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Versions: 00 01 02 03                                                   
SIPPING                                                     J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: July 23, 2007                                  January 19, 2007

  Identification of Communications Services in the Session Initiation
                             Protocol (SIP)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 23, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2007).


   This document considers the problem of how SIP endpoints can support
   a multiplicity of distinct SIP services within the context of a
   single user agent.  The principle problem to be addressed is that of
   dispatching of incoming requests to the right services, and how
   service contexts are matched up between calling and called parties.
   This document proposes the usage of service URN and service URI to
   solve the problem.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Concepts and Terminology . . . . . . . . . . . . . . . . . . .  4
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  8
   7.  UA Behavior  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Registration . . . . . . . . . . . . . . . . . . . . . . .  9
     7.2.  Publication  . . . . . . . . . . . . . . . . . . . . . . . 10
     7.3.  Session Initiation . . . . . . . . . . . . . . . . . . . . 10
     7.4.  Receipt of a Request . . . . . . . . . . . . . . . . . . . 11
   8.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Request Targeting  . . . . . . . . . . . . . . . . . . . . 12
     8.2.  Application Invocation . . . . . . . . . . . . . . . . . . 12
   9.  Guidelines for Using Service URN . . . . . . . . . . . . . . . 13
   10. Guidelines on Namespace Structure  . . . . . . . . . . . . . . 14
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   13. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     15.2. Informational References . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20

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1.  Introduction

   The Session Initiation Protocol (SIP) [2] defines mechanisms for
   initiating and managing communications sessions between agents.
   These agents can be entities such as hardphones, softphones, or
   gateways to other networks, such as the PSTN.  These agents are
   addressed by SIP URI, and in particular, a SIP Address-of-Record or

   However, in practice, the entities participating in a call can be
   more complicated.  An agent might be inside of a cell phone,
   supporting traditional telephony, Push-To-Talk, and voice and data
   content as part of an interactive game.  Furthermore, the servers
   within the network itself might provide additional functions, such as
   call screening or call recording.  These functions are often referred
   to as 'services', 'features' or 'applications'.  Their usage raises
   questions on how users invoke them, how they are identified, and how
   interoperability between them is provided.

   Section 3 defines the problem in more detail.  Section 4 defines
   concepts and terminology.  Section 5 introduces requirements for the
   solution.  Section 6 overviews the solution, and Section 7 defines
   detailed procedures for user agents, while Section 8 defines
   procedures for proxies.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [1] and indicate requirement levels for compliant STUN

3.  Problem Statement

   Consider a device that allows the user to select two applications.
   One of these applications is a traditional telephony application,
   which lets the user make and receive phone calls using telephone
   numbers.  The second application is a two-player chess game that
   utilizes voice commands to move pieces.  When one player says "Queen
   to D7", the software on their phone recognizes this and moves the
   piece.  At the same time, the user's voice is sent to the other
   player, where it is both rendered to the user, and interpreted
   locally in order to move the piece.

   If the user should make a call using the telephony application, the

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   result would be a SIP INVITE for a single voice media stream.
   Interestingly, if the user launches the game application, the same is
   true - the result would be a SIP INVITE for a single voice media
   stream.  However, depending on which application the caller selected,
   the appropriate application at the called party must also be
   selected.  It would be nonsensical for the user to invoke the
   telephony application and be connected to the gaming application on
   another user's device.  The user interface for the gaming application
   would not function properly, and the overall experience would be
   poor.  What is needed is some kind of way to differentiate these two

   A similar problem arises in the invocation of outbound applications
   that reside in the network.  Consider once more our user with the
   telephony and gaming application.  The user wishes to make a
   telephony call, but wants the call to be recorded.  The recording
   option is available on a call-by-call basis.  The recording
   application itself resides on a server in the domain of the caller,
   and acts as a back-to-back user agent (B2BUA) in order to perform the
   call recording (there are alternative models involving the
   application interaction framework [9] and conferencing [12], but the
   specific mechanism is not relevant to the discussion here).  When the
   user initiates the call, how do they signal to their outbound proxy
   that the call needs to pass through the recording application?

   In both cases, the problem at hand is the identification and
   invocation of applications which reside either on the endpoint (in
   the first example) or in the network (in the second example).

4.  Concepts and Terminology

   The problem of identifying and invoking services within SIP is not a
   new one.  The problem has been considered extensively in the context
   of presence.  In particular, the presence data model for SIP [14]
   defines the concept of a service as one of the core notions that
   presence describes.  Services are described in Section 3.3 of RFC
   4479, which has this to say on the topic:

   3.3.  Service

      Each presentity has access to a number of services.  Each of these
      represents a point of reachability for communications that can be
      used to interact with the user.  Examples of services are telephony
      (that is, traditional circuit-based telephone service), push-to-talk,
      instant messaging, Short Message Service (SMS), and Multimedia
      Message Service (MMS).

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      It is difficult to give a precise definition for service.  One
      reasonable approach is to model each software or hardware agent in
      the system as a service.  If a user starts a softphone application on
      their PC, then that represents a service.  If a user has a videophone
      device, then that represents another service.  This is effectively a
      physical view of services.  This definition, however, starts to fall
      apart when a service is spread across multiple software agents or
      devices.  For example, a SIP URI representing an address-of-record
      can be routed to a softphone or a videophone, or both.  In that case,
      one might attempt instead to define a service based on its address on
      the network.  This definition also falls apart when modeling devices
      or applications that receive calls and dispatch them to different
      "helpers" based on potentially complex logic.  For example, a
      cellular telephone might house multiple SIP applications, each of
      which can "register" different handlers based on the method or even
      body type of the request.  Each of those applications or handlers can
      rightfully be considered a service, but it doesn't have an address on
      the network distinct from the others.

      Because of this inherent difficulty in precisely defining a service,
      the data model doesn't try to constrain what can be considered a
      service.  Rather, anything can be considered a service so long as it
      exhibits a set of key properties defined by this model.  In
      particular, each service is associated with characteristics that
      identify the nature and capabilities of that service, with reach
      information that indicates how to connect to the service, with status
      information representing the state of that service, and relative
      information that describes the ways in which that service relates to
      others associated with the presentity.

      As a consequence, in this model, services are not explicitly
      enumerated.  There is no central registry where one finds identifiers
      for each service.  Consequently, each service does not have a single
      "service" attribute with values such as "ptt" or "telephony".  That
      doesn't mean that these consolidated monikers aren't useful; indeed,
      they represent an essential summary of what the service is.  Such
      summarization is useful in creating icons that allow a user to choose
      one service over another.  A watcher is free to create such
      summarization information from any of the information associated with
      a service.  The reach information often provides valuable information
      for creating such a summarization.  Oftentimes, the scheme of the URI
      is synonymous with the view of what a service is.  An "sms" URI [14]
      clearly indicates SMS, for example.  For some URIs, there may be many
      services available, for example, SIP or tel [15], in which case the
      scheme is less meaningful as a way of creating a summary.  The reach
      information could also indicate that certain application software has
      to be invoked (such as a videogame), in which case that aspect of the
      reach information would be useful for generating an iconic

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      representation of the game.

   Building upon this, we can model a user agent as containing a SIP
   processing layer ontop of which sit a number of different SIP
   services, as shown in Figure 2

                            |                                 |
                            | +-------------+ +-------------+ |
                            | |     UI      | |     UI      | |
                            | +-------------+ +-------------+ |
                            | +-------------+ +-------------+ |
                            | |             | |             | |
                            | |  Service 1  | |  Service 2  | |
                            | |             | |             | |
                            | +-------------+ +-------------+ |
                            | +-----------------------------+ |
                            | |                             | |
                            | |             SIP             | |
                            | |            Layer            | |
                            | |                             | |
                            | +-----------------------------+ |
                            |                                 |

                                      Physical Device

   Figure 2

   The role of the SIP layer is to parse incoming messages, handle the
   SIP state machinery for transactions and dialogs, and then dispatch
   request to the appropriate service.  The dispatching operation is
   based on any number of criteria in the SIP message itself.  For
   example, the method might be used to dispatch the request.  A
   messaging application on the phone would be dispatched when a MESSAGE
   request arrives.  Similarly, when a user interacts with the device,
   they would select a specific service, and then use that service to
   initiate communications.  The service would then request the SIP
   layer to send an appropriate message, depending on what was needed.
   Each service has a user interface (UI) that dictates how it interacts
   with the user.

   SIP uses URI, and in particular, SIP URI, to identify resources
   within the system.  The Address-of-Record, or AOR, identifies the
   user that is the originator or target of the request.  The Globally
   Routable User Agent URI (GRUU) [4] identifies a specific instance of
   a user agent.  In the model of Figure 2, there is still but one user

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   agent, and thus a single GRUU.  However, we have effectively
   introduced a layer of hierarchy into the system.  Within a particular
   UA instance, there can be one or more service instances.  Each
   service instance can be addressed by a URI.  This URI is formed by
   adding a parameter, at the discretion of the UA, to the GRUU.  The
   resulting URI is called a service instance URI.

   When a service spans multiple devices and multiple SIP UA instances,
   the aggregate set is represented by a service URI.  Typically, a
   domain will need to construct such a URI, and bind it to the various
   service instances that can be reached through the service URI.

   In addition, each service may or may not be a well-known service.  A
   well-known service is identified by a service URN [5].  The URN
   refers to the set of assumptions and processing requirements within
   the service layer that define how a request is processed.  For
   example, a service URN of urn:service:games:voice-chess could be used
   to identify the voice chess application described in Section 3.  In
   this case, the URN would need to be standardized, and there would be
   agreement that the "context" is that voice is interpreted by speech
   recognition for the purposes of performing chess moves, and a
   specific set of phrases would be agreed upon.  It is also possible to
   have vendor specific services, which would be identified using a URN
   such as "urn:service:vnd:example.com:foobar", which refers to the
   foobar service produced by a specific vendor.

   It is extremely important to note that this name refers to the
   additional logic that is required in the processing of a SIP session
   in order for it to be successfully utilized.  SIP assumes that the
   "normal" service is multimedia communications - the exchange of real
   time media between the users which generated and received the
   requests for the purpose of communications between humans or automata
   which act like a human.  The chess example is more than this, because
   the media is additionally consumed by an automata that is looking to
   do specialized processing, and because the media is not primarily for
   human communication, its for controlling moves on a chess board.
   Because of this, a traditional communications applications has no
   well-known service associated with it.

5.  Requirements

   REQ 1: It shall be possible for an incoming request to be dispatched
      to the correct service on a device.

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   REQ 2: When multiple services reside on a single device, sharing a
      single SIP layer, it must not require multiple registrations.
      This is primarily a performance and overhead requirement.

   REQ 3: It shall be possible to support cases where sessions initiated
      from a particular service purposefully fail unless they can be
      connected to a matching service for the called party.

   REQ 4: It must be possible for services to "match" based on
      proprietary and well-known identifiers.

   REQ 5: It must be possible for a user to initiate a session without
      knowledge of any information about the recipient except for their

   REQ 6: The mechanism must allow a presence server to determine the
      services on the phone, without requiring advanced knowledge of
      those services.

   REQ 7: It must be possible to support cases where sessions initiated
      from a service on the caller side connect to a different service
      on the other side in cases where the session is meaningful when
      the notions of service on each side do not match.

6.  Overview of Operation

   The proposed solution to the problem is relatively straightforward.

   The essential problem is that there are cases where a session cannot
   take place correctly unless the terminating party implements a
   certain piece of service logic.  This is directly analagous to the
   case where a session cannot take place correctly unless the
   terminating party implements a certain SIP extension correctly; the
   problem is just occurring at a different layer in the stack based on
   the model of Figure 2.  Consequently, the proposed solution is
   similar.  The Require header field is utilized, and the option tag is
   just the service URN for the well-known service, suitably escaped.

   When a request with a Require header arrives at the home proxy of the
   UAS, the home proxy utilizes implicit preferences, as described in
   RFC 3841 [3].  This will prefer routing of the request to contacts
   which have indicated support for those extensions.  The UAS itself
   uses the Require header field to dispatch the request to the correct
   service instance.

   In addition, the user agents make use of UA loose routing [6] and
   GRUU [4], and add an implementation-specific parameter to their GRUU

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   for each service instance on a device.  This allows future out-of-
   dialog and mid-dialog requests to be targeted at the right service
   instance, and provides a simple mechanism for dispatch in the device,
   based entirely on the URI.

   When a UAC wants the request to be processed by an application prior
   to reaching the terminating proxy, it includes the service URN in
   Route headers that get appended to the route set for the request.
   For example, if a UA wants a call to be recorded, it would include a
   service URN like "urn:service:comm:recording" to the bottom-most
   Route header.  This will cause an originating proxy to resolve the
   service URN to a URI for an application server, and then proxy the
   request there.

   In order to discover available services on a device, presence can be
   used.  A UA would just SUBSCRIBE to the presence of the AOR, and get
   back a document that contains a service element for each service
   available for that user.  The presence document includes the service
   URN for any well-known services (noting again that this is only
   needed when other information, such as method or media types, are
   insufficient to define the service).  The service URN can then be
   used for creating summary information about the service.  The URI
   present in the contact for that service is the service instance URI
   or service URI.  The former is published by the UA to the network in
   a presence document, and the latter may be constructed by the network
   when composing documents together.

7.  UA Behavior

7.1.  Registration

   When a UA supports numerous services, it SHOULD generate a single
   registration representing the entire UA instance.  The UA MUST
   utilize GRUU [4] and UA loose routing [6].  If any of the services on
   the UA are well-known services, the UA SHOULD include their URNs as
   option tags in the extensions media feature tag in the Contact header
   field parameter.

      The media feature tags can help the network construct presence
      documents when the UA doesn't publish them separately (though this
      is recommended as described below).  They are also used for
      routing of requests.

      NOTE: An alternative design would be to have each service instance
      be a separate registered Contact.  This would be more helpful for
      presence, though not needed if a PUBLISH is used.  However it has
      the drawback of adding more state to the network and exposing

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      "internal" routing within the UA to outside of the UA.  It would
      mean that the proxy would need to know the dispatch logic, which
      is more likely to be known by the UA.

7.2.  Publication

   If the UA supports presence, it SHOULD PUBLISH [7] a presence
   document for itself.  This document SHOULD include a service
   (represented by a tuple) for each service instance.  The contact of
   each tuple SHOULD be derived from the GRUU, and constructed by adding
   a UA-defined parameter to the GRUU for each service instance.  The
   parameter MUST be different for each service instance, and SHOULD
   persist over time.  The UA SHOULD include information that identifies
   what the service is, including supported methods and media types,
   when those are important for its definition.  For services that
   require well-known logic, the agent SHOULD include the service URN
   amongst the extensions listed for that service.

      The UA can do a better job constructing the presence document than
      the registrar.  This is because the UA knows what mechanisms are
      used to dispatch requests to each service, and knows what well-
      known service URN are associated with each service.  Having an
      explicit contact for each service allows a UA to unambiguously be
      reached based on a service selection made by a watcher.  This is
      important, since choice is a key concept provided by presence

7.3.  Session Initiation

   A UA can initiate a session either directly, or using presence.

   When using presence, the UA would start with the AOR for the target.
   It subscribes to the presence state for the AOR [8].  The result will
   be a presence document that includes a tuple for each service.  The
   services will include information that describe them with sufficient
   information for the user to choose one.  This may include well-known
   service URN associated with each service.  When the user selects a
   service for the target user, the UA will generate an INVITE to the
   contact listed there.  Since this contact is a service instance URI
   or service URI, the request will be routed towards the target UA and
   explicitly identify the desired service by the URI alone.

   Of course, when the UA selected the service to contact, the request
   would have been initiated from a matching service on the device.  In
   that case, if the initiating request is associated with a required
   well-known service, the corresponding escaped service URN MUST appear
   as an option tag in the Require header field.

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   When initiating a session directly, the user will select a service on
   the phone and then request communications by entering or selecting
   the target AOR.  If the service from which the request is being made
   is associated with a required well-known service, the corresponding
   escaped service URN MUST appear as an option tag in the Require
   header field.

      This has an important procedural side effect.  Based on the rules
      of the SIP change processs [15], the Require header field can only
      contain option tags defined in standards track documents.
      Otherwise, the resulting protocol cannot be considered SIP.  This
      also means that a UA can only require well-known services when
      they are IETF defined.  Otherwise, the resulting protocol is
      proprietary.  This was purposefully done to help temper usage of
      the mechanism, which can cause significant interop problems if

   RFC 3261 uses the 'token' construct for option tags.  The service URN
   is a valid token with the exception of the colon (:).  Consequently,
   when used as an option tag, a service URN MUST be escape coded by
   replacing the colon with an exclamation point (!).

   When initiating a session from a presence document, there is no need
   for the UA to insert any Require header fields or otherwise add any
   content to the request beyond what is implied by the contact URI.
   This does not prevent a UA from inserting one when the UA does in
   fact require that a specific well-known service be present.

   The Contact header field of a dialog forming request SHOULD be formed
   by taking the GRUU, and adding a URI parameter (at the discretion of
   the UA) which identifies the particular service invoking the request.
   The resulting URI is called the service instance URI.

   If a UA wants the network to pass the request through application
   servers that provide specific processing, the UA MUST include a
   service URN for that service as the bottom-most Route header.  The
   service URN that are available to the UA are learned through
   mechanisms outside the scope of this specification, and can include
   configuration [10] for example.  If the UA wants the request to be
   processed by multiple applications, it MUST include a Route header
   value for each service URN.  The UA SHOULD order them based on
   desired order of invocation, if known.

7.4.  Receipt of a Request

   When a UA receives a request, it MAY use any content of the request
   in order to determine which service on the device is appropriate for
   handling the request.  This includes the method, media types, and

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   required extensions, including any service URN that might be present
   in the Require header field.  The specific means by which a service
   "registers" itself with the underlying SIP layer to drive the
   dispatch logic is a matter of local implementation and outside the
   scope of this specification.

   Of course, if the UAS doesn't understand one of the option tags in
   the Require header field, it will generate a 420 response and include
   the list of unsupported option tags, including those which happen to
   be service URN.  This is helpful for diagnosing interoperability
   problems due to incompatible services.

   Once the request is delivered to the service instance for processing,
   any response SHOULD include the service URI derived from the GRUU in
   the Contact header field.

8.  Proxy Behavior

8.1.  Request Targeting

   When a home proxy receives a request and uses the location service to
   route the request, it SHOULD follow the procedures defined in RFC
   3841 [3] for preference and capability matching.  These SHOULD be
   done even if the request did not contain an Accept-Contact or Reject-
   Contact header field.  When neither was present, the proxy will
   construct implicit preferences based on the rules in Section 7.2.2 of
   RFC 3841.

   In addition, a proxy SHOULD construct an explicit preference for
   extensions when the request contains a Require header field.  For
   each option tag in the Require header field, the proxy adds a term to
   the conjunction of the following form:

                        (sip.extension=[option tag])

   This would include any option tags that were service URN.  The result
   will be that calls get routed to devices which understand the
   required service.

8.2.  Application Invocation

   When a proxy receives a request where the next Route header field
   value after the proxy itself contains a service URN, the proxy MUST
   resolve the service URN to a SIP URI that can be used to perform that
   service.  The specific mechanism for resolution is outside of the
   scope of this specification.  It can include standardized resolution
   services such as DDDS [16] or LoST [11], or can be done through local

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   If there are more than one consecutive Route header field values with
   service URN, a proxy MAY resolve all of them, and MAY reorder them
   based on localized knowledge of the required invocation sequence.
   This is particularly important when the proxy is aware of additional
   applications that need to be invoked, for which it needs to add
   additional Route header field values.

9.  Guidelines for Using Service URN

   This document introduces the concept of using a service URN to
   identify well-known logic that is required in order to successfully
   process a request.  Care must be taken in the usage of this
   mechanism, or serious interoperability problems can occur.

   For example, consider an extreme example whereby the vendor of a UA
   defines a service URN for each version of their software, under the
   assumption that the logic in the UA represents a well-known service.
   If multiple vendors do this, a request from one vendor's device will
   fail to interoperate with the devices from any other vendor, even if
   they would be interoperable otherwise.

   Consider a more realistic case where a service provider chooses to
   utilize a well-known service URN for voice telephony and another one
   for video telephony.  There is nothing unique about the actual
   service logic used to realize each.  However, calls made from the
   video telephony application include a Require header field, requiring
   the usage of video telephony on the other side.  If the call should
   reach a device that supports only voice, such as a PSTN gateway, the
   call will automatically fail.  However, had existing SIP negotiation
   techniques been utilized (in this case, the ability to reject media
   streams), the call would have succeeded.

   It is for this reason that the well-known service URN in the Require
   header field are restricted in several ways.  Firstly, they are meant
   specifically and exclusively for usage in cases where some service
   logic must be present and matching on both the originating and
   terminating sides in order for any type of reasonable communications
   to exist.  Secondly, it is limited to capabilities that cannot be
   negotiated or indicated by other SIP techniques (such as support for
   a specific media type).  One metric for this is that, absent the
   option tag in the Require header, a request to initiate the session
   would be identical to a request to or from a different service that
   is not actually interoperable.

   Furthermore, the SIP change process forbids the usage of vendor

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   proprietary option tags in the Require header field.  This means that
   IETF standardization is required for the definition of service URN
   that would be used with the mechanism proposed here.

10.  Guidelines on Namespace Structure

   The service URN [5] creates a basic namespace in which services can
   be registered.  When a new service is added, care should be taken to
   make sure it is as general purpose as possible while still preserving
   interoperability.  When variations are possible, but for which
   interoperability exists, these SHOULD be registered using

   This specification requests IANA to create the "vendor" top-level
   service for vendor specific services.  Each sub service MUST be
   constructed by taking the domain name of the vendor (example.com for
   example), and following that by a vendor-defined subservice that
   identifies their service.  For example, if vendor example.org wants
   to create a service called foo, its service URN would be
   "urn:service:vendor.example.org.foo".  Note that these subservices
   are not IANA registered, and that vendor-defined service URN are not
   IANA registered SIP option tags.

11.  Security Considerations

   This specification makes use of option tags and URI to facilitate
   routing of a request to the appropriate service instance.  An
   attacker in the network could modify these fields to cause the
   request to be routed to the wrong service instance, which would
   worsen user experience and possibly cause an interoperability
   failure.  Such an attack would require a man-in-the-middle to modify
   SIP requests.  An attacker capable of such modifications can launch
   far more disruptive attacks by manipulating other fields, such as
   Contact or the SDP.  Consequently, such attacks do not seem likely.

12.  IANA Considerations

   This specification registers a new service URN label per the
   guidelines in Section 4 of [5].  This represents vendor-proprietary
   services.  Allocation of subservices is done using hierarchical
   allocation [13] and requires no IANA action.

   Here is the information to be added to the table of service URN:

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   Service: vendor

   Specification: RFC XXXX [[NOTE TO RFC-EDITOR: Please replace XXXX
      with the RFC number of this specification.]]

   Brief Description: Vendor proprietary service tree

13.  Example

   Consider our example from Section 3.  A user, joe@example.com, starts
   their chess application and wishes to play with bob@example.com.
   Joe's INVITE would look like:

            INVITE sip:bob@example.com SIP/2.0
            Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK99a
            From: Joe <sip:joe@example.com>;tag=n88ah
            To: Bob <sip:bob@example.com>
            Call-ID: 1j9FpLxk3uxtma7@host.example.com
            CSeq: 1 INVITE
            Supported: gruu
            Require: urn!service!chess
            Content-Length: --
            Content-Type: application/sdp

            [SDP Not shown]

   Note that the request contains a Require header field with the
   service URN.  The Contact header field contains a GRUU, and Joe's UA
   has added a parameter, "service=chess" to this URI.  This parameter
   is used only by Joe's UA for dispatching the request to the chess
   application when a request is sent to that URI.

   In another example, a Joe receives a presence document indicating
   that the chess service is supported for Bob:

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           <?xml version="1.0" encoding="UTF-8"?>
           <presence xmlns="urn:ietf:params:xml:ns:pidf"
            <tuple id="sg89ae">
            <dm:person id="p1">
            <dm:device id="pc122">

   Joe's UA notices that the chess service is available by the service
   URN, and it renders an icon representing that service.  When Joe
   selects it, the chess application launches and generates an INVITE.
   Note that the chess application itself will include a Require header
   field, since chess has to be supported on the far end to proceed with
   the call:

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            INVITE sip:bob@example.com
              ;service=chess SIP/2.0
            Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK99a
            From: Joe <sip:joe@example.com>;tag=n88ah
            To: Bob <sip:bob@example.com>
            Call-ID: 1j9FpLxk3uxtma7@host.example.com
            CSeq: 1 INVITE
            Supported: gruu
            Require: urn!service!chess
            Content-Length: --
            Content-Type: application/sdp

            [SDP Not shown]

14.  Acknowledgements

   This document is based on discussions with Paul Kyzivat and Andrew
   Allen, who contributed significantly to the ideas here.

15.  References

15.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  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.

   [3]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
        Preferences for the Session Initiation Protocol (SIP)",
        RFC 3841, August 2004.

   [4]  Rosenberg, J., "Obtaining and Using Globally Routable User Agent
        (UA) URIs (GRUU) in the  Session Initiation Protocol (SIP)",
        draft-ietf-sip-gruu-11 (work in progress), October 2006.

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   [5]  Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
        draft-ietf-ecrit-service-urn-05 (work in progress), August 2006.

   [6]  Rosenberg, J., "Applying Loose Routing to Session Initiation
        Protocol (SIP) User Agents  (UA)",
        draft-rosenberg-sip-ua-loose-route-00 (work in progress),
        October 2006.

   [7]  Niemi, A., "Session Initiation Protocol (SIP) Extension for
        Event State Publication", RFC 3903, October 2004.

   [8]  Rosenberg, J., "A Presence Event Package for the Session
        Initiation Protocol (SIP)", RFC 3856, August 2004.

15.2.  Informational References

   [9]   Rosenberg, J., "A Framework for Application Interaction in the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sipping-app-interaction-framework-05 (work in
         progress), July 2005.

   [10]  Petrie, D., "A Framework for Session Initiation Protocol User
         Agent Profile Delivery", draft-ietf-sipping-config-framework-09
         (work in progress), October 2006.

   [11]  Hardie, T., "LoST: A Location-to-Service Translation Protocol",
         draft-ietf-ecrit-lost-02 (work in progress), October 2006.

   [12]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol (SIP)", RFC 4353, February 2006.

   [13]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.

   [14]  Rosenberg, J., "A Data Model for Presence", RFC 4479,
         July 2006.

   [15]  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.

   [16]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         One: The Comprehensive DDDS", RFC 3401, October 2002.

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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054

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

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