IMAP Extension for object identifiers

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. 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. defines a COPYUID response which allows a client which copies messages to know the mapping between the UIDs in the source and destination mailboxes, and hence update its local cache. If a mailbox is successfully renamed 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 copies (or moves) messages or renames a mailbox can update its local cache, but any other client connected to the same store can not know with certainty that the messages are identical, and so will re-download everything. This extension adds new properties to a message (EMAILID) and mailbox (MAILBOXID) which allow a client to quickly identify messages or mailboxes which have been renamed by another client. This extension also adds an optional thread identifier (THREADID) to messages, which can be used by the server to indicate messages which it has identified to be related. A server that does not implement threading will return NIL to all requests for THREADID.

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 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.

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

The MAILBOXID is a server-allocated unique identifer 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 does not obey all the invarients that defines for a mailbox which does not change name or UIDVALIDITY. The server SHOULD NOT change MAILBOXID when renaming a folder, as this loses the main benefit of having a unique identifier.

This document extends the CREATE command to have the response code MAILBOXID on successful mailbox creation. A server advertising the OBJECTID capability MUST include the MAILBOXID response code in the tagged OK response to all successful CREATE commands. Syntax: "MAILBOXID" SP "(" <objectid> ")"

Response code in tagged OK for successful CREATE command. Example:

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

This document adds a new untagged response code to the SELECT and EXAMINE commands. A server advertising the OBJECTID capability MUST return an untagged OK response with the MAILBOXID response code on all successful SELECT and EXAMINE commands. Syntax: "OK" SP "[" "MAILBOXID" SP "(" <objectid> ")" "]" text

Untagged OK response to SELECT or EXAMINE. Example:

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

This document adds the MAILBOXID attribute to the STATUS command using the extended syntax defined in . A server that advertises the OBJECTID capability MUST support the MAILBOXID status attribute. Syntax: "MAILBOXID"

The attribute in the STATUS command. Syntax: "MAILBOXID" SP "(" <objectid> ")"

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

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

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 MUST return the same EMAILID for the same 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 MOVE command. The server MAY assign the same EMAILID as an existing message upon APPEND (e.g. if it detects that the new message has exactly identical content to that of an existing message) NOTE: EMAILID only identifies the immutable content of the message. In particular, it is possible for different messages with the same EMAILID to have different keywords. This document does not specify a way to STORE by EMAILID.

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. describes some algorithms that could be used, however this specfication does not mandate any particular strategy. The server MUST return the same THREADID for all messages with the same EMAILID. The server SHOULD return the same THREADID for related messages even if they are in different mailboxes. THREADID is optional, if the server doesn't support THREADID or is unable to calculate relationships between messages, it MUST return NIL to 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 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.

This document defines two FETCH items: Syntax: EMAILID

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

The THREADID message data item causes the server to return THREADID FETCH response data items. And the following responses: Syntax: EMAILID ( <objectid> )

The EMAILID response data item contains the server-assigned objectid for each message. Syntax: THREADID ( <objectid> )

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

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

C: 5 append inbox "20-Mar-2018 03:07:37 +1100" {733} [...] Subject: Message A Message-ID: <fake.1521475657.54797@example.com> [...] S: 5 OK [APPENDUID 1521475658 1] Completed C: 11 append inbox "20-Mar-2018 03:07:37 +1100" {793} [...] Subject: Re: Message A Message-ID: <fake.1521475657.21213@example.org> References: <fake.1521475657.54797@example.com> [...] S: 11 OK [APPENDUID 1521475658 2] Completed C: 17 append inbox "20-Mar-2018 03:07:37 +1100" {736} [...] Subject: Message C Message-ID: <fake.1521475657.60280@example.com> [...] S: 17 OK [APPENDUID 1521475658 3] Completed C: 22 fetch 1:* (emailid threadid) S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9)) S: * 2 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) S: * 3 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5)) S: 22 OK Completed (0.000 sec) C: 23 move 2 foo S: * OK [COPYUID 1521475659 2 1] Completed S: * 2 EXPUNGE S: 23 OK Completed C: 24 fetch 1:* (emailid threadid) S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9)) S: * 2 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5)) S: 24 OK Completed (0.000 sec) C: 25 select "foo" C: 25 select "foo" [...] S: 25 OK [READ-WRITE] Completed C: 26 fetch 1:* (emailid threadid) S: * 1 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) S: 26 OK Completed (0.000 sec) Example: (no THREADID support)

C: 26 fetch 1:* (emailid threadid) S: * 1 FETCH (EMAILID (M00000001) THREADID NIL) S: * 2 FETCH (EMAILID (M00000002) THREADID NIL) S: 26 OK Completed (0.000 sec)

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

Messages whose EMAILID is exactly the specified objectid. THREADID <objectid>

Messages whose THREADID is exactly the specified objectid. Example: (as if run before the MOVE above when the mailbox had 3 messages)

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

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation. Elements not defined here can be found in the formal syntax of the ABNF , IMAP , and IMAP ABNF extensions 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 fetch-threadid-resp = "THREADID" SP "(" objectid ")" / "THREADID" NIL ; follows tagged-ext production from 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 search-key =/ "EMAILID" SP objectid / "THREADID" SP objectid status-att =/ "MAILBOXID" status-att-value =/ "MAILBOXID" SP "(" objectid ")" ; follows tagged-ext production from

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: ids starting with - ids starting with digits ids which contain only digits ids which differ only by ASCII case (A vs a) the specific sequence of 3 characters NIL A good solution to these issues is to prefix every ID with a single alphabetical character.

The case of RENAME INBOX may need special handling, as it has special behaviour as defined in section 6.3.5. It is advisable (though not required) to have MAILBOXID be globally unique, but it is only required to be unique within messages offered to a single client login to a single server hostname. For example, a proxy which aggregates multiple independent servers MUST NOT advertise the OBJECTID capability unless it can guarantee that different objects will never use the same identifiers, even if backend object collide.

Servers that implement both RFC 6154 and this specification SHOULD optimize their execution of command like UID SEARCH OR EMAILID 1234 EMAILID 4321. Clients can assume that searching the all-mail mailbox using OR/EMAILID or OR/THREADID is a fast way to find messages again if some other client has moved them out of the mailbox where they were previously seen. Clients that cache data offline SHOULD fetch the EMAILID of all new messages to avoid re-downloading already cached message details. Clients SHOULD fetch the MAILBOXID for any new mailboxes before discarding cache data for any mailbox which is no longer present on the server, so that they can detect renames and avoid re-downloading data.

This extension is intentionally defined to be compatible with the data model in . A future extension could be proposed to give a way to SELECT a mailbox by MAILBOXID rather than name. A future extension to 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.

IANA is requested to add "OBJECTID" to the "IMAP Capabilities" registry located at . IANA is requested to add "MAILBOXID" to the "IMAP Response Codes" registry located at with a Reference of [[THIS RFC]].

It is strongly advised that servers generate OBJECTIDs which are safe to use as filesystem names, and unlikely to be auto-detected as numbers. See implementation considerations. If a digest is used for ID generation, it must have a collision resistent property, so server implementations are advised to monitor current security research and choose secure digests. As the IDs are generated by the server, it will be possible to migrate to a new hash by just creating new IDs with the new algorithm. This is particularly true if a prefix is used on each ID, which can be changed when the algorithm changes. The use of a digest for ID generation may be used as proof that a particular sequence of bytes was seen by the server, however this is only a risk if IDs are leaked to clients who don't have permission to fetch the data directly. Servers that are expected to handle highly sensitive data should consider using a ID generation mechanism which doesn't derive from a digest. See also the security considerations in section 11.

To be removed by the editor before publication

described NIL THREADID in more detail (ad review) made RFC5256 a normative reference (ad review) fixed ABNF missing quote (ad review) documented hash upgrade process (ad review) referenced RFC3501 for INBOX rename (ad review) referenced RFC3501 security considerations (secdir review) turned mealy-mouthed "SHOULDs" in to "MUSTs" on immutability (genart review) remove suggested algorithms which are no longer legitimate (genart review) updated proxy advice to suggest rewriting ids (genart review) fixed minor gramatical issues (genart review) required that EMAILID and THREADID are not identical (own decision)

added RFC3501 to Abstract updated [[THIS RFC]] to not fail idnits changed jmap-mail to be informative rather than normative shortened IDs to stop wrapping and outdents in IMAP examples

added "Client usage" section

added "updates" for RFC3501 fixed domains in thread example described threading in more detail added IANA request for Response Code clarified RFC2119 references simplified some waffle in wording added security consideration to choose good digest added MAILBOXID-UID suggestion for EMAILID generation updated ABNF normative reference to RFC5234

renamed draft to be objectid rather than uniqueid renamed UNIQUEID (capability) to OBJECTID restricted objectid to 64 safe characters added security considerations and advice about choosing objectid wrapped all responses in () for RFC4466 compatibility signifiant rewrite of all sections

renamed draft to be an EXTRA document added example for LIST RETURN STATUS started work on ABNF attempted to add response codes for EMAILID and THREADID

renamed UNIQUEID (status item) to MAILBOXID renamed MSGID to EMAILID renamed THRID to THREADID added TODO section

initial upload with names UNIQUEID/MSGID/THRID

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 . Dovecot X-GUID implementation.

Ideas for calculating MAILBOXID: UUID Server assigned sequence number (guaranteed not to be reused) Ideas for implementing EMAILID: Digest of message content (RFC822 bytes) - expensive unless cached UUID Server assigned sequence number (guaranteed not to be reused) Ideas for implementing THREADID: Derive from EMAILID of first seen message in the thread. UUID 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.