Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Schulzrinne
draft-ietf-iptel-cpl-03.txt Columbia University
October 25, 2000
Expires: April, 2001
CPL: A Language for User Control of Internet Telephony Services
STATUS OF THIS MEMO
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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.
In examples, non-XML strings such as -action1, -action2, and so
forth, are sometimes used. These represent further parts of the
script which are not relevant to the example in question.
Some paragraphs are indented, like this; they give
motivations of design choices, or questions for future
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discussion in the development of the CPL, and are not
essential to the specification of the language.
2 Structure of CPL scripts
2.1 High-level structure
A CPL script consists of two types of information: ancillary
information about the script, and call processing actions.
A call processing action is a structured tree that describes the
decisions and actions a telephony signalling server performs on a
call set-up event. There are two types of call processing actions:
top-level actions are actions that are triggered by signalling events
that arrive at the server. Two top-level action names are defined:
incoming, the action performed when a call arrives whose destination
is the owner of the script; and outgoing, the action performed when a
call arrives whose originator is the owner of the script. Sub-actions
are actions which can be called from other actions. The CPL forbids
sub-actions from being called recursively: see section 9.
Ancillary information is information which is necessary for a server
to correctly process a script, but which does not directly describe
any actions. Currently, no ancillary information is defined, but the
section is reserved for use by extensions.
2.2 Abstract structure of a call processing action
Abstractly, a call processing action is described by a collection of
nodes, which describe actions that can be performed or choices which
can be made. A node may have several parameters, which specify the
precise behavior of the node; they usually also have outputs, which
depend on the result of the condition or action.
For a graphical representation of a CPL action, see Figure 1. Nodes
and outputs can be thought of informally as boxes and arrows; the CPL
is designed so that actions can be conveniently edited graphically
using this representation. Nodes are arranged in a tree, starting at
a single root node; outputs of nodes are connected to additional
nodes. When an action is run, the action or condition described by
the top-level node is performed; based on the result of that node,
the server follows one of the node's outputs, and that action or
condition is performed; this process continues until a node with no
specified outputs is reached. Because the graph is acyclic, this will
occur after a bounded and predictable number of nodes are visited.
If an output to a node is not specified, it indicates that the CPL
server should perform a node- or protocol-specific action. Some nodes
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have specific default actions associated with them; for others, the
default action is implicit in the underlying signalling protocol, or
can be configured by the administrator of the server. For further
details on this, see section 11.
_________________ ___________________ ________ busy
| Address-switch | | location | | proxy |--------\
Call --->| field: origin | ->| url: sip:jones@ |--->|timeout:| timeout|
| subfield: host | / | example.com | | 10s |--------|
|-----------------|/ |___________________| | | failure|
| subdomain-of: | |________|--------|
| example.com | |
|-----------------| _____________________________________________/
| otherwise | /..........................................
| |\|. Voicemail .
|_________________| \. ____________________ .
->| location | __________ .
. | url: sip:jones@ | | redirect | .
. | voicemail. |--->| | .
. | example.com | |__________| .
. |____________________| .
..........................................
Figure 1: Sample CPL Action: Graphical Version
2.3 Location model
For flexibility, one piece of information necessary for the function
of a CPL is not given as node parameters: the set of locations to
which a call is to be directed. Instead, this set of locations is
stored as an implicit global variable throughout the execution of a
processing action (and its sub-actions). This allows locations to be
retrieved from external sources, filtered, and so forth, without
requiring general language support for such actions (which could harm
the simplicity and tractability of understanding the language). The
specific actions which add, retrieve, or filter location sets are
given in section 6.
For the incoming top-level processing action, the location set is
initialized to the empty set. For the outgoing action, it is
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initialized to the destination address of the call.
2.4 XML structure
Syntactically, CPL scripts are represented by XML documents. XML is
thoroughly specified by [3], and implementors of this specification
should be familiar with that document, but as a brief overview, XML
consists of a hierarchical structure of tags; each tag can have a
number of attributes. It is visually and structurally very similar to
HTML [6], as both languages are simplifications of the earlier and
larger standard SGML [7].
See Figure 2 for the XML document corresponding to the graphical
representation of the CPL script in Figure 1. Both nodes and outputs
in the CPL are represented by XML tags; parameters are represented by
XML tag attributes. Typically, node tags contain output tags, and
vice-versa (with one exception; see section 6.1).
The connection between the output of a node and another node is
represented by enclosing the tag representing the pointed-to node
inside the tag for the outer node's output. Convergence (several
outputs pointing to a single node) is represented by sub-actions,
discussed further in section 9.
The higher-level structure of a CPL script is represented by tags
corresponding to each piece of meta-information, sub-actions, and
top-level actions, in order. This higher-level information is all
enclosed in a special tag cpl, the outermost tag of the XML document.
A complete Document Type Declaration for the CPL is provided in
Appendix C. The remainder of the main sections of this document
describe the semantics of the CPL, while giving its syntax
informally. For the formal syntax, please see the appendix.
3 Document information
This section gives meta-information about CPL scripts.
3.1 CPL Document Identifiers for XML
A CPL script list which appears as a top-level XML document is
identified with the formal public identifier "-//IETF//DTD RFCxxxx
CPL 1.0//EN". If this document is published as an RFC, "xxxx" will be
replaced by the RFC number.
A CPL embedded as a fragment within another XML document is
identified with the XML namespace identifier
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Figure 2: Sample CPL Script: XML Version
"http://www.ietf.org/internet-drafts/draft-ietf-iptel-cpl-03.txt".
If this document is published as an RFC, the namespace identifier
will be "http://www.rfc-editor.org/rfc/rfcxxxx.txt", where xxxx is
the RFC number.
Note that the URIs specifying XML namespaces are only
globally unique names; they do not have to reference any
particular actual object. The URI of a canonical source of
this specification meets the requirement of being globally
unique, and is also useful to document the format.
3.2 MIME Registration
As an XML type, CPL's MIME registration conforms with "XML Media
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Types" [8] as well as RFC 2048 [9].
MIME media type name: application
MIME subtype name: cpl+xml
Mandatory parameters: none
Optional parameters: charset
As for application/xml in "XML Media Types."
Encoding considerations: As for application/xml in "XML Media
Types."
Security considerations: See section 14, and section 10 of "XML
Media Types."
Interoperability considerations: Different CPL servers may use
incompatible address types. However, all potential
interoperability issues should be resolvable at the time a
script is uploaded; there should be no interoperability
issues which cannot be detected until runtime.
Published specification: This document.
Applications which use this media type: None publicly released
at this time, as far as the authors are aware.
Additional information:
Magic number: None
File extension: .cpl or .xml
Macintosh file type code: "TEXT"
Person and e-mail address for further information:
Jonathan Lennox
Henning Schulzrinne
Intended usage: COMMON
Author/Change Controller: The IETF.
4 Script structure: overview
As mentioned, a CPL script consists of ancillary information,
subactions, and top-level actions. The full syntax of the cpl node is
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given in Figure 3.
Tag: cpl
Parameters: none
Sub-tags: ancillary See section 10
subaction See section 9
outgoing Top-level actions to take on this user's
outgoing calls
incoming Top-level actions to take on this user's
incoming calls
Figure 3: Syntax of the top-level cpl tag
Call processing actions, both top-level actions and sub-actions,
consist of nodes and outputs. Nodes and outputs are both described by
XML tags. There are four categories of CPL nodes: switches , which
represent choices a CPL script can make; location modifiers , which
add or remove locations from the location set; signalling actions ,
which cause signalling events in the underlying protocol; and non-
signalling actions, which take an action but do not effect the
underlying protocol.
5 Switches
Switches represent choices a CPL script can make, based on either
attributes of the original call request or items independent of the
call.
All switches are arranged as a list of conditions that can match a
variable. Each condition corresponds to a node output; the output
points to the next node to execute if the condition was true. The
conditions are tried in the order they are presented in the script;
the output corresponding to the first node to match is taken.
There are two special switch outputs that apply to every switch type.
The output not-present, which MAY occur anywhere in the list of
outputs, is true if the variable the switch was to match was not
present in the original call setup request. (In this document, this
is sometimes described by saying that the information is "absent".)
The output otherwise, which MUST be the last output specified if it
is present, matches if no other condition matched.
If no condition matches and no otherwise output was present in the
script, the default script action is taken. See section 11 for more
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information on this.
5.1 Address switches
Address switches allow a CPL script to make decisions based on one of
the addresses present in the original call request. They are
summarized in Figure 4.
Node: address-switch
Outputs: address Specific addresses to match
Parameters: field origin, destination, or original-destination
subfield address-type, user, host, port, tel, or display,
(
also: password and alias-type)
Output: address
Parameters: is exact match
contains substring match (for display only)
subdomain-of sub-domain match (for host, tel only)
Figure 4: Syntax of the address-switch node
Address switches have two node parameters: field, and subfield. The
mandatory field parameter allows the script to specify which address
is to be considered for the switch: either the call's origin address
(field "origin"), its current destination address (field
"destination"), or its original destination (field "original-
destination"), the destination the call had before any earlier
forwarding was invoked. Servers MAY define additional field values.
The optional subfield specifies what part of the address is to be
considered. The possible subfield values are: address-type, user,
host, port, tel, and display. Additional subfield values MAY be
defined for protocol-specific values. (The subfield password is
defined for SIP in Section 5.1.1; the subfield alias-type is defined
for H.323 in Appendix B.1.) If no subfield is specified, the
"entire" address is matched; the precise meaning of this is defined
for each underlying signalling protocol. Servers MAY define
additional subfield values.
The subfields are defined as follows:
address-type This indicates the type of the underlying address;
i.e., the URI scheme, if the address can be represented by
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a URI. The types specifically discussed by this document
are sip, tel, and h323. The address type is not case-
sensitive. It has a value for all defined address types.
user This subfield of the address indicates, for e-mail style
addresses, the user part of the address. For telephone
number style address, it includes the subscriber number.
This subfield is case-sensitive; it may be absent.
host This subfield of the address indicates the Internet host
name or IP address corresponding to the address, in host
name, IPv4, or IPv6 format. For host names only, subdomain
matching is supported with the subdomain-of match operator.
It is not case sensitive, and may be absent.
port This subfield indicates the TCP or UDP port number of the
address, numerically in decimal format. It is not case
sensitive, as it MUST only contain decimal digits. It may
be absent; however, for address types with default ports,
an absent port matches the default port number.
tel This subfield indicates a telephone subscriber number, if
the address contains such a number. It is not case
sensitive (the telephone numbers may contain the symbols
`A' `B' `C' and `D'), and may be absent. It may be matched
using the subdomain-of match operator. Punctuation and
separator characters in telephone numbers are discarded.
display This subfield indicates a "display name" or user-visible
name corresponding to an address. It is a Unicode string,
and is matched using the case-insensitive algorithm
described in section 5.2. The contains operator may be
applied to it. It may be absent.
For any completely unknown subfield, the server MAY reject the script
at the time it is submitted with an indication of the problem; if a
script with an unknown subfield is executed, the server MUST consider
the not-present output to be the valid one.
The address output tag may take exactly one of three possible
parameters, indicating the kind of matching allowed.
is An output with this match operator is followed if the
subfield being matched in the address-switch exactly
matches the argument of the operator. It may be used for
any subfield, or for the entire address if no subfield was
specified.
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subdomain-of This match operator applies only for the subfields
host and tel. In the former case, it matches if the
hostname being matched is a subdomain of the domain given
in the argument of the match operator; thus, subdomain-
of="example.com" would match the hostnames "example.com",
"research.example.com", and
"zaphod.sales.internal.example.com". IP addresses may be
given as arguments to this operator; however, they only
match exactly. In the case of the tel subfield, the output
matches if the telephone number being matched has a prefix
that matches the argument of the match operator;
subdomain-of="1212555" would match the telephone number "1
212 555 1212."
contains This match operator applies only for the subfield
display. The output matches if the display name being
matched contains the argument of the match as a substring.
5.1.1 Usage of address-switch with SIP
For SIP, the origin address corresponds to the address in the From
header; destination corresponds to the Request-URI; and original-
destination corresponds to the To header.
The display subfield of an address is the display-name part of the
address, if it is present. Because of SIP's syntax, the destination
address field will never have a display subfield.
The address-type subfield of an address is the URI scheme of that
address. Other address fields depend on that address-type.
For sip URLs, the user, host, and port subfields correspond to the
"user," "host," and "port" elements of the URI syntax. The tel
subfield is defined to be the "user" part of the URI if and only if
the "user=phone" parameter is given to the URI. An additional
subfield, password is defined to correspond to the "password" element
of the SIP URI, and is case-sensitive. However, use of this field is
NOT RECOMMENDED for general security reasons.
For tel URLs, the tel and user subfields are the subscriber name; in
the former case, visual separators are stripped. The host and port
subfields are both not present.
For h323 URLs, subfields MAY be set according to the scheme described
in Appendix B.
For other URI schemes, only the address-type subfield is defined by
this specification; servers MAY set other pre-defined subfields, or
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MAY support additional subfields.
If no subfield is specified for addresses in SIP messages, the string
matched is the URI part of the address. For "sip" URLs, all
parameters are stripped; for other URLs, the URL is used verbatim.
5.2 String switches
String switches allow a CPL script to make decisions based on free-
form strings present in a call request. They are summarized in Figure
5.
Node: string-switch
Outputs: string Specific string to match
Parameters: field subject, organization, user-agent,
language, or display
Output: string
Parameters: is exact match
contains substring match
Figure 5: Syntax of the string-switch node
String switches have one node parameter: field. The mandatory field
parameter specifies which string is to be matched.
String switches are dependent on the call signalling protocol being
used.
Five fields are defined, listed below. The value of each of these
fields, except as specified, is a free-form Unicode string with no
other structure defined.
subject The subject of the call.
organization The organization of the originator of the call.
user-agent The name of the program or device with which the call
request was made.
language The languages in which the originator of the call
wishes to receive responses. This contains a list of RFC
1766 [10] language tags, separated by commas.
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Note that matching based on contains is likely to be
much more useful than matching based on is, for this
field.
display Free-form text associated with the call, intended to be
displayed to the recipient, with no other semantics defined
by the signalling protocol.
Strings are matched as case-insensitive Unicode strings, in the
following manner. First, strings are canonicalized to the
"Compatibility Composition" (KC) form, as specified in Unicode
Technical Report 15 [11]. Then, strings are compared using locale-
insensitive caseless mapping, as specified in Unicode Technical
Report 21 [12].
Code to perform the first step, in Java and Perl, is
available; see the links from Annex E of UTR 15 [11]. The
case-insensitive string comparison in the Java standard
class libraries already performs the second step; other
Unicode-aware libraries should be similar.
The output tags of string matching are named string, and have a
mandatory argument, one of is or contains, indicating whole-string
match or substring match, respectively.
5.2.1 Usage of string-switch with SIP
For SIP, the fields subject, organization, and user-agent correspond
to the SIP header fields with the same name. These are used verbatim
as they appear in the message.
The field language corresponds to the SIP Accept-Language header. It
is converted to a list of comma-separated languages as described
above.
The field display is not used, and is never present.
5.3 Time switches
Time switches allow a CPL script to make decisions based the time
and/or date the script is being executed. They are summarized in
Figure 6.
Time switches are independent of the underlying signalling protocol.
Time switches are based on a large subset of how recurring intervals
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Node: time-switch
Outputs: time Specific time to match
Parameters: tzid RFC 2445 Time Zone Identifier
tzurl RFC 2445 Time Zone URL
Output: time
Parameters: dtstart Start of interval (RFC 2445 DATE-TIME)
dtend End of interval (RFC 2445 DATE-TIME)
duration Length of interval (RFC 2445 DURATION)
freq Frequency of recurrence (one of "daily",
"weekly", "monthly", or "yearly")
interval How often the recurrence repeats
until Bound of recurrence (RFC 2445 DATE-TIME)
byday List of days of the week
bymonthday List of days of the month
byyearday List of days of the year
byweekno List of weeks of the year
bymonth List of months of the year
wkst First day of workweek
Figure 6: Syntax of the time-switch node
of time are specified in the Internet Calendaring and Scheduling Core
Object Specification (iCal COS), RFC 2445 [13].
This allows CPLs to be generated automatically from
calendar books. It also allows us to re-use the extensive
existing work specifying time intervals.
The subset was designed with the goal that a time-switch
can be evaluated -- an instant can be determined to fall
within an interval, or not -- in constant (O(1)) time.
An algorithm to whether an instant falls within a given recurrence is
given in Appendix A.
The time-switch tag takes two optional parameters, tzid and tzurl,
both of which are defined in RFC 2445 (sections 4.8.3.1 and 4.8.3.5
respectively). The TZID is the identifying label by which a time zone
definition is referenced. If it begins with a forward slash
(solidus), it references a to-be-defined global time zone registry;
otherwise it is locally-defined at the server. The TZURL gives a
network location from which an up-to-date VTIMEZONE definition for
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the timezone can be retrieved.
While TZID labels that do not begin with a forward slash are locally
defined, it is RECOMMENDED that servers support at least the naming
scheme used by Olson Time Zone database [14]. Examples of timezone
databases that use the Olson scheme are the zoneinfo files on most
Unix-like systems, and the standard Java TimeZone class.
If a script is uploaded with a tzid and tzurl which the CPL server
does not recognize or cannot resolve, it SHOULD diagnose and reject
this at script upload time. If neither tzid nor tzurl are present,
all non-UTC times within this time switch should be interpreted as
being "floating" times, i.e. that they are specified in the local
timezone of the CPL server.
Because of daylight-savings-time changes over the course of
a year, it is necessary to specify time switches in a given
timezone. UTC offsets are not sufficient, or a time-of-day
routing rule which held between 9 am and 5 pm in the
eastern United States would start holding between 8 am and
4 pm at the end of October.
Authors of CPL servers should be careful to handle correctly the
intervals when local time is discontinuous, at the beginning or end
of daylight-savings time. Note especially that some times may occur
more than once when clocks are set back. The algorithm in Appendix A
is believed to handle this correctly.
Time nodes specify a list of periods during which their output should
be taken. They have two required parameters: dtstart, which specifies
the beginning of the first period of the list, and exactly one of
dtend or duration, which specify the ending time or the duration of
the period, respectively. The dtstart and dtend parameters are
formatted as iCal COS DATE-TIME values, as specified in section 4.3.5
of RFC 2445 [13]. Because time zones are specified in the top-level
time-switch tag, only forms 1 or 2 (floating or UTC times) can be
used. The duration parameter is given as an iCal COS DURATION
parameter, as specified in section 4.3.6 of RFC 2445. Both the
DATE-TIME and the DURATION syntaxes are subsets of the corresponding
syntaxes from ISO 8601 [15].
For a recurring interval, the duration parameter MUST be less than
twenty-four hours. For non-recurring intervals, durations of any
length are permitted.
If no other parameters are specified, a time node indicates only a
single period of time. More complicated sets periods intervals are
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constructed as recurrences. A recurrence is specified by including
the freq parameter, which indicates the type of recurrence rule. No
parameters other than dtstart, dtend, and duration SHOULD be
specified unless freq is present.
The freq parameter takes one of the following values: daily, to
specify repeating periods based on an interval of a day or more;
weekly, to specify repeating periods based on an interval of a week
or more; monthly, to specify repeating periods based on an interval
of a month or more; and yearly, to specify repeating periods based on
an interval of a year or more. These values are not case-sensitive.
The values secondly, minutely, and hourly are present in
iCal, but were removed from CPL.
The interval parameter contains a positive integer representing how
often the recurrence rule repeats. The default value is "1", meaning
every second for a secondly rule, or every minute for a minutely
rule, every hour for an hourly rule, every day for a daily rule,
every week for a weekly rule, every month for a monthly rule and
every year for a yearly rule.
The until parameter defines an iCal COS DATE or DATE-TIME value which
bounds the recurrence rule in an inclusive manner. If the value
specified by until is synchronized with the specified recurrence,
this date or date-time becomes the last instance of the recurrence.
If specified as a date-time value, then it MUST be specified in an
UTC time format. If not present, the recurrence is considered to
repeat forever.
iCal also defines a count parameter, which allows an
alternate method of specifying a bound to a recurrence.
This bound has been removed from CPL. Translating from full
iCal recurrences to CPL recurrences requires that the count
parameter be converted to an until parameter, which can be
done by enumerating the recurrence and determining its
final date.
The byday parameter specifies a comma-separated list of days of the
week. MO indicates Monday; TU indicates Tuesday; WE indicates
Wednesday; TH indicates Thursday; FR indicates Friday; SA indicates
Saturday; SU indicates Sunday. These values are not case-sensitive.
Each byday value can also be preceded by a positive (+n) or negative
(-n) integer. If present, this indicates the nth occurrence of the
specific day within the monthly or yearly recurrence. For example,
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within a monthly rule, +1MO (or simply 1MO) represents the first
Monday within the month, whereas -1MO represents the last Monday of
the month. If an integer modifier is not present, it means all days
of this type within the specified frequency. For example, within a
monthly rule, MO represents all Mondays within the month.
The bymonthday parameter specifies a comma-separated list of days of
the month. Valid values are 1 to 31 or -31 to -1. For example, -10
represents the tenth to the last day of the month.
The byyearday parameter specifies a comma-separated list of days of
the year. Valid values are 1 to 366 or -366 to -1. For example, -1
represents the last day of the year (December 31st) and -306
represents the 306th to the last day of the year (March 1st).
The byweekno parameter specifies a comma-separated list of ordinals
specifying weeks of the year. Valid values are 1 to 53 or -53 to -1.
This corresponds to weeks according to week numbering as defined in
ISO 8601 [15]. A week is defined as a seven day period, starting on
the day of the week defined to be the week start (see wkst). Week
number one of the calendar year is the first week which contains at
least four (4) days in that calendar year. This parameter is only
valid for yearly rules. For example, 3 represents the third week of
the year.
Note: Assuming a Monday week start, week 53 can only occur
when Thursday is January 1 or if it is a leap year and
Wednesday is January 1.
The bymonth parameter specifies a comma-separated list of months of
the year. Valid values are 1 to 12.
The wkst parameter specifies the day on which the workweek starts.
Valid values are MO, TU, WE, TH, FR, SA and SU. This is significant
when a weekly recurrence has an interval greater than 1, and a byday
parameter is specified. This is also significant in a yearly
recurrence when a byweekno parameter is specified. The default value
is MO, following ISO 8601 [15].
iCal also includes the Byxxx parameters bysecond, byminute,
byhour, and bysetpos, which have been removed from CPL.
If byxxx parameter values are found which are beyond the available
scope (ie, bymonthday="30" in February), they are simply ignored.
Byxxx parameters modify the recurrence in some manner. Byxxx rule
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Internet Draft CPL October 25, 2000
parts for a period of time which is the same or greater than the
frequency generally reduce or limit the number of occurrences of the
recurrence generated. For example, freq="daily" bymonth="1" reduces
the number of recurrence instances from all days (if the bymonth
parameter is not present) to all days in January. Byxxx parameters
for a period of time less than the frequency generally increase or
expand the number of occurrences of the recurrence. For example,
freq="yearly" bymonth="1,2" increases the number of days within the
yearly recurrence set from 1 (if bymonth parameter is not present) to
2.
If multiple Byxxx parameters are specified, then after evaluating the
specified freq and interval parameters, the Byxxx parameters are
applied to the current set of evaluated occurrences in the following
order: bymonth, byweekno, byyearday, bymonthday, and byday; then
until is evaluated.
Here is an example of evaluating multiple Byxxx parameters.