Network Working Group J. Lennox Request for Comments: 3880 X. Wu Category: Standards Track H. Schulzrinne Columbia University October 2004 Call Processing Language (CPL): A Language for User Control of Internet Telephony Services Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This document defines the Call Processing Language (CPL), a language to describe and control Internet telephony services. It is designed to be implementable on either network servers or user agents. 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.
Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions of This Document. . . . . . . . . . . . . . 4 2. Structure of CPL Scripts . . . . . . . . . . . . . . . . . . . 4 2.1. High-level Structure. . . . . . . . . . . . . . . . . . 4 2.2. Abstract Structure of a Call Processing Action. . . . . 5 2.3. Location Model. . . . . . . . . . . . . . . . . . . . . 6 2.4. XML Structure . . . . . . . . . . . . . . . . . . . . . 6 3. Script Structure: Overview . . . . . . . . . . . . . . . . . . 7 4. Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Address Switches. . . . . . . . . . . . . . . . . . . . 9 4.1.1. Usage of "address-switch" with SIP. . . . . . . 11 4.2. String Switches . . . . . . . . . . . . . . . . . . . . 12 4.2.1. Usage of "string-switch" with SIP . . . . . . . 13 4.3. Language Switches . . . . . . . . . . . . . . . . . . . 14 4.3.1. Usage of "language-switch" with SIP . . . . . . 14 4.4. Time Switches . . . . . . . . . . . . . . . . . . . . . 15 4.4.1. iCalendar differences and implementation issues. . . . . . . . . . . . . . . . . . . . . 20 4.5. Priority Switches . . . . . . . . . . . . . . . . . . . 21 4.5.1. Usage of "priority-switch" with SIP . . . . . . 22 5. Location Modifiers . . . . . . . . . . . . . . . . . . . . . . 22 5.1. Explicit Location . . . . . . . . . . . . . . . . . . . 23 5.1.1. Usage of "location" with SIP. . . . . . . . . . 23 5.2. Location Lookup . . . . . . . . . . . . . . . . . . . . 24 5.2.1. Usage of "lookup" with SIP. . . . . . . . . . . 25 5.3. Location Removal. . . . . . . . . . . . . . . . . . . . 25 5.3.1. Usage of "remove-location" with SIP . . . . . . 26 6. Signalling Operations. . . . . . . . . . . . . . . . . . . . . 26 6.1. Proxy . . . . . . . . . . . . . . . . . . . . . . . . . 26 6.1.1. Usage of "proxy" with SIP . . . . . . . . . . . 29 6.2. Redirect. . . . . . . . . . . . . . . . . . . . . . . . 29 6.2.1. Usage of "redirect" with SIP. . . . . . . . . . 30 6.3. Reject. . . . . . . . . . . . . . . . . . . . . . . . . 30 6.3.1. Usage of "reject" with SIP. . . . . . . . . . . 30 7. Non-signalling Operations. . . . . . . . . . . . . . . . . . . 31 7.1. Mail. . . . . . . . . . . . . . . . . . . . . . . . . . 31 7.1.1. Suggested Content of Mailed Information . . . . 32 7.2. Log . . . . . . . . . . . . . . . . . . . . . . . . . . 32 8. Subactions . . . . . . . . . . . . . . . . . . . . . . . . . . 33 9. Ancillary Information. . . . . . . . . . . . . . . . . . . . . 34 10. Default Behavior . . . . . . . . . . . . . . . . . . . . . . . 35 11. CPL Extensions . . . . . . . . . . . . . . . . . . . . . . . . 35 12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 12.1. Example: Call Redirect Unconditional. . . . . . . . . . 37 12.2. Example: Call Forward Busy/No Answer. . . . . . . . . . 38 12.3. Example: Call Forward: Redirect and Default . . . . . . 39
12.4. Example: Call Screening . . . . . . . . . . . . . . . . 40 12.5. Example: Priority and Language Routing. . . . . . . . . 41 12.6. Example: Outgoing Call Screening. . . . . . . . . . . . 42 12.7. Example: Time-of-day Routing. . . . . . . . . . . . . . 43 12.8. Example: Location Filtering . . . . . . . . . . . . . . 44 12.9. Example: Non-signalling Operations. . . . . . . . . . . 45 12.10. Example: Hypothetical Extensions. . . . . . . . . . . . 46 12.11. Example: A Complex Example. . . . . . . . . . . . . . . 48 13. Security Considerations. . . . . . . . . . . . . . . . . . . . 49 14. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 49 14.1. URN Sub-Namespace Registration for urn:ietf:params:xml:ns:cpl. . . . . . . . . . . . . . . 49 14.2. Schema registration . . . . . . . . . . . . . . . . . . 50 14.3. MIME Registration . . . . . . . . . . . . . . . . . . . 50 15. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 51 A. An Algorithm for Resolving Time Switches . . . . . . . . . . . 52 B. Suggested Usage of CPL with H.323. . . . . . . . . . . . . . . 53 B.1. Usage of "address-switch" with H.323. . . . . . . . . . 53 B.2. Usage of "string-switch" with H.323 . . . . . . . . . . 55 B.3. Usage of "language-switch" with H.323 . . . . . . . . . 55 B.4. Usage of "priority-switch" with H.323 . . . . . . . . . 55 B.5. Usage of "location" with H.323. . . . . . . . . . . . . 56 B.6. Usage of "lookup" with H.323. . . . . . . . . . . . . . 56 B.7. Usage of "remove-location" with H.323 . . . . . . . . . 56 C. The XML Schema for CPL . . . . . . . . . . . . . . . . . . . . 56 Normative References . . . . . . . . . . . . . . . . . . . . . . . 70 Informative References . . . . . . . . . . . . . . . . . . . . . . 71 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 73 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 74 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 the Session Initiation Protocol (SIP)  and H.323 . 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 describe Internet telephony services. The language is not Turing-complete, and provides no way to write loops or recursion. CPL is also designed to be easily created and edited by graphical tools. It is based on the Extensible Markup Language (XML) , 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 the validity of a script when the server receives it, rather than discovering problems while a call is being processed. Implementations of 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" . 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 BCP 14, RFC 2119  and indicate requirement levels for compliant CPL implementations. Some paragraphs are indented, like this; they give motivations of design choices, advice to implementors, or thoughts on future development of or extensions to CPL. They are not essential to the specification of the language, and are non-normative. 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 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 actions 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. CPL forbids subactions from being called recursively: see Section 8. 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 that describe operations that can be performed or decisions that 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; 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 server. For further details on this, see Section 10.
_________________ ___________________ ________ 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 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 5. 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 Syntactically, CPL scripts are represented by XML documents. XML is thoroughly specified by the XML specification , and implementors of this specification should be familiar with that document. However, 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 , as both languages are simplifications of the earlier and larger standard SGML .
See Figure 2 for the XML document corresponding to the graphical representation of the CPL script in Figure 1. Both nodes and outputs in 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 5.1, 5.3, 7.1, and 7.2). 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 8. 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 XML Schema for CPL is provided in Appendix C. The remainder of the main sections of this document describe the semantics of CPL, while giving its syntax informally. For the formal syntax, please see the appendix. 3. 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. <?xml version="1.0" encoding="UTF-8"?> <cpl xmlns="urn:ietf:params:xml:ns:cpl" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:ietf:params:xml:ns:cpl cpl.xsd "> <subaction id="voicemail"> <location url="sip:email@example.com"> <redirect /> </location> </subaction> <incoming> <address-switch field="origin" subfield="host"> <address subdomain-of="example.com"> <location url="sip:firstname.lastname@example.org"> <proxy timeout="10"> <busy> <sub ref="voicemail" /> </busy> <noanswer> <sub ref="voicemail" /> </noanswer> <failure> <sub ref="voicemail" /> </failure> </proxy> </location>
</address> <otherwise> <sub ref="voicemail" /> </otherwise> </address-switch> </incoming> </cpl> Figure 2: Sample CPL Script: XML Version Tag: "cpl" Parameters: None Sub-tags: "ancillary" See Section 9 "subaction" See Section 8 "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 subactions, 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. 4. 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 that should be executed if the condition is 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 10 for more information on this. Switches MAY contain no outputs. They MAY only contain an "otherwise" output. Such switches are not particularly useful, but might be created by tools which automatically generate CPL scripts. 4.1. 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") 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 which 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 4.1.1; 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. user: This subfield of the address indicates, for e-mail style addresses, the user part of the address. For a 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  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. This subfield is not case sensitive, and may be absent. 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. 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. tel: This subfield indicates a telephone subscriber number, if the address contains such a number. It is not case sensitive (telephone numbers may contain the symbols 'A', 'B', 'C', or '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 4.2. 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. 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. 4.1.1. 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 URIs, the "user", "host", and "port" subfields correspond to the "user," "host," and "port" elements of the URI syntax. (Note that, following the definitions of RFC 3261 , a SIP URI which does not specify a port is not the same as an explicit port 5060; the former is indicated by an absent port subfield.) The "tel" subfield is defined to be the "user" part of the URI, with visual separators stripped, if the "user=phone" parameter is given to the URI, or if the server is otherwise configured to recognize the user part as a telephone number. 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 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. 4.2. 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. Four fields are defined and listed below. The value of each of these fields 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. 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 Standard Annex #15 . Then, strings are compared using locale- insensitive caseless mapping, as specified in Unicode Standard Annex #21 . Code to perform the first step, in Java and Perl, is available; see the links from Annex 5 of UAX 15 . 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 tag of string matching is named "string", and has a mandatory argument, one of "is" or "contains", indicating whole- string match or substring match, respectively. 4.2.1. 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.
4.3. 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. 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 Language switches take no parameters. The "language" output takes one parameter, "matches". The value of the parameter is a language-tag, as defined in RFC 3066 . The caller may have specified a set of language-ranges, also as defined in RFC 3066. The CPL server checks each language-tag 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 "-". If the caller specified the special language-range "*", it is ignored for the purpose of matching. Languages with a "q" value of 0 are also ignored. This switch MAY be not-present. 4.3.1. 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.
4.4. 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. 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 ("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 occurrences 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 work week "bysetpos" List of values within set of events specified Figure 7: Syntax of the "time-switch" node Time switches are based closely on the specification of recurring intervals of time in the Internet Calendaring and Scheduling Core Object Specification (iCalendar COS), RFC 2445 . This allows CPL scripts 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 update or obsolete RFC 2445, any changes or clarifications those documents make to recurrence handling apply to CPL time-switches as well.
An algorithm to determine 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 220.127.116.11 and 18.104.22.168 respectively). The "tzid" is the identifying label by which a time zone definition is referenced. If it begins with a forward slash (solidus), it references a to-be-defined global time zone registry; otherwise it is locally-defined at the server. The "tzurl" gives a network location from which an up-to-date VTIMEZONE definition for the timezone can be retrieved. 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 the Olson Time Zone database . 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 . 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 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 . 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 of period intervals are constructed as recurrences. A recurrence is specified by including the "freq" parameter, which indicates the type of recurrence rule. Parameters other than "dtstart", "dtend", and "duration" SHOULD NOT 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 second for a "secondly" rule, 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 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 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 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, and "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 . 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 January 1 is a Thursday or, for leap years, if January 1 is a Wednesday. 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 work week 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 . 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: <time -timerange- freq="monthly" byday="MO,TU,WE,TH,FR" bysetpos="-1"> Each "bysetpos" value can include a positive (+n) or negative (-n) integer. If present, this indicates the nth occurrence of the specific occurrence within the set of events specified by the rule. If byxxx parameter values are found which are beyond the available scope (i.e., bymonthday="30" in February), they are simply ignored. Byxxx parameters modify the recurrence in some manner. Byxxx rule parts for a period of time which is the same or greater than the frequency generally reduce or limit the number of occurrences of the recurrence generated. For example, freq="daily" bymonth="1" reduces the number of recurrence instances from all days (if the "bymonth" parameter is not present) to all days in January. Byxxx parameters for a period of time less than the frequency generally increase or expand the number of occurrences of the recurrence. For example, freq="yearly" bymonth="1,2" increases the number of days within the yearly recurrence set from 1 (if "bymonth" parameter is not present) to 2. If multiple Byxxx parameters are specified, then after evaluating the specified "freq" and "interval" parameters, the Byxxx parameters are applied to the current set of evaluated occurrences in the following order: "bymonth", "byweekno", "byyearday", "bymonthday", "byday", "byhour", "byminute", "bysecond", and "bysetpos"; then "count" and "until" are evaluated.
Here is an example of evaluating multiple Byxxx parameters. <time dtstart="19970105T083000" duration="10M" freq="yearly" interval="2" bymonth="1" byday="SU" byhour="8,9" byminute="30"> First, the interval="2" would be applied to freq="yearly" to arrive at "every other year." Then, bymonth="1" would be applied to arrive at "every January, every other year." Then, byday="SU" would be applied to arrive at "every Sunday in January, every other year." Then, byhour="8,9" would be applied to arrive at "every Sunday in January at 8 AM and 9 AM, every other year." Then, byminute="30" would be applied to arrive at "every Sunday in January at 8:30 AM and 9:30 AM, every other year." Then the second is derived from "dtstart" to end up in "every Sunday in January from 8:30:00 AM to 8:40:00 AM, and from and 9:30:00 AM to 9:40:00 AM, every other year." Similarly, 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 8) 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. 4.4.1. 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 , 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 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 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" parameter once, off-line, to determine 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 occurrence 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. 4.5. 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.
Node: "priority-switch" Outputs: "priority" Specific priority to match Parameters: None Output: "priority" Parameters: "less" Match if priority is less than that specified "greater" Match if priority is greater than that specified "equal" Match if priority is equal to that specified Figure 8: Syntax of the "priority-switch" node Priority switches take no parameters. The "priority" tag takes one of the three parameters "greater", "less", or "equal". The values of these parameters 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 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. 4.5.1. Usage of "priority-switch" with SIP The priority of a SIP message corresponds to the "Priority" header in the initial "INVITE" message.