wdiff rfc9698v3.txt rfc9698.txt

Internet Engineering Task Force (IETF) A. Gulbrandsen
Request for Comments: 9698 ICANN
Category: Standards Track B. Gondwana
ISSN: 2070-1721 Fastmail
December 2024

The JMAPACCESS Extension for IMAP

Abstract

This document defines an IMAP extension to let clients know that the
messages in this IMAP server are also available via the JSON Meta
Application Protocol (JMAP), and how. It is intended for clients
that want to migrate gradually to JMAP or use JMAP extensions within
an IMAP client.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for 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 this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9698.

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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. Requirements Language
3. Details
4. The GETJMAPACCESS Command and the JMAPACCESS Response
5. Examples
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Authors' Addresses

1. Introduction

An IMAP server can declare that the messages in its mailstore are
also available via JMAP. For simplicity, only a complete equivalence
is supported (the same set of messages are available via both IMAP
and JMAP).

2. Requirements Language

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. Details

By advertising the JMAPACCESS capability, the server asserts that if
a mailbox or message has a particular object ID when accessed via
either IMAP or JMAP (see [RFC3501], [RFC9051], and [RFC8620]), then
the same mailbox or message is accessible via the other protocol, and
it has the same ID.

The server MUST also advertise the OBJECTID extension, defined by
[RFC8474]. The JMAP session resource that allows access to the same
messages is called "the JMAP server" below.

This specification does not affect message lifetime: If a client
accesses a message via IMAP and half a second later via JMAP, then
the message may have been deleted between the two accesses.

When the server processes the client's LOGIN/AUTHENTICATE command and
enters Authenticated state, the server considers the way the client
authenticated. If the IMAP server can infer from the client's
authentication process that its credentials suffice to authenticate
via JMAP, then the server MUST include a JMAPACCESS capability in any
capability list sent after that point. This includes the capability
list that some servers send immediately when authentication succeeds.

Servers are encouraged to report the same message flags and other
data via both protocols, as far as possible.

This specification does not require mailboxes to have the same name
in IMAP and JMAP, even if they share a mailbox ID. However, the JMAP
specification regulates that in the text about the name and role
properties described in Section 2 of [RFC8620].

Note that all JMAP servers support internationalized email addresses
(see [RFC6530]). If this IMAP server does not or if the IMAP client
does not issue ENABLE UTF8=ACCEPT (see [RFC6855]), then it is
possible that the client will receive accurate address fields via
JMAP and downgraded fields via IMAP (see [RFC6857] and [RFC6858] for
examples). Issuing ENABLE UTF8=ACCEPT is a simple way to sidestep
the issue.

4. The GETJMAPACCESS Command and the JMAPACCESS Response

The GETJMAPACCESS command requests that the server respond with the
session URL for the JMAP server that provides access to the same
mail.

If such a JMAP server is known to this server, the server MUST
respond with an untagged JMAPACCESS response containing the JMAP
server's session resource (a URL) followed by a tagged OK response.

If such a JMAP server is not known, the server MUST respond with a
tagged BAD response (and MUST NOT include JMAPACCESS in the
capability list).

The JMAPACCESS response is followed by a single link to a JMAP
session resource.

The formal syntax in [RFC9051] is extended as follows:

command-auth =/ "GETJMAPACCESS"

mailbox-data =/ resp-jmapaccess

resp-jmapaccess = "JMAPACCESS" SP quoted

The syntax in [RFC3501] is extended similarly (this extension may be
used with IMAP4rev1 as well as IMAP4rev2).

5. Examples

Lines sent by the client are preceded by C: and lines sent by the
server are preceded by S:. Each example starts with the IMAP banner
issued by the server on connection, and generally abbreviates the
capability lists to what's required by the example itself.

Real connections use longer capability lists, much longer
AUTHENTICATE arguments and of course use TLS. However, these
examples focus on JMAPACCESS.

Example 1:

A client connects, sees that SASL OAuth [RFC7628] is available, and
authenticates in that way.

S: * OK [CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER SASL-IR] example1
C: 1 AUTHENTICATE OAUTHBEARER bixhPXVzZ...QEB

The server processes the command successfully. It knows that the
client used OAuth, and that it and its JMAP alter ego use the same
OAuth backend subsystem. Because of that it infers that the (next)
access token is just as usable via JMAP as via IMAP. It includes a
JMAPACCESS capability in its reply (again, real capability lists are
much longer):

S: 1 OK [CAPABILITY IMAP4rev1 JMAPACCESS] done
C: 1b GETJMAPACCESS
S: * JMAPACCESS "https://example.com/jmap" "https://example.com/.well-known/jmap"
S: 1b OK done

SASL OAuth is specified by [RFC7628], and the argument in this
example is abbreviated from the more realistic length used in RFC
7628.

Example 2:

A client connects, sees no SASL method it recognizes, and issues a
LOGIN command.

S: * OK [CAPABILITY IMAP4rev2] example2
C: 2 LOGIN "arnt" "trondheim"

The server sees that the password is accepted, knows that it and its
JMAP alter ego use the same password database, and issues a
JMAPACCESS capability:

S: * OK [CAPABILITY IMAP4rev2 JMAPACCESS] done
S: 2 OK done
C: 2b JMAPACCESS
S: * JMAPACCESS "https://example.com/.s/[jmap]" "https://example.com/.well-known/jmap"
S: 2b OK done

The URL uses the same quoting rules as most other IMAP strings.

Example 3:

A client connects, sees no SASL method it recognizes, and issues a
LOGIN command with a correct password.

S: * OK [CAPABILITY IMAP4rev1 IMAP4rev2] example3
C: 3 LOGIN "arnt" "trondheim"

The server operator has decided to disable password use with JMAP,
but allow it for a while with IMAP to cater to older clients.
Therefore, the login succeeds, but there is no JMAPACCESS capability.

S: 3 OK done

Example 4:

A client connects, sees no SASL method it recognizes, and issues a
LOGIN command. Its password is incorrect.

S: * OK [CAPABILITY IMAP4rev2 AUTH=GSS] example4
C: 4 LOGIN "arnt" "oslo"

The server does not enter Authenticated state, so nothing requires it
to mention JMAPACCESS. It replies curtly:

S: 4 NO done

6. IANA Considerations

The IANA has added the JMAPACCESS capability to the "Internet Message
Access Protocol (IMAP) Capabilities Registry" and listed this
document as the reference.

7. Security Considerations

JMAPACCESS reveals to authenticated IMAP clients that they would be
able to authenticate via JMAP using the same credentials and that the
object IDs match.

One does not normally reveal anything at all about authentication.
However, if the client is an attacker, then the attacker is known to
have valid credentials, and Section 2.2 of [RFC8620] tells the
attacker how to find the revealed URL without the help of this
extension. Therefore, it is believed that this document does not
benefit an attacker noticeably, and its value for migration far
outweighs its risk.

8. References

8.1. 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>.

[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>.

[RFC8474] Gondwana, B., Ed., "IMAP Extension for Object
Identifiers", RFC 8474, DOI 10.17487/RFC8474, September
2018, <https://www.rfc-editor.org/info/rfc8474>.

[RFC9051] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
Access Protocol (IMAP) - Version 4rev2", RFC 9051,
DOI 10.17487/RFC9051, August 2021,
<https://www.rfc-editor.org/info/rfc9051>.

8.2. Informative References

[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
February 2012, <https://www.rfc-editor.org/info/rfc6530>.

[RFC6855] Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
Support for UTF-8", RFC 6855, DOI 10.17487/RFC6855, March
2013, <https://www.rfc-editor.org/info/rfc6855>.

[RFC6857] Fujiwara, K., "Post-Delivery Message Downgrading for
Internationalized Email Messages", RFC 6857,
DOI 10.17487/RFC6857, March 2013,
<https://www.rfc-editor.org/info/rfc6857>.

[RFC6858] Gulbrandsen, A., "Simplified POP and IMAP Downgrading for
Internationalized Email", RFC 6858, DOI 10.17487/RFC6858,
March 2013, <https://www.rfc-editor.org/info/rfc6858>.

[RFC7628] Mills, W., Showalter, T., and H. Tschofenig, "A Set of
Simple Authentication and Security Layer (SASL) Mechanisms
for OAuth", RFC 7628, DOI 10.17487/RFC7628, August 2015,
<https://www.rfc-editor.org/info/rfc7628>.

[RFC8620] Jenkins, N. and C. Newman, "The JSON Meta Application
Protocol (JMAP)", RFC 8620, DOI 10.17487/RFC8620, July
2019, <https://www.rfc-editor.org/info/rfc8620>.

Authors' Addresses

Arnt Gulbrandsen
ICANN
6 Rond Point Schumann, Bd. 1
1040 Brussels
Belgium
Email: arnt@gulbrandsen.priv.no
URI: https://icann.org/ua

Bron Gondwana
Fastmail
Level 2, 114 William St.
Melbourne VIC 3000
Australia
Email: brong@fastmailteam.com
URI: https://fastmail.com