Network Working Group                                       W. Segmuller
Request for Comment: 3431                IBM T.J. Watson Research Center
Category: Standards Track                                  December 2002


                   Sieve Extension: Relational Tests

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 (2002).  All Rights Reserved.

Abstract

   This document describes the RELATIONAL extension to the Sieve mail
   filtering language defined in RFC 3028.  This extension extends
   existing conditional tests in Sieve to allow relational operators.
   In addition to testing their content, it also allows for testing of
   the number of entities in header and envelope fields.

1 Introduction

   Sieve [SIEVE] is a language for filtering e-mail messages at 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 Messages Access Protocol (IMAP) servers, as it has no
   variables, loops, nor the ability to shell out to external programs.

   The RELATIONAL extension provides relational operators on the
   address, envelope, and header tests.  This extension also provides a
   way of counting the entities in a message header or address field.

   With this extension, the sieve script may now determine if a field is
   greater than or less than a value instead of just equivalent.  One
   use is for the x-priority field: move messages with a priority
   greater than 3 to the "work on later" folder.  Mail could also be
   sorted by the from address.  Those userids that start with 'a'-'m' go
   to one folder, and the rest go to another folder.

   The sieve script can also determine the number of fields in the
   header, or the number of addresses in a recipient field.  For
   example:  are there more than 5 addresses in the to and cc fields.

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 BCP 14, RFC 2119.

   Conventions for notations are as in [SIEVE] section 1.1, including
   the use of [KEYWORDS] and "Syntax:" label for the definition of
   action and tagged arguments syntax, and the use of [ABNF].

   The capability string associated with extension defined in this
   document is "relational".

3 Comparators

   This document does not define any comparators or exempt any
   comparators from the require clause.  Any comparator used, other than
   "i;octet" and "i;ascii-casemap", MUST be declared a require clause as
   defined in [SIEVE].

   The "i;ascii-numeric" comparator, as defined in [ACAP], MUST be
   supported for any implementation of this extension.  The comparator
   "i;ascii-numeric" MUST support at least 32 bit unsigned integers.

   Larger integers MAY be supported.  Note: the "i;ascii-numeric"
   comparator does not support negative numbers.

4 Match Type

   This document defines two new match types.  They are the VALUE match
   type and the COUNT match type.

     The syntax is:

        MATCH-TYPE =/ COUNT / VALUE

        COUNT = ":count" relational-match

        VALUE = ":value" relational-match

        relational-match = DQUOTE ( "gt" / "ge" / "lt"
                                    / "le" / "eq" / "ne" ) DQUOTE

4.1  Match Type Value

   The VALUE match type does a relational comparison between strings.

   The VALUE match type may be used with any comparator which returns
   sort information.

   Leading and trailing white space MUST be removed from the value of
   the message for the comparison.  White space is defined as

                             SP / HTAB / CRLF

   A value from the message is considered the left side of the relation.
   A value from the test expression, the key-list for address, envelope,
   and header tests, is the right side of the relation.

   If there are multiple values on either side or both sides, the test
   is considered true, if any pair is true.

4.2  Match Type Count

   The COUNT match type first determines the number of the specified
   entities in the message and does a relational comparison of the
   number of entities to the values specified in the test expression.

   The COUNT match type SHOULD only be used with numeric comparators.

   The Address Test counts the number of recipients in the specified
   fields.  Group names are ignored.

   The Envelope Test counts the number of recipients in the specified
   envelope parts.  The envelope "to" will always have only one entry,
   which is the address of the user for whom the sieve script is
   running.  There is no way a sieve script can determine if the message
   was actually sent to someone else using this test.  The envelope
   "from" will be 0 if the MAIL FROM is blank, or 1 if MAIL FROM is not
   blank.

   The Header Test counts the total number of instances of the specified
   fields.  This does not count individual addresses in the "to", "cc",
   and other recipient fields.

   In all cases, if more than one field name is specified, the counts
   for all specified fields are added together to obtain the number for
   comparison.  Thus, specifying ["to", "cc"] in an address COUNT test,
   comparing the total number of "to" and "cc" addresses; if separate
   counts are desired, they must be done in two comparisons, perhaps
   joined by "allof" or "anyof".

5 Security Considerations

   Security considerations are discussed in [SIEVE].

   An implementation MUST ensure that the test for envelope "to" only
   reflects the delivery to the current user.  It MUST not be possible
   for a user to determine if this message was delivered to someone else
   using this test.

6 Example

   Using the message:

      received: ...
      received: ...
      subject: example
      to: foo@example.com.invalid, baz@example.com.invalid
      cc: qux@example.com.invalid

   The test:

        address :count "ge" :comparator "i;ascii-numeric" ["to", "cc"]
      ["3"]

      would be true and the test

         anyof ( address :count "ge" :comparator "i;ascii-numeric"
                         ["to"] ["3"],
                 address :count "ge" :comparator "i;ascii-numeric"
                         ["cc"] ["3"] )

      would be false.

      To check the number of received fields in the header, the
      following test may be used:

         header :count "ge" :comparator "i;ascii-numeric"
                        ["received"] ["3"]

      This would return false.  But

         header :count "ge" :comparator "i;ascii-numeric"
                          ["received", "subject"] ["3"]

      would return true.

   The test:

         header :count "ge" :comparator "i;ascii-numeric"
                       ["to", "cc"] ["3"]

   will always return false on an RFC 2822 compliant message [RFC2822],
   since a message can have at most one "to" field and at most one "cc"
   field.  This test counts the number of fields, not the number of
   addresses.

7 Extended Example

   require ["relational", "comparator-i;ascii-numeric"];

   if header :value "lt" :comparator "i;ascii-numeric"
             ["x-priority"] ["3"]
   {
      fileinto "Priority";
   }

   elseif address :count "gt" :comparator "i;ascii-numeric"
              ["to"] ["5"]
   {
      # everything with more than 5 recipients in the "to" field
      # is considered SPAM
      fileinto "SPAM";
   }

   elseif address :value "gt" :all :comparator "i;ascii-casemap"
              ["from"] ["M"]
   {
      fileinto "From N-Z";
   } else {
      fileinto "From A-M";
   }

   if allof ( address :count "eq" :comparator "i;ascii-numeric"
                      ["to", "cc"] ["1"] ,
              address :all :comparator "i;ascii-casemap"
                      ["to", "cc"] ["me@foo.example.com.invalid"]
   {
      fileinto "Only me";
   }

8 IANA Considerations

   The following template specifies the IANA registration of the Sieve
   extension specified in this document:

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

   Capability name: RELATIONAL
   Capability keyword: relational
   Capability arguments: N/A
   Standards Track/IESG-approved experimental RFC number: this RFC
   Person and email address to contact for further information:
    Wolfgang Segmuller
    IBM T.J. Watson Research Center
    30 Saw Mill River Rd
    Hawthorne, NY 10532

    Email: whs@watson.ibm.com

   This information should be added to the list of sieve extensions
   given on http://www.iana.org/assignments/sieve-extensions.

9 References

9.1 Normative References

   [SIEVE]     Showalter, T., "Sieve: A Mail Filtering Language", RFC
               3028, January 2001.

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

   [ABNF]      Crocker, D., "Augmented BNF for Syntax Specifications:
               ABNF", RFC 2234, November 1997.

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

9.2 Non-Normative References

   [ACAP]      Newman, C. and J. G. Myers, "ACAP -- Application
               Configuration Access Protocol", RFC 2244, November 1997.

10 Author's Address

   Wolfgang Segmuller
   IBM T.J. Watson Research Center
   30 Saw Mill River Rd
   Hawthorne, NY  10532

   EMail: whs@watson.ibm.com

11 Full Copyright Statement

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Acknowledgement

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   Internet Society.