Network Working Group M. Gahrns

Network Working Group M. Gahrns
Request for Comments: 2193 Microsoft
Category: Standards Track September 1997

IMAP4 Mailbox Referrals

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

1. Abstract

When dealing with large amounts of users, messages and geographically
dispersed IMAP4 [RFC-2060] servers, it is often desirable to
distribute messages amongst different servers within an organization.
For example an administrator may choose to store user's personal
mailboxes on a local IMAP4 server, while storing shared mailboxes
remotely on another server. This type of configuration is common
when it is uneconomical to store all data centrally due to limited
bandwidth or disk resources.

Mailbox referrals allow clients to seamlessly access mailboxes that
are distributed across several IMAP4 servers.

A referral mechanism can provide efficiencies over the alternative
"proxy method", in which the local IMAP4 server contacts the remote
server on behalf of the client, and then transfers the data from the
remote server to itself, and then on to the client. The referral
mechanism's direct client connection to the remote server is often a
more efficient use of bandwidth, and does not require the local
server to impersonate the client when authenticating to the remote
server.

2. Conventions used in this document

In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.

A home server, is an IMAP4 server that contains the user's inbox.

A remote mailbox is a mailbox that is not hosted on the user's home
server.

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RFC 2193 IMAP4 Mailbox Referrals September 1997

A remote server is a server that contains remote mailboxes.

A shared mailbox, is a mailbox that multiple users have access to.

An IMAP mailbox referral is when the server directs the client to
another IMAP mailbox.

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 [RFC-2119].

3. Introduction and Overview

IMAP4 servers that support this extension MUST list the keyword
MAILBOX-REFERRALS in their CAPABILITY response. No client action is
needed to invoke the MAILBOX-REFERRALS capability in a server.

A MAILBOX-REFERRALS capable IMAP4 server MUST NOT return referrals
that result in a referrals loop.

A referral response consists of a tagged NO response and a REFERRAL
response code. The REFERRAL response code MUST contain as an
argument a one or more valid URLs separated by a space as defined in
[RFC-1738]. If a server replies with multiple URLs for a particular
object, they MUST all be of the same type. In this case, the URL MUST
be an IMAP URL as defined in [RFC-2192]. A client that supports the
REFERRALS extension MUST be prepared for a URL of any type, but it
need only be able to process IMAP URLs.

A server MAY respond with multiple IMAP mailbox referrals if there is
more than one replica of the mailbox. This allows the implementation
of a load balancing or failover scheme. How a server keeps multiple
replicas of a mailbox in sync is not addressed by this document.

If the server has a preferred order in which the client should
attempt to access the URLs, the preferred URL SHOULD be listed in the
first, with the remaining URLs presented in descending order of
preference. If multiple referrals are given for a mailbox, a server
should be aware that there are synchronization issues for a client if
the UIDVALIDITY of the referred mailboxes are different.

An IMAP mailbox referral may be given in response to an IMAP command
that specifies a mailbox as an argument.

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RFC 2193 IMAP4 Mailbox Referrals September 1997

Example:

A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/REMOTE]Remote Mailbox

NOTE: user;AUTH=* is specified as required by [RFC-2192] to avoid a
client falling back to anonymous login.

Remote mailboxes and their inferiors, that are accessible only via
referrals SHOULD NOT appear in LIST and LSUB responses issued against
the user's home server. They MUST appear in RLIST and RLSUB
responses issued against the user's home server. Hierarchy referrals,
in which a client would be required to connect to the remote server
to issue a LIST to discover the inferiors of a mailbox are not
addressed in this document.

For example, if shared mailboxes were only accessible via referrals
on a remote server, a RLIST "" "#SHARED/%" command would return the
same response if issued against the user's home server or the remote
server.

Note: Mailboxes that are available on the user's home server do not
need to be available on the remote server. In addition, there may be
additional mailboxes available on the remote server, but they will
not accessible to the client via referrals unless they appear in the
LIST response to the RLIST command against the user's home server.

A MAILBOX-REFERRALS capable client will issue the RLIST and RLSUB
commands in lieu of LIST and LSUB. The RLIST and RLSUB commands
behave identically to their LIST and LSUB counterparts, except remote
mailboxes are returned in addition to local mailboxes in the LIST and
LSUB responses. This avoids displaying to a non MAILBOX-REFERRALS
enabled client inaccessible remote mailboxes.

4.1. SELECT, EXAMINE, DELETE, SUBSCRIBE, UNSUBSCRIBE, STATUS and APPEND
Referrals

An IMAP4 server MAY respond to the SELECT, EXAMINE, DELETE,
SUBSCRIBE, UNSUBSCRIBE, STATUS or APPEND command with one or more
IMAP mailbox referrals to indicate to the client that the mailbox is
hosted on a remote server.

When a client processes an IMAP mailbox referral, it will open a new
connection or use an existing connection to the remote server so that
it is able to issue the commands necessary to process the remote
mailbox.

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RFC 2193 IMAP4 Mailbox Referrals September 1997

Example: <IMAP4 connection to home server>

C: A001 DELETE "SHARED/FOO"
S: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/SHARED/FOO]
Remote mailbox. Try SERVER2.

<Client established a second connection to SERVER2 and
issues the DELETE command on the referred mailbox>

S: * OK IMAP4rev1 server ready
C: B001 AUTHENTICATE KERBEROS_V4
<authentication exchange>
S: B001 OK user is authenticated

C: B002 DELETE "SHARED/FOO"
S: B002 OK DELETE completed

Example: <IMAP4 connection to home server>

C: A001 SELECT REMOTE
S: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/REMOTE
IMAP://user;AUTH=*@SERVER3/REMOTE] Remote mailbox.
Try SERVER2 or SERVER3.

<Client opens second connection to remote server, and
issues the commands needed to process the items in the
remote mailbox>

S: * OK IMAP4rev1 server ready
C: B001 AUTHENTICATE KERBEROS_V4
<authentication exchange>
S: B001 OK user is authenticated

C: B002 SELECT REMOTE
S: * 12 EXISTS
S: * 1 RECENT
S: * OK [UNSEEN 10] Message 10 is first unseen
S: * OK [UIDVALIDITY 123456789]
S: * FLAGS (Answered Flagged Deleted Seen Draft)
S: * OK [PERMANENTFLAGS (Answered Deleted Seen ]
S: B002 OK [READ-WRITE] Selected completed

C: B003 FETCH 10:12 RFC822
S: * 10 FETCH . . .
S: * 11 FETCH . . .
S: * 12 FETCH . . .
S: B003 OK FETCH Completed

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RFC 2193 IMAP4 Mailbox Referrals September 1997

<Client is finished processing the REMOTE mailbox and
wants to process a mailbox on its home server>

C: B004 LOGOUT
S: * BYE IMAP4rev1 server logging out
S: B004 OK LOGOUT Completed

<Client continues with first connection>

C: A002 SELECT INBOX
S: * 16 EXISTS
S: * 2 RECENT
S: * OK [UNSEEN 10] Message 10 is first unseen
S: * OK [UIDVALIDITY 123456789]
S: * FLAGS (Answered Flagged Deleted Seen Draft)
S: * OK [PERMANENTFLAGS (Answered Deleted Seen ]
S: A002 OK [READ-WRITE] Selected completed

4.2. CREATE Referrals

An IMAP4 server MAY respond to the CREATE command with one or more
IMAP mailbox referrals, if it wishes to direct the client to issue
the CREATE against another server. The server can employ any means,
such as examining the hierarchy of the specified mailbox name, in
determining which server the mailbox should be created on.

Example:

C: A001 CREATE "SHARED/FOO"
S: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/SHARED/FOO]
Mailbox should be created on remote server

Alternatively, because a home server is required to maintain a
listing of referred remote mailboxes, a server MAY allow the creation
of a mailbox that will ultimately reside on a remote server against
the home server, and provide referrals on subsequent commands that
manipulate the mailbox.

Example:

C: A001 CREATE "SHARED/FOO"
S: A001 OK CREATE succeeded

C: A002 SELECT "SHARED/FOO"
S: A002 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/SHARED/FOO]
Remote mailbox. Try SERVER2

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RFC 2193 IMAP4 Mailbox Referrals September 1997

4.3. RENAME Referrals

An IMAP4 server MAY respond to the RENAME command with one or more
pairs of IMAP mailbox referrals. In each pair of IMAP mailbox
referrals, the first one is an URL to the existing mailbox name and
the second is an URL to the requested new mailbox name.

If within an IMAP mailbox referral pair, the existing and new mailbox
URLs are on different servers, the remote servers are unable to
perform the RENAME operation. To achieve the same behavior of
server RENAME, the client MAY issue the constituent CREATE, FETCH,
APPEND, and DELETE commands against both servers.

If within an IMAP mailbox referral pair, the existing and new mailbox
URLs are on the same server it is an indication that the currently
connected server is unable to perform the operation. The client can
simply re-issue the RENAME command on the remote server.

Example:

C: A001 RENAME FOO BAR
S: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER1/FOO
IMAP://user;AUTH=*@SERVER2/BAR] Unable to rename mailbox
across servers

Since the existing and new mailbox names are on different servers,
the client would be required to make a connection to both servers and
issue the constituent commands require to achieve the RENAME.

Example:

C: A001 RENAME FOO BAR
S: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/FOO
IMAP://user;AUTH=*@SERVER2/BAR] Unable to rename mailbox
located on SERVER2

Since both the existing and new mailbox are on the same remote
server, the client can simply make a connection to the remote server
and re-issue the RENAME command.

4.4. COPY Referrals

An IMAP4 server MAY respond to the COPY command with one or more IMAP
mailbox referrals. This indicates that the destination mailbox is on
a remote server. To achieve the same behavior of a server COPY, the
client MAY issue the constituent FETCH and APPEND commands against
both servers.

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RFC 2193 IMAP4 Mailbox Referrals September 1997

Example:

C: A001 COPY 1 "SHARED/STUFF"
S: A001 NO [REFERRAL IMAP://user;AUTH=*@SERVER2/SHARED/STUFF]
Unable to copy message(s) to SERVER2.

5.1 RLIST command

Arguments: reference name
mailbox name with possible wildcards

Responses: untagged responses: LIST

Result: OK - RLIST Completed
NO - RLIST Failure
BAD - command unknown or arguments invalid

The RLIST command behaves identically to its LIST counterpart, except
remote mailboxes are returned in addition to local mailboxes in the
LIST responses.

5.2 RLSUB Command

Arguments: reference name
mailbox name with possible wildcards

Responses: untagged responses: LSUB

Result: OK - RLSUB Completed
NO - RLSUB Failure
BAD - command unknown or arguments invalid

The RLSUB command behaves identically to its LSUB counterpart, except
remote mailboxes are returned in addition to local mailboxes in the
LSUB responses.

6. Formal Syntax

The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in [ABNF].

list_mailbox = <list_mailbox> as defined in [RFC-2060]

mailbox = <mailbox> as defined in [RFC-2060]

mailbox_referral = <tag> SPACE "NO" SPACE
<referral_response_code> (text / text_mime2)
; See [RFC-2060] for <tag>, text and text_mime2 definition

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RFC 2193 IMAP4 Mailbox Referrals September 1997

referral_response_code = "[" "REFERRAL" 1*(SPACE <url>) "]"
; See [RFC-1738] for <url> definition

rlist = "RLIST" SPACE mailbox SPACE list_mailbox

rlsub = "RLSUB" SPACE mailbox SPACE list_mailbox

6. Security Considerations

The IMAP4 referral mechanism makes use of IMAP URLs, and as such,
have the same security considerations as general internet URLs [RFC-
1738], and in particular IMAP URLs [RFC-2192].

With the MAILBOX-REFERRALS capability, it is potentially easier to
write a rogue server that injects a bogus referral response that
directs a user to an incorrect mailbox. Although referrals reduce
the effort to write such a server, the referral response makes
detection of the intrusion easier.

7. References

[RFC-2060], Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 2060, University of Washington, December 1996.

[RFC-2192], Newman, C., "IMAP URL Scheme", RFC 2192, Innosoft,
September 1997.

[RFC-1738], Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, CERN, Xerox Corporation,
University of Minnesota, December 1994.

[RFC-2119], Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.

[ABNF], DRUMS working group, Dave Crocker Editor, "Augmented BNF for
Syntax Specifications: ABNF", Work in Progress, Internet Mail
Consortium, April 1997.

8. Acknowledgments

Many valuable suggestions were received from private discussions and
the IMAP4 mailing list. In particular, Raymond Cheng, Mark Crispin,
Mark Keasling, Chris Newman and Larry Osterman made significant
contributions to this document.

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RFC 2193 IMAP4 Mailbox Referrals September 1997

9. Author's Address

Mike Gahrns
Microsoft
One Microsoft Way
Redmond, WA, 98072

Phone: (206) 936-9833
EMail: [email protected]

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