wdiff rfc8474.alt-original rfc8474.txt

EXTRA
Internet Engineering Task Force (IETF) B. Gondwana, Ed.
Internet-Draft
Request for Comments: 8474 FastMail
Updates: 3501 (if approved) August 2, September 2018
Intended status:
Category: Standards Track
Expires: February 3, 2019
ISSN: 2070-1721

IMAP Extension for object identifiers
draft-ietf-extra-imap-objectid-08 Object Identifiers

Abstract

This document updates RFC3501 RFC 3501 (IMAP4rev1) with persistent
identifiers on mailboxes and messages to allow clients to more
efficiently re-use reuse 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 an Internet Standards Track document.

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(IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
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This Internet-Draft will expire on February 3, 2019.
https://www.rfc-editor.org/info/rfc8474.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In in This Document . . . . . . . . . . . . . . 3
3. CAPABILITY Identification . . . . . . . . . . . . . . . . . . 3
4. MAILBOXID object identifier Object Identifier . . . . . . . . . . . . . . . . . 3
4.1. New response code Response Code for CREATE . . . . . . . . . . . . . . 4
4.2. New OK Untagged Response for SELECT and EXAMINE . . . . . 4
4.3. New attribute Attribute for STATUS . . . . . . . . . . . . . . . . 5
5. EMAILID object identifier Object Identifier and THREADID correlator Correlator . . . . . . 6
5.1. EMAILID identifier Identifier for identical messages Identical Messages . . . . . . . . 6
5.2. THREADID identifer Identifier for related messages . Related Messages . . . . . . . . 6
5.3. New Message Data Items in FETCH and UID FETCH Commands . 7
6. New Filters on SEARCH command Command . . . . . . . . . . . . . . . . 10 9
7. Formal syntax Syntax . . . . . . . . . . . . . . . . . . . . . . . . 10 9
8. Implementation considerations Considerations . . . . . . . . . . . . . . . . 11 10
8.1. Assigning object identifiers Object Identifiers . . . . . . . . . . . . . . 11 10
8.2. Interaction with special cases Special Cases . . . . . . . . . . . . . 12 11
8.3. Client usage . Usage . . . . . . . . . . . . . . . . . . . . . 12
9. Future considerations . . . . . . 11
8.4. Advice to Client Implementers . . . . . . . . . . . . . . 13
10. IANA 12
9. Future Considerations . . . . . . . . . . . . . . . . . . . . . 13
11. Security 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . 13
12. Changes . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13
12. References . . 14
12.1. draft-ietf-extra-imap-objectid-08 . . . . . . . . . . . 14
12.2. draft-ietf-extra-imap-objectid-07 . . . . . . . . . . . 14
12.3. draft-ietf-extra-imap-objectid-06 . . . . . . . . . . . 14
12.4. draft-ietf-extra-imap-objectid-05 . . . . . . . . . . . 15
12.5. draft-ietf-extra-imap-objectid-04 . . . . . . . . . . . 15
12.6. draft-ietf-extra-imap-objectid-03 . . . . . . . . . . . 15
12.7. draft-ietf-extra-imap-objectid-02 . . . . . . . . . . . 16
12.8. draft-ietf-extra-imap-objectid-01 . . . . . . . . . . . 16
12.9. draft-ietf-extra-imap-objectid-00 . . . . . . . . . . . 16
12.10. draft-ietf-extra-imap-uniqueid-00 . . . . . . . . . . . 16
12.11. draft-gondwana-imap-uniqueid-01 . . . . . . . . . . . . 17
12.12. draft-gondwana-imap-uniqueid-00 13
12.1. Normative References . . . . . . . . . . . . 17
13. Acknowledgments . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 17
13.1. 14
Appendix 1: ideas A. Ideas for implementing object identifiers . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. Normative References . . . . . Implementing Object Identifiers . . . . . 15
Acknowledgments . . . . . . . . 18
14.2. Informative References . . . . . . . . . . . . . . . . . 19 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 16

1. Introduction

IMAP stores are often used by many clients. Each client may cache
data from the server so that they don't it doesn't need to re-download redownload
information. [RFC3501] defines states 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. unique identifier (UID). The triple of mailbox
name, UIDVALIDITY UIDVALIDITY, and UID is guaranteed to be immutable.

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

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

This extension adds new properties to a message (EMAILID) and mailbox
(MAILBOXID) which
(MAILBOXID). These properties allow a client to quickly identify
messages or mailboxes which that 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 that
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 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. CAPABILITY Identification

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

4. MAILBOXID object identifier Object Identifier

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

The server MUST return the same MAILBOXID for a mailbox with the same
name and UIDVALIDITY.

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 that does
not obey all the invariants that [RFC3501] defines for a mailbox
which that
does not change name or UIDVALIDITY.

The server MUST keep the same MAILBOXID for the source and
destination when renaming a mailbox in a way which that keeps the same
messages (but see [RFC3501] for the special case regarding the
renaming of INBOX, which is treated as creating a new mailbox and
moving the
messages) messages).

4.1. New response code 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 response 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 ")" "]" SP 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 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 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 Object Identifier and THREADID correlator Correlator

5.1. EMAILID identifier Identifier for identical messages Identical Messages

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

The server MUST return the same EMAILID for the same triple, hence triple; hence,
EMAILID is immutable.

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

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

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

THREADID calculation is generally based on some combination of
References, In-Reply-To In-Reply-To, and Subject, but the exact logic is left up
to the server implementation. [RFC5256] describes some algorithms
that could be used, however used; however, this specfication specification 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 messages, even
if they are in different mailboxes, e.g. mailboxes; for example, messages which that would
appear in the same thread if they were in the same mailbox SHOULD
have the same THREADID THREADID, even if they are in different mailboxes.

The server MUST NOT change the THREADID of a message once reported.

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

The server MUST NOT use the same objectid ObjectID value for both EMAILIDs and
THREADIDs. If they are stored with the same value internally, the
server can generate prefixed values (as shown in 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

This document defines the following responses:

Syntax: "EMAILID" SP "(" objectid ")"

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

Syntax: "THREADID" SP "(" objectid ")"

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

Syntax: "THREADID" SP nil

The NIL value to is returned for 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 Command

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

Syntax: "EMAILID" SP objectid

Messages whose EMAILID is exactly the specified objectid. ObjectID.

Syntax: "THREADID" SP objectid

Messages whose THREADID is exactly the specified objectid. ObjectID.

Example: (as if run before the MOVE shown above when the mailbox had 3
three 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 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- case
insensitive. The use of upper- uppercase or lowercase characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.

Please note specifically that objectid ObjectID values are case sensitive.

capability =/ "OBJECTID"

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

fetch-emailid-resp = "EMAILID" SP "(" objectid ")"
; follows tagged-ext production from [@!RFC4466] [RFC4466]

fetch-threadid-resp = "THREADID" SP ( "(" objectid ")" / nil )
; follows tagged-ext production from [@!RFC4466] [RFC4466]

msg-att-static =/ fetch-emailid-resp / fetch-threadid-resp

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] [RFC3501]

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

status-att =/ "MAILBOXID"

status-att-value

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

8. Implementation considerations Considerations

8.1. Assigning object identifiers Object Identifiers

All objectid ObjectID values are allocated by the server.

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

An objectid 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. (e.g., filesystems, URIs, IMAP atoms).
These are the same characters defined as base64url in [RFC4648].

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. (e.g., on filesystems or in spreadsheets).
In particular particular, it is wise to avoid:

o ids IDs starting with - a dash

o ids IDs starting with digits

o ids which IDs that contain only digits

o ids which IDs that differ only by ASCII case (A vs (for example, A vs. a)

o the specific sequence of 3 three characters NIL in any case (as (because
this sequence can be confused with the IMAP protocol expression of
the null value)

A good solution to these issues is to prefix every ID with a single
alphabetical character.

8.2. Interaction with special cases Special Cases

The case of RENAME INBOX may need special handling, as handling because it has
special
behaviour behavior, as defined in [RFC3501] section [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 that aggregates multiple independent servers MUST NOT advertise
the OBJECTID capability unless it can guarantee that different
objects will never use the same identifiers, even if backend object
identifiers collide.

8.3. Client usage Usage

Servers that implement both RFC 6154 and this specification should
optimize their execution of command commands 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 redownloading already-cached message details.

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

8.4. Advice to Client Implementers

In cases of server failure and disaster recovery, or misbehaving
servers, it is possible that a client will be sent invalid
information, e.g., identical ObjectIDs or ObjectIDs that have changed
where they MUST NOT change according to this document.

In a case where a client detects inconsistent ObjectID responses from
a server, it SHOULD fall back to relying on the guarantees of RFC
3501. For simplicity, a client MAY instead choose to discard its
entire cache and resync all state from the server.

Client authors protecting against server misbehavior MUST ensure that
their design cannot get into an infinite loop of discarding cache and
fetching the same data repeatedly without user interaction.

9. Future considerations Considerations

This extension is intentionally defined to be compatible with the
data model in [I-D.ietf-jmap-mail]. [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 has added "OBJECTID" to the "IMAP Capabilities" registry located
at <http://www.iana.org/assignments/imap-
capabilities> <https://www.iana.org/assignments/imap-capabilities> with a Reference of [[THIS RFC]].

IANA is requested
reference to add this document.

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

11. Security Considerations

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

If a digest is used for ID generation, it must have a collision
resistent collision-
resistant 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 using the new algorith algorithm when creating new IDs. 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, however 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 this when choosing how to
create IDs.

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

14.

12. References

14.1.

12.1. 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>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

14.2.

12.2. Informative References

[I-D.ietf-jmap-mail]

[JMAP-MAIL]
Jenkins, N., N. and C. Newman, "JMAP for Mail", draft-ietf-jmap-mail-06
(work Work in progress), July
Progress, draft-ietf-jmap-mail-07, August 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>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

13.1.

Appendix 1: ideas A. Ideas for implementing object identifiers Implementing Object Identifiers

Ideas for calculating MAILBOXID:

o Universally Unique Identifier (UUID) [RFC4122] UUID

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

Ideas for implementing EMAILID:

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

o [RFC4122] UUID [RFC4122]

o Server assigned 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 [RFC4122]

o Server assigned 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 mailboxes, mailboxes or within each mailbox only.
The server has significant leeway here.

13.

Acknowledgments

The author would like to thank the EXTRA working group at IETF. In particular IETF for
feedback from and advice -- in particular, Arnt Gulbrandsen, Brandon Long,
Chris Newman Newman, and Josef Sipek.

The

This document drew inspiration from the Gmail X-GM-THRID and X-GM-MSGID implementation X-GM-
MSGID implementations as currently defined at <https://developers.google.com/gmail/imap/imap-
extensions>.

Dovecot
<https://developers.google.com/gmail/imap/imap-extensions>, as well
as the X-GUID implementation. implementation in the Dovecot server.

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