Sieve Working Group                                          A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Standards Track                                B. Leiba
Expires: November 19, December 11, 2011                           Huawei Technologies
                                                            May 18,
                                                            June 9, 2011

                Sieve Extension: Externally Stored Lists


   The Sieve scripting 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 hardcoded 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 LDAP (RFC 4510), ACAP (RFC 2244), CardDAV (work
   in progress), or relational databases.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 19, December 11, 2011.

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   document authors.  All rights reserved.

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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 . . . . . .  4
   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 . . . . . . . . . . . . . . . . . . . . . . . . .  9
   2.9.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . .  9
   2.9.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 10
   2.9.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . . . 10
   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 . . . . . . . 15 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

           Authors' Addresses . . . . . . . . . . . . . . . . . . . . 18

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
   [I-D.ietf-vcarddav-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",
   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


      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
   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"

   Implementations MAY support other tests but than the ones in this
   document.  Implementations MUST raise report an error
   (which SHOULD be a compile-time error, but MAY be a runtime error) when a script uses
   ":list" with a test for which it is that does not supported. 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, statement or using ihave

   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.

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

   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*",
   "*"), 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 the useful cases
   and situations at the time of this writing.  Other uses for external
   lists, using 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 [I-D.ietf-vcarddav-carddav], or Tag [RFC4151] to be useful
   for naming external lists.

   The "tag" URI scheme [RFC4151] can be used to represent opaque, but
   user friendlier identifiers.  Resolution of such identifiers is going
   to be implementation specific and it can help in hiding the
   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:,<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


   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:





   Address book names other than "default" MAY be case-sensitive,
   depending upon the implementation, so their case (after URI decoding)
   MUST be maintained.

   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

       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

       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:


       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

       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

       *   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 user's company's address book.

       *   department -- a subset of
           urn:ietf:params:sieve:addrbook:company, defined by the

       *   co-workers -- a subset of
           urn:ietf:params:sieve:addrbook:company, defined by the user.

       *   default -- the default address book, a reference to

   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
       implementation, so their case (after URI decoding) MUST be

   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 <>

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.

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 [RFC5435] the user via XMPP [RFC5437] about the

       require ["extlists", "date", "enotify"];
       if currentdate :list "date"
          ",2011-01-01:localHolidays" {
          notify "";

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"
                        ",2010-05-28:mylist") {
           redirect :list ",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}"
             ",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.

   [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" ];

         if header :mime :param "filename"
            :list ["Content-Type", "Content-Disposition"]
           # these attachment types are executable
           enclose :subject "Warning" :text
    WARNING! The enclosed message 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.

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

   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 IO and/or CPU
   intensive.  This can be used to deny service to the mailserver 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
   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 should be
   added to the list of sieve extensions given on


   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:  [[this RFC]]

   Contact address:  Sieve mailing list <>

4.2.  Registration of ManageSieve Capability

   The following requests IANA to register a new ManageSieve Capability
   according to the IANA registration template specified in [RFC5804]:


   Subject:  ManageSieve Capability Registration

   Capability name:  extlists

   Description:  This capability is returned if the server supports the
       "extlists" [[this RFC]] Sieve extension.

   Relevant publications:  [[this RFC]], Section 2.8
   Person & email address to contact for further information:  Sieve
       mailing list <>

   Author/Change controller:  IESG

4.3.  Creation of Sieve URN Parameters registry

   The following requests IANA to create a new registry under "Sieve
   Extensions" for Sieve URN Parameters.  Registration into this
   registry is according to the "Specification Required" policy

   The registry will contain 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

   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

   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.

4.4.  Registration of the "addrbook" URN parameter

   The following requests IANA to register a new Sieve URN parameter in
   the registry defined in Section 4.3.

   URN parameter name:  addrbook

   Reference:  [[this RFC]], Section 2.6

4.5.  Registration of "sieve" URN sub-namespace

   The following requests IANA to register a new URN sub-namespace
   within the IETF URN Sub-namespace for Registered Protocol Parameter
   Identifiers defined in [RFC3553].

   Registry name:  sieve

   Specification:  [[this RFC]]

   Repository:  [[the registry created in 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.

   [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

              Daboo, C., "vCard Extensions to WebDAV (CardDAV)",
              draft-ietf-vcarddav-carddav-10 (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.

   [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


   Barry Leiba
   Huawei Technologies

   Phone: +1 646 827 0648