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Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Schulzrinne
draft-ietf-iptel-cpl-03.txt Columbia University
October 25, 2000
Expires: April, 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

To view the list Internet-Draft Shadow Directories, see
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.

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1 Introduction

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

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discussion in the development of the CPL, and are not
essential to the specification of the language.

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.

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 use by 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.

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

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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.

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

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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 6.1).

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 C. 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.

A CPL embedded as a fragment within another XML document is
identified with the XML namespace identifier

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

Figure 2: Sample CPL Script: XML Version

"http://www.ietf.org/internet-drafts/draft-ietf-iptel-cpl-03.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

As an XML type, CPL's MIME registration conforms with "XML Media

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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 14, 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 detected until runtime.

Published specification: This document.

Applications which use this media type: None publicly 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,
subactions, and top-level actions. The full syntax of the cpl node is

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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

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
present in the original call setup request. (In this document, this
is sometimes described by saying that the information is "absent".)
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

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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, or display,
(
also: password and 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 for protocol-specific values. (The subfield password is
defined for SIP in Section 5.1.1; the subfield alias-type is defined
for H.323 in Appendix B.1.) 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.

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

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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 absent.

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 absent.

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 absent; 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 may be absent. 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 absent.

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
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.

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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 Usage of address-switch with 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, and is case-sensitive. 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.

For h323 URLs, subfields MAY be set according to the scheme described
in Appendix B.

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

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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.

5.2 String switches

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

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

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

String switches are dependent on the call signalling protocol being
used.

Five fields are defined, listed below. The value of each of these
fields, except as specified, is a free-form Unicode string with no
other structure defined.

subject The subject of the call.

organization The organization of the originator of the call.

user-agent The name of the program or device with which the call
request was made.

language The languages in which the originator of the call
wishes to receive responses. This contains a list of RFC
1766 [10] language tags, separated by commas.

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

display Free-form text associated with the call, intended to be
displayed to the recipient, with no other semantics defined
by the signalling protocol.

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 [11]. Then, strings are compared using locale-
insensitive caseless mapping, as specified in Unicode Technical
Report 21 [12].

Code to perform the first step, in Java and Perl, is
available; see the links from Annex E of UTR 15 [11]. 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.

5.2.1 Usage of string-switch with SIP

For SIP, the fields subject, organization, and user-agent correspond
to the SIP header fields with the same name. These are used verbatim
as they appear in the message.

The field language corresponds to the SIP Accept-Language header. It
is converted to a list of comma-separated languages as described
above.

The field display is not used, and is never present.

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.

Time switches are independent of the underlying signalling protocol.

Time switches are based on a large subset of how recurring intervals

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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 "daily",
"weekly", "monthly", or "yearly")
interval How often the recurrence repeats
until Bound of recurrence (RFC 2445 DATE-TIME)
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 workweek

Figure 6: Syntax of the time-switch node

of time are specified in the Internet Calendaring and Scheduling Core
Object Specification (iCal COS), RFC 2445 [13].

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 subset was designed with the goal that a time-switch
can be evaluated -- an instant can be determined to fall
within an interval, or not -- in constant (O(1)) time.

An algorithm to whether an instant falls within a given recurrence is
given in Appendix A.

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

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the timezone can be retrieved.

While TZID labels that do not begin with a forward slash are locally
defined, it is RECOMMENDED that servers support at least the naming
scheme used by Olson Time Zone database [14]. Examples of timezone
databases that use the Olson scheme are the zoneinfo files on most
Unix-like systems, and the standard Java TimeZone class.

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. Note especially that some times may occur
more than once when clocks are set back. The algorithm in Appendix A
is believed to handle this correctly.

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 [13]. Because time zones are specified in the top-level
time-switch tag, only forms 1 or 2 (floating or UTC times) can be
used. The duration parameter is given as an iCal COS DURATION
parameter, as specified in section 4.3.6 of RFC 2445. Both the
DATE-TIME and the DURATION syntaxes are subsets of the corresponding
syntaxes from ISO 8601 [15].

For a recurring interval, the duration parameter MUST be less than
twenty-four hours. For non-recurring intervals, durations of any
length are permitted.

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: 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 values secondly, minutely, and hourly are present in
iCal, but were removed from CPL.

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, the recurrence is considered to
repeat forever.

iCal also defines a count parameter, which allows an
alternate method of specifying a bound to a recurrence.
This bound has been removed from CPL. Translating from full
iCal recurrences to CPL recurrences requires that the count
parameter be converted to an until parameter, which can be
done by enumerating the recurrence and determining its
final date.

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
(-n) integer. If present, this indicates the nth occurrence of the
specific day within the monthly or yearly recurrence. For example,

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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 [15]. 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, following ISO 8601 [15].

iCal also includes the Byxxx parameters bysecond, byminute,
byhour, and bysetpos, which have been removed from CPL.

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

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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, and byday; then
until is evaluated.

Here is an example of evaluating multiple Byxxx parameters.

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 the time of day is derived from dtstart to end up in "every
Sunday in January from 8:30:00 AM to 8:40:00 AM, every other year."
Similarly, if the byday, bymonthday or bymonth parameter were
missing, the appropriate 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.

5.3.1 Motivations for the iCal subset

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(This sub-sub-section is non-normative.)

The syntax of the CPL time-switch was based on that of the iCal COS
RRULE, but as mentioned above, certain features were omitted and
restrictions were added. Specifically:

1. All recurrence intervals and rules describing periods less
than a day were removed. These were the frequencies
secondly, minutely, and hourly, and the Byxxx rules
bysecond, byminute, and byhour.

2. The count and bysetpos parameters were removed.

3. Durations were constrained to less than 24 hours for
recurring intervals.

These restrictions were added so that time switches could be resolved
efficiently, in O(1) time. This restriction means that it must be
possible to resolve a time switch without having to enumerate all its
recurrences from dtstart to the present interval. As far as we have
been able to determine, it is not possible to test whether the count
and bysetpos parameters are satisfied without performing such an
enumeration.

Constant running time of time switches also requires that a candidate
starting time for a recurrence can be established quickly and
uniquely, to check whether it satisfies the other restrictions. This
requires that a recurrence's duration not be longer than its
repetition interval, so that a given instant cannot fall within
several consecutive repetitions of the recurrence. We guaranteed this
by eliminating durations longer than 24 hours, and repetitions
shorter than that period. The one-day point seemed to be the most
generally useful place to place this division, as some investigation
showed that many common calendaring applications do not support
durations longer than a day, none that we found supported repetitions
shorter than a day. Eliminating sub-day repetitions also greatly
simplifies the handling of daylight-savings transitions.

The algorithm given in Appendix A runs in constant time, and
motivated the development of this iCal subset.

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. They are dependent on the underlying signalling protocol.

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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 one of the following priorities:
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
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.

5.4.1 Usage of priority-switch with SIP

The priority of a SIP message corresponds to the Priority header in
the message.

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

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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.

Explicit location nodes are dependent on the underlying signalling
protocol.

Node: location
Outputs: any node
Parameters: url URL of address to add to location set
priority Priority of this location (0.0-1.0)
clear Whether to clear the location set before adding
the new value

Figure 8: Syntax of the location node

Explicit location nodes have three 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 priority parameter specifies a priority for the
location. Its value is a floating-point number between 0.0 and 1.0.
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.

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.

6.1.1 Usage of location with SIP

All SIP locations are represented as URLs, so the locations specified
in location tags are interpreted directly.

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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.

Location lookup is dependent on the underlying signalling protocol.

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
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.

The lookup node also has four optional parameters. The timeout
parameter which specifies the time, in seconds, the script is willing

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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. By default,
a CPL server SHOULD invoke the appropriate caller preferences
filtering of the underlying signalling protocol, if the corresponding
information is available. The two parameters use and ignore allow the
script to modify how the script applies caller preferences filtering.
The specific meaning of the values of these parameters is
signalling-protocol dependent; see Section 6.2.1 for SIP and Appendix
B.5 for H.323.

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 C, but servers MAY accept them in any order.

6.2.1 Usage of lookup with SIP

Caller preferences for SIP are defined in "SIP Caller Preferences and
Callee Capabilities" [16]. By default, a CPL server SHOULD honor any
Accept-Contact and Reject-Contact headers of the original call
request, as specified in that document. The two parameters use and
ignore allow the script to modify the data input to the caller
preferences algorithm. These 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.

If a SIP server does not support caller preferences and callee
capabilities, if the call request does not contain any preferences,
or if the callee's registrations do not contain any capabilities, the
use and ignore parameters are ignored.

6.3 Location Removal

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A CPL script can also remove locations from the location set, through
the use of the remove-location node. The syntax of this node is
defined in Figure 10.

The meaning of this node is dependent on the underlying signalling
protocol.

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

Figure 10: Syntax of the remove-location node

A remove-location node removes locations from the location set. It is
primarily useful following a lookup node.

The remove-location node has three optional parameters. The parameter
location gives the URL (or a signalling-protocol-dependent URL
pattern) of location or locations to be removed from the set. If this
parameter is not given, all locations, subject to the constraints of
the other parameters, are removed from the set.

If param and value are present, their values are comma-separated
lists of caller preferences parameters and corresponding values,
respectively. The where the nth entry in the param list matches the
nth entry in the value list. There MUST be the same number of
parameters as values specified. The meaning of these parameters is
signalling-protocol dependent.

6.3.1 Usage of remove-location with SIP

For SIP-based CPL servers, the 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; thus,
an absent header is equivalent to an addr-spec of "*" in that
specification. The param and value parameters are treated as though
they appeared in the params field of a Reject-Location header, as ";
param=value" for each one.

If the CPL server does not support caller preferences and callee
capabilities, or if the callee did not supply any preferences, the

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param and value parameters are ignored.

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.

The specific actions invoked by the proxy node are signalling-
protocol-dependent, though the general concept should apply to any
signalling protocol.

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
default Default action for unspecified outputs
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

Output: default
Parameters: none

Figure 11: Syntax of the proxy node

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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, CPL execution terminates and the
server proceeds to its default behavior (normally, to allow the call
to be set up). Otherwise, the action corresponding to one of the
proxy node's outputs is taken. The busy output is followed if the
call was busy; noanswer is followed if the call was not answered
before the timeout parameter expired; redirection is followed if the
call was redirected; and failure is followed if the call setup failed
for any other reason.

If one of the conditions above is true, but the corresponding output
was not specified, the default output of the proxy node is followed
instead. If there is also no default node specified, CPL execution
terminates and the server returns to its default behavior (normally,
to forward the best response upstream to the originator).

Note: CPL extensions to allow in-call or end-of-call
actions will require an additional output, such as success,
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 noanswer or
default 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).

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

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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 CPL servers SHOULD honor the
priority parameter of the location tag. 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 of proxyable
locations if the ordering was parallel or sequential; the highest-
priority item in the set is removed from the set if ordering was
first-only. (In all cases, non-proxyable locations such as "http"
URIs remain.) In the case of a redirection output, the new addresses
to which the call was redirected are then added to the location set.

7.1.1 Usage of proxy with SIP

For SIP, the best response to a proxy node is determined by the
algorithm of the SIP specification. The node's outputs correspond to
the following events:

busy A 486 or 600 response was the best response received to the
call request.

redirection A 3xx response was the best response received to the
call request.

failure Any other 4xx, 5xx, or 6xx response was the best
response received to the call request.

no-answer No final response was received to the call request
before the timeout expired.

SIP servers SHOULD honor the q parameter of SIP registrations and the
output of the caller preferences lookup algorithm when determining
location priority.

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.

The specific behavior the redirect node invokes is dependent on the

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underlying signalling protocol involved, though its semantics are
generally applicable.

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

Figure 12: Syntax of the redirect node

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."

7.2.1 Usage of redirect with SIP

The SIP server SHOULD send a 3xx class response to a call request
upon executing a redirect tag. If permanent was yes, the server
SHOULD send the response "301 Moved permanently"; otherwise it SHOULD
send "302 Moved temporarily".

7.3 Reject

Reject nodes cause the server to reject the call attempt. Their
syntax is given in Figure 13. The specific behavior they invoke is
dependent on the underlying signalling protocol involved, though
their semantics are generally applicable.

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.

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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, or a signalling-protocol-defined status.

The reason argument optionally allows the script to specify a reason
for the rejection.

7.3.1 Usage of redirect with SIP

Servers which implement SIP SHOULD also allow the status field to be
a numeric argument corresponding to a SIP status in the 4xx, 5xx, or
6xx range.

They SHOULD send the "reason" parameter in the SIP reason phrase.

A suggested mapping of the named statuses is as follows. Servers MAY
use a different mapping, though similar semantics SHOULD be
preserved.

busy: 486 Busy Here

notfound: 404 Not Found

reject: 603 Decline

error: 500 Internal Server Error

8 Other actions

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

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

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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.

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" />.

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.

Note that the syntax of XML requires that ampersand characters, "&",
which are used as parameter separators in mailto URLs, be quoted as
"&" inside parameter values (see section C.12 of [3]).

8.1.1 Suggested Content of Mailed Information

This section presents suggested guidelines for the mail sent as a
result of the mail node, for requests triggered by SIP. The message
mailed (triggered by any protocol) SHOULD contain all this
information, but servers MAY elect to use a different format.

1. If the mailto URI did not specify a subject header, the
subject of the e-mail is "[CPL]" followed by the subject
header of the SIP request. If the URI specified a subject
header, it is used instead.

2. The From field of the e-mail is set to a CPL server
configured address, overriding any From field in the mailto
URI.

3. Any Reply-To header in the URI is honored. If none is
given, then an e-mail-ized version of the origin field of
the request is used, if possible (e.g., a SIP From header
with a sip: URI would be converted to an e-mail address by
stripping the URI scheme).

4. If the mailto URI specifies a body, it is used. If none was
specified, the body SHOULD contain at least the identity of
the caller (both the caller's display name and address),
the date and time of day, the call subject, and if
available, the call priority.

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The server SHOULD honor the user's requested languages, and send the
mail notification using an appropriate language and character set.

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

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 log file for the script owner. This specification does
not define how users may retrieve their logs from the server.

The name of a log is a logical name only, and does not necessarily
correspond to any physical file on the server. The interpretation of
the log file name is server defined, as is a mechanism to access
these logs. The CPL server SHOULD NOT directly map log names
uninterpreted onto local file names, for security reasons, lest a
security-critical file be overwritten.

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.

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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.

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 or extension defines external
linkages, it should probably use a different tag, perhaps
XLink [17]. Ensuring termination in the presence of
external links is a difficult problem.

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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.

Tag: ancillary
Parameters: none
Subtags: none

Figure 17: Syntax of the ancillary tag

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
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.

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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 CPL Extensions

Servers MAY support additional CPL features beyond those listed in
this document. Some of the extensions which have been suggested are a
means of querying how a call has been authenticated; richer control
over H.323 addressing; end-system or administrator-specific features;
regular-expression matching for strings and addresses; mid-call or
end-of-call controls; and the parts of iCal COS recurrence rules
omitted from time switches.

CPL extensions are indicated by XML namespaces [18]. Every extension
MUST have an appropriate XML namespace assigned to it. All XML tags
and attributes that are part of the extension MUST be appropriately
qualified so as to place them within that namespace.

Tags or attributes in a CPL script which are in the global namespace
(i.e., not associated with any namespace) are equivalent to tags and
attributes in the CPL namespace "http://www.ietf.org/internet-
drafts/draft-ietf-iptel-cpl-03.txt".

A CPL server MUST reject any script which contains a reference to a
namespace which it does not understand. It MUST reject any script
which contains an extension tag or attribute which is not qualified
to be in an appropriate namespace.

A CPL script SHOULD NOT specify any namespaces it does not use. For
compatibility with non-namespace-aware parsers, a CPL script SHOULD
NOT specify the base CPL namespace for a script which does not use
any extensions.

A syntax such as

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<extension-switch>
<extension has="http://www.example.com/foo">
[extended things]
</extension>
<otherwise>
[non-extended things]
</otherwise>
</extension-switch>

was suggested as an alternate way of handling extensions.
This would allow scripts to be uploaded to a server without
requiring a script author to somehow determine which
extensions a server supports. However, experience
developing other languages, notably Sieve [19], was that
this added excessive complexity to languages. The
extension-switch tag could, of course, itself be defined in
a CPL extension.

It is unfortunately true that XML DTDs, such as the CPL DTD
given in appendix C, are not powerful enough to encompass
namespaces, since the base XML specification (which defines
DTDs) predates the XML namespace specification. XML schemas
[20] are a work in progress to define a namespace-aware
method for validating XML documents, as well as improving
upon DTDs' expressive power in many other ways.

13 Examples

13.1 Example: Call Redirect Unconditional

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

13.2 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,
through the use of subactions.

13.3 Example: Call Forward: Redirect and Default

The script in Figure 20 illustrates further proxy behavior. The
server initially tries to proxy to a single address. If this attempt

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

Figure 18: Example Script: Call Redirect Unconditional

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

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

is redirected, a new redirection is generated using the locations
returned. In all other failure cases for the proxy node, a default
action -- forwarding to voicemail -- is performed.

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

Figure 20: Example Script: Call Forward: Redirect and Default

13.4 Example: Call Screening

The script in Figure 21 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.

13.5 Example: Priority and Language Routing

The script in Figure 22 illustrates service selection based on a
call's priority value and language settings. If the call request had
a priority of "urgent" or higher, the default script action is taken.
Otherwise, the language string field is checked for the string "es"
(Spanish). If it is present, the call is proxied to a Spanish-
speaking operator; other calls are proxied to an English-speaking
operator.

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

Figure 21: Example Script: Call Screening

13.6 Example: Outgoing Call Screening

The script in Figure 23 illustrates a script filtering outgoing
calls, in the form of a script which prevent 1-900 (premium) calls
from being placed. This script also illustrates subdomain matching.

13.7 Example: Time-of-day Routing

Figure 24 illustrates time-based conditions and timezones.

13.8 Example: Location Filtering

Figure 24 illustrates filtering actions on the location set. In this
example, we assume that version 0.9beta2 of the "Inadequate Software
SIP User Agent" mis-implements some features, and so we must work
around its problems. We assume, first, that the value of its
"feature" parameter in caller preferences is known to be unreliable,
so we ignore it; we also know that it cannot talk successfully to one
particular mobile device we may have registered, so we remove that
location from the location set. Once these two actions have been
completed, call setup is allowed to proceed normally.

13.9 Example: Non-call Actions

Figure 26 illustrates non-call actions; in particular, alerting a

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

Figure 22: Example Script: Priority and Language Routing

user by electronic mail if the lookup server failed. The primary
motivation for having 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.

13.10 Example: Hypothetical Extensions

The example in Figure 27 shows a hypothetical extension which
implements distinctive ringing. The XML namespace
"http://www.example.com/distinctive-ring" specifies a new node named
ring.

The example in Figure 28 implements a hypothetical new attribute for
address switches, to allow regular-expression matches. It defines a
new attribute regex for the standard address node. In this example,
the global namespace is not specified.

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

Figure 23: Example Script: Outgoing Call Screening

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

Figure 24: Example Script: Time-of-day Routing

13.11 Example: A Complex Example

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

Figure 25: Example Script: Location Filtering

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

Figure 26: Example Script: Non-call Actions

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

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

Figure 27: Example Script: Hypothetical Distinctive-Ringing Extension

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

Figure 28: Example Script: Hypothetical Regular-Expression Extension

are forwarded to his celphone, and all other calls are directed to
voicemail.

14 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.

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

Figure 29: Example Script: A Complex Example

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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details of what CPL actions are permitted.

15 IANA considerations

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

16 Acknowledgments

This document was reviewed and commented upon by IETF IP Telephony
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 [13], 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 [19], a language for user filtering of
electronic mail messages.

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

A An algorithm for resolving time switches

The following algorithm resolves, in constant time, whether a given
instant falls within a repetition of a time-switch recurrence. Open-
source Java code implementing this algorithm is available on the
world wide web at <http://www.cs.columbia.edu/~lennox/Cal-Code/>

1. Compute the time of the call, in the timezone of the time
switch. (No step after this needs to consider time zones
-- all calculations are done using continuously-running
standard Gregorian time.)

2. If the call time is earlier than dtstart, fail NOMATCH.

3. If the call time is less than duration after dtstart,
succeed MATCH.

4. Determine the smallest unit specified in a byxxx rule or by
the freq. Call this the Minimum Unit. Determine the
previous instant (before the call time) when all the time

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units smaller than the minimum unit are the same as those
of dtstart. (For all minimum units, the time-of-day must be
the same as dtstart. If the minimum unit is a week, the
day-of-the-week must be the same as dtstart. If the minimum
unit is a month, the day-of-the-month must be the same as
dtstart. If the minimum unit is a year, the month and day-
of-month must both be the same as dtstart. (Note that this
means it may be necessary to roll back more than one
minimum unit -- if the minimum unit is a month, then some
months do not have a 31st (or 30th or 29th) day; if the
minimum unit is a year, then some years do not have a
February 29th. In the Gregorian calendar, it is never
necessary to roll back more than two months, or eight years
(four years between 1904 and 2096).)

Call this instant the Candidate Start Time.

5. If the time between the candidate start time and the call
time is more than the duration, fail NOMATCH.

6. If the candidate start time is later than the until
parameter of the recurrence, fail NOMATCH.

7. Call the unit of the freq parameter of the recurrence the
Frequency Unit. Determine the frequency unit enclosing the
Candidate Start Time, and that enclosing dtstart. Calculate
the number of frequency units that have passed between
these two times. If this is not a multiple of the interval
parameter, fail NOMATCH.

8. For every byxxx rule, confirm that the candidate start time
matches one of the options specified by that byxxx rule. If
not, fail NOMATCH.

9. Succeed MATCH.

B Suggested Usage of CPL with H.323

This appendix gives a suggested usage of CPL with H.323 [2]. Study
Group 16 of the ITU, which developed H.323, is proposing to work on
official CPL mappings for that protocol. This section is therefore
not normative.

B.1 Usage of address-switch with H.323

Address switches are specified in section 5.1. This section specifies
the mapping between H.323 messages and the fields and subfields of
address-switches

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For H.323, the origin address corresponds to the alias addresses in
the sourceAddress field of the Setup-UUIE user-user information
element, and to the Q.931 [21] information element "Calling party
number." If both fields are present, or if multiple aliases addresses
for sourceAddress 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 alias addresses of the
destinationAddress field, and to the Q.931 information element
"Called party number."

The original-destination address corresponds to the "Redirecting
number" 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 two names SHOULD be treated as synonyms.

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 in Section 5.1.1 for SIP.

An H.323-aware CPL server SHOULD map the address subfields from the
primary alias used for routing. It MAY also map subfields from other
aliases, if subfields in the primary address are not present.

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: the same mappings are used as for SIP, in Section 5.1.1.

h323-ID: the user field is the string of characters, as is the
"entire-address" form. All other subfields are not present.

email-ID: the user and host subfields are set to the

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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.

H.323 version 4 [22] and the Internet-Draft draft-levin-iptel-h323-
url-scheme-00 [23] define a "h323" URI scheme. This appendix defines
a mapping for these URIs onto the CPL address-switch subfields, as
given in section 5.1. Neither of these documents has yet been
formally published in a final form, so this appendix is non-
normative.

For h323 URIs, the 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.

This mapping MAY be used both for h323 URIs in an h323 url-ID address
alias, and for h323 URIs in SIP messages.

B.2 Usage of string-switch with H.323

For H.323, the string-switch node (see Section 5.2) is used as
follows. The field language corresponds to the H.323 UUIE language,
translated to the format specified for that field. The field display
corresponds to the Q.931 information element of the same name, copied
verbatim. The fields subject, organization, and user-agent are not
used and are never present.

The display IE 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 [21] 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 allow it to be matched in a string-
switch instead.

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B.3 Usage of priority-switch with H.323

All H.323 messages are considered to have priority normal for the
purpose of a priority switch (see Section 5.4).

B.4 Usage of location with H.323

Locations in explicit location nodes (Section 6.1) are specified as
URLs. Therefore, all locations added in this manner are interpreted
as being of alias type url-ID in H.323.

Specifications of other H.323 address alias types will require a CPL
extension (see Section 12).

B.5 Usage of lookup with H.323

For location lookup nodes (Section 6.2), the registration lookup
source corresponds to the locations registered with the server using
RAS messages.

As H.323 currently has no counterpart of SIP caller preferences and
callee capabilities, the use and ignore parameters of the lookup node
are ignored.

B.6 Usage of remove-location with H.323

For location removal nodes (Section 6.3), only literal URLs can be
removed. No URL patterns are defined.

As H.323 currently has no counterpart of SIP caller preferences and
callee capabilities, the param and value parameters of the remove-
location node are ignored.

C 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" ?>

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

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

<!ENTITY % SignallingAction 'proxy|redirect|reject' >

<!ENTITY % OtherAction 'mail|log' >

<!ENTITY % Sub 'sub' >

<!ENTITY % Node '(%Location;|%Switch;|%SignallingAction;|
%OtherAction;|%Sub;)?' >

<!ELEMENT otherwise ( %Node; ) >

<!ELEMENT not-present ( %Node; ) >

<!ELEMENT address-switch ( (address|not-present)+, otherwise? ) >

<!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
>

<!ELEMENT string-switch ( (string|not-present)+, otherwise? ) >

<!ATTLIST string-switch
field CDATA #REQUIRED
>

<!ELEMENT string ( %Node; ) >
<!ATTLIST string
is CDATA #IMPLIED
contains CDATA #IMPLIED
>

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

<!ELEMENT time ( %Node; ) >

<!ATTLIST time
dtstart CDATA #REQUIRED
dtend CDATA #IMPLIED
duration CDATA #IMPLIED
freq CDATA #IMPLIED
until CDATA #IMPLIED
interval CDATA "1"

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byday CDATA #IMPLIED
bymonthday CDATA #IMPLIED
byyearday CDATA #IMPLIED
byweekno CDATA #IMPLIED
bymonth CDATA #IMPLIED
wkst CDATA "MO"
>

<!ELEMENT priority-switch ( (priority|not-present)+, otherwise? ) >

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

<!ELEMENT priority ( %Node; ) >

<!ATTLIST priority
less %PriorityVal; #IMPLIED
greater %PriorityVal; #IMPLIED
equal CDATA #IMPLIED
>

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

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

<!ELEMENT lookup ( success,notfound?,failure? ) >
<!ATTLIST lookup
source CDATA #REQUIRED
timeout CDATA "30"
use CDATA #IMPLIED
ignore CDATA #IMPLIED
%Clear;
>

<!ELEMENT success ( %Node; ) >

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

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

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

<!ATTLIST proxy
timeout CDATA #IMPLIED
recurse (yes|no) "yes"
ordering CDATA "parallel"
>

<!ELEMENT busy ( %Node; ) >
<!ELEMENT noanswer ( %Node; ) >
<!ELEMENT redirection ( %Node; ) >

<!ELEMENT default ( %Node; ) >

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

<!ELEMENT reject EMPTY >

<!ATTLIST reject
status CDATA #REQUIRED
reason CDATA #IMPLIED
>

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

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

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

<!ENTITY % Ancillary 'ancillary?' >

<!ELEMENT ancillary EMPTY >

<!ENTITY % Subactions 'subaction*' >

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

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

<!ELEMENT outgoing ( %Node; )>

<!ELEMENT incoming ( %Node; )>

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

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D Changes from earlier versions

D.1 Changes from draft -02

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

o Reduced time-switches from the full iCal recurrence to an iCal
subset. Added an appendix giving an algorithm to resolve
time-switches.

o Added the extension mechanism.

o Made explicit how each node is dependent on protocol handling.
Separated out protocol-specific information -- for SIP in
subsections of the main text, for H.323 in a non-normative
appendix.

o Clarified some address mapping rules for H.323.

o Corrected the name of the "Redirecting number" in Q.931.

o Clarified that address matching on the password subfield is
case-sensitive.

o Added a recommendation that TZID labels follow the usage of
the Olson database.

o Added the priority parameter to location nodes.

o Added the default output to the proxy node.

o Made the meaning of the proxy node's outputs explicit.

o Added suggested content for the e-mail generated by mail
nodes.

o Pointed out that "&" must be escaped in XML (this is relevant
for mailto URIs).

o Pointed out that log names are logical names, and should not
be interpreted as verbatim filenames.

o Added some examples.

o Clarified some wording.

o Fixed some typographical errors.

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D.2 Changes from draft -01

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.

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.

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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.

D.3 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.

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.

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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.

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

F 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

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Task Force, May 2000.

[5] S. Bradner, "Key words for use in RFCs to indicate requirement
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[6] D. Raggett, A. Le Hors, and I. Jacobs, "HTML 4.0 specification,"
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[16] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
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Full Copyright Statement

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of

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developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

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

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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Table of Contents

1 Introduction ........................................ 2
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 Usage of address-switch with SIP .................... 11
5.2 String switches ..................................... 12
5.2.1 Usage of string-switch with SIP ..................... 13
5.3 Time switches ....................................... 13
5.3.1 Motivations for the iCal subset ..................... 18
5.4 Priority switches ................................... 19
5.4.1 Usage of priority-switch with SIP ................... 20
6 Location modifiers .................................. 20
6.1 Explicit location ................................... 21
6.1.1 Usage of location with SIP .......................... 21
6.2 Location lookup ..................................... 22
6.2.1 Usage of lookup with SIP ............................ 23
6.3 Location Removal .................................... 23

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6.3.1 Usage of remove-location with SIP ................... 24
7 Signalling actions .................................. 25
7.1 Proxy ............................................... 25
7.1.1 Usage of proxy with SIP ............................. 27
7.2 Redirect ............................................ 27
7.2.1 Usage of redirect with SIP .......................... 28
7.3 Reject .............................................. 28
7.3.1 Usage of redirect with SIP .......................... 29
8 Other actions ....................................... 29
8.1 Mail ................................................ 29
8.1.1 Suggested Content of Mailed Information ............. 30
8.2 Log ................................................. 31
9 Subactions .......................................... 31
10 Ancillary information ............................... 33
11 Default actions ..................................... 33
12 CPL Extensions ...................................... 34
13 Examples ............................................ 35
13.1 Example: Call Redirect Unconditional ................ 35
13.2 Example: Call Forward Busy/No Answer ................ 35
13.3 Example: Call Forward: Redirect and Default ......... 35
13.4 Example: Call Screening ............................. 37
13.5 Example: Priority and Language Routing .............. 37
13.6 Example: Outgoing Call Screening .................... 38
13.7 Example: Time-of-day Routing ........................ 38
13.8 Example: Location Filtering ......................... 38
13.9 Example: Non-call Actions ........................... 38
13.10 Example: Hypothetical Extensions .................... 39
13.11 Example: A Complex Example .......................... 40
14 Security considerations ............................. 42
15 IANA considerations ................................. 44
16 Acknowledgments ..................................... 44
A An algorithm for resolving time switches ............ 44
B Suggested Usage of CPL with H.323 ................... 45
B.1 Usage of address-switch with H.323 .................. 45
B.2 Usage of string-switch with H.323 ................... 47
B.3 Usage of priority-switch with H.323 ................. 48
B.4 Usage of location with H.323 ........................ 48
B.5 Usage of lookup with H.323 .......................... 48
B.6 Usage of remove-location with H.323 ................. 48
C The XML DTD for CPL ................................. 48
D Changes from earlier versions ....................... 54
D.1 Changes from draft -02 .............................. 54
D.2 Changes from draft -01 .............................. 55
D.3 Changes from draft -00 .............................. 56
E Authors' Addresses .................................. 57
F Bibliography ........................................ 57

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