Internet Engineering Task Force                                  IPTEL WG
Internet Draft                                         Lennox/Schulzrinne
ietf-iptel-cpl-requirements-00.txt   Lucent Bell Labs/Columbia University
July 30, 1998
Expires: February 1999

                 Call Processing Language Requirements


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         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
         architecture for such a method, which we call a call
         processing language. It also outlines requirements for
         such a language.

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

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   and dramatic decentralization of the provisioning of telephone
   services so they can be under the user's control.

   Many of these services may reside on end devices. A broad set of
   services, however -- those involving user location, call
   distribution, behavior-on-busy, and the like -- are independent of a
   particular end device, or need to be operational even when an end
   device is unavailable.  These still best reside in a network device
   rather than an end system. To allow user control over such devices,
   we need a standardized way for end-users to specify the precise
   behavior of the servers. This document proposes an architecture in
   which network devices or end systems respond to call signalling
   events by triggering user-created programs which control the reaction
   to the events.

   For reasons discussed in section 3.7, this document proposes a
   relatively static, non-expressively-complete language to solve this
   problem. We call this a call processing language.  However, most of
   the requirements this document lists apply equally well to a library
   of call processing routines for an existing language.

2 Motivating examples

   These are some specific examples of services which we want to be able
   to create programatically. They are arranged roughly in order of
   increasing requirements they impose. Note that some of these examples
   are deliberately somewhat complicated, so as to demonstrate the level
   of decision logic that should be possible.

        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 superviser, in which case it
        should be proxied to the user's cellphone if it has registered.

        o Administrative screening -- firewall

        An outgoing call should be rejected if it is going to any
        destination that is on a "banned" list. Otherwise, it should be
        forwarded on to the appropriate destination; if the destination
        accepts the call, the firewall should be told to open up the UDP
        host/port pairs the two endpoints specified for their media. The
        same thing should be done for incoming calls, checking the
        origination address.

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        o Central phone server

        If a call comes in for a specific person, it should be
        redirected to the locations where they can currently be found.
        If a call comes in for the general "information" address we've
        advertised, 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 gets a recorded message indicating what times they can

        o Intelligent user location

        When a call comes in, it should ring at every station from which
        the user has registered. 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 Intelligent user location with mixer (home phone)

        When a call comes in, it should ring at every station from which
        the user has registered. If the call is picked up from more than
        one station, the media from each station should be transparently
        mixed together and sent to the caller.

        o Third-party registration control

        When a registration arrives for a user, make sure that the
        registration was authenticated, the person performing the
        registration has permission to perform the registration for the
        specified user, and the location registered is allowed for the
        registered user. If so, enter it in the registration database;
        if not, reject it.

        o Calendarbook access

        When a call comes in, the user's on-line calendar should be
        consulted. If it specifies that the user has a meeting scheduled

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        for this time, the caller should get a busy indication.
        Otherwise, the call should be directed to the user's office

        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 and the time and
        duration of the call are logged. If no corresponding client is
        found, the call is forwarded to the lawyer's secretary.

        o End system busy

        When a new call comes in, if the user is currently in a call, a
        call-waiting tone is generated, unless one of the calls
        currently in progress is with the user's boss or he or she has
        set "Do Not Disturb" in the user interface, in which case the
        caller gets a busy indication.

        o Phone bank (call distribution/queueing)

        Incoming calls should be distributed to the phone-bank workers,
        so that each worker handles approximately the same total number
        of calls. If all the phone-bank workers are busy, calls should
        be queued until someone is available. Calls coming from
        preferred customers should get priority in the queue. If the
        length of the queue grows to twice the size of the phone bank,
        calls should be directed to management as well until the queue
        length has decreased again. Each caller should be given an
        approximate indication of waiting time or number of calls ahead
        of them in the queue as they wait.  Callers should be given the
        option of listening to the music-on-hold of their choice.

3 Architecture

3.1 Network components

   A network which supports Internet telephony consists of two types of
   components: end systems, which originate and/or receive media, and
   network systems, which relay signalling information.

   End systems are either user agents, which reach actual people, or
   automated systems, which do not; this document will deal primarily
   with the former.  Network systems, in SIP, are proxy servers,
   redirect servers, or registrars; in H.323 they are gatekeepers. The
   functionality between the two protocols is largely equivalent, and
   this document will generally use the SIP names.  Proxy servers are
   network systems which receive a request and forward it on.  Redirect

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   servers tell the originating location an alternate location to try.
   Registrars track users' current locations; they will usually be the
   same devices as proxy or redirect servers, but do not necessarily
   have to be.  See the illustrations in [1]. End systems may also have
   some properties of network systems, most likely the ability to
   perform redirection.

3.2 Model of normal use

     Local Signalling Server                Remote Signalling Server
      locates destination                     finds location for
                                                  permanent address
         _______         Local server contacts           _______       _______
         |     |           permanent address             |     |       |     |
         |     | --------------------------------------> |     | ----->|     |
         |_____|  Local                         address  |_____|       |_____|
            ^     ----- server con-   permanent ------->    \  Search  Another
   Send to /           \-----------\ /---------/             \   for   Terminal
    local /                         X                         \  User
   server/               contacts  / \ tacts terminal          _|
     _______  Originator ---------/   \----------- address   _______
     |     |  ----------/                         \------->  |     |
     |     | ----------------------------------------------> |     |
     |_____|      Direct connection to terminal address      |_____|
    Originator                                          has terminal address

   Figure 1: Illustration of call signalling messages

   Internet telephony addresses can be divided into two broad
   categories:  terminal addresses and permanent user addresses. A
   terminal address is one that refers to a particular device, whose
   network-level (IP) address does not change. A permanent user address,
   on the other hand, refers on a more abstract level to an individual
   user, whose current location and network address may change. When a
   user becomes available at a location, his or her end system registers
   itself with a network server indicating this fact. (In SIP, users
   register via the REGISTER message; in H.323, via RRQ and related RAS

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   messages.) A user may register from more than one location

   Figure 1 shows how call signalling may flow. Calls may be placed
   either to terminal or permanent user addresses. Calls to terminal
   addresses may contact the corresponding device directly, or may
   travel through some signalling server. Calls to permanent user
   addresses must pass through the signalling server, which locates the
   user and proxies or redirects the call to the appropriate terminal
   addresses which the user has registered.

   Signalling servers may also be used on the originating side. Rather
   than locate a call's destination on its own, an end system, when
   originating a call, may have been configured to transfer all its call
   requests through a single, presumably local, server. This server can
   then perform the somewhat complex task of actually locating the
   destination, as well as other tasks such as firewall penetration or
   encryption of signalling information.

   Call requests may be forwarded between multiple signalling servers on
   both the origination and destination ends of a call. For example, a
   corporation could have a large company-wide server which forwards
   incoming call requests to individual departmental servers, which then
   perform the task of actually locating the desired user. This is
   similar to a typical configuration of e-mail forwarding.

   Different call invitations for a particular end system might travel
   through a different set of signalling servers; for instance, a user
   with several addresses might register his current end system with
   several different servers. Similarly, an end system placing a call
   might have several different outgoing signalling servers through
   which it could place the call.  Thus, in general, a signalling server
   does not see all the signalling events for a particular end system;
   and so it does not have enough information to be able to determine
   the end system's state.

3.3 Purpose of a call processing language

   A call processing language (CPL) is primarily intended to allow the
   user to modify the way an Internet telephony system handles call
   events. Call events include signalling events such as call setup,
   termination, or parameter changes, and also, for servers with an
   appropriate media path, in-band events such as DTMF tones. The user
   can modify either incoming or outgoing calls.

   The most common sort of modification will be for incoming call
   setups. Some ways a user might want to alter the call setup process
   include: to search terminal addresses for a given user address in an

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   alternate way; to specificy what happens when the initial search
   fails, either when it receives some sort of negative response (e.g.,
   busy), or does not receive any definitive response within a fixed
   time period (e.g., no answer); or to handle certain origination
   addresses specially, for instance by informing the caller that the
   call was refused. The useful changes to the outgoing call setup are
   somewhat more limited in scope, but one example is to translate a
   user's abbreviated addresses into an address specified with a fully-
   qualified domain name. The transformations to parameter changes or
   call terminations are generally only useful to complete the actions
   begun at call setup time; see for instance the lawyer's office
   example in section 2.

   Once a language with this level of power has been introduced, other
   applications of it present themselves. An administrator might wish to
   perform administrative restrictions on users' calls, for instance
   blocking incoming or outgoing calls from certain domains. The
   language could also be scripted on an end system; with minimal
   extensions, behavior specific to end systems, such as the specifics
   of how the user is alerted to incoming calls, could also be made

3.4 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 network systems.
   Scripts persist in network devices until changed or deleted, unless
   they are specifically given an expiration time; a network device
   which supports CPL scripting will need stable storage.

   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 access, SIP REGISTER message payloads, remote method invocation,
   SNMP, LDAP, and remote file systems such as NFS.

   Creation of a CPL script may be through the creation of a text file;
   or for a simpler user experience, a graphical user interface which
   allows the manipulation of some basic rules.

   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. The CPL also might not necessarily be created on a device
   near either the end device or the signalling server in network terms;
   a user might, for example, decide to forward his or her calls to a

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   remote location only after arriving at that location.

   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's calls will pass through multiple local signalling servers
   which know about that user (as discussed in section 3.2), the user
   may choose to upload scripts to any or all of those servers. These
   scripts can be entirely independent; see section 3.6 for some
   implications of this.

   If, as discussed in section 3.3, the call processing language is
   extended to control end systems, the script-creation mechanism
   described above should also be able to create such end-system
   scripts. It may be possible that the end system on which the script
   executes (the simple telephone mentioned before) is not the same
   device as the end system on which the script is created; in this
   case, the script should be transmitted from the script creation site
   to the end system in the same way it is transmitted from creation
   sites to network systems.

3.5 Execution process of a CPL script

   When a call event arrives, a CPL server considers the information in
   the request and determines if any of the scripts it has stored are
   applicable to the call in question. If so, it performs the actions
   corresponding to the matching scripts.

   The most common type of script defines a set of actions to be taken
   for the entire process of call set-up -- from the time a call request
   is initially received, to the time that (from the point of view of
   this device) the call is either definitively accepted or definitively
   rejected. This could be near-instantaneous, if, for instance, the
   script decides to reject the call; or it could be an arbitrarily long
   time, if we are waiting for a call pick-up without a timeout.

   Generally, we expect a script to be structured as a list of
   condition/action pairs; if an incoming invitation matches a given
   condition, then the corresponding action (or more properly set of
   actions) will be taken.  Whether this should be explicit in the
   language or just implicit in the normal usage remains to be seen. If
   no condition matches the invitation, the signalling server's standard
   action should be taken.

   Other types of scripts may define sets of actions to be taken for

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   other call events: call termination; changes to media format or other
   call parameters (re-invitations, in SIP); or in-band call events,
   such as a user sending DTMF tones. However, it is important to note
   that many, if not most, network servers cannot expect to be able to
   observe such events; subsequent signalling information may short-cut
   past the server, as media information almost certainly will.

   While many of the uses of a CPL script are specific to one particular
   user, there are a number of circumstances in which an administrator
   of a signalling server would wish to provide a script which applies
   to all users of the server, or a large set of them. For instance, a
   system might be configured to prevent calls from or to a list of
   banned incoming or outgoing addresses; these should presumably be
   configured for everyone, but users still need to be able to have
   their own custom scripts as well. Similarly, an administrative script
   might perform the necessary operations to allow media to traverse a
   firewall; but individual users' scripts should not have permission to
   perform these operations. See the next section for some implications
   of this.

3.6 Feature interaction behavior

   Feature interaction is the term used in telephony systems when two or
   more requested features produce ambiguous or conflicting behavior
   [3]. 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

   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.

   Script-to-script interactions can arise if both an originator and a
   destination have scripts specified on the same signalling server, or
   if an administrative script and a user's script are both specified.
   In the former case, the correct behavior is fairly obvious: a server
   should first execute the originator's script, and then, if that
   script placed a call to a destination, call the destination script
   with the appropriate conditions.

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   The correct behavior in the latter case depends on the scope of the
   administrative script; however, normally, the administrator's script
   should run after origination scripts, intercepting any proxy or
   redirection decisions, and before recipient scripts, to avoid a
   user's script evading administrative restrictions.

   The third case -- server-to-server interactions -- is the most
   complex of these three. Many such problems are unsolvable in an
   administratively heterogeneous network, even a "lightly"
   heterogeneous network such as current telephone systems. The
   canonical example of this is the interaction 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

   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.

   As an aside, [3] 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.

3.7 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
   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

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   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 5.1, 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. The
   second disadvantage with expressively complete languages is that they
   make automatic generation and parsing very difficult; an analogy can
   be drawn with the difference between markup languages like HTML or
   XML, which can easily be manipulated by smart editors, and powerful
   document programming languages such as Latex or Postscript which
   usually cannot be.

4 Related work

   A future revision of this document will discuss such items as
   decision tree languages, AT&T's TOPS language, the IN service
   creation language, timed state diagrams, the Java servlet API, cgi-
   bin, and active networks.

5 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.

5.1 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

        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).

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        Thus, it should be possible to verify, when the script 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 parseable 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

5.2 Base features -- call signalling

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

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        o Should execute an action script when a call request arrives

        See section 3, particularly 3.5.

        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

             - Event type

             It should be possible to handle call invitations, call
             terminations, user registrations, OPTIONS requests, and
             other distinct call events separately.

             - Originator address

             We want to be able to do originator-based screening or

             - Destination address

             Similarly, we want to be able to do destination-based
             screening or routing.  Note that in SIP we want to be able
             to filter on any or all of the addresses in the To header,
             the Location header, and the Request-URI.

             - 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.

             - Media description

             Requests specify the types of media that will flow, their
             bandwidth usage, their network destination addresses, etc.

             - Authentication/encryption status

             Requests 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.

           o Should be able to take action based on a request

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           There are a number of actions we can take in response to an
           incoming 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).

                - send a provisional response to it

                While a call request is being processed, provisional
                responses such as "Trying," "Ringing," and "Queued" are
                sent back to the caller. It is not clear whether the
                script should specify the sending of such responses
                explicitly, or whether they should be implicit in other
                actions performed.

                - redirect it

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

                - proxy it

                We should be able to send the request on to another
                location, or to several other locations, 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 request

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

                   - consider all its message fields

                   This consists of a similar set of fields as appear in
                   a request.

                   - relay it on to the requestor

                   If the response is satisfactory, it should be

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                   returned to the sender.

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

                   If we forked a request, 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 busy).

5.3 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 should use the same
        mechanism as the script server uses to report language errors to
        the user (see section 5.9).

        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
        3.2). The CPL should be able to refer to this information so a

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        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 the script should be able to 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

        o Creation of and access to local state

        A CPL script may wish to store state information, so that
        scripts invoked for future transactions related to this call or
        this user can have access to decisions made by an earlier
        invocation. For instance, a SIP re-invitation should be proxied
        to the same location as accepted the original invitation,
        regardless of the usual forwarding sequence; a server may wish
        to log the termination of a call in the same way it logged its
        initiation; or a user might want to limit the number of
        concurrent calls or calls per day allowed.  The persistence of
        this state information for a call should be time-limited, either
        explicitly or by default. See section 3.5 for some caveats for a
        network system of expecting to receive events other than call

5.4 Higher-level features

   There are some, more complex services which it would be quite useful
   to be able to describe with a CPL, but which require considerably
   more maintenance of state, elaborate inter-call event triggering, and
   so forth, than the features described earlier.

   It is not clear whether these features should be specified as
   primitives of the language, or whether they should be assembled from
   lower-level features. In the latter case, the language may need to
   have additional low-level features added, beyond those specified in
   the previous sections, so these features can be constructed.

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        o Queueing

        Calls which should go to end systems which aren't accepting
        calls currently can be queued to await delivery. This requires
        inter-call synchronization to know when to take calls off the

        It should be possible for the CPL to specify a priority for a
        queue entry.

        o Call distribution

        Calls can be spread to a number of end systems, in a "phone
        bank" style set-up. Calls need to be directed to exactly one,
        currently available destination, in some fair manner (e.g.,
        hierarchical, round-robin, randomly distributed, or weighted
        fair queueing). In many cases, this means that the proxy system
        needs to be able to track the state of end systems.

        This should also be able to interface with queueing -- if all
        end systems are busy, the call is queued, and when one becomes
        free, the call is taken off the queue.

5.5 Contingent features

   Some features are only useful if other network entities are

        o Access to media servers

        We want to be able to connect a remote call to recorded audio
        (or video) messages.

        o Firewall control

        If we are working in an environment with a firewall, we need to
        be able to tell it to open up a specific host/port 5-tuple for
        our media to flow through.

        We should be able to specify authentication so the firewall
        knows it can trust the proxy.

        o Mixer/translator control

        If we have a media mixer or translator available, we want to be
        able to tell it to mix media between several addresses, with
        fine-grained control over what media flows to where, in what

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5.6 End-system specific features

   Some features can only be implemented in end-systems, either because
   some end-system state is not generally communicated over the network,
   or because there is no protocol to signal that actions need to be
   performed. If we want these features to be implementable with the
   CPL, these additional operations will be necessary.

        o Access to current calling state

        We want to know how many other calls are in progress, who they
        are with, etc.

        o Access to additional user interface state

        The end system's user interface might present options which the
        user could specify on a per-call basis (for example, setting "do
        not disturb").

        o Control of user notification UI

        This will allow such features as custom distinctive ringing,
        call-waiting tones (in combination with the call-state query),
        "reminder ring" for call forwarding, etc.

5.7 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 Randomization

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

        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.

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5.8 Future features

   A number of services which don't exist yet or aren't widely deployed
   in the Internet will be relevant for Internet telephony. Once they
   are available, a CPL script should be able to control them.

        o Ability to specify quality-of-service

        Certain calls -- either on the basis of importance, or for
        known-troublesome destinations -- should be able to have their
        desired quality of service specified by the language.

        o Access to wide-area service location information

        A proxy doing call distribution might want to locate the service
        "closest" (in any one of a number of senses) to the caller; or
        we might want to find a PSTN gateway close to the destination of
        a PSTN-style call. In either case a script should be able to
        control these operations.

        o Control of payment authorization

        If any kind of per-call billing is required, a CPL might want to
        be able to decide whether to accept charges. This is obviously a
        rather delicate operation from a security standpoint.

5.9 Control

   As described in section 3.4, 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.

6 Security considerations

   The security considerations of transferring CPL scripts are discussed
   in sections 3.4 and 5.9. Some considerations about the execution of
   the language are discussed in section 5.1.

7 Acknowledgments

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

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8 Authors' Addresses

   Jonathan Lennox
   Lucent Technologies, Bell Laboratories
   Rm. 4F-520
   101 Crawfords Corner Road
   Holmdel, NJ 07733
   electronic mail:

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   electronic mail:

9 Bibliography

   [1] M. Handley, H. Schulzrinne, and E. Schooler, "SIP: session
   initiation protocol," Internet Draft, Internet Engineering Task
   Force, May 1998.  Work in progress.

   [2] International Telecommunication Union, "Visual telephone systems
   and equipment for local area networks which provide a non-guaranteed
   quality of service," Recommendation H.323, Telecommunication
   Standardization Sector of ITU, Geneva, Switzerland, May 1996.

   [3] E. J. Cameron, N. D. Griffeth, Y.-J. Lin, M. E. Nilson, and et
   al, "A feature interaction benchmark for IN and beyond," Feature
   Interactions in Telecommunications Systems, IOS Press , pp. 1--23,

   [4] T. Showalter, "Sieve -- a mail filtering language," Internet
   Draft, Internet Engineering Task Force, Jan. 1998.  Work in progress.

   Full Copyright Statement

   Copyright (c) The Internet Society (1998). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this

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   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

                           Table of Contents

   1          Introduction ........................................    1
   2          Motivating examples .................................    2
   3          Architecture ........................................    4
   3.1        Network components ..................................    4
   3.2        Model of normal use .................................    5
   3.3        Purpose of a call processing language ...............    6
   3.4        Creation and transport of a call processing
   language script ................................................    7
   3.5        Execution process of a CPL script ...................    8
   3.6        Feature interaction behavior ........................    9
   3.7        Relationship with existing languages ................   10
   4          Related work ........................................   11
   5          Necessary language features .........................   11
   5.1        Language characteristics ............................   11
   5.2        Base features -- call signalling ....................   12
            5.3        Base features -- non-signalling ............   15
   5.4        Higher-level features ...............................   16
   5.5        Contingent features .................................   17
   5.6        End-system specific features ........................   18
   5.7        Language features ...................................   18
   5.8        Future features .....................................   19
   5.9        Control .............................................   19
   6          Security considerations .............................   19

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   7          Acknowledgments .....................................   19
   8          Authors' Addresses ..................................   20
   9          Bibliography ........................................   20

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