2. Protocol Overview
2.1. Link Level
The IMAP4rev1 protocol assumes a reliable data stream such as
provided by TCP. When TCP is used, an IMAP4rev1 server listens on
port 143.
2.2. Commands and Responses
An IMAP4rev1 connection consists of the establishment of a
client/server network connection, an initial greeting from the
server, and client/server interactions. These client/server
interactions consist of a client command, server data, and a server
completion result response.
All interactions transmitted by client and server are in the form of
lines; that is, strings that end with a CRLF. The protocol receiver
of an IMAP4rev1 client or server is either reading a line, or is
reading a sequence of octets with a known count followed by a line.
2.2.1. Client Protocol Sender and Server Protocol Receiver
The client command begins an operation. Each client command is
prefixed with an identifier (typically a short alphanumeric string,
e.g. A0001, A0002, etc.) called a "tag". A different tag is
generated by the client for each command.
There are two cases in which a line from the client does not
represent a complete command. In one case, a command argument is
quoted with an octet count (see the description of literal in String
under Data Formats); in the other case, the command arguments require
server feedback (see the AUTHENTICATE command). In either case, the
server sends a command continuation request response if it is ready
for the octets (if appropriate) and the remainder of the command.
This response is prefixed with the token "+".
Note: If, instead, the server detected an error in the command, it
sends a BAD completion response with tag matching the command (as
It is also possible for the server to send a completion response
for some other command (if multiple commands are in progress), or
untagged data. In either case, the command continuation request
is still pending; the client takes the appropriate action for the
response, and reads another response from the server. In all
cases, the client MUST send a complete command (including
receiving all command continuation request responses and command
continuations for the command) before initiating a new command.
The protocol receiver of an IMAP4rev1 server reads a command line
from the client, parses the command and its arguments, and transmits
server data and a server command completion result response.
2.2.2. Server Protocol Sender and Client Protocol Receiver
Data transmitted by the server to the client and status responses
that do not indicate command completion are prefixed with the token
"*", and are called untagged responses.
Server data MAY be sent as a result of a client command, or MAY be
sent unilaterally by the server. There is no syntactic difference
between server data that resulted from a specific command and server
data that were sent unilaterally.
The server completion result response indicates the success or
failure of the operation. It is tagged with the same tag as the
client command which began the operation. Thus, if more than one
command is in progress, the tag in a server completion response
identifies the command to which the response applies. There are
three possible server completion responses: OK (indicating success),
NO (indicating failure), or BAD (indicating protocol error such as
unrecognized command or command syntax error).
The protocol receiver of an IMAP4rev1 client reads a response line
from the server. It then takes action on the response based upon the
first token of the response, which can be a tag, a "*", or a "+".
A client MUST be prepared to accept any server response at all times.
This includes server data that was not requested. Server data SHOULD
be recorded, so that the client can reference its recorded copy
rather than sending a command to the server to request the data. In
the case of certain server data, the data MUST be recorded.
This topic is discussed in greater detail in the Server Responses
section.
2.3. Message Attributes
In addition to message text, each message has several attributes
associated with it. These attributes may be retrieved individually
or in conjunction with other attributes or message texts.
2.3.1. Message Numbers
Messages in IMAP4rev1 are accessed by one of two numbers; the unique
identifier and the message sequence number.
2.3.1.1. Unique Identifier (UID) Message Attribute
A 32-bit value assigned to each message, which when used with the
unique identifier validity value (see below) forms a 64-bit value
that is permanently guaranteed not to refer to any other message in
the mailbox. Unique identifiers are assigned in a strictly ascending
fashion in the mailbox; as each message is added to the mailbox it is
assigned a higher UID than the message(s) which were added
previously.
Unlike message sequence numbers, unique identifiers are not
necessarily contiguous. Unique identifiers also persist across
sessions. This permits a client to resynchronize its state from a
previous session with the server (e.g. disconnected or offline access
clients); this is discussed further in [IMAP-DISC].
Associated with every mailbox is a unique identifier validity value,
which is sent in an UIDVALIDITY response code in an OK untagged
response at mailbox selection time. If unique identifiers from an
earlier session fail to persist to this session, the unique
identifier validity value MUST be greater than the one used in the
earlier session.
Note: Unique identifiers MUST be strictly ascending in the mailbox
at all times. If the physical message store is re-ordered by a
non-IMAP agent, this requires that the unique identifiers in the
mailbox be regenerated, since the former unique identifers are no
longer strictly ascending as a result of the re-ordering. Another
instance in which unique identifiers are regenerated is if the
message store has no mechanism to store unique identifiers.
Although this specification recognizes that this may be
unavoidable in certain server environments, it STRONGLY ENCOURAGES
message store implementation techniques that avoid this problem.
Another cause of non-persistance is if the mailbox is deleted and
a new mailbox with the same name is created at a later date, Since
the name is the same, a client may not know that this is a new
mailbox unless the unique identifier validity is different. A
good value to use for the unique identifier validity value is a
32-bit representation of the creation date/time of the mailbox.
It is alright to use a constant such as 1, but only if it
guaranteed that unique identifiers will never be reused, even in
the case of a mailbox being deleted (or renamed) and a new mailbox
by the same name created at some future time.
The unique identifier of a message MUST NOT change during the
session, and SHOULD NOT change between sessions. However, if it is
not possible to preserve the unique identifier of a message in a
subsequent session, each subsequent session MUST have a new unique
identifier validity value that is larger than any that was used
previously.
2.3.1.2. Message Sequence Number Message Attribute
A relative position from 1 to the number of messages in the mailbox.
This position MUST be ordered by ascending unique identifier. As
each new message is added, it is assigned a message sequence number
that is 1 higher than the number of messages in the mailbox before
that new message was added.
Message sequence numbers can be reassigned during the session. For
example, when a message is permanently removed (expunged) from the
mailbox, the message sequence number for all subsequent messages is
decremented. Similarly, a new message can be assigned a message
sequence number that was once held by some other message prior to an
expunge.
In addition to accessing messages by relative position in the
mailbox, message sequence numbers can be used in mathematical
calculations. For example, if an untagged "EXISTS 11" is received,
and previously an untagged "8 EXISTS" was received, three new
messages have arrived with message sequence numbers of 9, 10, and 11.
Another example; if message 287 in a 523 message mailbox has UID
12345, there are exactly 286 messages which have lesser UIDs and 236
messages which have greater UIDs.
2.3.2. Flags Message Attribute
A list of zero or more named tokens associated with the message. A
flag is set by its addition to this list, and is cleared by its
removal. There are two types of flags in IMAP4rev1. A flag of
either type may be permanent or session-only.
A system flag is a flag name that is pre-defined in this
specification. All system flags begin with "\". Certain system
flags (\Deleted and \Seen) have special semantics described
elsewhere. The currently-defined system flags are:
\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
\Draft Message has not completed composition (marked as a
draft).
\Recent Message is "recently" arrived in this mailbox. This
session is the first session to have been notified
about this message; subsequent sessions will not see
\Recent set for this message. This flag can not be
altered by the client.
If it is not possible to determine whether or not
this session is the first session to be notified
about a message, then that message SHOULD be
considered recent.
If multiple connections have the same mailbox
selected simultaneously, it is undefined which of
these connections will see newly-arrives messages
with \Recent set and which will see it without
\Recent set.
A keyword is defined by the server implementation. Keywords do
not begin with "\". Servers MAY permit the client to define new
keywords in the mailbox (see the description of the
PERMANENTFLAGS response code for more information).
A flag may be permanent or session-only on a per-flag basis.
Permanent flags are those which the client can add or remove
from the message flags permanently; that is, subsequent sessions
will see any change in permanent flags. Changes to session
flags are valid only in that session.
Note: The \Recent system flag is a special case of a
session flag. \Recent can not be used as an argument in a
STORE command, and thus can not be changed at all.
2.3.3. Internal Date Message Attribute
The internal date and time of the message on the server. This is not
the date and time in the [RFC-822] header, but rather a date and time
which reflects when the message was received. In the case of
messages delivered via [SMTP], this SHOULD be the date and time of
final delivery of the message as defined by [SMTP]. In the case of
messages delivered by the IMAP4rev1 COPY command, this SHOULD be the
internal date and time of the source message. In the case of
messages delivered by the IMAP4rev1 APPEND command, this SHOULD be
the date and time as specified in the APPEND command description.
All other cases are implementation defined.
2.3.4. [RFC-822] Size Message Attribute
The number of octets in the message, as expressed in [RFC-822]
format.
2.3.5. Envelope Structure Message Attribute
A parsed representation of the [RFC-822] envelope information (not to
be confused with an [SMTP] envelope) of the message.
2.3.6. Body Structure Message Attribute
A parsed representation of the [MIME-IMB] body structure information
of the message.
2.4. Message Texts
In addition to being able to fetch the full [RFC-822] text of a
message, IMAP4rev1 permits the fetching of portions of the full
message text. Specifically, it is possible to fetch the [RFC-822]
message header, [RFC-822] message body, a [MIME-IMB] body part, or a
[MIME-IMB] header.
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