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Network Working Group Mark Crispin Internet Draft: IMAP2bis University of Washington Obsoletes: RFC 1176, 1064 August 1993 INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 2bis Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of any this or any other Internet Draft. This draft document will be submitted to the RFC editor as a Proposed Standard for the Internet Community. Discussion and suggestions for improvement are requested. This document will expire before 31 December 1993. Distribution of this draft is unlimited. Comments are solicited and should be sent to imap@CAC.Washington.EDU. Introduction The Interactive Mail Access Protocol, Version 2bis (IMAP2bis) allows a client to access and manipulate electronic mail on a server. IMAP2bis is designed to permit manipulations of remote mailboxes as if they were local. IMAP2bis includes operations for creating, deleting, and renaming mailbox folders; checking for new mail; permanently removing messages; setting and clearing flags; RFC 822 and MIME parsing; searching; and selective fetching of message attributes, texts, and portions thereof. IMAP2bis does not specify a means of posting mail; this function is handled by a mail transfer protocol such as SMTP (RFC 821). IMAP2bis assumes a reliable data stream such as provided by TCP. When TCP is used, an IMAP2bis server listens on port 143. Crispin [Page 1] Internet Draft IMAP2bis August 13, 1993 System Model and Philosophy There are three fundamental models of client/server email: offline, online, and disconnected use. IMAP2bis can be used in any one of these three models. The offline model is the most familiar form of client/server email today, and is used by protocols such as POP-3 (RFC 1225) and UUCP. In this model, a client application periodically connects to a server. It downloads all of the pending messages to the client machine and deletes these from the server. Thereafter, all mail processing is local to the client. This model is store-and-forward; it moves mail on demand from an intermediate server (maildrop) to a single destination machine. The online model is most commonly used with remote filesystem protocols such as NFS. In this model, a client application manipulates mailbox data on a server machine. A connection to the server is maintained throughout the entire session. No mailbox data is kept on the client; the client retrieves data from the server as is needed. IMAP2bis introduces a form of the online model which requires considerably less network bandwidth than a remote filesystem protocol, and provides the opportunity for using the server for CPU or I/O intensive functions such as parsing and searching. The disconnected use model is a hybrid of the offline and online models, and is used by protocols such as PCMAIL (RFC 1056). In this model, a client user downloads some set of messages from the server, manipulates them offline, then at some later time uploads the changes. The server remains the authoritative repository of the messages. The problems of synchronization (particularly when multiple clients are involved) are handled through the means of unique identifiers for each message. Each of these models have their own strengths and weaknesses, as shown in the following table: Feature Offline Online Disc ------- ------- ------ ---- Can use multiple clients NO YES YES Minimum use of server connect time YES NO YES Minimum use of server resources YES NO NO Minimum use of client disk resources NO YES NO Multiple remote mailboxes NO YES YES Fast startup NO YES NO Mail processing when not online YES NO YES Although IMAP2bis was originally designed to accommodate the online Crispin [Page 2] Internet Draft IMAP2bis August 13, 1993 model, it can support the other two models as well. This makes possible the creation of clients which can be used in any of the three models. For example, a user may wish to switch between the online and disconnected models on a regular basis (e.g. due to travel). IMAP2bis is designed to transmit message data on demand, and to provide the facilities necessary for a client to make an intelligent decision on what data it needs at any particular time. There is generally no need to do a wholesale transfer of an entire mailbox or even of the complete text of a message. This makes a difference in situations where the mailbox is large, or when the link to the server is slow. More specifically, IMAP2bis provides the capability of server-based RFC 822 and MIME processing. With this information, it is possible for a client to determine in advance whether or not it wishes to retrieve a particular message or part of a message. For example, a user connected to an IMAP2bis server via a dialup link can determine that a message has a 2000 byte text segment and a 40 megabyte video segment, and elect to fetch only the text segment. In IMAP2bis, the client/server relationship lasts only for the duration of the TCP connection, and mailbox state is maintained on the server. There is no registration of clients. With the possible exception of unique identifiers used in disconnected use operation, the client initially has no knowledge of mailbox state and learns it from the IMAP2bis server when a mailbox is selected. This initial transfer is minimal; the client requests additional state data as it needs it. Crispin [Page 3] Internet Draft IMAP2bis August 13, 1993 The Protocol The IMAP2bis 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 the 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 "*". The unsolicited data mechanism is used to transmit most data in IMAP2bis. The term "unsolicited data" refers to the fact that the data may have been transmitted without any explicit request by the client for that data. No distinction is made in IMAP2bis between data transmitted as a result of a client command and data which is unilaterally transmitted by the server. One form of unilaterally transmitted data which commonly occurs is an alert of a change to the mailbox made by some process other than the IMAP2bis client or server; for example, changes in the size of the mailbox (new mail) or in the status of individual messages. There is another special reserved tag, "+", discussed below. The server must be listening for a connection. When a connection is opened the server sends a greeting message and then waits for commands. This greeting is either a PREAUTH (meaning that the user has already been identified and authenticated by an external mechanism such as rsh) or OK (meaning that the user is not yet authenticated) unsolicited response. The server may also send a BYE unsolicited response and close the connection if it rejects the connection. The client opens a connection and waits for the greeting. The client must not send any commands until it has received the greeting from the server. Once the greeting has been received, the client may begin sending commands. It is not under any obligation to wait for a server response to a command before sending another command, subject to the constraints of underlying flow control. When commands are received the server acts on them and responds with command responses, often interspersed with data. In general, the command responses do not themselves contain the requested data, but rather indicate the completion status of the request. There are three fundamental responses: success (OK), error Crispin [Page 4] Internet Draft IMAP2bis August 13, 1993 (NO), request faulty or not understood (BAD). The effect of a command can not be considered complete until a command response with a tag matching the command is received from the server. It is not required that a server process a command to completion before beginning processing of the next command, except when the processing of the previous command may affect the results of the next command by changing the state of the current mailbox. This has certain other effects; for example, this implies that an EXPUNGE response can not be transmitted as part of a response to a request that may be streamed with another response. If authentication has not yet been completed, it must now be done via the LOGIN command before any IMAP2bis commands are permitted. The only other permitted command prior to successful authentication is LOGOUT. See the section below on authentication. Once authenticated, the client must send a mailbox selection command to access the desired mailbox; no mailbox is selected by default. Mailbox names are implementation dependent. However, the word "INBOX" must be implemented to mean the primary or default mailbox for this user, independent of any other server semantics. It is permitted for a server not to have an INBOX if there is no concept of a primary or default mailbox for this user. The name "INBOX" MUST NOT be used for any other purpose. On a successful selection, the server will 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 selection command. The client requests mailbox data with FETCH commands, and receives it via the unsolicited data mechanism. Three major categories of mailbox data exist. The first category is data that is associated with a message as an entity in the mailbox. There are now four such items of data: the "internal date", the "RFC 822 size", the "flags", and the "unique id". The internal date is the date and time that the message was placed in the mailbox. The RFC 822 size is the size in bytes of the message, expressed as an RFC 822 text string. The flags are a list of status flags associated with the message. The unique id is an identifier which is guaranteed to refer to this message and to none other and which, unlike IMAP2bis sequence numbers, persists across sessions. The second category is data that describes the composition and Crispin [Page 5] Internet Draft IMAP2bis August 13, 1993 delivery information of a message; that is, information such as the message sender, recipient lists, message-ID, subject, MIME structure, etc. This is the information that is stored in the RFC 822 and MIME headers. In IMAP2bis, the RFC 822 header information that may be fetched is called the "envelope" (not to be confused with SMTP envelopes). Similarly, the MIME header information that may be fetched is called the "body". A client can use the parsed envelope and body and not worry about having to do its own RFC 822 or MIME parsing. The third category is textual data, some of which is intended for direct human viewing. IMAP2bis defines six such items: RFC822.HEADER, RFC822.HEADER.LINES, RFC822.HEADER.LINES.NOT, RFC822.TEXT, RFC822, and MIME body parts. It is possible to fetch an individual MIME body part of a message without fetching any other data associated with the message. In the simplest case, a client can "FETCH RFC822" to get the entire message without any processing. A more sophisticated client might fetch some combination of the first and second categories of data for use as a presentation menu. Then, when the user wishes to read a particular message, it will fetch the appropriate texts. Data structures in IMAP2bis are represented as an S-expression list similar to that used in the Lisp programming language. An S- expression consists of a sequence of data items delimited by space and bounded at each end by parentheses. An S-expression may itself contain other S-expressions, using parentheses to indicate nesting. S-expression syntax was chosen because it provides a concise and precise means of expressing nested data (e.g. MIME structure). The client can alter certain data with a STORE command. As an example, a message is deleted from a mailbox by setting the \DELETED flag with a STORE command. Other client operations that can be done to a mailbox include copying messages to other mailboxes, permanently removing deleted messages, checking for updated mailbox state, and searching for messages that match certain criteria. It is also possible to select different mailboxes, create a new mailbox, and rename or delete an existing mailbox. The client should terminate the session with the LOGOUT command. The server returns a "BYE" followed by an "OK", at which point both the client and the server close the connection. If the client closes the network connection without a LOGOUT command, the server should perform its normal logout procedures without attempting any further interaction with the client. Crispin [Page 6] Internet Draft IMAP2bis August 13, 1993 Authentication Pre-authentication is only possible when the connection to the IMAP2bis service is made through some link protocol which provides its own authentication mechanism. It is not used with a TCP connection to port 143. An example of pre-authentication is the BSD "RSH" protocol which provides authentication through a "trusted host" facility. Another example would be a manual invocation of an IMAP2bis server from a logged-in timesharing job. A pre-authenticated IMAP2bis server should recognize that authentication has already happened, and enter the post-login state. In its greeting message, it should use the unsolicited response "PREAUTH" instead of "OK" to indicate that external authentication has taken place. This is an example of a pre-authentication scenario. In this and all other examples in this document, S: indicates server dialog and C: indicates client dialog. S: * PREAUTH IMAP2bis Server pre-authenticated as user "Smith" C: A001 SELECT INBOX S: * FLAGS (\Answered \Flagged \Deleted \Seen) S: * 19 EXISTS S: * 2 RECENT S: A001 OK SELECT complete A connection that is not pre-authenticated is constrained to using the LOGIN command for establishing authentication. Authentication via the LOGIN command is with either a user name and password pair, or with an user identifier and Kerberos authenticator. See the description of the LOGIN command for more details. Servers may allow non-authenticated access to certain mailboxes or bulletin boards. By convention, this is accomplished with a LOGIN command using the userid "anonymous". A password is still required. It is implementation-dependent what requirements, if any, are placed upon the password and what access restrictions are placed on anonymous users. Implementations are NOT required to support pre-authentication, Kerberos authentication, or the anonymous convention in order to be considered IMAP2bis compliant. Crispin [Page 7] Internet Draft IMAP2bis August 13, 1993 Definitions of Commands and Responses Summary of Defined Commands and Responses Commands || Responses -------- || ------- tag NOOP || tag OK response_text tag LOGIN user password || tag NO response_text tag LOGOUT || tag BAD response_text tag CREATE mailbox || * PREAUTH response_text tag DELETE mailbox || * OK response_text tag RENAME old_mailbox new_mailbox || * NO response_text tag FIND MAILBOXES pattern || * BAD response_text tag FIND ALL.MAILBOXES pattern || * BYE response_text tag FIND BBOARDS pattern || * MAILBOX mstring tag FIND ALL.BBOARDS pattern || * BBOARD mstring tag SUBSCRIBE MAILBOX mailbox || * SEARCH 1#number tag UNSUBSCRIBE MAILBOX mailbox || * FLAGS flag_list tag SUBSCRIBE BBOARD mailbox || * number EXISTS tag UNSUBSCRIBE BBOARD mailbox || * number RECENT tag SELECT mailbox || * number EXPUNGE tag EXAMINE mailbox || * number FETCH data tag BBOARD mailbox || * number COPY tag CHECK || * number STORE data tag EXPUNGE || + text tag COPY sequence mailbox || tag APPEND mailbox string || tag FETCH sequence data || tag PARTIAL msgno data start count || tag STORE sequence data value || tag UID AFTER uniqueid || tag UID BEFORE uniqueid || tag UID COPY sequence mailbox || tag UID FETCH sequence data || tag UID STORE sequence data value || tag SEARCH search_program || tag x_command arguments || Note: there is no pairing between commands and responses on the same line. Any command may result in any number (including none at all) of any of responses beginning with "*" (referred to as "unsolicited data"), followed by one tagged response. Crispin [Page 8] Internet Draft IMAP2bis August 13, 1993 Commands Certain commands and facilities described below are listed as "optional". This term is used to identify server capabilities which may not be present in a minimal server implementation or a server implementation based upon an older version of this specification. Such a server is considered to be IMAP2bis compatible (but not necessarily compliant) if it implements all commands which are not labeled as "optional". An IMAP2bis compliant server implementation MUST implement all of the commands and facilities listed below, whether or not they are labelled as "optional". An IMAP2bis compliant client MUST be prepared to accept BAD as a response to any of the optional commands or facilities, and substitute alternate or default action in that case. If, during the execution of any command, the server observes that the mailbox size has changed, the server should output an unsolicited EXISTS and RECENT response reflecting the changed size to alert the client. Similarly, any observed change in message status should cause an unsolicited FETCH response with the new flag data. tag NOOP The NOOP command returns an OK to the client. By itself, it does nothing else. However, since any command can return a status update as unsolicited data, this command can be used to poll for new mail or for message status updates. Another possible use of this command is for the client to "ping" the server so that the client and server know that each other are still alive. This is useful with servers that have an inactivity autologout timer. 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. If a server supports authentication via Kerberos, it may accept Crispin [Page 9] Internet Draft IMAP2bis August 13, 1993 the string "@KERBEROS:" followed by the hexadecimal representation of a Kerberos authenticator. EXAMPLE: The following is a Kerberos login scenario (note that the line breaks in the sample authenticator are for editorial clarity and are not in a real authenticator): S: * OK Kerberos IMAP2bis Server C: a001 LOGIN smith @KERBEROS:040700414e445245572e434d552e4544550 038202c868f3890b377fc8266acc1bedb96b80d3fa76489898e74cd1c952dc 4003ea3428f29f1c470016cf5adc22f939e6deff2747254c1815d5b0b90d4c 5a2cba21eb0abe32f9acbf568d751bf4cc13f5ba4e6d82c638a8b5421 S: a001 OK [df84a4cb8323454f] Login OK via Kerberos The token in the brackets in the OK response is the Kerberos authentication response, encrypted with the session key in network byte order and an incremented checksum as in the usual Kerberos procedure. tag LOGOUT The LOGOUT command informs the server that the client is done with the session. The server should send an unsolicited BYE response before the (tagged) OK response, and then close the network connection. Mailbox manipulation commands: CREATE, DELETE, RENAME These commands permit the manipulation of entire mailboxes. Server support for mailbox manipulation commands is optional. tag CREATE mailbox The CREATE command creates a mailbox with the given name. This command returns an OK response only if a new mailbox with that name has been created. It is an error to attempt to create a mailbox with a name that refers to an extant mailbox. Any error in creation will return a NO response. Creating INBOX is not permitted. If there is a primary or default mailbox for this user, it MUST exist and be called INBOX. Crispin [Page 10] Internet Draft IMAP2bis August 13, 1993 tag DELETE mailbox The DELETE command deletes a mailbox with the given name. This command returns an OK response only if a mailbox with that name has been deleted. It is an error to attempt to delete a mailbox name that does not exist. Any error in deletion will return a NO response. A server SHOULD NOT attempt to test that a mailbox is empty prior to permitting deletion; this would prevent the deletion of a mailbox which for some reason can not be opened or expunged, leaving to possible denial of service problems. Any such checking should be left to the discretion of the client. Deleting INBOX is not permitted. tag RENAME old_mailbox new_mailbox The RENAME command changes the name of a mailbox. This command returns an OK response only if a mailbox with the old name exists and has been successfully renamed to the new name. It is an error to attempt to rename with an old mailbox name that does not exist or a new mailbox name which already exists. Any error in renaming will return a NO response. Renaming INBOX is permitted. A new, empty INBOX is created in its place. FIND commands The FIND commands return some set of unsolicited MAILBOX or BBOARD replies, depending upon the type of FIND, that have as their value a single mailbox name. Three wildcard characters are defined in the pattern argument. "*" specifies any number (including zero) characters may match at this position. "%" and "?" specify a single character may match at this position. For example, FOO*BAR will match FOOBAR, FOOD.ON.THE.BAR and FOO.BAR, whereas FOO%BAR and FOO?BAR match only FOO.BAR. "*" will match all mailboxes. Server support for all forms of FIND is optional. If a FIND command returns BAD as a response then the client can make no assumptions about what mailboxes exist on the server. Crispin [Page 11] Internet Draft IMAP2bis August 13, 1993 tag FIND MAILBOXES pattern The FIND MAILBOXES command accepts as an argument a pattern (including wildcards) that specifies some set of mailbox names that the user has declared as being "active" or "subscribed." The exact meaning of this is implementation-dependent, since the concept of a set of "active" or "subscribed" mailboxes that is preserved across sessions may not be meaningful for a particular server or server implementation. If the SUBSCRIBE MAILBOX and UNSUBSCRIBE MAILBOX commands are implemented then this command returns the list manipulated by those commands. EXAMPLE: C: A002 FIND MAILBOXES * S: * MAILBOX FOOBAR S: * MAILBOX GENERAL S: A002 OK FIND completed tag FIND ALL.MAILBOXES pattern The FIND ALL.MAILBOXES command is similar to FIND MAILBOXES; however, it should return a complete list of all mailboxes available to the user. Data is returned as in FIND MAILBOXES. The special name INBOX is included in the output from FIND ALL.MAILBOXES unless INBOX is not supported by this server or for this user. The criteria for omitting INBOX is whether or not SELECT INBOX will return failure; it is not relevant whether or not the user's actual INBOX resides on this or some other server. FIND MAILBOXES and SUBSCRIBE MAILBOX provide a mechanism for the user to identify that this is his or her real INBOX. If FIND ALL.MAILBOXES is implemented FIND MAILBOXES must also be implemented, even if it returns the same information. Also, FIND ALL.MAILBOXES must, at least, return all the mailbox names that are returned by FIND MAILBOXES. The exact meaning of this is implementation-dependent, since the concept of a bounded or deterministic set of "mailboxes available to the user" may not be meaningful for a particular server or server implementation. tag FIND BBOARDS pattern The FIND BBOARDS command accepts as an argument a pattern that specifies some set of bulletin board names that the user has Crispin [Page 12] Internet Draft IMAP2bis August 13, 1993 declared as being "active" or "subscribed." Wildcards are permitted as in FIND MAILBOXES. The FIND BBOARDS command will return some set of unsolicited BBOARD replies that have as their value a single bulletin board name. EXAMPLE: C: A002 FIND BBOARDS * S: * BBOARD FOOBAR S: * BBOARD GENERAL S: A002 OK FIND completed The exact meaning of this is implementation-dependent, since the concept of a set of "active" or "subscribed" bboards that is preserved across sessions may not be meaningful for a particular server or server implementation. If the SUBSCRIBE BBOARD and UNSUBSCRIBE BBOARD commands are implemented then this command returns the list manipulated by those commands. tag FIND ALL.BBOARDS pattern The FIND ALL.BBOARDS command is similar to FIND BBOARDS; however, it should return a complete list of all bulletin boards available to the user. Data is returned as in FIND BBOARDS. If FIND ALL.BBOARDS is implemented FIND BBOARDS must also be implemented, even if it returns the same information. Also, FIND ALL.BBOARDS must, at least, return all the bboard names that are returned by FIND BBOARDS. The exact meaning of this is implementation-dependent, since the concept of a bounded or deterministic set of "bboards available to the user" may not be meaningful for a particular server or server implementation. Subscription commands These commands permit the manipulation of mailbox or bulletin board subscriptions. Server support for subscription commands is optional. Subscription status should be preserved between sessions. tag SUBSCRIBE MAILBOX mailbox The SUBSCRIBE MAILBOX command adds the specified mailbox name Crispin [Page 13] Internet Draft IMAP2bis August 13, 1993 to the list of "active" or "subscribed" mailboxes as returned by the FIND MAILBOXES command. This command returns an OK response only if the subscription is successful. If SUBSCRIBE MAILBOX is implemented then FIND MAILBOXES must also be implemented, and FIND ALL.MAILBOXES should be implemented. tag UNSUBSCRIBE MAILBOX mailbox The UNSUBSCRIBE MAILBOX command removes the specified mailbox name from the list of "active" or "subscribed" mailboxes as returned by the FIND MAILBOXES command. This command returns an OK response only if the unsubscription is successful. If UNSUBSCRIBE MAILBOX is implemented then SUBSCRIBE MAILBOXES and FIND MAILBOXES must also be implemented, and FIND ALL.MAILBOXES should be implemented. tag SUBSCRIBE BBOARD bboard The SUBSCRIBE BBOARD command adds the specified mailbox name to the list of "active" or "subscribed" bulletin boards as returned by the FIND BBOARDS command. This command returns an OK response only if the subscription is successful. If SUBSCRIBE BBOARD is implemented then FIND BBOARDS must also be implemented, and FIND ALL.BBOARDS should be implemented. tag UNSUBSCRIBE BBOARD bboard The UNSUBSCRIBE BBOARD command removes the specified mailbox name from the list of "active" or "subscribed" bulletin boards as returned by the FIND BBOARDS command. This command returns an OK response only if the unsubscription is successful. If UNSUBSCRIBE BBOARD is implemented then SUBSCRIBE BBOARDS and FIND BBOARDS must also be implemented, and FIND ALL.BBOARDS should be implemented. tag SELECT mailbox This command select a particular mailbox. The server must check that the user is permitted read access to this mailbox. Before Crispin [Page 14] Internet Draft IMAP2bis August 13, 1993 returning an OK to the client, the server must send the following unsolicited data to the client: FLAGS mailbox's defined flags <n> EXISTS the number of messages in the mailbox <n> RECENT the number of new messages in the mailbox in order to define the initial state of the mailbox at the client. Multiple selection commands are permitted in a session, in which case the previous mailbox is automatically deselected when a new selection is made. The mailbox name INBOX is a special name reserved to mean "the primary mailbox for this user on this server". The format of other mailbox names is implementation dependent. The text of an OK response to the SELECT command should begin with either "[READ-ONLY]" or "[READ-WRITE]" to show the mailbox's access status. tag EXAMINE mailbox The EXAMINE command is similar to SELECT, and returns the same output; however, the selected mailbox is identified as read-only and no changes are permitted to this mailbox. EXAMINE has the same mailbox namespace as SELECT. Server support for EXAMINE is optional. tag BBOARD mailbox The BBOARD command is similar to SELECT, and returns the same output. Its argument is a shared mailbox (bulletin board) name instead of an ordinary mailbox. There is no requirement that the namespace for BBOARD be the same as that for SELECT and EXAMINE. BBOARD also differs from EXAMINE in that it may allow changes (e.g. marking a message as seen or deleted) to a mailbox; the exact handling of this is implementation dependent. Server support for BBOARD is optional. tag CHECK The CHECK command requests a checkpoint of the mailbox. CHECK may cause an operation which may take a non-instantaneous amount of real-time to complete. The exact meaning of a checkpoint is Crispin [Page 15] Internet Draft IMAP2bis August 13, 1993 implementation-dependent. Possible interpretations include forcing a update of the server's disk of all changes made to the selected mailbox, rescanning of the entire mailbox, etc. If an implementation has no such considerations, CHECK should be equivalent to NOOP. CHECK should NOT be used to poll for new mail; new mail checking happens implicitly as part of every command. NOOP should be used for any new mail polling. CHECK should NOT be used to get the current size of the mailbox; there is no guarantee that CHECK will cause an EXISTS or RECENT unsolicited response. tag EXPUNGE The EXPUNGE command permanently removes all messages with the \DELETED flag set from the currently selected mailbox. Before returning an OK to the client, for each message that is removed, an unsolicited EXPUNGE response is sent. The message number for each successive 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 unsolicited EXPUNGE responses for message 5. tag COPY sequence mailbox The COPY command copies the specified message(s) to the specified destination mailbox. If the destination mailbox does not exist, a server that supports the CREATE command SHOULD return an error. It SHOULD NOT automatically create the mailbox. Unless it is certain that the destination mailbox can not be created, the server MUST, prior to the tagged NO response, send an unsolicited OK response with text beginning with the string "[TRYCREATE]". This gives a hint to the client that it can attempt a CREATE command and retry the COPY if the CREATE is successful. If the COPY command is unsuccessful for any reason, IMAP2bis compliant server implementations MUST restore the destination mailbox to its state prior to the COPY. EXAMPLE: A003 COPY 2:4 MEETING copies messages 2, 3, and 4 to mailbox "MEETING". Crispin [Page 16] Internet Draft IMAP2bis August 13, 1993 tag APPEND mailbox string The APPEND command appends the string, which is in the format of an RFC 822 message, to the specified mailbox. If the append is unsuccessful for any reason the mailbox must be restored to its state prior to the append; no partial appending is permitted. If the mailbox is presently selected, the normal new mail actions should occur. If the destination mailbox does not exist, a server MUST return an error, and MUST NOT automatically create the mailbox. Unless it is certain that the destination mailbox can not be created, the server MUST, prior to the tagged NO response, send an unsolicited OK response with text beginning with the string "[TRYCREATE]". This gives a hint to the client that it can attempt a CREATE command and retry the APPEND if the CREATE is successful. Note that this functionality is unsuitable for message delivery, because it does not provide a mechanism to transfer RFC 821 (SMTP) envelope information. Server support for APPEND is optional. 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) BODY The MIME body structure of the message. The body is computed by the server by parsing the MIME header lines. Server support for this data item is optional. BODY[section] The text of a particular body section. The section specification is a set of one or more part numbers delimited by periods. Single-part messages only have a part 1. Multipart messages are assigned consecutive part numbers, as they occur in the message. Crispin [Page 17] Internet Draft IMAP2bis August 13, 1993 If a particular part is of type message or multipart, its parts must be indicated by a period followed by the part number within that nested multipart part. It is not permitted to fetch a multipart part itself, only its individual members. A part of type MESSAGE also has nested parts, which are the parts of the MESSAGE part's body. Every message has at least one part. EXAMPLE: Here is a very complex message with its associated section specifications. 1 TEXT/PLAIN 2 APPLICATION/OCTET-STREAM 3 MESSAGE/RFC822 3.1 TEXT/PLAIN 3.2 APPLICATION/OCTET-STREAM MULTIPART/MIXED 4.1 IMAGE/GIF 4.2 MESSAGE/RFC822 4.2.1 TEXT/PLAIN MULTIPART/ALTERNATIVE 4.2.2.1 TEXT/PLAIN 4.2.2.2 TEXT/RICHTEXT Note that there is no section specification for the Multi-part parts (no section 4 or 4.2.2). The \SEEN flag is implicitly set; if this causes the flags to change they should be included as part of the fetch results. Server support for this data item is optional. ENVELOPE The envelope of the message. The envelope is computed by the server by parsing the RFC 822 header into the component parts, defaulting various fields as necessary. FAST Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE) FLAGS The flags that are set for this message. FULL Macro equivalent to: (FLAGS INTERNALDATE RFC822.SIZE ENVELOPE BODY) Server support for this data item is optional. Crispin [Page 18] Internet Draft IMAP2bis August 13, 1993 INTERNALDATE The date and time the message was written to the mailbox. RFC822 The message in RFC 822 format. The \SEEN flag is implicitly set; if this causes the flags to change they should be included as part of the fetch results. This is the concatenation of RFC822.HEADER and RFC822.TEXT. RFC822.HEADER The RFC 822 format header of the message as stored on the server including the delimiting blank line between the header and the body. RFC822.HEADER.LINES header_line_list All header lines (including continuation lines) of the RFC 822 format header of the message with a field-name (as defined in RFC 822) that matches any of the strings in header_line_list. The matching is case-independent but otherwise exact. This is used to obtain a filtered header containing only desired header lines. Server support for this data item is optional. RFC822.HEADER.LINES.NOT header_line_list All header lines (including continuation lines) of the RFC 822 format header of the message with a field-name (as defined in RFC 822) that does not match any of the strings in header_line_list. The matching is case-independent but otherwise exact. This is used to obtain a filtered header not containing undesired header lines. Server support for this data item is optional. RFC822.SIZE The number of characters in the message as expressed in RFC 822 format. RFC822.TEXT The text body of the message, omitting the RFC 822 header. The \SEEN flag is implicitly set; if this causes the flags to change they should be included as part of the fetch results. Crispin [Page 19] Internet Draft IMAP2bis August 13, 1993 UID The unique identifier for the message. Server support for this data item is optional. EXAMPLES: A003 FETCH 2:4 ALL fetches the flags, internal date, RFC 822 size, and envelope for messages 2, 3, and 4. A004 FETCH 3 RFC822 fetches the RFC 822 representation for message 3. A005 FETCH 4 (FLAGS RFC822.HEADER) fetches the flags and RFC 822 format header for message 4. tag PARTIAL msgno data start_byte byte_count The PARTIAL command is equivalent to the associated FETCH command, with the added functionality that only the specified number of bytes, beginning at the specified starting byte, are returned. Note that only a single message can be fetched at a time. The first byte of a message, and hence the minimum for the starting byte, is byte 1. The following FETCH items are valid data for PARTIAL: RFC822, RFC822.HEADER, RFC822.TEXT, and BODY[section]. Any partial fetch which attempts to read beyond the end of the text is truncated as appropriate. If the starting byte is beyond the end of the text, an empty string is returned. The data is returned with the FETCH response. There is no indication of the range of the partial data in this response; thus it is generally not possible to implement caching with PARTIAL data. It is also not possible to stream multiple PARTIAL commands of the same data item without processing and synchronizing at each step, since each PARTIAL fetch of data replaces any prior (PARTIAL) FETCH of that data. Note that when partial fetching it is possible to break in the middle of a a line or a criticial sequence such as a BASE64 quadruple or QUOTED-PRINTABLE shift. Implementations using partial fetching should keep this in mind. There is no requirement that partial fetches follow any sequence; so if it turns out that a partial fetch of bytes 1 through 10000 breaks in an awkward place, it is permitted to continue with a partial fetch Crispin [Page 20] Internet Draft IMAP2bis August 13, 1993 of 9987 through 19987, etc. The handling of the \SEEN flag is the same as with the FETCH command. Server support for PARTIAL is optional. 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. EXAMPLE: A003 STORE 2:4 +FLAGS (\DELETED) marks messages 2, 3, and 4 for deletion. UID commands These commands use unique identifiers instead of message numbers in their arguments to reference a particular message or range of messages. Sequence ranges are permitted; note however that there is no guarantee that unique identifiers be contiguous. A non-existant unique identifier within a sequence range is ignored without any error message generated. Because of the potential for ambiguity, the UID command does not change responses. That is, the number after the "*" in an unsolicited FETCH response is a message number, not a unique identifier. However, servers MUST implicitly include UID as part of any FETCH response caused by a UID command, regardless of whether or not UID was specified. Crispin [Page 21] Internet Draft IMAP2bis August 13, 1993 EXAMPLE: C: A003 UID FETCH 4827313:4828442 FLAGS S: * 23 FETCH (FLAGS (\SEEN) UID 4827313) S: * 24 FETCH (FLAGS (\SEEN) UID 4827943) S: * 25 FETCH (FLAGS (\SEEN) UID 4828442) S: A003 UID FETCH completed tag UID AFTER uniqueid The UID AFTER command is used to determine what unique identifiers exist that are greater than the specified unique identifier. It returns unsolicited FETCH responses for each such message. For example, if the specified unique identifier refers to message 572 in a mailbox with 613 messages, the results returned are equivalent to doing "FETCH 573:613 UID". tag UID BEFORE uniqueid The UID BEFORE command is used to determine what unique identifiers exist that are less than the specified unique identifier. It returns unsolicited FETCH responses for each such message. For example, if the specified unique identifier refers to message 572 in a mailbox with 613 messages, the results returned are equivalent to doing "FETCH 1:571 UID". tag UID COPY sequence mailbox The UID COPY command is identical to the COPY command, with the exception that the numbers used in the sequence are unique identifiers instead of message numbers. tag UID FETCH sequence data The UID FETCH command is identical to the FETCH command, with the exception that the numbers used in the sequence are unique identifiers instead of message numbers. tag UID STORE sequence data value The UID STORE command is identical to the STORE command, with Crispin [Page 22] Internet Draft IMAP2bis August 13, 1993 the exception that the numbers used in the sequence are unique identifiers instead of message numbers. tag SEARCH search_criteria The SEARCH command searches the mailbox for messages that match the given set of criteria. The unsolicited SEARCH <1#number> response from the server is a list of messages that express the intersection (AND function) of all the messages which match that criteria. For 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. In all search criteria which use strings, a message matches the criteria if the string is a substring of that field. The matching is case-independent except as noted below. The string may be in RFC 1342 format to express a character set other than US-ASCII. In this case, the criteria matches if the string matches the character set and characters of a substring of that field. A server implementation may omit case-independent matching on RFC 1342 strings. The currently defined search criteria are: ALL All messages in the mailbox; the default initial criterion for ANDing. ANSWERED Messages with the \ANSWERED flag set. BCC istring 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 istring Messages which contain the specified string in the body of the message. CC istring 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. Crispin [Page 23] Internet Draft IMAP2bis August 13, 1993 FROM istring Messages which contain the specified string in the envelope's FROM field. 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 within 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 istring Messages which contain the specified string in the envelope's SUBJECT field. TEXT istring Messages which contain the specified string. TO istring 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. Crispin [Page 24] Internet Draft IMAP2bis August 13, 1993 Responses The first group of responses complete a request, and indicate whether or not the command was successful. The response text is a line of human readable text, optionally prefixed by an atom inside square brackets that conveys a special information token between cooperating servers and clients. The currently defined special information tokens are: PARSE An error occurred in parsing the RFC 822 or MIME headers of a message in the mailbox. READ-ONLY The mailbox is open read-only, or its access while open has changed from read-write to read-only. READ-WRITE The mailbox is open read-write, or its access while open has changed from read-only to read-write. TRYCREATE An APPEND or COPY attempt failed because the target mailbox does not exist. The server sends this as a hint to the client that the operation would probably succeed if the mailbox is first created by means of the CREATE command. hex string A hexadecimal string is returned as a special information token to represent a Kerberos return authenticator. This only occurs in response to a LOGIN command that uses Kerberos authentication. Other special information tokens defined by particular client or server implementations should be prefixed with an "X" until such time as they are added to a revision of this protocol. tag OK response_text This response identifies successful completion of the command with that tag. The response text may be useful in a protocol telemetry log for debugging purposes. tag NO response_text This response identifies unsuccessful completion of the command with that tag. The text is a line of human-readable text that probably should be displayed to the user in an error report by the client. Crispin [Page 25] Internet Draft IMAP2bis August 13, 1993 tag BAD response_text This response identifies faulty protocol received from the client; The text is a line of human-readable text that should be recorded in any telemetry as part of a bug report to the maintainer of the client. The second group of responses convey human-readable information. The response text is a line of human readable text, optionally prefixed by an atom inside square brackets that conveys special information between cooperating servers and clients. * PREAUTH response_text This response is one of three possible greetings at session startup. It indicates that the session has already been authenticated by external means and thus no LOGIN command is needed. * OK response_text This response identifies an information message from the server. It does not indicate completion of any particular request, nor is it necessarily related to any request. The text is a line of human-readable text that should be presented to the user as an information message. This response is also one of three possible greetings at session startup. It indicates that the session is not yet authenticated and that a LOGIN command is needed. * NO response_text This response identifies a warning message from the server. It does not indicate completion of any request, nor is it necessarily related to any request. The text is a line of human-readable text that should be presented to the user as a warning of improper operation. * BAD response_text This response identifies a serious error message from the server. It does not indicate completion of any request, nor is it Crispin [Page 26] Internet Draft IMAP2bis August 13, 1993 necessarily related to any request. It may also indicate a faulty command from the client in which a tag could not be parsed. The text is a line of human-readable text that should be presented to the user as a serious or possibly fatal error. * BYE text This response identifies that the server is about to close the connection. The text is a line of human-readable text that 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. This response is also one of three possible greetings at session startup. It indicates that the server is not willing to accept a session from this client at the present time. The third group of responses convey data about the mailbox or messages inside the mailbox. This is how message data is transmitted from the server to the client. * MAILBOX mstring This response occurs as a result of a FIND command for MAILBOXES and ALL.MAILBOXES. The string is a mailbox name that matches the pattern in the command. * BBOARD mstring This response occurs as a result of a FIND command for BBOARDS and ALL.BBOARDS. The string is a bulletin board name that matches the pattern in the command. * SEARCH number(s) This response occurs as a result of a SEARCH command. The number(s) refer to those messages that match the search criteria. Each number is delimited by a space, e.g., "SEARCH 2 3 6". Crispin [Page 27] Internet Draft IMAP2bis August 13, 1993 * FLAGS flag_list This response occurs as a result of a selection command (SELECT, BBOARD, and EXAMINE). The flag list are the list of flags (at a minimum, the system-defined flags) that are applicable for this mailbox. Flags other than the system flags are a function of the server implementation. * number message_data This response occurs as a result of any command when a mailbox is selected. 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 previous 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. An unsolicited EXPUNGE response MUST NOT be sent except while responding to a request other than FETCH, STORE, or SEARCH. All references to message numbers sent after an unsolicited EXPUNGE response are adjusted to reflect the effect of the expunge. Discussion: a potential ambiguity exists with the FETCH, STORE, and SEARCH requests if the server is permitted to send unsolicited EXPUNGE responses. This is because these requests can be streamed. If two successive FETCH requests are streamed, and if during the time of the processing of the first request there is an expunge response, then the sequence of the second request is no longer valid. FETCH data This is the principle means by which data about a message is returned to the client. The data is in a S-expression form, and consists of a sequence of pairs of data item name and their values. The current data Crispin [Page 28] Internet Draft IMAP2bis August 13, 1993 items are: BODY An S-expression format list that describes the body of a message. The body is computed by the server by parsing the RFC 822 header and body into the component parts, defaulting various fields as necessary. Multiple parts are indicated by S-expression nesting. In this case, instead of a body type as the first element of the list there is a nested body. The last element of the list is the multipart subtype (mixed, digest, parallel, alternative, etc.). The basic fields of a non-multipart body part are in the following order: body type - an atom giving the content type name as defined in MIME body subtype - a string giving the content subtype name as defined in MIME body parameter list - an S-expression list of attribute/value pairs [e.g. (foo bar baz rag) where "bar" is the value of "foo" and "rag" is the value of "baz"] as defined in MIME. body id - a string giving the content id as defined in MIME. body description - a string giving the content description as defined in MIME. body encoding - a string giving the content transfer encoding as defined in MIME. body size - a number giving the size of that body in bytes. Note that this size is the size in its transfer encoding and not the resulting size. A body type of type MESSAGE and subtype RFC822 contains, immediately after the basic fields, the envelope of the encapsulated message, its body structure, and its size in text lines. A body type of type TEXT contains, immediately after the basic fields, the size of the body in text lines. BODY[section] A string expressing the body contents of the specified section. This string is transmitted as 7-bit US-ASCII, and interpreted according to the content transfer encoding and the body type Crispin [Page 29] Internet Draft IMAP2bis August 13, 1993 and subtype. Note that 7-bit US-ASCII is the transport format and NOT necessarily the byte size or character set of the result. The resulting data may be 8-bit; it may also be some other character set or binary. ENVELOPE An S-expression format list that describes the envelope of a message. The envelope is computed by the server by parsing the RFC 822 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 that 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 that 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 that are set for this message. This may include the following system flags: \RECENT Message arrived since the previous time this mailbox was read \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 Crispin [Page 30] Internet Draft IMAP2bis August 13, 1993 INTERNALDATE A string containing the date and time the message was written to the mailbox. RFC822 A string expressing the message in RFC 822 format. RFC822.HEADER A string expressing the RFC 822 format header of the message, including the delimiting blank line between the header and the body. This is used for the FETCH data items RFC822.HEADER, RFC822.HEADER.LINES, and RFC822.HEADER.LINES.NOT (note that a blank line is always included regardless of header line restrictions). RFC822.SIZE A number indicating the number of characters in the message as expressed in RFC 822 format. RFC822.TEXT A string expressing the text body of the message, omitting the RFC 822 header. UID A string expressing the unique identifier of the message. COPY Obsolete. New server implementations MUST NOT transmit this response. Client implementations SHOULD ignore this response (not report an error). STORE data Obsolete and functionally equivalent to FETCH. New server implementations MUST NOT transmit this response. Client implementations SHOULD treat this response as equivalent to the FETCH response. The final group of responses contains a single, special purpose response. + text This response identifies that the server is ready to accept the text of a literal from the client. The text of this response is a line of human-readable text of the server's choosing (it is generally never seen by a client's human user). Crispin [Page 31] Internet Draft IMAP2bis August 13, 1993 The purpose of this command is to solve a synchronization problem which can occur if one of the strings in a command is a literal. This may occur when logging in (if the password contains "funny" characters) and especially with the APPEND command (since the message sent consists of multiple lines). 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 for commands that 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 expects it, preserving client/server synchronization. No such synchronization protection is provided for literals sent from the server to the client. Nor is any necessary. Any synchronization problems in this direction would be caused by a bug in the client or server. Crispin [Page 32] Internet Draft IMAP2bis August 13, 1993 Sample IMAP2bis session The following is a transcript of an IMAP2bis session. One very long long line in this sample is broken for editorial clarity. S: * OK IMAP2bis Service 7.2(62) at Thu, 29 Jul 1993 21:34:23 -0700 (PDT) C: a001 login mrc secret S: a001 OK LOGIN completed C: a002 select inbox S: * 18 EXISTS S: * FLAGS (\Answered \Flagged \Deleted \Seen) S: * 0 RECENT S: a002 OK [READ-WRITE] SELECT completed S: a003 fetch 12 full S: * 12 FETCH (FLAGS (\Seen) INTERNALDATE "14-Jul-1993 02:44:25 -0700" RFC822.SIZE 4282 ENVELOPE ("Wed, 14 Jul 1993 02:23:25 -0700 (PDT)" "IMAP2bis WG mtg summary and minutes" (("Terry Gray" NIL "gray" "cac.washington.edu")) ((NIL NIL "owner-imap" "cac.washington.edu")) (("Terry Gray" NIL "gray" "cac.washington.edu")) ((NIL NIL "imap" "cac.washington.edu")) ((NIL NIL "minutes" "CNRI.Reston.VA.US") ("John C Klensin" NIL "KLENSIN" "INFOODS.MIT.EDU")("Erik Huizer" NIL "Erik.Huizer" "SURFnet.nl")) NIL NIL "<Pine.3.84.9307140123.B27397-0100000@shiva2.cac.washington.edu>") BODY ("TEXT" "PLAIN" ("CHARSET" "US-ASCII") NIL NIL "7BIT" 3028 92)) S: a003 OK FETCH completed C: a004 fetch 12 rfc822.header S: * 12 FETCH (RFC822.HEADER {485} S: Date: Wed, 14 Jul 1993 02:23:25 -0700 (PDT) S: From: Terry Gray <gray@cac.washington.edu> S: Reply-To: Terry Gray <gray@cac.washington.edu> S: Subject: IMAP2bis WG mtg summary and minutes S: To: imap@cac.washington.edu S: Cc: minutes@CNRI.Reston.VA.US, S: John C Klensin <KLENSIN@INFOODS.MIT.EDU>, S: Erik Huizer <Erik.Huizer@SURFnet.nl> S: Message-Id: S: <Pine.3.84.9307140123.B27397-0100000@shiva2.cac.washington.edu> S: Mime-Version: 1.0 S: Content-Type: TEXT/PLAIN; CHARSET=US-ASCII S: S: ) S: a004 OK FETCH completed C: a005 store 12 +flags \deleted S: * 12 FETCH (FLAGS (\Seen \Deleted)) S: a005 OK +FLAGS completed C: a006 logout S: * BYE IMAP2bis server terminating connection S: a006 OK LOGOUT completed Crispin [Page 33] Internet Draft IMAP2bis August 13, 1993 Design Discussion IMAP2bis is a textual protocol. The use of MIME encoding in IMAP2bis makes it possible to support 8-bit textual and binary mail. IMAP2bis implementations MAY transmit 8-bit characters in literals, but should do so only when the character set is identified. For example, 8-bit characters are specifically permitted in MIME body parts (fetching BODY[section]) of type TEXT. 8-bit characters are also permitted in the argument to APPEND. For compatibility with the 7-bit world, servers MUST NOT transmit 8- bit characters in RFC822.HEADER fetch results, and SHOULD NOT transmit 8-bit characters in RFC822.TEXT (and by extension RFC822) fetch results. Because 8-bit characters are permitted in the argument to APPEND, a server in the 7-bit world MUST, if it implements the APPEND command, be able to make a reversible conversion of the 8-bit APPEND data to 7-bit data using MIME. Although a BINARY body encoding is defined, IMAP2bis does not permit unencoded binary strings. A "binary string" is any string with NUL characters; a string with an excessive number of CTL characters may also be considered to be binary. The mixing of unencoded binary in the same stream as textual commands would make the protocol more vulnerable to synchronization problems. Implementations MUST encode binary data into BASE64 prior to transmitting it with IMAP2bis. When operating in the online model, an IMAP2bis client should maintain a local cache of data from the mailbox. This cache is an incomplete model of the mailbox, and at startup is generally empty. As the client processes all unsolicited data, it updates the cache based on this data. When a tagged response arrives, the client's cache has been updated from the associated request. Note that a server can send data that the client did not request, such as mailbox size or flag updates. A server MUST send mailbox size updates automatically in the course of processing a command. A server SHOULD send message flag updates automatically, without requiring the client to request such updates explicitly. Regardless of what implementation decisions a client may take on caching, a client MUST record EXISTS and RECENT updates and MUST NOT assume that a CHECK or NOOP command will return EXISTS or RECENT information. Although it is permitted for a server to send an unsolicited response Crispin [Page 34] Internet Draft IMAP2bis August 13, 1993 while there is no command in progress, this practice SHOULD NOT be followed due to flow control considerations. It can cause an incautious implementation to deadlock. A deadlock is avoided if either of the following conditions are true: (1) with the exception of the greeting, the server never sends responses while there is no command in progress; (2) the client always reads responses from the IMAP2bis connection regardless of whether or not a command is in progress (or even if it is transmitting the command). The latter condition generally requires a multi-threading client implementation, which may not always be feasible. If a server has an inactivity autologout timer, that timer MUST be of at least 30 minutes' duration. The receipt of a NOOP command from the client during that interval should suffice to reset the autologout timer. Periodic transmission of a NOOP from the client during periods of inactivity also has the benefit of avoiding the possible deadlock noted above, or even act as a poll for updates. It is frequently asked why there is no message posting capabilities in IMAP2bis. Message posting is orthogonal to the scope of a mail access protocol and detracts from its primary focus. SMTP (RFC 821) provides the minimal functionality needed for message posting without losing valuable capabilities (such as blind carbon copies). Any message posting capability in IMAP2bis would need, at a minimum, to provide equivalent capabilities. At the time of the writing of this document, an extensive set of extensions to SMTP is in the Internet standards process. Should those extensions become an Internet Standard it would be necessary to revise IMAP2bis again to provide corresponding capabilities, if a message posting facility was included in IMAP2bis. In other words, a duplication of effort would be required each time a change is made to message transport technology. Another undesirable aspect of message posting in IMAP2bis occurs when a (very) remote server is used. It is unlikely that a client will support multiple means of posting a message; it adds excessive size and complexity which can not be afforded, particularly on smaller machines. Consider a client connecting to an IMAP2bis server over an interactive satellite link to a foreign country. A local message posting (SMTP) server is available which uses a lower-cost batched link. In this case, the use of the interactive link would be wasteful and costly. Message posting to IMAP2bis has been suggested as a means of authenticating postings. The problem is that access authentication credentials are not necessarily the same as posting authentication credentials. At some sites, the disclosure of a portion of access Crispin [Page 35] Internet Draft IMAP2bis August 13, 1993 authentication credentials in a mail message (as a "From" or "Sender" address) may be a serious security breach of greater significance than forged mail. The Internet message transport infrastructure has no concept of authentication credentials, and neither authentication syntax nor semantics are transferred within a message. As a result, any attempt at authenticating a message via posting authenticaton is completely ineffective once the message leaves the authenticating server; any indication of authentication in the message can easily be reproduced further down the line. It is for this reason that public-key based message authentication systems such as Privacy Enhanced Mail are presently under development. IMAP2bis does not address problems with multiple IMAP2bis servers at a single site, access control lists, and mobility of client configuration and address book information. These and other issues are being considered for a companion protocol (Interactive Mail Support Protocol) being developed at CMU. Crispin [Page 36] Internet Draft IMAP2bis August 13, 1993 Formal Syntax The following syntax specification uses the augmented Backus-Naur Form (BNF) notation as specified in RFC 822 with one exception; the delimiter used with the "#" construct is a single space (SPACE) and not a comma. Except as noted otherwise, all alphabetic characters in the IMAP2bis protocol are case-insensitive. For example, "LOGIN", "login" and "lOgIn" are all valid as the login command. Syntax marked as obsolete may be encountered with implementations written for an older version of this specification. New implementations SHOULD accept obsolete syntax as input, but MUST NOT otherwise use it. address ::= "(" addr_name SPACE addr_adl SPACE addr_mailbox SPACE addr_host ")" addr_adl ::= nil / string addr_host ::= nil / string addr_mailbox ::= nil / string addr_name ::= nil / string append ::= "APPEND" SPACE mailbox SPACE string astring ::= atom / string atom ::= 1*<any CHAR except specials, SPACE, and CTLs> bboard ::= "BBOARD" SPACE mailbox_bboard body ::= "(" body_structure ")" body_basic ::= body_type SPACE body_subtype SPACE body_parameter SPACE body_id SPACE body_description SPACE body_encoding SPACE body_size_byte body_description::= nil / string body_encoding ::= "7BIT" / "8BIT" / "BINARY" / "BASE64"/ "QUOTEDPRINTABLE" / atom body_id ::= nil / string Crispin [Page 37] Internet Draft IMAP2bis August 13, 1993 body_msg ::= body_basic SPACE envelope SPACE body body_parameter ::= nil / "(" 1#(string string) ")" body_section ::= number / number "." body_section body_size_byte ::= number body_size_line ::= number body_structure ::= body_basic / body_msg / body_text / "(" body_structure ")" SPACE body_subtype body_subtype ::= nil / string body_text ::= body_basic SPACE body_size_line body_type ::= "TEXT" / "MULTIPART" / "MESSAGE" / "APPLICATION" / "AUDIO" / "IMAGE" / "VIDEO" / atom CHAR ::= <any 7-bit US-ASCII character except NUL, 0x01 - 0x7f> CHAR8 ::= <any 8-bit octet except NUL, 0x01 - 0xff> check ::= "CHECK" copy ::= "COPY" SPACE sequence SPACE mailbox CR ::= <ASCII CR, carriage return, 0x0C> create ::= "CREATE" SPACE mailbox CRLF ::= CR LF CTL ::= <any ASCII control character and DEL, 0x00 - 0x1f, 0x7f> date ::= date_new / date_old date_day ::= 1*2DIGIT ;; day of month date_month ::= "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" / "Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec" date_new ::= date_day "-" date_month "-" date_year date_old ::= date_day "-" date_month "-" date_year_old ;; Obsolete Crispin [Page 38] Internet Draft IMAP2bis August 13, 1993 date_year ::= 4DIGIT / date_year_old date_year_old ::= 2DIGIT ;; Obsolete, (year - 1900) date_time ::= date_time_new / date_time_old date_time_new ::= date_new SPACE time SPACE zone date_time_old ::= date_old SPACE time "-" zone_old ;; Obsolete delete ::= "DELETE" SPACE mailbox_other DIGIT ::= <any ASCII decimal digit, 0x30 - 0x39> DIGIT_HEX :: DIGIT / "a" / "b" / "c" / "d" / "e" / "f" envelope ::= "(" env_date SPACE env_subject SPACE env_from SPACE env_sender SPACE env_reply-to SPACE env_to SPACE env_cc SPACE env_bcc SPACE env_in-reply-to SPACE 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 ")" examine ::= "EXAMINE" SPACE mailbox expunge ::= "EXPUNGE" Crispin [Page 39] Internet Draft IMAP2bis August 13, 1993 fetch ::= "FETCH" SPACE sequence SPACE ("ALL" / "FULL" / "FAST" / fetch_att / "(" 1#fetch_att ")") fetch_att ::= fetch_att_other / fetch_att_text fetch_att_other ::= "BODY" / "ENVELOPE" / "FLAGS" / "INTERNALDATE" / "RFC822.SIZE" / "UID" fetch_att_text ::= "BODY[" body_section "]" / "RFC822" / "RFC822.HEADER" / "RFC822.HEADER.LINES" SPACE header_line_list / "RFC822.HEADER.LINES.NOT" SPACE header_line_list / "RFC822.TEXT" find ::= find_mailbox / find_bboard find_bboard ::= find_bboards / find_boards_all find_bboards ::= "FIND" SPACE "BBOARDS" SPACE find_pattern find_bboards_all ::= "FIND" SPACE "ALL.BBOARDS" SPACE find_pattern find_mailbox ::= find_mailboxes / find_mailboxes_all find_mailboxes ::= "FIND" SPACE "MAILBOXES" SPACE find_pattern find_mailboxes_all ::= "FIND" SPACE "ALL.MAILBOXES" SPACE find_pattern find_pattern ::= astring ;; includes find_wildcards find_wildcards ::= "%" / "?" / "*" flag ::= user_flag / system_flag flag_list ::= "(" 1#flag ")" flags ::= 1#flag / flag_list greeting ::= "*" SPACE (human_data / "PREAUTH" SPACE response_text) header_line ::= string header_line_list ::= "(" 1#header_line ")" Crispin [Page 40] Internet Draft IMAP2bis August 13, 1993 human_data ::= "OK" SPACE response_text / "NO" SPACE response_text / "BAD" SPACE response_text / "BYE" SPACE resonse_text inbox ::= "INBOX" ;; case-independent, but SHOULD be upper-case istring ::= astring ;; possible RFC 1342 format data kerberos_authenticator ::= 1*DIGIT_HEX kerberos_response ::= 1*DIGIT_HEX LF ::= <ASCII LF, line feed, 0x0A> literal ::= "{" number "}" CRLF 1*CHAR8 ;; The number represents the number of CHAR8 octets. login ::= "LOGIN" SPACE userid SPACE password logout ::= "LOGOUT" mailbox ::= inbox / mailbox_other mailbox_bboard ::= astring ;; May not be INBOX (in any case). Should not ;; include find_wildcards. May be case-dependent ;; as a function of server implementation. May ;; be a different namespace from mailbox_other. mailbox_other ::= astring ;; May not be INBOX (in any case). Should not ;; include find_wildcards. May be case-dependent ;; as a function of server implementation mailbox_data ::= "MAILBOX" SPACE mstring / "BBOARD" SPACE mstring / "SEARCH" SPACE 1#number / "FLAGS" SPACE flag_list message_data ::= number SPACE (msg_exists / msg_recent / msg_expunge / msg_fetch / msg_obsolete) msg_copy ::= "COPY" ;; Obsolete msg_exists ::= "EXISTS" Crispin [Page 41] Internet Draft IMAP2bis August 13, 1993 msg_expunge ::= "EXPUNGE" msg_fetch ::= "FETCH" SPACE "(" 1#("BODY" SPACE body / "BODY[" body_section "]" string / "ENVELOPE" SPACE envelope / "FLAGS" SPACE "(" 1#(recent_flag / flag) ")" / "INTERNALDATE" SPACE date_time / "RFC822" SPACE string / "RFC822.HEADER" SPACE string / "RFC822.SIZE" SPACE number / "RFC822.TEXT" SPACE string / "UID" SPACE uniqueid) ")" msg_obsolete ::= msg_copy / msg_store ;; Obsolete unsolicited data responses msg_recent ::= "RECENT" msg_store ::= "STORE" SPACE "(" 1#("FLAGS" SPACE "(" 1#(recent_flag / flag) "))" ;; Obsolete mstring ::= text_line ;; Represents a mailbox nil ::= "NIL" noop ::= "NOOP" number ::= 1*DIGIT partial ::= "PARTIAL" SPACE number SPACE fetch_att_text SPACE number SPACE number password ::= astring / "@KERBEROS:" kerberos_authenticator QCHAR ::= <any CHAR except qspecials, CR, and LF> qspecials ::= <"> / "%" / "\" quoted_string ::= <"> *QCHAR <"> recent_flag ::= "\RECENT" ready ::= "+" SPACE response_text rename ::= "RENAME" SPACE mailbox SPACE mailbox_other Crispin [Page 42] Internet Draft IMAP2bis August 13, 1993 request ::= tag SPACE (noop / login / logout / create / delete / rename / find / subscribe / unsubscribe / select / examine / bboard / check / expunge / copy / append / fetch / partial / store / uid / search / x_command ) CRLF response ::= tag SPACE ("OK" / "NO" / "BAD") SPACE response_text CRLF response_code ::= "PARSE" / "READ-ONLY" / "READ-WRITE" / "TRYCREATE" / "X" atom / kerberos_response response_text ::= ("[" response_code "]" text_line) / text_line search ::= "SEARCH" SPACE 1#("ALL" / "ANSWERED" / "BCC" SPACE istring / "BEFORE" SPACE day / "BODY" SPACE istring / "CC" SPACE istring / "DELETED" / "FLAGGED" / "KEYWORD" SPACE atom / "NEW" / "OLD" / "ON" SPACE day / "RECENT" / "SEEN" / "SINCE" SPACE day / "TEXT" SPACE istring / "TO" SPACE istring / "UNANSWERED" / "UNDELETED" / "UNFLAGGED" / "UNKEYWORD" / "UNSEEN") select ::= "SELECT" SPACE mailbox sequence ::= number / (sequence "," sequence) / (number ":" number) ;; identifies a set of messages by consecutive numbers ;; from 1 to the number of messages in the mailbox. ;; Comma delimits individual numbers, colon delimits ;; between two numbers inclusive. ;; Example: 2,4:7,9,12:15 is 2,4,5,6,7,9,12,13,14,15 SPACE ::= <ASCII SP, space, 0x20> specials ::= "{" / qspecials store ::= "STORE" SPACE sequence SPACE store_att store_att ::= "+FLAGS" SPACE flags / "-FLAGS" SPACE flags / "FLAGS" SPACE flags string ::= quoted_string / literal subscribe ::= subscribe_mailbox / subscribe_bboard subscribe_bboard ::= "SUBSCRIBE" SPACE "BBOARD" SPACE mailbox_bboard Crispin [Page 43] Internet Draft IMAP2bis August 13, 1993 subscribe_mailbox ::= "SUBSCRIBE" SPACE "MAILBOX" SPACE mailbox system_flag ::= "\ANSWERED" / "\FLAGGED" / "\DELETED" / "\SEEN" tag ::= atom text_line ::= 1*<any CHAR except CR and LF> time ::= 2DIGIT ":" 2DIGIT ":" 2DIGIT ;; hours minutes seconds uid ::= "UID" SPACE (uid_after / uid_before / copy / fetch / store) ;; uniqueids used instead of message numbers uid_after ::= "AFTER" SPACE uniqueid uid_before ::= "BEFORE" SPACE uniqueid uniqueid ::= number ;;; strictly ascending unsolicited ::= "*" SPACE (human_data / mailbox_data / message_data) CRLF unsubscribe ::= unsubscribe_mailbox / unsubscribe_bboard unsubscribe_bboard ::= "UNSUBSCRIBE" SPACE "BBOARD" SPACE mailbox_bboard unsubscribe_mailbox ::= "UNSUBSCRIBE" SPACE "MAILBOX" SPACE mailbox_mailbox userid ::= astring user_flag ::= atom x_command ::= "X" atom <optional arguments> ;; experimental expansion commands zone ::= ("+" / "-") 4DIGIT ;; Signed four-digit value of hhmm representing ;; hours and minutes west of Greenwich (that is, ;; (the amount that the given time differs from ;; Universal Time). Subtracting the timezone ;; from the given time will give the UT form. ;; The Universal Time zone is "+0000". Crispin [Page 44] Internet Draft IMAP2bis August 13, 1993 zone_old ::= "UT" / "GMT" / "Z" / ;; +0000 "AST" / "EST" / "CST" / "MST" / ;; -0400 to -0700 "PST" / "YST" / "HST" / "BST" / ;; -0800 to -1100 "ADT" / "EDT" / "CDT" / "MDT" / ;; -0300 to -0600 "PDT" / "YDT" / "HDT" / "BDT" / ;; -0700 to -1000 "A" / "B" / "C" / "D" / "E" / "F" / ;; +0100 to +0600 "G" / "H" / "I" / "K" / "L" / "M" / ;; +0700 to +1200 "N" / "O" / "P" / "Q" / "R" / "S" / ;; -0100 to -0600 "T" / "U" / "V" / "W" / "X" / "Y" ;; -0700 to -1200 ;; Obsolete A protocol session is as follows: Server: greeting *<Client: request (first part, if it contains a literal) *<Server: ready Client: request (next part) > Server: *<unsolicited> Server: response > Crispin [Page 45] Internet Draft IMAP2bis August 13, 1993 Acknowledgements Bill Yeager and Rich Acuff contributed invaluable suggestions in the evolution of IMAP2 from the original IMAP. James Rice pointed out several ambiguities in the previous IMAP2 specification. My colleagues on the Pine team -- Steve Hubert, Laurence Lundblade, David Miller, and Mike Seibel -- worked long and hard to create a fantastic email user agent with worldwide popularity. Without their efforts, IMAP2 would have languished in obscurity. Terry Gray, our boss, provided much-needed moral support and guidance, while refusing to let us get away with "good enough" when "great" was possible. The present protocol would not have come into existance without the assistance of the IETF IMAP2 working group, and in particular Ned Freed, John Myers, Chris Newman, and Adam Treister. Any mistakes, flaws, or sins of omission in this IMAP2bis protocol specification are, however, strictly my own; and the mention of any name above does not imply an endorsement. Security Considerations Security issues are discussed in this memo only as far as authentication for the purpose of accessing a server are concerned. Author's Address Mark R. Crispin Networks and Distributed Computing, JE-30 University of Washington Seattle, WA 98195 Phone: (206) 543-5762 EMail: MRC@CAC.Washington.EDU Crispin [Page 46]