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Network Working Group                                          J. Lennox
Request for Comments: 2824                                H. Schulzrinne
Category: Informational                              Columbia University
                                                                May 2000

          Call Processing Language Framework and Requirements

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   A large number of the services we wish to make possible for Internet
   telephony require fairly elaborate combinations of signalling
   operations, often in network devices, to complete. We want a simple
   and standardized way to create such services to make them easier to
   implement and deploy.  This document describes an architectural
   framework for such a mechanism, which we call a call processing
   language. It also outlines requirements for such a language.

Table of Contents

   1        Introduction ........................................    2
   2        Terminology .........................................    3
   3        Example services ....................................    4
   4        Usage scenarios .....................................    6
   5        CPL creation ........................................    6
   6        Network model .......................................    7
   6.1      Model components ....................................    7
   6.1.1    End systems .........................................    7
   6.1.2    Signalling servers ..................................    8
   6.2      Component interactions ..............................    8
   7        Interaction of CPL with network model ...............   10
   7.1      What a script does ..................................   10
   7.2      Which script is executed ............................   11
   7.3      Where a script runs .................................   12
   8        Creation and transport of a call processing
            language script .....................................   12
   9        Feature interaction behavior ........................   13
   9.1      Feature-to-feature interactions .....................   13

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   9.2      Script-to-script interactions .......................   14
   9.3      Server-to-server interactions .......................   15
   9.4      Signalling ambiguity ................................   15
   10       Relationship with existing languages ................   15
   11       Related work ........................................   17
   11.1     IN service creation environments ....................   17
   11.2     SIP CGI .............................................   17
   12       Necessary language features .........................   17
   12.1     Language characteristics ............................   17
   12.2     Base features -- call signalling ....................   19
   12.3     Base features -- non-signalling .....................   21
   12.4     Language features ...................................   22
   12.5     Control .............................................   23
   13       Security Considerations .............................   23
   14       Acknowledgments .....................................   23
   15       Authors' Addresses ..................................   23
   16       Bibliography ........................................   24
   17       Full Copyright Statement ............................   25

1 Introduction

   Recently, several protocols have been created to allow telephone
   calls to be made over IP networks, notably SIP [1] and H.323 [2].
   These emerging standards have opened up the possibility of a broad
   and dramatic decentralization of the provisioning of telephone
   services so they can be under the user's control.

   Many Internet telephony services can, and should, be implemented
   entirely on end devices. Multi-party calls, for instance, or call
   waiting alert tones, or camp-on services, depend heavily on end-
   system state and on the specific content of media streams,
   information which often is only available to the end system. A
   variety of services, however -- those involving user location, call
   distribution, behavior when end systems are busy, and the like -- are
   independent of a particular end device, or need to be operational
   even when an end device is unavailable. These services are still best
   located in a network device, rather than in an end system.

   Traditionally, network-based services have been created only by
   service providers. Service creation typically involved using
   proprietary or restricted tools, and there was little range for
   customization or enhancement by end users.  In the Internet
   environment, however, this changes. Global connectivity and open
   protocols allow end users or third parties to design and implement
   new or customized services, and to deploy and modify their services
   dynamically without requiring a service provider to act as an

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   A number of Internet applications have such customization
   environments -- the web has CGI [3], for instance, and e-mail has
   Sieve [4] or procmail. To create such an open customization
   environment for Internet telephony, we need a standardized, safe way
   for these new service creators to describe the desired behavior of
   network servers.

   This document describes an architecture in which network devices
   respond to call signalling events by triggering user-created programs
   written in a simple, static, non-expressively-complete language. We
   call this language a call processing language.

   The development of this document has been substantially informed by
   the development of a particular call processing language, as
   described in [5]. In general, when this document refers to "a call
   processing language," it is referring to a generic language that
   fills this role; "the call processing language" or "the CPL" refers
   to this particular language.

2 Terminology

   In this section we define some of the terminology used in this

   SIP [1] terminology used includes:

      invitation: The initial INVITE request of a SIP transaction, by
           which one party initiates a call with another.

      redirect server: A SIP device which responds to invitations and
           other requests by informing the request originator of an
           alternate address to which the request should be sent.

      proxy server: A SIP device which receives invitations and other
           requests, and forwards them to other SIP devices. It then
           receives the responses to the requests it forwarded, and
           forwards them back to the sender of the initial request.

      user agent: A SIP device which creates and receives requests, so
           as to set up or otherwise affect the state of a call. This
           may be, for example, a telephone or a voicemail system.

      user agent client: The portion of a user agent which initiates

      user agent server: The portion of a user agent which responds to

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   H.323 [2] terminology used includes:

      terminal: An H.323 device which originates and receives calls, and
           their associated media.

      gatekeeper: An H.323 entity on the network that provides address
           translation and controls access to the network for H.323
           terminals and other endpoints. The gatekeeper may also
           provide other services to the endpoints such as bandwidth
           management and locating gateways.

      gateway: A device which translates calls between an H.323 network
           and another network, typically the public-switched telephone

      RAS: The Registration, Admission and Status messages communicated
           between two H.323 entities, for example between an endpoint
           and a gatekeeper.

   General terminology used in this document includes:

      user location: The process by which an Internet telephony device
           determines where a user named by a particular address can be

      CPL: A Call Processing Language, a simple language to describe how
           Internet telephony call invitations should be processed.

      script: A particular instance of a CPL, describing a particular
           set of services desired.

      end system: A device from which and to which calls are
           established.  It creates and receives the call's media
           (audio, video, or the like). This may be a SIP user agent or
           an H.323 terminal.

      signalling server: A device which handles the routing of call
           invitations. It does not process or interact with the media
           of a call. It may be a SIP proxy or redirect server, or an
           H.323 gatekeeper.

3 Example services

   To motivate the subsequent discussion, this section gives some
   specific examples of services which we want users to be able to
   create programmatically.  Note that some of these examples are
   deliberately somewhat complicated, so as to demonstrate the level of
   decision logic that should be possible.

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      o  Call forward on busy/no answer

         When a new call comes in, the call should ring at the user's
         desk telephone.  If it is busy, the call should always be
         redirected to the user's voicemail box. If, instead, there's no
         answer after four rings, it should also be redirected to his or
         her voicemail, unless it's from a supervisor, in which case it
         should be proxied to the user's cell phone if it is currently

      o  Information address

         A company advertises a general "information" address for
         prospective customers. When a call comes in to this address, if
         it's currently working hours, the caller should be given a list
         of the people currently willing to accept general information
         calls. If it's outside of working hours, the caller should get
         a webpage indicating what times they can call.

      o  Intelligent user location

         When a call comes in, the list of locations where the user has
         registered should be consulted. Depending on the type of call
         (work, personal, etc.), the call should ring at an appropriate
         subset of the registered locations, depending on information in
         the registrations. If the user picks up from more than one
         station, the pick-ups should be reported back separately to the
         calling party.

      o  Intelligent user location with media knowledge

         When a call comes in, the call should be proxied to the station
         the user has registered from whose media capabilities best
         match those specified in the call request. If the user does not
         pick up from that station within four rings, the call should be
         proxied to the other stations from which he or she has
         registered, sequentially, in order of decreasing closeness of

      o  Client billing allocation -- lawyer's office

         When a call comes in, the calling address is correlated with
         the corresponding client, and client's name, address, and the
         time of the call is logged. If no corresponding client is
         found, the call is forwarded to the lawyer's secretary.

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4 Usage scenarios

   A CPL would be useful for implementing services in a number of
   different scenarios.

      o  Script creation by end user

         In the most direct approach for creating a service with a CPL,
         an end user simply creates a script describing their service.
         He or she simply decides what service he or she wants,
         describes it using a CPL script, and then uploads it to a

      o  Third party outsourcing

         Because a CPL is a standardized language, it can also be used
         to allow third parties to create or customize services for
         clients. These scripts can then be run on servers owned by the
         end user or the user's service provider.

      o  Administrator service definition

         A CPL can also be used by server administrators to create
         simple services or describe policy for servers they control.
         If a server is implementing CPL services in any case, extending
         the service architecture to allow administrators as well as
         users to create scripts is a simple extension.

      o  Web middleware

         Finally, there have been a number of proposals for service
         creation or customization using web interfaces. A CPL could be
         used as the back-end to such environments: a web application
         could create a CPL script on behalf of a user, and the
         telephony server could then implement the services without
         either component having to be aware of the specifics of the

5 CPL creation

   There are also a number of means by which CPL scripts could be
   created.  Like HTML, which can be created in a number of different
   manners, we envision multiple creation styles for a CPL script.

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      o  Hand authoring

         Most directly, CPL scripts can be created by hand, by
         knowledgeable users.  The CPL described in [5] has a text
         format with an uncomplicated syntax, so hand authoring will be

      o  Automated scripts

         CPL features can be created by automated means, such as in the
         example of the web middleware described in the previous
         section. With a simple, text-based syntax, standard text-
         processing languages will be able to create and edit CPL
         scripts easily.

      o  GUI tools

         Finally, users will be able to use GUI tools to create and edit
         CPL scripts.  We expect that most average-experience users will
         take this approach once the CPL gains popularity.  The CPL will
         be designed with this application in mind, so that the full
         expressive power of scripts can be represented simply and
         straightforwardly in a graphical manner.

6 Network model

   The Call Processing Language operates on a generalized model of an
   Internet telephony network. While the details of various protocols
   differ, on an abstract level all major Internet telephony
   architectures are sufficiently similar that their major features can
   be described commonly. This document generally uses SIP terminology,
   as its authors' experience has mainly been with that protocol.

6.1 Model components

   In the Call Processing Language's network model, an Internet
   telephony network contains two types of components.

6.1.1 End systems

   End systems are devices which originate and/or receive signalling
   information and media. These include simple and complex telephone
   devices, PC telephony clients, and automated voice systems. The CPL
   abstracts away the details of the capabilities of these devices. An
   end system can originate a call; and it can accept, reject, or
   forward incoming calls. The details of this process (ringing, multi-
   line telephones, and so forth) are not important for the CPL.

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   For the purposes of the CPL, gateways -- for example, a device which
   connects calls between an IP telephony network and the PSTN -- are
   also considered to be end systems. Other devices, such as mixers or
   firewalls, are not directly dealt with by the CPL, and they will not
   be discussed here.

6.1.2 Signalling servers

   Signalling servers are devices which relay or control signalling
   information. In SIP, they are proxy servers, redirect servers, or
   registrars; in H.323, they are gatekeepers.

   Signalling servers can perform three types of actions on call setup
   information. They can:

      proxy it: forward it on to one or more other network or end
           systems, returning one of the responses received.

      redirect it: return a response informing the sending system of a
           different address to which it should send the request.

      reject it: inform the sending system that the setup request could
           not be completed.

   RFC 2543 [1] has illustrations of proxy and redirect functionality.
   End systems may also be able to perform some of these actions: almost
   certainly rejection, and possibly redirection.

   Signalling servers also normally maintain information about user
   location.  Whether by means of registrations (SIP REGISTER or H.323
   RAS messages), static configuration, or dynamic searches, signalling
   servers must have some means by which they can determine where a user
   is currently located, in order to make intelligent choices about
   their proxying or redirection behavior.

   Signalling servers are also usually able to keep logs of transactions
   that pass through them, and to send e-mail to destinations on the
   Internet, under programmatic control.

6.2 Component interactions

   When an end system places a call, the call establishment request can
   proceed by a variety of routes through components of the network. To
   begin with, the originating end system must decide where to send its
   requests. There are two possibilities here: the originator may be
   configured so that all its requests go to a single local server; or
   it may resolve the destination address to locate a remote signalling
   server or end system to which it can send the request directly.

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   Once the request arrives at a signalling server, that server uses its
   user location database, its local policy, DNS resolution, or other
   methods, to determine the next signalling server or end system to
   which the request should be sent. A request may pass through any
   number of signalling servers: from zero (in the case when end systems
   communicate directly) to, in principle, every server on the network.
   What's more, any end system or signalling server can (in principle)
   receive requests from or send them to any other.

   For example, in figure 1, there are two paths the call establishment
   request information may take. For Route 1, the originator knows only
   a user address for the user it is trying to contact, and it is
   configured to send outgoing calls through a local outgoing proxy
   server.  Therefore, it forwards the request to its local server,
   which finds the server of record for that address, and forwards it on
   to that server.

   In this case, the organization the destination user belongs to uses a
   multi-stage setup to find users. The corporate server identifies
   which department a user is part of, then forwards the request to the
   appropriate departmental server, which actually locates the user.
   (This is similar to the way e-mail forwarding is often configured.)
   The response to the request will travel back along the same path.

   For Route 2, however, the originator knows the specific device
   address it is trying to contact, and it is not configured to use a
   local outgoing proxy.  In this case, the originator can directly
   contact the destination without having to communicate with any
   network servers at all.

   We see, then, that in Internet telephony signalling servers cannot in
   general know the state of end systems they "control," since
   signalling information may have bypassed them. This architectural
   limitation implies a number of restrictions on how some services can
   be implemented. For instance, a network system cannot reliably know
   if an end system is currently busy or not; a call may have been
   placed to the end system without traversing that network system.
   Thus, signalling messages must explicitly travel to end systems to
   find out their state; in the example, the end system must explicitly
   return a "busy" indication.

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      Outgoing                           Corporate        Departmental
        Proxy                              Server            Server
       _______  Outgoing proxy contacts   _______            _______
       |     |     corporate server       |     |            |     |
       |     | -------------------------> |     | ---------> |     |
       |_____|                            |_____|            |_____|
Route 1   ^                                                    \Searches
         /                                                      \   for
Sends to/                                                        \ User
 proxy /                                                         _|
   _______                                                      _______
   |     |   Route 2                                            |     |
   |     | ---------------------------------------------------> |     |
   |_____|      Originator directly contacts destination        |_____|

  Originator                                                 Destination

         Figure 1: Possible paths of call setup messages

7 Interaction of CPL with network model

7.1 What a script does

   A CPL script runs in a signalling server, and controls that system's
   proxy, redirect, or rejection actions for the set-up of a particular
   call. It does not attempt to coordinate the behavior of multiple
   signalling servers, or to describe features on a "Global Functional
   Plane" as in the Intelligent Network architecture [6].

   More specifically, a script replaces the user location functionality
   of a signalling server. As described in section 6.1.2, a signalling
   server typically maintains a database of locations where a user can
   be reached; it makes its proxy, redirect, and rejection decisions
   based on the contents of that database. A CPL script replaces this
   basic database lookup functionality; it takes the registration
   information, the specifics of a call request, and other external
   information it wants to reference, and chooses the signalling actions
   to perform.

   Abstractly, a script can be considered as a list of condition/action
   pairs; if some attribute of the registration, request, and external
   information matches a given condition, then the corresponding action
   (or more properly set of actions) is taken. In some circumstances,
   additional actions can be taken based on the consequences of the
   first action and additional conditions. If no condition matches the
   invitation, the signalling server's standard action -- its location
   database lookup, for example -- is taken.

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7.2 Which script is executed

   CPL scripts are usually associated with a particular Internet
   telephony address. When a call establishment request arrives at a
   signalling server which is a CPL server, that server associates the
   source and destination addresses specified in the request with its
   database of CPL scripts; if one matches, the corresponding script is

   Once the script has executed, if it has chosen to perform a proxy
   action, a new Internet telephony address will result as the
   destination of that proxying. Once this has occurred, the server
   again checks its database of scripts to see if any of them are
   associated with the new address; if one is, that script as well is
   executed (assuming that a script has not attempted to proxy to an
   address which the server has already tried). For more details of this
   recursion process, and a description of what happens when a server
   has scripts that correspond both to a scripts origination address and
   its destination address, see section 9.2.

   In general, in an Internet telephony network, an address will denote
   one of two things: either a user, or a device. A user address refers
   to a particular individual, for example sip:joe@example.com,
   regardless of where that user actually is or what kind of device he
   or she is using. A device address, by contrast, refers to a
   particular physical device, such as sip:x26063@phones.example.com.
   Other, intermediate sorts of addresses are also possible, and have
   some use (such as an address for "my cell phone, wherever it
   currently happens to be registered"), but we expect them to be less
   common. A CPL script is agnostic to the type of address it is
   associated with; while scripts associated with user addresses are
   probably the most useful for most services, there is no reason that a
   script could not be associated with any other type of address as
   well.  The recursion process described above allows scripts to be
   associated with several of a user's addresses; thus, a user script
   could specify an action "try me at my cell phone," whereas a device
   script could say "I don't want to accept cell phone calls while I'm
   out of my home area."

   It is also possible for a CPL script to be associated not with one
   specific Internet telephony address, but rather with all addresses
   handled by a signalling server, or a large set of them. For instance,
   an administrator might configure a system to prevent calls from or to
   a list of banned incoming or outgoing addresses; these should
   presumably be configured for everyone, but users should still to be
   able to have their own custom scripts as well. Exactly when such

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   scripts should be executed in the recursion process depends on the
   precise nature of the administrative script. See section 9.2 for
   further discussion of this.

7.3 Where a script runs

   Users can have CPL scripts on any network server which their call
   establishment requests pass through and with which they have a trust
   relationship. For instance, in the example in figure 1, the
   originating user could have a script on the outgoing proxy, and the
   destination user could have scripts on both the corporate server and
   the departmental server. These scripts would typically perform
   different functions, related to the role of the server on which they
   reside; a script on the corporate-wide server could be used to
   customize which department the user wishes to be found at, for
   instance, whereas a script at the departmental server could be used
   for more fine-grained location customization. Some services, such as
   filtering out unwanted calls, could be located at either server. See
   section 9.3 for some implications of a scenario like this.

   This model does not specify the means by which users locate a CPL-
   capable network server. In general, this will be through the same
   means by which they locate a local Internet telephony server to
   register themselves with; this may be through manual configuration,
   or through automated means such as the Service Location Protocol [7].
   It has been proposed that automated means of locating such servers
   should include a field to indicate whether the server allows users to
   upload CPLs.

8 Creation and transport of a call processing language script

   Users create call processing language scripts, typically on end
   devices, and transmit them through the network to signalling servers.
   Scripts persist in signalling servers until changed or deleted,
   unless they are specifically given an expiration time; a network
   system which supports CPL scripting will need stable storage.

   The end device on which the user creates the CPL script need not bear
   any relationship to the end devices to which calls are actually
   placed. For example, a CPL script might be created on a PC, whereas
   calls might be intended to be received on a simple audio-only
   telephone.  Indeed, the device on which the script is created may not
   be an "end device" in the sense described in section 6.1.1 at all;
   for instance, a user could create and upload a CPL script from a
   non-multimedia-capable web terminal.

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   The CPL also might not necessarily be created on a device near either
   the end device or the signalling server in network terms. For
   example, a user might decide to forward his or her calls to a remote
   location only after arriving at that location.

   The exact means by which the end device transmits the script to the
   server remains to be determined; it is likely that many solutions
   will be able to co-exist. This method will need to be authenticated
   in almost all cases.  The methods that have been suggested include
   web file upload, SIP REGISTER message payloads, remote method
   invocation, SNMP, ACAP, LDAP, and remote file systems such as NFS.

   Users can also retrieve their current script from the network to an
   end system so it can be edited. The signalling server should also be
   able to report errors related to the script to the user, both static
   errors that could be detected at upload time, and any run-time errors
   that occur.

   If a user has trust relationships with multiple signalling servers
   (as discussed in section 7.3), the user may choose to upload scripts
   to any or all of those servers. These scripts can be entirely

9 Feature interaction behavior

   Feature interaction is the term used in telephony systems when two or
   more requested features produce ambiguous or conflicting behavior
   [8]. Feature interaction issues for features implemented with a call
   processing language can be roughly divided into three categories:
   feature-to-feature in one server, script-to-script in one server, and

9.1 Feature-to-feature interactions

   Due to the explicit nature of event conditions discussed in the
   previous section, feature-to-feature interaction is not likely to be
   a problem in a call processing language environment. Whereas a
   subscriber to traditional telephone features might unthinkingly
   subscribe to both "call waiting" and "call forward on busy," a user
   creating a CPL script would only be able to trigger one action in
   response to the condition "a call arrives while the line is busy."
   Given a good user interface for creation, or a CPL server which can
   check for unreachable code in an uploaded script, contradictory
   condition/action pairs can be avoided.

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9.2 Script-to-script interactions

   Script-to-script interactions arise when a server invokes multiple
   scripts for a single call, as described in section 7.2.  This can
   occur in a number of cases: if both the call originator and the
   destination have scripts specified on a single server; if a script
   forwards a request to another address which also has a script; or if
   an administrative script is specified as well as a user's individual

   The solution to this interaction is to determine an ordering among
   the scripts to be executed. In this ordering, the "first" script is
   executed first; if this script allows or permits the call to be
   proxied, the script corresponding to the next address is executed.
   When the first script says to forward the request to some other
   address, those actions are considered as new requests which arrive at
   the second script. When the second script sends back a final
   response, that response arrives at the first script in the same
   manner as if a request arrived over the network. Note that in some
   cases, forwarding can be recursive; a CPL server must be careful to
   prevent forwarding loops.

   Abstractly, this can be viewed as equivalent to having each script
   execute on a separate signalling server. Since the CPL architecture
   is designed to allow scripts to be executed on multiple signalling
   servers in the course of locating a user, we can conceptually
   transform script-to-script interactions into the server-to-server
   interactions described in the next section, reducing the number of
   types of interactions we need to concern ourselves with.

   The question, then, is to determine the correct ordering of the
   scripts.  For the case of a script forwarding to an address which
   also has a script, the ordering is obvious; the other two cases are
   somewhat more subtle. When both originator and destination scripts
   exist, the originator's script should be executed before the
   destination script; this allows the originator to perform address
   translation, call filtering, etc., before a destination address is
   determined and a corresponding script is chosen.

   Even more complicated is the case of the ordering of administrative
   scripts. Many administrative scripts, such as ones that restrict
   source and destination addresses, need to be run after originator
   scripts, but before destination scripts, to avoid a user's script
   evading administrative restrictions through clever forwarding;
   however, others, such as a global address book translation function,
   would need to be run earlier or later.  Servers which allow

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   administrative scripts to be run will need to allow the administrator
   to configure when in the script execution process a particular
   administrative script should fall.

9.3 Server-to-server interactions

   The third case of feature interactions, server-to-server
   interactions, is the most complex of these three. The canonical
   example of this type of interaction is the combination of Originating
   Call Screening and Call Forwarding: a user (or administrator) may
   wish to prevent calls from being placed to a particular address, but
   the local script has no way of knowing if a call placed to some
   other, legitimate address will be proxied, by a remote server, to the
   banned address. This type of problem is unsolvable in an
   administratively heterogeneous network, even a "lightly"
   heterogeneous network such as current telephone systems. CPL does not
   claim to solve it, but the problem is not any worse for CPL scripts
   than for any other means of deploying services.

   Another class of server-to-server interactions are best resolved by
   the underlying signalling protocol, since they can arise whether the
   signalling servers are being controlled by a call processing language
   or by some entirely different means. One example of this is
   forwarding loops, where user X may have calls forwarded to Y, who has
   calls forwarded back to X. SIP has a mechanism to detect such loops.
   A call processing language server thus does not need to define any
   special mechanisms to prevent such occurrences; it should, however,
   be possible to trigger a different set of call processing actions in
   the event that a loop is detected, and/or to report back an error to
   the owner of the script through some standardized run-time error
   reporting mechanism.

9.4 Signalling ambiguity

   As an aside, [8] discusses a fourth type of feature interaction for
   traditional telephone networks, signalling ambiguity. This can arise
   when several features overload the same operation in the limited
   signal path from an end station to the network: for example, flashing
   the switch-hook can mean both "add a party to a three-way call" and
   "switch to call waiting." Because of the explicit nature of
   signalling in both the Internet telephony protocols discussed here,
   this issue does not arise.

10 Relationship with existing languages

   This document's description of the CPL as a "language" is not
   intended to imply that a new language necessarily needs to be
   implemented from scratch.  A server could potentially implement all

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   the functionality described here as a library or set of extensions
   for an existing language; Java, or the various freely-available
   scripting languages (Tcl, Perl, Python, Guile), are obvious

   However, there are motivations for creating a new language. All the
   existing languages are, naturally, expressively complete; this has
   two inherent disadvantages. The first is that any function
   implemented in them can take an arbitrarily long time, use an
   arbitrarily large amount of memory, and may never terminate. For call
   processing, this sort of resource usage is probably not necessary,
   and as described in section 12.1, may in fact be undesirable. One
   model for this is the electronic mail filtering language Sieve [4],
   which deliberately restricts itself from being Turing-complete.

   Similar levels of safety and protection (though not automatic
   generation and parsing) could also be achieved through the use of a
   "sandbox" such as is used by Java applets, where strict bounds are
   imposed on the amount of memory, cpu time, stack space, etc., that a
   program can use. The difficulty with this approach is primarily in
   its lack of transparency and portability:  unless the levels of these
   bounds are imposed by the standard, a bad idea so long as available
   resources are increasing exponentially with Moore's Law, a user can
   never be sure whether a particular program can successfully be
   executed on a given server without running into the server's resource
   limits, and a program which executes successfully on one server may
   fail unexpectedly on another. Non-expressively-complete languages, on
   the other hand, allow an implicit contract between the script writer
   and the server:  so long as the script stays within the rules of the
   language, the server will guarantee that it will execute the script.

   The second disadvantage with expressively complete languages is that
   they make automatic generation and parsing of scripts very difficult,
   as every parsing tool must be a full interpreter for the language. An
   analogy can be drawn from the document-creation world: while text
   markup languages like HTML or XML can be, and are, easily manipulated
   by smart editors, powerful document programming languages such as
   LaTeX or Postscript usually cannot be. While there are word
   processors that can save their documents in LaTeX form, they cannot
   accept as input arbitrary LaTeX documents, let alone preserve the
   structure of the original document in an edited form. By contrast,
   essentially any HTML editor can edit any HTML document from the web,
   and the high-quality ones preserve the structure of the original
   documents in the course of editing them.

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11 Related work

11.1 IN service creation environments

   The ITU's IN series describe, on an abstract level, service creation
   environments [6]. These describe services in a traditional circuit-
   switched telephone network as a series of decisions and actions
   arranged in a directed acyclic graph. Many vendors of IN services use
   modified and extended versions of this for their proprietary service
   creation environments.

11.2 SIP CGI

   SIP CGI [9] is an interface for implementing services on SIP servers.
   Unlike a CPL, it is a very low-level interface, and would not be
   appropriate for services written by non-trusted users.

   The paper "Programming Internet Telephony Services" [10] discusses
   the similarities and contrasts between SIP CGI and CPL in more

12 Necessary language features

   This section lists those properties of a call processing language
   which we believe to be necessary to have in order to implement the
   motivating examples, in line with the described architecture.

12.1 Language characteristics

   These are some abstract attributes which any proposed call processing
   language should possess.

      o  Light-weight, efficient, easy to implement

         In addition to the general reasons why this is desirable, a
         network server might conceivably handle very large call
         volumes, and we don't want CPL execution to be a major
         bottleneck. One way to achieve this might be to compile scripts
         before execution.

      o  Easily verifiable for correctness

         For a script which runs in a server, mis-configurations can
         result in a user becoming unreachable, making it difficult to
         indicate run-time errors to a user (though a second-channel
         error reporting mechanism such as e-mail could ameliorate
         this). Thus, it should be possible to verify, when the script

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         is committed to the server, that it is at least syntactically
         correct, does not have any obvious loops or other failure
         modes, and does not use too many server resources.

      o  Executable in a safe manner

         No action the CPL script takes should be able to subvert
         anything about the server which the user shouldn't have access
         to, or affect the state of other users without permission.
         Additionally, since CPL scripts will typically run on a server
         on which users cannot normally run code, either the language or
         its execution environment must be designed so that scripts
         cannot use unlimited amounts of network resources, server CPU
         time, storage, or memory.

      o  Easily writeable and parsable by both humans and machines.

         For maximum flexibility, we want to allow humans to write their
         own scripts, or to use and customize script libraries provided
         by others. However, most users will want to have a more
         intuitive user-interface for the same functionality, and so
         will have a program which creates scripts for them.  Both cases
         should be easy; in particular, it should be easy for script
         editors to read human-generated scripts, and vice-versa.

      o  Extensible

         It should be possible to add additional features to a language
         in a way that existing scripts continue to work, and existing
         servers can easily recognize features they don't understand and
         safely inform the user of this fact.

      o  Independent of underlying signalling details

         The same scripts should be usable whether the underlying
         protocol is SIP, H.323, a traditional telephone network, or any
         other means of setting up calls. It should also be agnostic to
         address formats. (We use SIP terminology in our descriptions of
         requirements, but this should map fairly easily to other
         systems.) It may also be useful to have the language extend to
         processing of other sorts of communication, such as e-mail or

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12.2 Base features -- call signalling

   To be useful, a call processing language obviously should be able to
   react to and initiate call signalling events.

      o  Should execute actions when a call request arrives

         See section 7, particularly 7.1.

      o  Should be able to make decisions based on event properties

         A number of properties of a call event are relevant for a
         script's decision process. These include, roughly in order of

         -  Destination address

            We want to be able to do destination-based routing or
            screening.  Note that in SIP we want to be able to filter on
            either or both of the addresses in the To header and the

         -  Originator address

            Similarly, we want to be able to do originator-based
            screening or routing.

         -  Caller Preferences

            In SIP, a caller can express preferences about the type of
            device to be reached -- see [11]. The script should be able
            to make decisions based on this information.

         -  Information about caller or call

            SIP has textual fields such as Subject, Organization,
            Priority, etc., and a display name for addresses; users can
            also add non-standard additional headers. H.323 has a single
            Display field. The script should be able to make decisions
            based on these parameters.

         -  Media description

            Call invitations specify the types of media that will flow,
            their bandwidth usage, their network destination addresses,
            etc. The script should be able to make decisions based on
            these media characteristics.

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         -  Authentication/encryption status

            Call invitations can be authenticated. Many properties of
            the authentication are relevant: the method of
            authentication/encryption, who performed the authentication,
            which specific fields were encrypted, etc.  The script
            should be able to make decisions based on these security

      o  Should be able to take action based on a call invitation

         There are a number of actions we can take in response to an
         incoming call setup request. We can:

         -  reject it

            We should be able to indicate that the call is not
            acceptable or not able to be completed. We should also be
            able to send more specific rejection codes (including, for
            SIP, the associated textual string, warning codes, or
            message payload).

         -  redirect it

            We should be able to tell the call initiator sender to try a
            different location.

         -  proxy it

            We should be able to send the call invitation on to another
            location, or to several other locations ("forking" the
            invitation), and await the responses. It should also be
            possible to specify a timeout value after which we give up
            on receiving any definitive responses.

      o  Should be able to take action based a response to a proxied or
         forked call invitation

         Once we have proxied an invitation, we need to be able to make
         decisions based on the responses we receive to that invitation
         (or the lack thereof).  We should be able to:

         -  consider its message fields

            We should be able to consider the same fields of a response
            as we consider in the initial invitation.

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         -  relay it on to the call originator

            If the response is satisfactory, it should be returned to
            the sender.

         -  for a fork, choose one of several responses to relay back

            If we forked an invitation, we obviously expect to receive
            several responses. There are several issues here -- choosing
            among the responses, and how long to wait if we've received
            responses from some but not all destinations.

         -  initiate other actions

            If we didn't get a response, or any we liked, we should be
            able to try something else instead (e.g., call forward on

12.3 Base features -- non-signalling

   A number of other features that a call processing language should
   have do not refer to call signalling per se; however, they are still
   extremely desirable to implement many useful features.

   The servers which provide these features might reside in other
   Internet devices, or might be local to the server (or other
   possibilities). The language should be independent of the location of
   these servers, at least at a high level.

      o  Logging

         In addition to the CPL server's natural logging of events, the
         user will also want to be able to log arbitrary other items.
         The actual storage for this logging information might live
         either locally or remotely.

      o  Error reporting

         If an unexpected error occurs, the script should be able to
         report the error to the script's owner. This may use the same
         mechanism as the script server uses to report language errors
         to the user (see section 12.5).

      o  Access to user-location info

         Proxies will often collect information on users' current
         location, either through SIP REGISTER messages, the H.323 RRQ
         family of RAS messages, or some other mechanism (see section

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         6.2). The CPL should be able to refer to this information so a
         call can be forwarded to the registered locations or some
         subset of them.

      o  Database access

         Much information for CPL control might be stored in external
         databases, for example a wide-area address database, or
         authorization information, for a CPL under administrative
         control. The language could specify some specific database
         access protocols (such as SQL or LDAP), or could be more

      o  Other external information

         Other external information a script could access includes web
         pages, which could be sent back in a SIP message body; or a
         clean interface to remote procedure calls such as Corba, RMI,
         or DCOM, for instance to access an external billing database.
         However, for simplicity, these interfaces may not be in the
         initial version of the protocol.

12.4 Language features

   Some features do not involve any operations external to the CPL's
   execution environment, but are still necessary to allow some standard
   services to be implemented. (This list is not exhaustive.)

      o  Pattern-matching

         It should be possible to give special treatment to addresses
         and other text strings based not only on the full string but
         also on more general or complex sub-patterns of them.

      o  Address filtering

         Once a set of addresses has been retrieved through one of the
         methods in section 12.3, the user needs to be able to choose a
         sub-set of them, based on their address components or other

      o  Randomization

         Some forms of call distribution are randomized as to where they
         actually end up.

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      o  Date/time information

         Users may wish to condition some services (e.g., call
         forwarding, call distribution) on the current time of day, day
         of the week, etc.

12.5 Control

   As described in section 8, we must have a mechanism to send and
   retrieve CPL scripts, and associated data, to and from a signalling
   server. This method should support reporting upload-time errors to
   users; we also need some mechanism to report errors to users at
   script execution time. Authentication is vital, and encryption is
   very useful. The specification of this mechanism can be (and probably
   ought to be) a separate specification from that of the call
   processing language itself.

13 Security Considerations

   The security considerations of transferring CPL scripts are discussed
   in sections 8 and 12.5. Some considerations about the execution of
   the language are discussed in section 12.1.

14 Acknowledgments

   We would like to thank Tom La Porta and Jonathan Rosenberg for their
   comments and suggestions.

15 Authors' Addresses

   Jonathan Lennox
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY 10027

   EMail: lennox@cs.columbia.edu

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY 10027

   EMail: schulzrinne@cs.columbia.edu

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16 Bibliography

   [1]  Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
        "SIP: Session Initiation Protocol", RFC 2543, March 1999.

   [2]  International Telecommunication Union, "Packet based multimedia
        communication systems," Recommendation H.323, Telecommunication
        Standardization Sector of ITU, Geneva, Switzerland, Feb. 1998.

   [3]  K. Coar and D. Robinson, "The WWW common gateway interface
        version 1.1", Work in Progress.

   [4]  T. Showalter, "Sieve: A mail filtering language", Work in

   [5]  J. Lennox and H. Schulzrinne, "CPL: a language for user control
        of internet telephony services", Work in Progress.

   [6]  International Telecommunication Union, "General recommendations
        on telephone switching and signaling -- intelligent network:
        Introduction to intelligent network capability set 1,"
        Recommendation Q.1211, Telecommunication Standardization Sector
        of ITU, Geneva, Switzerland, Mar. 1993.

   [7]  Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
        Location Protocol, Version 2", RFC 2608, June 1999.

   [8]  E. J. Cameron, N. D. Griffeth, Y.-J. Lin, M. E. Nilson, W. K.
        Schure, and H. Velthuijsen, "A feature interaction benchmark for
        IN and beyond," Feature Interactions in Telecommunications
        Systems, IOS Press, pp. 1-23, 1994.

   [9]  J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
        interface for SIP", Work in Progress.

   [10] J. Rosenberg, J. Lennox, and H. Schulzrinne, "Programming
        internet telephony services," Technical Report CUCS-010-99,
        Columbia University, New York, New York, Mar. 1999.

   [11] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
        callee capabilities", Work in Progress.

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17 Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
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