EXTRA                                                   B. Gondwana, Ed.
Internet-Draft                                                  FastMail
Updates: 3501 (if approved)                                 May 28,                                July 17, 2018
Intended status: Standards Track
Expires: November 29, 2018 January 18, 2019

                 IMAP Extension for object identifiers
                   draft-ietf-extra-imap-objectid-03
                   draft-ietf-extra-imap-objectid-04

Abstract

   This document updates RFC3501 (IMAP4rev1) with persistent identifiers
   on mailboxes and messages to allow clients to more efficiently re-use
   cached data when resources have changed location on the server.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 29, 2018. January 18, 2019.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used In This Document . . . . . . . . . . . . . .   3
   3.  CAPABILITY Identification . . . . . . . . . . . . . . . . . .   3
   4.  MAILBOXID object identifier . . . . . . . . . . . . . . . . .   3
     4.1.  New response code for CREATE  . . . . . . . . . . . . . .   4
     4.2.  New OK Untagged Response for SELECT and EXAMINE . . . . .   4
     4.3.  New attribute for STATUS  . . . . . . . . . . . . . . . .   5
   5.  EMAILID object identifier and THREADID correlator . . . . . .   6
     5.1.  EMAILID identifier for identical messages . . . . . . . .   6
     5.2.  THREADID identifer for related messages . . . . . . . . .   6
     5.3.  New Message Data Items in FETCH and UID FETCH Commands  .   7
   6.  New Filters on SEARCH command . . . . . . . . . . . . . . . .   9
   7.  Formal syntax . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Implementation considerations . . . . . . . . . . . . . . . .  10
     8.1.  Assigning object identifiers  . . . . . . . . . . . . . .  10
     8.2.  Interaction with special cases  . . . . . . . . . . . . .  11
     8.3.  Client usage  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Future considerations . . . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   12. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     12.1.  draft-ietf-extra-imap-objectid-03  draft-ietf-extra-imap-objectid-04  . . . . . . . . . . .  12
     12.2.  draft-ietf-extra-imap-objectid-02  draft-ietf-extra-imap-objectid-03  . . . . . . . . . . .  12  13
     12.3.  draft-ietf-extra-imap-objectid-01  draft-ietf-extra-imap-objectid-02  . . . . . . . . . . .  12  13
     12.4.  draft-ietf-extra-imap-objectid-00  draft-ietf-extra-imap-objectid-01  . . . . . . . . . . .  13
     12.5.  draft-ietf-extra-imap-uniqueid-00  draft-ietf-extra-imap-objectid-00  . . . . . . . . . . .  13  14
     12.6.  draft-ietf-extra-imap-uniqueid-00  . . . . . . . . . . .  14
     12.7.  draft-gondwana-imap-uniqueid-01  . . . . . . . . . . . .  13
     12.7.  14
     12.8.  draft-gondwana-imap-uniqueid-00  . . . . . . . . . . . .  14
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
     13.1.  Appendix 1: ideas for implementing object identifiers  .  14  15
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     14.2.  Informative References . . . . . . . . . . . . . . . . .  15  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   IMAP stores are often used by many clients.  Each client may cache
   data from the server so that they don't need to re-download
   information.  [RFC3501] defines that a mailbox can be uniquely
   referenced by its name and UIDVALIDITY, and a message within that
   mailbox can be uniquely referenced by its mailbox (name +
   UIDVALIDITY) and UID.  The triple of mailbox name, UIDVALIDITY and
   UID is guaranteed to be immutable.

   [RFC4315] defines a COPYUID response which allows a client which
   copies messages to know the mapping between the UIDs in the source
   and destination mailboxes, and hence update its local cache.

   If a mailbox is successfully renamed, the renamed by a client, that client knows will
   know that the same messages exist in the destination mailbox name as
   previously existed in the source mailbox name.

   The result is that the client which copies (or [RFC6851] moves)
   messages or renames a mailbox can update its local cache, but any
   other client connected to the same store can not know with certainty
   that the messages are identical, and so will re-download everything.

   This extension adds new properties to a message (EMAILID) and mailbox
   (MAILBOXID) which allow a client to quickly identify messages or
   mailboxes which have been renamed by another client.

   This extension also adds an optional thread identifier (THREADID) to
   messages, which can be used by the server to indicate messages which
   it has identified to be related.  A server that does not implement
   threading will return NIL to all requests for THREADID.

2.  Conventions Used In This Document

   In examples, "C:" indicates lines sent by a client that is connected
   to a server.  "S:" indicates lines sent by the server to the client.

   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] when they
   appear in ALL CAPS.  These words may also appear in this document in
   lower case as plain English words, absent their normative meanings.

3.  CAPABILITY Identification

   IMAP servers that support this extension MUST include "OBJECTID" in
   the response list to the CAPABILITY command.

4.  MAILBOXID object identifier

   The MAILBOXID is a server-allocated unique identifer for each
   mailbox.

   The server SHOULD MUST return the same MAILBOXID for a server mailbox with the same mailbox
   name and UIDVALIDITY.  This is almost MUST, but weakened
   to allow for the possibility of loss of OBJECTID data during disaster
   recovery while still keeping the name and UIDVALIDITY the same.

   The server MUST NOT report the same MAILBOXID for two mailboxes at
   the same time.

   The server MUST NOT reuse the same MAILBOXID for a mailbox which does
   not obey all the invarients that [RFC3501] defines for a mailbox
   which does not change name or UIDVALIDITY.

   The server MAY choose to create a SHOULD NOT change MAILBOXID value in a way that does
   not survive RENAME (e.g. when renaming a digest of name + uidvalidity could be
   used), however the client then folder, as
   this loses the major main benefit of having an a unique identifier.

4.1.  New response code for CREATE

   This document extends the CREATE command to have the response code
   MAILBOXID on successful mailbox creation.

   A server advertising the OBJECTID capability MUST include the
   MAILBOXID response code in the tagged OK response to all successful
   CREATE commands.

   Syntax: "MAILBOXID" SP "(" <objectid> ")"

         Response code in tagged OK for successful CREATE command.

   Example:

     C: 3 create foo
     S: 3 OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Completed
     C: 4 create bar
     S: 4 OK [MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)] Completed
     C: 5 create foo
     S: 5 NO Mailbox already exists

4.2.  New OK Untagged Response for SELECT and EXAMINE

   This document adds a new untagged response code to the SELECT and
   EXAMINE commands.

   A server advertising the OBJECTID capability MUST return an untagged
   OK response with the MAILBOXID response code on all successful SELECT
   and EXAMINE commands.

   Syntax: "OK" SP "[" "MAILBOXID" SP "(" <objectid> ")" "]" text

                Untagged OK response to SELECT or EXAMINE.

   Example:

        C: 27 select "foo"
        [...]
        S: * OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Ok
        [...]
        S: 27 OK [READ-WRITE] Completed

4.3.  New attribute for STATUS

   This document adds the MAILBOXID attribute to the STATUS command
   using the extended syntax defined in [RFC4466].

   A server that advertises the OBJECTID capability MUST support the
   MAILBOXID status attribute.

   Syntax: "MAILBOXID"

                   The attribute in the STATUS command.

   Syntax: "MAILBOXID" SP "(" <objectid> ")"

      The response item in the STATUS response contains the objectid
      assigned by the server for this mailbox.

   Example:

    C: 6 status foo (mailboxid)
    S: * STATUS foo (MAILBOXID (F2212ea87-6097-4256-9d51-71338625))
    S: 6 OK Completed
    C: 7 status bar (mailboxid)
    S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3))
    S: 7 OK Completed
    C: 8 rename foo renamed
    S: * OK rename foo renamed
    S: 8 OK Completed
    C: 9 status renamed (mailboxid)
    S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625))
    S: 9 OK Completed
    C: 10 status bar (mailboxid)
    S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3))
    S: 10 OK Completed

   When the LIST-STATUS IMAP capability defined in [RFC5819] is also
   available, the STATUS command can be combined with the LIST command.

   Example:

   C: 11 list "" "*" return (status (mailboxid))
   S: * LIST (\HasNoChildren) "." INBOX
   S: * STATUS INBOX (MAILBOXID (Ff8e3ead4-9389-4aff-adb1-d8d89efd8cbf))
   S: * LIST (\HasNoChildren) "." bar
   S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3))
   S: * LIST (\HasNoChildren) "." renamed
   S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625))
   S: 11 OK Completed (0.001 secs 3 calls)

5.  EMAILID object identifier and THREADID correlator

   This document defines the data items EMAILID and THREADID on
   messages.

5.1.  EMAILID identifier for identical messages

   The EMAILID data item is an objectid which uniquely identifies the
   content of a single message.  Anything which must remain immutable on
   a {name, uidvalidity, uid} triple must also be the same between
   messages with the same EMAILID.

   The server SHOULD MUST return the same EMAILID for the same UID triple.
   As with MAILBOXID above, this triple, hence
   EMAILID is almost a MUST, but allows for the
   possibility of loss of OBJECTID data in disaster recovery without
   having to change UIDVALIDITY. immutable.

   The server SHOULD MUST return the same EMAILID as the source message for the exact same
   matching destination message
   content in different folders the COPYUID pairing after a COPY or
   [RFC6851] MOVE command.

   The server MAY assign the same EMAILID as an existing message upon
   APPEND (e.g. if it detects that the new message has exactly identical
   content to that of an existing message)

   NOTE: EMAILID only identifies the immutable content of the message.
   In particular, it is possible for different messages with the same
   EMAILID to have different keywords.  This document does not specify a
   way to STORE by EMAILID.

5.2.  THREADID identifer for related messages

   The THREADID data item is an objectid which uniquely identifies a set
   of messages which the server believes should be grouped together when
   presented.

   THREADID calculation is generally based on some combination of
   References, In-Reply-To and Subject, but the exact logic is left up
   to the server implementation.  [RFC5256] describes some algorithms
   that MAY could be used, however this specfication does not mandate any
   particular strategy.

   The server MUST return the same THREADID for all messages with the
   same EMAILID.

   The server SHOULD return the same THREADID for related messages even
   if they are in different mailboxes.

   THREADID is optional, if the server doesn't support THREADID or is
   unable to calculate relationships between messages, it MUST return
   NIL to in all FETCH responses for the THREADID data item, and a SEARCH
   for THREADID MUST NOT match any messages.

   The server MAY MUST NOT use the same objectid value for both EMAILID EMAILIDs and
   THREADID, for example the THREADID could be the EMAILID of
   THREADIDs.  If they are stored with the first
   message that same value internally, the
   server has seen can generate prefixed values (as shown in each thread. the examples below
   with M and T prefixes) to avoid clashes.

5.3.  New Message Data Items in FETCH and UID FETCH Commands

   This document defines two FETCH items:

   Syntax: EMAILID

     The EMAILID message data item causes the server to return EMAILID
     FETCH response data items.

   Syntax: THREADID

    The THREADID message data item causes the server to return THREADID
    FETCH response data items.

   And the following responses:

   Syntax: EMAILID ( <objectid> )

   The EMAILID response data item contains the server-assigned objectid
   for each message.

   Syntax: THREADID ( <objectid> )

   The THREADID response data item contains the server-assigned objectid
   for the set of related messages to which this message belongs.

   Syntax: THREADID NIL

     The NIL value to the THREADID response data item is returned when
     the server mailbox does not support THREADID calculation.

   Example:

    C: 5 append inbox "20-Mar-2018 03:07:37 +1100" {733}
    [...]
    Subject: Message A
    Message-ID: <fake.1521475657.54797@example.com>
    [...]
    S: 5 OK [APPENDUID 1521475658 1] Completed

    C: 11 append inbox "20-Mar-2018 03:07:37 +1100" {793}
    [...]
    Subject: Re: Message A
    Message-ID: <fake.1521475657.21213@example.org>
    References: <fake.1521475657.54797@example.com>
    [...]
    S: 11 OK [APPENDUID 1521475658 2] Completed

    C: 17 append inbox "20-Mar-2018 03:07:37 +1100" {736}
    [...]
    Subject: Message C
    Message-ID: <fake.1521475657.60280@example.com>
    [...]
    S: 17 OK [APPENDUID 1521475658 3] Completed

    C: 22 fetch 1:* (emailid threadid)
    S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9))
    S: * 2 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9))
    S: * 3 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5))
    S: 22 OK Completed (0.000 sec)

    C: 23 move 2 foo
    S: * OK [COPYUID 1521475659 2 1] Completed
    S: * 2 EXPUNGE
    S: 23 OK Completed

    C: 24 fetch 1:* (emailid threadid)
    S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9))
    S: * 2 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5))
    S: 24 OK Completed (0.000 sec)
    C: 25 select "foo"

    C: 25 select "foo"
    [...]
    S: 25 OK [READ-WRITE] Completed
    C: 26 fetch 1:* (emailid threadid)
    S: * 1 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9))
    S: 26 OK Completed (0.000 sec)

   Example: (no THREADID support)
              C: 26 fetch 1:* (emailid threadid)
              S: * 1 FETCH (EMAILID (M00000001) THREADID NIL)
              S: * 2 FETCH (EMAILID (M00000002) THREADID NIL)
              S: 26 OK Completed (0.000 sec)

6.  New Filters on SEARCH command

   This document defines filters EMAILID and THREADID on the SEARCH
   command.

   EMAILID <objectid>

         Messages whose EMAILID is exactly the specified objectid.

   THREADID <objectid>

        Messages whose THREADID is exactly the specified objectid.

   Example: (as if run before the MOVE above when the mailbox had 3
   messages)

                 C: 27 search emailid M6d99ac3275bb4e
                 S: * SEARCH 1
                 S: 27 OK Completed (1 msgs in 0.000 secs)
                 C: 28 search threadid T64b478a75b7ea9
                 S: * SEARCH 1 2
                 S: 28 OK Completed (2 msgs in 0.000 secs)

7.  Formal syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) [RFC5234] notation.  Elements not defined here can be
   found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and
   IMAP ABNF extensions [RFC4466] specifications.

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

   capability =/ "OBJECTID"

   fetch-att =/ "EMAILID" / "THREADID"

   fetch-emailid-resp = "EMAILID" SP "(" objectid ")" ; follows tagged-
   ext production from [RFC4466]
   fetch-threadid-resp = "THREADID" SP "(" objectid ")" / "THREADID "THREADID" NIL
   ; follows tagged-ext production from [RFC4466]

   objectid = 1*255(ALPHA / DIGIT / "_" / "-") ; characters in object
   identifiers are case ; significant

   resp-text-code =/ "MAILBOXID" SP "(" objectid ")" ; incorporated
   before the expansion rule of ; atom [SP 1*<any TEXT-CHAR except "]">]
   ; that appears in [RFC3501]

   search-key =/ "EMAILID" SP objectid / "THREADID" SP objectid

   status-att =/ "MAILBOXID"

   status-att-value =/ "MAILBOXID" SP "(" objectid ")" ; follows tagged-
   ext production from [RFC4466]

8.  Implementation considerations

8.1.  Assigning object identifiers

   All objectid values are allocated by the server.

   In the interests of reducing the possibilities of encoding mistakes,
   objectids are restricted to a safe subset of possible byte values,
   and in order to allow clients to allocate storage, they are
   restricted in length.

   An objectid is a string of 1 to 255 characters from the following set
   of 64 codepoints. a-z, A-Z, 0-9, '_', '-'.  These characters are safe
   to use in almost any context (e.g. filesystems, URIs, IMAP atoms).

   For maximum safety, servers SHOULD also follow defensive allocation
   strategies to avoid creating risks where glob completion or data type
   detection may be present (e.g. on filesystems or in spreadsheets).
   In particular it is wise to avoid:

   o  ids starting with -

   o  ids starting with digits

   o  ids which contain only digits

   o  ids which differ only by ASCII case (A vs a)

   o  the specific sequence of 3 characters NIL
   A good solution to these issues is to prefix every ID with a single
   alphabetical character.

8.2.  Interaction with special cases

   The case of RENAME INBOX may need special handling for unique ids. handling, as it has special
   behaviour as defined in [RFC3501] section 6.3.5.

   It is advisable (though not required) to have MAILBOXID be globally
   unique, but it is only required to be unique within messages offered
   to a single client login to a single server hostname.  For example, a
   proxy which aggregates multiple independent servers MUST NOT
   advertise the OBJECTID capability unless it can guarantee that the
   backends
   different objects will not never use the same identifiers for different objects. identifiers, even if
   backend object collide.

8.3.  Client usage

   Servers that implement both RFC 6154 and this specification SHOULD
   optimize their execution of command like UID SEARCH OR EMAILID 1234
   EMAILID 4321.

   Clients can assume that searching the all-mail mailbox using OR/
   EMAILID or OR/THREADID is a fast way to find messages again if some
   other client has moved them out of the mailbox where they were
   previously seen.

   Clients that cache data offline SHOULD fetch the EMAILID of all new
   messages to avoid re-downloading already cached message details.

   Clients SHOULD fetch the MAILBOXID for any new mailboxes before
   discarding cache data for any mailbox which is no longer present on
   the server, so that they can detect renames and avoid re-downloading
   data.

9.  Future considerations

   This extension is intentionally defined to be compatible with the
   data model in [I-D.ietf-jmap-mail].

   A future extension could be proposed to give a way to SELECT a
   mailbox by MAILBOXID rather than name.

   A future extension to [RFC5228] could allow fileinto by MAILBOXID
   rather than name.

   An extension to allow fetching message content directly via EMAILID
   and message listings by THREADID could be proposed.

10.  IANA Considerations

   IANA is requested to add "OBJECTID" to the "IMAP Capabilities"
   registry located at <http://www.iana.org/assignments/imap-
   capabilities>.

   IANA is requested to add "MAILBOXID" to the "IMAP Response Codes"
   registry located at <https://www.iana.org/assignments/imap-response-
   codes> with a Reference of [[THIS RFC]].

11.  Security Considerations

   It is strongly advised that servers generate OBJECTIDs which are safe
   to use as filesystem names, and unlikely to be auto-detected as
   numbers.  See implementation considerations.

   If a digest is used for ID generation, it must have a collision
   resistent property, so server implementations are advised to monitor
   current security research and choose secure digests.  As the IDs are
   generated by the server, it will be possible to migrate to a new hash
   by just creating new IDs with the new algorithm.  This is
   particularly true if a prefix is used on each ID, which can be
   changed when the algorithm changes.

   The use of a digest for ID generation may be used as proof that a
   particular sequence of bytes was seen by the server. server, however this is
   only a risk if IDs are leaked to clients who don't have permission to
   fetch the data directly.  Servers that are expected to handle highly
   sensitive data should consider using a ID generation mechanism which
   doesn't derive from a digest.

   See also the security considerations in [RFC3501] section 11.

12.  Changes

   To be removed by the editor before publication

12.1.  draft-ietf-extra-imap-objectid-04

   o  described NIL THREADID in more detail (ad review)

   o  made RFC5256 a normative reference (ad review)

   o  fixed ABNF missing quote (ad review)

   o  documented hash upgrade process (ad review)

   o  referenced RFC3501 for INBOX rename (ad review)
   o  referenced RFC3501 security considerations (secdir review)

   o  turned mealy-mouthed "SHOULDs" in to "MUSTs" on immutability
      (genart review)

   o  remove suggested algorithms which are no longer legitimate (genart
      review)

   o  updated proxy advice to suggest rewriting ids (genart review)

   o  fixed minor gramatical issues (genart review)

   o  required that EMAILID and THREADID are not identical (own
      decision)

12.2.  draft-ietf-extra-imap-objectid-03

   o  added RFC3501 to Abstract

   o  updated [[THIS RFC]] to not fail idnits

   o  changed jmap-mail to be informative rather than normative

   o  shortened IDs to stop wrapping and outdents in IMAP examples

12.2.

12.3.  draft-ietf-extra-imap-objectid-02

   o  added "Client usage" section

12.3.

12.4.  draft-ietf-extra-imap-objectid-01

   o  added "updates" for RFC3501

   o  fixed domains in thread example

   o  described threading in more detail

   o  added IANA request for Response Code

   o  clarified RFC2119 references

   o  simplified some waffle in wording

   o  added security consideration to choose good digest

   o  added MAILBOXID-UID suggestion for EMAILID generation

   o  updated ABNF normative reference to RFC5234

12.4.

12.5.  draft-ietf-extra-imap-objectid-00

   o  renamed draft to be objectid rather than uniqueid

   o  renamed UNIQUEID (capability) to OBJECTID

   o  restricted objectid to 64 safe characters

   o  added security considerations and advice about choosing objectid

   o  wrapped all responses in () for RFC4466 compatibility

   o  signifiant rewrite of all sections

12.5.

12.6.  draft-ietf-extra-imap-uniqueid-00

   o  renamed draft to be an EXTRA document

   o  added example for LIST RETURN STATUS

   o  started work on ABNF

   o  attempted to add response codes for EMAILID and THREADID

12.6.

12.7.  draft-gondwana-imap-uniqueid-01

   o  renamed UNIQUEID (status item) to MAILBOXID

   o  renamed MSGID to EMAILID

   o  renamed THRID to THREADID

   o  added TODO section

12.7.

12.8.  draft-gondwana-imap-uniqueid-00

   o  initial upload with names UNIQUEID/MSGID/THRID

13.  Acknowledgments

   The EXTRA working group at IETF.  In particular feedback from Arnt
   Gulbrandsen, Brandon Long, Chris Newman and Josef Sipek.

   The Gmail X-GM-THRID and X-GM-MSGID implementation as currently
   defined at <https://developers.google.com/gmail/imap/imap-
   extensions>.

   Dovecot X-GUID implementation.

13.1.  Appendix 1: ideas for implementing object identifiers

   Ideas for implementing calculating MAILBOXID:

   o  Digest of (MailboxName/UIDVALIDITY) - not kept when renaming, but
      is guaranteed unique and doesn't require storage.

   o  [RFC4122] UUID

   o  Server assigned sequence number (guaranteed not to be reused)

   Ideas for implementing EMAILID:

   o  Digest of (MailboxName/UIDVALIDITY/UID) - is not kept when copying
      messages, but is guaranteed unique and doesn't require storage.

   o  Concatenation of MAILBOXID-UID - for servers which store MAILBOXID
      but not EMAILID.

   o  Digest of message content (RFC822 bytes) - expensive unless cached

   o  [RFC4122] UUID

   o  Server assigned sequence number (guaranteed not to be reused)

   Ideas for implementing THREADID:

   o  Derive from EMAILID of first seen message in the thread.

   o  [RFC4122] UUID

   o  Server assigned sequence number (guaranteed not to be reused)

   There is a need to index and look up reference/in-reply-to data at
   message creation to efficiently find matching messages for threading.
   Threading may be either across folders, or within each folder only.
   The server has significant leeway here.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC4315]  Crispin, M., "Internet Message Access Protocol (IMAP) -
              UIDPLUS extension", RFC 4315, DOI 10.17487/RFC4315,
              December 2005, <https://www.rfc-editor.org/info/rfc4315>.

   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
              <https://www.rfc-editor.org/info/rfc4466>.

   [RFC5228]  Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
              Filtering Language", RFC 5228, DOI 10.17487/RFC5228,
              January 2008, <https://www.rfc-editor.org/info/rfc5228>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5256]  Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256,
              DOI 10.17487/RFC5256, June 2008,
              <https://www.rfc-editor.org/info/rfc5256>.

   [RFC5819]  Melnikov, A. and T. Sirainen, "IMAP4 Extension for
              Returning STATUS Information in Extended LIST", RFC 5819,
              DOI 10.17487/RFC5819, March 2010,
              <https://www.rfc-editor.org/info/rfc5819>.

   [RFC6851]  Gulbrandsen, A. and N. Freed, Ed., "Internet Message
              Access Protocol (IMAP) - MOVE Extension", RFC 6851,
              DOI 10.17487/RFC6851, January 2013,
              <https://www.rfc-editor.org/info/rfc6851>.

14.2.  Informative References

   [I-D.ietf-jmap-mail]
              Jenkins, N., "JMAP for Mail", draft-ietf-jmap-mail-05 draft-ietf-jmap-mail-06
              (work in progress), May July 2018.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC5256]  Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256,
              DOI 10.17487/RFC5256, June 2008,
              <https://www.rfc-editor.org/info/rfc5256>.

Author's Address
   Bron Gondwana (editor)
   FastMail
   Level 2, 114 William St
   Melbourne  VIC 3000
   Australia

   Email: brong@fastmailteam.com
   URI:   https://www.fastmail.com