-send:es
a=hlang-recv:es
An alternative answer to the above offer indicating spoken Italian
both ways (as the callee does not support any of the requested
languages but chose to proceed with the call):
m=audio 49250 RTP/AVP 20
a=hlang-send:it
a=hlang-recv:it
An offer or answer indicating written Greek both ways:
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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, 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 had 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, 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
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a=hlang-recv:sp
m=audio 49250 RTP/AVP 20
a=hlang-send:sp
m=video 51372 RTP/AVP 31 32
Note that, even though the examples show the same (or essentially the
same) language being used in both directions (even when the modality
differs), there is no requirement that this be the case. However, in
practice, doing so is likely to increase the chances of successful
matching.
6. IANA Considerations
6.1. att-field Table in SDP Parameters
The syntax in this section uses ABNF per RFC 5234 [RFC5234].
IANA is kindly requested to add two entries to the 'att-field (media
level only)' table of the SDP parameters registry:
The first entry is for hlang-recv:
Attribute Name: hlang-recv
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 BCP 47
SP = 1*" " ; one or more space (%x20) characters
hlang-ansv = Language-Tag
Attribute Semantics: Described in Section 5.1 of TBD: THIS
DOCUMENT
Usage Level: media
Mux Category: NORMAL
Charset Dependent: No
Purpose: See Section 5.1 of TBD: THIS DOCUMENT
O/A Procedures: See Section 5.1 of TBD: THIS DOCUMENT
Reference: TBD: THIS DOCUMENT
The second entry is for hlang-send:
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Attribute Name: hlang-send
Contact Name: Randall Gellens
Contact Email Address: rg+ietf@coretechnologyconsulting.com
Attribute Value: hlang-value
Attribute Syntax:
hlang-value = hlang-offv / hlang-ansv
Attribute Semantics: Described in Section 5.1 of TBD: THIS
DOCUMENT
Usage Level: media
Mux Category: NORMAL
Charset Dependent: No
Purpose: See Section 5.1 of TBD: THIS DOCUMENT
O/A Procedures: See Section 5.1 of TBD: THIS DOCUMENT
Reference: TBD: THIS DOCUMENT
6.2. Warn-Codes Sub-Registry of SIP Parameters
IANA is requested to add a new value in the warn-codes sub-registry
of SIP parameters in the 300 through 329 range that is 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 and media are: [list of supported languages and media]."
7. Security Considerations
The Security Considerations of BCP 47 [RFC5646] apply here. An
attacker with the ability to modify signaling could prevent a call
from succeeding by altering any of several crucial elements,
including the 'hlang-send' or 'hlang-recv' values. RFC 5069
[RFC5069] discusses such threats. Use of TLS or IPSec can protect
against such threats. Emergency calls are of particular concern; RFC
6881 [RFC6881], which is specific to emergency calls, mandates use of
TLS or IPSec (in ED-57/SP-30).
8. Privacy Considerations
Language and media information can suggest a user's nationality,
background, abilities, disabilities, etc.
9. Changes from Previous Versions
RFC EDITOR: Please remove this section prior to publication.
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9.1. Changes from draft-ietf-slim-...-04 to draft-ietf-slim-...-06
o Deleted Section 3 ("Expected Use")
o Reworded modalities in Introduction from "voice, video, text" to
"spoken, signed, written"
o Reworded text about "increasingly fine-grained distinctions" to
instead merely point to BCP 47 Section 4.1's advice to "tag
content wisely" and not include unnecessary subtags
o Changed IANA registration of new SDP attributes to follow RFC 4566
template with extra fields suggested in 4566-bis (expired draft)
o Deleted "(known as voice carry over)"
o Changed textual instanced of RFC 5646 to BCP 47, although actual
reference remains RFC due to xml2rfc limitations
9.2. 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
9.3. Changes from draft-ietf-slim-...-01 to draft-ietf-slim-...-02
o Deleted most of Section 4 and replaced with a very short summary
o Replaced "wishes to" with "is willing to" in Section 5.1
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
9.4. Changes from draft-ietf-slim-...-00 to draft-ietf-slim-...-01
o Editorial changes to wording in Section 5.
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9.5. Changes from draft-gellens-slim-...-03 to draft-ietf-slim-...-00
o Updated title to reflect WG adoption
9.6. 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
9.7. 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
9.8. Changes from draft-gellens-slim-...-00 to draft-gellens-
slim-...-01
o Revision to keep draft from expiring
9.9. 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 [RFC8255] which addresses the non-real-
time side.
9.10. 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).
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o Added new section ("Alternative Proposal: Caller-prefs")
discussing use of SIP-level Caller-prefs instead of SDP-level.
9.11. 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.
9.12. 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 'hlang' attribute with 'hlang-send' and
'hlang-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.
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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.
9.13. Changes from draft-gellens-...-01 to -02
o Updated text for (possible) new attribute "hlang" 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 "hlang" to
attempt to better describe the role of language tags in media in
an offer and an answer.
9.14. Changes from draft-gellens-...-00 to -01
o Changed name of (possible) new attribute from 'humlang" to "hlang"
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 and
assistance.
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,
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
reviews, corrections, suggestions, and participating in in-person and
email discussions.
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12. References
12.1. Normative References
[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>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
12.2. Informational References
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002, <https://www.rfc-
editor.org/info/rfc3264>.
[RFC5069] Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
Shanmugam, "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069,
DOI 10.17487/RFC5069, January 2008, <https://www.rfc-
editor.org/info/rfc5069>.
[RFC6881] Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in Support of Emergency Calling",
BCP 181, RFC 6881, DOI 10.17487/RFC6881, March 2013,
<https://www.rfc-editor.org/info/rfc6881>.
[RFC8255] Tomkinson, N. and N. Borenstein, "Multiple Language
Content Type", RFC 8255, DOI 10.17487/RFC8255, October
2017, <https://www.rfc-editor.org/info/rfc8255>.
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Author's Address
Randall Gellens
Core Technology Consulting
Email: rg+ietf@coretechnologyconsulting.com
URI: http://www.coretechnologyconsulting.com
Gellens Expires August 24, 2018 [Page 17]
