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Internet Engineering Task Force                                 IPTEL WG
Internet Draft                                        Lennox/Schulzrinne
draft-ietf-iptel-cpl-02.txt                          Columbia University
July 14, 2000
Expires: January, 2001


    CPL: A Language for User Control of Internet Telephony Services

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".


     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.



Abstract

   The Call Processing Language (CPL) is a language that can be used to
   describe and control Internet telephony services. It is designed to
   be implementable on either network servers or user agent servers. It
   is meant to be simple, extensible, easily edited by graphical
   clients, and independent of operating system or signalling protocol.
   It is suitable for running on a server where users may not be allowed
   to execute arbitrary programs, as it has no variables, loops, or
   ability to run external programs.

   This document is a product of the IP Telephony (IPTEL) working group
   of the Internet Engineering Task Force. Comments are solicited and
   should be addressed to the working group's mailing list at
   iptel@lists.research.bell-labs.com and/or the authors.


1 Introduction

   The Call Processing Language (CPL) is a language that can be used to



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   describe and control Internet telephony services. It is not tied to
   any particular signalling architecture or protocol; it is anticipated
   that it will be used with both SIP [1] and H.323 [2].

   The CPL is powerful enough to describe a large number of services and
   features, but it is limited in power so that it can run safely in
   Internet telephony servers. The intention is to make it impossible
   for users to do anything more complex (and dangerous) than describing
   Internet telephony services. The language is not Turing-complete, and
   provides no way to write loops or recursion.

   The CPL is also designed to be easily created and edited by graphical
   tools.  It is based on XML [3], so parsing it is easy and many
   parsers for it are publicly available. The structure of the language
   maps closely to its behavior, so an editor can understand any valid
   script, even ones written by hand. The language is also designed so
   that a server can easily confirm scripts' validity at the time they
   are delivered to it, rather that discovering them while a call is
   being processed.

   Implementations of the CPL are expected to take place both in
   Internet telephony servers and in advanced clients; both can usefully
   process and direct users' calls.  This document primarily addresses
   the usage in servers. A mechanism will be needed to transport scripts
   between clients and servers; this document does not describe such a
   mechanism, but related documents will.

   The framework and requirements for the CPL architecture are described
   in RFC 2824, "Call Processing Language Framework and Requirements."
   [4].

1.1 Conventions of this document

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [5] and
   indicate requirement levels for compliant CPL implementations.

   In examples, non-XML strings such as -action1, -action2, and so
   forth, are sometimes used. These represent further parts of the
   script which are not relevant to the example in question.


        Some paragraphs are indented, like this; they give
        motivations of design choices, or questions for future
        discussion in the development of the CPL, and are not
        essential to the specification of the language.




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2 Structure of CPL scripts

2.1 High-level structure

   A CPL script consists of two types of information: ancillary
   information about the script, and call processing actions.

   A call processing action is a structured tree that describes the
   decisions and actions a telephony signalling server performs on a
   call set-up event.  There are two types of call processing actions:
   top-level actions are actions that are triggered by signalling events
   that arrive at the server.  Two top-level action names are defined:
   incoming, the action performed when a call arrives whose destination
   is the owner of the script; and outgoing, the action performed when a
   call arrives whose originator is the owner of the script. Sub-actions
   are actions which can be called from other actions. The CPL forbids
   sub-actions from being called recursively: see section 9.


        Note: The names "action," "sub-action," and "top-level
        action" are probably not ideal. Suggestions for better
        names for these concepts are welcomed.

   Ancillary information is information which is necessary for a server
   to correctly process a script, but which does not directly describe
   any actions. Currently, no ancillary information is defined, but the
   section is reserved for future extensions.

2.2 Abstract structure of a call processing action

   Abstractly, a call processing action is described by a collection of
   nodes, which describe actions that can be performed or choices which
   can be made. A node may have several parameters, which specify the
   precise behavior of the node; they usually also have outputs, which
   depend on the result of the condition or action.

   For a graphical representation of a CPL action, see figure 1.  Nodes
   and outputs can be thought of informally as boxes and arrows; the CPL
   is designed so that actions can be conveniently edited graphically
   using this representation. Nodes are arranged in a tree, starting at
   a single root node; outputs of nodes are connected to additional
   nodes. When an action is run, the action or condition described by
   the top-level node is performed; based on the result of that node,
   the server follows one of the node's outputs, and that action or
   condition is performed; this process continues until a node with no
   specified outputs is reached.  Because the graph is acyclic, this
   will occur after a bounded and predictable number of nodes are
   visited.



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   If an output to a node is not specified, it indicates that the CPL
   server should perform a node- or protocol-specific action. Some nodes
   have specific default actions associated with them; for others, the
   default action is implicit in the underlying signalling protocol, or
   can be configured by the administrator of the server. For further
   details on this, see section 11.




          _________________      ___________________      ________  busy
         | Address-switch  |    | location          |    | proxy  |--------\
Call --->|  field: origin  |  ->|   url: sip:jones@ |--->|timeout:| timeout|
         |  subfield: host | /  |     example.com   |    |  10s   |--------|
         |-----------------|/   |___________________|    |        | failure|
         | subdomain-of:   |                             |________|--------|
         |   example.com   |                                               |
         |-----------------|  _____________________________________________/
         | otherwise       | /..........................................
         |                 |\|. Voicemail                              .
         |_________________| \.  ____________________                  .
                              ->| location           |     __________  .
                              . |   url: sip:jones@  |    | redirect | .
                              . |        voicemail.  |--->|          | .
                              . |        example.com |    |__________| .
                              . |____________________|                 .
                              ..........................................




   Figure 1: Sample CPL Action: Graphical Version



2.3 Location model

   For flexibility, one piece of information necessary for the function
   of a CPL is not given as node parameters: the set of locations to
   which a call is to be directed. Instead, this set of locations is
   stored as an implicit global variable throughout the execution of a
   processing action (and its sub-actions). This allows locations to be
   retrieved from external sources, filtered, and so forth, without
   requiring general language support for such actions (which could harm
   the simplicity and tractability of understanding the language). The
   specific actions which add, retrieve, or filter location sets are
   given in section 6.




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   For the incoming top-level processing action, the location set is
   initialized to the empty set. For the outgoing action, it is
   initialized to the destination address of the call.

2.4 XML structure

   Syntactically, CPL scripts are represented by XML documents. XML is
   thoroughly specified by [3], and implementors of this specification
   should be familiar with that document, but as a brief overview, XML
   consists of a hierarchical structure of tags; each tag can have a
   number of attributes. It is visually and structurally very similar to
   HTML [6], as both languages are simplifications of the earlier and
   larger standard SGML [7].

   See figure 2 for the XML document corresponding to the graphical
   representation of the CPL script in figure 1. Both nodes and outputs
   in the CPL are represented by XML tags; parameters are represented by
   XML tag attributes. Typically, node tags contain output tags, and
   vice-versa (with one exception; see section 2.3).

   The connection between the output of a node and another node is
   represented by enclosing the tag representing the pointed-to node
   inside the tag for the outer node's output. Convergence (several
   outputs pointing to a single node) is represented by sub-actions,
   discussed further in section 9.

   The higher-level structure of a CPL script is represented by tags
   corresponding to each piece of meta-information, sub-actions, and
   top-level actions, in order. This higher-level information is all
   enclosed in a special tag cpl, the outermost tag of the XML document.

   A complete Document Type Declaration for the CPL is provided in
   Appendix A. The remainder of the main sections of this document
   describe the semantics of the CPL, while giving its syntax
   informally. For the formal syntax, please see the appendix.


3 Document information

   This section gives meta-information about CPL scripts.

3.1 CPL Document Identifiers for XML

   A CPL script list which appears as a top-level XML document is
   identified with the formal public identifier "-//IETF//DTD RFCxxxx
   CPL 1.0//EN". If this document is published as an RFC, "xxxx" will be
   replaced by the RFC number.




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   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <subaction id="voicemail">
       <location url="sip:jones@voicemail.example.com">
         <redirect />
       </location>
     </subaction>

     <incoming>
       <address-switch field="origin" subfield="host">
         <address subdomain-of="example.com">
           <location url="sip:jones@example.com">
             <proxy timeout="10">
               <busy> <sub ref="voicemail" /> </busy>
               <noanswer> <sub ref="voicemail" /> </noanswer>
               <failure> <sub ref="voicemail" /> </failure>
             </proxy>
           </location>
         </address>
         <otherwise>
           <sub ref="voicemail" />
         </otherwise>
       </address-switch>
     </incoming>
   </cpl>


   Figure 2: Sample CPL Script: XML Version


   An CPL embedded as a fragment within another XML document is
   identified with the XML namespace identifier
   "http://www.ietf.org/internet-drafts/draft-ietf-iptel-cpl-02.txt".
   If this document is published as an RFC, the namespace identifier
   will be "http://www.rfc-editor.org/rfc/rfcxxxx.txt", where xxxx is
   the RFC number.


        Note that the URIs specifying XML namespaces are only
        globally unique names; they do not have to reference any
        particular actual object.  The URI of a canonical source of
        this specification meets the requirement of being globally
        unique, and is also useful to document the format.

3.2 MIME Registration



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   As an XML type, CPL's MIME registration conforms with "XML Media
   Types" [8] as well as RFC 2048 [9].

        MIME media type name: application

        MIME subtype name: cpl+xml

        Mandatory parameters: none

        Optional parameters: charset
             As for application/xml in "XML Media Types."

        Encoding considerations: As for application/xml in "XML Media
             Types."

        Security considerations: See section 13, and section 10 of "XML
             Media Types."

        Interoperability considerations: Different CPL servers may use
             incompatible address types. However, all potential
             interoperability issues should be resolvable at the time a
             script is uploaded; there should be no interoperability
             issues which cannot be detetected until runtime.

        Published specification: This document.

        Applications which use this media type: None publically released
             at this time, as far as the authors are aware.

        Additional information:

             Magic number: None

             File extension: .cpl or .xml

             Macintosh file type code: "TEXT"

        Person and e-mail address for further information:
             Jonathan Lennox <lennox@cs.columbia.edu>
             Henning Schulzrinne <hgs@cs.columbia.edu>

        Intended usage: COMMON

        Author/Change Controller: The IETF.

4 Script structure: overview

   As mentioned, a CPL script consists of ancillary information,



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   subactions, and top-level actions. The full syntax of the cpl node is
   given in figure 3.


            Tag:  cpl
     Parameters:  none
       Sub-tags:  ancillary  See section 10
                  subaction  See section 9
                  outgoing   Top-level actions to take on this user's
                             outgoing calls
                  incoming   Top-level actions to take on this user's
                             incoming calls

         Output:  outgoing
     Parameters:  none

         Output:  incoming
     Parameters:  none


   Figure 3: Syntax of the top-level cpl tag



   Call processing actions, both top-level actions and sub-actions,
   consist of nodes and outputs. Nodes and outputs are both described by
   XML tags. There are four categories of CPL nodes:  switches , which
   represent choices a CPL script can make; location modifiers , which
   add or remove locations from the location set; signalling actions ,
   which cause signalling events in the underlying protocol; and non-
   signalling actions, which take an action but do not effect the
   underlying protocol.

5 Switches

   Switches represent choices a CPL script can make, based on either
   attributes of the original call request or items independent of the
   call.

   All switches are arranged as a list of conditions that can match a
   variable. Each condition corresponds to a node output; the output
   points to the next node to execute if the condition was true.  The
   conditions are tried in the order they are presented in the script;
   the output corresponding to the first node to match is taken.

   There are two special switch outputs that apply to every switch type.
   The output not-present, which MAY occur anywhere in the list of
   outputs, is true if the variable the switch was to match was not



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   present in the original call setup request.  The output otherwise,
   which MUST be the last output specified if it is present, matches if
   no other condition matched.

   If no condition matches and no otherwise output was present in the
   script, the default script action is taken. See section 11 for more
   information on this.

5.1 Address switches

   Address switches allow a CPL script to make decisions based on one of
   the addresses present in the original call request. They are
   summarized in figure 4.


         Node:  address-switch
      Outputs:  address         Specific addresses to match
   Parameters:  field           origin, destination, or original-destination
                subfield        address-type, user, host, port, tel, display,
                                password, or alias-type

       Output:  address
   Parameters:  is              exact match
                contains        substring match (for display only)
                subdomain-of    sub-domain match (for host, tel only)


   Figure 4: Syntax of the address-switch node



   Address switches have two node parameters: field, and subfield.  The
   mandatory field parameter allows the script to specify which address
   is to be considered for the switch: either the call's origin address
   (field "origin"), its current destination address (field
   "destination"), or its original destination (field "original-
   destination"), the destination the call had before any earlier
   forwarding was invoked. Servers MAY define additional field values.

   The optional subfield specifies what part of the address is to be
   considered. The possible subfield values are: address-type, user,
   host, port, tel, and display.  Additional subfield values MAY be
   defined: two additional ones, password and asn1 are defined
   specifically for SIP and H.323 respectively, in sections 5.1.1 and
   5.1.2 below. If no subfield is specified, the "entire" address is
   matched; the precise meaning of this is defined for each underlying
   signalling protocol.  Servers MAY define additional subfield values.




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   The subfields are defined as follows:

        address-type This indicates the type of the underlying address;
             i.e., the URI scheme, if the address can be represented by
             a URI. The types specifically discussed by this document
             are sip, tel, and h323. The address type is not case-
             sensitive.  It has a value for all defined address types.

        user This subfield of the address indicates, for e-mail style
             addresses, the user part of the address. For telephone
             number style address, it includes the subscriber number.
             This subfield is case-sensitive; it may be not present.

        host This subfield of the address indicates the Internet host
             name or IP address corresponding to the address, in host
             name, IPv4, or IPv6 format. For host names only, subdomain
             matching is supported with the subdomain-of match operator.
             It is not case sensitive, and may be not present.

        port This subfield indicates the TCP or UDP port number of the
             address, numerically in decimal format. It is not case
             sensitive, as it MUST only contain decimal digits. It may
             be not present; however, for address types with default
             ports, an absent port matches the default port number.

        tel This subfield indicates a telephone subscriber number, if
             the address contains such a number. It is not case
             sensitive (the telephone numbers may contain the symbols
             `A' `B' `C' and `D'), and might not be present. It may be
             matched using the subdomain-of match operator.  Punctuation
             and separator characters in telephone numbers are
             discarded.

        display This subfield indicates a "display name" or user-visible
             name corresponding to an address. It is a Unicode string,
             and is matched using the case-insensitive algorithm
             described in section 5.2. The contains operator may be
             applied to it. It may be not present.

   For any completely unknown subfield, the server MAY reject the script
   at the time it is submitted with an indication of the problem; if a
   script with an unknown subfield is executed, the server MUST consider
   the not-present output to be the valid one.

   The address output tag may take exactly one of three possible
   parameters, indicating the kind of matching allowed.

        is An output with this match operator is followed if the



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             subfield being matched in the address-switch exactly
             matches the argument of the operator. It may be used for
             any subfield, or for the entire address if no subfield was
             specified.

        subdomain-of This match operator applies only for the subfields
             host and tel. In the former case, it matches if the
             hostname being matched is a subdomain of the domain given
             in the argument of the match operator; thus, subdomain-
             of="example.com" would match the hostnames "example.com",
             "research.example.com", and
             "zaphod.sales.internal.example.com". IP addresses may be
             given as arguments to this operator; however, they only
             match exactly. In the case of the tel subfield, the output
             matches if the telephone number being matched has a prefix
             that matches the argument of the match operator;
             subdomain-of="1212555" would match the telephone number "1
             212 555 1212."

        contains This match operator applies only for the subfield
             display. The output matches if the display name being
             matched contains the argument of the match as a substring.

5.1.1 Address switch mapping for SIP

   For SIP, the origin address corresponds to the address in the From
   header; destination corresponds to the Request-URI; and original-
   destination corresponds to the To header.

   The display subfield of an address is the display-name part of the
   address, if it is present. Because of SIP's syntax, the destination
   address field will never have a display subfield.

   The address-type subfield of an address is the URI scheme of that
   address. Other address fields depend on that address-type.

   For sip URLs, the user, host, and port subfields correspond to the
   "user," "host," and "port" elements of the URI syntax. The tel
   subfield is defined to be the "user" part of the URI if and only if
   the "user=phone" parameter is given to the URI. An additional
   subfield, password is defined to correspond to the "password" element
   of the SIP URI; however, use of this field is NOT RECOMMENDED for
   general security reasons.

   For tel URLs, the tel and user subfields are the subscriber name; in
   the former case, visual separators are stripped. The host and port
   subfields are both not present.




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   For h323 URLs, the subfields are set as in section 5.1.2 below.

   For other URI schemes, only the address-type subfield is defined by
   this specification; servers MAY set other pre-defined subfields, or
   MAY support additional subfields.

   If no subfield is specified for addresses in SIP messages, the string
   matched is the URI part of the address.  For "sip" URLs, all
   parameters are stripped; for other URLs, the URL is used verbatim.

5.1.2 Address switch mapping for H.323

   For H.323, the origin address corresponds to the primary alias
   address in the sourceAddress field of the Setup-UUIE user-user
   information element, and to the Q.931 information element
   callingPartyNumber. If both fields are present, which one has
   priority is a matter of local server policy; the server SHOULD use
   the same resolution as it would use for routing decisions in this
   case. Similarly, the destination address corresponds to the primary
   alias address of the destinationAddress field, and to the Q.931
   information element calledPartyNumber.

   This discussion is based on H.323 version 4 [10], which is expected
   to be approved in November 2000.

   The original-destination address corresponds to the redirectedNumber
   Q.931 information element, if it is present; otherwise it is the same
   as the destination address.

   The mapping of H.323 addresses into subfields depends on the type of
   the alias address. An additional subfield type, alias-type, is
   defined for H.323 servers, corresponding to the type of the address.
   Possible values are dialedDigits, h323-ID, url-ID, transportID,
   email-ID, partyNumber, mobileUIM, and Q.931IE. If future versions of
   the H.323 specification define additional types of alias addresses,
   those names MAY also be used.

   In versions of H.323 prior to version 4, dialedDigits was known as
   e164. The new name should be used.

   The value of the address-type subfield for H.323 messages is "h323"
   unless the alias type is url-ID and the URL scheme is something other
   than h323; in this case the address-type is the URL scheme, as
   specified above for SIP.

   If an alias address of type h323-ID is present anywhere among the
   sequence of aliases, the first such h323-ID alias address is used for
   the display subfield of the address. The values of all other



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   subfields depend only on the first alias address in the sequence.

   The following mappings are used for H.323 alias types:

        dialedDigits, partyNumber, mobileUIM, and Q.931IE: the tel and
             user subfields are the string of digits, as is the
             "entire-address" form. The host and port subfields are not
             present.

        url-ID with a "h323" URI: the user, host, and port subfields are
             set to the corresponding parts of the H.323 URL.  The tel
             subfield is not present. The "entire-address" form
             corresponds to the entire URI.

        url-ID with other URI schemes: the same mapping is used as for
             SIP, above.

        email-ID: the user and host subfields are set to the
             corresponding parts of the e-mail address. The port and tel
             subfields are not present. The "entire-address" form
             corresponds to the entire e-mail address.

        transportID: if the TransportAddress is of type "ipAddress,"
             "ipSourceRoute," or "ip6Address," the host subfield is set
             to the "ip" element of the sequence, translated into the
             standard IPv4 or IPv6 textual representation, and the port
             subfield is set to the "port" element of the sequence
             represented in decimal. The tel and user fields are not
             present. The "entire-address" form is not defined. The
             representation and mapping of transport addresses is not
             defined for non-IP addresses.

5.2 String switches

   String switches allow a CPL script to make decisions based on free-
   form Unicode strings present in a call request. They are summarized
   in figure 5.


   String switches have one node parameter: field. The mandatory field
   parameter specifies which string is to be matched.

   Currently five fields are defined. Three fields are currently
   applicable only to SIP, one is currently applicable only to H.323,
   and one is applicable to both.

   The three fields which are applicable only to SIP are: subject,
   indicating the subject of the call; organization, indicating the



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            Node:  string-switch
         Outputs:  string         Specific string to match
      Parameters:  field          subject, organization, user-agent,
                                  language, or display

          Output:  string
      Parameters:  is             exact match
                   contains       substring match


   Figure 5: Syntax of the string-switch node


   originator's organization; and user-agent, indicating the program or
   device with which the call request was made. All these fields
   correspond to the contents of the SIP header fields with the same
   names.

   The field applicable only to H.323 is display, which corresponds to
   the Q.931 information element of the same name.


        This is conventionally used for Caller-ID purposes, so
        arguably it should be mapped to the display subfield of an
        address-match with the field originator. However, since a)
        it is a message-level information element, not an address-
        level one, and b) the Q.931 specification [11] says only
        that "[t]he purpose of the Display information element is
        to supply display information that may be displayed by the
        user," it seems to be more appropriate to match it as a
        string instead.

   The field appropriate both to SIP and H.323 is language. This field
   contains a list of RFC 1766 [12] language tags, separated by commas,
   corresponding to the SIP Accept-Language header and the H.323
   language UUIE.


        Note that matching based on contains is likely to be much
        more useful than matching based on is, for this field.

   Strings are matched as case-insensitive Unicode strings, in the
   following manner. First, strings are canonicalized to the
   "Compatibility Composition" (KC) form, as specified in Unicode
   Technical Report 15 [13]. Then, strings are compared using locale-
   insensitive caseless mapping, as specified in Unicode Technical
   Report 21 [14].



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        Code to perform the first step, in Java and Perl, is
        available; see the links from Annex E of UTR 15 [13]. The
        case-insensitive string comparison in the Java standard
        class libraries already performs the second step; other
        Unicode-aware libraries should be similar.

   The output tags of string matching are named string, and have a
   mandatory argument, one of is or contains, indicating whole-string
   match or substring match, respectively.

5.3 Time switches

   Time switches allow a CPL script to make decisions based the time
   and/or date the script is being executed. They are summarized in
   figure 6.


         Node:  time-switch
      Outputs:  time         Specific time to match
   Parameters:  tzid         RFC 2445 Time Zone Identifier
                tzurl        RFC 2445 Time Zone URL

       Output:  time
   Parameters:  dtstart      Start of interval (RFC 2445 DATE-TIME)
                dtend        End of interval (RFC 2445 DATE-TIME)
                duration     Length of interval (RFC 2445 DURATION)
                freq         Frequency of recurrence (one of "secondly",
                             "minutely", "hourly", "daily", "weekly",
                             "monthly", or "yearly")
                interval     How often the recurrence repeats
                until        Bound of recurrence (RFC 2445 DATE-TIME)
                count        Number of occurences of recurrence
                bysecond     List of seconds within a minute
                byminute     List of minutes within an hour
                byhour       List of hours of the day
                byday        List of days of the week
                bymonthday   List of days of the month
                byyearday    List of days of the year
                byweekno     List of weeks of the year
                bymonth      List of months of the year
                wkst         First day of week
                bysetpos     List of values within set of events specified


   Figure 6: Syntax of the time-switch node






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   Time switches are based closely on the specification of recurring
   intervals of time from the Internet Calendaring and Scheduling Core
   Object Specification (iCal COS), RFC 2445 [15].


        This allows CPLs to be generated automatically from
        calendar books. It also allows us to re-use the extensive
        existing work specifying time intervals.

   The time-switch tag takes two optional parameters, tzid and tzurl,
   both of which are defined in RFC 2445 (sections 4.8.3.1 and 4.8.3.5
   respectively). The TZID is the identifying label by which a time zone
   definition is referenced. If it begins with a forward slash
   (solidus), it references a to-be-defined global time zone registry;
   otherwise it is locally-defined at the server. The TZURL gives a
   network location from which an up-to-date VTIMEZONE definition for
   the timezone can be retrieved.

   If a script is uploaded with a tzid and tzurl which the CPL server
   does not recognize or cannot resolve, it SHOULD diagnose and reject
   this at script upload time. If neither tzid nor tzurl are present,
   all non-UTC times within this time switch should be interpreted as
   being "floating" times, i.e. that they are specified in the local
   timezone of the CPL server.


        Because of daylight-savings-time changes over the course of
        a year, it is necessary to specify time switches in a given
        timezone. UTC offsets are not sufficient, or a time-of-day
        routing rule which held between 9 am and 5 pm in the
        eastern United States would start holding between 8 am and
        4 pm at the end of October.

   Authors of CPL servers should be careful to handle correctly the
   intervals when local time is discontinuous, at the beginning or end
   of daylight-savings time.

   Time nodes specify a list of periods during which their output should
   be taken. They have two required parameters: dtstart, which specifies
   the beginning of the first period of the list, and exactly one of
   dtend or duration, which specify the ending time or the duration of
   the period, respectively. The dtstart and dtend parameters are
   formatted as iCal COS DATE-TIME values, as specified in section 4.3.5
   of RFC 2445. The duration parameter is given as an iCal COS DURATION
   parameter, as specified in section 4.3.6 of RFC 2445.

   If no other parameters are specified, a time node indicates only a
   single period of time. More complicated sets periods intervals are



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   constructed as recurrences. A recurrence is specified by including
   the freq parameter, which indicates the type of recurrence rule. No
   parameters other than dtstart, dtend, and duration SHOULD be
   specified unless freq is present.

   The freq parameter takes one of the following values: secondly, to
   specify repeating periods based on an interval of a second or more;
   minutely, to specify repeating periods based on an interval of a
   minute or more; hourly, to specify repeating periods based on an
   interval of an hour or more; daily, to specify repeating periods
   based on an interval of a day or more; weekly, to specify repeating
   periods based on an interval of a week or more; monthly, to specify
   repeating periods based on an interval of a month or more; and
   yearly, to specify repeating periods based on an interval of a year
   or more. These values are not case-sensitive.

   The interval parameter contains a positive integer representing how
   often the recurrence rule repeats. The default value is "1", meaning
   every second for a secondly rule, or every minute for a minutely
   rule, every hour for an hourly rule, every day for a daily rule,
   every week for a weekly rule, every month for a monthly rule and
   every year for a yearly rule.

   The until parameter defines an iCal COS DATE or DATE-TIME value which
   bounds the recurrence rule in an inclusive manner. If the value
   specified by until is synchronized with the specified recurrence,
   this date or date-time becomes the last instance of the recurrence.
   If specified as a date-time value, then it MUST be specified in an
   UTC time format. If not present, and the count parameter is also not
   present, the recurrence is considered to repeat forever.

   The count parameter defines the number of occurrences at which to
   range-bound the recurrence. The dtstart parameter counts as the first
   occurrence. The until and count parameters MUST NOT occur in the same
   time output.

   The bysecond parameter specifies a comma-separated list of seconds
   within a minute. Valid values are 0 to 59. The byminute parameter
   specifies a comma-separated list of minutes within an hour. Valid
   values are 0 to 59. The byhour parameter specifies a comma-separated
   list of hours of the day. Valid values are 0 to 23.

   The byday parameter specifies a comma-separated list of days of the
   week. MO indicates Monday; TU indicates Tuesday; WE indicates
   Wednesday; TH indicates Thursday; FR indicates Friday; SA indicates
   Saturday; SU indicates Sunday. These values are not case-sensitive.

   Each byday value can also be preceded by a positive (+n) or negative



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   (-n) integer. If present, this indicates the nth occurrence of the
   specific day within the monthly or yearly recurrence. For example,
   within a monthly rule, +1MO (or simply 1MO) represents the first
   Monday within the month, whereas -1MO represents the last Monday of
   the month. If an integer modifier is not present, it means all days
   of this type within the specified frequency. For example, within a
   monthly rule, MO represents all Mondays within the month.

   The bymonthday parameter specifies a comma-separated list of days of
   the month. Valid values are 1 to 31 or -31 to -1. For example, -10
   represents the tenth to the last day of the month.

   The byyearday parameter specifies a comma-separated list of days of
   the year. Valid values are 1 to 366 or -366 to -1. For example, -1
   represents the last day of the year (December 31st) and -306
   represents the 306th to the last day of the year (March 1st).

   The byweekno parameter specifies a comma-separated list of ordinals
   specifying weeks of the year. Valid values are 1 to 53 or -53 to -1.
   This corresponds to weeks according to week numbering as defined in
   [ISO 8601]. A week is defined as a seven day period, starting on the
   day of the week defined to be the week start (see wkst). Week number
   one of the calendar year is the first week which contains at least
   four (4) days in that calendar year. This parameter is only valid for
   yearly rules. For example, 3 represents the third week of the year.


        Note: Assuming a Monday week start, week 53 can only occur
        when Thursday is January 1 or if it is a leap year and
        Wednesday is January 1.

   The bymonth parameter specifies a comma-separated list of months of
   the year. Valid values are 1 to 12.

   The wkst parameter specifies the day on which the workweek starts.
   Valid values are MO, TU, WE, TH, FR, SA and SU. This is significant
   when a weekly recurrence has an interval greater than 1, and a byday
   parameter is specified. This is also significant in a yearly
   recurrence when a byweekno parameter is specified. The default value
   is MO.

   The bysetpos parameter specifies a comma-separated list of values
   which corresponds to the nth occurrence within the set of events
   specified by the rule. Valid values are 1 to 366 or -366 to -1. It
   MUST only be used in conjunction with another byxxx parameter. For
   example "the last work day of the month" could be represented as:





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     <time ... freq="monthly" byday="MO,TU,WE,TH,FR" bysetpos="-1">



   Each bysetpos value can include a positive (+n) or negative (-n)
   integer. If present, this indicates the nth occurrence of the
   specific occurrence within the set of events specified by the rule.

   If byxxx parameter values are found which are beyond the available
   scope (ie, bymonthday="30" in February), they are simply ignored.

   Byxxx parameters modify the recurrence in some manner. Byxxx rule
   parts for a period of time which is the same or greater than the
   frequency generally reduce or limit the number of occurrences of the
   recurrence generated. For example, freq="daily" bymonth="1" reduces
   the number of recurrence instances from all days (if the bymonth
   parameter is not present) to all days in January. Byxxx parameters
   for a period of time less than the frequency generally increase or
   expand the number of occurrences of the recurrence. For example,
   freq="yearly" bymonth="1,2" increases the number of days within the
   yearly recurrence set from 1 (if bymonth parameter is not present) to
   2.

   If multiple Byxxx parameters are specified, then after evaluating the
   specified freq and interval parameters, the Byxxx parameters are
   applied to the current set of evaluated occurrences in the following
   order:  bymonth, byweekno, byyearday, bymonthday, byday, byhour,
   byminute, bysecond and bysetpos; then count and until are evaluated.

   Here is an example of evaluating multiple Byxxx parameters.


     <time dtstart="19970105T083000" duration="10M"
           freq="yearly" interval="2" bymonth="1" byday="SU" byhour="8,9"
           byminute="30">



   First, the interval="2" would be applied to freq="YEARLY" to arrive
   at "every other year." Then, bymonth="1" would be applied to arrive
   at "every January, every other year." Then, byday="SU" would be
   applied to arrive at "every Sunday in January, every other year."
   Then, byhour="8,9" would be applied to arrive at "every Sunday in
   January at 8 AM and 9 AM, every other year." Then, byminute="30"
   would be applied to arrive at "every Sunday in January at 8:30 AM and
   9:30 AM, every other year." Then the second is derived from dtstart
   to end up in "every Sunday in January from 8:30:00 AM to 8:40:00 AM,
   and from and 9:30:00 AM to 9:40:00 AM, every other year." Similarly,



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   if the byminute, byhour, byday, bymonthday or bymonth parameter were
   missing, the appropriate minute, hour, day or month would have been
   retrieved from the dtstart parameter.

   The iCal COS RDATE, EXRULE and EXDATE recurrence rules are not
   specifically mapped to components of the time-switch node. Equivalent
   functionality to the exception rules can be attained by using the
   ordering of switch rules to exclude times using earlier rules;
   equivalent functionality to the additional-date RDATE rules can be
   attained by using sub nodes (see section 9) to link multiple outputs
   to the same subsequent node.

   The not-present output is never true for a time switch.  However, it
   MAY be included, to allow switch processing to be more regular.

5.4 Priority switches

   Priority switches allow a CPL script to make decisions based on the
   priority specified for the original call. They are summarized in
   figure 7.


         Node:  priority-switch
      Outputs:  priority         Specific priority to match
   Parameters:  none

       Output:  priority
   Parameters:  less             Match if priority is less than specified
                greater          Match if priority is greater than specified
                equal            Match if priority is equal to specified


   Figure 7: Syntax of the priority-switch node



   Priority switches take no parameters.

   The priority tags take one of the three parameters greater, less, and
   equal. The values of these tags are the priorities specified in SIP
   [1]: in decreasing order, emergency, urgent, normal, and non-urgent.
   These values are matched in a case-insensitive manner. Outputs with
   the less parameter are taken if the priority of the message is less
   than the priority given in the argument; and so forth.

   If no priority header is specified in a message, the priority is
   considered to be normal. If an unknown priority is given, the
   priority is considered to be equivalent to normal for the purposes of



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   greater and less comparisons, but it is compared literally for equal
   comparisons.

   Since every message has a priority, the not-present output is never
   true for a priority switch.  However, it MAY be included, to allow
   switch processing to be more regular.

6 Location modifiers

   The abstract location model of the CPL is described in section 2.3.
   The behavior of several of the signalling actions (defined in section
   7) is dependent on the current location set specified. Location nodes
   add or remove locations from the location set.

   There are three types of location nodes defined. Explicit locations
   add literally-specified locations to the current location set;
   location lookups obtain locations from some outside source; and
   location filters remove locations from the set, based on some
   specified criteria.

6.1 Explicit location

   Explicit location nodes specify a location literally. Their syntax is
   described in figure 8.


           Node:  location
        Outputs:  any node
     Parameters:  url       URL of address to add to location set
                  clear     Whether to clear the location set before adding
   the new value


   Figure 8: Syntax of the location node



   Explicit location nodes have two node parameters. The mandatory url
   parameter's value is the URL of the address to add to the location
   set.  Only one address may be specified per location node; multiple
   locations may be specified by cascading these nodes.  The optional
   clear parameter specifies whether the location set should be cleared
   before adding the new location to it. Its value can be "yes" or "no",
   with "no" as the default.

   Since the location is specified as a URL, all locations added in this
   manner are interpreted as url-ID addresses in H.323.




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   Basic location nodes have only one possible output, since there is no
   way that they can fail. (If a basic location node specifies a
   location which isn't supported by the underlying signalling protocol,
   the script server SHOULD detect this and report it to the user at the
   time the script is submitted.) Therefore, its XML representation does
   not have explicit output nodes; the <location> tag directly contains
   another node tag.

6.2 Location lookup

   Locations can also be specified up through external means, through
   the use of location lookups. The syntax of these tags is given in
   figure 9.


            Node:  lookup
         Outputs:  success   Action if lookup was successful
                   notfound  Action if lookup found no addresses
                   failure   Action if lookup failed
      Parameters:  source    Source of the lookup
                   timeout   Time to try before giving up on the lookup
                   use       Caller preferences fields to use
                   ignore    Caller preferences fields to ignore
                   clear     Whether to clear the location set before adding
   the new values

          Output:  success
      Parameters:  none

          Output:  notfound
      Parameters:  none

          Output:  failure
      Parameters:  none


   Figure 9: Syntax of the lookup node



   Location lookup nodes have one mandatory parameter, and four optional
   parameters. The mandatory parameter is source, the source of the
   lookup. This can either be a URL, or a non-URL value. If the value of
   source is a URL, it indicates a location which returns the
   application/url media type. The server adds the locations returned by
   the URL to the location set.

   Non-URL sources indicate a source not specified by a URL which the



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   server can query for addresses to add to the location set. The only
   non-URL source currently defined is registration, which specifies all
   the locations currently registered with the server, using SIP
   REGISTER or H.323 RAS messages.

   The lookup node also has four optional parameters. The timeout
   parameter which specifies the time, in seconds, the script is willing
   to wait for the lookup to be performed. If this is not specified, its
   default value is 30.  The clear parameter specifies whether the
   location set should be cleared before the new locations are added.

   The other two optional parameters affect the interworking of the CPL
   script with caller preferences and caller capabilities. These are
   defined in "SIP Caller Preferences and Callee Capabilities" [16]. By
   default, a CPL server SHOULD invoke caller preferences filtering when
   performing a lookup action; that is, it should honor any Accept-
   Location and Reject-Location headers of the original call request.
   The two parameters use and ignore allow the script to modify how the
   script applies caller preferences filtering. The use and ignore
   parameters both take as their arguments comma-separated lists of
   caller preferences parameters. If use is given, the server applies
   the caller preferences resolution algorithm only to those preference
   parameters given in the use parameter, and ignores all others; if the
   ignore parameter is given, the server ignores the specified
   parameters, and uses all the others. Only one of use and ignore can
   be specified. The addr-spec part of the caller preferences is always
   applied, and the script cannot modify it.


        Note: this is very SIP-specific.  H.323 has no similar
        endpoint-capabilities and requested-capabilities mechanism.

   Lookup has three outputs: success, notfound, and failure. Notfound is
   taken if the lookup process succeeded but did not find any locations;
   failure is taken if the lookup failed for some reason, including that
   specified timeout was exceeded.  If a given output is not present,
   script execution terminates and the default action is taken.

   Clients SHOULD specify the three outputs success, notfound, and
   failure in that order, so their script complies with the DTD given in
   Appendix A, but servers MAY accept them in any order.

6.3 Location filtering

   A CPL script can also filter addresses out of the address set,
   through the use of a mechanism very similar to caller preferences:
   the remove-location node. The syntax of these nodes is defined in
   figure 10.



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          Node:  remove-location
       Outputs:  any node
    Parameters:  param            Caller preference parameter to apply
                 value            Value of caller preference parameter
                 location         Caller preference location to apply


   Figure 10: Syntax of the remove-location node



   A remove-location node has the same effect on the location set as a
   Reject-Contact header in caller preferences [16]. The value of the
   location parameter is treated as though it were the addr-spec field
   of a Reject-Contact header; an absent header is equivalent to an
   addr-spec of "*" in that specification. If param and value are
   present, their values are comma-separated lists of caller preferences
   parameters and corresponding values, respectively, where the nth
   entry in the name list matches the nth entry in the value list.
   There MUST be the same number of parameters as values specified.
   These are treated, for location filtering purposes, as though they
   appeared in the params field of a Reject-Location header, as ";
   param=value" for each one.


        Note: this is also very SIP-specific.  H.323 has no similar
        endpoint-capabilities mechanism.

7 Signalling actions

   Signalling action nodes cause signalling events in the underlying
   signalling protocol. Three signalling actions are defined: "proxy,"
   "redirect," and "reject."

7.1 Proxy

   Proxy causes the triggering call to be forwarded on to the currently
   specified set of locations. The syntax of the proxy node is given in
   figure 11.


   After a proxy action has completed, the CPL server chooses the "best"
   response to the call attempt, as defined by the signalling protocol
   or the server's administrative configuration rules.

   If the call attempt was successful, or if no output was specified
   which corresponded to the the best response, CPL execution terminates



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         Node:  proxy
      Outputs:  busy         Action if call attempt returned "busy"
                noanswer     Action if call attempt was not answered before timeout
                redirection  Action if call attempt was redirected
                failure      Action if call attempt failed
   Parameters:  timeout      Time to try before giving up on the call attempt
                recurse      Whether to recursively look up redirections
                ordering     What order to try the location set in.

       Output:  busy
   Parameters:  none

       Output:  noanswer
   Parameters:  none

       Output:  redirection
   Parameters:  none

       Output:  failure
   Parameters:  none


   Figure 11: Syntax of the proxy node


   and the server returns to its default behavior (normally, to forward
   the best response upstream to the originator). Otherwise, one of the
   four outputs busy, noanswer, redirection, or failure is taken.


        Note: future extension of the CPL to allow in-call or end-
        of-call actions will require success outputs to be added.

   If no locations were present in the set, or if the only locations in
   the set were locations to which the server cannot proxy a call (for
   example, "http" URLs), the failure output is taken.

   Proxy has three optional parameters. The timeout parameter specifies
   the time, in seconds, to wait for the call to be completed or
   rejected; after this time has elapsed, the call attempt is terminated
   and the noanswer branch is taken. If this parameter is not specified,
   the default value is 20 seconds if the proxy node has a no-answer
   output specified; otherwise the server SHOULD allow the call to ring
   for a reasonably long period of time (to the maximum extent that
   server policy allows).





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        Question: is having the default value dependent on script
        structure too ugly?

   The second optional parameter is recurse, which can take two values,
   yes or no. This specifies whether the server should automatically
   attempt to place further call attempts to telephony addresses in
   redirection responses that were returned from the initial server.
   Note that if the value of recurse is yes, the redirection output to
   the script is never taken. In this case this output SHOULD NOT be
   present. The default value of this parameter is yes.

   The third optional parameter is ordering. This can have three
   possible values: parallel, sequential, and first-only.  This
   parameter specifies in what order the locations of the location set
   should be tried. Parallel asks that they all be tried simultaneously;
   sequential asks that the one with the highest priority be tried
   first, the one with the next-highest priority second, and so forth,
   until one succeeds or the set is exhausted. First-only instructs the
   server to try only the highest-priority address in the set, and then
   follow one of the outputs.  The priority of locations in a set is
   determined by server policy, though SIP servers SHOULD honor the q
   parameter of SIP registrations and the output of the caller
   preferences lookup algorithm.  The default value of this parameter is
   parallel.

   Once a proxy action completes, if control is passed on to other
   actions, all locations which have been used are cleared from the
   location set. That is, the location set is emptied if ordering was
   parallel or sequential; the highest-priority item in the set is
   removed from the set if ordering was first-only. In the case of a
   redirection output, the new addresses to which the call was
   redirected are then added to the location set.

7.2 Redirect

   Redirect causes the server to direct the calling party to attempt to
   place its call to the currently specified set of locations. The
   syntax of this node is specified in figure 12.


   Redirect immediately terminates execution of the CPL script, so this
   node has no outputs.  It has one parameter, permanent, which
   specifies whether the result returned should indicate that this is a
   permanent redirection. The value of this parameter is either "yes" or
   "no" and its default value is "no."


        This corresponds to the SIP "moved permanently and "moved



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              Node:  redirect
           Outputs:  none
        Parameters:  permanent  Whether the redirection should be
                                considered permanent


   Figure 12: Syntax of the redirect node


        temporarily" (301 and 302) redirections. The contents of
        the location set are placed into the response's Contact
        header.

7.3 Reject

   Reject nodes cause the server to reject the call attempt. Their
   syntax is given in figure 13.


                     Node:  reject
                  Outputs:  none
               Parameters:  status  Status code to return
                            reason  Reason phrase to return


   Figure 13: Syntax of the reject node



   This immediately terminates execution of the CPL script, so this node
   has no outputs.

   This node has two arguments: status and reason. The status argument
   is required, and can take one of the values busy, notfound, reject,
   and error. Servers which implement SIP MAY also allow a numeric
   argument corresponding to a SIP status in the 4xx, 5xx, or 6xx range,
   but scripts SHOULD NOT use them if they wish to be portable.

   The reason argument optionally allows the script to specify a reason
   for the rejection. CPL servers MAY ignore the reason, but ones that
   implement SIP SHOULD send them in the SIP reason phrase.

8 Other actions

   In addition to the signalling actions, the CPL defines several
   actions which do not affect the telephony signalling protocol.




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8.1 Mail

   The mail node causes the server to notify a user of the status of the
   CPL script through electronic mail. Its syntax is given in figure 14.


          Node:  mail
       Outputs:  any node
    Parameters:  url       Mailto url to which the mail should be sent


   Figure 14: Syntax of the mail node



   The mail node takes one argument: a mailto URL giving the address,
   and any additional desired parameters, of the mail to be sent.  The
   server sends the message containing the content to the given url; it
   SHOULD also include other status information about the original call
   request and the CPL script at the time of the notification.

   Mail nodes have only one output, since failure of e-mail delivery
   cannot reliably be known in real-time. Therefore, its XML
   representation does not have explicit output nodes: the <mail> tag
   directly contains another node tag.


        Using a full mailto URL rather than just an e-mail address
        allows additional e-mail headers to be specified, such as
        <mail
        url="mailto:jones@example.com?subject=lookup%20failed" />.

8.2 Log

   The Log node causes the server to log information about the call to
   non-volatile storage. Its syntax is specified in figure 15.


                Node:  log
             Outputs:  any node
          Parameters:  name      Name of the log file to use
                       comment   Comment to be placed in log file


   Figure 15: Syntax of the log node






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   Log takes two arguments, both optional: name, which specifies the
   name of the log, and comment, which gives a comment about the
   information being logged. Servers SHOULD also include other
   information in the log, such as the time of the logged event,
   information that triggered the call to be logged, and so forth. Logs
   are specific to the owner of the script which logged the event.  If
   the name parameter is not given, the event is logged to a standard,
   server-defined logfile for the script owner.  This specification does
   not define how users may retrieve their logs from the server.

   A correctly operating CPL server SHOULD NOT ever allow the log event
   to fail. As such, log nodes have only one output, and their XML
   representation does not have explicit output nodes. A CPL <log> tag
   directly contains another node tag.

9 Subactions

   XML syntax defines a tree. To allow more general call flow diagrams,
   and to allow script re-use and modularity, we define subactions.

   Two tags are defined for subactions: subaction definitions and
   subaction references. Their syntax is given in figure 16.


               Tag:  subaction
           Subtags:  any node
        Parameters:  id                Name of this subaction

       Pseudo-node:  sub
           Outputs:  none in XML tree
        Parameters:  ref               Name of subaction to execute


   Figure 16: Syntax of subactions and sub pseudo-nodes



   Subactions are defined through subaction tags. These tags are placed
   in the CPL after any ancillary information (see section 10) but
   before any top-level tags. They take one argument: id, a token
   indicating a script-chosen name for the subaction.

   Subactions are called from sub tags. The sub tag is a "pseudo-node":
   it can be used anyplace in a CPL action that a true node could be
   used. It takes one parameter, ref, the name of the subaction to be
   called. The sub tag contains no outputs of its own; control instead
   passes to the subaction.




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   References to subactions MUST refer to subactions defined before the
   current action. A sub tag MUST NOT refer to the action which it
   appears in, or to any action defined later in the CPL script. Top-
   level actions cannot be called from sub tags, or through any other
   means. Script servers MUST verify at the time the script is submitted
   that no sub node refers to any sub-action which is not its proper
   predecessor.


        Allowing only back-references of subs forbids any sort of
        recursion. Recursion would introduce the possibility of
        non-terminating or non-decidable CPL scripts, a possibility
        our requirements specifically excluded.

   Every sub MUST refer to a subaction ID defined within the same CPL
   script. No external links are permitted.


        If any subsequent version ever defines external linkages,
        it will use a different tag, perhaps XLink [17]. Ensuring
        termination in the presence of external links is a
        difficult problem.

10 Ancillary information

   No ancillary information is currently defined for CPL scripts. If
   ancillary information, not part of any action, is found to be
   necessary for scripts in the future, it will be added to this
   section.

   The (trivial) definition of the ancillary information section is
   given in figure 17.


        It may be useful to include timezone definitions inside CPL
        scripts directly, rather than referencing them externally
        with tzid and tzurl parameters. If it is, they will be
        included here.


11 Default actions

   When a CPL action reaches an unspecified output, the action it takes
   is dependent on the current state of script execution. This section
   gives the actions that should be taken in each case.

        no location or signalling actions performed, location set empty:
             Look up the user's location through whatever mechanism the



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                                Tag:  ancillary
                         Parameters:  none
                            Subtags:  none



   Figure 17: Syntax of the ancillary tag


             server would use if no CPL script were in effect. Proxy,
             redirect, or send a rejection message, using whatever
             policy the server would use in the absence of a CPL script.

        no location or signalling actions performed, location set non-
             empty:  (This can only happen for outgoing calls.) Proxy
             the call to the addresses in the location set.

        location actions performed, no signalling actions: Proxy or
             redirect the call, whichever is the server's standard
             policy, to the addresses in the current location set. If
             the location set is empty, return notfound rejection.

        noanswer output of proxy, no timeout given: (This is a special
             case.)  If the noanswer output of a proxy node is
             unspecified, and no timeout parameter was given to the
             proxy node, the call should be allowed to ring for the
             maximum length of time allowed by the server (or the
             request, if the request specified a timeout).

        proxy action previously taken: Return whatever the "best"
             response is of all accumulated responses to the call to
             this point, according to the rules of the underlying
             signalling protocol.

12 Examples

12.1 Example: Call Redirect Unconditional

   The script in figure 18 is a simple script which redirects all calls
   to a single fixed location.


12.2 Example: Call Forward Busy/No Answer

   The script in figure 19 illustrates some more complex behavior. We
   see an initial proxy attempt to one address, with further actions if
   that fails. We also see how several outputs take the same action,



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   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <incoming>
       <location url="sip:smith@phone.example.com">
        <redirect />
       </location>
     </incoming>
   </cpl>


   Figure 18: Example Script: Call Redirect Unconditional


   through the use of subactions.


   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <subaction id="voicemail">
       <location url="sip:jones@voicemail.example.com" >
         <proxy />
       </location>
     </subaction>

     <incoming>
       <location url="sip:jones@jonespc.example.com">
          <proxy timeout="8">
            <busy>
              <sub ref="voicemail" />
            </busy>
            <noanswer>
              <sub ref="voicemail" />
            </noanswer>
          </proxy>
       </location>
     </incoming>
   </cpl>


   Figure 19: Example Script: Call Forward Busy/No Answer






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12.3 Example: Call Screening

   The script in figure 20 illustrates address switches and call
   rejection, in the form of a call screening script. Note also that
   because the address-switch lacks an otherwise clause, if the initial
   pattern did not match, the script does not define any action. The
   server therefore proceeds with its default action, which would
   presumably be to contact the user.


   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <incoming>
       <address-switch field="origin" subfield="user">
         <address is="anonymous">
            <reject status="reject"
                    reason="I don't accept anonymous calls" />
         </address>
       </address-switch>
     </incoming>
   </cpl>


   Figure 20: Example Script: Call Screening



12.4 Example: Outgoing Call Screening

   The script in figure 21 illustrates a script filtering outgoing
   calls, in the form of a script which prevent 1-900 (premium) calls
   from being placed.


12.5 Example: Time-of-day Routing

   Figure 22 illustrates time-based conditions and timezones.


12.6 Example: Non-call Actions

   Figure 23 illustrates non-call actions; in particular, alerting a
   user by electronic mail if the lookup server failed. The primary
   reason for the mail node is to allow this sort of out-of-band
   notification of error conditions, as the user might otherwise be
   unaware of any problem.



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   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <outgoing>
       <address-switch field="original-destination" subfield="tel">
         <address subdomain-of="1900">
           <reject status="reject" reason="Not allowed to make 1-900 calls." />
         </address>
       </address-switch>
     </outgoing>
   </cpl>


   Figure 21: Example Script: Outgoing Call Screening



   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <incoming>
       <time-switch tzid="America/New-York"
           tzurl="http://zones.stds_r_us.net/tz/America/New-York">
         <time dtstart="20000703T090000" duration="8H"
               freq="weekly" byday="MO,TU,WE,TH,FR">
           <lookup source="registration">
             <success>
               <proxy />
             </success>
           </lookup>
         </time>
         <otherwise>
           <location url="sip:jones@voicemail.example.com">
             <proxy />
           </location>
         </otherwise>
       </time-switch>
     </incoming>
   </cpl>


   Figure 22: Example Script: Time-of-day Routing






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   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <incoming>
       <lookup source="http://www.example.com/cgi-bin/locate.cgi?user=jones"
               timeout="8">
         <success>
           <proxy />
         </success>
         <failure>
           <mail url="mailto:jones@example.com?subject=lookup%20failed" />
         </failure>
       </lookup>
     </incoming>
   </cpl>


   Figure 23: Example Script: Non-call Actions



12.7 Example: A Complex Example

   Finally, figure 24 is a complex example which shows the sort of
   sophisticated behavior which can be achieved by combining CPL nodes.
   In this case, the user attempts to have his calls reach his desk; if
   he does not answer within a small amount of time, calls from his boss
   are forwarded to his celphone, and all other calls are directed to
   voicemail.


13 Security considerations

   The CPL is designed to allow services to be specified in a manner
   which prevents potentially hostile or mis-configured scripts from
   launching security attacks, including denial-of-service attacks.
   Because script runtime is strictly bounded by acyclicity, and because
   the number of possible script actions are strictly limited, scripts
   should not be able to inflict damage upon a CPL server.

   Because scripts can direct users' telephone calls, the method by
   which scripts are transmitted from a client to a server MUST be
   strongly authenticated. Such a method is not specified in this
   document.

   Script servers SHOULD allow server administrators to control the



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   <?xml version="1.0" ?>
   <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

   <cpl>
     <subaction id="voicemail">
       <location url="sip:jones@voicemail.example.com">
         <redirect />
       </location>
     </subaction>

     <incoming>
       <location url="sip:jones@phone.example.com">
         <proxy timeout="8">
           <busy>
             <sub ref="voicemail" />
           </busy>
           <noanswer>
             <address-switch field="origin">
               <address contains="boss@example.com">
                 <location url="tel:+19175551212">
                   <proxy />
                 </location>
               </address>
               <otherwise>
                 <sub ref="voicemail" />
               </otherwise>
             </address-switch>
           </noanswer>
         </proxy>
       </location>
     </incoming>
   </cpl>


   Figure 24: Example Script: A Complex Example


   details of what CPL actions are permitted.

14 IANA considerations

   This document registers the MIME type application/cpl+xml. See
   section 3.2.

15 Acknowledgments

   This document was reviewed and commented upon by IETF IP Telephony



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   Working Group. We specifically acknowledge the following people for
   their help:

   The outgoing call screening script was written by Kenny Hom.

   Paul E. Jones contributed greatly to the mappings of H.323 addresses.

   The text of the time-switch section was taken (lightly modified) from
   RFC 2445 [15], by Frank Dawson and Derik Stenerson.

   We drew a good deal of inspiration, notably the language's lack of
   Turing-completeness and the syntax of string matching, from the
   specification of Sieve [18], a language for user filtering of
   electronic mail messages.

   Thomas F. La Porta and Jonathan Rosenberg had many useful
   discussions, contributions, and suggestions.

A The XML DTD for CPL

   This section includes a full DTD describing the XML syntax of the
   CPL.  Every script submitted to a CPL server SHOULD comply with this
   DTD.  However, CPL servers MAY allow minor variations from it,
   particularly in the ordering of output branches of nodes. Note that
   compliance with this DTD is not a sufficient condition for
   correctness of a CPL script, as many of the conditions described
   above are not expressible in DTD syntax.
























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   <?xml version="1.0" encoding="US-ASCII" ?>

   <!--
       Draft DTD for CPL, corresponding to
       draft-ietf-iptel-cpl-01.
   -->

   <!-- Nodes. -->
   <!-- Switch nodes -->
   <!ENTITY % Switch 'address-switch|string-switch|time-switch|
                      priority-switch' >

   <!-- Location nodes -->
   <!ENTITY % Location 'location|lookup|remove-location' >

   <!-- Signalling action nodes -->
   <!ENTITY % SignallingAction 'proxy|redirect|reject' >

   <!-- Other actions -->
   <!ENTITY % OtherAction 'mail|log' >

   <!-- Links to subactions -->
   <!ENTITY % Sub 'sub' >

   <!-- Nodes are one of the above four categories, or a subaction.
        This entity (macro) describes the contents of an output.
        Note that a node can be empty, implying default action. -->
   <!ENTITY % Node     '(%Location;|%Switch;|%SignallingAction;|
                        %OtherAction;|%Sub;)?' >


   <!-- Switches: choices a CPL script can make. -->

   <!-- All switches can have an 'otherwise' output. -->
   <!ELEMENT otherwise ( %Node; ) >

   <!-- All switches can have a 'not-present' output. -->
   <!ELEMENT not-present ( %Node; ) >

   <!-- Address-switch makes choices based on addresses. -->
   <!ELEMENT address-switch ( (address|not-present)+, otherwise? ) >
   <!-- <not-present> must appear at most once -->
   <!ATTLIST address-switch
      field         CDATA    #REQUIRED
      subfield      CDATA    #IMPLIED
   >




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   <!ELEMENT address ( %Node; ) >

   <!ATTLIST address
      is            CDATA    #IMPLIED
      contains      CDATA    #IMPLIED
      subdomain-of  CDATA    #IMPLIED
   > <!-- Exactly one of these three attributes must appear -->


   <!-- String-switch makes choices based on strings. -->

   <!ELEMENT string-switch ( (string|not-present)+, otherwise? ) >
   <!-- <not-present> must appear at most once -->
   <!ATTLIST string-switch
      field         CDATA    #REQUIRED
   >

   <!ELEMENT string ( %Node; ) >
   <!ATTLIST string
      is            CDATA    #IMPLIED
      contains      CDATA    #IMPLIED
   >  <!-- Exactly one of these two attributes must appear -->

   <!-- Time-switch makes choices based on the current time. -->

   <!ELEMENT time-switch ( (time|not-present)+, otherwise? ) >
   <!ATTLIST time-switch
      tzid          CDATA    #IMPLIED
      tzurl         CDATA    #IMPLIED
   >

   <!ELEMENT time ( %Node; ) >

   <!-- Exactly one of the two attributes "dtend" and "duration"
        must occur. -->
   <!-- The value of "freq" is
           (secondly|minutely|hourly|daily|weekly|monthly|yearly).  It is
           case-insensitive, so it is not given as a DTD switch. -->
   <!-- None of the attributes following freq is meaningful unless freq
            appears. -->
   <!-- At most one of "until" and "count" may appear. -->
   <!-- The value of "wkst" is (MO|TU|WE|TH|FR|SA|SU).  It is
           case-insensitive, so it is not given as a DTD switch. -->
   <!ATTLIST time
      dtstart       CDATA  #REQUIRED
      dtend         CDATA  #IMPLIED
      duration      CDATA  #IMPLIED
      freq          CDATA  #IMPLIED



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      until         CDATA  #IMPLIED
      count         CDATA  #IMPLIED
      interval      CDATA  "1"
      bysecond      CDATA  #IMPLIED
      byminute      CDATA  #IMPLIED
      byhour        CDATA  #IMPLIED
      byday         CDATA  #IMPLIED
      bymonthday    CDATA  #IMPLIED
      byyearday     CDATA  #IMPLIED
      byweekno      CDATA  #IMPLIED
      bymonth       CDATA  #IMPLIED
      wkst          CDATA  "MO"
      bysetpos      CDATA  #IMPLIED
   >


   <!-- Priority-switch makes choices based on message priority. -->

   <!ELEMENT priority-switch ( (priority|not-present)+, otherwise? ) >
   <!-- <not-present> must appear at most once -->

   <!ENTITY % PriorityVal '(emergency|urgent|normal|non-urgent)' >

   <!ELEMENT priority ( %Node; ) >

   <!-- Exactly one of these three attributes must appear -->
   <!ATTLIST priority
      less          %PriorityVal;  #IMPLIED
      greater       %PriorityVal;  #IMPLIED
      equal         CDATA          #IMPLIED
   >


   <!-- Locations: ways to specify the location a subsequent action
        (proxy, redirect) will attempt to contact. -->

   <!ENTITY % Clear  'clear (yes|no) "no"' >

   <!ELEMENT location ( %Node; ) >
   <!ATTLIST location
      url           CDATA    #REQUIRED
      %Clear;
   >

   <!ELEMENT lookup ( success,notfound?,failure? ) >
   <!ATTLIST lookup
     source         CDATA     #REQUIRED
     timeout        CDATA     "30"



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     use            CDATA     #IMPLIED
     ignore         CDATA     #IMPLIED
     %Clear;
   >

   <!ELEMENT success  ( %Node; ) >
   <!ELEMENT notfound ( %Node; ) >
   <!ELEMENT failure ( %Node; ) >

   <!ELEMENT remove-location ( %Node; ) >
   <!ATTLIST remove-location
      param         CDATA    #IMPLIED
      value         CDATA    #IMPLIED
      location      CDATA    #IMPLIED
   >


   <!-- Signalling Actions: call-signalling actions the script can
        take. -->

   <!ELEMENT proxy ( busy?,noanswer?,redirection?,failure? ) >

   <!-- The default value of timeout is "20" if the <noanswer> output
        exists. -->
   <!ATTLIST proxy
      timeout       CDATA    #IMPLIED
      recurse       (yes|no) "yes"
      ordering      CDATA    "parallel"
   >

   <!ELEMENT busy ( %Node; ) >
   <!ELEMENT noanswer ( %Node; ) >
   <!ELEMENT redirection ( %Node; ) >
   <!-- "failure" repeats from lookup, above. -->

   <!ELEMENT redirect EMPTY >
   <!ATTLIST redirection
      permanent     (yes|no) "no"
   >


   <!-- Statuses we can return -->

   <!ELEMENT reject EMPTY >
   <!-- The value of "status" is (busy|notfound|reject|error), or a SIP
        4xx-6xx status. -->
   <!ATTLIST reject
      status        CDATA    #REQUIRED



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      reason        CDATA    #IMPLIED
   >

   <!-- Non-signalling actions: actions that don't affect the call -->

   <!ELEMENT mail ( %Node; ) >
   <!ATTLIST mail
      url           CDATA    #REQUIRED
   >

   <!ELEMENT log ( %Node; ) >
   <!ATTLIST log
      name          CDATA    #IMPLIED
      comment       CDATA    #IMPLIED
   >


   <!-- Calls to subactions. -->

   <!ELEMENT sub EMPTY >
   <!ATTLIST sub
      ref           IDREF    #REQUIRED
   >


   <!-- Ancillary data -->

   <!ENTITY % Ancillary 'ancillary?' >

   <!ELEMENT ancillary EMPTY >


   <!-- Subactions -->

   <!ENTITY % Subactions 'subaction*' >

   <!ELEMENT subaction ( %Node; )>
   <!ATTLIST subaction
      id            ID       #REQUIRED
   >


   <!-- Top-level actions -->

   <!ENTITY % TopLevelActions 'outgoing?,incoming?' >

   <!ELEMENT outgoing ( %Node; )>




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   <!ELEMENT incoming ( %Node; )>

   <!-- The top-level element of the script. -->

   <!ELEMENT cpl  ( %Ancillary;,%Subactions;,%TopLevelActions; ) >



B TODO

        o Add many more examples, especially for the caller preferences
          parts of lookup and remove-location, and for time-switch.
          Other areas that are not currently addressed (or not addressed
          adequately) are address-switch subfields, string-switch,
          priority-switch, and log, and a number of parameters.

        o Investigate if there is some way that H.323 endpoint
          characteristics can be usefully mapped to SIP caller
          preferences and callee capabilities.

C Changes from earlier versions

C.1 Changes from draft -01

   The changebars in the Postscript and PDF versions of this document
   indicate significant changes from this version.

        o Completely re-wrote changes to time switches: they are now
          based on iCal rather than on crontab.

        o Timezone references are now defined within time switches
          rather than in the ancillary section. The ancillary section is
          now empty, but still defined for future use. To facilitate
          this, an explicit ancillary tag was added.

        o Added XML document type identifiers (the public identifier and
          the namespace), and MIME registration information.

        o Clarified that the not-present output can appear anywhere in a
          switch.

        o Re-wrote H.323 address mappings. Added the alias-type subfield
          for H.323 addresses.

        o Added the language and display string switch fields.

        o Clarified why useless not-present outputs can appear in time
          and priority switches.



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        o Added the clear parameter to location and lookup nodes. (It
          had been in the DTD previously, but not in the text.)

        o Weakened support for non-validating scripts from SHOULD to
          MAY, to allow the use of validating XML parsers.

        o Added redirection output of proxy nodes.

        o Clarified some aspects of how proxy nodes handle the location
          set.

        o Added permanent parameter of redirect nodes.

        o Add example script for outgoing call screening (from Kenny
          Hom)

        o Updated example scripts to use the public identifier.

        o Add omitted tag to example script for call forward busy/no
          answer

        o Clarified in introduction that this document mainly deals with
          servers.

        o Updated reference to RFC 2824 now that it has been published.

        o Added explanatory text to the introduction to types of nodes.

        o Numerous minor clarifications and wording changes.

        o Fixed copy-and-paste errors, typos.

C.2 Changes from draft -00

        o Added high-level structure; script doesn't just start at a
          first action.

        o Added a section giving a high-level explanation of the
          location model.

        o Added informal syntax specifications for each tag so people
          don't have to try to understand a DTD to figure out the
          syntax.

        o Added subactions, replacing the old link tags. Links were far
          too reminiscent of gotos for everyone's taste.

        o Added ancillary information section, and timezone support.



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        o Added not-present switch output.

        o Added address switches.

        o Made case-insensitive string matching locale-independent.

        o Added priority switch.

        o Deleted "Other switches" section. None seem to be needed.

        o Unified url and source parameters of lookup.

        o Added caller prefs to lookup.

        o Added location filtering.

        o Eliminated "clear" parameter of location setting. Instead,
          proxy "eats" locations it has used.

        o Added recurse and ordering parameters to proxy.

        o Added default value of timeout for proxy.

        o Renamed response to reject.

        o Changed notify to mail, and simplified it.

        o Simplified log, eliminating its failure output.

        o Added description of default actions at various times during
          script processing.

        o Updated examples for these changes.

        o Updated DTD to reflect new syntax.

D Authors' Addresses

   Jonathan Lennox
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY 10027
   USA
   electronic mail: lennox@cs.columbia.edu

   Henning Schulzrinne
   Dept. of Computer Science



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   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY 10027
   USA
   electronic mail: schulzrinne@cs.columbia.edu

E Bibliography

   [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 2543, Internet
   Engineering Task Force, Mar. 1999.

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

   [3] T. Bray, J. Paoli, and C. M. Sperberg-McQueen, "Extensible markup
   language (XML) 1.0," W3C Recommendation REC-xml-19980210, World Wide
   Web Consortium (W3C), Feb. 1998.  Available at
   http://www.w3.org/TR/REC-xml.

   [4] J. Lennox and H. Schulzrinne, "Call processing language framework
   and requirements," Request for Comments 2824, Internet Engineering
   Task Force, May 2000.

   [5] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [6] D. Raggett, A. Le Hors, and I. Jacobs, "HTML 4.0 specification,"
   W3C Recommendation REC-html40-19980424, World Wide Web Consortium
   (W3C), Apr.  1998.  Available at http://www.w3.org/TR/REC-html40/.

   [7] ISO (International Organization for Standardization),
   "Information processing -- text and office systems -- standard
   generalized markup language (SGML)," ISO Standard ISO 8879:1986(E),
   International Organization for Standardization, Geneva, Switzerland,
   Oct. 1986.

   [8] M. Murata, S. S. Laurent, and D. Kohn, "XML media types,"
   Internet Draft, Internet Engineering Task Force, June 2000.  Work in
   progress.

   [9] N. Freed, J. Klensin, and J. Postel, "Multipurpose internet mail
   extensions (MIME) part four: Registration procedures," Request for
   Comments 2048, Internet Engineering Task Force, Nov. 1996.

   [10] International Telecommunication Union, "Packet based multimedia



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   communication systems," Recommendation H.323 Draft v4,
   Telecommunication Standardization Sector of ITU, Geneva, Switzerland,
   July 2000.  To be published November 2000.

   [11] International Telecommunication Union, "Digital subscriber
   signalling system no. 1 (dss 1) - isdn user-network interface layer 3
   specification for basic call control," Recommendation Q.931,
   Telecommunication Standardization Sector of ITU, Geneva, Switzerland,
   Mar. 1993.

   [12] H. Alvestrand, "Tags for the identification of languages,"
   Request for Comments 1766, Internet Engineering Task Force, Mar.
   1995.

   [13] M. Davis and M. Drst, "Unicode normalization forms," Unicode
   Technical Report 15, Unicode Consortium, Nov. 1999.  Revision 18.0.
   Available at http://www.unicode.org/unicode/reports/tr15/.

   [14] M. Davis, "Case mapping," Unicode Technical Report 21, Unicode
   Consortium, Nov. 1999.  Revision 3.0. Available at
   http://www.unicode.org/unicode/reports/tr21/.

   [15] F. Dawson and D. Stenerson, "Internet calendaring and scheduling
   core object specification (icalendar)," Request for Comments 2445,
   Internet Engineering Task Force, Nov. 1998.

   [16] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
   callee capabilities," Internet Draft, Internet Engineering Task
   Force, Mar. 2000.  Work in progress.

   [17] S. DeRose, E. Maler, D. Orchard, and B. Trafford, "XML linking
   language (XLink)," Working Draft WD-xlink-20000221, World Wide Web
   Consortium (W3C), Feb. 2000.  Available at
   http://www.w3.org/TR/xlink/.

   [18] T. Showalter, "Sieve: A mail filtering language," Internet
   Draft, Internet Engineering Task Force, May 2000.  Work in progress.


   Full Copyright Statement

   Copyright (c) The Internet Society (2000). 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



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   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
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                           Table of Contents



   1          Introduction ........................................    1
   1.1        Conventions of this document ........................    2
   2          Structure of CPL scripts ............................    3
   2.1        High-level structure ................................    3
   2.2        Abstract structure of a call processing action ......    3
   2.3        Location model ......................................    4
   2.4        XML structure .......................................    5
   3          Document information ................................    5
   3.1        CPL Document Identifiers for XML ....................    5
   3.2        MIME Registration ...................................    6
   4          Script structure: overview ..........................    7
   5          Switches ............................................    8
   5.1        Address switches ....................................    9
   5.1.1      Address switch mapping for SIP ......................   11
   5.1.2      Address switch mapping for H.323 ....................   12
   5.2        String switches .....................................   13
   5.3        Time switches .......................................   15
   5.4        Priority switches ...................................   20
   6          Location modifiers ..................................   21
   6.1        Explicit location ...................................   21
   6.2        Location lookup .....................................   22
   6.3        Location filtering ..................................   23



Lennox/Schulzrinne                                           [Page 48]

Internet Draft                    CPL                      July 14, 2000


   7          Signalling actions ..................................   24
   7.1        Proxy ...............................................   24
   7.2        Redirect ............................................   26
   7.3        Reject ..............................................   27
   8          Other actions .......................................   27
   8.1        Mail ................................................   28
   8.2        Log .................................................   28
   9          Subactions ..........................................   29
   10         Ancillary information ...............................   30
   11         Default actions .....................................   30
   12         Examples ............................................   31
   12.1       Example: Call Redirect Unconditional ................   31
   12.2       Example: Call Forward Busy/No Answer ................   31
   12.3       Example: Call Screening .............................   33
   12.4       Example: Outgoing Call Screening ....................   33
   12.5       Example: Time-of-day Routing ........................   33
   12.6       Example: Non-call Actions ...........................   33
   12.7       Example: A Complex Example ..........................   35
   13         Security considerations .............................   35
   14         IANA considerations .................................   36
   15         Acknowledgments .....................................   36
   A          The XML DTD for CPL .................................   37
   B          TODO ................................................   43
   C          Changes from earlier versions .......................   43
   C.1        Changes from draft -01 ..............................   43
   C.2        Changes from draft -00 ..............................   44
   D          Authors' Addresses ..................................   45
   E          Bibliography ........................................   46























Lennox/Schulzrinne                                           [Page 49]


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