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IETF RFC 5231
Sieve Email Filtering: Relational Extension
Last modified on Monday, January 14th, 2008
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Network Working Group W. Segmuller
Request for Comments: 5231 B. Leiba
Obsoletes: 3431 IBM T.J. Watson Research Center
Category: Standards Track January 2008
Sieve Email Filtering: Relational Extension
 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 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.
This document obsoletes RFC 3431.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 2
3. Comparators . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Match Types . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Match Type VALUE . . . . . . . . . . . . . . . . . . . . . 3
4.2. Match Type COUNT . . . . . . . . . . . . . . . . . . . . . 3
5. Interaction with Other Sieve Actions . . . . . . . . . . . . . 4
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Extended Example . . . . . . . . . . . . . . . . . . . . . . . 6
8. Changes since RFC 3431 . . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
11. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Segmuller & Leiba Standards Track PAGE 1 
RFC 5231 Sieve: Relational Extension January 2008
1. Introduction
The RELATIONAL extension to the Sieve mail filtering language [Sieve]
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,
whether there are more than 5 addresses in the to and cc fields.
The capability string associated with the extension defined in this
document is "relational".
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 [Kwds] and the use of [ABNF].
3. Comparators
This document does not define any comparators or exempt any
comparators from the require clause. Any comparator used must be
treated as defined in [Sieve].
The "i;ascii-numeric" comparator, as defined in [RFC 4790], 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.
Segmuller & Leiba Standards Track PAGE 2
RFC 5231 Sieve: Relational Extension January 2008
4. Match Types
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
; "gt" means "greater than", the C operator ">".
; "ge" means "greater than or equal", the C operator ">=".
; "lt" means "less than", the C operator "<".
; "le" means "less than or equal", the C operator "<=".
; "eq" means "equal to", the C operator "==".
; "ne" means "not equal to", the C operator "!=".
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 that returns
sort information.
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, as defined below, 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 addresses (the number of
"mailbox" elements, as defined in [RFC 2822]) in the specified fields.
Group names are ignored, but the contained mailboxes are counted.
Segmuller & Leiba Standards Track PAGE 3
RFC 5231 Sieve: Relational Extension January 2008
The Envelope Test counts the number of addresses 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. Using this test, there is no way a Sieve script can
determine if the message was actually sent to someone else. The
envelope "from" will be 0 if the MAIL FROM is empty, or 1 if MAIL
FROM is not empty.
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
compares 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. Interaction with Other Sieve Actions
This specification adds two match types. The VALUE match type only
works with comparators that return sort information. The COUNT match
type only makes sense with numeric comparators.
There is no interaction with any other Sieve operations, nor with any
known extensions. In particular, this specification has no effect on
implicit KEEP, nor on any explicit message actions.
6. Example
Using the message:
received: ...
received: ...
subject: example
to: foo@example.com, baz@example.com
cc: qux@example.com
The test:
address :count "ge" :comparator "i;ascii-numeric"
["to", "cc"] ["3"]
would evaluate to true, and the test
Segmuller & Leiba Standards Track PAGE 4
RFC 5231 Sieve: Relational Extension January 2008
anyof ( address :count "ge" :comparator "i;ascii-numeric"
["to"] ["3"],
address :count "ge" :comparator "i;ascii-numeric"
["cc"] ["3"] )
would evaluate to 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 evaluate to false. But
header :count "ge" :comparator "i;ascii-numeric"
["received", "subject"] ["3"]
would evaluate to true.
The test:
header :count "ge" :comparator "i;ascii-numeric"
["to", "cc"] ["3"]
will always evaluate to false on an RFC 2822 compliant message
[RFC 2822], 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.
Segmuller & Leiba Standards Track PAGE 5
RFC 5231 Sieve: Relational Extension January 2008
7. Extended Example
require ["relational", "comparator-i;ascii-numeric", "fileinto"];
if header :value "lt" :comparator "i;ascii-numeric"
["x-priority"] ["3"]
{
fileinto "Priority";
}
elsif address :count "gt" :comparator "i;ascii-numeric"
["to"] ["5"]
{
# everything with more than 5 recipients in the "to" field
# is considered SPAM
fileinto "SPAM";
}
elsif 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"] )
{
fileinto "Only me";
}
8. Changes since RFC 3431
Apart from several minor editorial/wording changes, the following
list describes the notable changes to this specification since RFC
3431.
o Updated references, including changing the comparator reference
from the Application Configuration Access Protocol (ACAP) to the
"Internet Application Protocol Collation Registry" document
[RFC 4790].
o Updated and corrected the examples.
Segmuller & Leiba Standards Track PAGE 6
RFC 5231 Sieve: Relational Extension January 2008
o Added definition comments to ABNF for "gt", "lt", etc.
o Clarified what RFC 2822 elements are counted in the COUNT test.
o Removed the requirement to strip white space from header fields
before comparing; a more general version of this requirement has
been added to the Sieve base spec.
9. IANA Considerations
The following template specifies the IANA registration of the
relational Sieve extension specified in this document:
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: relational
Description: Extends existing conditional tests in Sieve language
to allow relational operators
RFC number: RFC 5231
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
10. Security Considerations
An implementation MUST ensure that the test for envelope "to" only
reflects the delivery to the current user. Using this test, it MUST
not be possible for a user to determine if this message was delivered
to someone else.
Additional security considerations are discussed in [Sieve].
11. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[Kwds] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC 4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790,
March 2007.
[Sieve] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
Filtering Language", RFC 5228, January 2008.
Segmuller & Leiba Standards Track PAGE 7
RFC 5231 Sieve: Relational Extension January 2008
Authors' Addresses
Wolfgang Segmuller
IBM T.J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532
US
Phone: +1 914 784 7408
EMail: werewolf@us.ibm.com
Barry Leiba
IBM T.J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532
US
Phone: +1 914 784 7941
EMail: leiba@watson.ibm.com
Segmuller & Leiba Standards Track PAGE 8
RFC 5231 Sieve: Relational Extension January 2008
Full Copyright Statement
Copyright © The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Segmuller & Leiba Standards Track PAGE 9
Sieve Email Filtering: Relational Extension
RFC TOTAL SIZE: 15243 bytes
PUBLICATION DATE: Monday, January 14th, 2008
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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