Sieve Email Filtering: Date and Index Extensions
RFC 5260

Network Working Group                                           N. Freed
Request for Comments: 5260                              Sun Microsystems
Category: Standards Track                                      July 2008

            Sieve Email Filtering: Date and Index Extensions

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.

Abstract

   This document describes the "date" and "index" extensions to the
   Sieve email filtering language.  The "date" extension gives Sieve the
   ability to test date and time values in various ways.  The "index"
   extension provides a means to limit header and address tests to
   specific instances of header fields when header fields are repeated.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  2
   3.  Capability Identifiers . . . . . . . . . . . . . . . . . . . .  3
   4.  Date Test  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     4.1.  Zone and Originalzone Arguments  . . . . . . . . . . . . .  4
     4.2.  Date-part Argument . . . . . . . . . . . . . . . . . . . .  4
     4.3.  Comparator Interactions with Date-part Arguments . . . . .  5
     4.4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Currentdate Test . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Index Extension  . . . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Julian Date Conversions . . . . . . . . . . . . . . . 11
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   Sieve [RFC5228] is a language for filtering email messages at or
   around the time of final delivery.  It is designed to be
   implementable on either a mail client or mail server.  It is meant to
   be extensible, simple, and independent of access protocol, mail
   architecture, and operating system.  It is suitable for running on a
   mail server where users may not be allowed to execute arbitrary
   programs, such as on black box Internet Message Access Protocol
   [RFC3501] servers, as it does not have user-controlled loops or the
   ability to run external programs.

   The "date" extension provides a new date test to extract and match
   date/time information from structured header fields.  The date test
   is similar in concept to the address test specified in [RFC5228],
   which performs similar operations on addresses in header fields.

   The "date" extension also provides a currentdate test that operates
   on the date and time when the Sieve script is executed.

   Some header fields containing date/time information, e.g., Received:,
   naturally occur more than once in a single header.  In such cases it
   is useful to be able to restrict the date test to some subset of the
   fields that are present.  For example, it may be useful to apply a
   date test to the last (earliest) Received: field.  Additionally, it
   may also be useful to apply similar restrictions to either the header
   or address tests specified in [RFC5228].

   For this reason, this specification also defines an "index"
   extension.  This extension adds two additional tagged arguments
   :index and :last to the header, address, and date tests.  If present,
   these arguments specify which occurrence of the named header field is
   to be tested.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The terms used to describe the various components of the Sieve
   language are taken from Section 1.1 of [RFC5228].  Section 2 of the
   same document describes basic Sieve language syntax and semantics.
   The date-time syntactic element defined using ABNF notation [RFC5234]
   in [RFC3339] is also used here.

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3.  Capability Identifiers

   The capability strings associated with the two extensions defined in
   this document are "date" and "index".

4.  Date Test

   Usage:   date [<":zone" <time-zone: string>> / ":originalzone"]
                 [COMPARATOR] [MATCH-TYPE] <header-name: string>
                 <date-part: string> <key-list: string-list>

   The date test matches date/time information derived from headers
   containing [RFC2822] date-time values.  The date/time information is
   extracted from the header, shifted to the specified time zone, and
   the value of the given date-part is determined.  The test returns
   true if the resulting string matches any of the strings specified in
   the key-list, as controlled by the comparator and match keywords.
   The date test returns false unconditionally if the specified header
   field does not exist, the field exists but does not contain a
   syntactically valid date-time specification, the date-time isn't
   valid according to the rules of the calendar system (e.g., January
   32nd, February 29 in a non-leap year), or the resulting string fails
   to match any key-list value.

   The type of match defaults to ":is" and the default comparator is
   "i;ascii-casemap".

   Unlike the header and address tests, the date test can only be
   applied to a single header field at a time.  If multiple header
   fields with the same name are present, only the first field that is
   found is used.  (Note, however, that this behavior can be modified
   with the "index" extension defined below.)  These restrictions
   simplify the test and keep the meaning clear.

   The "relational" extension [RFC5231] adds a match type called
   ":count".  The count of a date test is 1 if the specified field
   exists and contains a valid date; 0, otherwise.

   Implementations MUST support extraction of RFC 2822 date-time
   information that either makes up the entire header field (e.g., as it
   does in a standard Date: header field) or appears at the end of a
   header field following a semicolon (e.g., as it does in a standard
   Received: header field).  Implementations MAY support extraction of
   date and time information in RFC2822 or other formats that appears in
   other positions in header field content.  In the case of a field
   containing more than one date or time value, the last one that
   appears SHOULD be used.

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4.1.  Zone and Originalzone Arguments

   The :originalzone argument specifies that the time zone offset
   originally in the extracted date-time value should be retained.  The
   :zone argument specifies a specific time zone offset that the date-
   time value is to be shifted to prior to testing.  It is an error to
   specify both :zone and :originalzone.

   The value of time-zone MUST be an offset relative to UTC with the
   following syntax:

       time-zone  =  ( "+" / "-" ) 4DIGIT

   The "+" or "-" indicates whether the time-of-day is ahead of (i.e.,
   east of) or behind (i.e., west of) UTC.  The first two digits
   indicate the number of hours difference from Universal Time, and the
   last two digits indicate the number of minutes difference from
   Universal Time.  Note that this agrees with the RFC 2822 format for
   time zone offsets, not the ISO 8601 format.

   If both the :zone and :originalzone arguments are omitted, the local
   time zone MUST be used.

4.2.  Date-part Argument

   The date-part argument specifies a particular part of the resulting
   date/time value to match against the key-list.  Possible case-
   insensitive values are:

     "year"      => the year, "0000" .. "9999".
     "month"     => the month, "01" .. "12".
     "day"       => the day, "01" .. "31".
     "date"      => the date in "yyyy-mm-dd" format.
     "julian"    => the Modified Julian Day, that is, the date
                    expressed as an integer number of days since
                    00:00 UTC on November 17, 1858 (using the Gregorian
                    calendar).  This corresponds to the regular
                    Julian Day minus 2400000.5.  Sample routines to
                    convert to and from modified Julian dates are
                    given in Appendix A.
     "hour"      => the hour, "00" .. "23".
     "minute"    => the minute, "00" .. "59".
     "second"    => the second, "00" .. "60".
     "time"      => the time in "hh:mm:ss" format.
     "iso8601"   => the date and time in restricted ISO 8601 format.
     "std11"     => the date and time in a format appropriate
                    for use in a Date: header field [RFC2822].

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     "zone"      => the time zone in use.  If the user specified a
                    time zone with ":zone", "zone" will
                    contain that value.  If :originalzone is specified
                    this value will be the original zone specified
                    in the date-time value.  If neither argument is
                    specified the value will be the server's default
                    time zone in offset format "+hhmm" or "-hhmm".  An
                    offset of 0 (Zulu) always has a positive sign.
     "weekday"   => the day of the week expressed as an integer between
                    "0" and "6". "0" is Sunday, "1" is Monday, etc.

   The restricted ISO 8601 format is specified by the date-time ABNF
   production given in [RFC3339], Section 5.6, with the added
   restrictions that the letters "T" and "Z" MUST be in upper case, and
   a time zone offset of zero MUST be represented by "Z" and not
   "+00:00".

4.3.  Comparator Interactions with Date-part Arguments

   Not all comparators are suitable with all date-part arguments.  In
   general, the date-parts can be compared and tested for equality with
   either "i;ascii-casemap" (the default) or "i;octet", but there are
   two exceptions:

   julian  This is an integer, and may or may not have leading zeros.
           As such, "i;ascii-numeric" is almost certainly the best
           comparator to use with it.

   std11   This is provided as a means to obtain date/time values in a
           format appropriate for inclusion in email header fields.  The
           wide range of possible syntaxes for a std11 date/time --
           which implementations of this extension are free to use when
           composing a std11 string -- makes this format a poor choice
           for comparisons.  Nevertheless, if a comparison must be
           performed, this is case-insensitive, and therefore "i;ascii-
           casemap" needs to be used.

   "year", "month", "day", "hour", "minute", "second" and "weekday" all
   use fixed-width string representations of integers, and can therefore
   be compared with "i;octet", "i;ascii-casemap", and "i;ascii-numeric"
   with equivalent results.

   "date" and "time" also use fixed-width string representations of
   integers, and can therefore be compared with "i;octet" and "i;ascii-
   casemap"; however, "i;ascii-numeric" can't be used with it, as
   "i;ascii-numeric" doesn't allow for non-digit characters.

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4.4.  Examples

   The Date: field can be checked to test when the sender claims to have
   created the message and act accordingly:

     require ["date", "relational", "fileinto"];
     if allof(header :is "from" "boss@example.com",
              date :value "ge" :originalzone "date" "hour" "09",
              date :value "lt" :originalzone "date" "hour" "17")
     { fileinto "urgent"; }

   Testing the initial Received: field can provide an indication of when
   a message was actually received by the local system:

     require ["date", "relational", "fileinto"];
     if anyof(date :is "received" "weekday" "0",
              date :is "received" "weekday" "6")
     { fileinto "weekend"; }

5.  Currentdate Test

   Usage:   currentdate [":zone" <time-zone: string>]
                        [COMPARATOR] [MATCH-TYPE]
                        <date-part: string>
                        <key-list: string-list>

   The currentdate test is similar to the date test, except that it
   operates on the current date/time rather than a value extracted from
   the message header.  In particular, the ":zone" and date-part
   arguments are the same as those in the date test.

   All currentdate tests in a single Sieve script MUST refer to the same
   point in time during execution of the script.

   The :count value of a currentdate test is always 1.

5.1.  Examples

   The simplest use of currentdate is to have an action that only
   operates at certain times.  For example, a user might want to have
   messages redirected to their pager after business hours and on
   weekends:

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     require ["date", "relational"];
     if anyof(currentdate :is "weekday" "0",
              currentdate :is "weekday" "6",
              currentdate :value "lt" "hour" "09",
              currentdate :value "ge" "hour" "17")
     { redirect "pager@example.com"; }

   Currentdate can be used to set up vacation [RFC5230] responses in
   advance and to stop response generation automatically:

     require ["date", "relational", "vacation"];
     if allof(currentdate :value "ge" "date" "2007-06-30",
              currentdate :value "le" "date" "2007-07-07")
     { vacation :days 7  "I'm away during the first week in July."; }

   Currentdate may also be used in conjunction with the variables
   extension to pass time-dependent arguments to other tests and
   actions.  The following Sieve places messages in a folder named
   according to the current month and year:

     require ["date", "variables", "fileinto"];
     if currentdate :matches "month" "*" { set "month" "${1}"; }
     if currentdate :matches "year"  "*" { set "year"  "${1}"; }
     fileinto "${month}-${year}";

   Finally, currentdate can be used in conjunction with the editheader
   extension to insert a header-field containing date/time information:

      require ["variables", "date", "editheader"];
      if currentdate :matches "std11" "*"
        {addheader "Processing-date" "${0}";}

6.  Index Extension

   The "index" extension, if specified, adds optional :index and :last
   arguments to the header, address, and date tests as follows:

   Syntax:   date [":index" <fieldno: number> [":last"]]
                  [<":zone" <time-zone: string>> / ":originalzone"]
                  [COMPARATOR] [MATCH-TYPE] <header-name: string>
                  <date-part: string> <key-list: string-list>

   Syntax:   header [":index" <fieldno: number> [":last"]]
                    [COMPARATOR] [MATCH-TYPE]
                    <header-names: string-list> <key-list: string-list>

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   Syntax:   address [":index" <fieldno: number> [":last"]]
                     [ADDRESS-PART] [COMPARATOR] [MATCH-TYPE]
                     <header-list: string-list> <key-list: string-list>

   If :index <fieldno> is specified, the attempts to match a value are
   limited to the header field fieldno (beginning at 1, the first named
   header field).  If :last is also specified, the count is backwards; 1
   denotes the last named header field, 2 the second to last, and so on.
   Specifying :last without :index is an error.

   :index only counts separate header fields, not multiple occurrences
   within a single field.  In particular, :index cannot be used to test
   a specific address in an address list contained within a single
   header field.

   Both header and address allow the specification of more than one
   header field name.  If more than one header field name is specified,
   all the named header fields are counted in the order specified by the
   header-list.

6.1.  Example

   Mail delivery may involve multiple hops, resulting in the Received:
   field containing information about when a message first entered the
   local administrative domain being the second or subsequent field in
   the message.  As long as the field offset is consistent, it can be
   tested:

     # Implement the Internet-Draft cutoff date check assuming the
     # second Received: field specifies when the message first
     # entered the local email infrastructure.
     require ["date", "relational", "index"];
     if date :value "gt" :index 2 :zone "-0500" "received"
             "iso8601" "2007-02-26T09:00:00-05:00",
     { redirect "aftercutoff@example.org"; }

7.  Security Considerations

   The facilities defined here, like the facilities in the base Sieve
   specification, operate on message header information that can easily
   be forged.  Note, however, that some fields are inherently more
   reliable than others.  For example, the Date: field is typically
   inserted by the message sender and can be altered at any point.  By
   contrast, the uppermost Received: field is typically inserted by the
   local mail system and is therefore difficult for the sender or an
   intermediary to falsify.

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   Use of the currentdate test makes script behavior inherently less
   predictable and harder to analyze.  This may have consequences for
   systems that use script analysis to try and spot problematic scripts.

   All of the security considerations given in the base Sieve
   specification also apply to these extensions.

8.  IANA Considerations

   The following templates specify the IANA registrations of the two
   Sieve extensions specified in this document:

      To: iana@iana.org
      Subject: Registration of new Sieve extensions

      Capability name: date
      Description:     The "date" extension gives Sieve the ability
                       to test date and time values.
      RFC number:      RFC 5260
      Contact address: Sieve discussion list <ietf-mta-filters@imc.org>

      Capability name: index
      Description:     The "index" extension provides a means to
                       limit header and address tests to specific
                       instances when more than one field of a
                       given type is present.
      RFC number:      RFC 5260
      Contact address: Sieve discussion list <ietf-mta-filters@imc.org>

9.  References

9.1.  Normative References

   [CALGO199]  Tantzen, R., "Algorithm 199: Conversions Between Calendar
               Date and Julian Day Number", Collected Algorithms from
               CACM 199.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2822]   Resnick, P., "Internet Message Format", RFC 2822,
               April 2001.

   [RFC3339]   Klyne, G., Ed. and C. Newman, "Date and Time on the
               Internet: Timestamps", RFC 3339, July 2002.

   [RFC5228]   Guenther, P. and T. Showalter, "Sieve: An Email Filtering
               Language", RFC 5228, January 2008.

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   [RFC5231]   Segmuller, W. and B. Leiba, "Sieve Email Filtering:
               Relational Extension", RFC 5231, January 2008.

   [RFC5234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", STD 68, RFC 5234, January 2008.

9.2.  Informative References

   [RFC3501]   Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
               4rev1", RFC 3501, March 2003.

   [RFC5230]   Showalter, T. and N. Freed, "Sieve Email Filtering:
               Vacation Extension", RFC 5230, January 2008.

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Appendix A.  Julian Date Conversions

   The following C routines show how to translate day/month/year
   information to and from modified Julian dates.  These routines are
   straightforward translations of the Algol routines specified in CACM
   Algorithm 199 [CALGO199].

   Given the day, month, and year, jday returns the modified Julian
   date.

   int jday(int year, int month, int day)
   {
       int j, c, ya;

       if (month > 2)
           month -= 3;
       else
       {
           month += 9;
           year--;
       }
       c = year / 100;
       ya = year - c * 100;
       return (c * 146097 / 4 + ya * 1461 / 4 + (month * 153 + 2) / 5 +
               day + 1721119);
   }

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   Given j, the modified Julian date, jdate returns the day, month, and
   year.

   void jdate(int j, int *year, int *month, int *day)
   {
       int y, m, d;

       j -= 1721119;
       y = (j * 4 - 1) / 146097;
       j = j * 4 - y * 146097 - 1;
       d = j / 4;
       j = (d * 4 + 3) / 1461;
       d = d * 4 - j * 1461 + 3;
       d = (d + 4) / 4;
       m = (d * 5 - 3) / 153;
       d = d * 5 - m * 153 - 3;
       *day = (d + 5) / 5;
       *year = y * 100 + j;
       if (m < 10)
           *month = m + 3;
       else
       {
           *month = m - 9;
           *year += 1;
       }
   }

Appendix B.  Acknowledgements

   Dave Cridland contributed the text describing the proper comparators
   to use with different date-parts.  Cyrus Daboo, Frank Ellerman,
   Alexey Melnikov, Chris Newman, Dilyan Palauzov, and Aaron Stone
   provided helpful suggestions and corrections.

Author's Address

   Ned Freed
   Sun Microsystems
   800 Royal Oaks
   Monrovia, CA  91016-6347
   USA

   Phone: +1 909 457 4293
   EMail: ned.freed@mrochek.com

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