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rfc:rfc6134

Internet Engineering Task Force (IETF) A. Melnikov Request for Comments: 6134 Isode Limited Category: Standards Track B. Leiba ISSN: 2070-1721 Huawei Technologies

                                                             July 2011
              Sieve Extension: Externally Stored Lists

Abstract

 The Sieve email filtering language can be used to implement email
 whitelisting, blacklisting, personal distribution lists, and other
 sorts of list matching.  Currently, this requires that all members of
 such lists be hard-coded in the script itself.  Whenever a member of
 a list is added or deleted, the script needs to be updated and
 possibly uploaded to a mail server.
 This document defines a Sieve extension for accessing externally
 stored lists -- lists whose members are stored externally to the
 script, such as using the Lightweight Directory Access Protocol
 (LDAP), the Application Configuration Access Protocol (ACAP), vCard
 Extensions to WebDAV (CardDAV), or relational databases.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6134.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect

Melnikov & Leiba Standards Track [Page 1] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
 2.  Extlists Extension . . . . . . . . . . . . . . . . . . . . . .  3
   2.1.  Capability Identifier  . . . . . . . . . . . . . . . . . .  3
   2.2.  :list Match Type for Supported Tests . . . . . . . . . . .  3
   2.3.  :list Tagged Argument to the "redirect" Action . . . . . .  5
   2.4.  Other Uses for External Lists  . . . . . . . . . . . . . .  5
   2.5.  Syntax of an Externally Stored List Name . . . . . . . . .  5
   2.6.  Definition of "addrbook" URN Parameter . . . . . . . . . .  7
   2.7.  Test valid_ext_list  . . . . . . . . . . . . . . . . . . .  9
   2.8.  Interaction with ManageSieve . . . . . . . . . . . . . . .  9
   2.9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
     2.9.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . 10
     2.9.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . 10
     2.9.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . 11
     2.9.4.  Example 4  . . . . . . . . . . . . . . . . . . . . . . 11
     2.9.5.  Example 5  . . . . . . . . . . . . . . . . . . . . . . 11
 3.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
 4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   4.1.  Registration of Sieve Extension  . . . . . . . . . . . . . 14
   4.2.  Registration of ManageSieve Capability . . . . . . . . . . 14
   4.3.  Creation of Sieve URN Parameters Registry  . . . . . . . . 15
   4.4.  Registration of the "addrbook" URN parameter . . . . . . . 16
   4.5.  Registration of "sieve" URN sub-namespace  . . . . . . . . 16
 5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
 6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
   6.2.  Informative References . . . . . . . . . . . . . . . . . . 17

Melnikov & Leiba Standards Track [Page 2] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

1. Introduction

 This document specifies an extension to the Sieve language [RFC5228]
 for checking membership in an external list or for redirecting
 messages to an external list of recipients.  An "external list" is a
 list whose members are stored externally to the Sieve script, such as
 using LDAP [RFC4510], ACAP [RFC2244], CardDAV [CardDAV], or
 relational databases.
 This extension adds a new match type to apply to supported tests and
 a new tagged argument to the "redirect" action.

1.1. Conventions Used in This Document

 Conventions for notations are as in [RFC5228], Section 1.1, including
 the use of ABNF [RFC5234].
 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 [RFC2119].

2. Extlists Extension

2.1. Capability Identifier

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

2.2. :list Match Type for Supported Tests

 ABNF:
       MATCH-TYPE  =/ ":list"
            ; only valid for supported tests
 The new ":list" match type changes the interpretation of the "key-
 list" parameter (the second parameter) in supported tests.  When the
 match type is ":list", the key-list becomes a list of names of
 externally stored lists.  The external lists are queried, perhaps
 through a list-specific mechanism, and the test evaluates to "true"
 if any of the specified values matches any member of one or more of
 the lists.
 Comparators are not allowed together with the ":list" match type, so
 if both are specified in a test, that MUST result in an error.
 Queries done through list-specific mechanisms might have the effect

Melnikov & Leiba Standards Track [Page 3] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 of built-in comparators; for example, queries to certain lists might
 be case-sensitive, while queries to other lists might be done without
 regard to case.
 Implementations MUST support the use of ":list" in "address",
 "envelope", and "header" tests.  Implementations that include the
 Variables extension [RFC5229] MUST also support its use in "string"
 tests.
 Implementations MAY support other tests than the ones in this
 document.  Implementations MUST report an error when a script uses
 ":list" with a test that does not support ":list".  This error SHOULD
 be reported at compile-time but MAY be reported at run-time.  To
 maintain interoperability, other tests that can be used with ":list"
 SHOULD be documented in a specification that defines a capability
 string that can be tested (in a "require" statement or using "ihave"
 [RFC5463]).
 For example, testing 'header ["to", "cc"]' against a list would cause
 each "to" and "cc" value, ignoring leading and trailing whitespace,
 to be queried.  If any value is found to belong to the list, the test
 returns "true".  If no value belongs to the list, the test returns
 "false".  Once a value is found in the list, there is no need for the
 query mechanism to look further.
 For some lists, the Sieve engine might directly retrieve the list and
 make its own comparison.  Other lists might not work that way -- they
 might provide a way to ask if a value is in the list, but not permit
 retrieval of the list itself.  It is up to the Sieve implementation
 to understand how to interact with any supported list.  If the Sieve
 engine is permanently unable to query the list (perhaps because the
 list doesn't support the required operation), the test MUST result in
 a runtime error in the Sieve script.
 See Section 2.5 for the detailed description of syntax used for
 naming externally stored lists.
 The ":list" match type uses the concept of "match variables" as
 defined in Section 3.2 of the Variables extension [RFC5229].
 Implementations that also support that extension MUST set the ${0}
 match variable to the value in the list that matched the query.
 Other numbered match variables (${1}, ${2}, and so on) MAY be set
 with list-specific information that might be of use to the script.

Melnikov & Leiba Standards Track [Page 4] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

2.3. :list Tagged Argument to the "redirect" Action

 Usage:  redirect :list <ext-list-name: string>
 The "redirect" action with the ":list" argument is used to send the
 message to the set of email addresses in the externally stored list
 named by the ext-list-name string.  This variant of the redirect
 command can be used to implement a personal distribution list.
 For this feature to work, one of the following conditions has to be
 true:
 1.  The list resolves to a list of email addresses, and the Sieve
     engine is able to enumerate those addresses.
 2.  The list handler is able to take care of the redirection on
     behalf of the Sieve engine.
 In cases where, for example, a list contains hashed email address
 values or an email address pattern ("sz*@example.com",
 "*+ietf@example.net"), the Sieve engine will not be able to redirect
 to that list, and responsibility must pass to the list handler.
 If neither the Sieve engine nor the list handler can enumerate (or
 iterate) the list, or the list does not resolve to email addresses,
 the situation MUST result in a runtime error in the Sieve script.
 See Section 2.5 for the detailed description of syntax used for
 naming externally stored lists.

2.4. Other Uses for External Lists

 The uses for external lists specified here represent known cases and
 situations at the time of this writing.  Other uses for external
 lists, employing other Sieve features, might be devised in the
 future, and such uses can be described in extensions to this
 document.

2.5. Syntax of an Externally Stored List Name

 A name of an externally stored list is always an absolute URI
 [RFC3986].  Implementations might find URIs such as LDAP [RFC4510],
 CardDAV [CardDAV], or "tag" [RFC4151] to be useful for naming
 external lists.
 The "tag" URI scheme [RFC4151] can be used to represent opaque, but
 more user-friendly identifiers.  Resolution of such identifiers is
 going to be implementation specific and it can help in hiding the

Melnikov & Leiba Standards Track [Page 5] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 complexity of an implementation from end users.  For example, an
 implementation can provide a web interface for managing lists of
 users stored in LDAP.  Requiring users to know generic LDAP URI
 syntax might not be very practical, due to its complexity.  An
 implementation can instead use a fixed tag URI prefix such as "tag:
 example.com,<date>:" (where <date> can be, for example, a date
 generated once on installation of the web interface and left
 untouched upon upgrades), and the prefix doesn't even need to be
 shown to end users.
 The "addrbook" URNs defined in Section 2.6 (in particular, the
 reserved URI "urn:ietf:params:sieve:addrbook:default") MUST be
 supported.  To make it easier to use registered Sieve URN parameters,
 we define a shorthand way to specify them in a Sieve script: a list
 name that begins with ":" is taken as referencing a Sieve URN
 parameter, with the initial ":" expanding to
 "urn:ietf:params:sieve:".  So we have the following equivalences:
    :addrbook:default == urn:ietf:params:sieve:addrbook:default
    :addrbook:personal == urn:ietf:params:sieve:addrbook:personal
 and so on.
 The mandatory-to-implement URI
    urn:ietf:params:sieve:addrbook:default
 gives access to the user's default address book (usually the user's
 personal address book).  Note that these are URIs, subject to normal
 URI encoding rules, including percent-encoding.  The reserved name
 "default" MUST be considered case-insensitive after decoding.  That
 means that the following URIs are all equivalent:
    :addrbook:default
    :ADDRBOOK:DEFAULT
    :aDdRbOOk:DeFauLt
    :AddrBook:%44%65%66ault
 Address book names other than "default" MAY be case-sensitive,
 depending upon the implementation, so their case (after URI decoding)
 MUST be maintained.

Melnikov & Leiba Standards Track [Page 6] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 It's possible that a server will have no access to anything
 resembling an address book (perhaps in an implementation where
 address books are only client-side things), but the server can still
 provide access to other sorts of lists -- consider the list of dates
 in Example 2 (Section 2.9.2), or lists of important keywords and the
 like.  It might sometimes make sense to map ":addrbook:default" into
 some available list, but that might not always be reasonable.  If
 there really is no concept of an address book in a particular server
 implementation, the server MAY support ":addrbook:default" by having
 all matches to it fail.  Such an implementation SHOULD NOT be done
 except as a last resort.
 Queries against address books SHOULD be done without regard to case.

2.6. Definition of "addrbook" URN Parameter

 This section gives the details of the "addrbook" Sieve URN parameter
 that's registered in Section 4.4.  URIs that use this parameter begin
 with "urn:ietf:params:sieve:addrbook:".
 URN parameter name:  addrbook
 URN parameter syntax:  The "addrbook" parameter is defined by the
     <addrbook-urn> rule, defined using ABNF [RFC5234]:
        addrbook-urn = "addrbook:" addrbook [ "?" extensions ]
        addrbook = segment
             ; <segment> defined in [RFC3986]
        extensions = query
             ; <query> defined in [RFC3986]
 Intended usage:  "addrbook" URNs are used for designating references
 to address books.  An address book is a concept used by different
 applications (such as Sieve interpreters) for describing a list of
 named entries, and may be translated into other types of address
 books, such as LDAP groups.  Address books may be private or shared;
 they may be personal, organizational, or perhaps even "crowdsourced".
 The address book name (the "addrbook" element in the ABNF above)
 refers to a specifically named address book, as defined by the
 implementation.  A user might, for example, have access to a number
 of different address books, such as a personal one, a family one, a
 company one, and one for the town where the user lives.
 The extension information (the "extensions" element in the ABNF
 above) is available for use in future extensions.  It might allow for
 things such as dynamic subsets of an address book -- for example,
 something such as this might be defined in the future:

Melnikov & Leiba Standards Track [Page 7] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

     urn:ietf:params:sieve:addrbook:personal?name.contains=fred
     There are no extensions defined at this time.
     An "addrbook" URN is designed to be used by applications for
     referencing address books.  Each URN is intended to represent a
     grouping of addresses that can be logically thought of as one
     "book".  Any given address can belong to more than one book --
     that is, can be referred to by more than one URN.
     The URI "urn:ietf:params:sieve:addrbook" has no meaning in
     itself.  It MUST be used with sub-parameters representing the
     address book name and extension information, as shown in the ABNF
     above.
     The sub-parameter "default" (creating the URN
     "urn:ietf:params:sieve:addrbook:default") is a reserved (case-
     insensitive) name that MUST be implemented, representing a
     default grouping (book) of addresses.  Other names, representing
     the same or other groupings MAY be implemented.  For example, an
     implementation might use the following sub-parameters:
  • personal – a book representing the user's personal address

book.

  • friends – a subset of

urn:ietf:params:sieve:addrbook:personal, defined by the user.

  • family – a subset of

urn:ietf:params:sieve:addrbook:personal, defined by the user.

  • company – a book representing the user's company's address

book.

  • department – a subset of

urn:ietf:params:sieve:addrbook:company, defined by the

         company.
  • co-workers – a subset of

urn:ietf:params:sieve:addrbook:company, defined by the user.

  • default – the default address book, a reference to

urn:ietf:params:sieve:addrbook:personal.

 Interoperability considerations:  Applications are only REQUIRED to
     support "addrbook:default", where all cases and encodings of
     "default" are considered equivalent.  Address book names other
     than "default" MAY be case-sensitive, depending upon the

Melnikov & Leiba Standards Track [Page 8] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

     implementation, so their case (after URI decoding) MUST be
     maintained.
 Security considerations:  Applications SHOULD ensure appropriate
     restrictions are in place to protect sensitive information that
     might be revealed by "addrbook" URNs from access or modification
     by untrusted sources.
 Contact:  Sieve mailing list <sieve@ietf.org>

2.7. Test valid_ext_list

 Usage: valid_ext_list <ext-list-names: string-list>
 The "valid_ext_list" test is true if all of the external list names
 in the ext-list-names argument are supported, and they are valid both
 syntactically (including URI parameters) and semantically (including
 implementation-specific semantic restrictions).  Otherwise, the test
 returns false.
 This test MUST perform exactly the same validation of an external
 list name as would be performed by the "header :list" test.

2.8. Interaction with ManageSieve

 This extension defines the following new capability for ManageSieve
 (see [RFC5804], Section 1.7):
 EXTLISTS - A space-separated list of URI schema parts [RFC3986] for
 supported externally stored list types.  This capability MUST be
 returned if the corresponding Sieve implementation supports the
 "extlists" extension defined in this document.
 This also extends the ManageSieve ABNF as follows:
 single-capability  =/ DQUOTE "EXTLISTS" DQUOTE SP ext-list-types CRLF
         ; single-capability is defined in [RFC5804]
 ext-list-types  = string
         ; space separated list of URI schema parts
         ; for supported externally stored list types.
         ; MUST NOT be empty.

Melnikov & Leiba Standards Track [Page 9] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

2.9. Examples

2.9.1. Example 1

 This example uses a personal address book, along with the Spamtest
 [RFC5235] and Relational [RFC5231] extensions to give a different
 level of spam tolerance to known senders.
     require ["envelope", "extlists", "fileinto", "spamtest",
              "relational", "comparator-i;ascii-numeric"];
     if envelope :list "from" ":addrbook:default"
       { /* Known: allow high spam score */
         if spamtest :value "ge" :comparator "i;ascii-numeric" "8"
           {
             fileinto "spam";
           }
       }
     elsif spamtest :value "ge" :comparator "i;ascii-numeric" "3"
       { /* Unknown: less tolerance in spam score */
         fileinto "spam";
       }
 The same example can also be written another way, if the Variables
 extension [RFC5229] is also supported:
     require ["envelope", "extlists", "fileinto", "spamtest",
         "variables", "relational", "comparator-i;ascii-numeric"];
     if envelope :list "from" ":addrbook:default" {
       set "lim" "8";  /* Known: allow high spam score */
     } else {
       set "lim" "3";  /* Unknown: less tolerance in spam score */
     }
     if spamtest :value "ge" :comparator "i;ascii-numeric" "${lim}" {
       fileinto "spam";
     }

2.9.2. Example 2

 This example uses the "currentdate" test [RFC5260] and a list
 containing the dates of local holidays.  If today is a holiday, the
 script will notify (as described in [RFC5435]) the user via the
 Extensible Messaging and Presence Protocol (XMPP) [RFC5437] about the
 message.

Melnikov & Leiba Standards Track [Page 10] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

     require ["extlists", "date", "enotify"];
     if currentdate :list "date"
        "tag:example.com,2011-01-01:localHolidays" {
        notify "xmpp:romeo@im.example.com";
     }

2.9.3. Example 3

 This example also uses the "envelope" option [RFC5228] and the
 Subaddress extension [RFC5233].  If mail is sent with the list name
 as a subaddress of the recipient (to, say, "alexey+mylist"), and the
 message comes from a member of the list, it will be redirected to all
 members of the list.  Variants of this technique might be useful for
 creating private mailing lists.
     require ["extlists", "envelope", "subaddress"];
     # Submission from list members is sent to all members
     if allof (envelope :detail "to" "mylist",
               header :list "from"
                      "tag:example.com,2010-05-28:mylist") {
         redirect :list "tag:example.com,2010-05-28:mylist";
     }

2.9.4. Example 4

 This example uses variable matching [RFC5229] to extract the IP
 address from the last "Received" header field.  It then checks that
 against a "block list" of undesirable IP addresses, and rejects the
 message if there's a match.
     require ["variables", "extlists", "index", "reject"];
     if header :index 1 :matches "received" "*(* [*.*.*.*])*" {
       set "ip" "${3}.${4}.${5}.${6}";
       if string :list "${ip}"
           "tag:example.com,2011-04-10:DisallowedIPs" {
         reject "Message not allowed from this IP address";
       }
     }

2.9.5. Example 5

 This example uses several features of the MIME parts extension
 [RFC5703] to scan for unsafe attachment types.  To make it easily
 extensible, the unsafe types are kept in an external list, which
 would be shared among all users and all scripts, avoiding the need to
 change scripts when the list changes.

Melnikov & Leiba Standards Track [Page 11] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 [Note that this is an illustrative example, and more rigorous malware
 filtering is advisable.  It is insufficient to base email security on
 checks of filenames alone.]
     require [ "extlists", "foreverypart", "mime", "enclose" ];
     foreverypart
     {
       if header :mime :param "filename"
          :list ["Content-Type", "Content-Disposition"]
            "tag:example.com,2011-04-10:BadFileNameExts"
       {
         # these attachment types are executable
         enclose :subject "Warning" :text
  WARNING! The enclosed message has attachments that might be unsafe.
  These attachment types may contain a computer virus program
  that can infect your computer and potentially damage your data.
  Before clicking on these message attachments, you should verify
  with the sender that this message was sent intentionally, and
  that the attachments are safe to open.
  .
  ;
         break;
       }
     }

3. Security Considerations

 Security considerations related to the "address"/"envelope"/"header"
 tests and "redirect" action discussed in Sieve [RFC5228] also apply
 to this document.
 External list memberships ought to be treated as if they are an
 integral part of the script, so a temporary failure to access an
 external list SHOULD be handled in the same way as a temporary
 failure to retrieve the Sieve script itself.
 For example, if the Sieve script is stored in the Lightweight
 Directory Access Protocol [RFC4510] and the script can't be retrieved
 when a message is processed (perhaps the LDAP server is unavailable),
 then the Sieve engine might delay message delivery until the script
 can be retrieved successfully.  Similarly, if an external list is
 stored in LDAP and that LDAP server is unavailable, the Sieve engine
 would take the same action -- delay message delivery and try again
 later.

Melnikov & Leiba Standards Track [Page 12] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 Protocols/APIs used to retrieve/verify external list membership MUST
 provide an appropriate level of confidentiality and authentication.
 Usually, that will be at least the same level of confidentiality as
 protocols/APIs used to retrieve Sieve scripts, but only the
 implementation (or deployment) will know what is appropriate.
 There's a difference, for example, between making an LDAP request on
 a closed LAN that's only used for trusted servers (it may be that
 neither encryption nor authentication is needed), on a firewalled LAN
 internal to a company (it might be OK to skip encryption, depending
 upon policy), and on the open Internet (encryption and authentication
 are probably both required).  It also matters whether the list being
 accessed is private or public (no encryption or authentication may be
 needed for public data, even on the Internet).
 Having the processing and outcome of a Sieve script depend on the
 contents of external data can allow someone with control of the
 external data to have unusual, and perhaps unauthorized, control of
 the script -- and, consequently, of the disposition of the user's
 email.  A user using such a list for spam control, for example, might
 find important mail being discarded because of tampering with the
 list.  Someone using redirect to an external list could have her
 email redirected to the wrong eyes because of such tampering.
 Security and integrity protection of external lists is as important
 as protection of the Sieve script itself.
 Implementations of this extension should keep in mind that matching
 values against an externally stored list can be I/O and/or CPU
 intensive.  This can be used to deny service to the mail server
 and/or to servers providing access to externally stored mailing
 lists.  A naive implementation, such as the one that tries to
 retrieve content of the whole list to perform matching, can make this
 worse.
 But note that many protocols that can be used for accessing
 externally stored lists support flexible searching features that can
 be used to minimize network traffic and load on the directory
 service.  For example, LDAP allows for search filters.
 Implementations SHOULD use such features whenever they can.
 Many organizations support external lists with thousands of
 recipients.  In order to avoid mailbombs when redirecting a message
 to an externally stored list, implementations SHOULD enforce limits
 on the number of recipients and/or on domains to which such
 recipients belong.
 Note, in particular, that it can be too easy for a script to use
      redirect :list ":addrbook:default";
 to send messages to "everyone in your address book", and one can

Melnikov & Leiba Standards Track [Page 13] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 easily imagine both intentional and accidental abuse.  The situation
 can be even worse for, say, ":addrbook:corporate".  Warnings, as well
 as enforced limits, are appropriate here.
 Applications SHOULD ensure appropriate restrictions are in place to
 protect sensitive information that might be revealed by "addrbook"
 URNs from access or modification by untrusted sources.

4. IANA Considerations

4.1. Registration of Sieve Extension

 The following template specifies the IANA registration of the Sieve
 extension specified in this document.  This information has been
 added to the Sieve Extensions registry on http://www.iana.org.
 To: iana@iana.org
 Subject:  Registration of new Sieve extension
 Capability name:  extlists
 Description:  Adds the ":list" match type to certain Sieve tests, and
     the ":list" argument to the "redirect" action.  The ":list" match
     type changes tests to match values against values stored in one
     or more externally stored lists.  The ":list" argument to the
     redirect action changes the redirect action to forward the
     message to email addresses stored in the externally stored list.
 RFC number:  RFC 6134
 Contact address:  Sieve mailing list <sieve@ietf.org>

4.2. Registration of ManageSieve Capability

 IANA has registered a new ManageSieve Capability according to the
 IANA registration template specified in [RFC5804]:
 To: iana@iana.org
 Subject:  ManageSieve Capability Registration
 Capability name:  extlists
 Description:  This capability is returned if the server supports the
     "extlists" RFC 6134 Sieve extension.
 Relevant publications:  RFC 6134, Section 2.8

Melnikov & Leiba Standards Track [Page 14] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 Person & email address to contact for further information:
     Sieve mailing list <sieve@ietf.org>
 Author/Change controller:  IESG

4.3. Creation of Sieve URN Parameters Registry

 IANA has created a new registry under "Sieve Extensions" for Sieve
 URN Parameters.  Registration into this registry is according to the
 "Specification Required" policy [RFC5226].
 The registry contains the following two items:
 URN parameter name:  The name of the URN parameter.  If the name is
     "paramname", the resulting top-level URN will be
     "urn:ietf:params:sieve:paramname".
 Reference:  The document and section where the definition of the
     parameter can be found.  Be sure to include the section number as
     well as the document reference, so the documentation is easy to
     find.
 The documentation -- which will be in the referenced document and
 section, and will not be included in the registry -- MUST include the
 following information (see Section 2.6 for an example):
    URN parameter name:  The name of the URN parameter.
    URN parameter syntax:  The syntax of the parameter and any sub-
        parameters, which SHOULD be specified using ABNF [RFC5234].
    Intended usage:  A detailed description of how the parameter and
        any sub-parameters are expected to be used.  This is the place
        to define static sub-parameters, registries for sub-
        parameters, options, registries for options, and so on.
    Interoperability considerations:  Any notes specific to
        interoperability issues.  This is where to put mandatory-to-
        implement sub-parameters and the like.
    Security considerations:  Any notes specific to security and
        privacy issues.
    Contact:  Contact information, in case there are questions.

Melnikov & Leiba Standards Track [Page 15] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

4.4. Registration of the "addrbook" URN parameter

 IANA has registered a new Sieve URN parameter in the registry defined
 in Section 4.3.
 URN parameter name:  addrbook
 Reference:  RFC 6134, Section 2.6

4.5. Registration of "sieve" URN sub-namespace

 IANA has registered a new URN sub-namespace within the IETF URN Sub-
 namespace for Registered Protocol Parameter Identifiers defined in
 [RFC3553].
 Registry name:  sieve
 Specification:  RFC 6134
 Repository:  Sieve URN Parameters registry (Section 4.3)
 Index value:  Sub-parameters MUST be specified in UTF-8, using
     standard URI encoding where necessary.

5. Acknowledgements

 Thanks to Alexandros Vellis, Nigel Swinson, Ned Freed, Kjetil Torgrim
 Homme, Dave Cridland, Cyrus Daboo, Pete Resnick, and Robert Burrell
 Donkin for ideas, comments, and suggestions.  Kristin Hubner also
 helped greatly with the examples.

6. References

6.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, January 2005.
 [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
            RFC 4151, October 2005.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.

Melnikov & Leiba Standards Track [Page 16] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
            Language", RFC 5228, January 2008.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5804]  Melnikov, A. and T. Martin, "A Protocol for Remotely
            Managing Sieve Scripts", RFC 5804, July 2010.

6.2. Informative References

 [CardDAV]  Daboo, C., "vCard Extensions to WebDAV (CardDAV)", Work
            in Progress, November 2009.
 [RFC2244]  Newman, C. and J. Myers, "ACAP -- Application
            Configuration Access Protocol", RFC 2244, November 1997.
 [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
            IETF URN Sub-namespace for Registered Protocol
            Parameters", BCP 73, RFC 3553, June 2003.
 [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
            (LDAP): Technical Specification Road Map", RFC 4510,
            June 2006.
 [RFC5229]  Homme, K., "Sieve Email Filtering: Variables Extension",
            RFC 5229, January 2008.
 [RFC5231]  Segmuller, W. and B. Leiba, "Sieve Email Filtering:
            Relational Extension", RFC 5231, January 2008.
 [RFC5233]  Murchison, K., "Sieve Email Filtering: Subaddress
            Extension", RFC 5233, January 2008.
 [RFC5235]  Daboo, C., "Sieve Email Filtering: Spamtest and Virustest
            Extensions", RFC 5235, January 2008.
 [RFC5260]  Freed, N., "Sieve Email Filtering: Date and Index
            Extensions", RFC 5260, July 2008.
 [RFC5435]  Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,
            "Sieve Email Filtering: Extension for Notifications",
            RFC 5435, January 2009.
 [RFC5437]  Saint-Andre, P. and A. Melnikov, "Sieve Notification
            Mechanism: Extensible Messaging and Presence Protocol
            (XMPP)", RFC 5437, January 2009.

Melnikov & Leiba Standards Track [Page 17] RFC 6134 Sieve Extension: Externally Stored Lists July 2011

 [RFC5463]  Freed, N., "Sieve Email Filtering: Ihave Extension",
            RFC 5463, March 2009.
 [RFC5703]  Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
            Tests, Iteration, Extraction, Replacement, and Enclosure",
            RFC 5703, October 2009.

Authors' Addresses

 Alexey Melnikov
 Isode Limited
 5 Castle Business Village
 36 Station Road
 Hampton, Middlesex  TW12 2BX
 UK
 EMail: Alexey.Melnikov@isode.com
 Barry Leiba
 Huawei Technologies
 Phone: +1 646 827 0648
 EMail: barryleiba@computer.org
 URI:   http://internetmessagingtechnology.org/

Melnikov & Leiba Standards Track [Page 18]

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