IMAP Extension for object identifiers
draft-ietf-extra-imap-objectid-01
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (extra WG) | |
|---|---|---|---|
| Author | Bron Gondwana | ||
| Last updated | 2018-04-23 (Latest revision 2018-04-20) | ||
| Replaces | draft-ietf-extra-imap-uniqueid | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-extra-imap-objectid-01
EXTRA B. Gondwana
Internet-Draft FastMail
Intended status: Standards Track April 24, 2018
Expires: October 26, 2018
IMAP Extension for object identifiers
draft-ietf-extra-imap-objectid-01
Abstract
This document adds new properties to IMAP mailboxes and messages to
allow clients to more efficiently re-use cached data for resources
which 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 26, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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
9. Future considerations . . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11
12. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. draft-ietf-extra-imap-objectid-01 . . . . . . . . . . . 12
12.2. draft-ietf-extra-imap-objectid-00 . . . . . . . . . . . 12
12.3. draft-ietf-extra-imap-uniqueid-00 . . . . . . . . . . . 13
12.4. draft-gondwana-imap-uniqueid-01 . . . . . . . . . . . . 13
12.5. draft-gondwana-imap-uniqueid-00 . . . . . . . . . . . . 13
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
13.1. Appendix 1: ideas for implementing object identifiers . 13
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
14.1. Normative References . . . . . . . . . . . . . . . . . . 14
14.2. Informative References . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
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.
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If a mailbox is successfully renamed, the client knows 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.
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 return the same MAILBOXID for a server 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.
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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 MAILBOXID value in a way that does
not survive RENAME (e.g. a digest of name + uidvalidity could be
used), however the client then loses the major benefit of having an
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 (F2212ea87-6097-4256-9d51-716686338625)] Completed
C: 4 create bar
S: 4 OK [MAILBOXID (F6352ae03-b7f5-463c-896f-d47cf8b48ee3)] 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:
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C: 27 select "foo"
[...]
S: * OK [UIDVALIDITY 1524195797] Ok
S: * OK [MAILBOXID (F2212ea87-6097-4256-9d51-716686338625)] 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 (F2212ea87-6097-4256-9d51-716686338625))
S: 6 OK Completed
C: 7 status bar (mailboxid)
S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d47cf8b48ee3))
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 (F2212ea87-6097-4256-9d51-716686338625))
S: 9 OK Completed
C: 10 status bar (mailboxid)
S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d47cf8b48ee3))
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:
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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-d47cf8b48ee3))
S: * LIST (\HasNoChildren) "." renamed
S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-716686338625))
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 return the same EMAILID for the same UID triple.
As with MAILBOXID above, this is almost a MUST, but allows for the
possibility of loss of OBJECTID data in disaster recovery without
having to change UIDVALIDITY.
The server SHOULD return the same EMAILID for the exact same message
content in different folders after a COPY or [RFC6851] MOVE command.
The server MAY assign the same EMAILID as an existing message upon
APPEND if it detects that the new message has exactly identical
content to that existing message.
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 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.
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The server SHOULD return the same THREADID for related messages even
if they are in different mailboxes.
THREADID is optional, if the server 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 use the same objectid value for both EMAILID and
THREADID, for example the THREADID could be the EMAILID of the first
message that the server has seen in each thread.
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:
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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 (M8976d99ac3275bb4e) THREADID (T4964b478a75b7ea9))
S: * 2 FETCH (EMAILID (Md3c288836c4c7a762) THREADID (T4964b478a75b7ea9))
S: * 3 FETCH (EMAILID (M2e25fdc09b49ea703) THREADID (T6311863d02dd95b5))
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 (M8976d99ac3275bb4e) THREADID (T4964b478a75b7ea9))
S: * 2 FETCH (EMAILID (M2e25fdc09b49ea703) THREADID (T6311863d02dd95b5))
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 (Md3c288836c4c7a762) THREADID (T4964b478a75b7ea9))
S: 26 OK Completed (0.000 sec)
Example: (no THREADID support)
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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 M8976d99ac3275bb4e
S: * SEARCH 1
S: 27 OK Completed (1 msgs in 0.000 secs)
C: 28 search threadid T4964b478a75b7ea9
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]
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fetch-threadid-resp = "THREADID" SP "(" objectid ")" /
"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
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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.
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 will not use the same identifiers for different objects.
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.
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.
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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.
The use of a digest for ID generation may be used as proof that a
particular sequence of bytes was seen by the server.
12. Changes
To be removed by the editor before publication
12.1. 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
12.2. 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
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12.3. 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
12.4. 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
12.5. 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 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:
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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
[I-D.ietf-jmap-mail]
Jenkins, N., "JMAP for Mail", draft-ietf-jmap-mail-04
(work in progress), March 2018.
[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>.
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[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>.
[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>.
[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
[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
FastMail
Level 2, 114 William St
Melbourne VIC 3000
Australia
Email: brong@fastmailteam.com
URI: https://www.fastmail.com
Gondwana Expires October 26, 2018 [Page 15]