path: root/doc/rfc/rfc7462.txt

Internet Engineering Task Force (IETF)                     L. Liess, Ed.
Request for Comments: 7462                                     R. Jesske
Updates: 3261                                        Deutsche Telekom AG
Category: Standards Track                                    A. Johnston
ISSN: 2070-1721                                                    Avaya
                                                               D. Worley
                                                                 Ariadne
                                                              P. Kyzivat
                                                                  Huawei
                                                              March 2015


              URNs for the Alert-Info Header Field of the
                   Session Initiation Protocol (SIP)

Abstract

   The Session Initiation Protocol (SIP) supports the capability to
   provide a reference to a specific rendering to be used by the User
   Agent (UA) as an alerting signal (e.g., a ring tone or ringback tone)
   when the user is alerted.  This is done using the Alert-Info header
   field.  However, the reference (typically a URL) addresses only a
   specific network resource with specific rendering properties.  There
   is currently no support for standard identifiers for describing the
   semantics of the alerting situation or the characteristics of the
   alerting signal, without being tied to a particular rendering.  To
   overcome these limitations and support new applications, a new family
   of URNs for use in Alert-Info header fields (and situations with
   similar requirements) is defined in this specification.

   This document normatively updates RFC 3261, which defines the Session
   Initiation Protocol (SIP).  It changes the usage of the Alert-Info
   header field defined in RFC 3261 by additionally allowing its use in
   any non-100 provisional response to INVITE.  This document also
   permits proxies to add or remove an Alert-Info header field and to
   add or remove Alert-Info header field values.















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Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7462.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.











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Table of Contents

   1. Introduction ....................................................5
   2. Requirements Language ...........................................7
   3. Terminology .....................................................7
   4. Updates to RFC 3261 .............................................7
      4.1. Allow Alert-Info in Provisional Responses ..................7
      4.2. Proxies May Alter Alert-Info Header Fields .................8
   5. Requirements ....................................................8
   6. Use Cases ......................................................10
      6.1. PBX Ring Tones ............................................10
           6.1.1. Normal .............................................10
           6.1.2. External ...........................................10
           6.1.3. Internal ...........................................11
           6.1.4. Priority ...........................................11
           6.1.5. Short ..............................................11
           6.1.6. Delayed ............................................11
      6.2. Service Tones .............................................11
           6.2.1. Call Waiting .......................................11
           6.2.2. Forward ............................................12
           6.2.3. Transfer Recall ....................................12
           6.2.4. Auto Callback ......................................12
           6.2.5. Hold Recall ........................................12
      6.3. Country-Specific Ringback Tone Indications for the
           Public Switched ...........................................12
   7. URN Specification for the "alert" Namespace Identifier .........12
   8. "alert" URN Values .............................................18
      8.1. <alert-category> Values ...................................18
      8.2. <alert-indication> Values .................................18
           8.2.1. <alert-indication> Values for the
                  <alert-category> "service" .........................19
           8.2.2. <alert-indication> Values for the
                  <alert-category> "source" ..........................19
           8.2.3. <alert-indication> Values for the
                  <alert-category> "priority" ........................19
           8.2.4. <alert-Indication> Values for the
                  <alert-category> "duration" ........................20
           8.2.5. <alert-indication> Values for the
                  <alert-category> "delay" ...........................20
           8.2.6. <alert-indication> Values for the
                  <alert-category> "locale" ..........................20
   9. IANA Considerations ............................................20
      9.1. URN Namespace Identifier "alert" ..........................20
      9.2. 'Alert URN Identifiers' Registry ..........................20
           9.2.1. Initial IANA Registration ..........................21
                  9.2.1.1. The "service" <alert-category> and
                           <alert-identifier>s .......................22




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                  9.2.1.2. The "source" <alert-category> and
                           <alert-identifier>s .......................23
                  9.2.1.3. The "priority" <alert-category>
                           and <alert-identifier>s ...................24
                  9.2.1.4. The "duration" <alert-category>
                           and <alert-identifier>s ...................24
                  9.2.1.5. The "delay" <alert-category> and
                           <alert-identifier>s .......................25
                  9.2.1.6. The "locale" <alert-category> and
                           <alert-identifier>s .......................25
      9.3. 'Alert URN Providers' Registry ............................26
   10. Extension Rules ...............................................26
      10.1. General Extension Rules ..................................26
      10.2. Private Extension Rules ..................................27
      10.3. Examples .................................................28
           10.3.1. Subsetting an Existing URN ........................28
           10.3.2. A New Value within an <alert-category> ............29
           10.3.3. A New <alert-category> ............................29
           10.3.4. Subsetting a Private Extension URN ................29
   11. Combinations of "alert" URNs ..................................30
      11.1. Priority Rules ...........................................30
      11.2. Multi-mode Signals .......................................31
   12. Non-normative Algorithm for Handling Combinations of URNs .....32
      12.1. Algorithm Description ....................................32
      12.2. Examples of How the Algorithm Works ......................34
           12.2.1. Example 1 .........................................34
           12.2.2. Example 2 .........................................35
           12.2.3. Example 3 .........................................37
           12.2.4. Example 4 .........................................38
           12.2.5. Example 5 .........................................39
   13. User Agent Behaviour ..........................................40
   14. Proxy Behaviour ...............................................41
   15. Internationalization Considerations ...........................42
   16. Security Considerations .......................................42
   17. References ....................................................43
      17.1. Normative References .....................................43
      17.2. Informative References ...................................44
   Acknowledgements ..................................................45
   Authors' Addresses ................................................46












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1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] includes a means to
   suggest to a User Agent (UA) a particular ringback tone or ring tone
   to be used during session establishment.  In [RFC3261], this is done
   by including a URI, in the Alert-Info header field, that specifies a
   reference to the tone.  The URI is most commonly the HTTP URL to an
   audio file.  On the receipt of the Alert-Info header field, the UA
   may fetch the referenced ringback tone or ring tone and play it to
   the user.

   This mechanism hinders interoperability when there is no common
   understanding of the meaning of the referenced tone, which might be
   country- or vendor-specific.  It can lead to problems for the user
   trying to interpret the tone and for the UA wanting to substitute its
   own tone (e.g., in accordance with user preferences) or provide an
   alternative alerting mode (e.g., for deaf and hard-of-hearing users).
   If the caller and the callee are from different countries, their
   understanding of the tones may differ significantly.  Deaf or hard-
   of-hearing users may not sense the specific tone if it is provided as
   an audio file.  The tone, per se, is also not useful for automata.

   Another limitation of using URLs of audio files is that the
   referenced tones are tied to particular renderings.  There is no
   method to signal the semantic intention of the alert while enabling
   the recipient UA to choose the specific alert indication (such as a
   particular tone, vibration, or visual display) to use to signal the
   intention.  Similarly, there is no method to signal particular
   rendering features (such as short duration, delay, or country-
   specific conventions).

   The issues with URLs that reference audio files can be avoided by
   using fixed URLs with specific meanings.  However, this approach has
   its own interoperability issues.  For example, consider the Private
   Branch Exchange (PBX) special ring tone for an external (to the PBX)
   caller.  Different vendors use different approaches such as:

      Alert-Info: <file://ring.pcm>;alert=external

   where ring.pcm is a dummy file name, or:

      Alert-Info: <file://external.ring.pcm>

      Alert-Info: <sip:external-ringtone@example.com>

   As a result, the Alert-Info header field currently only works when
   the same vendor provides a PBX and UA, and only then if the same
   artificial proprietary URI convention is used.



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   To solve the described issues, this specification defines the new URN
   namespace "alert" for the SIP Alert-Info header field that allows for
   programmatic user interface adaptation and for conversion of
   equivalent alerting tones in the Public Switched Telephone Network
   (PSTN) when the client is a gateway.  The work to standardize an
   "alert" URN will increase SIP interoperability for this header field
   by replacing proprietary conventions used today.

   The "alert" namespace provides a syntax for several different
   application spaces, for example:

   o  Names for service indications, such as call waiting or automatic
      callback, not tied to any particular rendering.

   o  Names for common ring tones generated by PBX phones for cases such
      as an internal enterprise caller, external caller, ringback tone
      after a transfer failure or expiration of a hold timer, etc.

   o  Names for country-specific ringback tones.

   o  Names for things with specific renderings that aren't purely
      audio.  They might be static icons, video sequences, text, etc.

   Some advantages of a URN rather than a URL of a downloadable
   resource:

   o  There is no need to download it or deal with security issues
      associated with dereferencing.

   o  There are no formatting or compatibility issues.

   o  There is no security risk of rendering something unexpected and
      undesirable.

   o  The tone can be stored locally in whatever format and at whatever
      quality level is appropriate, because it is specified "by name"
      rather than "by value".

   o  It is easier to make policy decisions about whether or not to use
      it.

   o  It facilitates translation for the deaf and hard of hearing.

   The downside is that if the recipient does not understand the URN,
   then it will only be able to render a default ringback tone or ring
   tone.





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   This document creates a new URN namespace and registry for alert
   indications and registers some initial values.

   In practice, this specification extends the usage of the Alert-Info
   header field in that it will cause the use of a new class of URIs and
   the use of multiple URIs.  Backward compatibility issues are not
   expected, as devices that do not understand an "alert" URN should
   ignore it, and devices should not malfunction upon receiving multiple
   Alert-Info header field values (<alert-param>s in [RFC3261]) (which
   was syntactically permitted before, but rarely used).

2.  Requirements Language

   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 [RFC2119].

3.  Terminology

   This specification uses a number of terms to refer to the roles
   involved in the use of alerting indications in SIP.  A "specifier"
   sends an "alerting indication" (one or more URNs in an Alert-Info
   header field) to a "renderer", which then "renders" a "signal" or
   "rendering" based on the indication to a human user.  A "category" is
   a characteristic whose "values" can be used to classify indications.

   This specification uses the terms "ring tone" and "ringback tone".  A
   "ring tone" or "calling signal" (terminology used in [E182]) is a
   signal generated by the callee's end device, advising the callee
   about an incoming call.  A "ringback tone" or "ringing tone"
   (terminology used in [E182]) is a signal advising the caller that a
   connection has been made and that a ring tone is being rendered to
   the callee.

4.  Updates to RFC 3261

4.1.  Allow Alert-Info in Provisional Responses

   This specification changes the usage of the Alert-Info header field
   defined in [RFC3261] by additionally allowing its use in any non-100
   provisional response to INVITE.

   Previously, the Alert-Info header field was only permitted in 180
   (Ringing) responses.  But in telephony, other situations indicated by
   SIP provisional responses, such as 181 (Call Is Being Forwarded) and
   182 (Call Is Being Queued), are often indicated by tones.  Extending
   the applicability of the Alert-Info header field allows the telephony
   practice to be implemented in SIP.



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   To support this change, the following paragraph replaces the the
   first paragraph of Section 20.4 of [RFC3261]:

      When present in an INVITE request, the Alert-Info header field
      specifies an alternative ring tone to the User Agent Server (UAS).
      When present in a non-100 provisional response, the Alert-Info
      header field specifies an alternative ringback tone to the UAC.  A
      typical usage is for a proxy to insert this header field to
      provide a distinctive ring feature.

4.2.  Proxies May Alter Alert-Info Header Fields

   A SIP proxy MAY add or remove an Alert-Info header field, and it MAY
   add or remove Alert-Info header field values, in a SIP request or a
   non-100 provisional response.

5.  Requirements

   This section discusses the requirements for an alerting indication to
   transport the semantics of the alerting situation or the
   characteristics of the rendering.

   REQ-1:  The mechanism will allow UAs and proxies to provide in the
           Alert-Info header field an alerting indication that describes
           the semantics of the signaling situation or the
           characteristics of the rendering and allows the recipient to
           decide how to render the received information to the user.

   REQ-2:  The mechanism will allow the alerting indication to be
           specified "by name" rather than "by value", to enable local
           policy decisions whether or not to use it.

   REQ-3:  The mechanism will enable alerting indications to represent a
           wide variety of signals, which have many largely orthogonal
           characteristics.

   REQ-4:  The mechanism will enable the set of alerting indications to
           support extensibility by a wide variety of organizations that
           are not coordinated with each other.  Extensions will be able
           to:

               add further values to any existing category

               add further categories that are orthogonal to existing
               categories

               semantically subdivide the meaning provided by any
               existing indication



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   REQ-5:   The mechanism will be flexible, so new alerting indications
            can be defined in the future, when SIP-applications evolve.
            For example, "alert" URNs could identify specific media by
            name, such as "Beethoven's Fifth", and the end device could
            render some small part of it as a ring tone.

   REQ-6:   The mechanism will provide only an indication capability,
            not a negotiation capability.

   REQ-7:   The mechanism will not require an alerting indication to
            depend on context provided by a previous alerting indication
            in either direction.

   REQ-8:   The mechanism will allow transmission in the Alert-Info
            header field of SIP INVITE requests and provisional 1xx
            responses excepting the 100 responses.

   REQ-9:   The mechanism will be able to accommodate both renderers
            that are customized with a limited or uncommon set of
            signals that they can render and renderers that are provided
            with a set of signals that have uncommon semantics.  (The
            canonical example is a UA for the deaf and hard of hearing,
            customized with an alternative set of signals, video or text
            instead of audio.  By REQ-6, the renderer has no way of
            transmitting this fact to the specifier.)

   REQ-10:  The mechanism will allow an alerting indication to reliably
            carry all extensions if the specifier and the renderer have
            designs that are properly coordinated.

   REQ-11:  The mechanism will allow a renderer to select a tone that
            approximates to that intended by the specifier if the
            renderer is unable to provide the precise tone indicated.

   REQ-12:  The mechanism will support alerting indications relating to
            services such as call waiting, call forwarding, transfer
            recall, auto callback, and hold recall.

   REQ-13:  The mechanism will allow rendering common PBX ring tone
            types.

   REQ-14:  The mechanism will allow rendering specific country ringback
            tones.

   REQ-15:  The mechanism will allow rendering tones for emergency
            alerts.  (Use cases and definitions of URN values for
            emergency calls are not a subject of this specification.)




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   REQ-16:  The mechanism will allow rendering using other means than
            tones, e.g., text or images.

   REQ-17:  The mechanism will allow PSTN gateways to map ring/ringback
            tones from legacy protocols to SIP at the edge of a network,
            e.g., national ring tones as defined in TIA/EIA-41-D and
            3GPP2 A.S0014.  (Use cases and values definition for this
            situation are not a subject of this specification.)

   REQ-18:  The mechanism will ensure that if an UA receives "alert"
            URNs or portions of an "alert" URN it does not understand,
            it can ignore them.

   REQ-19:  The mechanism will allow storage of the actual encoding of
            the rendering locally rather than fetching it.

   REQ-20:  The mechanism must provide a simple way to combine two or
            more alerting indications to produce an alerting indication
            that requests a combination of the intentions of the two
            alerting indications, where any contradictions or conflicts
            between the two alerting indications are resolved in favor
            of the intention of the first alerting indication.

6.  Use Cases

   This section describes some use cases for which the "alert" URN
   mechanism is needed today.

6.1.  PBX Ring Tones

   This section defines some commonly encountered ring tones on PBX or
   business phones.  They are as listed in the following subsections.

6.1.1.  Normal

   This tone indicates that the default or normal ring tone should be
   rendered.  This is essentially a no-operation "alert" URN and should
   be treated by the UA as if no "alert" URN is present.  This is most
   useful when Alert-Info header field parameters are being used.  For
   example, in [RFC7463], an Alert-Info header field needs to be present
   containing the "appearance" parameter, but no special ring tone needs
   to be specified.

6.1.2.  External

   This tone is used to indicate that the caller is external to the
   enterprise or PBX system.  This could be a call from the PSTN or from
   a SIP trunk.



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6.1.3.  Internal

   This tone is used to indicate that the caller is internal to the
   enterprise or PBX system.  The call could have been originated from
   another user on this PBX or on another PBX within the enterprise.

6.1.4.  Priority

   A PBX tone needs to indicate that a priority level alert should be
   applied for the type of alerting specified (e.g., internal alerting).

6.1.5.  Short

   In this case, the alerting type specified (e.g., internal alerting)
   should be rendered shorter than normal.  In contact centers, this is
   sometimes referred to as "abbreviated ringing" or a "zip tone".

6.1.6.  Delayed

   In this case, the alerting type specified should be rendered after a
   short delay.  In some bridged-line/shared-line-appearance
   implementations, this is used so that the bridged line does not ring
   at exactly the same time as the main line but is delayed a few
   seconds.

6.2.  Service Tones

   These tones are used to indicate specific PBX and public network
   telephony services.

6.2.1.  Call Waiting

   The call-waiting service [TS24.615] permits a callee to be notified
   of an incoming call while the callee is engaged in an active or held
   call.  Subsequently, the callee can either accept, reject, or ignore
   the incoming call.  There is an interest on the caller side to be
   informed about the call-waiting situation on the callee side.  Having
   this information the caller can decide whether to continue waiting
   for callee to pickup or better to call some time later when it is
   estimated that the callee could have finished the ongoing
   conversation.  To provide this information, a callee's UA (or proxy)
   that is aware of the call-waiting condition can add the call-waiting
   indication to the Alert-Info header field in the 180 (Ringing)
   response.







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6.2.2.  Forward

   This feature is used in a 180 (Ringing) response when a call
   forwarding feature has been initiated on an INVITE.  Many PBX system
   implement a forwarding "beep" followed by normal ringing to indicate
   this.  Note that a 181 response can be used in place of this URN.

6.2.3.  Transfer Recall

   This feature is used when a blind transfer [RFC5589] has been
   performed by a server on behalf of the transferor and fails.  Instead
   of failing the call, the server calls back the transferor, giving
   them another chance to transfer or otherwise deal with the call.
   This service tone is used to distinguish this INVITE from a normal
   incoming call.

6.2.4.  Auto Callback

   This feature is used when a user has utilized a server to implement
   an automatic callback service [RFC6910].  When the user is available,
   the server calls back the user and utilizes this service tone to
   distinguish this INVITE from a normal incoming call.

6.2.5.  Hold Recall

   This feature is used when a server implements a call hold timer on
   behalf of an endpoint.  After a certain period of time of being on
   hold, the user who placed the call on hold is alerted to either
   retrieve the call or otherwise dispose of the call.  This service
   tone is used to distinguish this case from a normal incoming call.

6.3.  Country-Specific Ringback Tone Indications for the Public Switched
      Telephone Network

   In the PSTN, different tones are used in different countries.  End
   users are accustomed to hear the callee's country ringback tone and
   would like to have this feature for SIP.

7.  URN Specification for the "alert" Namespace Identifier

   This section provides the registration template for the "alert" URN
   namespace identifier (NID) according to [RFC2141] and [RFC3406].

   Namespace ID:  alert

   Registration Information:
      Registration version:  1
      Registration date:  2014-12-10



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   Declared registrant of the namespace:
      Registering organization:  Real-time Applications and
         Infrastructure Area, IETF
      Designated contact:  RAI Area Director
      Designated contact email:  rai-ads@ietf.org

   Declaration of syntactic structure:

      The Namespace Specific String (NSS) for the "alert" URNs is called
      an <alert-identifier> and has a hierarchical structure.  The first
      colon-separated part after "alert" is called the <alert-category>;
      the parts to the right of that are <alert-ind-part>s, and together
      form the <alert-indication>.  The general form is
      urn:alert:<alert-category>:<alert-indication>.

      The following <alert-category> identifiers are defined in this
      document: "service" , "priority" , "source" , "duration", "delay",
      and "locale".  The <alert-category> set can be extended in the
      future, either by standardization or by private action.  The
      <alert-category>s describe distinct features of alerting signals.

      Any "alert" URN defined in this specification is syntactically
      valid for ring and ringback tones and can be used in SIP INVITE
      requests or in provisional 1xx responses excepting the 100
      response.

      The ABNF [RFC5234] for the "alert" URNs is shown below:

         alert-URN         = "urn:alert:" alert-identifier
         alert-identifier  = alert-category ":" alert-indication
         alert-category    = alert-name
         alert-indication  = alert-ind-part *(":" alert-ind-part)
         alert-ind-part    = alert-name
         alert-name        = alert-label / private-name
         private-name      = alert-label "@" provider
         provider          = alert-label
         alert-label       = let-dig [ *let-dig-hyp let-dig ]
         let-dig-hyp       = let-dig / "-"
         let-dig           = ALPHA / DIGIT
         ALPHA             = %x41-5A / %x61-7A   ; A-Z / a-z
         DIGIT             = %x30-39 ; 0-9

      <alert-label>s MUST comply with the syntax for Non-Reserved LDH
      labels [RFC5890].  Registered URNs and components thereof MUST be
      transmitted as registered (including case).

   Relevant ancillary documentation:  RFC 7462




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   Namespace considerations:  This specification defines a URN namespace
      "alert" for URNs representing signals or renderings that are
      presented to users to inform them of events and actions.  The
      initial usage is to specify ring tones and ringback tones when
      dialogs are established in SIP, but they can also be used for
      other communication-initiation protocols (e.g., H.323), and more
      generally, in any situation (e.g., web pages or endpoint device
      software configurations) to describe how a user should be
      signaled.

      An "alert" URN does not describe a complete signal, but rather it
      describes a particular characteristic of the event it is signaling
      or a feature of the signal to be presented.  The complete
      specification of the signal is a sequence of "alert" URNs
      specifying the desired characteristics/significance of the signal
      in priority order, with the most important aspects specified by
      the earlier URNs.  This allows the sender of a sequence of URNs to
      compose very detailed specifications from a restricted set of
      URNs, and to clearly specify which aspects of the specification it
      considers most important.

      The initial scope of usage is in the Alert-Info header field, in
      initial INVITE requests (to indicate how the called user should be
      alerted regarding the call) and non-100 provisional (1xx)
      responses to those INVITE requests (to indicate the ringback, how
      the calling user should be alerted regarding the progress of the
      call).

      In order to ensure widespread adoption of these URNs for
      indicating ring tones and ringback tones, the scheme must allow
      replication of the current diversity of these tones.  Currently,
      these tones vary between the PSTNs of different nations and
      between equipment supplied by different vendors.  Thus, the scheme
      must accommodate national variations and proprietary extensions in
      a way that minimizes the information that is lost during
      interoperation between systems that follow different national
      variations or that are supplied by different vendors.

      The scheme allows definition of private extension URNs that refine
      and extend the information provided by standard URNs.  Private
      extension URNs can also refine and extend the information provided
      by other private extension URNs.  Private extensions can also
      define entirely new categories of information about calls.  We
      expect these extensions to be used extensively when existing PBX
      products are converted to support SIP operation.






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      The device that receives an Alert-Info header field containing a
      sequence of "alert" URNs provides to the user a rendering that
      represents the semantic content of the URNs.  The device is given
      great leeway in choosing the rendering, but it is constrained by
      rules that maximize interoperability between systems that support
      different sets of private extensions.  In particular, earlier URNs
      in the sequence have priority of expression over later URNs in the
      sequence, and URNs that are not usable in their entirety (because
      they contain unknown extensions or are incompatible with previous
      URNs) are successively truncated in attempt to construct a URN
      that retains some information and is renderable in the context.

      Due to the practical importance of private extensions for the
      adoption of URNs for alerting calls and the very specific rules
      for private extensions and the corresponding processing rules that
      allow quality interoperation in the face of private extensions,
      the requirements of the "alert" URN scheme cannot be met by a
      fixed enumeration of URNs and corresponding meanings.  In
      particular, the existing namespace "urn:ietf:params" does not
      suffice (unless the private extension apparatus is applied to that
      namespace).

      There do not appear to be other URN namespaces that uniquely
      identify the semantic of a signal or rendering feature.  Unlike
      most other currently registered URN namespaces, the "alert" URN
      does not identify documents and protocol objects (e.g., [RFC3044],
      [RFC3120], [RFC3187], [RFC3188], [RFC4179], [RFC4195], [RFC4198]),
      types of telecommunications equipment [RFC4152], people, or
      organizations [RFC3043].

      The <alert-URN>s are hierarchical identifiers.  An <alert-URN>
      asserts some fact or feature of the offered SIP dialog, or some
      fact or feature of how it should be presented to a user, or of how
      it is being presented to a user.  Removing an <alert-ind-part>
      from the end of an <alert-URN> (which has more than one <alert-
      ind-part>) creates a shorter <alert-URN> with a less specific
      meaning; the set of dialogs to which the longer <alert-URN>
      applies is necessarily a subset of the set of dialogs to which the
      shorter <alert-URN> applies.  (If the starting <alert-URN>
      contains only one <alert-ind-part>, and thus the <alert-ind-part>
      cannot be removed to make a shorter <alert-URN>, we can consider
      the set of dialogs to which the <alert-URN> applies to be a subset
      of the set of all dialogs.)

      The specific criteria defining the subset to which the longer
      <alert-URN> applies, within the larger set of dialogs, is
      considered to be the meaning of the final <alert-ind-part>.  This
      meaning is relative to and depends upon the preceding <alert-



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      category> and <alert-ind-part>s (if any).  The meanings of two
      <alert-ind-part>s that are textually the same but are preceded by
      different <alert-category>s or <alert-ind-part>s have no necessary
      connection.  (An <alert-category> considered alone has no meaning
      in this sense.)

      The organization owning the <provider> within a <private-name>
      specifies the meaning of that <private-name> when it is used as an
      <alert-ind-part>.  (The organization owning a <provider> is
      specified by the registry described in Section 9.3.)

      The organization owning the <provider> within a <private-name> (in
      either an <alert-category> or an <alert-ind-part>) specifies the
      meaning of each <alert-ind-part>, which is an <alert-label> that
      follows that <private-name> and that precedes the next <alert-ind-
      part> which is a <private-name> (if any).

      The meaning of all other <alert-ind-part>s (i.e., those that are
      not <private-name>s and do not follow a <private-name>) is defined
      by standardization.

   Community considerations:  The "alert" URNs are relevant to a large
      cross-section of Internet users, namely those that initiate and
      receive communication connections via the Session Initiation
      Protocol.  These users include both technical and non-technical
      users, on a variety of devices and with a variety of perception
      capabilities.  The "alert" URNs will allow Internet users to
      receive more information about offered calls and enable them to
      better make decisions about accepting an offered call, and to get
      better feedback on the progress of a call they have made.

      User interfaces that utilize alternative sensory modes can better
      render the ring and ringback tones based on the "alert" URNs
      because the URNs provide more detailed information regarding the
      intention of communications than is provided by current SIP
      mechanisms.

   Process of identifier assignment:

      Assignment of standardized "alert" URNs is by insertion into the
      IANA registry described in Section 9.2.  This process defines the
      meanings of <alert-ind-part>s that have standardized meanings, as
      described in "Namespace Considerations".

      A new URN MUST NOT be registered if it is equal by the comparison
      rules to an already registered URN.





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      Private extensions are "alert" URNs that include <alert-ind-part>s
      that are <private-name>s and <alert-label>s that appear after a
      <private-name> (either as an <alert-category> or an <alert-
      indication>).  If such an <alert-ind-part> is a <private-name>,
      its meaning is defined by the organization that owns the
      <provider> that appears in the <private-name>.  If the <alert-ind-
      part> is an <alert-label>, its meaning is defined by the
      organization that owns the <provider> that appears in the closest
      <private-name> preceding the <alert-label>.  The organization
      owning a <provider> is specified by the registry described in
      Section 9.3.

   Identifier uniqueness and persistence considerations:  An "alert" URN
      identifies a semantic feature of a call or a sensory feature of
      how the call alerting should be a rendered at the caller's or
      callee's end device.

      For standardized <alert-ind-part>s in URNs, uniqueness and
      persistence of their meanings is guaranteed by the fact that they
      are registered with IANA in accordance with the procedures of
      Section 9.2; the feature identified by a particular "alert" URN is
      distinct from the feature identified by any other standardized
      "alert" URN.

      Assuring uniqueness and persistence of the meanings of private
      extensions is delegated to the organizations that define private
      extension <alert-ind-part>s.  The organization responsible for a
      particular <alert-ind-part> in a particular "alert" URN is the
      owner of a syntactically determined <provider> part within the
      URN.

      An organization SHOULD use only one <provider> value for all of
      the <private-name>s it defines.

   Process for identifier resolution:  The process of identifier
      resolution is the process by which a rendering device chooses a
      rendering to represent a sequence of "alert" URNs.  The device is
      allowed great leeway in making this choice, but the process MUST
      obey the rules of Section 11.1.  The device is expected to provide
      renderings that users associate with the meanings assigned to the
      URNs within their cultural context.  A non-normative example
      resolution algorithm is given in Section 12.1.

   Rules for lexical equivalence:  "alert" URNs are compared according
      to case-insensitive string equality.






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   Conformance with URN syntax:  All "alert" URNs must conform to the
      ABNF in the "Declaration of Syntactic Structure" in Section 7.
      That ABNF is a subset of the generic URN syntax [RFC2141].
      <alert-label>s are constrained to be Non-Reserved LDH labels
      [RFC5890], that is, "ordinary ASCII labels".  Future
      standardization may allow <alert-label>s that are A-labels
      [RFC5890], and so interpreters of "alert" URNs MUST operate
      correctly (per Section 11.1) when given such URNs as input.

   Validation mechanism:  An "alert" URN containing no private
      extensions can be validated based on the IANA registry of
      standardized "alert" URNs.  Validating an "alert" URN containing
      private extensions requires obtaining information regarding the
      private extensions defined by the organization that owns the
      <provider> in the relevant <private-name>.  The identity of the
      organization can be determined from the IANA registry described in
      Section 9.2.  However, if an "alert" URN contains at least one
      <alert-identifier> that precedes the first <private-name>, the
      portion of the "alert" URN that precedes the first <private-name>
      must itself be a valid standardized "alert" URN, which may be
      validated as above.

   Scope:  The scope for this URN is public and global.

8.  "alert" URN Values

8.1.  <alert-category> Values

   The following <alert-category> values are defined in this document:

   - service
   - source
   - priority
   - duration
   - delay
   - locale

8.2.  <alert-indication> Values

   This section describes the "alert" URN indication values for the
   <alert-category>s defined in this document.

   For each <alert-category>, a default <alert-indication> is defined,
   which is essentially a no-operation "alert" URN and should be treated
   by the UA as if no "alert" URN for the respective category is
   present.  "alert" URN default indications are most useful when Alert-
   Info header field parameters are being used.  For example, in




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   [RFC7463], an Alert-Info header field needs to be present containing
   the "appearance" parameter, but no special ringtone need be
   specified.

   The <private-name> syntax is used for extensions defined by
   independent organizations, as described in Section 10.2.

8.2.1.  <alert-indication> Values for the <alert-category> "service"

   - normal (default)
   - call-waiting
   - forward
   - recall:callback
   - recall:hold
   - recall:transfer
   - <private-name>

   Examples: <urn:alert:service:call-waiting> or
   <urn:alert:service:recall:transfer>.

8.2.2.  <alert-indication> Values for the <alert-category> "source"

   - unclassified (default)
   - internal
   - external
   - friend
   - family
   - <private-name>

   (These <alert-indication>s will rarely be provided by the sending UA;
   rather they will usually be inserted by a proxy acting on behalf of
   the recipient UA to inform the recipient UA about the origins of a
   call.)

   Examples: <urn:alert:source:external>.

8.2.3.  <alert-indication> Values for the <alert-category> "priority"

   - normal (default)
   - low
   - high
   - <private-name>

   Examples: <urn:alert:priority:high>.







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8.2.4.  <alert-Indication> Values for the <alert-category> "duration"

   - normal (default)
   - short
   - long
   - <private-name>

   Examples: <urn:alert:duration:short>.

8.2.5.  <alert-indication> Values for the <alert-category> "delay"

   - none (default)
   - yes
   - <private-name>

   Examples: <urn:alert:delay:yes>.

8.2.6.  <alert-indication> Values for the <alert-category> "locale"

   - default (default)
   - country:<ISO 3166-1 country code>
   - <private-name>

   The ISO 3166-1 country code [ISO3166-1] is used to inform the
   renderer on the other side of the call that a country-specific
   rendering should be used.  For example, to indicate ringback tones
   from South Africa, the following URN would be used:
   <urn:alert:locale:country:za>.

9.  IANA Considerations

9.1.  URN Namespace Identifier "alert"

   This section registers a new URN namespace identifier (NID), "alert",
   in accordance with [RFC3406] with the registration template provided
   in Section 7.

9.2.  'Alert URN Identifiers' Registry

   Standard "alert" URNs are recorded as <alert-identifier>s in a new
   registry called "Alert URN Identifiers".  Thus, creating a new
   standard "alert" URN requires IANA action.  IANA manages the "Alert
   URN Identifiers" registry under the policy 'Specification Required'
   [RFC5226] following the guidelines in Section 10.1.







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   The registry contains entries in the following formats:

      <alert-category>/      Reference    Description
      <alert-identifier>
      ---------------------------------------------------------------
      foo                    [RFCxyz]     Description of the 'foo'
                                          <alert-category>;
      foo:bar                [RFCabc]     Description of the 'foo:bar'
                                          <alert-identifier>

      foo:<range>            [RFCdef]     Description of the
      'foo:<category>'                    <alert-identifer>s (which will
                                          reference the <range> value)

   The first value in each row is the value that is registered, which is
   either: (1) an <alert-category> value, (2) an <alert-identifier>
   value, composed of an <alert-category> followed by an <alert-
   indication>, in turn composed of one or more <alert-label>s, or (3) a
   pattern for <alert-identifier> values (e.g., for the "locale" <alert-
   category> in Section 9.2.1.6).

   The second value in each row is the reference to the required
   specification for the value.

   The third value in each row is a short description of the semantics
   of the value.

   A new URN MUST NOT be registered if it is equal by the comparison
   rules (that is, case-insensitive string comparison) to an already
   registered URN.

   <alert-category> and <alert-identifier> values that contain <private-
   name>s are not managed by IANA.  The process of assigning these
   values is described in Section 10.2.

9.2.1.  Initial IANA Registration

   This document defines the <alert-category>s 'service', 'source',
   'priority', 'duration', 'delay' and 'locale'.  The entries to be
   added to the 'Alert URN Identifiers' registry table for each <alert-
   category> are given in the respective sections below.










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9.2.1.1.  The "service" <alert-category> and <alert-identifier>s

   The following table contains the initial IANA registration for the
   "service" <alert-category> and <alert-identifier>s.  The value of
   this indicator is set to a value different from "normal" if the
   caller or callee is informed that a specific telephony service has
   been initiated.

   <alert-category>/              Reference  Description
   <alert-identifier>
   -----------------------------------------------------------
   service                        RFC 7462   Specific telephony
                                             service used in this
                                             call

   service:normal                 RFC 7462   Normal ring/ringback
                                             rendering (default value)

   service:call-waiting           RFC 7462   Call waiting was
                                             initiated at the other side
                                             of the call

   service:forward                RFC 7462   Call has been forwarded

   service:recall:callback        RFC 7462   Recall due to callback

   service:recall:hold            RFC 7462   Recall due to call hold

   service:recall:transfer        RFC 7462   Recall due to transfer






















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9.2.1.2.  The "source" <alert-category> and <alert-identifier>s

   The following table contains the initial IANA registration for the
   "source" <alert-category> and <alert-identifier>.  The value of this
   indicator provides information about the user at the other side of
   the call.

   <alert-category>/             Reference  Description
   <alert-identifier>
   -----------------------------------------------------------
   source                        RFC 7462   Classification
                                            of the other party
                                            to the call

   source:unclassified           RFC 7462   Unclassified ring/ringback
                                            rendering (default value)

   source:internal               RFC 7462   User at the other side of
                                            the call is internal to the
                                            enterprise or PBX system

   source:external               RFC 7462   User at the other side of
                                            the call is external to the
                                            enterprise or PBX system

   source:friend                 RFC 7462   User at the other side of
                                            the call is a friend

   source:family                 RFC 7462   User at the other side of
                                            the call is a family member





















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9.2.1.3.  The "priority" <alert-category> and <alert-identifier>s

   The following table contains the initial IANA registration for the
   "priority" <alert-category> and <alert-identifier>s.  The value of
   this indicator provides information about the priority the alerted
   user should give to the call.

   <alert-category>/               Reference  Description
   <alert-identifier>
   -----------------------------------------------------------
   priority                        RFC 7462   Priority of the
                                              call

   priority:normal                 RFC 7462   Normal ring/ringback
                                              rendering (default value)

   priority:low                    RFC 7462   Low priority call

   priority:high                   RFC 7462   High priority call

9.2.1.4.  The "duration" <alert-category> and <alert-identifier>s

   The following table contains the initial IANA registration for the
   "duration" <alert-category> and <alert-identifier>s.  The value of
   this indicator provides information about the duration of the
   alerting signals compared to the default alerting signals.

  <alert-category>/               Reference  Description
  <alert-identifier>
  -----------------------------------------------------------
  duration                        RFC 7462   Duration of alerting signal

  duration:normal                 RFC 7462   Normal ring/ringback
                                             rendering (default value)

  duration:short                  RFC 7462   Shorter than normal

  duration:long                   RFC 7462   Longer than normal













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9.2.1.5.  The "delay" <alert-category> and <alert-identifier>s

   The following table contains the initial IANA registration for the
   "delay" <alert-category> and <alert-identifier>s.  The value of this
   indicator provides information about whether the presentation of the
   alerting signal should be delayed compared to the default
   presentation process.  For more details see Section 6.1.6.

   <alert-category>/            Reference  Description
   <alert-identifier>
   -----------------------------------------------------------
   delay                        RFC 7462   Delay of rendering
                                           of alerting signal

   delay:none                   RFC 7462   Immediate alerting
                                           (default value)

   delay:yes                    RFC 7462   Delayed alerting


9.2.1.6.  The "locale" <alert-category> and <alert-identifier>s

   The following table contains the initial IANA registration for the
   "locale" <alert-category> and <alert-identifier>s.  The value of this
   indicator provides information about whether the alerting signals
   characteristic of the specified location should be used.

   <alert-category>/             Reference  Description
   <alert-identifier>
   -----------------------------------------------------------
   locale                        RFC 7462   Location-specific
                                            alerting signals

   locale:default                RFC 7462   Alerting not location
                                            specific
                                            (default value)

   locale:country:<ISO 3166-1 country code>
                                 RFC 7462   Alerting according to the
                                            conventions of the specified
                                            country










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9.3.  'Alert URN Providers' Registry

   Values of <provider>, which are used to create <private-name>s, are
   recorded in a new registry called "Alert URN Providers".  (Private
   extension "alert" URNs that are defined are not recorded by IANA.)
   The registry is managed by IANA under the policy 'First Come First
   Served' [RFC5226].

   The registry contains entries in the following format:

   <provider>             Registrant       Contact URI
   ---------------------------------------------------------------------
   example                IETF             rai-ads@ietf.org

   The first value in each row is the <provider> value that is
   registered.  This value is case-insensitive and MUST comply with the
   syntax for Non-Reserved LDH labels [RFC5890].

   The second value in each row is the name of the registrant of the
   value.

   The third value is a contact URI for the registrant.

   The registry initially contains the one entry shown above, which can
   be used for constructing examples of private extension URNs.

10.  Extension Rules

10.1.  General Extension Rules

   The set of "alert" URNs is extensible.  An extension "at the top
   level" creates a new <alert-category> (which represents a new
   alerting characteristic), an extension "at the second level" creates
   a new <alert-indication> value for an existing <alert-category>, an
   extension "at the third level" creates a subdivision of an existing
   <alert-indication> (that has one <alert-ind-part>), etc.  URNs allow
   (in principle) indefinite subdivision of existing <alert-indication>
   values, although most of the standard "alert" URNs have only one
   level of subdivision and a few have two levels of subdivision.

   Extensions, either standard or private, MUST conform to the following
   principles:

   A new <alert-category> is independent of all previously existing
   <alert-category>s: For any combination of one <alert-identifier> in
   the new <alert-category> with any one <alert-identifier> in any of
   the previously existing <alert-category>s, there are potential calls
   to which the combination can be meaningfully applied.



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   A new <alert-identifier> that has more than one <alert-ind-part> is a
   semantic refinement of a parent <alert-identifier>, the parent being
   obtained by deleting the final <alert-ind-part>.  The new <alert-
   identifier> has as parent the most specific previously existing
   <alert-identifier> whose meaning includes all potential calls to
   which the new <alert-identifier> could be meaningfully applied.

   A new <alert-identifier> has no semantic overlap with any sibling
   <alert-identifier> (<alert-identifier>s that differ only in the final
   <alert-ind-part>).  That is, there could be no call to which both
   <alert-identifier>s could be meaningfully applied.

   The process for defining new standard "alert" URNs is described in
   Section 9.2; all such definitions require registering a publicly
   available specification.  The process for defining new "alert" URNs
   via the private extension mechanism is described in Section 10.2.

10.2.  Private Extension Rules

   The <private-name> syntax is used to create private extensions,
   extensions that are not registered with IANA.  The <private-name> has
   the form of an <alert-label> followed by "@" and then a <provider>
   that designates the organization defining the extension.  Both
   <alert-label> and <provider> have the same syntax as an ordinary
   ASCII DNS label.  A private extension URN is created by using a
   <private-name> as either an <alert-category> or an <alert-ind-part>.

   If the <private-name> is used as an <alert-category>, the
   characteristic of the alerting signal that the <alert-category>
   describes is defined by the organization.  If the <private-name> is
   used as the first <alert-ind-part>, the organization defines an
   alternative value for the standardized <alert-category> of the URN.
   If the <private-name> is used as the second or later <alert-ind-
   part>, the organization defines the meaning of the URN as a subset of
   the meaning of the shorter URN resulting when the <private-name> (and
   any subsequent <alert-ind-part>s) are removed.

   Within a URN, all <alert-label> components that follow a <private-
   name> but are before any following <private-name>s are additional
   private extensions whose meaning is defined by the organization
   defining the nearest preceding <private-name>.

   A URN that contains a private extension can be further subdivided by
   the private extension of a different organization: the second
   organization appends an <alert-ind-part> that is a <private-name>
   containing a the <provider> value for the second organization.





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   The meaning of a <private-name> or an <alert-label> that is defined
   privately (because of a preceding <private-name>) is only fixed
   within the context provided by the sequence of preceding
   <alert-name>s; these components have no meaning in isolation and
   there is no necessary relationship between the meaning of textually
   identical <alert-name>s that are preceded by different sequences of
   <alert-name>s.

   Creating private extension "alert" URNs is not a Standards Action and
   they are not registered with IANA.

   The organization defining a private extension is responsible for
   ensuring persistence of the meaning of the private extension.

   Private extensions MUST conform to the principles of Section 10.1,
   both in regard to previously existing standard <alert-URN>s and in
   regard to any previously existing private extensions using the same
   <provider> value, and any other private extensions that the
   organization is aware of.  In particular, a private extension MUST
   NOT duplicate any standard URN or any private extension that the
   organization is aware of.  (In either of those cases, the
   organization MUST use the existing URN for its purposes.)

   An organization obtains a <provider> value for constructing <private-
   name>s by registering the value with IANA as provided in Section 9.3.

10.3.  Examples

10.3.1.  Subsetting an Existing URN

   The organization registering the <provider> "example" can define
   distinctive versions of <urn:alert:service:call-waiting>:

      urn:alert:service:call-waiting:abc@example

      urn:alert:service:call-waiting:def@example

   It can create a more specialized URN that applies to a subset of the
   situations to which the first URN above applies:

      urn:alert:service:call-waiting:abc@example:xyz

   Because "xyz" follows "abc@example" (and there is no intervening
   <private-name>), its meaning is defined by the owner of the
   <provider> "example".






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10.3.2.  A New Value within an <alert-category>

   The organization registering the <provider> "example" can define URNs
   in the "service" category to express a new service that is not
   covered by any of the standardized URNs:

      urn:alert:service:ghi@example

   However, before defining such a URN, the organization should verify
   that the set of calls to which the URN applies is not a subset of the
   set of calls for some existing URN.  If it is a subset, the extension
   URN should be a subdivision of the existing URN.

10.3.3.  A New <alert-category>

   The organization registering the <provider> "example" can define an
   extension <alert-category> named "jkl@example" with two <alert-
   indication>s "a1" and "a2":

      urn:alert:jkl@example:a1

      urn:alert:jkl@example:a2

10.3.4.  Subsetting a Private Extension URN

   The organization registering the <provider> "foo" wants to define a
   set of URNs that specify the different ring patterns used by a
   "distinctive ring" service to alert for incoming calls that are
   directed to different directory numbers.  These ring patterns are
   composed of groups of ring sounds that have particular patterns of
   lengths.

   The company can create a private <alert-category> "distinctive@foo",
   and within it assign three 'alert' URNs that indicate the three
   different ring patterns used by the company's service:

      urn:alert:distinctive@foo:long-long

      urn:alert:distinctive@foo:short-long-short

      urn:alert:distinctive@foo:short-short-long

   Later, the company registering the <provider> "bar" wants to define
   an additional 'alert' URN for the ring pattern "short short", which
   it uses to support a fourth directory number for a phone instrument.






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   The company can create a <private-name> to be used with the
   "distinctive@foo" <alert-category>:

      urn:alert:distinctive@foo:short-short@bar

11.  Combinations of "alert" URNs

11.1.  Priority Rules

   This section describes combination rules for the case when all the
   Alert-Info header fields only contain "alert" URNs.  Other
   combinations of URIs in the Alert-Info header fields of the same SIP
   message are not defined in this specification.

   In many cases, more than one URN will be needed to fully define a
   particular tone.  This is done by including multiple "alert" URNs, in
   one or more Alert-Info header fields in a request or a response.  For
   example, an internal, priority call could be indicated by Alert-Info:
   <urn:alert:source:internal>, <urn:alert:priority:high>.  A priority
   call-waiting tone could be indicated by Alert-Info:
   <urn:alert:service:call-waiting>, <urn:alert:priority:high>.

   The sender of the Alert-Info header field may include an arbitrary
   list of "alert" URNs, even if they are redundant or contradictory.
   An earlier URN has priority over any later contradictory URN.  This
   allows any element to modify a list of URNs to require a feature
   value (by adding a URN at the beginning of the list) or to suggest a
   feature value (by adding a URN at the end of the list).

   The receiving UA matches the received "alert" URN combination with
   the signal(s) it is able to render.

   The implementation is free to ignore an "alert" URN if it does not
   recognize the URN, or if it is incapable of rendering its effect in
   the context.  Similarly, it can remove a final series of one or more
   <alert-ind-part>s of an "alert" URN to create a "more generic" URN
   that it recognizes and whose meaning it can render in the context.

   The exact way in which a UA renders a received combination of "alert"
   URNs is left as an implementation issue.  However, the implementation
   MUST comply to following rules:

   (a)  Each "alert" URN has precedence over all URNs that follow it,
        and its interpretation is subordinate to all URNs that precede
        it.






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   (b)  If the UA cannot implement the effect of a URN (because it does
        not recognize the URN or the URN's effect is precluded by
        preceding URNs), the UA repeatedly removes the final <alert-ind-
        part> of the URN until either:

        (1)  the resulting URN is recognized and can be given effect by
             some signal (without reducing the degree of expression of
             any preceding URN), or

        (2)  the resulting URN is reduced to having no <alert-ind-part>
             in which case, that URN in the series cannot be given
             effect, and so is ignored.

   (c)  In case that after processing all the received URNs, the UA can
        generate more than one signal that are equally effective at
        expressing the URNs (under the preceding rules), one of those
        signals is selected.  When selecting from the set of equally
        effective signals, the least specific signal in the set should
        be chosen: a signal should not be chosen if a less-specific
        signal is also in the set.  (Specificity is to be judged based
        on the defined meanings of the signals to the user.)  (For
        example, if each signal is considered to express certain <alert-
        indication>s of certain <alert-category>s, one signal is less-
        specific than a second signal if the first signal's <alert-
        indication>s are a subset or are prefixes of the second signal's
        <alert-indication>s.)  However, a more-specific signal may be
        chosen if the choice is based on information derived from the
        containing SIP message.  For example, a signal implying
        <urn:alert:priority:high> may be chosen if the SIP message
        contains the header field "Priority: urgent".

   In all situations, the set of signals that can be rendered and their
   significances may change based on user preferences and local policy.
   In addition, the chosen signal may change based on the status of the
   UA.  For example, if a call is active on the UA, all audible signals
   may become unavailable, or audible signals may be available only if
   <urn:alert:priority:high> is specified.

11.2.  Multi-mode Signals

   There are cases when the device can render two signal modes (e.g.,
   audio and visual, or video and text) at the same time.

   Formally, the device must be considered to be making its choice from
   the set of all combined signals that it can render (pairs of one
   signal from the first mode and one signal from the second mode), and
   that choice must conform to the above rules.  However, it can be
   proven that if the device makes its rendering choice for each of the



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   two modes independently, with each choice separately conforming to
   the above rules, its combined choice also conforms to the above
   rules, when it is regarded as a choice from among all possible
   combinations.

   In such a situation, it may simplify implementation to make each
   choice separately.  It is an implementation decision whether to chose
   from among combined signals or to combine choices made from each
   signal mode.

12.  Non-normative Algorithm for Handling Combinations of URNs

   The following text is a non-normative example of an algorithm for
   handling combinations of URNs that complies with the rules in
   Sections 10 and 11.  Thus, it demonstrates that the rules are
   consistent and implementable.  (Of course, a device may use any other
   algorithm that complies with Sections 10 and 11.)

12.1.  Algorithm Description

   For each <alert-category> (feature) known by the implementation,
   there is a "feature tree" of the known <alert-indication>s for that
   <alert-category>, with the sequence of <alert-ind-part>s in an
   <alert-indication> specifying the path in the tree from the root to
   the node representing the <alert-indication>.  For this description,
   we will name each tree and its root node by the <alert-category>
   name, and name each non-root node by the <alert-identifier>.  Each
   URN thus corresponds to one non-root node in one feature tree.  For
   example, there is a tree named "source", whose root node is also
   named "source", and which has the children source:internal,
   source:external, source:friend, and source:family.  The URN
   <urn:alert:source:external> is placed at the node "source:external"
   in the "source" tree.  If the implementation understands
   <urn:alert:source:foo@example>, there is a node source:foo@example
   that is a child of node "source".  If the implementation understands
   <urn:alert:source:external:bar@example>, there is a node
   source:external:bar@example that is a child of node source:external.
   (Of course, there are an infinite number of potential additional
   nodes in the tree for private values, but we don't have to represent
   those nodes explicitly unless the device has a signal representing
   the private value.)

   We assign similar locations to signals, but each signal has a
   position in *every* tree, describing the specific combination of
   meanings that it carries.  If a signal has a simple meaning, such as
   "external source", its place in the "source" tree is source:external,





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   showing that it carries the "external source" meaning, but its place
   in every other feature tree is at the root node, meaning that it has
   no particular meaning for those features.

   A signal that has a complex meaning may have non-root positions in
   more than one feature tree.  For example, an "external, high
   priority" signal would be placed at source:external and priority:high
   in those trees, but be at the root in all other feature trees.

   In order to assure that the algorithm always selects at least one
   signal, we require that there is a "default" signal, whose position
   in every feature tree is at the root.  This default signal will never
   be excluded from the set of acceptable signals for any set of URNs,
   but will be the lowest priority signal for any set of URNs.

   The algorithm proceeds by considering each URN in the received Alert-
   Info header fields from left to right, while revising a set of
   signals.  The set of signals starts as the entire set of signals
   available to the device.  Each URN excludes some signals from the
   set, and "sorts" the signals that remain in the set according to how
   well they represent the URN.  (The details of these operations are
   described below.)  The first URN is the "major sort", and has the
   most influence on the position of a signal in the set.  The second
   URN is a "minor sort", in that it arranges the orders of the signals
   that are tied within the first sort, the third URN arranges the
   orders of the signals that are tied within the first two sorts, etc.

   At the end of the algorithm, a final, "most minor" sort is done,
   which orders the signals that remain tied under all the sorts driven
   by the URNs.  This final sort places the least specific signals
   (within their tied groups) "first".  (If one signal's position in
   each feature tree is ancestral or the same as a second signal's
   position in that tree, the first signal is "less specific" than the
   second signal.  Other cases are left to the implementation to
   decide.)

   Once all the URNs are processed and the sorting of the signals that
   have not been excluded is done, the device selects the first signal
   in the set.

   Here is how a single sort step proceeds, examining a single URN to
   modify the set of signals (by excluding some signals and further
   sorting the signals that remain):

   o  The URN specifies a specific node in a specific feature tree.






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   o  All signals in the set that are, within that feature tree,
      positioned at the URN's node, or at an ancestor node of the URN's
      node, are kept.  All other signals are removed from the set
      (because they have meanings that are incompatible with the URN's
      meaning).

   o  Each group of signals that are tied under the previous sorts are
      further sorted into groups based on how much of the URN's meaning
      they represent: those which are positioned at the node of the URN
      are tied for first position, those which are positioned at the
      parent node of the URN are tied for second position, etc., and
      those which are positioned at the root node of the feature tree
      are tied for last position.

12.2.  Examples of How the Algorithm Works

   The following examples show how the algorithm described in the
   previous section works:

12.2.1.  Example 1

   The device has a set of four alerting signals.  We list their primary
   meanings, and the locations that they are placed in the feature
   trees:

   Signal 1

      Meaning: external
      Locations:
      - source:external
      - priority (that is, the root node of the priority tree)

   Signal 2

      Meaning: internal
      Locations:
      - source:internal
      - priority

   Signal 3

      Meaning: low
      Locations:
      - source
      - priority:low






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   Signal 4

      Meaning: high
      Locations:
      - source
      - priority:high

   To which we add:

   Signal 5

      Meaning: default
      Locations:
      - source
      - priority

   If the device receives <urn:alert:source:internal>, then the sort is:

   Signals at source:internal: (this is, first place)

      Signal 2: internal

   Signals at source: (tied for second place)

      Signal 3: low
      Signal 4: high
      Signal 5: default

   And these signals are excluded from the set:

      Signal 1: external

   So, in this example, the sorting algorithm properly gives first place
   to Signal 2 "internal".

12.2.2.  Example 2

   Let us add to the set of signals in Example 1 ones that express
   combinations like "internal, high priority", but let us specifically
   exclude the combination "internal, low priority" so as to set up some
   tricky examples.  This enlarges our set of signals:

   Signal 1

      Meaning: default
      Locations:
      - source
      - priority



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   Signal 2

      Meaning: external
      Locations:
      - source:external
      - priority

   Signal 3

      Meaning: internal
      Locations:
      - source:internal
      - priority

   Signal 4

      Meaning: low
      Locations:
      - source
      - priority:low

   Signal 5

      Meaning: high
      Locations:
      - source
      - priority:high

   Signal 6

      Meaning: external high
      Locations:
      - source:external
      - priority:high

   Signal 7

      Meaning: external low
      Locations:
      - source:external
      - priority:low

   Signal 8

      Meaning: internal high
      Locations:
      - source:internal
      - priority:high



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   If the device receives <urn:alert:source:internal>, then the sort is:

   Signals at source:internal: (that is, tied for first place)

      - Signal 3: internal
      - Signal 8: internal high

   Signals at source: (tied for second place)

      - Signal 4: low
      - Signal 5: high
      - Signal 1: default

   Signals excluded from the set:

      - Signal 2: external
      - Signal 7: external low
      - Signal 6: external high

   Two signals are tied for the first place, but the final sort orders
   them:

      - Signal 3: internal
      - Signal 8: internal high

   because it puts the least-specific signal first.  So, the Signal 3
   "internal" is chosen.

12.2.3.  Example 3

   The same device receives <urn:alert:source:external>,
   <urn:alert:priority:low>.  The first sort (due to
   <urn:alert:source:external>) is:

   Signals at source:external:

      - Signal 2: external
      - Signal 7: external low
      - Signal 6: external high

   Signals at source:

      - Signal 4: low
      - Signal 5: high
      - Signal 1: default






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   Signals excluded:

      - Signal 3: internal
      - Signal 8: internal high

   The second sort (due to <urn:alert:priority:low>) puts signals at
   priority:low before signals at priority, and excludes signal at
   priority:high:

      - Signal 7: external low
      - Signal 2: external
      - Signal 4: low
      - Signal 1: default

   Excluded:

      - Signal 6: external high
      - Signal 5: high
      - Signal 3: internal
      - Signal 8: internal high

   So, we choose Signal 7 "external low".

12.2.4.  Example 4

   Suppose the same device receives <urn:alert:source:internal>,
   <urn:alert:priority:low>.  Note that there is no signal that
   corresponds to this combination.

   The first sort is based on source:internal, and results in this
   order:

      - Signal 3: internal
      - Signal 8: internal high
      - Signal 4: low
      - Signal 5: high
      - Signal 1: default

   Excluded:

      - Signal 2: external
      - Signal 7: external low
      - Signal 6: external high








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   The second sort is based on priority:low, and results in this order:

      - Signal 3: internal
      - Signal 4: low
      - Signal 1: default

   Excluded:

      - Signal 8: internal high
      - Signal 5: high
      - Signal 7: external low
      - Signal 2: external
      - Signal 6: external high

   So, we choose the Signal 3 "internal".

   Note that <urn:alert:priority:low> could not be given effect because
   it followed <urn:alert:source:internal>.  If the two URNs had
   appeared in the reverse order, the Signal 2 "external" would have
   been chosen, because <urn:alert:priority:low> would have been given
   precedence.

12.2.5.  Example 5

   Let us set up a simple set of signals, with three signals giving
   priority:

   Signal 1

      Meaning: default
      Locations:
      - priority

   Signal 2

      Meaning: low
      Locations:
      - priority:low

   Signal 3

      Meaning: high
      Locations:
      - priority:high







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   Notice that we've used the "default" signal to cover "normal
   priority".  That is so the signal will cover situations where no
   priority URN is present, as well as the ones with
   <urn:alert:priority:normal>.  So, we're deliberately failing to
   distinguish "priority:normal" from the default priority.

   If the device receives <urn:alert:priority:low>, the sort is:

      - Signal 2: low
      - Signal 1: default

   Excluded:

      - Signal 3: high

   and Signal 2 "low" is chosen.

   Similarly, if the device receives <urn:alert:priority:high>, Signal 3
   is chosen.

   If the device receives <urn:alert:priority:normal>, the sort is:

      -Signal 1 :default

   Excluded:

      - Signal 2: low
      - Signal 3: high

   and Signal 1 "default" is chosen.

   If no "priority" URN is received, Signal 1 "default" will be put
   before Signal 2 "low" and Signal 3 "high" by the final sort, and so
   it will be chosen.

13.  User Agent Behaviour

   A SIP UA MAY add a URN or multiple URNs to the Alert-Info header
   field in a SIP request or a provisional 1xx response (excepting a 100
   response) when it needs to provide additional information about the
   call or about the provided service.

   Upon receiving a SIP INVITE request or a SIP provisional response
   with an Alert-Info header field that contains a combination of Alert-
   Info URNs, the UA attempts to match the received Alert- Info URNs
   combination with a signal it can render.  The process the UA uses
   MUST conform to the rules described in Section 11.  (A non-normative
   algorithm example for the process is described in Section 12.)



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   The UA must produce a reasonable rendering regardless of the
   combination of URIs (of any schemes) in the Alert-Info header field:
   it MUST produce a rendering based on the URIs that it can understand
   and act on (if any), interpreted as prescribed by local policy, and
   ignore the other URIs.  In particular, unless the containing message
   is a request and is immediately rejected, the UA SHOULD provide some
   alert unless it is instructed not to (for example, by Alert-Info URIs
   that it understands, the presence of a Replaces or Joins header
   field, local policy, or direction of the user).

   Subsequent provisional responses, even within the same dialog, may
   contain different Alert-Info header field values.  The Alert-Info
   header field values received within different provisional responses
   are treated independently.  If subsequent provisional responses
   containing different Alert-Info header field values were received
   within the same dialog, the UA SHOULD render, at any time, the last
   received Alert-Info header field value.  The handling of provisional
   responses containing different Alert-Info header field values that
   were not received within the same dialog is left as an implementation
   issue.

14.  Proxy Behaviour

   A SIP proxy MAY add or remove an Alert-Info header field, and MAY add
   or remove Alert-Info header field values, in a SIP request or a
   non-100 provisional response when it needs to modify the information
   about the call or about the provided services.

   There are many reasons a proxy may choose do this, for example, (1)
   to add indications based on information that the proxy can determine
   about the call, such as that it is coming from an external source, or
   that the INVITE contains a "Priority: urgent" header field; (2) to
   add indication that a particular service is being invoked at this end
   of the call; (3) to remove undesirable indications, such as possibly
   deceptive indications from untrusted sources; and (4) to remove
   indications that contain information that should be suppressed for
   privacy reasons.

   The following example shows a typical example of a 180 (Ringing)
   provisional response that has been modified by a proxy.  The response
   sent by the UAS to the proxy was very similar, but had no Alert-Info
   header field.  The proxy has added Alert-Info header field values
   specifying both a network audio resource referenced by the HTTP URI
   and the URN indication for the call-waiting service.  This allows the
   UAC to render the network audio resource, to choose a rendering based
   on the URN, or to perform some combination of these actions.  Due to
   Section 10, the UAC must produce some reasonable rendering in this
   situation.



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   SIP/2.0 180 Ringing
   Alert-Info: <http://www.example.com/sound/moo.wav>,
                <urn:alert:service:call-waiting>
   To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   Contact: <sip:bob@192.0.2.4>
   CSeq: 314159 INVITE
   Via: SIP/2.0/UDP server10.biloxi.example.com;
               branch=z9hG4bK4b43c2ff8.1
   Content-Length: 0

15.  Internationalization Considerations

   The <alert-identifier> labels are protocol elements [RFC6365] and are
   not normally seen by users.  Thus, the character set for these
   elements is restricted, as described in Section 7.

   Allowance has been made for the possibility of internationalizing
   <alert-identifier>s by allowing them to be A-labels: a processor that
   does not understand such <alert-identifier>s is required to ignore
   them as specified in Sections 7 and 11.1.

   The URNs <urn:alert:locale:country:<ISO 3166-1 country code>> select
   renderings that are conventional in the specified country.

16.  Security Considerations

   As an identifier, the "alert" URN does not appear to raise any
   particular security issues.  The indications described by the "alert"
   URN are meant to be well-known.

   However, the provision of specific indications may raise privacy
   issues by revealing information about the source UA, e.g., its
   nature, its dialog state, or services initiated at its end of the
   call.  For example, call-waiting (Section 6.2.1) and call-forwarding
   (Section 6.2.2) services can reveal the dialog state of the UA.  Such
   a provision SHALL always require authorization on behalf of the user
   of the source UA (usually through accessing configured policy).
   Authorization SHALL NOT assume that there is any limitation of the
   potential recipients of the indications without obtaining specific
   information about the SIP transaction.

   Based on local policy, a UA MAY choose to ignore undesirable
   indications (e.g., possibly deceptive indications from untrusted
   sources), and it MAY choose not to send indications that are





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   otherwise valid in the context (e.g., for privacy reasons).  A proxy
   acting on behalf of a UA MAY add or delete indications going to or
   from the UA for the same reasons.

   Since the alert indications can be sensitive, end-to-end SIP
   encryption mechanisms using S/MIME MAY be used to protect it.  UAs
   that implement alert indications SHOULD also implement SIP over TLS
   [RFC5246] and the sips: scheme [RFC5630].

17.  References

17.1.  Normative References

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

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997,
              <http://www.rfc-editor.org/info/rfc2141>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002, <http://www.rfc-editor.org/info/rfc3261>.

   [RFC3406]  Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
              "Uniform Resource Names (URN) Namespace Definition
              Mechanisms", BCP 66, RFC 3406, October 2002,
              <http://www.rfc-editor.org/info/rfc3406>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008, <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630, October 2009,
              <http://www.rfc-editor.org/info/rfc5630>.






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17.2.  Informative References

   [E182]     ITU-T, "Application of tones and recorded announcements in
              telephone services", ITU-T Recommendation E.182, 1998,
              <http://www.itu.int/rec/T-REC-E.182-199803-I/en>.

   [ISO3166-1]
              ISO, "English country names and code elements", ISO
              3166-1, <http://www.iso.org/iso/
              english_country_names_and_code_elements>.

   [RFC3043]  Mealling, M., "The Network Solutions Personal Internet
              Name (PIN): A URN Namespace for People and Organizations",
              RFC 3043, January 2001,
              <http://www.rfc-editor.org/info/rfc3043>.

   [RFC3044]  Rozenfeld, S., "Using The ISSN (International Serial
              Standard Number) as URN (Uniform Resource Names) within an
              ISSN-URN Namespace", RFC 3044, January 2001,
              <http://www.rfc-editor.org/info/rfc3044>.

   [RFC3120]  Best, K. and N. Walsh, "A URN Namespace for XML.org", RFC
              3120, June 2001, <http://www.rfc-editor.org/info/rfc3120>.

   [RFC3187]  Hakala, J. and H. Walravens, "Using International Standard
              Book Numbers as Uniform Resource Names", RFC 3187, October
              2001, <http://www.rfc-editor.org/info/rfc3187>.

   [RFC3188]  Hakala, J., "Using National Bibliography Numbers as
              Uniform Resource Names", RFC 3188, October 2001,
              <http://www.rfc-editor.org/info/rfc3188>.

   [RFC4152]  Tesink, K. and R. Fox, "A Uniform Resource Name (URN)
              Namespace for the Common Language Equipment Identifier
              (CLEI) Code", RFC 4152, August 2005,
              <http://www.rfc-editor.org/info/rfc4152>.

   [RFC4179]  Kang, S., "Using Universal Content Identifier (UCI) as
              Uniform Resource Names (URN)", RFC 4179, October 2005,
              <http://www.rfc-editor.org/info/rfc4179>.

   [RFC4195]  Kameyama, W., "A Uniform Resource Name (URN) Namespace for
              the TV-Anytime Forum", RFC 4195, October 2005,
              <http://www.rfc-editor.org/info/rfc4195>.

   [RFC4198]  Tessman, D., "A Uniform Resource Name (URN) Namespace for
              Federated Content", RFC 4198, November 2005,
              <http://www.rfc-editor.org/info/rfc4198>.



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   [RFC5589]  Sparks, R., Johnston, A., and D. Petrie, "Session
              Initiation Protocol (SIP) Call Control - Transfer", BCP
              149, RFC 5589, June 2009,
              <http://www.rfc-editor.org/info/rfc5589>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              September 2011, <http://www.rfc-editor.org/info/rfc6365>.

   [RFC6910]  Worley, D., Huelsemann, M., Jesske, R., and D. Alexeitsev,
              "Completion of Calls for the Session Initiation Protocol
              (SIP)", RFC 6910, April 2013,
              <http://www.rfc-editor.org/info/rfc6910>.

   [RFC7463]  Johnston, A., Ed., Soroushnejad, M., Ed., and V.
              Venkataramanan, "Shared Appearances of a Session
              Initiation Protocol (SIP) Address of Record (AOR)", RFC
              7463, March 2015,
              <http://www.rfc-editor.org/info/rfc7463>.

   [TS24.615]
              3GPP, "Communication Waiting (CW) using IP Multimedia (IM)
              Core Network (CN) subsystem; Protocol Specification", 3GPP
              TS 24.615, September 2015.

Acknowledgements

   The authors wish to thank Denis Alexeitsev, the editor of the initial
   version in BLISS, Anwar Siddiqui for his contributions to the
   document, Christer Holmberg for his careful review of the document,
   Adam Roach, Dean Willis, Martin Huelsemann, Shida Schubert, John
   Elwell, and Tom Taylor for their comments and suggestions and Alfred
   Hoenes for his extensive comments and proposals related to new
   namespace identifiers for URNs.












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Authors' Addresses

   Laura Liess (editor)
   Deutsche Telekom AG
   Heinrich-Hertz Str 3-7
   Darmstadt, Hessen  64295
   Germany

   Phone: +49 6151 5812761
   EMail: laura.liess.dt@gmail.com


   Roland  Jesske
   Deutsche Telekom AG
   Heinrich-Hertz Str. 3-7
   Darmstadt, Hessen  64295
   Germany

   Phone: +49 6151 5812766
   EMail: r.jesske@telekom.de


   Alan Johnston
   Avaya, Inc.
   St. Louis, MO
   United States

   EMail: alan.b.johnston@gmail.com


   Dale R. Worley
   Ariadne Internet Services, Inc.
   738 Main St.
   Waltham, MA    02451
   United States

   Phone: +1 781 647 9199
   EMail: worley@ariadne.com


   Paul Kyzivat
   Huawei
   United States

   EMail: pkyzivat@alum.mit.edu






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