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Versions: (draft-rosenberg-sipping-callerprefs-usecases) 00 01 02 03 04 05 RFC 4596

SIPPING                                                     J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: January 17, 2005                                     P. Kyzivat
                                                           Cisco Systems
                                                           July 19, 2004


  Guidelines for Usage of the Session Initiation Protocol (SIP) Caller
                         Preferences Extension
               draft-ietf-sipping-callerprefs-usecases-02

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on January 17, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document contains guidelines for usage of the Caller Preferences
   Extension to the Session Initiation Protocol (SIP).  It motivates the
   benefits of caller preferences with specific example applications,
   provides use cases to show proper operation, provides guidance on the
   applicability of the registered feature tags, and describes a
   straightforward implementation of the matching algorithm.




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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.   Motivations for Caller Preferences . . . . . . . . . . . . .   5
     2.1  One-Number . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2  Direct-to-Voicemail  . . . . . . . . . . . . . . . . . . .   7
   3.   Caller Preference Use Cases  . . . . . . . . . . . . . . . .   9
     3.1  Routing of INVITE and MESSAGE to different UA  . . . . . .   9
       3.1.1  Desired Behavior . . . . . . . . . . . . . . . . . . .   9
       3.1.2  Solution . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2  Single Contact Not Matching Implicit Preferences . . . . .  10
       3.2.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  10
       3.2.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3  Package-Based Routing  . . . . . . . . . . . . . . . . . .  11
       3.3.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  11
       3.3.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  11
     3.4  Package Routing II . . . . . . . . . . . . . . . . . . . .  13
       3.4.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  13
       3.4.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  13
     3.5  Audio/Video vs. Audio Only . . . . . . . . . . . . . . . .  13
       3.5.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  13
       3.5.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  14
     3.6  Forcing Audio/Video  . . . . . . . . . . . . . . . . . . .  15
       3.6.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  15
       3.6.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  15
     3.7  Third Party Call Control - Forcing Media . . . . . . . . .  16
       3.7.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  16
       3.7.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  16
     3.8  Maximizing Media Overlaps  . . . . . . . . . . . . . . . .  17
       3.8.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  17
       3.8.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  17
     3.9  Multilingual Lines . . . . . . . . . . . . . . . . . . . .  19
       3.9.1  Desired Behavior . . . . . . . . . . . . . . . . . . .  19
       3.9.2  Solution . . . . . . . . . . . . . . . . . . . . . . .  19
     3.10   I Hate Voicemail!  . . . . . . . . . . . . . . . . . . .  20
       3.10.1   Desired Behavior . . . . . . . . . . . . . . . . . .  20
       3.10.2   Solution . . . . . . . . . . . . . . . . . . . . . .  21
     3.11   I Hate People! . . . . . . . . . . . . . . . . . . . . .  21
       3.11.1   Desired Behavior . . . . . . . . . . . . . . . . . .  22
       3.11.2   Solution . . . . . . . . . . . . . . . . . . . . . .  22
     3.12   Prefer Voicemail . . . . . . . . . . . . . . . . . . . .  22
       3.12.1   Desired Behavior . . . . . . . . . . . . . . . . . .  22
       3.12.2   Solution . . . . . . . . . . . . . . . . . . . . . .  22
     3.13   Routing to an Executive  . . . . . . . . . . . . . . . .  22
       3.13.1   Desired Behavior . . . . . . . . . . . . . . . . . .  22
       3.13.2   Solution . . . . . . . . . . . . . . . . . . . . . .  23
     3.14   Speak to the Executive . . . . . . . . . . . . . . . . .  23
       3.14.1   Desired Behavior . . . . . . . . . . . . . . . . . .  24



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       3.14.2   Solution . . . . . . . . . . . . . . . . . . . . . .  24
     3.15   Mobile Phone Only  . . . . . . . . . . . . . . . . . . .  24
       3.15.1   Desired Behavior . . . . . . . . . . . . . . . . . .  24
       3.15.2   Solution . . . . . . . . . . . . . . . . . . . . . .  24
     3.16   Simultaneous Languages . . . . . . . . . . . . . . . . .  25
       3.16.1   Desired Behavior . . . . . . . . . . . . . . . . . .  25
       3.16.2   Solution . . . . . . . . . . . . . . . . . . . . . .  25
     3.17   UA Routing . . . . . . . . . . . . . . . . . . . . . . .  26
       3.17.1   Desired Behavior . . . . . . . . . . . . . . . . . .  26
       3.17.2   Solution . . . . . . . . . . . . . . . . . . . . . .  26
     3.18   The Number you Have Called.. . . . . . . . . . . . . . .  27
       3.18.1   Desired Behavior . . . . . . . . . . . . . . . . . .  27
       3.18.2   Solution . . . . . . . . . . . . . . . . . . . . . .  27
     3.19   The Number you Have Called, Take Two . . . . . . . . . .  28
       3.19.1   Desired Behavior . . . . . . . . . . . . . . . . . .  29
       3.19.2   Solution . . . . . . . . . . . . . . . . . . . . . .  29
     3.20   Forwarding to a Colleague  . . . . . . . . . . . . . . .  29
       3.20.1   Desired Behavior . . . . . . . . . . . . . . . . . .  29
       3.20.2   Solution . . . . . . . . . . . . . . . . . . . . . .  29
   4.   Capability Use Cases . . . . . . . . . . . . . . . . . . . .  32
     4.1  Web Redirect . . . . . . . . . . . . . . . . . . . . . . .  32
     4.2  Voicemail Icon . . . . . . . . . . . . . . . . . . . . . .  32
   5.   Usage of the Feature Tags  . . . . . . . . . . . . . . . . .  34
     5.1  Traditional Cell Phone . . . . . . . . . . . . . . . . . .  34
     5.2  Traditional Work Phone . . . . . . . . . . . . . . . . . .  35
     5.3  PC Messenging Application  . . . . . . . . . . . . . . . .  35
     5.4  Standalone Videophone  . . . . . . . . . . . . . . . . . .  35
   6.   Security Considerations  . . . . . . . . . . . . . . . . . .  37
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  38
   8.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  39
   9.   TODO . . . . . . . . . . . . . . . . . . . . . . . . . . . .  40
   10.  Informative References . . . . . . . . . . . . . . . . . . .  40
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  41
        Intellectual Property and Copyright Statements . . . . . . .  42

















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1.  Introduction

   The Session Initiation Protocol (SIP) [1] extension for Callee
   Capabilities [3]  describes mechanisms that allow a UA to register
   its capabilities in a REGISTER request.  A caller can express
   preferences [2], either explicitly or implicitly, about how that
   request is to be handled.  This is accomplished with the
   Accept-Contact and Reject-Contact header fields.

   The caller preferences extension can serve as a useful tool for
   supporting many applications.  However, it generality makes it
   difficult to correctly and effectively use in any one situation.  To
   remedy that, this document serves as an compendium to the caller
   preferences extension.  This additional information is broken into
   four sections.

   First, Section 2 motivates the usage of caller preferences by
   describing several concrete applications which are enabled by the
   extension.  Section 3 describes a set of detailed use cases for
   expressing caller preferences.  Each use case presents a situation,
   describes how caller preferences can be used to handle the
   requirements for the situation, and verifies that the desired
   behavior occurs by showing the results of the matching operation.
   These use cases validate that the caller preferences specification is
   complete, and capable of meeting a specific set of requirements.
   Since the caller preferences specification pre-dates the SIP change
   process [4], no requirements work was ever done for it.  To some
   degree, this document "backfills" requirements.  However, this is not
   an academic exercise only, since the use cases described here did
   result in changes in the caller preferences document as it evolved.
   These use cases also help implementors figure out how to use it in
   their own applications.

   Section 4 discusses applications for the callee capabilities
   specification.  Section 5 discusses the example registrations of the
   feature tags described in [3].  Proper usage of the caller
   preferences extension depends on proper interpretation of the
   semantics of these tags.  More detail is provided on the tags, and
   example registrations are included that show typical usage.












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2.  Motivations for Caller Preferences

   At its core, SIP is protocol to facilitate rendezvous of users.  The
   caller and callee need to meet up in order to exchange session
   information, so that they may communicate.  The rendezvous process is
   complicated by the fact that a user has multiple points of attachment
   to the network.  A user can have a cell phone, a PDA, a work phone, a
   home phone, and one of several PC-based communications applications.
   When someone calls a user, to which of these devices is the call
   routed?

   Certainly, the call can be routed to all of them at the same time, a
   process known as parallel forking.  However, that is not always the
   desired behavior.  A user will frequently have preferences for their
   devices, and wish the call to be routed to one of them first.  As an
   example, a user might prefer that their cell phone ring first, and if
   no one answers, the call rings their work phone next.  As an
   alternative, a user might prefer that their cell phone ring first,
   and then their home and work phones ring at the same time, and then,
   if neither answer, the call is forwarded to voicemail.  These
   variations are all referred to as as find-me/follow-me features.

   SIP supports find-me/follow-me features in many ways.  The most basic
   is through the SIP registration process.  Each device at which a user
   can be contacted registers to the network.  This registration
   associates the device with the canonical name of the user - called
   the address-of-record (AOR), which is a SIP URI.  Each registration
   can include a preference value, indicating the relative preference
   for receiving calls at that device, compared to other devices.  When
   someone makes a call to the AOR, proxies compliant to RFC 3261 will
   try the registered devices in order of preference, unless
   administrative policy overrides user preferences.

   Preference values in SIP registrations can only provide basic
   find-me/follow-me features.  To support more complex features, the
   Call Processing Language (CPL) [5] has been specified.  It is an XML
   script that provides specific call routing instructions.  Users can
   upload these scripts to the network, instructing the servers how
   calls should be routed.  As an example, a CPL script can instruct a
   proxy to route a call to the work phone during work hours (9am - 5pm)
   and then to the cell phone after hours, unless the call is from a
   family member, in which case it always goes to the cell phone.

   It is important to note that both CPL scripts and preference values
   in registrations describe operation of a service from the perspective
   of the called party.  That is, they describe how a call made to them
   should be routed by the network.  However, the called party is not
   the only one with preferences.  A caller will also have preferences



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   for how they want their call to be routed.  As an example, a caller
   will often want to reach a user on their cell phone.  In the current
   telephone network, this is accomplished by requiring a user to have a
   separate number for each device.  This way, when a caller wishes to
   reach the cell phone, they dial the number for the cell phone.  This
   requires users to maintain lists of potential reach numbers for a
   user, and then select the appropriate one.  A far better approach is
   for a user to maintain a single address-of-record.  When someone
   wishes to reach them on their cell phone, they call the AOR, but
   indicate a preference for the call to be routed to the cell phone.

   A caller may actually have a wide variety of preferences for how a
   call should be routed.  They may prefer to go right to voicemail.
   They may prefer to never reach voicemail.  The may prefer to reach
   the user on a device which supports video (because a video-conference
   is desired).  They may wish to reach a device that has an attendant
   who can answer if the user is not there.

   The SIP caller preferences extension allows a caller to express these
   preferences for the way in which their calls are handled.  These
   preferences are expressed in terms of properties of the desired
   device.  These properties are name-value pairs that convey some kind
   of information about a device.  One example is the property
   "mobility" which can have the values "mobile" or "fixed".  When a
   caller wishes to reach a cell phone, they include information in
   their call setup request (the INVITE method) which indicates that the
   call should be routed to a device that has the property "mobility"
   set to "mobile".  When devices register to the network, they include
   their properties - also known as callee capabilities - as part of the
   registration.  In this way, a proxy can match the caller's
   preferences against the capabilities of the various devices
   registered to the user, and route the call appropriately.

   The caller preferences extension can support a wide variety of call
   routing applications and features.  Two particularly important
   examples are "one-number" and ``direct-to-voicemail''.

2.1  One-Number

   In today's circuit-switched telephony networks, users have multiple
   devices, and each device is associated with its own phone number.  A
   user will typically list all of these numbers on a business card -
   cell phone, work phone, home office phone, and so on.  Other users
   need to store and manage all of these numbers.  It is difficult to
   keep these numbers complete and up-to-date.  Worse, when you want to
   call someone, you need to pick a number to try.  Sometimes, you want
   a specific device (the cell phone), and other times, you just want to
   reach them wherever they are.  In the latter case, a user is forced



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   to try each number, one at a time.  This is inefficient, and
   difficult to do while driving, for example.

   As an alternative, a user can have a single address.  This is the one
   and only address they give out to other users on their business
   cards.  If a caller wishes to reach that user on their cell phone,
   they select that one address, and then access a pull-down menu of
   device types.  This menu would include home phone, work phone, and
   cell phone.  The caller can select cell-phone, and then the call is
   placed to the cell phone.  There is no need to manage or maintain
   more than one number for the user - a single one will suffice.

   If, on the other hand, the caller wishes to reach the user wherever
   they are, they make a call to that one number without a selection of
   a preferred device.  The network will ring all devices at the same
   time, and therefore reach the user as fast as possible.

   This one number service makes use of caller preferences.  To express
   a preference for the cell phone, the caller's device would include a
   header in the SIP INVITE request indicating a desire to reach a
   device with "mobility" equal to "mobile".

2.2  Direct-to-Voicemail

   Frequently, a busy executive on the road wants to quickly pass a
   message to a colleague by voice.  As an example, a boss might want to
   instruct an employee to call a specific customer and resolve a
   pending issue.  In such a case, the user doesn't actually want to
   talk to the person; they just want to leave them a voice message.
   Having a phone conversation may require too much time, whereas a
   voice message can be quick and to the point.  The voice message can
   also serve as a record of exactly what is desired, whereas a fleeting
   voice conversation can be forgotten or misremembered.

   In today's circuit-switched telephone networks, there is often no way
   to go directly to someones voicemail and leave a message.  Sometimes,
   you can dial the main number for the voicemail system, enter in the
   extension of the desired party, and leave a message by entering a
   specific prompt.  This is time consuming, and requires the caller to
   know the main voicemail number.

   Instead, an address book in a cell phone can have an option called
   "leave voice message", available for each entry in the address book.
   When this option is selected, a call is made directly to the
   voicemail for that user, which immediately picks up and prompts for a
   message.  In fact, a rapid greeting is played, so that the caller can
   go directly to the recording procedure.




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   This saves time for the caller, making it very easy to quickly leave
   recorded messages for a large number of people.

   This feature is possible using the caller preferences extension.
   When the user selects the "leave voice message" option, the phone
   sends a SIP INVITE request, and includes a caller preferences header
   field that indicates a preference for devices whose "msgserver"
   attribute has a value of "true".  This will cause the proxy to route
   the call directly to a registered voicemail service.  Furthermore,
   the voicemail server will see that the caller asked to go directly to
   voicemail, and can therefore play an abbreviated greeting explicitly
   designed for this case.







































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3.  Caller Preference Use Cases

   Each use case is described as a situation along with a desired
   behavior.  Then, it demonstrates how the various caller preferences
   headers and the proxy processing logic would result in the
   appropriate decision being made.

3.1  Routing of INVITE and MESSAGE to different UA

3.1.1  Desired Behavior

   Address of Record (AOR) Y has two contacts Y1 and Y2.  Y1 is a phone,
   and supports the standard operations INVITE, ACK, OPTIONS, BYE, and
   so on, while Y2 is a pager and supports only OPTIONS and MESSAGE.
   Caller X wants to send pages to Y.  There is a lot of traffic in the
   network of both calls and pages, so there is a goal not to
   unnecessarily fork messages to devices that can't support them.  So,
   ensure that INVITEs of Y are delivered only to Y1, while MESSAGEs to
   Y are delivered only to Y2.

3.1.2  Solution

   Y1 will create a registration which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact:<sip:Y1@pc.example.com>
     ;methods="INVITE,ACK,OPTIONS,BYE,CANCEL"
     ;uri-user="<Y1>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip"
     ;mobility="mobile"

   Y2 will create a registration which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y2@pc.example.com>
     ;methods="OPTIONS,MESSAGE"
     ;uri-user="<Y2>"
     ;uri-domain="example.com"
     ;+message
     ;schemes="sip,im"
     ;mobility="mobile"




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   When a UAC sends an INVITE, it will arrive at the proxy for
   example.com.  There are no caller preferences in the request.
   However, per Section 7.2.2 of [2], the proxy will construct an
   implicit require-flagged Accept-Contact preference that looks like:


   (& (methods="INVITE"))

   Applying the matching algorithm of RFC 2533 [6] to this feature set
   and those registered by Y1 and Y2, the feature set of Y1 alone
   matches.  Because the Accept-Contact predicate has its require flag
   set, Y2 is discarded and the INVITE is routed to Y1.

   If the request was MESSAGE, the proxy constructs an implicit
   require-tagged Accept-Contact preference that looks like:


   (& (methods="MESSAGE"))

   which matches the feature set of Y2, but not Y1.  Thus, Y1 is
   discarded, and the request is routed to Y2.

3.2  Single Contact Not Matching Implicit Preferences

3.2.1  Desired Behavior

   AOR Y has a single contact Y1.  Its a phone, and therefore supports
   the INVITE, BYE, OPTIONS, CANCEL and ACK methods, but not MESSAGE.  A
   caller X sends a MESSAGE request.  The desired behavior is that the
   request is still routed to the solitary contact so that it can
   generate a 405 response.

3.2.2  Solution

   The single contact Y1 will generate a registration which looks like,
   in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y1@pc.example.com>;methods="INVITE,ACK,OPTIONS,BYE,CANCEL"
     ;uri-user="<Y1>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip"
     ;mobility="fixed"
     ;class="personal"




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   X sends a MESSAGE request.  There are no explicit caller preferences.
   This results in an implicit require-flagged Accept-Contact
   preference:


   (& (methods="MESSAGE"))

   Since Y1 doesn't match and the Accept-Contact predicate is
   require-flagged, it is discarded.  However, according to the
   specifications, if there are no matching targets, the original target
   set is used, with its original q-values.  Thus, the request is sent
   to the one original target, Y1, as desired.  Y1 then responds with a
   405.

3.3  Package-Based Routing

3.3.1  Desired Behavior

   AOR Y has a number of contacts, Y1, Y2, ..., Yn that can each support
   normal calls - INVITE, ACK, BYE, etc., and can also support SUBSCRIBE
   for the "dialog" event package [7].  Y also has another contact Yp
   that is a presence agent (PA) [8] - it can accept SUBSCRIBE requests
   for the "presence" event package.  The goal is for subscribe requests
   for presence to be routed to Yp while invites and subscribes for the
   dialog package are forked to Y1...Yn.

3.3.2  Solution

   Y1..Yn will generate REGISTER requests which look like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Yi@pc.example.com>
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL,SUBSCRIBE"
     ;events="dialog"
     ;uri-user="<Yi>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip"
     ;mobility="fixed"
     ;class="personal"

   and Yp will generate a REGISTER request which looks like, in part:







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   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Yp@pc.example.com>;methods="SUBSCRIBE"
     ;events="presence"
     ;uri-user="<Yp>"
     ;uri-domain="example.com"
     ;schemes="sip,pres"
     ;mobility="fixed"
     ;class="business"

   A SUBSCRIBE request for presence will arrive at the proxy for
   example.com.  Since there are no explicit preferences, it constructs
   an implicit require-tagged Accept-Contact preference from the
   request:


   (& (methods="SUBSCRIBE") (events="presence"))

   This feature set only matches the one registered by Yp.  Because the
   require flag is set, the contacts which do not match are removed from
   the target set.  Therefore, Y1..Yn are eliminated.  The request is
   sent to the remaining contact, Yp, representing the PA.

   An INVITE request without explicit preferences results in an implicit
   require-flagged Accept-Contact preference:


   (& (methods="INVITE"))

   The implicit Accept-Contact feature set matches Y1..Yn, but not Yp.
   The score for Y1..Yn against this predicate is 1.0.  As a result, the
   caller preference Qa for each contact is 1.0.  The registrations did
   not contain q-values, so the default q-value of 1.0 is applied to
   each Contact URI.  Since the caller and callee preferences are the
   same, and all equal to 1.0, there is no reordering of contacts.  The
   result is that the proxy will consider Y1..Yn each as equally good
   targets for the request, and possibly fork the request to each.

   A SUBSCRIBE request for the dialog event package without explicit
   preferences will result in an implicit require-flagged Accept-Contact
   preference:


   (& (methods="SUBSCRIBE") (events="dialog"))

   This only matches Y1..Yn, so Yp is discarded, and the request is
   routed to the remaining contacts just as the INVITE was.




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3.4  Package Routing II

3.4.1  Desired Behavior

   This case is nearly identical to that of Section Section 3.3.
   However, Y1..Yn omit the "events" feature tag from their
   registration.  Yp registers as in Section Section 3.3.  A SUBSCRIBE
   for the presence event package should still preferentially route to
   Yp.

3.4.2  Solution

   The registration from Y1..Yn will look like:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Yi@pc.example.com>
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL,SUBSCRIBE"
     ;uri-user="<Yi>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip"
     ;mobility="fixed"
     ;class="personal"

   When the caller sends a SUBSCRIBE for the presence event package
   (without explicit preferences), the proxy computes an implicit
   preference:


   (& (methods="SUBSCRIBE") (events="presence"))

   This predicate matches Y1..Yn and Yp.  However, the score for Y1..Yn
   against this predicate is 0.5, and the score of Yp is 1.0.  The
   result is a caller preference Qa of 0.5 for Y1..Yn, and a caller
   preference Qa of 1.0 for Yp.  Since the callee provided no q-values,
   the proxy will assume a default of 1.0.  Thus, all contacts are in
   the same equivalence class.  They are then sorted by Qa, so that Yp
   is first, followed by Y1 through Yn.  It will therefore route the
   request first to Yp, and if that should fail, to Y1..Yn.

3.5  Audio/Video vs. Audio Only

3.5.1  Desired Behavior

   X sends an invitation to Y to initiate an audio/video call, including
   both m=audio and m=video lines in the SDP.  AOR Y has two contacts,



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   Y1 and Y2.  Y1 represents a normal audio phone, where Y prefers to
   receive their calls.  It will answer an audio/video call, refusing
   the video.  Y2 represents an audio/video phone that should only used
   when needed.  The caller really wants the called answered by a device
   that supports video, but will take an audio-only call as a second
   choice.

3.5.2  Solution

   Y1 will generate a registration which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y1@pc.example.com>;q=1.0
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
     ;uri-user="<Y1>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip,tel"
     ;mobility="fixed"
     ;class="business"

   Y2 will generate a registration which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y2@pc.example.com>;q=0.6
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
     ;uri-user="<Y2>"
     ;uri-domain="example.com"
     ;audio
     ;video
     ;schemes="sip,tel"
     ;mobility="fixed"
     ;class="business"

   Note the different q-values, allowing Y2 to be selected as a device
   of "last resort".

   To have the call is preferentially routed to a device that supports
   video, the caller X sends an INVITE that looks like, in part:



   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *



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     ;methods="INVITE"
     ;video

   The proxy will convert this to a feature set.  This feature set
   matches Y2 and Y1.  However, the score for Y2 is 1.0, and 0.5 for Y1.
   The two contacts are then ordered by q-value, and broken into
   equivalence classes.  There are two equivalence classes, each with
   one contact.  As a result, the caller preference values have no
   impact on the ordering.  The call will first try the higher priority
   Y1, which will answer the call and reject the video stream.  Thus,
   the desired behavior is not achieved.

   The desired behavior could be achieved by adding the "explicit" and
   "require" tags to the Accept-Contact header field in the INVITE, as
   is done in Section 3.6.  However, doing so may result in calls
   failing when they could occur, but without video.  As discussed in
   [2], both the "require" and "explicit" tags are generally used only
   when the request cannot be serviced in any way unless the preferences
   are met.  That is not the case here.
      OPEN ISSUE: This is a use case that used to work, but no longer
      works because of the change in the way q-values are used.  I think
      the loss of support for this use case is acceptable.  I also
      believe it is worthwhile to keep this case in the use cases spec
      to illustrate that not everything is possible.

3.6  Forcing Audio/Video

3.6.1  Desired Behavior

   This case is similar to that of Section 3.5.  However, X requires an
   audio/video call, and would like the call to fail if this is not
   possible, rather than succeeding with audio only.

3.6.2  Solution

   The solution is similar to that of Section 3.5, however the
   Accept-Contact header field now includes the explicit and require
   tags, guaranteeing that the call is never established to any UA that
   had not explicitly indicated support for video:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;video;require;explicit

   This arrives at the example.com proxy.  This explicit feature set
   matches the feature set for Y2 and Y1.  However, the match for Y1 did
   not have a score of 1.  Since the explicit and require tags are
   present, the contact is discarded.  That leaves Y2 only.  The call



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   will therefore get routed to the videophone, and if the user is not
   there, the audio phone will never ring.

   Because both the "require" and "explicit" flags are present, a
   contact will also be discarded if it didn't say anything about
   support for video.  Thus, a UA that can do video, but neglected to
   indicate it, would not be reached in this case.  This is why it is
   important for a UA to indicate all of its capabilities.  Note that
   this is only true for a contact that indicated other capabilities,
   just not video.  Contacts which don't indicate any capabilities are
   "immune" from caller preferences filtering, and would not be
   discarded.

3.7  Third Party Call Control - Forcing Media

3.7.1  Desired Behavior

   Z is a third party call control controller [9] trying to establish an
   audio/video call from X to Y.  X has contacts X1 and X2, and Y has
   contacts Y1 and Y2.  X1 and X2 have capabilities identical to Y1 and
   Y2, respectively.  Z needs to send an offerless invite to X and use
   the offer proposed by X to send an invite to Y.  When sending the
   offerless invite to X the 3pcc controller must ensure that an audio/
   video contact (X2) is chosen over an audio only contact (X1).

3.7.2  Solution

   X1 will generate a registration which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:X@example.com
   Contact: <sip:X1@pc.example.com>;q=1.0
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
     ;uri-user="<X1>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip,tel"
     ;mobility="fixed"
     ;class="business"

   X2 will generate a registration which looks like, in part:









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   REGISTER sip:example.com SIP/2.0
   To: sip:X@example.com
   Contact: <sip:X2@pc.example.com>;q=0.6
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
     ;uri-user="<X2>"
     ;uri-domain="example.com"
     ;audio
     ;video
     ;schemes="sip,tel"
     ;mobility="fixed"
     ;class="business"

   Z would include, in its INVITE, an Accept-Contact header field:


   INVITE sip:X@example.com SIP/2.0
   Accept-Contact: *;audio;video;require;explicit

   This caller preference matches both X1 and X2.  However, it matches
   X1 with a score of .5 and X2 with a score of 1.  Because of the
   require and explicit tags, X1 is discarded despite X's preference for
   it.  Thus, the call is routed to X2.

   The same caveats apply here as do in Section 3.6.  Generally, it is
   not advisable to mandate support for features (such as video), which
   are not strictly neccesary for the request to proceed.

3.8  Maximizing Media Overlaps

3.8.1  Desired Behavior

   AOR Y has two contacts, Y1 that is a regular audio phone, and Y2 that
   is a PC capable of supporting both audio and session oriented IM
   [10].  X is a PC with capability to support audio, video and session
   oriented IM.  X calls Y for the purpose of establishing a voice call.
   However, X wishes to connect to the device which has the maximal
   overlap with its media capabilities, in order to maximize the
   functionality available to the caller.

3.8.2  Solution

   Y1 will generate a registration which looks like, in part:









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   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y1@phone.example.com>
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL"
     ;uri-user="<Y1>"
     ;uri-domain="example.com"
     ;audio
     ;schemes="sip,tel"
     ;mobility="fixed"
     ;class="business"

   Y2 will generate a registration which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: <sip:Y2@pc.example.com>
     ;methods="INVITE,BYE,OPTIONS,ACK,CANCEL,MESSAGE"
     ;uri-user="<Y2>"
     ;uri-domain="example.com"
     ;audio
     ;+message
     ;schemes="sip,tel"
     ;mobility="fixed"
     ;class="business"

   The solution requires the caller to support caller preferences.  They
   would include, in their INVITE, an Accept-Contact header field that
   lists all the media types they support.  In this case:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;audio;video;+message

   Both Y1 and Y2 match the predicate.  Y1 matches with a score of 0.33,
   and Y2 matches with a score of 0.66.  Since there is only one
   Accept-Contact predicate, the Qa for each contact is equal to the
   score.  The registered contacts are then sorted by q-value, and
   broken into equivalence classes.  There is a single equivalence class
   with q-value of 1.0.  The two contacts in that class are then
   re-ordered based on the values of Qa.  Y2 has a higher Qa, so it is
   used first, followed by Y1.  The result is that the call is routed to
   the device with the maximum overlap in media capabilities, as
   desired.
      EDITORS NOTE: This is a case that didn't work well before caller
      prefs -09, but works very well now - as long as the registered
      contacts have the same q-value.




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3.9  Multilingual Lines

3.9.1  Desired Behavior

   AOR Y represents a shared line in an office.  Several employees in
   the office have phones registered for Y.  Some of the employees speak
   only English, some speak Spanish fluently and have some limited
   capability for English, and some speak both English and Spanish
   fluently.  Calls from callers that speak only English should be
   parallel forked to all office workers that speak fluent English.  If
   the call isn't picked up, then the phones of workers that speak
   English marginally should be rung.  Calls from callers that speak
   only Spanish should be forked only to workers that speak Spanish.

3.9.2  Solution

   A user at phone Y1 that speaks English only would generate a REGISTER
   which looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y1@pc.example.com;languages="en"

   A user at a phone Y2 that speak Spanish and a little bit of English
   would generate a REGISTER that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y2-es@pc2.example.com;languages="es"
   Contact: <sip:Y2-en@pc2.example.com>;languages="en";q=0.2

   Y2 has registered two contacts.  Both of them route to the same
   device (pc2.example.com), but they differ in their language support
   and relative q-values.  Multiple contacts are needed whenever a UA
   wishes to express differing preferences for being reached for
   different feature collections.

   A user at phone Y3 that speaks English and Spanish fluently would
   generate a REGISTER that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y3@pc3.example.com;languages="es,en"

   Notice that only a single contact is needed because the same q-value



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   is applied across all feature collections.

   For the language based routing to occur, the caller must indicate its
   language preferences explicitly:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;languages="en";require

   The predicate derived from this looks like:


   (& (languages="en"))

   This matches all Y1 phones, the second contact registered by Y2
   phones, and Y3 phones, all with a score of 1.0.  The first contact
   registered by Y2 does not match, and because of the "require" flag,
   is discarded.  The remaining contacts are sorted by q-value, and
   divided into equivalence classes.  There are two equivalence classes.
   The first contains Y1 and Y3 with a q-value of 1.0, and the second
   contains Y2-en with a q-value of 0.2.  The contacts in the first
   class are ordered by Qa.  However, since all contacts have the same
   value of Qa (1.0), there is no change in ordering.  Thus, Y1 and Y3
   are tried first, followed by Y2-en.  This is the desired behavior.

   A caller that speaks Spanish only would specify their preference
   thusly:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;languages="es";require

   This matches the first contact of Y2 phones, and Y3 phones, all with
   a score of 1.0.  The English contact of Y2, Y2-en, doesn't match, and
   is discarded because of the "require" flag.  The remaining contacts
   are sorted by q-values (Y3, Y2-es), and broken into a single
   equivalence class containing both contacts.  Since the Qa for both
   contacts is the same - 1.0 - there is no reordering.  The result is
   that the call is routed to either Y3 or Y2-es.

3.10  I Hate Voicemail!

3.10.1  Desired Behavior

   AOR Y has two contacts, a phone Y1 and a voicemail service Y2.  X
   wishes to call Y and talk in person.  X does not want to be sent to
   voicemail under any circumstance.




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3.10.2  Solution

   The phone would register with a Contact that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y1@pc.example.com
     ;audio
     ;mobility="fixed"

   and the voicemail server would register with a Contact that looks
   like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y2@pc.example.com
      ;msgserver
      ;automata
      ;attendant
      ;audio
      ;q=0.2

   The voicemail server registers with a lower q-value so that it is
   used only after the phone itself is rung.  Note that the voicemail
   server need not actually register.  There can be a configured contact
   and feature set defined for it instead.

   A caller that wishes to avoid voicemail can include an explicit
   preference to avoid it.  It would do this with the Reject-Contact
   header field:


   INVITE sip:Y@example.com SIP/2.0
   Reject-Contact: *;msgserver

   Since this feature set contains a feature tag that is not contained
   in the registration for Y1, the feature set is discarded when
   examining Y1.  However, the registration for Y2 contains all feature
   tags listed in the feature set, and so the rule is considered.  There
   is a match, and therefore, Y2 is discarded.  The result is that the
   user is never routed to voicemail.

3.11  I Hate People!






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3.11.1  Desired Behavior

   The situation is similar to Section 3.10, except the caller wishes to
   only leave a message, not actually speak to the person.

3.11.2  Solution

   The caller would send an INVITE which looks like, in part:




   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;msgserver;require;explicit

   This caller preference matches both Y1 and Y2.  Y1 matches, but with
   a score of zero.  Y2 matches with a score of 1.  Since both the
   require and explicit flags are set, Y1 is discarded.  Therefore, the
   call is routed to Y2, the voicemail server, as desired.

   If these preferences are used with a user that doesn't have voicemail
   at all, the call will fail completely, rather than connecting to the
   user.

3.12  Prefer Voicemail

3.12.1  Desired Behavior

   The situation is similar to that of Section 3.10.  However, the
   caller prefers to leave a message.  If voicemail is not available,
   they are willing to talk to a person.

3.12.2  Solution
      OPEN ISSUE: This cannot be done with caller prefs -09, as it would
      require a re-ordering of the callee contacts, which is not done.
      Is there another way? Is it OK to set a callee contact to q=0.0?

3.13  Routing to an Executive

3.13.1  Desired Behavior

   Y is the AOR of an executive.  It has three contacts.  Y1 is the
   phone on the executive's desk.  Y2 is the phone on the desk of the
   executive's assistant.  Y3 is the address of an auto-attendant system
   that can answer general questions, route calls to other parties, etc.
   By default, calls to Y should be directed to Y2, and if that fails,
   to Y3.  If Y3 doesn't answer then Y1 should ring.




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3.13.2  Solution

   This is primarily a called party feature, and is best accomplished
   with a CPL script [5].  However, it can be accomplished with caller
   preferences alone by properly setting the q-values across the three
   devices.  Assuming this coordination is possible, here are the
   settings that would be made:

   Y1 would generate a REGISTER that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y1@pc.example.com;q=0.1

   Y2 would generate a REGISTER that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y2@pc2.example.com;attendant;q=1.0

   Y3 would generate a REGISTER that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y3@pc3.example.com;attendant;automata;q=0.5

   Note that, in reality, the automated attendant would probably not use
   REGISTER.  Since the attendant would be used for every employee in
   the company, a static contact would probably be added
   administratively for each user in the enterprise.  However, the
   information in that static contact would be identical to the
   information in the registration above.

   When X makes a call to the executive, Y, and expresses no preference,
   the proxy computes an implicit preference to support INVITE.  All
   three contacts match such a preference, even though they have not
   indicated explicit support for INVITE.  Thus, no contacts are
   discarded.  Since the contacts each have a different q-value, the
   caller preferences do not cause any reordering.  The result is that
   the call is first routed to Y2, then Y3, then Y1, all as a result of
   the proper setting of the q-values.

3.14  Speak to the Executive





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3.14.1  Desired Behavior

   This case is similar to that of Section 3.13, but this time the
   caller, X, has a preference.  X calls Y, but wants to speak directly
   to the executive.  X doesn't want the call to ring either the
   assistant or the auto attendant (automata).

3.14.2  Solution

   X's INVITE would look like, in part:


   INVITE sip:Y@example.com SIP/2.0
   Reject-Contact: *;attendant
   Reject-Contact: *;automata

   Note that the caller uses two separate Reject-Contact header field
   values, rather than a single one with two separate feature
   parameters.  The distinction is important.  If X had use a single
   value with two parameters, a matching UA would need to declare that
   it was BOTH an attendant and an automata.  If it only declared that
   it was one of these, based on the matching rules in the caller
   preferences specification, it would not be rejected.

   The above request would result in the elimination of both Y2 and Y3
   as contacts.  The call would then be routed to Y1, as desired.

   This case indicates why a CPL script, or some other programmed
   version of the feature, is preferrable.  With caller preferences, a
   caller can override the desired ring sequence, and disturb the
   executive without any kind of authorization.  A proper version of
   this service would simply not permit caller preferences to force the
   call to go directly to the executive.
      OPEN ISSUE: Should we discard this use case as a result?

3.15  Mobile Phone Only

3.15.1  Desired Behavior

   The situation is similar to that in Section 3.13.  However, the
   executive also has a mobile phone which they have registered.  Caller
   X knows that the owner of Y is traveling, and that an assistant is
   covering the office phone.  X wants to call Y and ring only the
   mobile phone.

3.15.2  Solution

   The mobile phone would generate a registration which looks like, in



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   part:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y4@mobile.example.com;mobility="mobile";q=0.5

   The caller would express their preference by generating an INVITE
   which looks like, in part:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;mobility="mobile";require;explicit

   All four contacts match.  However, Y1 through Y3 match with a score
   of zero.  Y4 matches with a score of 1.  Because of the require and
   explicit tags, Y1 through Y3 are discarded, and only Y4 is used, as
   desired.

3.16  Simultaneous Languages

3.16.1  Desired Behavior

   AOR Y is as in Section 3.9.  Caller X, fluent in both English and
   Spanish, has discovered that company's Spanish language documentation
   is inconsistent with the English language documentation, and wants to
   discuss the differences between the two.  So X wants to speak with
   one of the workers that is fluent in both English and Spanish.

3.16.2  Solution

   The caller would generate an INVITE which looks like, in part:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;language="en";require
   Accept-Contact: *;language="es";require

   This will require a Contact URI to match both constraints.  That
   means it needs to support English and Spanish.  This will achieve the
   desired property.

   Note that there are two separate Accept-Contact header fields.  If
   the caller had instead used this INVITE:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;language="en,es";require



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   It would have connected them to a UA that speaks either English or
   Spanish, which is not what is desired here.

3.17  UA Routing

3.17.1  Desired Behavior

   AOR Y has contacts Y1 and Y2.  The addresses Y1 and Y2 are behind a
   firewall and are not addressable by all callers.  Caller X makes a
   call to Y and is connected to Y1.  The call fails for some reason,
   and X wants to reestablish it, reaching only Y1.

3.17.2  Solution

   There is currently no generally workable solution to this problem.
   The best solution that exists does make use of caller preferences.
   Lets say that the Contact URI for Y1 was sip:Y1@host.example.com.
   The caller would generate an INVITE which looks like, in part:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *
     ;require
     ;explicit
     ;uri-user="<Y1>"
     ;uri-domain="host.example.com"

   When this request arrives at the proxy for example.com, it attempts
   to apply the caller preferences.  Following the guidelines in the
   caller preferences extension, Y1 would have included a uri-user and
   uri-domain feature tag in its registration:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y1@host.example.com;uri-user="<Y1>"
     ;uri-domain="host.example.com"

   as would have Y2:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:Y2@pc.example.com;uri-user="<Y2>"
     ;uri-domain="pc.example.com"

   The proxy then applies the caller preferences.  Only Y1 is a match.
   So, Y2 is discarded, and the request is routed to Y1 as desired.



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   The difficulty is that this solution won't always work.  In a
   multi-proxy scenario, it is possible that the routing logic changes,
   and therefore the request is never even routed to the proxy where Y1
   has registered.

3.18  The Number you Have Called..

3.18.1  Desired Behavior

   Consider once more the case of the executive, where the caller wishes
   to reach only their mobile phone (Section 3.15).  However, there is a
   twist.  The callee Y has moved to new address YY, and all the
   configuration described for the callee now applies to YY.  The old
   address Y remains with a pair of statically assigned contacts.  One
   contact is YY.  The other is M referencing an automaton that
   generates a voice message reporting that the number has been changed.
   The caller is unaware of the move and calls Y, requesting to reach
   the mobile phone in exactly the same way they did in Section 3.15.
   The call should connect to the mobile.

3.18.2  Solution

   There would be four registrations against YY:

   YY1, the executive, would generate a REGISTER that looks like, in
   part:


   REGISTER sip:example.com SIP/2.0
   To: sip:YY@example.com
   Contact: sip:YY1@pc.example.com;q=0.1

   YY2, the attendant, would generate a REGISTER that looks like, in
   part:


   REGISTER sip:example.com SIP/2.0
   To: sip:YY@example.com
   Contact: sip:YY2@pc2.example.com;attendant;q=1.0

   YY3, the answering service, would generate a REGISTER that looks
   like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:YY@example.com
   Contact: sip:YY3@pc3.example.com;attendant;automata;q=0.5




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   YY4, the mobile, would generate a REGISTER that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:YY@example.com
   Contact: sip:YY4@mobile.example.com;mobility="mobile";q=0.5

   Athough it would be configured administratively, there are two
   registered contacts for Y.  The first is for the forwarding:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:YY@example.com;q=1.0

   and the second for the automated answering service:


   REGISTER sip:example.com SIP/2.0
   To: sip:Y@example.com
   Contact: sip:machine@example.com;automata;q=0.5

   The caller, not knowing that Y has moved, calls Y and asks for their
   mobile phone:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;mobility="mobile";require;explicit

   This reaches the example.com proxy, which finds two registrations.
   Only one of these is associated with feature parameters (the
   automata).  The other has no feature parameters, and is therefore
   immune from caller preferences processing.  The caller preferences
   are applied to the the automata's contact.  The feature sets match,
   but have a score of zero.  Since the require and explicit tags are
   present, the contact for the automata is dropped.  The other contact,
   YY@example.com, is then added back in as the sole contact.  The proxy
   therefore sends the call to sip:YY@example.com.  There, there are
   four registrations, all of which are associated with feature
   parameters.  The caller preferences are applied.  Only YY4 matches
   explicitly, however.  Because of the presence of the require and
   explicit flags, all other contacts are dropped.  As such, the call is
   forwarded to YY4, and the mobile phone rings.

3.19  The Number you Have Called, Take Two






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3.19.1  Desired Behavior

   This use case is nearly identical to that of Section 3.18.  However,
   this time, the caller wishes to contact the personal phone of Y.
   They don't feel strongly about it, and will accept other devices.

3.19.2  Solution

   The INVITE generated by the caller in this case will look like:


   INVITE sip:Y@example.com SIP/2.0
   Accept-Contact: *;class="personal"

   This reaches the example.com proxy.  Once more, the first
   registration (which forwards to the address-of-record for YY) is
   unaffected by the caller preferences computation.  The other contact,
   for the automata, is a match, but its score is zero.  Its caller
   preference Qa equals zero.  The other contact is added back in with a
   Qa of 1.0.  The contacts are sorted based on q-value, resulting in YY
   (q=1.0) followed by machine (q=0.5).  These are broken into
   equivalence classes.  There are two classes, one for each contact.
   As a result, the caller's preferences have no impact on the ordering,
   and the call is routed to YY.

   When processing the request for YY@example.com, all four contacts
   match.  However, the score for all of them is zero (none are the
   personal phone).  As such, the contacts are ordered based on q-value.
   Each contact has a different q-value, so no reordering based on
   caller preference is possible (not that the caller preference would
   cause a reordering - all contacts have a Qa of 0.0).  Thus, the
   highest q-value contact is tried, which is the executive assistant.

3.20  Forwarding to a Colleague

3.20.1  Desired Behavior

   Alice wants to forward her phone to Bob, but doesn't want folks
   calling her to get Bob's voicemail if he doesn't answer.  She wants
   her callers to get her voicemail.

3.20.2  Solution

   Alice would create three registrations.  The first, Y1, represents
   Alice's phone.  The second is Bob's AOR.  The third is a voicemail
   server.  The three contacts have decreasing q-values.  The
   registration for Bob's AOR contains an embedded Reject-Contact header
   field, which rejects message servers.



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   REGISTER sip:example.com
   To: <sip:alice@example.com>
   Contact: <sip:Y1@192.0.2.150>;q=1.0



   REGISTER sip:example.com
   To: <sip:alice@example.com>
   Contact: <sip:bob@example.com?Reject-Contact=*;msgserver>;q=0.3



   REGISTER sip:example.com
   To: <sip:alice@example.com>
   Contact: <sip:alice-drop@msgcenter.example.com>
     ;msgserver;
     ;automata
     ;attendant
     ;q=0.1

   Meanwhile, Bob is registered as follows:


   REGISTER sip:example.com
   To: <sip:bob@example.com>
   Contact: <sip:bob3@192.0.2.212>;q=0.8

   REGISTER sip:example.com
   To: <sip:bob@example.com>
   Contact: <sip:bob-drop@msgcenter.example.com>
     ;msgserver
     ;automata
     ;attendant
     ;q=0.2

   Carol calls Alice, and doesn't include any caller preference
   parameters.  As such, the example.com proxy constructs an implicit
   preference for INVITE.  This preference matches all three registered
   contacts, with a score of zero.  Because each contact has a different
   q-value, there is no reordering of contacts.  So, the proxy tries the
   highest q-value Contact, Alice's desk phone (Y1).  The proxy cancels
   after a few seconds (no answer).  The proxy then tries the next
   Contact, which is Bob's AOR.  When constructing the request for this
   Contact, the proxy includes the embedded Reject-Contact header field
   in the INVITE.  This INVITE undergoes caller preferences processing
   based on Bob's registered Contacts.

   Bob has two registered Contacts.  The second is a message server, and



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   it matches the Reject-Contact in the INVITE.  Thus, this contact is
   discarded.  The other remaining Contact, Bob's phone, is tried.  Bob
   is not around, and so his phone rings for a while.  Upon timeout, the
   proxy determines it is unable to reach Bob's AOR.  So, the proxy
   handling Alice tries the final remaining contact, which is Alice's
   message server.













































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4.  Capability Use Cases

   The callee capabilities spec [3] allows the Contact header field in
   OPTIONS responses and dialog initiating messages to contain
   capabilities of the UA.  These capabilities can be very useful for
   developing new applications.  In the subsections below, several
   usages are outlined.

4.1  Web Redirect

   A caller sends an INVITE to the called party.  However, the called
   party is not present.  The proxy server representing the called party
   would like to redirect the caller to a web page, where they can find
   out more information on how to reach the called party.  However, the
   proxy needs to know whether or not the caller supports redirects to
   web pages.  If it doesn't, the proxy would connect the user to an
   IVR, which would execute an answering machine application.

   The proxy could make such a determination if the caller included the
   "schemes" feature tag in the Contact header field of its INVITE:


   INVITE sip:callee@example.com SIP/2.0
   Contact: sip:host22.example.com;schemes="http,sip,sips,tel"

   This tells the proxy that the UAC can be redirected to an http URI.
   The INVITE from a normal "black phone" which lacked this capability
   would look like:


   INVITE sip:callee@example.com SIP/2.0
   Contact: sip:host22.example.com;schemes="sip,sips,tel"

   which indicates that it needs to be connected to the IVR.

4.2  Voicemail Icon

   On the circuit network, when a user makes a call, and an answering
   machine picks up, the caller usually requires several seconds to make
   the determination that they are speaking to an answering machine.  It
   would be helpful if a phone could display an icon immediately on call
   completion that indicated that an answering machine was reached.

   This indication can be provided by the "msgserver" feature parameter.
   When the answering machine picks up, its 200 OK looks like, in part:


   SIP/2.0 200 OK



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   Contact: sip:server33.example.com;msgserver;automata;attendant

   This tells the caller that its an answering machine.
















































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5.  Usage of the Feature Tags

   The caller preferences extension briefly enumerates a list of media
   feature tags which can be registered by a device, and included in the
   Accept-Contact and Reject-Contact header fields in a request.  Proper
   operation of caller preferences depends strongly on consistent
   interpretation of these feature tags by the caller and the callee.
   In this section, we provide some guidelines on the usage of these
   feature tags.

   Generally speaking, the more information a device provides when it
   registers, the more effective the caller preferences extension is.
   This is why the callee capabilities extension recommends that a
   device register as much information as it can.  This point cannot be
   understated.
      OPEN ISSUE: Should we advise devices to register things they can't
      do? As an example, if a phone doesn't support video, it would
      include a "video=FALSE" feature parameter in its registration to
      indicate that.  This provides more information, and would improve
      the operation of caller preferences, but can seriously bloat
      registrations.  It also seems odd to have to say what you can't
      do.

   The subsections below show example registrations from typical
   devices.

5.1  Traditional Cell Phone

   A VoIP cell phone capable of making voice calls would generate a
   registration that looks like, in part:


   REGISTER sip:example.com SIP/2.0
   To: sip:user@example.com
   Contact: sip:cell-phone@example.com
     ;audio
     ;class="business"
     ;duplex="full"
     ;sip-extensions="100rel,path"
     ;mobility="mobile"
     ;methods="INVITE,BYE,OPTIONS,CANCEL,ACK"
     ;schemes="sip,sips,tel"
     ;uri-user="<cell-phone>"
     ;uri-domain="example.com"







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5.2  Traditional Work Phone

   A traditional landline IP PBX phone would generate a registration
   that looks like:


   REGISTER sip:example.com SIP/2.0
   To: sip:user@example.com
   Contact: sip:ippbx-phone@example.com
     ;audio
     ;class="business"
     ;duplex="full"
     ;events="dialog"
     ;sip-extensions="100rel,privacy"
     ;mobility="fixed"
     ;methods="INVITE,BYE,OPTIONS,CANCEL,ACK,SUBSCRIBE"
     ;schemes="sip,sips,tel"
     ;uri-user="<ippbx-phone>"
     ;uri-domain="example.com"

   This device also supports the dialog event package and several SIP
   extensiosn that would be typical in an IP PBX phone.

5.3  PC Messenging Application

   A PC messenger client, capable of just doing presence and IM (no
   voice) would generate a registration that looks like:


   REGISTER sip:example.com SIP/2.0
   To: sip:user@example.com
   Contact: sip:pc-msgr@example.com
     ;class="personal"
     ;mobility="fixed"
     ;methods="OPTIONS,MESSAGE,NOTIFY"
     ;schemes="sip,sips,im,pres"
     ;uri-user="<pc-msgr>"
     ;uri-domain="example.com"


5.4  Standalone Videophone

   A standalone IP videophone, capable of audio and video would generate
   a registration that looks like, in part







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   REGISTER sip:example.com SIP/2.0
   To: sip:user@example.com
   Contact: sip:vp@example.com
     ;audio
     ;video
     ;class="business"
     ;duplex="full"
     ;mobility="fixed"
     ;methods="INVITE,BYE,OPTIONS,CANCEL,ACK"
     ;schemes="sip,sips,tel"
     ;uri-user="<vp>"
     ;uri-domain="example.com"







































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6.  Security Considerations

   There are no security considerations associated with this
   specification.















































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7.  IANA Considerations

   There are no IANA considerations associated with this specification.
















































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8.  Acknowledgements

   The authors would like to thank Rohan Mahy for his input in this
   specification.















































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9.  TODO
   o  Add a section showing an example implementation of the matching
      algorithm in caller preferences [[OPEN ISSUE: Do we want this?]].
   o  Provide additional explanations in each routing use case on why
      require and/or explicit tags were used.
   o  Show examples using the message server, automata, and attendant
      tags.

10  Informative References

   [1]   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.

   [2]   Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)",
         draft-ietf-sip-callerprefs-10 (work in progress), October 2003.

   [3]   Rosenberg, J., "Indicating User Agent Capabilities in the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sip-callee-caps-03 (work in progress), January 2004.

   [4]   Mankin, A., Bradner, S., Mahy, R., Willis, D., Ott, J. and B.
         Rosen, "Change Process for the Session Initiation Protocol
         (SIP)", BCP 67, RFC 3427, December 2002.

   [5]   Lennox, J. and H. Schulzrinne, "Call Processing Language
         Framework and Requirements", RFC 2824, May 2000.

   [6]   Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
         2533, March 1999.

   [7]   Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
         Event Package for the Session Initiation  Protocol (SIP)",
         draft-ietf-sipping-dialog-package-04 (work in progress),
         February 2004.

   [8]   Rosenberg, J., "A Presence Event Package for the Session
         Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
         in progress), January 2003.

   [9]   Rosenberg, J., Peterson, J., Schulzrinne, H. and G. Camarillo,
         "Best Current Practices for Third Party Call Control in the
         Session  Initiation Protocol", draft-ietf-sipping-3pcc-06 (work
         in progress), January 2004.

   [10]  Campbell, B., "The Message Session Relay Protocol",
         draft-ietf-simple-message-sessions-06 (work in progress), May



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         2004.


Authors' Addresses

   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net


   Paul Kyzivat
   Cisco Systems
   Mail Stop LWL3/12/2
   900 Chelmsford St.
   Lowell, MA  01851
   US

   EMail: pkzivat@cisco.com



























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