IMAP ABNF extensions: Collected Extensions to IMAP4 ABNF

Network Working Group                                        A. Melnikov
Request for Comments: 4466                                    Isode Ltd.
Updates: 2088, 2342, 3501, 3502, 3516                           C. Daboo
Category: Standards Track                                     April 2006


                   Collected Extensions to IMAP4 ABNF

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 (2006).

Abstract

   Over the years, many documents from IMAPEXT and LEMONADE working
   groups, as well as many individual documents, have added syntactic
   extensions to many base IMAP commands described in RFC 3501.  For
   ease of reference, this document collects most of such ABNF changes
   in one place.

   This document also suggests a set of standard patterns for adding
   options and extensions to several existing IMAP commands defined in
   RFC 3501.  The patterns provide for compatibility between existing
   and future extensions.

   This document updates ABNF in RFCs 2088, 2342, 3501, 3502, and 3516.
   It also includes part of the errata to RFC 3501.  This document
   doesn't specify any semantic changes to the listed RFCs.

Table of Contents

   1. Introduction ....................................................2
      1.1. Purpose of This Document ...................................2
      1.2. Conventions Used in This Document ..........................3
   2. IMAP ABNF Extensions ............................................3
      2.1. Optional Parameters with the SELECT/EXAMINE Commands .......3
      2.2. Extended CREATE Command ....................................4
      2.3. Extended RENAME Command ....................................5
      2.4. Extensions to FETCH and UID FETCH Commands .................6
      2.5. Extensions to STORE and UID STORE Commands .................6
      2.6. Extensions to SEARCH Command ...............................7
           2.6.1. Extended SEARCH Command .............................7
           2.6.2. ESEARCH untagged response ...........................8
      2.7. Extensions to APPEND Command ...............................8
   3. Formal Syntax ...................................................9
   4. Security Considerations ........................................14
   5. Normative References ...........................................15
   6. Acknowledgements ...............................................15

1.  Introduction

1.1.  Purpose of This Document

   This document serves several purposes:

      1.  rationalize and generalize ABNF for some existing IMAP
          extensions;
      2.  collect the ABNF in one place in order to minimize cross
          references between documents;
      3.  define building blocks for future extensions so that they can
          be used together in a compatible way.

   It is expected that a future revision of this document will be
   incorporated into a revision of RFC 3501.

   This document updates ABNF in RFCs 2088, 2342, 3501, 3502, and 3516.
   It also includes part of the errata to RFC 3501.  This document
   doesn't specify any semantic changes to the listed RFCs.

   The ABNF in section 6 of RFC 2342 got rewritten to conform to the
   ABNF syntax as defined in RFC 4234 and to reference new non-terminals
   from RFC 3501.  It was also restructured to allow for better
   readability.  There were no changes "on the wire".

   Section 2 extends ABNF for SELECT, EXAMINE, CREATE, RENAME, FETCH/UID
   FETCH, STORE/UID STORE, SEARCH, and APPEND commands in a consistent
   manner.  Extensions to all the commands but APPEND have the same
   structure.  Extensibility for the APPEND command was done slightly
   differently in order to preserve backward compatibility with existing
   extensions.

   Section 2 also defines a new ESEARCH response, whose purpose is to
   define a better version of the SEARCH response defined in RFC 3501.

   Section 3 defines the collected ABNF that replaces pieces of ABNF in
   the aforementioned RFCs.  The collected ABNF got generalized to allow
   for easier future extensibility.

1.2.  Conventions Used in This Document

   In examples, "C:" and "S:" indicate lines sent by the client and
   server, respectively.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [KEYWORDS].

2.  IMAP ABNF Extensions

   This section is not normative.  It provides some background on the
   intended use of different extensions and it gives some guidance about
   how future extensions should extend the described commands.

2.1.  Optional Parameters with the SELECT/EXAMINE Commands

   This document adds the ability to include one or more parameters with
   the IMAP SELECT (section 6.3.1 of [IMAP4]) or EXAMINE (section 6.3.2
   of [IMAP4]) commands, to turn on or off certain standard behaviors,
   or to add new optional behaviors required for a particular extension.

   There are two possible modes of operation:

   o  A global state change where a single use of the optional parameter
      will affect the session state from that time on, irrespective of
      subsequent SELECT/EXAMINE commands.

   o  A per-mailbox state change that will affect the session only for
      the duration of the new selected state.  A subsequent
      SELECT/EXAMINE without the optional parameter will cancel its
      effect for the newly selected mailbox.

   Optional parameters to the SELECT or EXAMINE commands are added as a
   parenthesized list of attribute/value pairs, and appear after the
   mailbox name in the standard SELECT or EXAMINE command.  The order of
   individual parameters is arbitrary.  A parameter value is optional
   and may consist of atoms, strings, or lists in a specific order.  If
   the parameter value is present, it always appears in parentheses (*).
   Any parameter not defined by extensions that the server supports must
   be rejected with a BAD response.

      Example:

              C: a SELECT INBOX (ANNOTATE)
              S: ...
              S: a OK SELECT complete

      In the above example, a single parameter is used with the SELECT
      command.

      Example:

              C: a EXAMINE INBOX (ANNOTATE RESPONSES ("UID Responses")
                 CONDSTORE)
              S: ...
              S: a OK EXAMINE complete

      In the above example, three parameters are used with the EXAMINE
      command.  The second parameter consists of two items: an atom
      "RESPONSES" followed by a quoted string.

      Example:

              C: a SELECT INBOX (BLURDYBLOOP)
              S: a BAD Unknown parameter in SELECT command

      In the above example, a parameter not supported by the server is
      used.  This results in the BAD response from the server.

   (*) - if a parameter has a mandatory value, which can always be
   represented as a number or a sequence-set, the parameter value does
   not need the enclosing ().  See ABNF for more details.

2.2.  Extended CREATE Command

   Arguments:  mailbox name
               OPTIONAL list of CREATE parameters

   Responses:  no specific responses for this command

   Result:     OK - create completed
               NO - create failure: cannot create mailbox with
                    that name
               BAD - argument(s) invalid

   This document adds the ability to include one or more parameters with
   the IMAP CREATE command (see section 6.3.3 of [IMAP4]), to turn on or
   off certain standard behaviors, or to add new optional behaviors
   required for a particular extension.  No CREATE parameters are
   defined in this document.

   Optional parameters to the CREATE command are added as a
   parenthesized list of attribute/value pairs after the mailbox name.
   The order of individual parameters is arbitrary.  A parameter value
   is optional and may consist of atoms, strings, or lists in a specific
   order.  If the parameter value is present, it always appears in
   parentheses (*).  Any parameter not defined by extensions that the
   server supports must be rejected with a BAD response.

   (*) - if a parameter has a mandatory value, which can always be
   represented as a number or a sequence-set, the parameter value does
   not need the enclosing ().  See ABNF for more details.

2.3.  Extended RENAME Command

   Arguments:  existing mailbox name
               new mailbox name
               OPTIONAL list of RENAME parameters

   Responses:  no specific responses for this command

   Result:     OK - rename completed
               NO - rename failure: cannot rename mailbox with
                    that name, cannot rename to mailbox with
                    that name, etc.
               BAD - argument(s) invalid

   This document adds the ability to include one or more parameters with
   the IMAP RENAME command (see section 6.3.5 of [IMAP4]), to turn on or
   off certain standard behaviors, or to add new optional behaviors
   required for a particular extension.  No RENAME parameters are
   defined in this document.

   Optional parameters to the RENAME command are added as a
   parenthesized list of attribute/value pairs after the new mailbox
   name.  The order of individual parameters is arbitrary.  A parameter
   value is optional and may consist of atoms, strings, or lists in a
   specific order.  If the parameter value is present, it always appears
   in parentheses (*).  Any parameter not defined by extensions that the
   server supports must be rejected with a BAD response.

   (*) - if a parameter has a mandatory value, which can always be
   represented as a number or a sequence-set, the parameter value does
   not need the enclosing ().  See ABNF for more details.

2.4.  Extensions to FETCH and UID FETCH Commands

   Arguments:  sequence set
               message data item names or macro
               OPTIONAL fetch modifiers

   Responses:  untagged responses: FETCH

   Result:     OK - fetch completed
               NO - fetch error: cannot fetch that data
               BAD - command unknown or arguments invalid

   This document extends the syntax of the FETCH and UID FETCH commands
   (see section 6.4.5 of [IMAP4]) to include optional FETCH modifiers.
   No fetch modifiers are defined in this document.

   The order of individual modifiers is arbitrary.  Each modifier is an
   attribute/value pair.  A modifier value is optional and may consist
   of atoms and/or strings and/or lists in a specific order.  If the
   modifier value is present, it always appears in parentheses (*).  Any
   modifiers not defined by extensions that the server supports must be
   rejected with a BAD response.

   (*) - if a modifier has a mandatory value, which can always be
   represented as a number or a sequence-set, the modifier value does
   not need the enclosing ().  See ABNF for more details.

2.5.  Extensions to STORE and UID STORE Commands

   Arguments:  message set
               OPTIONAL store modifiers
               message data item name
               value for message data item

   Responses:  untagged responses: FETCH

   Result:     OK - store completed
               NO - store error: cannot store that data
               BAD - command unknown or arguments invalid

   This document extends the syntax of the STORE and UID STORE commands
   (see section 6.4.6 of [IMAP4]) to include optional STORE modifiers.
   No store modifiers are defined in this document.

   The order of individual modifiers is arbitrary.  Each modifier is an
   attribute/value pair.  A modifier value is optional and may consist
   of atoms and/or strings and/or lists in a specific order.  If the
   modifier value is present, it always appears in parentheses (*).  Any
   modifiers not defined by extensions that the server supports must be
   rejected with a BAD response.

   (*) - if a modifier has a mandatory value, which can always be
   represented as a number or a sequence-set, the modifier value does
   not need the enclosing ().  See ABNF for more details.

2.6.  Extensions to SEARCH Command

2.6.1.  Extended SEARCH Command

   Arguments:  OPTIONAL result specifier
               OPTIONAL [CHARSET] specification
               searching criteria (one or more)

   Responses:  REQUIRED untagged response: SEARCH (*)

   Result:     OK - search completed
               NO - search error: cannot search that [CHARSET] or
                    criteria
               BAD - command unknown or arguments invalid

   This section updates definition of the SEARCH command described in
   section 6.4.4 of [IMAP4].

   The SEARCH command is extended to allow for result options.  This
   document does not define any result options.

   The order of individual options is arbitrary.  Individual options may
   contain parameters enclosed in parentheses (**).  If an option has
   parameters, they consist of atoms and/or strings and/or lists in a
   specific order.  Any options not defined by extensions that the
   server supports must be rejected with a BAD response.

   (*) - An extension to the SEARCH command may require another untagged
   response, or no untagged response to be returned.  Section 2.6.2
   defines a new ESEARCH untagged response that replaces the SEARCH
   untagged response.  Note that for a given extended SEARCH command the
   SEARCH and ESEARCH responses SHOULD be mutually exclusive, i.e., only
   one of them should be returned.

   (**) - if an option has a mandatory parameter, which can always be
   represented as a number or a sequence-set, the option parameter does
   not need the enclosing ().  See ABNF for more details.

2.6.2.  ESEARCH untagged response

   Contents:   one or more search-return-data pairs

   The ESEARCH response SHOULD be sent as a result of an extended SEARCH
   or UID SEARCH command specified in section 2.6.1.

   The ESEARCH response starts with an optional search correlator.  If
   it is missing, then the response was not caused by a particular IMAP
   command, whereas if it is present, it contains the tag of the command
   that caused the response to be returned.

   The search correlator is followed by an optional UID indicator.  If
   this indicator is present, all data in the ESEARCH response refers to
   UIDs, otherwise all returned data refers to message numbers.

   The rest of the ESEARCH response contains one or more search data
   pairs.  Each pair starts with unique return item name, followed by a
   space and the corresponding data.  Search data pairs may be returned
   in any order.  Unless specified otherwise by an extension, any return
   item name SHOULD appear only once in an ESEARCH response.

   Example:    S: * ESEARCH UID COUNT 5 ALL 4:19,21,28

   Example:    S: * ESEARCH (TAG "a567") UID COUNT 5 ALL 4:19,21,28

   Example:    S: * ESEARCH COUNT 5 ALL 1:17,21

2.7.  Extensions to APPEND Command

   The IMAP BINARY extension [BINARY] extends the APPEND command to
   allow a client to append data containing NULs by using the <literal8>
   syntax.  The ABNF was rewritten to allow for easier extensibility by
   IMAP extensions.  This document hasn't specified any semantical
   changes to the [BINARY] extension.

   In addition, the non-terminal "literal8" defined in [BINARY] got
   extended to allow for non-synchronizing literals if both [BINARY] and
   [LITERAL+] extensions are supported by the server.

   The IMAP MULTIAPPEND extension [MULTIAPPEND] extends the APPEND
   command to allow a client to append multiple messages atomically.
   This document defines a common syntax for the APPEND command that
   takes into consideration syntactic extensions defined by both
   [BINARY] and [MULTIAPPEND] extensions.

3.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [ABNF].

   Non-terminals referenced but not defined below are as defined by
   [IMAP4].

   Except as noted otherwise, all alphabetic characters are case-
   insensitive.  The use of uppercase or lowercase characters to define
   token strings is for editorial clarity only.  Implementations MUST
   accept these strings in a case-insensitive fashion.

   append          = "APPEND" SP mailbox 1*append-message
                     ;; only a single append-message may appear
                     ;; if MULTIAPPEND [MULTIAPPEND] capability
                     ;; is not present

   append-message  = append-opts SP append-data

   append-ext      = append-ext-name SP append-ext-value
                     ;; This non-terminal define extensions to
                     ;; to message metadata.

   append-ext-name = tagged-ext-label

   append-ext-value= tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.


   append-data     = literal / literal8 / append-data-ext

   append-data-ext = tagged-ext
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions,
                     ;; i.e., a mandatory label followed
                     ;; by parameters.

   append-opts     = [SP flag-list] [SP date-time] *(SP append-ext)
                     ;; message metadata

   charset         = atom / quoted
                     ;; Exact syntax is defined in [CHARSET].

   create          = "CREATE" SP mailbox
                     [create-params]
                     ;; Use of INBOX gives a NO error.

   create-params   = SP "(" create-param *( SP create-param) ")"

   create-param-name = tagged-ext-label

   create-param      = create-param-name [SP create-param-value]

   create-param-value= tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.


   esearch-response  = "ESEARCH" [search-correlator] [SP "UID"]
                        *(SP search-return-data)
                      ;; Note that SEARCH and ESEARCH responses
                      ;; SHOULD be mutually exclusive,
                      ;; i.e., only one of the response types
                      ;; should be
                      ;; returned as a result of a command.


   examine         = "EXAMINE" SP mailbox [select-params]
                     ;; modifies the original IMAP EXAMINE command
                     ;; to accept optional parameters

   fetch           = "FETCH" SP sequence-set SP ("ALL" / "FULL" /
                     "FAST" / fetch-att /
                     "(" fetch-att *(SP fetch-att) ")")
                     [fetch-modifiers]
                     ;; modifies the original IMAP4 FETCH command to
                     ;; accept optional modifiers

   fetch-modifiers = SP "(" fetch-modifier *(SP fetch-modifier) ")"

   fetch-modifier  = fetch-modifier-name [ SP fetch-modif-params ]

   fetch-modif-params  = tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.

   fetch-modifier-name = tagged-ext-label

   literal8        = "~{" number ["+"] "}" CRLF *OCTET
                      ;; A string that might contain NULs.
                      ;; <number> represents the number of OCTETs
                      ;; in the response string.
                      ;; The "+" is only allowed when both LITERAL+ and
                      ;; BINARY extensions are supported by the server.

   mailbox-data      =/ Namespace-Response /
                        esearch-response

   Namespace         = nil / "(" 1*Namespace-Descr ")"

   Namespace-Command = "NAMESPACE"

   Namespace-Descr   = "(" string SP
                          (DQUOTE QUOTED-CHAR DQUOTE / nil)
                           *(Namespace-Response-Extension) ")"

   Namespace-Response-Extension = SP string SP
                     "(" string *(SP string) ")"

   Namespace-Response = "NAMESPACE" SP Namespace
                        SP Namespace SP Namespace
         ;; This response is currently only allowed
         ;; if the IMAP server supports [NAMESPACE].
         ;; The first Namespace is the Personal Namespace(s)
         ;; The second Namespace is the Other Users' Namespace(s)
         ;; The third Namespace is the Shared Namespace(s)

   rename          = "RENAME" SP mailbox SP mailbox
                     [rename-params]
                     ;; Use of INBOX as a destination gives
                     ;; a NO error, unless rename-params
                     ;; is not empty.

   rename-params     = SP "(" rename-param *( SP rename-param) ")"

   rename-param      = rename-param-name [SP rename-param-value]

   rename-param-name = tagged-ext-label

   rename-param-value= tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.


   response-data   = "*" SP response-payload CRLF

   response-payload= resp-cond-state / resp-cond-bye /
                     mailbox-data / message-data / capability-data

   search          = "SEARCH" [search-return-opts]
                     SP search-program

   search-correlator  = SP "(" "TAG" SP tag-string ")"
   search-program     = ["CHARSET" SP charset SP]
                        search-key *(SP search-key)
                        ;; CHARSET argument to SEARCH MUST be
                        ;; registered with IANA.

   search-return-data = search-modifier-name SP search-return-value
                        ;; Note that not every SEARCH return option
                        ;; is required to have the corresponding
                        ;; ESEARCH return data.

   search-return-opts = SP "RETURN" SP "(" [search-return-opt
                        *(SP search-return-opt)] ")"

   search-return-opt = search-modifier-name [SP search-mod-params]

   search-return-value = tagged-ext-val
                        ;; Data for the returned search option.
                        ;; A single "nz-number"/"number" value
                        ;; can be returned as an atom (i.e., without
                        ;; quoting).  A sequence-set can be returned
                        ;; as an atom as well.

   search-modifier-name = tagged-ext-label

   search-mod-params = tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.


   select          = "SELECT" SP mailbox [select-params]
                     ;; modifies the original IMAP SELECT command to
                     ;; accept optional parameters

   select-params   = SP "(" select-param *(SP select-param) ")"

   select-param    = select-param-name [SP select-param-value]
                     ;; a parameter to SELECT may contain one or
                     ;; more atoms and/or strings and/or lists.

   select-param-name= tagged-ext-label

   select-param-value= tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.


   status-att-list = status-att-val *(SP status-att-val)
                     ;; Redefines status-att-list from RFC 3501.

                     ;; status-att-val is defined in RFC 3501 errata

   status-att-val  = ("MESSAGES" SP number) /
                     ("RECENT" SP number) /
                     ("UIDNEXT" SP nz-number) /
                     ("UIDVALIDITY" SP nz-number) /
                     ("UNSEEN" SP number)
                     ;; Extensions to the STATUS responses
                     ;; should extend this production.
                     ;; Extensions should use the generic
                     ;; syntax defined by tagged-ext.

   store           = "STORE" SP sequence-set [store-modifiers]
                     SP store-att-flags
                     ;; extend [IMAP4] STORE command syntax
                     ;; to allow for optional store-modifiers

   store-modifiers =  SP "(" store-modifier *(SP store-modifier)
                       ")"

   store-modifier  = store-modifier-name [SP store-modif-params]

   store-modif-params = tagged-ext-val
                     ;; This non-terminal shows recommended syntax
                     ;; for future extensions.

   store-modifier-name = tagged-ext-label

   tag-string         = string
                        ;; tag of the command that caused
                        ;; the ESEARCH response, sent as
                        ;; a string.

   tagged-ext          = tagged-ext-label SP tagged-ext-val
                          ;; recommended overarching syntax for
                          ;; extensions

   tagged-ext-label    = tagged-label-fchar *tagged-label-char
                         ;; Is a valid RFC 3501 "atom".

   tagged-label-fchar  = ALPHA / "-" / "_" / "."

   tagged-label-char   = tagged-label-fchar / DIGIT / ":"

   tagged-ext-comp     = astring /
                         tagged-ext-comp *(SP tagged-ext-comp) /
                         "(" tagged-ext-comp ")"
                          ;; Extensions that follow this general
                          ;; syntax should use nstring instead of
                          ;; astring when appropriate in the context
                          ;; of the extension.
                          ;; Note that a message set or a "number"
                          ;; can always be represented as an "atom".
                          ;; An URL should be represented as
                          ;; a "quoted" string.

   tagged-ext-simple   = sequence-set / number

   tagged-ext-val      = tagged-ext-simple /
                         "(" [tagged-ext-comp] ")"

4.  Security Considerations

   This document updates ABNF in RFCs 2088, 2342, 3501, 3502, and 3516.
   The updated documents must be consulted for security considerations
   for the extensions that they define.

   As a protocol gets more complex, parser bugs become more common
   including buffer overflow, denial of service, and other common
   security coding errors.  To the extent that this document makes the
   parser more complex, it makes this situation worse.  To the extent
   that this document makes the parser more consistent and thus simpler,
   the situation is improved.  The impact will depend on how many
   deployed IMAP extensions are consistent with this document.
   Implementers are encouraged to take care of these issues when
   extending existing implementations.  Future IMAP extensions should
   strive for consistency and simplicity to the greatest extent
   possible.

   Extensions to IMAP commands that are permitted in NOT AUTHENTICATED
   state are more sensitive to these security issues due to the larger
   possible attacker community prior to authentication, and the fact
   that some IMAP servers run with elevated privileges in that state.
   This document does not extend any commands permitted in NOT
   AUTHENTICATED state.  Future IMAP extensions to commands permitted in
   NOT AUTHENTICATED state should favor simplicity over consistency or
   extensibility.

5.  Normative References

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

   [IMAP4]       Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
                 VERSION 4rev1", RFC 3501, March 2003.

   [ABNF]        Crocker, D., Ed., and P. Overell, "Augmented BNF for
                 Syntax Specifications: ABNF", RFC 4234, October 2005.

   [CHARSET]     Freed, N. and J. Postel, "IANA Charset Registration
                 Procedures", BCP 19, RFC 2978, October 2000.

   [MULTIAPPEND] Crispin, M., "Internet Message Access Protocol (IMAP) -
                 MULTIAPPEND Extension", RFC 3502, March 2003.

   [NAMESPACE]   Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342,
                 May 1998.

   [LITERAL+]    Myers, J., "IMAP4 non-synchronizing literals", RFC
                 2088, January 1997.

   [BINARY]      Nerenberg, L., "IMAP4 Binary Content Extension", RFC
                 3516, April 2003.

6.  Acknowledgements

   This documents is based on ideas proposed by Pete Resnick, Mark
   Crispin, Ken Murchison, Philip Guenther, Randall Gellens, and Lyndon
   Nerenberg.

   However, all errors and omissions must be attributed to the authors
   of the document.

   Thanks to Philip Guenther, Dave Cridland, Mark Crispin, Chris Newman,
   Elwyn Davies, and Barry Leiba for comments and corrections.

   literal8 syntax was taken from RFC 3516.

Authors' Addresses

   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex, TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com


   Cyrus Daboo

   EMail: cyrus@daboo.name

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