SIPPING                                                     J. Rosenberg
Internet-Draft                                                     Cisco
Intended status: Best Current                             August 1, 2007
Expires: February 2, 2008

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

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document considers the problem of service identification in the
   Session Initiation Protocol (SIP).  Service identification is the
   process of determining the user-level use case that is driving the
   signaling being utilized by the user agent.  While seemingly simple,
   this process is quite complex, and when not addressed properly, can
   lead to fraud, interoperability problems, and stifling of innovation.

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   This document discusses these problems and makes recommendations on
   how to address them.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Services and Service Identification  . . . . . . . . . . . . .  4
   4.  Example Services . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  IPTV vs. Multimedia  . . . . . . . . . . . . . . . . . . .  6
     4.2.  Gaming vs. Voice Chat  . . . . . . . . . . . . . . . . . .  7
     4.3.  Configuration vs. Pager Messaging  . . . . . . . . . . . .  7
   5.  Using Service Identification . . . . . . . . . . . . . . . . .  7
     5.1.  Application Invocation in the User Agent . . . . . . . . .  8
     5.2.  Application Invocation in the Network  . . . . . . . . . .  9
     5.3.  Network Quality of Service Authorization . . . . . . . . .  9
     5.4.  Service Authorization  . . . . . . . . . . . . . . . . . . 10
     5.5.  Accounting and Billing . . . . . . . . . . . . . . . . . . 10
     5.6.  Negotiation of Service . . . . . . . . . . . . . . . . . . 10
     5.7.  Dispatch to Devices  . . . . . . . . . . . . . . . . . . . 11
   6.  Key Principles of Service Identification . . . . . . . . . . . 11
     6.1.  Services are a By-Product of Signaling . . . . . . . . . . 11
     6.2.  Perils of Explicit Identifiers . . . . . . . . . . . . . . 13
       6.2.1.  Fraud  . . . . . . . . . . . . . . . . . . . . . . . . 13
       6.2.2.  Systematic Interoperability Failures . . . . . . . . . 14
       6.2.3.  Stifling of Service Innovation . . . . . . . . . . . . 16
   7.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informational References . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
   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.  SIP
   allows for a broad array of session types between agents.  It can
   manage audio sessions, ranging from low bitrate voice-only up to
   multi-channel hi fidelity music.  It can manage video sessions,
   ranging from small, "talking-head" style video chat, up to high
   definition multipoint video conferencing, to low bandwidth user-
   generated content, up to high definition movie and TV content.  SIP
   endpoints can be anything - adaptors that convert an old analog
   telephone to Voice over IP (VoIP), dedicated hardphones, fancy
   hardphones with rich displays and user entry capabilities, softphones
   on a PC, buddylist and presence applications on a PC, dedicated
   videoconferencing peripherals, and speakerphones.

   This breadth of applicability is SIPs greatest asset, but it also
   introduces numerous challenges.  One of these is that, when an
   endpoint generates a SIP INVITE for a session, or receives one, that
   session can potentially be within the context of any number of
   different use cases and endpoint types.  For example, a SIP INVITE
   with a single audio stream could represent a Push-To-Talk session
   between mobile devices, a VoIP session between softphones, or audio-
   based access to stored content on a server.

   These differing use cases have driven implementors and system
   designers to seek techniques for service identification.  Service
   identification is the process of determining and/or signaling the
   specific use case that is driving the signaling being generated by a
   user agent.  At first glance, this seems harmless and easy enough.
   It is tempting to define a new header, "Service-ID", for example, and
   have a user agent populate it with any number of well-known tokens
   which define what the service is.  This information could then be
   consumed for any number of purposes.

   However, as this document will demonstrate, service identification is
   a very complex and difficult process, and can very easily lead to
   fraud, systemic interoperability failures, and a complete stifling of
   the innovation that SIP was meant to achieve.

   Section 3 begins by defining a service and the service identification
   problem.  Section 4 gives some concrete examples of services and why
   they can be challenging to identify.  Section 5 explores the ways in
   which a service identification can be utilized within a network.
   Next, Section 6 discusses the key architectural principles of service
   identification, and how explicit service identifiers can lead to
   fraud, interoperability failures, and stifling of service innovation.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

3.  Services and Service Identification

   The problem of identifying 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 [3] 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).

      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

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      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
      representation of the game.

   Essentially, the service is the user-visible use case that is driving
   the behavior of the user-agents and servers in the SIP network.
   Being user-visible means that there is a difference in user
   experience between two services that are different.  That user
   experience can be part of the call, or outside of the call.  Within a
   call, the user experience can be based on different media types (an
   audio call vs. a video chat), different content within a particular
   media type (stored content, such as a movie or TV session), different
   devices (a wireless device for "telephony" vs. a PC application for
   "voice-chat"), different user interfaces (a buddy list view of voice
   on a PC application vs. a software emulation of a hard phone),
   different communities that can be accessed (voice chat with other
   users that have the same voice chat client, vs. voice communications
   with any endpoint on the PSTN), or different applications that are
   invoked by the user (manually selecting a push-to-talk application
   from a wireless phone vs. a telephony application).  Outside of a
   call, the difference in user experience can be a billing one (cheaper
   for one service than other), a notification feature for one and not
   another (for example, an IM that gets sent whenever a user makes a

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   call), and so on.

   In some cases, there is very little difference in the underlying
   technology that will support two different services, and in other
   cases, there are big differences.  However, for purposes of this
   discussion, the key definition is that two services are distinct when
   there is a perceived difference by the user in the two services.

   This leads naturally to the desire to perform service identification.
   Service identification is defined as the process of (1) determination
   of the underlying service which is driving a particular signaling
   exchange, (2) associating that service with some kind of moniker, and
   (3) attaching that moniker to a signaling message (typically a SIP
   INVITE), and then utilizing it for various purposes within the
   network.  Service identification can be done in the endpoints, in
   which case the UA would insert the moniker directly into the
   signaling message based on its awareness of the service.  Or, it can
   be done within a proxy in the network, based on inspection of the SIP
   message, or based on hints placed into the message by the user.

4.  Example Services

   It is very useful to consider several example services, especially
   ones that appear difficult to differentiate from each other.

4.1.  IPTV vs. Multimedia

   IP Television (IPTV) is the usage of IP networks to access
   traditional television content, such as movies and shows.  SIP can be
   utilized to establish a session to a media server in a network, which
   then serves up multimedia content and streams it as an audio and
   video stream towards the client.  Whether SIP is ideal for IPTV is,
   in itself, a good question.  However, such a discussion is outside
   the scope of this document.

   Consider multimedia conferencing.  The user accesses a voice and
   video conference at a conference server.  The user might join in
   listen-only mode, in which case the user receives audio and video
   streams, but does not send.

   These two services - IPTV and multimedia conferencing, clearly appear
   as different services.  They have different user experiences and
   applications.  A user is unlikely to ever be confused about whether a
   session is IPTV or multimedia conferencing.  Indeed, they are likely
   to have different software applications or endpoints for the two

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   However, these two services look remarkably alike based on the
   signaling.  Both utilize audio and video.  Both could utilize the
   same codecs.  Both are unidirectional streams (from a server in the
   network to the client).  Thus, it would appear on the surface that
   there is no way to differentiate them, based on inspection of the
   signaling alone.

4.2.  Gaming vs. Voice Chat

   Consider an interactive game, played between two users from their
   mobile devices.  The game involves the users sending each other game
   moves, using a messaging channel, in addition to voice.  In another
   service, users have a voice and IM chat conversation using a buddy
   list application on their PC.

   In both services, there are two media streams - audio and messaging.
   The audio uses the same codecs.  Both use the Message Session Relay
   Protocol (MSRP) [5].  In both cases, the caller would send an INVITE
   to the AOR of the target user.  However, these represent fairly
   different services, in terms of user experience.

4.3.  Configuration vs. Pager Messaging

   The SIP MESSAGE method [8] provides a way to send one-shot messages
   to a particular AOR.  This specification is primarily aimed at Short
   Message Service (SMS) style messaging, commonly found in wireless
   phones.  Receipt of a MESSAGE request would cause the messaging
   application on a phone to launch, allowing the user to browse message
   history and respond.

   However, MESSAGE is sometimes used for the delivery of content to a
   device for other purposes.  For example, some providers use it to
   deliver configuration updates, such as new phone settings or
   parameters, or to indicate that a new version of firmware is
   available.  Though not designed for this purpose, MESSAGE gets used
   since, in existing wireless networks, SMS are used for this purpose,
   and MESSAGE is the SIP equivalent of SMS.

   Consequently, the MESSAGE request sent to a phone can be for two
   different services.  One would require invocation of a messaging app,
   whereas the other would be consumed by the software in the phone,
   without any user interaction at all.

5.  Using Service Identification

   It is important to understand what the service identity would be
   utilized for, if known.  The discussions in Section 4 give some hints

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   to the possible usages.  Here, we explicitly discuss them.

5.1.  Application Invocation in the User Agent

   In some of the examples above, there were multiple software
   applications running within a single user agent.  When an incoming
   INVITE or MESSAGE arrives, it must be delivered to the appropriate
   application software.  When each service is bound to a distinct
   software application, it would seem that the service identity is
   needed to dispatch the message to the appropriate piece of software.
   This is 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.  For the example services in
   Section 4.2, an incoming INVITE for the gaming service would be
   delivered to the gaming application software.  An incoming INVITE for
   the voice chat service would be delivered to the voice chat
   application software.  For the examples in Section 4.3, a MESSAGE
   request for user to user messaging would be delivered to the
   messaging or SMS app, and a MESSAGE request containing configuration
   data would be delivered to a configuration update application.

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5.2.  Application Invocation in the Network

   Another usage of a service identifier would be to cause servers in
   the SIP network to provide additional processing, based on the
   service.  For example, an INVITE issued by a user agent for IPTV
   would pass through a server that does some kind of content rights
   management, authorizing whether the user is allowed to access that
   content.  On the other hand, an INVITE issued by a user for
   multimedia conferencing would pass through a server providing
   "traditional" telephony features, such as outbound call screening and
   call recording.  It would make no sense for the INVITE associated
   with IPTV to have outbound call screening and call recording applied,
   and it would make no sense for the multimedia conferencing INVITE to
   be processed by the content rights management server.  Indeed, in
   these cases, its not just an efficiency issue (invoking servers when
   not needed), but rather, truly incorrect behavior can occur.  For
   example, if an outbound call screening application is set to block
   outbound calls to everything except for the phone numbers of friends
   and family, an IPTV request that gets processed by such a server
   would be blocked (as its not targeted to the AOR of a friend or
   family member).  This would block a user's attempt to access IPTV
   services, when that was not the goal at all.

   Similarly, a MESSAGE request from Section 4.3 might need to pass
   through a message server for filtering when it is associated with
   chat, but not when it is associated with config.  Consider a filter
   which gets applied to MESSAGE requests, and that filter runs in a
   server in the network.  The filter operation prevents user Joe from
   sending messages to user Bob that contain the words "stock" or
   "purchase", due to some regulations that disallow Joe and Bob from
   discussing stock trading.  However, a MESSAGE for configuration
   purposes might contain an XML document that uses the token "stock" as
   some kind of attribute.  This configuration update would be discarded
   by the filtering server, when it should not have been.

5.3.  Network Quality of Service Authorization

   The IP network can provide differing levels of Quality of Service
   (QoS) to IP packets.  This service can include guaranteed throughput,
   latency, or loss characteristics.  Typically, the user agent will
   make some kind of QoS request, either using explicit signaling
   protocols (such as RSVP) or through marking of Diffserv value in
   packets.  The network will need to make a policy decision based on
   whether these QoS treatments are authorized or not.  One common
   authorization policy is to check if the user has invoked a service
   using SIP that they are authorized to invoke, and that this service
   requires the level of QoS treatment the user has requested.

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   For example, consider IPTV and multimedia conferencing as described
   in Section 4.1.  IPTV is a non-real time service.  Consequently,
   media traffic for IPTV would be authorized for bandwidth guarantees,
   but not for latency or loss guarantees.  On the other hand,
   multimedia conferencing is real time.  Its traffic would require
   bandwidth, loss and latency guarantees from the network.

   Consequently, if a user should make an RSVP reservation for a media
   stream, and ask for latency guarantees for that stream, the network
   would like to be able to authorize it if the service was multimedia
   conferencing, but not if it was IPTV.  This would require the server
   performing the QoS authorization to know the service associated with
   the INVITE that set up the session.

5.4.  Service Authorization

   Frequently, a network administrator will want to authorize whether a
   user is allowed to invoke a particular service.  Not all users will
   be authorized to use all services that are provided.  For example, a
   user may not be authorized to access IPTV services, whereas they are
   authorized to utilize multimedia processing.  A user might not be
   able to utilize a multiplayer gaming service, whereas they are
   authorized to utilize voice chat services.

   Consequently, when an INVITE arrives at a proxy in the network, the
   proxy will need to determine what the requested service is, so that
   the proxy can make an authorization decision.

5.5.  Accounting and Billing

   Service authorization and accounting/billing go hand in hand.
   Presumably, one of the primary reasons for authorizing that a user
   can utilize a service is that they are being billed differently based
   on the type of service.  Consequently, one of the goals of a service
   identity is to be able to include it in accounting records, so that
   the appropriate billing model can be applied.

   For example, in the case of IPTV, a service provider can bill based
   on the content (US $5 per movie, perhaps), whereas for multimedia
   conferencing, they can bill by the minute.  This requires the
   accounting streams to indicate which service was invoked for the
   particular session.

5.6.  Negotiation of Service

   In some cases, when the caller initiates a session, they don't
   actually know which service will be utilized.  Rather, they might
   like to offer up all of the services they have available to the

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   called party, and then let the called party decide, or let the system
   make a decision based on overlapping service capabilities.

   As an example, s user can do both the game and the voice chat service
   of Section 4.2.  They initiate a session to a target AOR, but the
   devices used by that user can only support voice chat.  Consequently,
   voice chat gets utilized for the session.

5.7.  Dispatch to Devices

   When a user has multiple devices, each with varying capabilities in
   terms of service, it is useful to dispatch an incoming request to the
   right device based on whether the device can support the service that
   has been requested.

   For example, if a user initiates a gaming session with voice chat,
   and the target user has two devices - one that can support the gaming
   service, and the other that cannot, the INVITE should be dispatched
   to the device which supports the gaming session.

6.  Key Principles of Service Identification

   In this section, we describe some of the key principles of performing
   service identification.

6.1.  Services are a By-Product of Signaling

   Almost always, the first solution that people consider is to add some
   kind of field to the signaling messages which indicates what the
   service is.  This field would then be inserted by the user agent, and
   then can be used by the proxies and other user agent as a service

   This approach, however, misses a key point, which cannot be stressed
   enough, and which represents the core architectural principle to be
   understood here:

      A service is the by-product of the signaling and the context
      around it (the user profile, time-of-day and so on) - the effects
      of the signaling message once launched into the network.  The
      service identity is therefore always derivable from the signaling
      and its context without additional identifiers.

   When a user sends an INVITE request to the network, and targets that
   request at an IPTV server, and includes SDP for audio and video
   streaming, the *result* of sending such an INVITE is that an IPTV
   session occurs.  The entire purpose of the INVITE is to establish

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   such a session, and therefore, invoke the service.  Thus, a service
   is not something that is different from the rest of the signaling
   message.  A service is what the user gets after the network and other
   user agents have processed a signaling message.

   This principle leads to another important conclusion:

      If two services are different, but their signaling appears to be
      the same, it is because there is in fact something different that
      has been overlooked, or something has been implied from the
      signaling which should have been signaled explicitly.

   This makes sense; if a service is the byproduct of signaling, how can
   a user have different experiences and different services when the
   signaling message is the same?  There has to be something different
   in the messages, if the user experience was in fact different.

   To illustrate this, let us take each of the example services in
   Section 4 and investigate whether there is, or should be, something
   different in the signaling in each case.

   IPTV vs. Multimedia Conferencing:  The two services in Section 4.1
      appear to have identical signaling.  They both involve audio and
      video streams, both of which are unidirectional.  Both might
      utilize the same codecs.  However, there is another important
      difference in the signaling - the target URI.  In the case of
      IPTV, the request is targeted at a media server or to a particular
      piece of content to be viewed.  In the case of multimedia
      conferencing, the target is a conference server.  The
      administrator of the domain can therefore examine the two Request-
      URI, and figure out whether it is targeted for a conference server
      or a content server, and use that to derive the service associated
      with the request.

   Gaming vs. Voice Chat:  Though both sessions involve MSRP and voice,
      and both are targeted to the same AOR of the called user, there is
      a difference.  The MSRP messages for the gaming session carry
      content which is game specific, whereas the MSRP messages for the
      voice chat are just regular text, meant for rendering to a user.
      Thus, the MSRP session in the SDP will indicate the specific
      content type that MSRP is carrying, and this type will differ in
      both cases.  Even if the game moves look like text, since they are
      being consumed by an automata there is an underlying schema that
      dictates their content, and therefore, this schema represents the
      actual content type that should be signaled.

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   Configuration vs. Pager Messaging:  Just as in the case of gaming vs.
      voice chat, the content type of the messages differentiates the
      service that occurs as a consequence of the messages.

   This is ultimately an expression of the principle of DWIM vs. DWIS
   (Do-What-I-Mean vs. Do-What-I-Say).  Explicit signaling is DWIS - the
   user is asking for a service by invoking the signaling that results
   in the desired effect.  A service identifier is DWIM - an unspecific
   request for something that is ill-defined and non-interoperable.

6.2.  Perils of Explicit Identifiers

   Given that the information in the signaling message always conveys
   enough information to identify the service, another important
   conclusion can be drawn:

      Inclusion of an explicit service identifier within a message is,
      at best, redundant, and at worst, an avenue for fraud, loss of
      interoperability, and stifling of service innovation.

   By "explicit service identifier", we mean a field included in the
   signaling message that contains a token whose value indicates the
   specific service invoked by the calling user.  This would be "IPTV"
   or "voice chat" or "shoot-em game" or "short message service".  This
   explicit identifier would typically be inserted by the originating
   user agent, and carried in the signaling message.

   Clearly, if the signaling message itself contains enough information
   to identify the service, inclusion of an extra field to say the same
   thing is going to be redundant.  Redundancy by itself is not a big
   deal.  However, redundancy can lead to other,more significant

6.2.1.  Fraud

   First and foremost, it can lead to fraud.  If a provider uses the
   service identifier for billing and accounting purposes, or for
   authorization purposes, it opens an avenue for attack.  The user can
   construct the signaling message so that its actual effect (which is
   the service the user will receive), is what the user desires, but the
   service identity (which is what is used for billing and
   authorization) doesn't match, and indicates a cheaper service, or one
   that the user is authorized to receive.  If, however, the service
   identity used by the domain admistrator is derived from the signaling
   itself, the user cannot lie.  If they did lie, they wouldn't get the
   desired service.

   Consider the example of IPTV vs. multimedia conferencing.  If

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   multimedia conferencing is cheaper, the user could send an INVITE for
   an IPTV session, but include a service identifier which indicates
   multimedia conferencing.  They get the service associated with IPTV,
   but at the cost of multimedia conferencing.

   This same principle shows up in other places.  For example, in the
   identification of an emergency services call [6].  It is desirable to
   give emergency services calls special treatment, such as being free,
   authorized even when the user cannot otherwise make calls, and to
   give them priority.  If emergency calls where indicated through
   something other than the target of the call being an emergency
   services URN [7], it would open an avenue for fraud.  The user could
   place any desired URI in the request-URI, and indicate that the call
   is an emergency services call.  This could would then get special
   treatment, but of course get routed to the target URI.  The only way
   to prevent this fraud is to consider an emergency call as any call
   whose target is an emergency services URN.  Thus, the service
   identification here is based on the target of the request.  When the
   target is an emergency services URN, the request can get special
   treatment.  The user cannot lie, since there is no way to separately
   indicate this is an emergency call, besides targeting it to an
   emergency URN.

6.2.2.  Systematic Interoperability Failures

   How can inclusion of an explicit service identifier cause loss of
   interoperability?  When such an identifier is used to drive
   functionality - such as dispatch on the phones, in the network, or
   QoS authorization, it means that the wrong thing can happen when this
   field is not set properly.  Consider a user in domain 1, calling a
   user in domain 2.  Domain 1 provides the user with a service they
   call "voice chat", which utilizes voice and IM for real time
   conversation, driven off of a buddy list application on a PC.  Domain
   2 provides their users with a service they call, "text telephony",
   which is a voice service on a wireless device that also allows the
   user to send text messages.  Consider the case where domain 1 and
   domain 2 both have their user agents insert a service identifiers
   into the request, and then use that to derive QoS authorization,
   accounting, and invocation of applications in the network and in the
   device.  The user in domain 1 calls the user in domain 2, and inserts
   the identifier "Voice Chat" into the INVITE.  When this arrives at
   the proxy in domain 2, the service is unknown.  Consequently, the
   request does not get the proper QoS treatment.  When it gets
   delivered to the User Agent of the user in domain 2, the user agent
   does not see a service it understands, and so consequently, does not
   know to dispatch the request to the right application software.
   Thus, this call has completely failed, even when it could have
   succeeded.  This illustrates the following key point:

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      Explicit service identifiers, used between domains, cause
      interoperability failures unless all interconnected domains agree
      on exactly the same set of services and how to name them.

   Of course, lack of service identifiers does not guarantee service
   interoperability.  However, SIP was built with rich tools for
   negotiation of capabilities at a finely granular level.  One user
   agent can make a call using audio and video, but if the receiving UA
   only supports audio, SIP allows both sides to negotiate down to the
   lowest common denominator.  Thus, communications is still provided.
   As another example, if one agent initiates a Push-To-Talk session
   (which is audio with a companion floor control mechanism), and the
   other side only did regular audio, SIP would be able to negotiate
   back down to a regular voice call.  As another example, if a calling
   user agent is running a high-definition video conferencing endpoint,
   and the called user agent supports just a regular video endpoint, the
   codecs themselves can negotiate downward to a lower rate, picture
   size, and so on.  Thus, interoperability is achieved.  Interestingly,
   the final "service" may no longer be well characterized by the
   service identifier that would have been placed in the original
   INVITE.  For example, in this case, of the original INVITE from the
   caller had contained the service identifier, "hi-fi video", but the
   video gets negotiated down to a lower rate and picture size, the
   service identifier is no longer really appropriate.

   This illustrates another key aspect of the interoperability problem:

      Usage of explicit service identifiers in the request will result
      in inconsistencies with results of any SIP negotiation that might
      otherwise be applied in the session.

   Of course, there are cases where negotiating to a common baseline is
   not what is desired.  SIP provides tools (such as Require), to force
   the call to fail unless the desired capabilities are supported.
   However, this is not recommended as a general rule [4].

   When a service identifier becomes something that both proxies and the
   user agent need to understand in order to properly treat a request,
   it becomes equivalent to including a token in the Proxy-Require and
   Require header fields of every single SIP request.  The very reason
   that RFC 4485 frowns upon usage of Require and certainly Proxy-
   Require is the huge impact on interoperability it causes.  It is for
   this same reason that explicit service identifiers need to be

      The usage of explicit service identifiers is equivalent to the
      usage of Require and Proxy-Require in the request, and has the
      same negative impact on interoperability as those headers have.

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6.2.3.  Stifling of Service Innovation

   The probability that any two pair of service providers end up with
   the same set of services, and give them the same names, becomes
   decreasingly small as the number of providers grow.  Indeed, it would
   almost certainly require a centralized authority to identify what the
   services are, how they work, and what they are named.  This, in turn,
   leads to a requirement for complete homogeneity in order to
   facilitate interconnection.  Two providers cannot usefully
   interconnect unless they agree on the set of services they are
   offering to their customers, and each do the same thing.  This is, in
   a very real sense, anathema to the entire notion of SIP, which is
   built on the idea that heterogeneous domains can interconnect and
   still get interoperability:

      Explicit service identifiers lead to a requirement for homogeneity
      in service definitions across providers that interconnect, ruining
      the very service heterogeneity that SIP was meant to bring.

   Indeed, Metcalfe's law says that the value of a network grows with
   the square of the number of participants.  As a consequence of this,
   once a bunch of large domains did get together, agree on a set of
   services, and then a set of well-known identifiers for those
   services, it would force other providers to also deploy the same
   services, in order to obtain the value that interconnection brings.
   This, in turn, will stifle innovation, and quickly force the set of
   services in SIP to become fixed and never expand beyond the ones
   initially agreed upon.  This, too, is anathema to the very framework
   on which SIP is built, and defeats much of the purpose of why
   providers have chosen to deploy SIP in their own networks:

      Metcalfe's law, when combined with explicit service identifiers,
      will stifle the ability of providers to develop new SIP services,
      since they have no hope of interconnecting them with anyone else.

   Consider the following example.  Several providers get together, and
   standardize on a bunch of service identifiers.  One of these uses
   audio and video (say, "multimedia conversation").  This service is
   successful, and is widely utilized.  Endpoints look for this
   identifier to dispatch calls to the right software applications, and
   the network looks for it to invoke features, perform accouting, and
   QoS.  A new provider gets the idea for a new service, say, avatar-
   enhanced multimedia conversation.  In this service, there is audio
   and video, but there is a third stream, which renders an avatar.  A
   caller can press buttons on their phone, to cause the avatar on the
   other person's device to show emotion, make noise, and so on.  This
   is similar to the way emoticons are used today in IM.  This service
   is enabled by adding a third media stream (and consequently, third

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   m-line) to the SDP.

   Normally, this service would be backwards compatible with a regular
   audio-video endpoint, which would just reject the third media stream.
   However, because a large network has been deployed that is expecting
   to see the token, "multimedia conversation" and its associated audio+
   video service, it is nearly impossible for the new provider to roll
   out this new service.  If they did, it would fail completely, or
   partially fail, when their users call users in other provider

7.  Recommendations

   From these principles, several recommendations can be made:

   o  Systems needing to perform service identification must examine
      existing signaling constructs to identify the service based on
      fields that exist within the signaling message already.

   o  If it appears that the signaling currently defined in standards is
      not sufficient to identify the service, it may be due to lack of
      sufficient signaling to convey what is needed, and new standards
      work should be undertaken to fill this gap.

   o  The usage of an explicit service identifier does make sense as a
      way to cache a decision made by a network element, for usage by
      another network element within the same domain.  However, service
      identifiers are fundamentally useful within a particular domain,
      and any such header must be stripped at a network boundary.

8.  Security Considerations

   Oftentimes, the service associated with a request is utilized for
   purposes such as authorization, accounting, and billing.  When
   service identification is not done properly, the possibility of
   network fraud is introduced.  It is for this reason, discussed
   extensively in Section 6.2.1, that the usage of explicit service
   identifiers inserted by a UA is NOT RECOMMENDED.

9.  IANA Considerations

   There are no IANA considerations associated with this specification.

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10.  Acknowledgements

   This document is based on discussions with Paul Kyzivat and Andrew
   Allen, who contributed significantly to the ideas here.  Much of the
   content in this draft is a result of discussions amongst participants
   in the SIPPING mailing list, including Dean Willis, Tom Taylor, Eric
   Burger, Dale Worley, Christer Holmberg, and John Elwell, amongst many

11.  References

11.1.  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.

11.2.  Informational References

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

   [4]  Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors of
        Extensions to the Session Initiation Protocol (SIP)", RFC 4485,
        May 2006.

   [5]  Campbell, B., "The Message Session Relay Protocol",
        draft-ietf-simple-message-sessions-19 (work in progress),
        February 2007.

   [6]  Rosen, B., "Framework for Emergency Calling in Internet
        Multimedia", draft-ietf-ecrit-framework-01 (work in progress),
        March 2007.

   [7]  Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
        draft-ietf-ecrit-service-urn-06 (work in progress), March 2007.

   [8]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
        D. Gurle, "Session Initiation Protocol (SIP) Extension for
        Instant Messaging", RFC 3428, December 2002.

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Author's Address

   Jonathan Rosenberg
   Edison, NJ


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Full Copyright Statement

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