draft-ietf-extra-imap-objectid-08.txt   rfc8474.txt 
EXTRA B. Gondwana, Ed. Internet Engineering Task Force (IETF) B. Gondwana, Ed.
Internet-Draft FastMail Request for Comments: 8474 FastMail
Updates: 3501 (if approved) August 2, 2018 Updates: 3501 September 2018
Intended status: Standards Track Category: Standards Track
Expires: February 3, 2019 ISSN: 2070-1721
IMAP Extension for object identifiers IMAP Extension for Object Identifiers
draft-ietf-extra-imap-objectid-08
Abstract Abstract
This document updates RFC3501 (IMAP4rev1) with persistent identifiers This document updates RFC 3501 (IMAP4rev1) with persistent
on mailboxes and messages to allow clients to more efficiently re-use identifiers on mailboxes and messages to allow clients to more
cached data when resources have changed location on the server. efficiently reuse cached data when resources have changed location on
the server.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
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time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
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Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on February 3, 2019. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8474.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. CAPABILITY Identification . . . . . . . . . . . . . . . . . . 3 3. CAPABILITY Identification . . . . . . . . . . . . . . . . . . 3
4. MAILBOXID object identifier . . . . . . . . . . . . . . . . . 3 4. MAILBOXID Object Identifier . . . . . . . . . . . . . . . . . 3
4.1. New response code for CREATE . . . . . . . . . . . . . . 4 4.1. New Response Code for CREATE . . . . . . . . . . . . . . 4
4.2. New OK Untagged Response for SELECT and EXAMINE . . . . . 4 4.2. New OK Untagged Response for SELECT and EXAMINE . . . . . 4
4.3. New attribute for STATUS . . . . . . . . . . . . . . . . 5 4.3. New Attribute for STATUS . . . . . . . . . . . . . . . . 5
5. EMAILID object identifier and THREADID correlator . . . . . . 6 5. EMAILID Object Identifier and THREADID Correlator . . . . . . 6
5.1. EMAILID identifier for identical messages . . . . . . . . 6 5.1. EMAILID Identifier for Identical Messages . . . . . . . . 6
5.2. THREADID identifer for related messages . . . . . . . . . 6 5.2. THREADID Identifier for Related Messages . . . . . . . . 6
5.3. New Message Data Items in FETCH and UID FETCH Commands . 7 5.3. New Message Data Items in FETCH and UID FETCH Commands . 7
6. New Filters on SEARCH command . . . . . . . . . . . . . . . . 10 6. New Filters on SEARCH Command . . . . . . . . . . . . . . . . 9
7. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Implementation considerations . . . . . . . . . . . . . . . . 11 8. Implementation Considerations . . . . . . . . . . . . . . . . 10
8.1. Assigning object identifiers . . . . . . . . . . . . . . 11 8.1. Assigning Object Identifiers . . . . . . . . . . . . . . 10
8.2. Interaction with special cases . . . . . . . . . . . . . 12 8.2. Interaction with Special Cases . . . . . . . . . . . . . 11
8.3. Client usage . . . . . . . . . . . . . . . . . . . . . . 12 8.3. Client Usage . . . . . . . . . . . . . . . . . . . . . . 11
9. Future considerations . . . . . . . . . . . . . . . . . . . . 13 8.4. Advice to Client Implementers . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9. Future Considerations . . . . . . . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13 11. Security Considerations . . . . . . . . . . . . . . . . . . . 13
12. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. draft-ietf-extra-imap-objectid-08 . . . . . . . . . . . 14 12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. draft-ietf-extra-imap-objectid-07 . . . . . . . . . . . 14 12.2. Informative References . . . . . . . . . . . . . . . . . 14
12.3. draft-ietf-extra-imap-objectid-06 . . . . . . . . . . . 14 Appendix A. Ideas for Implementing Object Identifiers . . . . . 15
12.4. draft-ietf-extra-imap-objectid-05 . . . . . . . . . . . 15 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
12.5. draft-ietf-extra-imap-objectid-04 . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
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 . . . . . . . . . . . . 17
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Appendix 1: ideas for implementing object identifiers . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. Normative References . . . . . . . . . . . . . . . . . . 18
14.2. Informative References . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
IMAP stores are often used by many clients. Each client may cache 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 data from the server so that it doesn't need to redownload
information. [RFC3501] defines that a mailbox can be uniquely information. [RFC3501] states that a mailbox can be uniquely
referenced by its name and UIDVALIDITY, and a message within that referenced by its name and UIDVALIDITY, and a message within that
mailbox can be uniquely referenced by its mailbox (name + mailbox can be uniquely referenced by its mailbox (name +
UIDVALIDITY) and UID. The triple of mailbox name, UIDVALIDITY and UIDVALIDITY) and unique identifier (UID). The triple of mailbox
UID is guaranteed to be immutable. name, UIDVALIDITY, and UID is guaranteed to be immutable.
[RFC4315] defines a COPYUID response which allows a client which [RFC4315] defines a COPYUID response that allows a client that copies
copies messages to know the mapping between the UIDs in the source messages to know the mapping between the UIDs in the source and
and destination mailboxes, and hence update its local cache. destination mailboxes and, hence, update its local cache.
If a mailbox is successfully renamed by a client, that client will If a mailbox is successfully renamed by a client, that client will
know that the same messages exist in the destination mailbox name as know that the same messages exist in the destination mailbox name as
previously existed in the source mailbox name. previously existed in the source mailbox name.
The result is that the client which copies (or [RFC6851] moves) The result is that the client that copies (or moves [RFC6851])
messages or renames a mailbox can update its local cache, but any messages or renames a mailbox can update its local cache, but any
other client connected to the same store can not know with certainty other client connected to the same store cannot know with certainty
that the messages are identical, and so will re-download everything. that the messages are identical, so it will redownload everything.
This extension adds new properties to a message (EMAILID) and mailbox This extension adds new properties to a message (EMAILID) and mailbox
(MAILBOXID) which allow a client to quickly identify messages or (MAILBOXID). These properties allow a client to quickly identify
mailboxes which have been renamed by another client. messages or mailboxes that have been renamed by another client.
This extension also adds an optional thread identifier (THREADID) to This extension also adds an optional thread identifier (THREADID) to
messages, which can be used by the server to indicate messages which messages, which can be used by the server to indicate messages that
it has identified to be related. A server that does not implement it has identified to be related. A server that does not implement
threading will return NIL to all requests for THREADID. threading will return NIL to all requests for THREADID.
2. Conventions Used In This Document 2. Conventions Used in This Document
In examples, "C:" indicates lines sent by a client that is connected 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. to a server. "S:" indicates lines sent by the server to the client.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
3. CAPABILITY Identification 3. CAPABILITY Identification
IMAP servers that support this extension MUST include "OBJECTID" in IMAP servers that support this extension MUST include "OBJECTID" in
the response list to the CAPABILITY command. the response list to the CAPABILITY command.
4. MAILBOXID object identifier 4. MAILBOXID Object Identifier
The MAILBOXID is a server-allocated unique identifer for each The MAILBOXID is a server-allocated unique identifier for each
mailbox. mailbox.
The server MUST return the same MAILBOXID for a mailbox with the same The server MUST return the same MAILBOXID for a mailbox with the same
name and UIDVALIDITY. name and UIDVALIDITY.
The server MUST NOT report the same MAILBOXID for two mailboxes at The server MUST NOT report the same MAILBOXID for two mailboxes at
the same time. the same time.
The server MUST NOT reuse the same MAILBOXID for a mailbox which does The server MUST NOT reuse the same MAILBOXID for a mailbox that does
not obey all the invariants that [RFC3501] defines for a mailbox not obey all the invariants that [RFC3501] defines for a mailbox that
which does not change name or UIDVALIDITY. does not change name or UIDVALIDITY.
The server MUST keep the same MAILBOXID for the source and The server MUST keep the same MAILBOXID for the source and
destination when renaming a mailbox in a way which keeps the same destination when renaming a mailbox in a way that keeps the same
messages (but see [RFC3501] for the special case regarding renaming messages (but see [RFC3501] for the special case regarding the
of INBOX, which is treated as creating a new mailbox and moving the renaming of INBOX, which is treated as creating a new mailbox and
messages) moving the messages).
4.1. New response code for CREATE 4.1. New Response Code for CREATE
This document extends the CREATE command to have the response code This document extends the CREATE command to have the response code
MAILBOXID on successful mailbox creation. MAILBOXID on successful mailbox creation.
A server advertising the OBJECTID capability MUST include the A server advertising the OBJECTID capability MUST include the
MAILBOXID response code in the tagged OK response to all successful MAILBOXID response code in the tagged OK response to all successful
CREATE commands. CREATE commands.
Syntax: "MAILBOXID" SP "(" objectid ")" Syntax: "MAILBOXID" SP "(" objectid ")"
Response code in tagged OK for successful CREATE command. Response code in tagged OK response for successful CREATE command.
Example: Example:
C: 3 create foo C: 3 create foo
S: 3 OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Completed S: 3 OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Completed
C: 4 create bar C: 4 create bar
S: 4 OK [MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)] Completed S: 4 OK [MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)] Completed
C: 5 create foo C: 5 create foo
S: 5 NO Mailbox already exists S: 5 NO Mailbox already exists
skipping to change at page 5, line 14 skipping to change at page 5, line 5
Untagged OK response to SELECT or EXAMINE. Untagged OK response to SELECT or EXAMINE.
Example: Example:
C: 27 select "foo" C: 27 select "foo"
[...] [...]
S: * OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Ok S: * OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Ok
[...] [...]
S: 27 OK [READ-WRITE] Completed S: 27 OK [READ-WRITE] Completed
4.3. New attribute for STATUS 4.3. New Attribute for STATUS
This document adds the MAILBOXID attribute to the STATUS command This document adds the MAILBOXID attribute to the STATUS command
using the extended syntax defined in [RFC4466]. using the extended syntax defined in [RFC4466].
A server that advertises the OBJECTID capability MUST support the A server that advertises the OBJECTID capability MUST support the
MAILBOXID status attribute. MAILBOXID status attribute.
Syntax: "MAILBOXID" Syntax: "MAILBOXID"
The attribute in the STATUS command. The attribute in the STATUS command.
Syntax: "MAILBOXID" SP "(" objectid ")" Syntax: "MAILBOXID" SP "(" objectid ")"
The response item in the STATUS response contains the objectid The response item in the STATUS response contains the ObjectID
assigned by the server for this mailbox. assigned by the server for this mailbox.
Example: Example:
C: 6 status foo (mailboxid) C: 6 status foo (mailboxid)
S: * STATUS foo (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) S: * STATUS foo (MAILBOXID (F2212ea87-6097-4256-9d51-71338625))
S: 6 OK Completed S: 6 OK Completed
C: 7 status bar (mailboxid) C: 7 status bar (mailboxid)
S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3))
S: 7 OK Completed S: 7 OK Completed
skipping to change at page 6, line 16 skipping to change at page 6, line 5
C: 11 list "" "*" return (status (mailboxid)) C: 11 list "" "*" return (status (mailboxid))
S: * LIST (\HasNoChildren) "." INBOX S: * LIST (\HasNoChildren) "." INBOX
S: * STATUS INBOX (MAILBOXID (Ff8e3ead4-9389-4aff-adb1-d8d89efd8cbf)) S: * STATUS INBOX (MAILBOXID (Ff8e3ead4-9389-4aff-adb1-d8d89efd8cbf))
S: * LIST (\HasNoChildren) "." bar S: * LIST (\HasNoChildren) "." bar
S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3))
S: * LIST (\HasNoChildren) "." renamed S: * LIST (\HasNoChildren) "." renamed
S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625))
S: 11 OK Completed (0.001 secs 3 calls) S: 11 OK Completed (0.001 secs 3 calls)
5. EMAILID object identifier and THREADID correlator 5. EMAILID Object Identifier and THREADID Correlator
5.1. EMAILID identifier for identical messages 5.1. EMAILID Identifier for Identical Messages
The EMAILID data item is an objectid which uniquely identifies the The EMAILID data item is an ObjectID that uniquely identifies the
content of a single message. Anything which must remain immutable on content of a single message. Anything that must remain immutable on
a {name, uidvalidity, uid} triple must also be the same between a {name, uidvalidity, uid} triple must also be the same between
messages with the same EMAILID. messages with the same EMAILID.
The server MUST return the same EMAILID for the same triple, hence The server MUST return the same EMAILID for the same triple; hence,
EMAILID is immutable. EMAILID is immutable.
The server MUST return the same EMAILID as the source message for the The server MUST return the same EMAILID as the source message for the
matching destination message in the COPYUID pairing after a COPY or matching destination message in the COPYUID pairing after a COPY or
[RFC6851] MOVE command. MOVE command [RFC6851].
The server MAY assign the same EMAILID as an existing message upon 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 APPEND (e.g., if it detects that the new message has exactly
content to that of an existing message) identical content to that of an existing message).
NOTE: EMAILID only identifies the immutable content of the message. NOTE: EMAILID only identifies the immutable content of the message.
In particular, it is possible for different messages with the same In particular, it is possible for different messages with the same
EMAILID to have different keywords. This document does not specify a EMAILID to have different keywords. This document does not specify a
way to STORE by EMAILID. way to STORE by EMAILID.
5.2. THREADID identifer for related messages 5.2. THREADID Identifier for Related Messages
The THREADID data item is an objectid which uniquely identifies a set The THREADID data item is an ObjectID that uniquely identifies a set
of messages which the server believes should be grouped together when of messages that the server believes should be grouped together when
presented. presented.
THREADID calculation is generally based on some combination of THREADID calculation is generally based on some combination of
References, In-Reply-To and Subject, but the exact logic is left up References, In-Reply-To, and Subject, but the exact logic is left up
to the server implementation. [RFC5256] describes some algorithms to the server implementation. [RFC5256] describes some algorithms
that could be used, however this specfication does not mandate any that could be used; however, this specification does not mandate any
particular strategy. particular strategy.
The server MUST return the same THREADID for all messages with the The server MUST return the same THREADID for all messages with the
same EMAILID. same EMAILID.
The server SHOULD return the same THREADID for related messages even The server SHOULD return the same THREADID for related messages, even
if they are in different mailboxes, e.g. messages which would appear if they are in different mailboxes; for example, messages that would
in the same thread if they were in the same mailbox SHOULD have the appear in the same thread if they were in the same mailbox SHOULD
same THREADID even if they are in different mailboxes. have the same THREADID, even if they are in different mailboxes.
The server MUST NOT change the THREADID of a message once reported. The server MUST NOT change the THREADID of a message once reported.
THREADID is OPTIONAL; if the server doesn't support THREADID or is THREADID is OPTIONAL; if the server doesn't support THREADID or is
unable to calculate relationships between messages, it MUST return unable to calculate relationships between messages, it MUST return
NIL to all FETCH responses for the THREADID data item, and a SEARCH NIL to all FETCH responses for the THREADID data item, and a SEARCH
for THREADID MUST NOT match any messages. for THREADID MUST NOT match any messages.
The server MUST NOT use the same objectid value for both EMAILIDs and The server MUST NOT use the same ObjectID value for both EMAILIDs and
THREADIDs. If they are stored with the same value internally, the THREADIDs. If they are stored with the same value internally, the
server can generate prefixed values (as shown in the examples below server can generate prefixed values (as shown in the examples below
with M and T prefixes) to avoid clashes. with M and T prefixes) to avoid clashes.
5.3. New Message Data Items in FETCH and UID FETCH Commands 5.3. New Message Data Items in FETCH and UID FETCH Commands
This document defines two FETCH items: This document defines two FETCH items:
Syntax: "EMAILID" Syntax: "EMAILID"
The EMAILID message data item causes the server to return EMAILID The EMAILID message data item causes the server to return EMAILID
FETCH response data items. FETCH response data items.
Syntax: "THREADID" Syntax: "THREADID"
The THREADID message data item causes the server to return THREADID The THREADID message data item causes the server to return THREADID
FETCH response data items. FETCH response data items.
And the following responses: This document defines the following responses:
Syntax: "EMAILID" SP "(" objectid ")" Syntax: "EMAILID" SP "(" objectid ")"
The EMAILID response data item contains the server-assigned objectid The EMAILID response data item contains the server-assigned ObjectID
for each message. for each message.
Syntax: "THREADID" SP "(" objectid ")" Syntax: "THREADID" SP "(" objectid ")"
The THREADID response data item contains the server-assigned objectid The THREADID response data item contains the server-assigned ObjectID
for the set of related messages to which this message belongs. for the set of related messages to which this message belongs.
Syntax: "THREADID" SP nil Syntax: "THREADID" SP nil
The NIL value to the THREADID response data item is returned when
the server mailbox does not support THREADID calculation. The NIL value is returned for the THREADID response data item when
the server mailbox does not support THREADID calculation.
Example: Example:
C: 5 append inbox "20-Mar-2018 03:07:37 +1100" {733} C: 5 append inbox "20-Mar-2018 03:07:37 +1100" {733}
[...] [...]
Subject: Message A Subject: Message A
Message-ID: <fake.1521475657.54797@example.com> Message-ID: <fake.1521475657.54797@example.com>
[...] [...]
S: 5 OK [APPENDUID 1521475658 1] Completed S: 5 OK [APPENDUID 1521475658 1] Completed
skipping to change at page 10, line 4 skipping to change at page 9, line 6
C: 25 select "foo" C: 25 select "foo"
C: 25 select "foo" C: 25 select "foo"
[...] [...]
S: 25 OK [READ-WRITE] Completed S: 25 OK [READ-WRITE] Completed
C: 26 fetch 1:* (emailid threadid) C: 26 fetch 1:* (emailid threadid)
S: * 1 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) S: * 1 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9))
S: 26 OK Completed (0.000 sec) S: 26 OK Completed (0.000 sec)
Example: (no THREADID support) Example: (no THREADID support)
C: 26 fetch 1:* (emailid threadid) C: 26 fetch 1:* (emailid threadid)
S: * 1 FETCH (EMAILID (M00000001) THREADID NIL) S: * 1 FETCH (EMAILID (M00000001) THREADID NIL)
S: * 2 FETCH (EMAILID (M00000002) THREADID NIL) S: * 2 FETCH (EMAILID (M00000002) THREADID NIL)
S: 26 OK Completed (0.000 sec) S: 26 OK Completed (0.000 sec)
6. New Filters on SEARCH command 6. New Filters on SEARCH Command
This document defines filters EMAILID and THREADID on the SEARCH This document defines the filters EMAILID and THREADID on the SEARCH
command. command.
Syntax: "EMAILID" SP objectid Syntax: "EMAILID" SP objectid
Messages whose EMAILID is exactly the specified objectid. Messages whose EMAILID is exactly the specified ObjectID.
Syntax: "THREADID" SP objectid Syntax: "THREADID" SP objectid
Messages whose THREADID is exactly the specified objectid. Messages whose THREADID is exactly the specified ObjectID.
Example: (as if run before the MOVE above when the mailbox had 3 Example: (as if run before the MOVE shown above when the mailbox had
messages) three messages)
C: 27 search emailid M6d99ac3275bb4e C: 27 search emailid M6d99ac3275bb4e
S: * SEARCH 1 S: * SEARCH 1
S: 27 OK Completed (1 msgs in 0.000 secs) S: 27 OK Completed (1 msgs in 0.000 secs)
C: 28 search threadid T64b478a75b7ea9 C: 28 search threadid T64b478a75b7ea9
S: * SEARCH 1 2 S: * SEARCH 1 2
S: 28 OK Completed (2 msgs in 0.000 secs) S: 28 OK Completed (2 msgs in 0.000 secs)
7. Formal syntax 7. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) [RFC5234] notation. Elements not defined here can be Form (ABNF) [RFC5234] notation. Elements not defined here can be
found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and
IMAP ABNF extensions [RFC4466] specifications. IMAP ABNF extensions [RFC4466] specifications.
Except as noted otherwise, all alphabetic characters are case- Except as noted otherwise, all alphabetic characters are case
insensitive. The use of upper- or lowercase characters to define insensitive. The use of uppercase or lowercase characters to define
token strings is for editorial clarity only. Implementations MUST token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion. accept these strings in a case-insensitive fashion.
Please note specifically that objectid values are case sensitive. Please note specifically that ObjectID values are case sensitive.
capability =/ "OBJECTID" capability =/ "OBJECTID"
fetch-att =/ "EMAILID" / "THREADID" fetch-att =/ "EMAILID" / "THREADID"
fetch-emailid-resp = "EMAILID" SP "(" objectid ")" fetch-emailid-resp = "EMAILID" SP "(" objectid ")"
; follows tagged-ext production from [@!RFC4466] ; follows tagged-ext production from [RFC4466]
fetch-threadid-resp = "THREADID" SP ( "(" objectid ")" / nil ) fetch-threadid-resp = "THREADID" SP ( "(" objectid ")" / nil )
; follows tagged-ext production from [@!RFC4466] ; follows tagged-ext production from [RFC4466]
msg-att-static =/ fetch-emailid-resp / fetch-threadid-resp msg-att-static =/ fetch-emailid-resp / fetch-threadid-resp
objectid = 1*255(ALPHA / DIGIT / "_" / "-") objectid = 1*255(ALPHA / DIGIT / "_" / "-")
; characters in object identifiers are case ; characters in object identifiers are case
; significant ; significant
resp-text-code =/ "MAILBOXID" SP "(" objectid ")" resp-text-code =/ "MAILBOXID" SP "(" objectid ")"
; incorporated before the expansion rule of ; incorporated before the expansion rule of
; atom [SP 1*<any TEXT-CHAR except "]">] ; atom [SP 1*<any TEXT-CHAR except "]">]
; that appears in [@!RFC3501] ; that appears in [RFC3501]
search-key =/ "EMAILID" SP objectid / "THREADID" SP objectid search-key =/ "EMAILID" SP objectid / "THREADID" SP objectid
status-att =/ "MAILBOXID" status-att =/ "MAILBOXID"
status-att-value =/ "MAILBOXID" SP "(" objectid ")" status-att-val =/ "MAILBOXID" SP "(" objectid ")"
; follows tagged-ext production from [@!RFC4466] ; follows tagged-ext production from [RFC4466]
8. Implementation considerations 8. Implementation Considerations
8.1. Assigning object identifiers 8.1. Assigning Object Identifiers
All objectid values are allocated by the server. All ObjectID values are allocated by the server.
In the interests of reducing the possibilities of encoding mistakes, In the interest of reducing the possibilities of encoding mistakes,
objectids are restricted to a safe subset of possible byte values, ObjectIDs are restricted to a safe subset of possible byte values; in
and in order to allow clients to allocate storage, they are order to allow clients to allocate storage, they are restricted in
restricted in length. length.
An objectid is a string of 1 to 255 characters from the following set 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 of 64 codepoints: a-z, A-Z, 0-9, _, -. These characters are safe to
to use in almost any context (e.g. filesystems, URIs, IMAP atoms). use in almost any context (e.g., filesystems, URIs, IMAP atoms).
These are the same characters defined as base64url in [RFC4648]. These are the same characters defined as base64url in [RFC4648].
For maximum safety, servers should also follow defensive allocation For maximum safety, servers should also follow defensive allocation
strategies to avoid creating risks where glob completion or data type strategies to avoid creating risks where glob completion or data type
detection may be present (e.g. on filesystems or in spreadsheets). detection may be present (e.g., on filesystems or in spreadsheets).
In particular it is wise to avoid: In particular, it is wise to avoid:
o ids starting with - o IDs starting with a dash
o ids starting with digits o IDs starting with digits
o ids which contain only digits o IDs that contain only digits
o ids which differ only by ASCII case (A vs a) o IDs that differ only by ASCII case (for example, A vs. a)
o the specific sequence of 3 characters NIL in any case (as this o the specific sequence of three characters NIL in any case (because
sequence can be confused with the IMAP protocol expression of the this sequence can be confused with the IMAP protocol expression of
null value) the null value)
A good solution to these issues is to prefix every ID with a single A good solution to these issues is to prefix every ID with a single
alphabetical character. alphabetical character.
8.2. Interaction with special cases 8.2. Interaction with Special Cases
The case of RENAME INBOX may need special handling, as it has special The case of RENAME INBOX may need special handling because it has
behaviour as defined in [RFC3501] section 6.3.5. special behavior, as defined in [RFC3501], Section 6.3.5.
It is advisable (though not required) to have MAILBOXID be globally It is advisable (though not required) to have MAILBOXID be globally
unique, but it is only required to be unique within messages offered 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 to a single client login to a single server hostname. For example, a
proxy which aggregates multiple independent servers MUST NOT proxy that aggregates multiple independent servers MUST NOT advertise
advertise the OBJECTID capability unless it can guarantee that the OBJECTID capability unless it can guarantee that different
different objects will never use the same identifiers, even if objects will never use the same identifiers, even if backend object
backend object identifiers collide. identifiers collide.
8.3. Client usage 8.3. Client Usage
Servers that implement both RFC 6154 and this specification should Servers that implement both RFC 6154 and this specification should
optimize their execution of command like UID SEARCH OR EMAILID 1234 optimize their execution of commands like UID SEARCH OR EMAILID 1234
EMAILID 4321. EMAILID 4321.
Clients can assume that searching the all-mail mailbox using OR/ 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 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 other client has moved them out of the mailbox where they were
previously seen. previously seen.
Clients that cache data offline should fetch the EMAILID of all new Clients that cache data offline should fetch the EMAILID of all new
messages to avoid re-downloading already cached message details. messages to avoid redownloading already-cached message details.
Clients should fetch the MAILBOXID for any new mailboxes before Clients should fetch the MAILBOXID for any new mailboxes before
discarding cache data for any mailbox which is no longer present on discarding cache data for any mailbox that is no longer present on
the server, so that they can detect renames and avoid re-downloading the server so that they can detect renames and avoid redownloading
data. data.
9. Future considerations 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
This extension is intentionally defined to be compatible with the This extension is intentionally defined to be compatible with the
data model in [I-D.ietf-jmap-mail]. data model in [JMAP-MAIL].
A future extension could be proposed to give a way to SELECT a A future extension could be proposed to give a way to SELECT a
mailbox by MAILBOXID rather than name. mailbox by MAILBOXID rather than name.
A future extension to [RFC5228] could allow fileinto by MAILBOXID A future extension to [RFC5228] could allow fileinto by MAILBOXID
rather than name. rather than name.
An extension to allow fetching message content directly via EMAILID An extension to allow fetching message content directly via EMAILID
and message listings by THREADID could be proposed. and message listings by THREADID could be proposed.
10. IANA Considerations 10. IANA Considerations
IANA is requested to add "OBJECTID" to the "IMAP Capabilities" IANA has added "OBJECTID" to the "IMAP Capabilities" registry located
registry located at <http://www.iana.org/assignments/imap- at <https://www.iana.org/assignments/imap-capabilities> with a
capabilities> with a Reference of [[THIS RFC]]. reference to this document.
IANA is requested to add "MAILBOXID" to the "IMAP Response Codes" IANA has added "MAILBOXID" to the "IMAP Response Codes" registry
registry located at <https://www.iana.org/assignments/imap-response- located at <https://www.iana.org/assignments/imap-response-codes>
codes> with a Reference of [[THIS RFC]]. with a reference to this document.
11. Security Considerations 11. Security Considerations
It is strongly advised that servers generate OBJECTIDs which are safe It is strongly advised that servers generate ObjectIDs that are safe
to use as filesystem names, and unlikely to be auto-detected as to use as filesystem names and unlikely to be autodetected as
numbers. See implementation considerations. numbers. See implementation considerations.
If a digest is used for ID generation, it must have a collision If a digest is used for ID generation, it must have a collision-
resistent property, so server implementations are advised to monitor resistant property, so server implementations are advised to monitor
current security research and choose secure digests. As the IDs are 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 generated by the server, it will be possible to migrate to a new hash
by just using the new algorith when creating new IDs. This is by just using the new algorithm when creating new IDs. This is
particularly true if a prefix is used on each ID, which can be particularly true if a prefix is used on each ID, which can be
changed when the algorithm changes. changed when the algorithm changes.
The use of a digest for ID generation may be used as proof that a 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 this is particular sequence of bytes was seen by the server. However, this
only a risk if IDs are leaked to clients who don't have permission to is only a risk if IDs are leaked to clients who don't have permission
fetch the data directly. Servers that are expected to handle highly to fetch the data directly. Servers that are expected to handle
sensitive data should consider this when choosing how to create IDs. highly sensitive data should consider this when choosing how to
create IDs.
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-08
o added reference to RFC4648 for base64url (Adam Roach review)
o less prescriptive instruction for digests (Adam Roach review)
o minor punctuation fix (Alissa Cooper review)
o clarified SHOULD around THREADID in different mailboxes (Alissa
Cooper review)
o clarified that objectids are case sensitive (Eric Rescorla review)
o clarified why NIL is a bad objectid (Eric Rescorla review)
o upper case OPTIONAL for threadid (Pete Resnick genart review)
o missing word in object identifiers clash (Pete Resnick genart
review)
12.2. draft-ietf-extra-imap-objectid-07
o updated boilerplate to RFC8174 (Benjamin Kaduk review)
o fixed spelling of invariants (Benjamin Kaduk review)
o block quoted ABNF for better text formatting (Benjamin Kaduk
review)
o clarified that servers can just switch to a new digest without
changing old IDs (Benjamin Kaduk review)
o changed use of folder to mailbox to avoid confusion (Warren Kumari
review)
o made both IANA requests say "reference of this RFC" (Warren Kumari
review)
12.3. draft-ietf-extra-imap-objectid-06
o fixed one more missing space in ABNF (ad review)
o made one more MUST for mailbox being retained on rename (genart
review)
o updated ABNF to also extend msg-att-static (validator review)
o lowercased NIL => nil in ABNF (validator review)
12.4. draft-ietf-extra-imap-objectid-05
o changed some SHOULD to lower case in advice sections (genart
review)
o clarified that THREADID MUST NOT change
12.5. 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)
12.6. 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
12.7. draft-ietf-extra-imap-objectid-02
o added "Client usage" section
12.8. 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
12.9. 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
12.10. 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
12.11. 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
12.12. 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 calculating MAILBOXID:
o [RFC4122] UUID
o 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
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) See also the security considerations in [RFC3501], Section 11.
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, or within each mailbox
only. The server has significant leeway here.
14. References 12. References
14.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>. <https://www.rfc-editor.org/info/rfc3501>.
skipping to change at page 19, line 14 skipping to change at page 14, line 29
[RFC6851] Gulbrandsen, A. and N. Freed, Ed., "Internet Message [RFC6851] Gulbrandsen, A. and N. Freed, Ed., "Internet Message
Access Protocol (IMAP) - MOVE Extension", RFC 6851, Access Protocol (IMAP) - MOVE Extension", RFC 6851,
DOI 10.17487/RFC6851, January 2013, DOI 10.17487/RFC6851, January 2013,
<https://www.rfc-editor.org/info/rfc6851>. <https://www.rfc-editor.org/info/rfc6851>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
14.2. Informative References 12.2. Informative References
[I-D.ietf-jmap-mail] [JMAP-MAIL]
Jenkins, N., "JMAP for Mail", draft-ietf-jmap-mail-06 Jenkins, N. and C. Newman, "JMAP for Mail", Work in
(work in progress), July 2018. Progress, draft-ietf-jmap-mail-07, August 2018.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122, Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005, DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>. <https://www.rfc-editor.org/info/rfc4122>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>. <https://www.rfc-editor.org/info/rfc4648>.
Appendix A. Ideas for Implementing Object Identifiers
Ideas for calculating MAILBOXID:
o Universally Unique Identifier (UUID) [RFC4122]
o Server-assigned sequence number (guaranteed not to be reused)
Ideas for implementing EMAILID:
o Digest of message content (RFC822 bytes) -- expensive unless
cached
o UUID [RFC4122]
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 UUID [RFC4122]
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 mailboxes or within each mailbox only.
The server has significant leeway here.
Acknowledgments
The author would like to thank the EXTRA working group at IETF for
feedback and advice -- in particular, Arnt Gulbrandsen, Brandon Long,
Chris Newman, and Josef Sipek.
This document drew inspiration from the Gmail X-GM-THRID and X-GM-
MSGID implementations as currently defined at
<https://developers.google.com/gmail/imap/imap-extensions>, as well
as the X-GUID implementation in the Dovecot server.
Author's Address Author's Address
Bron Gondwana (editor) Bron Gondwana (editor)
FastMail FastMail
Level 2, 114 William St Level 2, 114 William St
Melbourne VIC 3000 Melbourne VIC 3000
Australia Australia
Email: brong@fastmailteam.com Email: brong@fastmailteam.com
URI: https://www.fastmail.com URI: https://www.fastmail.com
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