Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Schulzrinne
draft-ietf-iptel-cpl-05.txt Columbia University
November 21, 2001
Expires: May, 2002

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.

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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.

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

2 Structure of CPL Scripts

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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
operations and decisions a telephony signalling server performs on a
call set-up event. There are two types of call processing actions:
top-level actions and subactions. 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. Subactions are actions which can be
called from other actions. The CPL forbids subactions 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 operations or decisions. 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 operations that can be performed or decisions
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 decision 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 decision described by the
action's top-level node is performed; based on the result of that
node, the server follows one of the node's outputs, and the
subsequent node it points to 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 does not point to another node, it indicates
that the CPL server should perform a node- or protocol-specific
action. Some nodes have specific default behavior associated with
them; for others, the default behavior is implicit in the underlying
signalling protocol, or can be configured by the administrator of the

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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 subactions). This allows locations to be
retrieved from external sources, filtered, and so forth, without
requiring general language support for such operations (which could
harm the simplicity and tractability of understanding the language).
The specific operations which add, retrieve, or filter location sets
are given in Section 6.

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

2.4 XML Structure

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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 a few exceptions: see Sections 6.1, 6.3, 8.1, and
8.2).

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 subactions,
discussed further in Section 9.

The higher-level structure of a CPL script is represented by tags
corresponding to each piece of ancillary information, subactions, 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 information describing how CPL scripts are
identified.

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".

A CPL embedded as a fragment within another XML document is
identified with the XML namespace identifier "http://www.rfc-
editor.org/rfc/rfcxxxx.txt".

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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

[Note to RFC editor: please replace "xxxx" above with the
number of this RFC.]

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
Types," RFC 3023 [8].

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MIME media type name: application

MIME subtype name: cpl+xml

Mandatory parameters: none

Optional parameters: charset
As for application/xml in RFC 3023.

Encoding considerations: As for application/xml in RFC 3023.

Security considerations: See Section 14, and Section 10 of RFC
3023.

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 given in Figure 3.

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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 a tree 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 operations, which cause signalling events in the
underlying protocol; and non-signalling operations, which trigger
behavior which does 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 behavior is taken. See Section 11 for more
information on this.

5.1 Address Switches

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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
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.

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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 [9] textual representation format.
Host names are compared as strings. IP addresses are
compared numerically. (In particular, the presence or
location of an IPv6 :: omitted-zero-bits block is not
significant for matching purposes.) Host names are never
equal to IP addresses -- no DNS resolution is performed.
IPv4 addresses are never equal to IPv6 addresses, even if
the IPv6 address is a v4-in-v6 embedding.

For host names only, subdomain matching is supported with
the "subdomain-of" match operator. The "subdomain-of"
operator ignores leading dots in the hostname or match
pattern, if any. This subfield 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. Leading
zeros are ignored. This subfield 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

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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.

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, with visual
separators stripped, 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.

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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
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 "is" matches, standard
SIP URI matching rules are used; for "contains" matches, the URI 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",
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.

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"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.

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

Code to perform the first step, in Java and Perl, is
available; see the links from Annex E of UTR 15 [10]. 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 "display" is not used, and is never present.

5.3 Language Switches

Language switches allow a CPL script to make decisions based on the
languages in which the originator of the call wishes to communicate.
They are summarized in Figure 6.

Language switches take no parameters.

The "language" outputs take one parameter, "matches". The value of
one of these parameters is a language-tag, as defined in RFC 3066
[12]. The caller may have specified a set of language-ranges, also as
defined in RFC 3066. The CPL server checks each language-tag

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Node: "language-switch"
Outputs: "language" Specific string to match
Parameters: None

Output: "language"
Parameters: "matches" Match if the given language matches a
language-range of the call.

Figure 6: Syntax of the "language-switch" node

specified by the script against the language-ranges specified in the
request.

See RFC 3066 for the details of how language-ranges match language-
tags. Briefly, a language-range matches a language-tag if it exactly
equals the tag, or if it exactly equals a prefix of the tag such that
the first character following the prefix is "-".

The special language-range "*" is ignored for the purpose of
matching. Languages with a "q" value of 0 are also ignored.

This switch MAY be not-present.

5.3.1 Usage of "language-switch" with SIP

The language-ranges for the "language-switch" switch are obtained
from the SIP "Accept-Language" header field. The switch is not-
present if the initial SIP request did not contain this header field.

Note that because of CPL's first-match semantics in
switches, "q" values other than 0 of the "Accept-Language"
header fields are ignored.

5.4 Time Switches

Time switches allow a CPL script to make decisions based on the time
and/or date the script is being executed. They are summarized in
Figure 7.

Time switches are independent of the underlying signalling protocol.

Time switches are based closely on the specification of recurring
intervals of time in the Internet Calendaring and Scheduling Core

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

Figure 7: Syntax of the "time-switch" node

Object Specification (iCalendar 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.

If future standards-track documents are published that obsolete RFC
2445, any changes or clarifications those documents make to
recurrence handling apply to CPL time-switches as well.

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

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time zone definition is referenced. If it begins with a forward slash
(solidus), it references a to-be-defined global time zone registry;
otherwise it is locally-defined at the server. The TZURL gives a
network location from which an up-to-date VTIMEZONE definition for
the timezone can be retrieved.

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.

Servers SHOULD resolve TZID and TZURL references to time zone
definitions at the time the script is uploaded. They MAY periodically
refresh these resolutions to obtain the most up-to-date definition of
a time zone. If a TZURL becomes invalid, servers SHOULD remember the
most recent valid data retrieved from the URL.

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 iCalendar 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

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given as an iCalendar 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 small
enough such that subsequent intervals do not overlap.

For non-recurring intervals, durations of any positive length are
permitted. Zero-length and negative-length durations are not allowed.

If no other parameters are specified, a time node indicates only a
single period of time. More complicated sets periods intervals are
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, though CPL servers SHOULD accept
scripts with such parameters present, and ignore the other
parameters.

The "freq" parameter takes one of the following values: "secondly",
to specify repeating periods based on an interval of a second or
more; "minutely", to specify repeating periods based on an interval
of a minute or more; "hourly", to specify repeating periods based on
an interval of an hour or more;

"daily", to specify repeating periods based on an interval of a day
or more; "weekly", to specify repeating periods based on an interval
of a week or more; "monthly", to specify repeating periods based on
an interval of a month or more; and "yearly", to specify repeating
periods based on an interval of a year or more. These values are not
case-sensitive.

The "interval" parameter contains a positive integer representing how
often the recurrence rule repeats. The default value is "1", meaning
every 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 iCalendar COS DATE or DATE-TIME
value which bounds the recurrence rule in an inclusive manner. If the
value specified by "until" is synchronized with the specified
recurrence, this date or date-time becomes the last instance of the
recurrence. If specified as a date-time value, then it MUST be
specified in an UTC time format. If not present, and the "count"
parameter is not also present, the recurrence is considered to repeat
forever.

The "count" parameter defines the number of occurrences at which to
range-bound the recurrence. The "dtstart" parameter counts as the

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first occurrence. The "until" and "count" parameters MUST NOT occur
in the same "time" output.

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

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

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

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

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

The "byweekno" parameter specifies a comma-separated list of ordinals
specifying weeks of the year. Valid values are 1 to 53 or -53 to -1.
This corresponds to weeks according to week numbering as defined in
ISO 8601 [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.

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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].

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

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

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

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

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

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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, byhour="8,9" would be applied to arrive at "every Sunday in
January at 8 AM and 9 AM, every other year." Then, byminute="30"
would be applied to arrive at "every Sunday in January at 8:30 AM and
9:30 AM, every other year." Then the second is derived from "dtstart"
to end up in "every Sunday in January from 8:30:00 AM to 8:40:00 AM,
and from and 9:30:00 AM to 9:40:00 AM, every other year." Similarly,
if the "byminute", "byhour", "byday", "bymonthday" or "bymonth"
parameter were missing, the appropriate minute, hour, day or month
would have been retrieved from the "dtstart" parameter.

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

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

5.4.1 iCalendar differences and implementation issues

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

The specification of recurring events in this section is identical
(except for syntax and formatting issues) to that of RFC 2445 [13],
with only one additional restriction. That one restriction is that
consecutive instances of recurrence intervals may not overlap.

It was a matter of some debate, during the design of the CPL, whether
the entire iCalendar COS recurrence specification should be included
in CPL, or whether only a subset should be included. It was
eventually decided that compatibility between the two protocols was
of primary importance. This imposes some additional implementation

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issues on implementors of CPL servers.

It does not appear to be possible to determine, in constant time,
whether a given instant of time falls within one of the intervals
defined by a full iCalendar COS recurrence. The primary concerns are
as follows:

o The "count" parameter cannot be checked in constant running
time, since it requires that the server enumerate all
recurrences from "dtstart" to the present time, in order to
determine whether the current recurrence satisfies the
parameter. However, a server can expand a "count" paramter
once, off-line, to determime the date of the last recurrence.
This date can then be treated as a virtual "until" parameter
for the server's internal processing.

o Similarly, the "bysetpos" parameter requires that the server
enumerate all instances of the currence from the start of the
current recurrence set until the present time. This requires
somewhat more complex pre-processing, but generally, a single
recurrence with a "bysetpos" parameter can be split up into
several recurrences without them.

o Finally, 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 potential repetitions of the recurrence. The
restriction that consecutive intervals not overlap partially
satisfies this condition, but does not fully ensure it. Again,
to some extent pre-processing can help resolve this.

The algorithm given in Appendix A runs in constant time after these
pre-processing steps.

Servers ought to check that recurrence rules do not create any absurd
run-time or memory requirements, and reject those that do, just as
they ought to check that CPL scripts in general are not absurdly
large.

5.5 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 8. 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 8: 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 call 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 specified in the
call, it 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.

5.5.1 Usage of "priority-switch" with SIP

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

6 Location Modifiers

The abstract location model of the CPL is described in Section 2.3.
The behavior of several of the signalling operations (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

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add literally-specified locations to the current location set;
location lookups obtain locations from some outside source; and
location filters remove locations from the set, based on some
specified criteria.

6.1 Explicit Location

Explicit location nodes specify a location literally. Their syntax is
described in Figure 9.

Explicit location nodes are dependent on the underlying signalling
protocol.

Node: "location"
Outputs: None (next node follows directly)
Next node: 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 9: 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.
If it is not specified, the server SHOULD assume a default priority
of 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 result, 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, their XML representations
do not have explicit output tags; the <location> tag directly
contains another node.

6.1.1 Usage of "location" with SIP

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

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 10.

Location lookup is dependent on the underlying signalling protocol.

Node: "lookup"
Outputs: "success" Next node if lookup was successful
"notfound" Next node if lookup found no addresses
"failure" Next node 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 10: 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 URI, or a non-URI value. If the value of
"source" is a URI, it indicates a location which the CPL server can
query to obtain an object with the text/uri-list media type (see the
IANA registration of this type, which also appears in RFC 2483 [16]).
The query is performed verbatim, with no additional information (such
as URI parameters) added. The server adds the locations contained in
this object to the location set.

CPL servers MAY refuse to allow URI-based sources for location

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queries for some or all URI schemes. In this case, they SHOULD reject
the script at script upload time.

There has been discussion of having CPL servers add URI
parameters to the location request, so that (for instance)
CGI scripts could be used to resolve them. However, the
consensus was that this should be a CPL extension, not a
part of the base specification.

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 specifies the time, as a positive integer number of
seconds, the script is willing to wait for the lookup to be
performed. If this is not specified, its default value is 30. The
"clear" parameter specifies whether the location set should be
cleared before the new locations are added.

The other two optional parameters affect the interworking of the CPL
script with caller preferences and caller capabilities. 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.6 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 behavior is
performed.

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" [17]. By default, a CPL server SHOULD honor any
"Accept-Contact" and "Reject-Contact" headers of the original call

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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

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 11.

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

Node: "remove-location"
Outputs: None (next node follows directly)
Next node: Any node
Parameters: "location" Location to remove
"param" Caller preference parameters to apply
"value" Value of caller preference parameters

Figure 11: Syntax of the "remove-location" node

A "remove-location" node removes locations from the location set. It
is primarily useful following a "lookup" node. An example of this is
given in Section 13.8.

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

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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 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.

The "remove-location" node has no explicit output tags. In the XML
syntax, the XML "remove-location" tag directly encloses the next
node's tag.

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 [17]. 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
"param" and "value" parameters are ignored.

7 Signalling Operations

Signalling operation nodes cause signalling events in the underlying
signalling protocol. Three signalling operations 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 12.

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

After a proxy operation 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.

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Node: "proxy"
Outputs: "busy" Next node if call attempt returned "busy"
"noanswer" Next node if call attempt was not answered
before timeout
"redirection" Next node if call attempt was redirected
"failure" Next node if call attempt failed
"default" Default next node 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 12: Syntax of the "proxy" node

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 next node 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

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operations 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, as a positive integer number of 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
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 operation completes, if control is passed on to other
nodes, 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.

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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 13.

The specific behavior the redirect node invokes is dependent on the
underlying signalling protocol involved, though its semantics are
generally applicable.

Node: "redirect"
Outputs: None (no node may follow)
Next node: None
Parameters: "permanent" Whether the redirection should be
considered permanent

Figure 13: Syntax of the "redirect" node

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

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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 14. The specific behavior they invoke is
dependent on the underlying signalling protocol involved, though
their semantics are generally applicable.

Node: "reject"
Outputs: None (no node may follow)
Next node: None
Parameters: "status" Status code to return
"reason" Reason phrase to return

Figure 14: Syntax of the "reject" node

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

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 "reject" 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

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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 Non-signalling Operations

In addition to the signalling operations, the CPL defines several
operations 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 15.

Node: "mail"
Outputs: None (next node follows directly)
Next node: Any node
Parameters: "url" Mailto url to which the mail should be sent

Figure 15: Syntax of the "mail" node

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

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 possible result, since failure of e-mail
delivery cannot reliably be known in real-time. Therefore, its XML
representation does not have output tags: the <mail> tag directly

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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
"&amp;" 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.

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 16.

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

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Node: "log"
Outputs: None (next node follows directly)
Next node: Any node
Parameters: "name" Name of the log file to use
"comment" Comment to be placed in log file

Figure 16: Syntax of the "log" node

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 can have only one possible result,
and their XML representation does not have explicit output tags. 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 17.

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. The "id" value for
every "subaction" tag in a script MUST be unique within that script.

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

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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 17: Syntax of subactions and "sub" pseudo-nodes

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 subaction 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.

Subaction IDs are case sensitive.

If any subsequent version or extension defines external
linkages, it should probably use a different tag, perhaps
XLink [18]. Ensuring termination in the presence of
external links is a difficult problem.

10 Ancillary Information

No ancillary information is defined in the base CPL specification. If
ancillary information, not part of any operation, is found to be
necessary for a CPL extension, it SHOULD be placed within this tag.

The (trivial) definition of the ancillary information tag is given in
Figure 18.

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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, an extension
could be defined to include them here.

Tag: "ancillary"
Parameters: None
Subtags: None

Figure 18: Syntax of the "ancillary" tag

11 Default Behavior

When a CPL node reaches an unspecified output, either because the
output tag is not present, or because the tag is present but does not
contain a node, the CPL server's behavior is dependent on the current
state of script execution. This section gives the operations that
should be taken in each case.

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

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

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

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proxy operation 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 iCalendar COS recurrence rules
omitted from time switches.

CPL extensions are indicated by XML namespaces [19]. 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.rfc-
editor.org/rfc/rfcxxxx.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

<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.

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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 [20], 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
[21] 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 19 is a simple script which redirects all calls
to a single fixed location.

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

Figure 19: Example Script: Call Redirect Unconditional

13.2 Example: Call Forward Busy/No Answer

The script in Figure 20 illustrates some more complex behavior. We
see an initial proxy attempt to one address, with further operations
if that fails. We also see how several outputs take the same action
subtree, through the use of subactions.

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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 Busy/No Answer

13.3 Example: Call Forward: Redirect and Default

The script in Figure 21 illustrates further proxy behavior. The
server initially tries to proxy to a single address. If this attempt
is redirected, a new redirection is generated using the locations
returned. In all other failure cases for the proxy node, a default
operation -- forwarding to voicemail -- is performed.

13.4 Example: Call Screening

The script in Figure 22 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
operations. The server therefore proceeds with its default behavior,
which would presumably be to contact the user.

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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 Forward: Redirect and Default

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

Figure 22: Example Script: Call Screening

13.5 Example: Priority and Language Routing

The script in Figure 23 illustrates service selection based on a

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call's priority value and language settings. If the call request had
a priority of "urgent" or higher, the default script behavior is
performed. Otherwise, the language field is checked for the language
"es" (Spanish). If it is present, the call is proxied to a Spanish-
speaking operator; other calls are proxied to an English-speaking
operator.

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

Figure 23: Example Script: Priority and Language Routing

13.6 Example: Outgoing Call Screening

The script in Figure 24 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 25 illustrates time-based conditions and timezones.

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

Figure 24: Example Script: Outgoing Call Screening

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

Figure 25: Example Script: Time-of-day Routing

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13.8 Example: Location Filtering

Figure 26 illustrates filtering operations 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 operations have been
completed, call setup is allowed to proceed normally.

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

Figure 26: Example Script: Location Filtering

13.9 Example: Non-signalling Operations

Figure 27 illustrates non-signalling operations; in particular,
alerting a 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

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

Figure 27: Example Script: Non-signalling Operations

The example in Figure 28 shows a hypothetical extension which
implements distinctive ringing. The XML namespace
"http://www.example.com/distinctive-ring" specifies a new node named
"ring".

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

Figure 28: Example Script: Hypothetical Distinctive-Ringing Extension

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The example in Figure 29 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.

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

Figure 29: Example Script: Hypothetical Regular-Expression Extension

13.11 Example: A Complex Example

Finally, Figure 30 is a complex example which shows the sort of
sophisticated behavior which can be achieved by combining CPL nodes.
In this case, the user attempts to have his calls reach his desk; if
he does not answer within a small amount of time, calls from his boss
are forwarded to his mobile phone, and all other calls are directed
to voicemail. If the call setup failed, no operation is specified,
so the server's default behavior is performed.

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.
Because script runtime is strictly bounded by acyclicity, and because
the number of possible script operations 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

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

Figure 30: Example Script: A Complex Example

strongly authenticated. Such a method is not specified in this
document.

Script servers SHOULD allow server administrators to control the
details of what CPL operations are permitted.

15 IANA Considerations

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

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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 [20], a language for user filtering of
electronic mail messages.

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

Richard Gumpertz performed a very useful last-minute technical and
editorial review of the specification.

A An Algorithm for Resolving Time Switches

The following algorithm determines whether a given instant falls
within a repetition of a "time-switch" recurrence. If the pre-
processing described in Section 5.4.1 has been done, it operates in
constant time. Open-source Java code implementing this algorithm is
available on the world wide web at
<http://www.cs.columbia.edu/~lennox/Cal-Code/>

This algorithm is believed to be correct, but this section is non-
normative. Section 5.4, and RFC 2445 [13], are the definitive
definitions of recurrences.

1. Compute the time of the call, in the timezone of the time
switch.

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." If the minimum unit is a second, this time is
the same as the instant. If the minimum unit is a minute or
an hour, the minutes or the minutes and hours,
respectively, must be the same as "dtstart". For all other
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 if the minimum
unit is a month, or eight years if the minimum unit is a
year. Between 1904 and 2096, it is never necessary to roll
back more than four years -- the eight-year rollback can
only occur when the Gregorian calendar "skips" a leap year.

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 (or the virtual "until"
computed off-line from "count") , 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 so, succeed MATCH.

9. Calculate a previous candidate start time. Repeat until the
difference between the candidate start time and the call
time is more than the duration. If no candidate start time
has been validated, fail NOMATCH.

B Suggested Usage of CPL with H.323

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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

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 [22] 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:

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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
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 [23] and the Internet-Draft draft-levin-iptel-h323-
url-scheme-00 [24] 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 "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

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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 [22] 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.

B.3 Usage of "language-switch" with H.323

The language-ranges for the "language-switch" switch are obtained
from the H.323 UUIE "language". The switch is not-present if the
initial message did not contain this UUIE.

B.4 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 5.5).

B.5 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.6 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.7 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.

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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 the outputs 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 in
this specification are not expressible in DTD syntax.

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

<!ENTITY % Switch 'address-switch|string-switch|language-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 language-switch ( (language|not-present)*, otherwise? ) >

<!ELEMENT language ( %Node; ) >
<!ATTLIST language
matches CDATA #REQUIRED
>

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

<!ELEMENT time ( %Node; ) >

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

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%Clear;
>

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

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

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

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

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

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

<!ELEMENT default ( %Node; ) >

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

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<!ELEMENT reject EMPTY >

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

<!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

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id ID #REQUIRED
>

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

<!ELEMENT outgoing ( %Node; )>

<!ELEMENT incoming ( %Node; )>

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

D Changes from Earlier Versions

[Note to RFC Editor: please remove this appendix before
publication as an RFC.]

D.1 Changes from Draft -04

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

o Broke out language switches into their own switch node.

o Restored the full iCalendar COS recurrence specification.
Added text describing the consequences of this for
implementors, and expanded somewhat on the recurrence
algorithm.

o Clarified when time zones are resolved.

o Spelled out "iCalendar" rather than abbreviating it "iCal."

o Clarified some points about host and port matching.

o Whole-address matching in SIP uses the standard SIP URL-match
rules.

o Specified that proxy and lookup timeouts are positive integer
number of seconds.

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Internet Draft CPL November 21, 2001

o Specified that "subaction" "id" parameters must be unique.

o Corrected example scripts' namespace and DTD references
indicating older drafts of this document.

o Deleted an unused subaction from the "Call Forward: Redirect
and Default" example script.

o Made empty switches legal in the DTD.

o Made the legal values for the "proxy" "ordering" parameter
explicit in the DTD.

o Made the "success" output of "lookup" optional in the DTD. It
can trigger a default action, just like anything else.

o Clarified that the time-switch resolution algorithm is non-
normative.

o Updated references to previously-unpublished RFCs, now
published.

o Thanked Richard Gumpertz.

D.2 Changes from Draft -03

o Removed an obsolete reference to a usage in examples which
wasn't actually used anywhere.

o Added forward references to "remove-location", "mail" and
"log", as well as "location", in the XML syntax as examples of
nodes that don't have explicit output tags.

o Made the usage of some terminology more consistent: "output"
vs. "next node"; "action" vs. "operation" vs. "behavior";
"sub-actions" and "subactions"; "other operations" and "non-
call operations" and "non-signalling operations"; "meta-
information" and "ancillary information."

o The "tel" subfield of addresses which come from sip URIs
should have its visual separators stripped.

o The default value of the "priority" value of the "location"
node is 1.0.

o Corrected the media type of a set of URIs to text/uri-list,
and added a reference to it.

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o Added some wording clarifying how URI-based lookup queries
work.

o Corrected the syntax of "duration" parameter in the examples.

o Performed some pre-RFC textual cleanups (e.g. removing the
reference to the Internet-Draft URL from the XML namespace
identifier).

o Re-worded text in the description of the Ancillary tag which
implied that information could be placed in that node in the
base CPL specification. Clarified that the tag is for use by
extensions only.

o Expunged some references to sub-daily recurrences which had
accidentally been left in the text.

o Updated bibliography to refer to the latest versions of the
cited documents.

o Fixed a number of typographical errors.

D.3 Changes from Draft -02

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.

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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.

D.4 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.

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Internet Draft CPL November 21, 2001

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

o Added "permanent" parameter of "redirect" nodes.

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

o Updated example scripts to use the public identifier.

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

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

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

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

o Numerous minor clarifications and wording changes.

o Fixed copy-and-paste errors, typos.

D.5 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.

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

o Unified "url" and "source" parameters of "lookup".

o Added caller prefs to "lookup".

o Added location filtering.

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

o Added "recurse" and "ordering" parameters to "proxy".

o Added default value of "timeout" for proxy.

o Renamed "response" to "reject".

o Changed "notify" to "mail", and simplified it.

o Simplified "log", eliminating its "failure" output.

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

o Updated examples for these changes.

o Updated DTD to reflect new syntax.

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

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[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 (second edition)," W3C Recommendation REC-xml-
20001006, World Wide Web Consortium (W3C), Oct. 2000. Available at
http://www.w3.org/XML/.

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

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

[6] D. Raggett, A. Le Hors, and I. Jacobs, "HTML 4.01 specification,"
W3C Recommendation REC-html401-19991224, World Wide Web Consortium
(W3C), Dec. 1999. Available at http://www.w3.org/TR/html4/.

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

[8] M. Murata, S. S. Laurent, and D. Kohn, "XML media types," Request
for Comments 3023, Internet Engineering Task Force, Jan. 2001.

[9] R. Hinden and S. Deering, "IP version 6 addressing architecture,"
Request for Comments 2373, Internet Engineering Task Force, July
1998.

[10] M. Davis and M. Duerst, "Unicode normalization forms," Unicode
Technical Report 15, Unicode Consortium, Aug. 2000. Revision 19;
part of Unicode 3.0.1. Available at
http://www.unicode.org/unicode/reports/tr15/.

[11] M. Davis, "Case mappings," Unicode Technical Report 21, Unicode
Consortium, Oct. 2000. Revision 4.3. Available at
http://www.unicode.org/unicode/reports/tr21/.

[12] H. Alvestrand, "Tags for the identification of languages,"

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Request for Comments 3066, Internet Engineering Task Force, Jan.
2001.

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

[14] P. Eggert, "Sources for time zone and daylight saving time
data." Available at http://www.twinsun.com/tz/tz-link.htm.

[15] ISO (International Organization for Standardization), "Data
elements and interchange formats -- information interchange --
representation of dates and times," ISO Standard ISO 8601:1988(E),
International Organization for Standardization, Geneva, Switzerland,
June 1986.

[16] M. Mealling and R. Daniel, "URI resolution services necessary
for URN resolution," Request for Comments 2483, Internet Engineering
Task Force, Jan. 1999.

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

[18] S. DeRose, E. Maler, D. Orchard, and B. Trafford, "XML linking
language (XLink) version 1.0," W3C Candidate Recommendation CR-
xlink-20000703, World Wide Web Consortium (W3C), July 2000.
Available at http://www.w3.org/TR/xlink/.

[19] T. Bray, D. Hollander, and A. Layman, "Namespaces in XML," W3C
Recommendation REC-xml-names-19900114, World Wide Web Consortium
(W3C), Jan. 1999. Available at http://www.w3.org/TR/REC-xml-names/.

[20] T. Showalter, "Sieve: A mail filtering language," Request for
Comments 3028, Internet Engineering Task Force, Jan. 2001.

[21] D. C. Fallside, "XML schema part 0: Primer," W3C Candidate
Recommendation CR-xmlschema-0-20001024, World Wide Web Consortium
(W3C), Oct. 2000. Available at http://www.w3.org/TR/xmlschema-0/.

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

[23] International Telecommunication Union, "Packet based multimedia
communication systems," Recommendation H.323, Telecommunication

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Standardization Sector of ITU, Geneva, Switzerland, Nov. 2000.

[24] O. Levin, "H.323 URL scheme definition," Internet Draft,
Internet Engineering Task Force, Nov. 2001. Work in progress.

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
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.

This document and the information contained herein is provided on an
"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 ............................ 2
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 ....................................... 4

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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 .................................... 8
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 Language Switches ................................... 13
5.3.1 Usage of "language-switch" with SIP ................. 14
5.4 Time Switches ....................................... 14
5.4.1 iCalendar differences and implementation issues ..... 20
5.5 Priority Switches ................................... 21
5.5.1 Usage of "priority-switch" with SIP ................. 22
6 Location Modifiers .................................. 22
6.1 Explicit Location ................................... 23
6.1.1 Usage of "location" with SIP ........................ 23
6.2 Location Lookup ..................................... 24
6.2.1 Usage of "lookup" with SIP .......................... 25
6.3 Location Removal .................................... 26
6.3.1 Usage of "remove-location" with SIP ................. 27
7 Signalling Operations ............................... 27
7.1 Proxy ............................................... 27
7.1.1 Usage of "proxy" with SIP ........................... 30
7.2 Redirect ............................................ 30
7.2.1 Usage of "redirect" with SIP ........................ 31
7.3 Reject .............................................. 31
7.3.1 Usage of "reject" with SIP .......................... 31
8 Non-signalling Operations ........................... 32
8.1 Mail ................................................ 32
8.1.1 Suggested Content of Mailed Information ............. 33
8.2 Log ................................................. 33
9 Subactions .......................................... 34
10 Ancillary Information ............................... 35
11 Default Behavior .................................... 36
12 CPL Extensions ...................................... 37
13 Examples ............................................ 38
13.1 Example: Call Redirect Unconditional ................ 38
13.2 Example: Call Forward Busy/No Answer ................ 38
13.3 Example: Call Forward: Redirect and Default ......... 39
13.4 Example: Call Screening ............................. 39
13.5 Example: Priority and Language Routing .............. 40
13.6 Example: Outgoing Call Screening .................... 41
13.7 Example: Time-of-day Routing ........................ 41
13.8 Example: Location Filtering ......................... 43
13.9 Example: Non-signalling Operations .................. 43
13.10 Example: Hypothetical Extensions .................... 43

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13.11 Example: A Complex Example .......................... 45
14 Security Considerations ............................. 45
15 IANA Considerations ................................. 46
16 Acknowledgments ..................................... 47
A An Algorithm for Resolving Time Switches ............ 47
B Suggested Usage of CPL with H.323 ................... 48
B.1 Usage of "address-switch" with H.323 ................ 49
B.2 Usage of "string-switch" with H.323 ................. 50
B.3 Usage of "language-switch" with H.323 ............... 51
B.4 Usage of "priority-switch" with H.323 ............... 51
B.5 Usage of "location" with H.323 ...................... 51
B.6 Usage of "lookup" with H.323 ........................ 51
B.7 Usage of "remove-location" with H.323 ............... 51
C The XML DTD for CPL ................................. 52
D Changes from Earlier Versions ....................... 58
D.1 Changes from Draft -04 .............................. 58
D.2 Changes from Draft -03 .............................. 59
D.3 Changes from Draft -02 .............................. 60
D.4 Changes from Draft -01 .............................. 61
D.5 Changes from Draft -00 .............................. 62
E Authors' Addresses .................................. 63
F Bibliography ........................................ 63

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