rum                                                             B. Rosen
Internet-Draft                                              29 July                                            26 August 2021
Intended status: Standards Track
Expires: 30 January 27 February 2022

           Interoperability Profile for Relay User Equipment


   Video Relay Service (VRS) is a term used to describe a method by
   which a hearing persons can communicate with deaf/Hard of Hearing
   user using an interpreter ("Communications Assistant") connected via
   a videophone to the deaf/HoH user and an audio telephone call to the
   hearing user.  The CA interprets using sign language on the
   videophone link and voice on the telephone link.  Often the
   interpreters may be supplied by a company or agency termed a
   "provider" in this document.  The provider also provides a video
   service that allows users to connect video devices to their service,
   and subsequently to CAs and other deaf/HoH users.  It is desirable
   that the videophones used by the deaf/HoH/H-I user conform to a
   standard so that any device may be used with any provider and that
   video calls direct between deaf/HoH users work.  This document
   describes the interface between a videophone and a provider.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   4.  General Requirements  . . . . . . . . . . . . . . . . . . . .   6
   5.  SIP Signaling . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Registration  . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Session Establishment . . . . . . . . . . . . . . . . . .   8
       5.2.1.  Normal Call Origination . . . . . . . . . . . . . . .   8
       5.2.2.  Dial-Around Origination . . . . . . . . . . . . . . .   9
       5.2.3.  RUE Contact Information . . . . . . . . . . . . . . .  10
       5.2.4.  Incoming Calls  . . . . . . . . . . . . . . . . . . .  10
       5.2.5.  Emergency Calls . . . . . . . . . . . . . . . . . . .  11
     5.3.  Mid Call Signaling  . . . . . . . . . . . . . . . . . . .  11
     5.4.  URI Representation of Phone Numbers . . . . . . . . . . .  12
     5.5.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Media . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  SRTP and SRTCP  . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Text-Based Communication  . . . . . . . . . . . . . . . .  13
     6.3.  Video . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.4.  Audio . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.5.  DTMF Digits . . . . . . . . . . . . . . . . . . . . . . .  13
     6.6.  Session Description Protocol  . . . . . . . . . . . . . .  13
     6.7.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.8.  Negative Acknowledgment, Packet Loss Indicator, and Full
           Intraframe Request Features . . . . . . . . . . . . . . .  14
   7.  Contacts  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  CardDAV Login and Synchronization . . . . . . . . . . . .  14
     7.2.  Contacts Import/Export Service  . . . . . . . . . . . . .  15
   8.  Mail Waiting Indicator (MWI)  . . . . . . . . . . . . . . . .  15
   9.  Provisioning and Provider Selection . . . . . . . . . . . . .  15
     9.1.  RUE Provider Selection  . . . . . . . . . . . . . . . . .  16
     9.2.  RUE Configuration Service . . . . . . . . . . . . . . . .  18
       9.2.1.  Provider Configuration  . . . . . . . . . . . . . . .  18
       9.2.2.  RUE Configuration . . . . . . . . . . . . . . . . . .  19
       9.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .  20
       9.2.4.  Using the Provider Selection and RUE Configuration
               Services Together . . . . . . . . . . . . . . . . . .  21

     9.3.  OpenAPI Interface Descriptions  . . . . . . . . . . . . .  21
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     11.1.  RUE Provider List Registry . . . . . . . . . . . . . . .  27
     11.2.  Registration of rue-owner purpose parameter  . . . . . .  27
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   Video Relay Service (VRS) is a form of Telecommunications Relay
   Service (TRS) that enables persons with hearing disabilities who use
   sign language, such as American Sign Language (ASL), to communicate
   with voice telephone users through video equipment.  These services
   also enable communication between such individuals directly in
   suitable modalities, including any combination of sign language via
   video, real-time text (RTT), and speech.

   This Interoperability Profile for Relay User Equipment (RUE) is a
   profile of the Session Initiation Protocol (SIP) and related media
   protocols that enables end-user equipment registration and calling
   for VRS calls.  It specifies the minimal set of call flows, Internet
   Engineering Task Force (IETF) and ITU-T standards that must be
   supported, provides guidance where the standards leave multiple
   implementation options, and specifies minimal and extended
   capabilities for RUE calls.

   Both deaf/HoH to provider (interpreted) and direct deaf/HoH to deaf/
   HoH calls are supported on this interface.  While there are some
   accommodations in this document to maximize backwards compatibility
   with other devices and services that are used to provide VRS service,
   backwards compatibility is not a requirement, and some interwork may
   be required to allow direct video calls to older devices.  This
   document only describes the interface between the device and the
   provider, and not any other interface the provider may have.

2.  Terminology

   Communication Assistant (CA): A sign-language interpreter working for
   a VRS Provider, providing functionally equivalent phone service.

   Communication modality (modality): A specific form of communication
   that may be employed by two users, e.g., English voice, Spanish
   voice, American Sign Language, English lip-reading, or French real-
   time-text.  Here, one communication modality is assumed to encompass
   both the language and the way that language is exchanged.  For
   example, English voice and French voice are two different
   communication modalities.

   Default video relay service: The video relay service operated by a
   subscriber's default VRS provider.

   Default video relay service Provider (Default Provider): The VRS
   provider that registers, and assigns a telephone number to a specific
   subscriber, and by default provides the VRS for incoming voice calls
   to the user.  The default Provider also by default provides VRS for
   outgoing relay calls.  The user can have more than one telephone
   number and each has a default provider.

   Outbound Dial-around call: A relay call where the subscriber
   specifies the use of a VRS provider other than the default VRS
   provider.  This can be accomplished by the user dialing a "front-
   door" number for a VRS provider and signing or texting a phone number
   to call ("two-stage").  Alternatively, this can be accomplished by
   the user's RUE software instructing the server of its default VRS
   provider to automatically route the call through the alternate
   Provider to the desired public switched telephone network (PSTN)
   directory number ("one-stage").  Dial-around is per-call -- for any
   call, a user can use the default VRS provider or any dial-around VRS

   Full Intra Request (FIR): A request to a video media sender,
   requiring that media sender to send a Decoder Refresh Point at the
   earliest opportunity.  FIR is sometimes known as "instantaneous
   decoder refresh request", "video fast update request", or "fast
   update request".  Point-to-Point Call (P2P Call): A call between two
   RUEs, without including a CA.

   Relay call: A call that allows persons with hearing or speech
   disabilities to use a RUE to talk to users of traditional voice
   services with the aid of a communication assistant (CA) to relay the
   communication.  Please refer to FCC-VRS-GUIDE.

   Relay service (RS): A service that allow a registered subscriber to
   use a RUE to make and receive relay calls, point-to-point calls, and
   relay-to-relay calls.  The functions provided by the relay service
   include the provision of media links supporting the communication
   modalities used by the caller and callee, and user registration and
   validation, authentication, authorization, automatic call distributor
   (ACD) platform functions, routing (including emergency call routing),
   call setup, mapping, call features (such as call forwarding and video
   mail), and assignment of CAs to relay calls.

   Relay service Provider (Provider): An organization that operates a
   relay service.  A subscriber selects a relay service Provider to
   assign and register a telephone number for their use, to register
   with for receipt of incoming calls, and to provide the default
   service for outgoing calls.

   Relay user: Please refer to "subscriber".

   Relay user E.164 Number (user E.164): The telephone number (in ITU-T
   E.164 format) assigned to the user.

   Relay user equipment (RUE): A SIP user agent (UA) enhanced with extra
   features to support a subscriber in requesting, receiving and using
   relay calls.  A RUE may take many forms, including a stand-alone
   device; an application running on a general-purpose computing device
   such as a laptop, tablet or smart phone; or proprietary equipment
   connected to a server that provides the RUE interface.

   RUE Interface: the interfaces described in this document between a
   RUE and a VRS provider who supports it

   Sign language: A language that uses hand gestures and body language
   to convey meaning including, but not limited to, American Sign
   Language (ASL).

   Subscriber: An individual who has registered with a Provider and who
   obtains service by using relay user equipment.  This is the
   traditional telecom term for an end-user customer, which in our case
   is a relay user.  A user may be a subscriber to more than one VRS

   Video relay service (VRS): A relay service for people with hearing or
   speech disabilities who use sign language to communicate using video
   equipment (video RUE) with other people in real time.  The video link
   allows the CA to view and interpret the subscriber's signed
   conversation and relay the conversation back and forth with the other

3.  Requirements Language

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119]

4.  General Requirements

   All HTTP/HTTPS connections specified throughout this document MUST
   use HTTPS.  Both HTTPS and all SIP connections MUST use TLS
   conforming to at least [RFC7525] and must support [RFC8446]

   All text data payloads not otherwise constrained by a specification
   in another standards document MUST be encoded as Unicode UTF/8.

   Implementations MUST support IPv4 and IPv6.  Dual stack support is
   NOT required and provider implementations MAY support separate
   interfaces for IPv4 and IPv6 by having more than one server in the
   appropriate SRV record where there is either an A or AAAA record in
   each server DNS record but not both.  The same version of IP MUST be
   used for both signaling and media of a call. call unless ICE ([RFC5245]) is
   used, in which case candidates may explicitly offer IPv4, IPv6 or
   both for any media stream.

5.  SIP Signaling

   Implementations of the RUE Interface MUST conform to the following
   core SIP standards [RFC3261] (Base SIP) [RFC3263] (Locating SIP
   Servers), [RFC3264] (Offer/Answer), [RFC3840] (User Agent
   Capabilities), [RFC5626] (Outbound), [RFC8866] (Session Description
   Protocol), [RFC3323] (Privacy), [RFC3605] (RTCP Attribute in SDP),
   [RFC6665] (SIP Events), [RFC3311] (UPDATE Method), [RFC5393] (Loop-
   Fix), [RFC5658] (Record Route fix), [RFC5954] (ABNF fix), [RFC3960]
   (Early Media), and [RFC6442] (Geolocation Header).

   In the above documents the RUE device conforms to the requirements of
   a SIP user Agent, and the provider conforms to the requirements of
   Registrar and Proxy Server where the document specifies different
   behavior for different roles.  The only requirement on providers for
   RFC6655 (Events) is support for the Message Waiting Indicator (See
   Section Section 8), which is optional and providers not supporting
   voicemail need not support RFC6665.

   In addition, implementation MUST conform to [RFC3327] (Path),
   [RFC5245] (ICE), [RFC3326] (Reason header), [RFC3515] (REFER Method),
   [RFC3891] (Replaces Header), [RFC3892] (Referred-By).

   Implementations MUST include a "User-Agent" header field uniquely
   identifying the RUE application, platform, and version in all SIP
   requests, and MUST include a "Server" header field with the same
   content in SIP responses.

   Implementations intended to support mobile platforms MUST support
   [RFC8599] and MUST use it as at least one way to support waking up
   the client from background state.

5.1.  Registration

   The RUE MUST register with a SIP registrar, following [RFC3261] and
   [RFC5626] at a provider it has an account with.  If the configuration
   (see Section 9.2) contains multiple "outbound-proxies", then the RUE
   MUST use them as specified in [RFC5626] to establish multiple flows.

   The request-URI for the REGISTER request MUST contain the "provider-
   domain" from the configuration.  The To-URI and From-URI MUST be
   identical URIs, formatted as specified in Section 13, using the
   "phone-number" and "provider-domain" from the configuration.

   The RUE determines the URI to resolve by initially determining if an
   outbound proxy is configured.  If it is, the URI will be that of the
   outbound proxy.  If no outbound proxy is configured, the URI will be
   the Request-URI from the REGISTER request.  The RUE extracts the
   domain from that URI and consults the DNS record for that domain.
   The DNS entry MUST contain NAPTR records conforming to RFC3263.  One
   of those NAPTR records MUST specify TLS as the preferred transport
   for SIP.  For example, a DNS NAPTR query for "sip:" could return:

         IN NAPTR 50  50 "s" "SIPS+D2T" ""
         IN NAPTR 90  50 "s" "SIP+D2T"  ""

   If the RUE receives a 439 (First Hop Lacks Outbound Support) response
   to a REGISTER request, it MUST re-attempt registration without using
   the outbound mechanism.

   The registrar MAY authenticate using SIP digest authentication.  The
   credentials to be used (username and password) MUST be supplied
   within the credentials section of the configuration and identified by
   the realm the registrar uses in a digest challenge.  This username/
   password combination SHOULD NOT be the same as that used for other
   purposes, such as retrieving the RUE configuration or logging into
   the Provider's customer service portal.  [RFC8760] MUST be supported
   by all implementations and SHA-512 digest algorithms MUST be

   If the registration request fails with an indication that credentials
   from the configuration are invalid, then the RUE SHOULD retrieve a
   fresh version of the configuration.  If credentials from a freshly
   retrieved configuration are found to be invalid, then the RUE MUST
   cease attempts to register and SHOULD inform the RUE User of the

   Support for multiple simultaneous registrations with multiple
   providers by the RUE is OPTIONAL for the RUE (and providers do not
   need any support for this option).

   Multiple simultaneous RUE SIP registrations from different RUE
   devices with the same SIP URI SHOULD be permitted by the Provider.
   The Provider MAY limit the total number of simultaneous
   registrations.  When a new registration request is received that
   results in exceeding the limit on simultaneous registrations, the
   Provider MAY then prematurely terminate another registration;
   however, it SHOULD NOT do this if it would disconnect an active call.

   If a Provider prematurely terminates a registration to reduce the
   total number of concurrent registrations with the same URI, it SHOULD
   take some action to prevent the affected RUE from automatically re-
   registering and re-triggering the condition.

5.2.  Session Establishment

5.2.1.  Normal Call Origination

   After initial SIP registration, the RUE adheres to SIP [RFC3261]
   basic call flows, as documented in [RFC3665].

   A RUE device MUST route all outbound calls through an outbound proxy
   if configured.

   The SIP URIs in the To field and the Request-URI MUST be formatted as
   specified in subsection Section 5.4 using the destination phone
   number, or as SIP URIs, as provided in the configuration
   (Section 9.2.  The domain field of the URIs SHOULD be the "provider-
   domain" from the configuration (e.g.,;user=phone) except that an anonymous
   call would not use the provider domain.

   Anonymous calls MUST be supported by all implementations.  An
   anonymous call is signaled per [RFC3323].

   The From-URI MUST be formatted as specified in Section 5.4, using the
   phone-number and "provider-domain" from the configuration.  It SHOULD
   also contain the display-name from the configuration when present.
   (Please refer to Section 9.2.)

   Negotiated media MUST follow the guidelines specified in Section 6 of
   this document.

   To allow time to timeout an unanswered call and direct it to a
   videomail server, the User Agent Client MUST NOT impose a time limit
   less than the default SIP Invite transaction timeout of 3 minutes.

5.2.2.  Dial-Around Origination

   Providers and RUE devices MUST support both One-Stage and Two-Stage

   Outbound dial-around calls allow a RUE user to select any Provider to
   provide interpreting services for any call.  "Two-stage" dial-around
   calls involve the RUE calling a telephone number that reaches the
   dial-around Provider and using signing or DTMF to provide the called
   party telephone number.  In two-stage dial-around, the To URI is the
   front door URI (see Section Section 9.2 of the dial-around Provider
   and the domain of the URI is the Provider domain from the
   configuration.  The provider list service (Section 9.1) can be used
   to by the RUE to obtain a list of providers and then the
   configuration service (xref target="providerConfig"/>) without
   credentials can be used to find the front door URI for each of these

   One-stage dial-around is a method where the called party telephone
   number is provided in the To URI and the Request-URI, using the
   domain of the dial-around Provider.

   For one-stage dial-around, the RUE MUST follow the procedures in
   Section 5.2.1 with the following exception: the domain part of the
   SIP URIs in the To field and the Request-URI MUST be the domain of
   the dial-around Provider, discovered according to Section 9.1.

   The following is a partial example of a one-stage dial-around call
   from VRS user +1-555-222-0001 hosted by to a hearing
   user +1-555-123-4567 using dial-around to for the
   relay service.  Only important details of the messages are shown and
   many header fields have been omitted:

   One Stage Dial-Around
     ,-+-.        ,----+----.    ,-----+-----.
     |RUE|        |Default  |    |Dial-Around|
     |   |        |Provider |    | Provider  |
     `-+-'        `----+----'    `-----+-----'
       |               |               |
       | [1] INVITE    |               |
       |-------------->| [2] INVITE    |
       |               |-------------->|

     Message Details:

     [1] INVITE Rue -> Default Provider

     INVITE;user=phone SIP/2.0
     To: <;user=phone>
     From: "Bob Smith" <;user=phone>

     [2] INVITE Default Provider -> Dial-Around Provider

     INVITE;user=phone SIP/2.0
     To: <;user=phone>
     From: "Bob Smith";user=phone

                                  Figure 1

5.2.3.  RUE Contact Information

   To identify the owner of a RUE, the initial INVITE for a call from a
   RUE, or the 200 OK accepting a call by a RUE, identifies the owner by
   sending a Call-Info header with a purpose parameter of "rue-owner".
   The URI MAY be an HTTPS URI or Content-Indirect URL.  The latter is
   defined by [RFC2392] to locate message body parts.  This URI type is
   present in a SIP message to convey the RUE ownership information as a
   MIME body.  The form of the RUE ownership information is a jCard
   [RFC7095].  Please refer to [RFC6442] xCard
   [RFC6351].  for an example of using Content-Indirect URLs in SIP
   messages.  Note that use of the Content-
   Indirect Content-Indirect URL usually implies
   multiple message bodies ("mime/
   multipart"). ("mime/multipart").

5.2.4.  Incoming Calls

   The RUE MUST only accept inbound calls sent to it by a proxy
   mentioned in the configuration.

   If Multiple simultaneous RUE SIP registrations from different RUE
   devices with the same SIP URI exist, the Provider MUST parallel fork
   the call to all registered RUEs so that they ring at the same time.
   The first RUE to reply with a 200 OK answers the call and the
   Provider MUST CANCEL other call branches.

5.2.5.  Emergency Calls

   Implementations MUST conform to [RFC6881] for handling of emergency
   calls, except that if the device is unable to determine its own
   location, it MAY send the emergency call without a Geolocation header
   and without a Route header (since it would be unable to query the
   LoST server for a route per RFC6881).  If an emergency call arrives
   at the provider without a Geolocation header, the provider MUST
   supply location by adding the Geolocation header, and MUST supply the
   route by querying the LoST server with that location.

   If the emergency call is to be handled using existing country
   specific procedures, the Provider is responsible for modifying the
   INVITE to conform to the country-specific requirements.  In this
   case, location MAY be extracted from the RFC6881 conformant INVITE
   and used to propagate it to the appropriate country-specific
   entities.  Because the RUE may have a more accurate and timely
   location of the device than the a manual entry location for nomadic
   RUE devices, but country-specific procedures require the location to
   be pre-loaded in some entity prior to placing an emergency call,
   implementations of a RUE device MAY send a Geolocation header
   containing its location in the REGISTER request if the configuration
   specifies it.  That information MAY be used to populate the location
   to appropriate country-specific entities.  Re-registration SHOULD be
   used to update the location, so long as the rate of re-registration
   is limited if the device is moving.

   Implementations MUST implement Additional Data, [RFC7852].  RUE
   devices MUST implement Data Provider, Device Implementation and
   Owner/Subscriber Information blocks.

5.3.  Mid Call Signaling

   Implementations MUST support re-INVITE to renegotiate media session
   parameters (among other uses).  Per Section 6.1, implementations
   MUST, be able to support an INFO request for full frame refresh for
   devices that do not support RTCP mechanisms (please refer to
   Section 6.8).  Implementations MUST support an in-dialog REFER
   ([RFC3515] updated by [RFC7647] and including support for norefersub
   per [RFC4488]) with the Replaces header [RFC3891] to enable call

5.4.  URI Representation of Phone Numbers

   SIP URIs constructed from non-URI sources (dial strings) and sent to
   SIP proxies by the RUE MUST be represented as follows, depending on
   whether they can be represented as an E.164 number.  In this section
   "expressed as an E.164 number" includes numbers such as toll free
   numbers that are not actually E.164 numbers, but have the same

   A dial string that can be expressed as an E.164 phone number MUST be
   represented as a SIP URI with a URI ";user=phone" tag.  The user part
   of the URI MUST be in conformance with 'global-number' defined in
   [RFC3966].  The user part MUST NOT contain any 'visual-separator'

   Dial strings that cannot be expressed as E.164 numbers MUST be
   represented as dialstring URIs, as specified by [RFC4967], e.g.,;user=dialstring.

   The domain part of Relay Service URIs and User Address of Records
   (AoR) MUST resolve (per [RFC3263]) to globally routable IPv4 and/or
   IPv6 addresses.

5.5.  Transport

   Implementations MUST conform to [RFC8835] except that that this
   specification does not use the WebRTC data channel.  See
   Section Section 6.2 for how RUE supports real time text without the
   data channel.

   Implementations MUST support SIP outbound [RFC5626] (please also
   refer to Section 5.1).

6.  Media

   This specification adopts the media specifications for WebRTC
   ([RFC8825]).  Where WebRTC defines how interactive media
   communications may be established using a browser as a client, this
   specification assumes a normal SIP call.  The RTP, RTCP, SDP and
   specific media requirements specified for WebRTC are adopted for this
   document.  The RUE is a WebRTC non-browser" endpoint, except as noted
   expressly below.

   The following sections specify the WebRTC documents to which
   conformance is required.  "Mandatory to Implement" means a conforming
   implementation must implement the specified capability.  It does not
   mean that the capability must be used in every session.  For example,
   OPUS is a mandatory to implement audio codec, and all conforming
   implementations must support OPUS.  However, implementation
   presenting a call across the RUE Interface where the call originates
   in the Public Switched Telephone Network, or an older, non-RUE-
   compatible device, which only offers G.711 audio, does not need to
   include the OPUS codec in the offer, since it cannot be used with
   that call.

6.1.  SRTP and SRTCP

   Implementations MUST support [RFC8834] except that MediaStreamTracks
   are not used.  Implementations MUST conform to Section 6.4 of

6.2.  Text-Based Communication

   Implementations MUST support real-time text ([RFC4102] and [RFC4103])
   via T.140 media.  One original and two redundant generations MUST be
   transmitted and supported, with a 300 ms transmission interval.  Note
   that this is not how real time text is transmitted in WebRTC and some
   form of transcoder would be required to interwork real time text in
   the data channel of WebRTC to RFC4103 real time text.

6.3.  Video

   Implementations MUST conform to [RFC7742] with the exception that,
   since backwards compatibility is desirable and older devices do not
   support VP8, that only H.264, as specified in [RFC7742] is Mandatory
   to Implement and VP8 support is OPTIONAL at both the device and

6.4.  Audio

   Implementations MUST conform to [RFC7874].

6.5.  DTMF Digits

   Implementations MUST support the "audio/telephone-event" [RFC4733]
   media type.  They MUST support conveying event codes 0 through 11
   (DTMF digits "0"-"9", "*","#") defined in Table 7 of [RFC4733].
   Handling of other tones is OPTIONAL.

6.6.  Session Description Protocol

   The SDP offers and answers MUST conform to the SDP rules in [RFC8829]
   except that the RUE Interface uses SIP transport for SDP and the SDP
   for real time text MUST specify [RFC4103].

6.7.  Privacy

   The RUE MUST be able to control privacy of the user by implementing a
   one-way mute of audio and or video, without signaling, locally, but
   MUST maintain any NAT bindings by periodically sending media packets
   on all active media sessions containing silence/comfort noise/black
   screen/etc. per [RFC6263].

6.8.  Negative Acknowledgment, Packet Loss Indicator, and Full
      Intraframe Request Features

   NACK SHOULD be used when negotiated and conditions warrant its use.
   Signaling picture losses as Packet Loss Indicator (PLI) SHOULD be
   preferred, as described in [RFC5104].

   FIR SHOULD be used only in situations where not sending a decoder
   refresh point would render the video unusable for the users, as per
   RFC5104 subsection

   For backwards compatibility with calling devices that do not support
   the foregoing methods, implementations MUST implement SIP INFO
   messages to send and receive XML encoded Picture Fast Update messages
   according to [RFC5168].

7.  Contacts

7.1.  CardDAV Login and Synchronization

   Support of CardDAV by Providers is OPTIONAL.

   The RUE MUST and Providers MAY be able to synchronize the user's
   contact directory between the RUE endpoint and one maintained by the
   user's VRS provider using CardDAV ([RFC6352] and [RFC6764]).

   The configuration MAY supply a username and domain identifying a
   CardDAV server and address book for this account.

   To access the CardDAV server and address book, the RUE MUST follow
   Section 6 of RFC6764, using the chosen username and domain in place
   of an email address.  If the request triggers a challenge for digest
   authentication credentials, the RUE MUST attempt to continue using
   matching "credentials" from the configuration.  If no matching
   credentials are configured, the RUE MUST use the SIP credentials from
   the configuration.  If the SIP credentials fail, the RUE MUST query
   the user.

   Synchronization using CardDAV MUST be a two-way synchronization
   service, with proper handling of asynchronous adds, changes, and
   deletes at either end of the transport channel.

7.2.  Contacts Import/Export Service

   Implementations MUST be able to export/import the list of contacts in
   jCard [RFC7095] [RFC6351] json format.

   The RUE accesses this service via the "contacts" URI in the
   configuration.  The URL MUST resolve to identify a web server
   resource that imports/exports contact lists for authorized users.

   The RUE stores/retrieves the contact list (address book) by issuing
   an HTTPS POST or GET request.  If the request triggers a challenge
   for digest authentication credentials, the RUE MUST attempt to
   continue using matching "credentials" from the configuration.  If no
   credentials are configured, the RUE MUST query the user.

8.  Mail Waiting Indicator (MWI)

   Providers MUST support MWI if they support video mail.  RUE devices
   MUST support MWI.

   Implementations MUST support subscriptions to "message-summary"
   events [RFC3842] to the URI specified in the configuration.

   In notification bodies, videomail messages SHOULD be reported using
   "message-context-class multimedia-message" defined in [RFC3458].

9.  Provisioning and Provider Selection

   To simplify how users interact with RUE devices, the RUE interface
   separates provisioning into two parts.  One provides a directory of
   providers so that a user interface that allows easy provider
   selection either for registering or for dial-around.  The other
   provides configuration data for the device for each provider.

9.1.  RUE Provider Selection

   To allow the user to select a relay service, the RUE MAY at any time
   obtain (typically on startup) a list of Providers that provide
   service in a country.  IANA has established a registry that contains
   a two letter country code and a URI.  The URI, when used with the
   following interface, returns a list of provider names for a country
   code suitable for display, with a corresponding a URI to obtain
   information about that provider.  The provider URI is the entry point
   of a simple web service that returns contact information for that

   Each country that supports video relay service using this
   specification MAY support the provider list.  This document does not
   specify who maintains the list.  Some possibilities are a regulator
   or entity designated by a regulator, an agreement among providers to
   provide the list, or a user group.

   The web service also has a simple version mechanism that returns a
   list of versions of the web service it supports.  This document
   describes version 1.0.  Versions are described as a major version,
   the period "." and a minor version, where major and minor versions
   are integers.  A backwards compatible change within a major version
   MAY increment only the minor version number.  A non-backwards
   compatible change MUST increment the major version number.  To
   achieve backwards compatibility, implementations MUST ignore any
   object members they do not implement and minor version definitions
   SHALL only add objects, non-required members of existing objects, and
   non-mandatory-to use functions and SHALL NOT delete any objects,
   members of objects or functions.  This means an implementation of a
   specific major version and minor version is backwards compatible with
   all minor versions of the major version.  The versions mechanism
   returns an array of supported versions, one for each major version
   supported, with the minor version listed being the highest supported
   minor version.

   The V1 provider list is a json object consisting of an array where
   each entry describes one Provider.  Each entry consists of the
   following items:

   *  name: This parameter contains the text label identifying the
      Provider and is meant to be displayed to the human VRS user.

   *  domain: The domain parameter is used for configuration purposes by
      the RUE (as discussed in Section 9.2)

   The VRS user interacts with the RUE to select from the Provider list
   one or more Providers with whom the user has already established an
   account, wishes to establish an account, or wishes to use the
   provider for a one stage dial around.

       "providers": [
           "name": "Red",
           "domain": "",
           "name": "Green",
           "domain": "",
           "name": "Blue",
           "domain": ""

              Figure 2: Example of a Provider list JSON object

       "versions": [
           "major": 1,
           "minor": 6,
           "major": 2,
           "minor": 13,
           "major": 3,
           "minor": 2

                 Figure 3: Example of a Version JSON object

9.2.  RUE Configuration Service

   A RUE device may retrieve a provider configuration the using a simple
   HTTPs web service.  There are two entry points.  One is used without
   user credentials, the response includes configuration data for new
   user sign up and dial around.  The other uses the userid/password to
   authenticate to the interface and returns configuration data for the

   An optional parameter may be provided to the interface which is an
   API Key.  The implementation MAY have an API Key obtained from the
   provider and specific to the implementation.  The method the API Key
   is obtained is not specified in this document.  The provider MAY
   refuse to provide service to an implementation presenting an API Key
   it does not recognize.

   A required parameter which contains an instance identifier.  This
   parameter MUST be the same value each time this instance (same
   implementation on same device) queries the interface.  This may be
   used by the provider, for example, to associate a location with the
   instance for emergency calls.

   The data returned is a json object consisting of an array of key/
   value configuration parameters to be used by the RUE.

   The configuration API also provides the same version mechanism as
   specified above in Section 9.1.  The version of the configuration
   service MAY be different than the version of the provider list

   The configuration data payload includes the following data items.
   Items not noted as (OPTIONAL) are REQUIRED.  If other unexpected
   items are found, they MUST be ignored.

9.2.1.  Provider Configuration

   *  signup: (OPTIONAL) an array of json objects consisting of:

      -  language: entry from the IANA language subtag registry

      -  uri: a URI to the website for creating a new account in the
         supported language.  The new user signup URI may only initiate
         creation of a new account.  Various vetting, approval and other
         processes may be needed, which could take time, before the
         account is established.  The result of creating a new account
         would be a username and password, which would be manually
         entered into the RUE device to allow connection to the

   *  dialAround: an array of json objects consisting of:

      -  language: entry from the IANA language subtag registry

      -  frontDoor: a URI to a queue of interpreters supporting the
         specified language for a two stage dial-around

      -  oneStage: a URI that can be used with a one-stage dial-around
         Section 5.2.2 using an interpreter supporting the specified

   *  helpDesk: (OPTIONAL) an array of json objets consisting of:

      -  language: entry from the IANA language subtag registry

      -  uri: URI that reaches a help desk for callers supporting the
         specified language.

9.2.2.  RUE Configuration

   *  lifetime: Specifies how long (in seconds) the RUE MAY cache the
      configuration values.  Values may not be valid when lifetime
      expires.  If the RUE caches configuration values, it MUST
      cryptographically protect them.  The RUE SHOULD retrieve a fresh
      copy of the configuration before the lifetime expires or as soon
      as possible after it expires.  The lifetime is not guaranteed: the
      configuration may change before the lifetime value expires.  In
      that case, the Provider MAY indicate this by generating
      authorization challenges to requests and/or prematurely
      terminating a registration.Emergency Calls MUST continue to work.

   *  sip-password: (OPTIONAL) a password used for SIP, STUN and TURN
      authentication.  The RUE device retains this data, which must be
      stored securely.  If it is not supplied, but was supplied on a
      prior invocation of this interface, the most recently supplied
      password MUST be used.  If it was never supplied, the password
      used to authenticate to the configuration service is used for SIP,
      STUN and TURN.

   *  phone-number: (OPTIONAL) The telephone number (in E.164 format)
      assigned to this subscriber.  This becomes the user portion of the
      SIP URI identifying the subscriber.

   *  outbound-proxy: (OPTIONAL) A URI of a SIP proxy to be used when
      sending requests to the Provider.

   *  mwi: (OPTIONAL) A URI identifying a SIP event server that
      generates "message-summary" events for this subscriber.

   *  videomail: (OPTIONAL) A SIP URI that can be called to retrieve
      videomail messages.

   *  contacts: (CONDITIONAL) An HTTPS URI that may be used to export
      (retrieve) the subscriber's complete contact list managed by the

   *  carddav: (OPTIONAL) A username, password and domain name
      (separated by ""@"") identifying a "CardDAV" server that can be
      used to synchronize the RUE's contact list with the contact list
      managed by the Provider.  Optionally contains a user name and/or
      password that may be used with the server.  If username or
      password are not supplied, the main account credentials are used.

   *  sendLocationWithRegistration: (OPTIONAL) True if the RUE should
      send a Geolocation Header with REGISTER, false if it should not.
      Defaults to false if not present.

   *  ice-servers: (OPTIONAL) An array of URLs identifying STUN and TURN
      servers available for use by the RUE for establishing media
      streams in calls via the Provider.

9.2.3.  Examples

   Example JSON provider configuration payload
       "signUp": [
          { "language" : "en", "uri" : ""} ,
          { "language" : "es", "uri" : ""} ] ,
       "dialAround": [
          { "language" : "en", "frontDoor" : "",
               "oneStage" : "" } ,
          { "language" : "es", "frontDoor" : "",
               "oneStage" : "" } ] ,
       "helpDesk": [
          { "language" : "en", "uri" : ""} ,
          { "language" : "es", "uri" : ""} ] ,

                                  Figure 4

   Example JSON RUE configuration payload
       "lifetime": 86400,
       "display-name" : "Bob Smith",
       "phone-number": "+18135551212",
       "provider-domain": "",
       "outbound-proxies": [
       "mwi": "",
       "videomail": "",
       "contacts": "",
       "carddav": "" ,
       "sendLocationWithRegistration": false,
       "ice-servers": [
          {"stun":"" },

                                  Figure 5

9.2.4.  Using the Provider Selection and RUE Configuration Services

   One way to use these two services is:

   *  At startup, the RUE retrieves the provider list for the country it
      is located in.

   *  For each provider in the list:

      -  If the RUE does not have credentials for that provider, use the
         configuration service without credentials to obtain signup,
         dial around and helpdesk information.

      -  If the RUE has credentials for that provider, use the
         configuration service with credentials to obtain all
         configuration data.

9.3.  OpenAPI Interface Descriptions

   The interfaces in Sections Section 9.1 and Section 9.2 are formally
   specified with OpenAPI 3.0 descriptions in yaml form.

 openapi: 3.0.1
   title: RUM API
   version: "1.0"
   - url: http://localhost/rum/v1
       summary: Get a list of providers and domains to get
                config data from
       operationId: GetProviderList
           description: List of providers for a country
                 $ref: '#/components/schemas/ProviderList'
       summary: Configuration data for one provider
       operationId: GetProviderConfiguration
         - in: query
           name: apiKey
             type: string
           description: API Key assigned to this implementation
         - in: query
           name: instanceId
             type: string
           required: true
           description: Unique string for this implementation
                        on this device
           description: configuration object
                 $ref: '#/components/schemas/ProviderConfigurationData'
       summary: Configuration data for one RUE
       operationId: GetRueConfiguration

         - in: query
           name: apiKey
             type: string
           description: API Key assigned to this implementation
         - in: query
           name: instanceId
             type: string
           required: true
           description: Unique string for this implementation
                        on this device
           description: configuration object
                 $ref: '#/components/schemas/RueConfigurationData'
       - url:
         description: Override base path for Versions query
       summary: Retrieves all supported versions
       operationId: RetrieveVersions
           description: Versions supported
                 $ref: '#/components/schemas/VersionsArray'
       type: object
         - providers
           type: array
             type: object
               - name
               - domain

                 type: string
                 description: Human readable provider name
                 type: string
                 description: provider domain for interface
       type: object
         - versions
           type: array
             type: object
               - major
               - minor
                 type: integer
                 format: int32
                 description: Version major number
                 type: integer
                 format: int32
                 description: Version minor number
       type: object
           type: object
             - language
             - uri
               type: string
               description: entry from IANA language-subtag-registry
               type: string
               format: uri
               description: uri to signup website supporting language
           type: object
             - language
             - frontDoor

               type: string
               description: entry from IANA language-subtag-registry
               type: string
               format: uri
               description: SIP uri for 2 stage dial around
               type: string
               format: uri
               description: SIP uri for 1 stage dial around
           type: object
             - language
             - uri
               type: string
               description: entry from IANA language-subtag-registry
               type: string
               format: uri
               description: SIP uri of helpdesk supporting language
       type: object
           type: integer
           description: how long (in seconds) the RUE MAY cache the
                        configuration values
           type: string
           type: string
           description: telephone number assigned this subscriber
           type: string
           format: uri
           description: SIP uri of proxy to be used when sending
                        requests to the Provider
           type: string
           format: uri
           description: A URI identifying a SIP event server that
               generates "message-summary" events for this subscriber.

           type: string
           format: uri
           description: A SIP URI that can be called to retrieve
                        videomail messages.
           type: string
           format: uri
           description: An HTTPS URI that may be used to export
              (retrieve) the subscriber's complete contact list
              managed by the Provider.
           type: object
           description: CardDAV server and user information that can be
                used to synchronize the RUE's contact list with the
                contact list managed by the Provider.
               type: string
               description: CardDAV server address
               type: string
               description: username for authentication with CardDAV
                  server.  Use provider username if not provided
               type: string
               description: password for authentication to the CardDAV
                  server. Use provider password if not provided
           type: boolean
           description: True if the RUE should send a Geolocation Header
                 with REGISTER, false if it should not.
                  Defaults to false if not present.
           type: array
             type: string
             format: uri
             description: URIs identifying STUN and TURN servers
                available for use by the RUE for establishing
                media streams in calls via the Provider.

              Figure 6: Provider List OpenAPI description

10.  Acknowledgements

   Brett Henderson and Jim Malloy provided many helpful edits to prior
   versions of this document.

11.  IANA Considerations

11.1.  RUE Provider List Registry

   IANA has created the "RUE Provider List" registry.  The management
   policy for this registry is "Expert Review" [RFC8126].  The expert
   should prefer a regulator operated or designated list interface
   operator.  Otherwise, evidence that the proposed list interface
   operator will provide a complete list of providers is required to add
   the entry to the registry.  Updates to the registry are permitted if
   the expert judges the new proposed uri to be better than the existing
   entry.  Each entry has two fields, values for both of which MUST be
   provided when registering or updating an entry:

   *  country code: a two letter ISO93166 country code

   *  list uri: a uri that implements the provider list interface for
      that country

11.2.  Registration of rue-owner purpose parameter

   This document defines the new predefined value "rue-owner" for the
   "purpose" header field parameter of the Call-Info header field.  This
   modifies the "Header Field Parameters and Parameter Values"
   subregistry of the "Session Initiation Protocol (SIP) Parameters"
   registry by adding this RFC as a reference to the line for the header
   field "Call-Info" and parameter name "purpose"

   *  Header Field: Call-Info

   *  Parameter Name: purpose

   *  Predefined Values: Yes

12.  Security Considerations

   The RUE is required to communicate with servers on public IP
   addresses and specific ports to perform its required functions.  If
   it is necessary for the RUE to function on a corporate or other
   network that operates a default-deny firewall between the RUE and
   these services, the user must arrange with their network manager for
   passage of traffic through such a firewall in accordance with the
   protocols and associated SRV records as exposed by the Provider.
   Because VRS providers may use different ports for different services,
   these port numbers may differ from Provider to Provider.

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,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [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,

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              DOI 10.17487/RFC3263, June 2002,

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,

   [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,

   [RFC5626]  Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
              "Managing Client-Initiated Connections in the Session
              Initiation Protocol (SIP)", RFC 5626,
              DOI 10.17487/RFC5626, October 2009,

   [RFC8866]  Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
              Session Description Protocol", RFC 8866,
              DOI 10.17487/RFC8866, January 2021,

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323,
              DOI 10.17487/RFC3323, November 2002,

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              DOI 10.17487/RFC3605, October 2003,

   [RFC6665]  Roach, A.B., "SIP-Specific Event Notification", RFC 6665,
              DOI 10.17487/RFC6665, July 2012,

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
              2002, <>.

   [RFC5393]  Sparks, R., Ed., Lawrence, S., Hawrylyshen, A., and B.
              Campen, "Addressing an Amplification Vulnerability in
              Session Initiation Protocol (SIP) Forking Proxies",
              RFC 5393, DOI 10.17487/RFC5393, December 2008,

   [RFC5658]  Froment, T., Lebel, C., and B. Bonnaerens, "Addressing
              Record-Route Issues in the Session Initiation Protocol
              (SIP)", RFC 5658, DOI 10.17487/RFC5658, October 2009,

   [RFC5954]  Gurbani, V., Ed., Carpenter, B., Ed., and B. Tate, Ed.,
              "Essential Correction for IPv6 ABNF and URI Comparison in
              RFC 3261", RFC 5954, DOI 10.17487/RFC5954, August 2010,

   [RFC3960]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
              Tone Generation in the Session Initiation Protocol (SIP)",
              RFC 3960, DOI 10.17487/RFC3960, December 2004,

   [RFC6442]  Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
              for the Session Initiation Protocol", RFC 6442,
              DOI 10.17487/RFC6442, December 2011,

   [RFC3327]  Willis, D. and B. Hoeneisen, "Session Initiation Protocol
              (SIP) Extension Header Field for Registering Non-Adjacent
              Contacts", RFC 3327, DOI 10.17487/RFC3327, December 2002,

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,

   [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
              Header Field for the Session Initiation Protocol (SIP)",
              RFC 3326, DOI 10.17487/RFC3326, December 2002,

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, DOI 10.17487/RFC3515, April 2003,

   [RFC4488]  Levin, O., "Suppression of Session Initiation Protocol
              (SIP) REFER Method Implicit Subscription", RFC 4488,
              DOI 10.17487/RFC4488, May 2006,

   [RFC7647]  Sparks, R. and A.B. Roach, "Clarifications for the Use of
              REFER with RFC 6665", RFC 7647, DOI 10.17487/RFC7647,
              September 2015, <>.

   [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
              Protocol (SIP) "Replaces" Header", RFC 3891,
              DOI 10.17487/RFC3891, September 2004,

   [RFC3892]  Sparks, R., "The Session Initiation Protocol (SIP)
              Referred-By Mechanism", RFC 3892, DOI 10.17487/RFC3892,
              September 2004, <>.

   [RFC3665]  Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and
              K. Summers, "Session Initiation Protocol (SIP) Basic Call
              Flow Examples", BCP 75, RFC 3665, DOI 10.17487/RFC3665,
              December 2003, <>.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,

   [RFC4967]  Rosen, B., "Dial String Parameter for the Session
              Initiation Protocol Uniform Resource Identifier",
              RFC 4967, DOI 10.17487/RFC4967, July 2007,

   [RFC4102]  Jones, P., "Registration of the text/red MIME Sub-Type",
              RFC 4102, DOI 10.17487/RFC4102, June 2005,

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              DOI 10.17487/RFC4733, December 2006,

   [RFC6263]  Marjou, X. and A. Sollaud, "Application Mechanism for
              Keeping Alive the NAT Mappings Associated with RTP / RTP
              Control Protocol (RTCP) Flows", RFC 6263,
              DOI 10.17487/RFC6263, June 2011,

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
              February 2008, <>.

   [RFC5168]  Levin, O., Even, R., and P. Hagendorf, "XML Schema for
              Media Control", RFC 5168, DOI 10.17487/RFC5168, March
              2008, <>.

   [RFC6352]  Daboo, C., "CardDAV: vCard Extensions to Web Distributed
              Authoring and Versioning (WebDAV)", RFC 6352,
              DOI 10.17487/RFC6352, August 2011,

   [RFC6764]  Daboo, C., "Locating Services for Calendaring Extensions
              to WebDAV (CalDAV) and vCard Extensions to WebDAV
              (CardDAV)", RFC 6764, DOI 10.17487/RFC6764, February 2013,

   [RFC7095]  Kewisch, P., "jCard: The JSON Format for vCard", RFC 7095,
              DOI 10.17487/RFC7095, January 2014,

   [RFC3842]  Mahy, R., "A Message Summary and Message Waiting
              Indication Event Package for the Session Initiation
              Protocol (SIP)", RFC 3842, DOI 10.17487/RFC3842, August
              2004, <>.

   [RFC3458]  Burger, E., Candell, E., Eliot, C., and G. Klyne, "Message
              Context for Internet Mail", RFC 3458,
              DOI 10.17487/RFC3458, January 2003,

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <>.

   [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,

   [RFC7852]  Gellens, R., Rosen, B., Tschofenig, H., Marshall, R., and
              J. Winterbottom, "Additional Data Related to an Emergency
              Call", RFC 7852, DOI 10.17487/RFC7852, July 2016,

   [RFC7874]  Valin, JM. and C. Bran, "WebRTC Audio Codec and Processing
              Requirements", RFC 7874, DOI 10.17487/RFC7874, May 2016,

   [RFC7742]  Roach, A.B., "WebRTC Video Processing and Codec
              Requirements", RFC 7742, DOI 10.17487/RFC7742, March 2016,

   [RFC6351]  Perreault, S., "xCard: vCard XML Representation",
              RFC 6351, DOI 10.17487/RFC6351, August 2011,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8825]  Alvestrand, H., "Overview: Real-Time Protocols for
              Browser-Based Applications", RFC 8825,
              DOI 10.17487/RFC8825, January 2021,

   [RFC8827]  Rescorla, E., "WebRTC Security Architecture", RFC 8827,
              DOI 10.17487/RFC8827, January 2021,

   [RFC8829]  Uberti, J., Jennings, C., and E. Rescorla, Ed.,
              "JavaScript Session Establishment Protocol (JSEP)",
              RFC 8829, DOI 10.17487/RFC8829, January 2021,

   [RFC8834]  Perkins, C., Westerlund, M., and J. Ott, "Media Transport
              and Use of RTP in WebRTC", RFC 8834, DOI 10.17487/RFC8834,
              January 2021, <>.

   [RFC8835]  Alvestrand, H., "Transports for WebRTC", RFC 8835,
              DOI 10.17487/RFC8835, January 2021,

   [RFC8760]  Shekh-Yusef, R., "The Session Initiation Protocol (SIP)
              Digest Access Authentication Scheme", RFC 8760,
              DOI 10.17487/RFC8760, March 2020,

   [RFC8599]  Holmberg, C. and M. Arnold, "Push Notification with the
              Session Initiation Protocol (SIP)", RFC 8599,
              DOI 10.17487/RFC8599, May 2019,

Author's Address

   Brian Rosen
   470 Conrad Dr
   Mars, PA 16046
   United States of America

   Phone: +1 724 382 1051