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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. Abstract The Call Processing Language (CPL) is a language that can be used to describe and control Internet telephony services. It is designed to be implementable on either network servers or user agent servers. It is meant to be simple, extensible, easily edited by graphical clients, and independent of operating system or signalling protocol. It is suitable for running on a server where users may not be allowed to execute arbitrary programs, as it has no variables, loops, or ability to run external programs. This document is a product of the IP Telephony (IPTEL) working group of the Internet Engineering Task Force. Comments are solicited and should be addressed to the working group's mailing list at iptel@lists.research.bell-labs.com and/or the authors. Lennox/Schulzrinne [Page 1] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 2] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 3] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 4] Internet Draft CPL November 21, 2001 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". Lennox/Schulzrinne [Page 5] Internet Draft CPL November 21, 2001
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]. Lennox/Schulzrinne [Page 6] Internet Draft CPL November 21, 2001 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 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 given in Figure 3. Lennox/Schulzrinne [Page 7] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 8] Internet Draft CPL November 21, 2001 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. Lennox/Schulzrinne [Page 9] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 10] Internet Draft CPL November 21, 2001 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. Lennox/Schulzrinne [Page 11] Internet Draft CPL November 21, 2001 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. Lennox/Schulzrinne [Page 12] Internet Draft CPL November 21, 2001 "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 Lennox/Schulzrinne [Page 13] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 14] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 15] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 16] Internet Draft CPL November 21, 2001 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 Lennox/Schulzrinne [Page 17] Internet Draft CPL November 21, 2001 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. Lennox/Schulzrinne [Page 18] Internet Draft CPL November 21, 2001 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: