Network Working Group M. Crispin
Request for Comments: 1064 SUMEX-AIM
July 1988
INTERACTIVE MAIL Access PROTOCOL - VERSION 2
Status of this Memo
This RFCsuggests a method for workstations to dynamically access
mail from a mailbox server ("repository"). This RFCspecifies a
standard for the SUMEX-AIM community and a proposed eXPerimental
protocol for the Internet community. Discussion and suggestions for
improvement are requested. Distribution of this memo is unlimited.
IntrodUCtion
The intent of the Interactive Mail Access Protocol, Version 2 (IMAP2)
is to allow a workstation or similar small machine to access
electronic mail from a mailbox server. IMAP2 is the protocol used by
the SUMEX-AIM MM-D (MM Distributed) mail system.
Although different in many ways from POP2 (RFC937), IMAP2 may be
thought of as a functional superset of POP2, and the POP2 RFCwas
used as a model for this RFC. There was a cognizant reason for this;
RFC937 deals with an identical problem and it was desirable to offer
a basis for comparison.
Like POP2, IMAP2 specifies a means of accessing stored mail and not
of posting mail; this function is handled by a mail transfer protocol
such as SMTP (RFC821). A comparison with the DMSP protocol of
PCMAIL can be found at the end of "System Model and Philosophy"
section.
This protocol assumes a reliable data stream such as provided by TCP
or any similar protocol. When TCP is used, the IMAP2 server listens
on port 143.
System Model and Philosophy
Electronic mail is a primary means of communication for the widely
spread SUMEX-AIM community. The advent of distributed workstations
is forcing a significant rethinking of the mechanisms employed to
manage such mail. With mainframes, each user tends to receive and
process mail at the computer he used most of the time, his "primary
host". The first inclination of many users when an independent
workstation is placed in front of them is to begin receiving mail at
the workstation, and, in fact, many vendors have implemented
facilities to do this. However, this approach has several
disadvantages:
(1) Workstations (especially Lisp workstations) have a software
design that gives full control of all ASPects of the system to the
user at the console. As a result, background tasks, like
receiving mail, could well be kept from running for long periods
of time either because the user is aSKINg to use all of the
machine's resources, or because, in the course of working, the
user has (perhaps accidentally) manipulated the environment in
such a way as to prevent mail reception. This could lead to
repeated failed delivery attempts by outside agents.
(2) The hardware failure of a single workstation could keep its
user "off the air" for a considerable time, since repair of
individual workstation units might be delayed. Given the growing
number of workstations spread throughout Office environments,
quick repair would not be assured, whereas a centralized mainframe
is generally repaired very soon after failure.
(3) It is more difficult to keep track of mailing addresses when
each person is associated with a distinct machine. Consider the
difficulty in keeping track of a large number of postal addresses
or phone numbers, particularly if there was no single address or
phone number for an organization through which you could reach any
person in that organization. Traditionally, electronic mail on
the ARPANET involved remembering a name and one of several "hosts"
(machines) whose name reflected the organization in which the
individual worked. This was suitable at a time when most
organizations had only one central host. It is less satisfactory
today unless the concept of a host is changed to refer to an
organizational entity and not a particular machine.
(4) It is very difficult to keep a multitude of heterogeneous
workstations working properly with complex mailing protocols,
making it difficult to move forward as progress is made in
electronic communication and as new standards emerge. Each system
has to worry about receiving incoming mail, routing and delivering
outgoing mail, formatting, storing, and providing for the
stability of mailboxes over a variety of possible filing and
mailing protocols.
Consequently, while the workstation may be viewed as an Internet host
in the sense that it implements IP, it should not be viewed as the
entity which contains the user's mailbox. Rather, a mail server
machine (sometimes called a "repository") should hold the mailbox,
and the workstation (hereafter referred to as a "client") should
access the mailbox via mail transactions. Because the mail server
machine would be isolated from direct user manipulation, it could
achieve high software reliability easily, and, as a shared resource,
it could achieve high hardware reliability, perhaps through
redundancy. The mail server could be used from arbitrary locations,
allowing users to read mail across campus, town, or country using
more and more commonly available clients. Furthermore, the same user
may access his mailbox from different clients at different times, and
multiple users may access the same mailbox simultaneously.
The mail server acts an an interface among users, data storage, and
other mailers. The mail access protocol is used to retrieve
messages, access and change properties of messages, and manage
mailboxes. This differs from some approaches (e.g., Unix mail via
NFS) in that the mail access protocol is used for all message
manipulations, isolating the user and the client from all knowledge
of how the data storage is used. This means that the mail server can
utilize the data storage in whatever way is most efficient to
organize the mail in that particular environment, without having to
worry about storage representation compatibility across different
machines.
In defining a mail access protocol, it is important to keep in mind
that the client and server form a macrosystem, in which it should be
possible to exploit the strong points of both while compensating for
each other's weaknesses. Furthermore, it's desirable to allow for a
growth path beyond the hoary text-only RFC822 protocol. Unlike
POP2, IMAP2 has extensive features for remote searching and parsing
of messages on the server. For example, a free text search
(optionally in conjunction with other searching) can be made
throughout the entire mailbox by the server and the results made
available to the client without the client having to transfer the
entire mailbox and searching itself. Since remote parsing of a
message into a structured (and standard format) "envelope" is
available, a client can display envelope information and implement
commands such as REPLY without having any understanding of how to
parse RFC822, etc. headers.
Additionally, IMAP2 offers several facilities for managing a mailbox
beyond the simple "delete message" functionality of POP2.
In spite of this, IMAP2 is a relatively simple protocol. Although
servers should implement the full set of IMAP2 functions, a simple
client can be written which uses IMAP2 in much the way as a POP2
client.
IMAP2 differs from the DMSP protocol of PCMAIL (RFC1056) in a more
fundamental manner, reflecting the differing architectures of MM-D
and PCMAIL. PCMAIL is either an online ("interactive mode"), or
offline ("batch mode") system. MM-D is primarily an online system in
which real-time and simultaneous mail access were considered
important.
In PCMAIL, there is a long-term client/server relationship in which
some mailbox state is preserved on the client. There is a
registration of clients used by a particular user, and the client
keeps a set of "descriptors" for each message which summarize the
message. The server and client synchronize their states when the
DMSP connection starts up, and, if a client has not accessed the
server for a while, the client does a complete reset (reload) of its
state from the server.
In MM-D, the client/server relationship lasts only for the duration
of the IMAP2 connection. All mailbox state is maintained on the
server. There is no registration of clients. The function of a
descriptor is handled by a structured representation of the message
"envelope". This structure makes it unnecessary for a client to know
anything about RFC822 parsing. There is no synchronization since
the client does not remember state between IMAP2 connections. This
is not a problem since in general the client never needs the entire
state of the mailbox in a single session, therefore there isn't much
overhead in fetching the state information that is needed as it is
needed.
There are also some functional differences between IMAP2 and DMSP.
DMSP has explicit support for bulletin boards which are only handled
implicitly in IMAP2. DMSP has functions for sending messages,
printing messages, listing mailboxes, and changing passWords, all of
which are done outside of IMAP2. DMSP has 16 binary flags of which 8
are defined by the system. IMAP has flag names; there are currently
5 defined system flag names and a facility for some number (30 in the
current implementations) of user flag names. IMAP2 has a
sophisticated message search facility in the server to identify
interesting messages based on dates, addresses, flag status, or
textual contents without compelling the client to fetch this data for
every message.
It was felt that maintaining state on the client is advantageous only
in those cases where the client is only used by a single user, or if
there is some means on the client to restrict access to another
user's data. It can be a serious disadvantage in an environment in
which multiple users routinely use the same client, the same user
routinely uses different clients, and where there are no access
restrictions on the client. It was also observed that most user mail
access is to a relatively small set of "interesting" messages, which
were either "new" mail or mail based upon some user-selected
criteria. Consequently, IMAP2 was designed to easily identify those
"interesting" messages so that the client could fetch the state of
those messages and not those that were not "interesting".
The Protocol
The IMAP2 protocol consists of a sequence of client commands and
server responses, with server data interspersed between the
responses. Unlike most Internet protocols, commands and responses
are tagged. That is, a command begins with a unique identifier
(typically a short alphanumeric sequence such as a Lisp "gensym"
function would generate e.g., A0001, A0002, etc.), called a tag. The
response to this command is given the same tag from the server.
Additionally, the server may send an arbitrary amount of "unsolicited
data", which is identified by the special reserved tag of "*". There
is another special reserved tag, "+", discussed below.
The server must be listening for a connection. When a connection is
opened the server sends an unsolicited OK response as a greeting
message and then waits for commands. When commands are received the
server acts on them and responds with responses, often interspersed
with data.
The client opens a connection, waits for the greeting, then sends a
LOGIN command with user name and password arguments to establish
authorization. Following an OK response from the server, the client
then sends a SELECT command to access the desired mailbox. The
user's default mailbox has a special reserved name of "INBOX" which
is independent of the operating system that the server is implemented
on. The server will generally send a list of valid flags, number of
messages, and number of messages arrived since last access for this
mailbox as unsolicited data, followed by an OK response. The client
may terminate access to this mailbox and access a different one with
another SELECT command.
The client reads mailbox information by means of FETCH commands. The
actual data is transmitted via the unsolicited data mechanism (that
is, FETCH should be viewed as poking the server to include the
desired data along with any other data it wishes to transmit to the
client). There are three major categories of data which may be
fetched.
The first category is that data which is associated with a message as
an entity in the mailbox. There are presently three such items of
data: the "internal date", the "RFC822 size", and the "flags". The
internal date is the date and time that the message was placed in the
mailbox. The RFC822 size is subject to deletion in the future; it
is the size in bytes of the message, expressed as an RFC822 text
string. Current clients only use it as part of a status display
line. The flags are a list of status flags associated with the
message (see below). All of the first category data can be fetched
by using the macro-fetch word "FAST"; that is, "FAST" expands to
"(FLAGS INTERNALDATE RFC822.SIZE)".
The second category is that data which describes the composition and
delivery information of a message; that is, information such as the
message sender, recipient lists, message-ID, subject, etc. This is
the information which is stored in the message header in RFC822
format message and is traditionally called the "envelope". [Note:
this should not be confused with the SMTP (RFC821) envelope, which
is strictly limited to delivery information.] IMAP2 defines a
structured and unambiguous representation for the envelope which is
particularly nice for Lisp-based parsers. A client can use the
envelope for operations such as replying and not worry about RFC822
at all. Envelopes are discussed in more detail below. The first and
second category data can be fetched together by using the macro-fetch
word "ALL"; that is, "ALL" expands to "(FLAGS INTERNALDATE
RFC822.SIZE ENVELOPE)".
The third category is that data which is intended for direct human
viewing. The present RFC822 based IMAP2 defines three such items:
RFC822.HEADER, RFC822.TEXT, and RFC822 (the latter being the two
former appended together in a single text string). Fetching "RFC822"
is equivalent to typing the RFC822 representation of the message as
stored on the mailbox without any filtering or processing.
Typically, a client will "FETCH ALL" for some or all of the messages
in the mailbox for use as a presentation menu, and when the user
wishes to read a particular message will "FETCH RFC822.TEXT" to get
the message body. A more primitive client could, of course, simply
"FETCH RFC822" a la POP2-type functionality.
The client can alter certain data (presently only the flags) by means
of a STORE command. As an example, a message is deleted from a
mailbox by a STORE command which includes the \DELETED flag as one of
the flags being set.
Other client operations include copying a message to another mailbox
(COPY command), permanently removing deleted messages (EXPUNGE
command), checking for new messages (CHECK command), and searching
for messages which match certain criteria (SEARCH command).
The client terminates the session with the LOGOUT command. The
server returns a "BYE" followed by an "OK".
A Typical Scenario
Client Server
------ ------
{Wait for Connection}
{Open Connection} -->
<-- * OK IMAP2 Server Ready
{Wait for command}
A001 LOGIN Fred Secret -->
<-- A001 OK User Fred logged in
{Wait for command}
A002 SELECT INBOX -->
<-- * FLAGS (Meeting Notice \Answered
\Flagged \Deleted \Seen)
<-- * 19 EXISTS
<-- * 2 RECENT
<-- A0002 OK Select complete
{Wait for command}
A003 FETCH 1:19 ALL -->
<-- * 1 Fetch (......)
...
<-- * 18 Fetch (......)
<-- * 19 Fetch (......)
<-- A003 OK Fetch complete
{Wait for command}
A004 FETCH 8 RFC822.TEXT -->
<-- * 8 Fetch (RFC822.TEXT {893}
...893 characters of text...
<-- )
<-- A004 OK Fetch complete
{Wait for command}
A005 STORE 8 +Flags \Deleted -->
<-- * 8 Store (Flags (\Deleted
\Seen))
<-- A005 OK Store complete
{Wait for command}
A006 EXPUNGE -->
<-- * 19 EXISTS
<-- * 8 EXPUNGE
<-- * 18 EXISTS
<-- A006 Expunge complete
{Wait for command}
A007 LOGOUT -->
<-- * BYE IMAP2 server quitting
<-- A007 OK Logout complete
{Close Connection} --><-- {Close connection}
{Go back to start}
Conventions
The following terms are used in a meta-sense in the syntax
specification below:
An ASCII-STRING is a sequence of arbitrary ASCII characters.
An ATOM is a sequence of ASCII characters delimited by SP or CRLF.
A CHARACTER is any ASCII character except """", "{", CR, LF, "%",
or "\".
A CRLF is an ASCII carriage-return character followed immediately
by an ASCII linefeed character.
A NUMBER is a sequence of the ASCII characters which represent
decimal numerals ("0" through "9"), delimited by SP, CRLF, ",", or
":".
A SP is the ASCII space character.
A TEXT_LINE is a human-readable sequence of ASCII characters up to
but not including a terminating CRLF.
One of the most common fields in the IMAP2 protocol is a STRING,
which may be an ATOM, QUOTED-STRING (a sequence of CHARACTERs inside
double-quotes), or a LITERAL. A literal consists of an open brace
("{"), a number, a close brace ("}"), a CRLF, and then an ASCII-
STRING of n characters, where n is the value of the number inside the
brace. In general, a string should be represented as an ATOM or
QUOTED-STRING if at all possible. The semantics for QUOTED-STRING or
LITERAL are checked before those for ATOM; therefore an ATOM used in
a STRING may only contain CHARACTERs. Literals are most often sent
from the server to the client; in the rare case of a client to server
literal there is a special consideration (see the "+ text" response
below).
Another important field is the SEQUENCE, which identifies a set of
messages by consecutive numbers from 1 to n where n is the number of
messages in the mailbox. A sequence may consist of a single number,
a pair of numbers delimited by colon indicating all numbers between
those two numbers, or a list of single numbers and/or number pairs.
For example, the sequence 2,4:7,9,12:15 is equivalent to
2,4,5,6,7,9,12,13,14,15 and identifies all of those messages.
Definitions of Commands and Responses
Summary of Commands and Responses
Commands Responses
-------- -------
tag NOOP tag OK text
tag LOGIN user password tag NO text
tag LOGOUT tag BAD text
tag SELECT mailbox * message_number data
tag CHECK * FLAGS flag_list
tag EXPUNGE * SEARCH sequence
tag COPY sequence mailbox * BYE text
tag FETCH sequence data * OK text
tag STORE sequence data value * NO text
tag SEARCH search_program * BAD text
+ text
Commands
tag NOOP
The NOOP command returns an OK to the client. By itself, it does
nothing, but certain things may happen as side effects. For
example, server implementations which implicitly check the mailbox
for new mail may do so as a result of this command. The primary
use of this command is to for the client to see if the server is
still alive (and notify the server that the client is still alive,
for those servers which have inactivity autologout timers).
tag LOGIN user password
The LOGIN command identifies the user to the server and carries
the password authenticating this user. This information is used
by the server to control access to the mailboxes.
EXAMPLE: A001 LOGIN SMITH SESAME logs in as user SMITH with
password SESAME.
tag LOGOUT
The LOGOUT command indicates the client is done with the session.
The server sends an unsolicited BYE response before the (tagged)
OK response, and then closes the connection.
tag SELECT mailbox
The SELECT command selects a particular mailbox. The server must
check that the user is permitted read access to this mailbox.
Prior to returning an OK to the client, the server must send an
unsolicited FLAGS and <n> EXISTS response to the client giving the
flags list for this mailbox (simply the system flags if this
mailbox doesn't have any special flags) and the number of messages
in the mailbox. It is also recommended that the server send a <n>
RECENT unsolicited response to the client for the benefit of
clients which make use of the number of new messages in a mailbox.
Multiple SELECT commands are permitted in a session, in which case
the prior mailbox is deselected first.
The default mailbox for the SELECT command is INBOX, which is a
special name reserved to mean "the primary mailbox for this user
on this server". The format of other mailbox names is operating
system dependent (as of this writing, it reflects the filename
path of the mailbox file on the current servers).
EXAMPLE: A002 SELECT INBOX selects the default mailbox.
tag CHECK
The CHECK command forces a check for new messages and a rescan of
the mailbox for internal change for those implementations which
allow multiple simultaneous read/write access to the same mailbox
(e.g., TOPS-20). It is recommend that periodic implicit checks
for new mail be done by servers as well. The server should send
an unsolicited <n> EXISTS response prior to returning an OK to the
client.
tag EXPUNGE
The EXPUNGE command permanently removes all messages with the
\DELETED flag set in its flags from the mailbox. Prior to
returning an OK to the client, for each message which is removed,
an unsolicited <n> EXPUNGE response is sent indicating which
message was removed. The message number of each subsequent
message in the mailbox is immediately decremented by 1; this means
that if the last 5 messages in a 9-message mail file are expunged
you will receive 5 "* 5 EXPUNGE" responses. To ensure mailbox
integrity and server/client synchronization, it is recommended
that the server do an implicit check prior to commencing the
expunge and again when the expunge is completed. Furthermore, if
the server allows multiple simultaneous access to the same mail
file the server must lock the mail file for exclusive access while
an expunge is taking place.
EXPUNGE is not allowed if the user does not have write access to
this mailbox.
tag COPY sequence mailbox
The COPY command copies the specified message(s) to the specified
destination mailbox. If the destination mailbox does not exist,
the server should create it. Prior to returning an OK to the
client, the server should return an unsolicited <n> COPY response
for each message copied. A copy should set the \SEEN flag for all
messages which were successfully copied (provided, of course, that
the user has write access to this mailbox).
EXAMPLE: A003 COPY 2:4 MEETING copies messages 2, 3, and 4 to
mailbox "MEETING".
COPY is not allowed if the user does not have write access to the
destination mailbox.
tag FETCH sequence data
The FETCH command retrieves data associated with a message in the
mailbox. The data items to be fetched may be either a single atom
or an S-expression list. The currently defined data items that
can be fetched are:
ALL Macro equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE ENVELOPE)
ENVELOPE The envelope of the message. The envelope is
computed by the server by parsing the RFC822
header into the component parts, defaulting
various fields as necessary.
FAST Macro equivalent to:
(FLAGS INTERNALDATE RFC822.SIZE)
FLAGS The flags which are set for this message.
This may include the following system flags:
\RECENT Message arrived since
last read of this mail
file
\SEEN Message has been read
\ANSWERED Message has been answered
\FLAGGED Message is "flagged" for
urgent/special attention
\DELETED Message is "deleted" for
removal by later EXPUNGE
INTERNALDATE The date and time the message was written to
the mailbox.
RFC822 The message in RFC822 format.
RFC822.HEADER The RFC822 format header of the message.
RFC822.SIZE The number of characters in the message as
expressed in RFC822 format.
RFC822.TEXT The text body of the message, omitting the
RFC822 header.
EXAMPLES:
A003 FETCH 2:4 ALL
fetches the flags, internal date, RFC822 size, and envelope
for messages 2, 3, and 4.
A004 FETCH 3 RFC822
fetches the RFC822 representation for message 3.
A005 FETCH 4 (FLAGS RFC822.HEADER)
fetches the flags and RFC822 format header for message 4.
tag STORE sequence data value
The STORE command alters data associated with a message in the
mailbox. The currently defined data items that can be stored are:
FLAGS Replace the flags for the message with the
argument (in flag list format).
+FLAGS Add the flags in the argument to the
message's flag list.
-FLAGS Remove the flags in the argument from the
message's flag list.
STORE is not allowed if the user does not have write access to
this mailbox.
EXAMPLE: A003 STORE 2:4 +FLAGS (\DELETED)
marks messages 2, 3, and 4 for deletion.
tag SEARCH search_criteria
The SEARCH command searches the mailbox for messages which match
the given set of criteria. The unsolicited SEARCH <1#number>
response from the server is a list of messages which express the
intersection (AND function) of all the messages. The currently
defined criteria are:
ALL All messages in the mailbox; the default
initial criterion for ANDing.
ANSWERED Messages with the \ANSWERED flag set.
BCC string Messages which contain the specified string
in the envelope's BCC field.
BEFORE date Messages whose internal date is earlier than
the specified date.
BODY string Messages which contain the specified string
in the body of the message.
CC string Messages which contain the specified string
in the envelope's CC field.
DELETED Messages with the \DELETED flag set.
FLAGGED Messages with the \FLAGGED flag set.
KEYWORD flag Messages with the specified flag set.
NEW Messages which have the \RECENT flag set but
not the \SEEN flag. This is functionally
equivalent to "RECENT UNSEEN".
OLD Messages which do not have the \RECENT flag
set.
ON date Messages whose internal date is the same as
the specified date.
RECENT Messages which have the \RECENT flag set.
SEEN Messages which have the \SEEN flag set.
SINCE date Messages whose internal date is later than
the specified date.
SUBJECT string Messages which contain the specified string
in the envelope's SUBJECT field.
TEXT string Messages which contain the specified string.
TO string Messages which contain the specified string in
the envelope's TO field.
UNANSWERED Messages which do not have the \ANSWERED flag
set.
UNDELETED Messages which do not have the \DELETED flag
set.
UNFLAGGED Messages which do not have the \FLAGGED flag
set.
UNKEYWORD flag Messages which do not have the specified flag
set.
UNSEEN Messages which do not have the \SEEN flag set.
EXAMPLE: A003 SEARCH DELETED FROM "SMITH" SINCE 1-OCT-87
returns the message numbers for all deleted messages from Smith
that were placed in the mail file since October 1, 1987.
Responses
tag OK text
This response identifies successful completion of the command with
the indicated tag. The text is a line of human-readable text
which may be useful in a protocol telemetry log for debugging
purposes.
tag NO text
This response identifies unsuccessful completion of the command
with the indicated tag. The text is a line of human-readable text
which probably should be displayed to the user in an error report
by the client.
tag BAD text
This response indicates faulty protocol received from the client
and indicates a bug in the client. The text is a line of human-
readable text which should be recorded in any telemetry as part of
a bug report to the maintainer of the client.
* number message_data
This response occurs as a result of several different commands.
The message_data is one of the following:
EXISTS The specified number of messages exists in the mailbox.
RECENT The specified number of messages have arrived since the
last time this mailbox was read.
EXPUNGE The specified message number has been permanently
removed from the mailbox, and the next message in the
mailbox (if any) becomes that message number.
STORE data
Functionally equivalent to FETCH, only it happens as a
result of a STORE command.
FETCH data
This is the principle means by which data about a
message is returned to the client. The data is in a
Lisp-like S-expression property list form. The current
properties are:
ENVELOPE An S-expression format list which describes the
envelope of a message. The envelope is computed
by the server by parsing the RFC822 header into
the component parts, defaulting various fields
as necessary.
The fields of the envelope are in the following
order: date, subject, from, sender, reply-to, to,
cc, bcc, in-reply-to, and message-id. The date,
subject, in-reply-to, and message-id fields are
strings. The from, sender, reply-to, to, cc,
and bcc fields are lists of addresses.
An address is an S-expression format list which
describes an electronic mail address. The fields
of an address are in the following order:
personal name, source-route (a.k.a. the
at-domain-list in SMTP), mailbox name, and
host name.
Any field of an envelope or address which is
not applicable is presented as the atom NIL.
Note that the server must default the reply-to
and sender fields from the from field; a client is
not expected to know to do this.
FLAGS An S-expression format list of flags which are set
for this message. This may include the following
system flags:
\RECENT Message arrived since last
read of this mail file
\SEEN Message has been read
\ANSWERED Message has been answered
\FLAGGED Message is "flagged" for
urgent/special attention
\DELETED Message is "deleted" for
removal by later EXPUNGE
INTERNALDATE A string containing the date and time the
message was written to the mailbox.
RFC822 A string expressing the message in RFC822
format.
RFC822.HEADER A string expressing the RFC822 format
header of the message
RFC822.SIZE A number indicating the number of
characters in the message as expressed
in RFC822 format.
RFC822.TEXT A string expressing the text body of the
message, omitting the RFC822 header.
* FLAGS flag_list
This response occurs as a result of a SELECT command. The flag
list are the list of flags (at a minimum, the system-defined
flags) which are applicable for this mailbox. Flags other than
the system flags are a function of the server implementation.
* SEARCH number(s)
This response occurs as a result of a SEARCH command. The
number(s) refer those messages which match the search criteria.
Each number is delimited by a space, e.g., "SEARCH 2 3 6".
* BYE text
This response indicates that the server is about to close the
connection. The text is a line of human-readable text which
should be displayed to the user in a status report by the client.
This may be sent as part of a normal logout sequence, or as a
panic shutdown announcement by the server. It is also used by
some servers as an announcement of an inactivity autologout.
* OK text
This response indicates that the server is alive. No special
action on the part of the client is called for. This is presently
only used by servers at startup as a greeting message indicating
that they are ready to accept the first command. The text is a
line of human-readable text which may be logged in protocol
telemetry.
* NO text
This response indicates some operational error at the server which
cannot be traced to any protocol command. The text is a line of
human-readable text which should be logged in protocol telemetry
for the maintainer of the server and/or the client. No known
server currently outputs such a response.
* BAD text
This response indicates some protocol error at the server which
cannot be traced to any protocol command. The text is a line of
human-readable text which should be logged in protocol telemetry
for the maintainer of the server and/or the client. This
generally indicates a protocol synchronization problem on the part
of the client, and examination of the protocol telemetry is
advised to determine the cause of the problem.
+ text
This response indicates that the server is ready to accept the
text of a literal from the client. Normally, a command from the
client is a single text line. If the server detects an error in
the command, it can simply discard the remainder of the line. It
cannot do this in the case of commands which contain literals,
since a literal can be an arbitrarily long amount of text, and the
server may not even be expecting a literal. This mechanism is
provided so the client knows not to send a literal until the
server definitely expects it, preserving client/server
synchronization.
In actual practice, this situation is rarely encountered. In the
current protocol, the only client command likely to contain a
literal is the LOGIN command. Consider a situation in which a
server validates the user before checking the password. If the
password contains "funny" characters and hence is sent as a
literal, then if the user is invalid an error would occur before
the password is parsed.
No such synchronization protection is provided for literals sent
from the server to the client, for performance reasons. Any
synchronization problems in this direction would be due to a bug
in the client or server and not for some operational problem.
Sample IMAP2 session
The following is a transcript of an actual IMAP2 session. Server
output is identified by "S:" and client output by "U:". In cases
where lines were too long to fit within the boundaries of this
document, the line was continued on the next line preceded by a tab.
S: * OK SUMEX-AIM.Stanford.EDU Interim Mail Access Protocol II
Service 6.1(349) at Thu, 9 Jun 88 14:58:30 PDT
U: a001 login crispin secret
S: a002 OK User CRISPIN logged in at Thu, 9 Jun 88 14:58:42 PDT,
job 76
U: a002 select inbox
S: * FLAGS (Bugs SF Party Skating Meeting Flames Request AI
Question Note \XXXX \YYYY \Answered \Flagged \Deleted
\Seen)
S: * 16 EXISTS
S: * 0 RECENT
S: a002 OK Select complete
U: a003 fetch 16 all
S: * 16 Fetch (Flags (\Seen) InternalDate " 9-Jun-88 12:55:
RFC822.Size 637 Envelope ("Sat, 4 Jun 88 13:27:11 PDT"
"INFO-MAC Mail Message" (("Larry Fagan" NIL "FAGAN"
"SUMEX-AIM.Stanford.EDU")) (("Larry Fagan" NIL "FAGAN"
"SUMEX-AIM.Stanford.EDU")) (("Larry Fagan" NIL "FAGAN"
"SUMEX-AIM.Stanford.EDU")) ((NIL NIL "rindflEISCH"
"SUMEX-AIM.Stanford.EDU")) NIL NIL NIL
"<12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>"))
S: a003 OK Fetch completed
U: a004 fetch 16 rfc822
S: * 16 Fetch (RFC822 {637}
S: Mail-From: RINDFLEISCH created at 9-Jun-88 12:55:43
S: Mail-From: FAGAN created at 4-Jun-88 13:27:12
S: Date: Sat, 4 Jun 88 13:27:11 PDT
S: From: Larry Fagan <FAGAN@SUMEX-AIM.Stanford.EDU>
S: To: rindflEISCH@SUMEX-AIM.Stanford.EDU
S: Subject: INFO-MAC Mail Message
S: Message-ID: <12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>
S: ReSent-Date: Thu, 9 Jun 88 12:55:43 PDT
S: ReSent-From: TC Rindfleisch <Rindfleisch@SUMEX-AIM.Stanford.EDU>
S: ReSent-To: Yeager@SUMEX-AIM.Stanford.EDU,
Crispin@SUMEX-AIM.Stanford.EDU
S: ReSent-Message-ID:
<12405133897.80.RINDFLEISCH@SUMEX-AIM.Stanford.EDU>
S:
S: The file is <info-mac>usenetv4-55.arc ...
S: Larry
S: -------
S: )
S: pa004 OK Fetch completed
U: a005 logout
S: * BYE DEC-20 IMAP II server terminating connection
S: a005 OK SUMEX-AIM.Stanford.EDU Interim Mail Access Protocol
Service logout
Implementation Discussion
As of this writing, SUMEX has completed an IMAP2 client for Xerox
Lisp machines written in hybrid Interlisp/CommonLisp and is beta-
testing a client for TI Explorers written entirely in CommonLisp.
SUMEX has also completed a portable IMAP2 client protocol library
module written in C. This library, with the addition of a small main
program (primarily user interface) and a TCP/IP driver, became a
rudimentary remote system mail-reading program under Unix. The first
production use of this library will be as a part of a MacII client
which is under development.
As of this writing, SUMEX has completed IMAP2 servers for TOPS-20
written in DEC-20 assembly language and 4.2/3 BSD Unix written in C.
The TOPS-20 server is fully compatible with MM-20, the standard
TOPS-20 mailsystem, and requires no special action or setup on the
part of the user. The INBOX under TOPS-20 is the user's MAIL.TXT.
The TOPS-20 server also supports multiple simultaneous access to the
same mailbox, including simultaneous access between the IMAP2 server
and MM-20. The 4.2/3 BSD Unix server requires that the user use
mail.txt format which is compatible only with SRI MM-32 or Columbia
MM-C. The 4.2/3 BSD Unix server only allows simultaneous read
access; write access must be exclusive.
The Xerox Lisp client and DEC-20 server have been in production use
for over a year; the Unix server was put into production use a few
months ago. IMAP2 has been used to access mailboxes at remote sites
from a local workstation via the Internet. For example, from the
Stanford local network the author has read his mailbox at a Milnet
site.
This specification does not make any formal definition of size
restrictions, but the DEC-20 server has the following limitations:
. length of a mailbox: 7,077,888 characters
. maximum number of messages: 18,432 messages
. length of a command line: 10,000 characters
. length of the local host name: 64 characters
. length of a "short" argument: 39 characters
. length of a "long" argument: 491,520 characters
. maximum amount of data output in a single fetch:
655,360 characters
To date, nobody has run up against any of these limitations, many of
which are substantially larger than most current user mail reading
programs.
There are several advantages to the scheme of tags and unsolicited
responses. First, the infamous synchronization problems of SMTP and
similar protocols do not happen with tagged commands; a command is
not considered satisfied until a response with the same tag is seen.
Tagging allows an arbitrary amount of other responses ("unsolicited"
data) to be sent by the server with no possibility of the client
losing synchronization. Compare this with the problems that FTP or
SMTP clients have with continuation, partial completion, and
commentary reply codes.
Another advantage is that a non-lockstep client implementation is
possible. The client could send a command, and entrust the handling
of the server responses to a different process which would signal the
client when the tagged response comes in. Under certain
circumstances, the client could even have more than one command
outstanding.
It was observed that synchronization problems can occur with literals
if the literal is not recognized as such. Fortunately, the cases in
which this can happen are relatively rare; a mechanism (the special
"+" tag response) was introduced to handle those few cases which
could happen. The proper way to address this problem in all cases is
probably to move towards a record-oriented architecture instead of
the text stream model provided by TCP.
Unsolicited data needs some discussion. Unlike most protocols, in
which the server merely does the client's bidding, an IMAP2 server
has a semi-autonomous role. By means of sending "unsolicited data",
the server is in effect sending a command to the client -- to update
and/or extend its (incomplete) model of the mailbox with new
information from the server. In this viewpoint, a "fetch" command is
merely a request to the server to include the desired data in any
other "unsolicited" data the server may send, and a server
acknowledgement to the "fetch" is a statement that all the requested
data has been sent.
In terms of implementation, the client may have a local cache of data
from the mailbox. This cache is incomplete, and at startup is empty.
A listener processes all unsolicited data, and updates the cache
based on this data. If a tagged response arrives, the listener
unblocks the process which sent the tagged request.
Perhaps as a result of opening a mailbox, unsolicited data from the
server arrives. The first piece of data is the number of messages.
This is used to size the cache; note that by sending a new "number of
messages" unsolicited data message the cache would be re-sized (this
is how newly arrived mail is handled). If the client attempts to
access information from the cache, it will encounter empty spots
which will trigger "fetch" requests. The request would be sent, some
unsolicited data including the answer to the fetch will flow back,
and then the "fetch" response will unblock the client.
People familiar with demand-paged virtual memory operating system
design will recognize this model as being very similar to page-fault
handling on a demand-paged system.
Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in RFC822 with one exception; the
delimiter used with the "#" construct is a single space (SP) and not
a comma.
address ::= "(" addr_name SP addr_adl SP addr_mailbox SP
addr_host ")"
addr_adl ::= nil / string
addr_host ::= nil / string
addr_mailbox ::= nil / string
addr_name ::= nil / string
check ::= "CHECK"
copy ::= "COPY" SP sequence SP mailbox
data ::= ("FLAGS" SP flag_list / "SEARCH" SP 1#number /
"BYE" SP text_line / "OK" SP text_line /
"NO" SP text_line / "BAD" SP text_line)
date ::= string in form "dd-mmm-yy hh:mm:ss-zzz"
envelope ::= "(" env_date SP env_subject SP env_from SP
env_sender SP env_reply-to SP env_to SP
env_cc SP env_bcc SP env_in-reply-to SP
env_message-id ")"
env_bcc ::= nil / "(" 1*address ")"
env_cc ::= nil / "(" 1*address ")"
env_date ::= string
env_from ::= nil / "(" 1*address ")"
env_in-reply-to ::= nil / string
env_message-id ::= nil / string
env_reply-to ::= nil / "(" 1*address ")"
env_sender ::= nil / "(" 1*address ")"
env_subject ::= nil / string
env_to ::= nil / "(" 1*address ")"
expunge ::= "EXPUNGE"
fetch ::= "FETCH" SP sequence SP ("ALL" / "FAST" /
fetch_att / "(" 1#fetch_att ")")
fetch_att ::= "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
"RFC822" / "RFC822.HEADER" / "RFC822.SIZE" /
"RFC822.TEXT"
flag_list ::= ATOM / "(" 1#ATOM ")"
literal ::= "{" NUMBER "}" CRLF ASCII-STRING
login ::= "LOGIN" SP userid SP password
logout ::= "LOGOUT"
mailbox ::= "INBOX" / string
msg_copy ::= "COPY"
msg_data ::= (msg_exists / msg_recent / msg_expunge /
msg_fetch / msg_copy)
msg_exists ::= "EXISTS"
msg_expunge ::= "EXPUNGE"
msg_fetch ::= ("FETCH" / "STORE") SP "(" 1#("ENVELOPE" SP
envelope / "FLAGS" SP "(" 1#(recent_flag
flag_list) ")" / "INTERNALDATE" SP date /
"RFC822" SP string / "RFC822.HEADER" SP string /
"RFC822.SIZE" SP NUMBER / "RFC822.TEXT" SP
string) ")"
msg_recent ::= "RECENT"
msg_num ::= NUMBER
nil ::= "NIL"
noop ::= "NOOP"
password ::= string
recent_flag ::= "\RECENT"
ready ::= "+" SP text_line
request ::= tag SP (noop / login / logout / select / check /
expunge / copy / fetch / store / search) CRLF
response ::= tag SP ("OK" / "NO" / "BAD") SP text_line CRLF
search ::= "SEARCH" SP 1#("ALL" / "ANSWERED" /
"BCC" SP string / "BEFORE" SP string /
"BODY" SP string / "CC" SP string / "DELETED" /
"FLAGGED" / "KEYWORD" SP atom / "NEW" / "OLD" /
"ON" SP string / "RECENT" / "SEEN" /
"SINCE" SP string / "TEXT" SP string /
"TO" SP string / "UNANSWERED" / "UNDELETED" /
"UNFLAGGED" / "UNKEYWORD" / "UNSEEN")
select ::= "SELECT" SP mailbox
sequence ::= NUMBER / (NUMBER "," sequence) / (NUMBER ":"
sequence)
store ::= "STORE" SP sequence SP store_att
store_att ::= ("+FLAGS" SP flag_list / "-FLAGS" SP flag_list /
"FLAGS" SP flag_list)
string ::= atom / """" 1*character """" / literal
system_flags ::= "\ANSWERED" SP "\FLAGGED" SP "\DELETED" SP
"\SEEN"
tag ::= atom
unsolicited ::= "*" SP (msg_num SP msg_data / data) CRLF
userid ::= string
Acknowledgements
Bill Yeager and Rich Acuff both contributed invaluable suggestions in
the evolution of IMAP2 from the original IMAP. The SUMEX IMAP2
software was written by Mark Crispin (DEC-20 server, Xerox Lisp
client, C client), Frank Gilmurray (Common Lisp client), Christopher
Lane (Xerox Lisp client), and Bill Yeager (Unix server). Any
mistakes or flaws in this IMAP2 protocol specification are, however,
strictly my own.
