This RFC is a discussion of the Pcmail workstation based distributed mail system. It is identical to the discussion in RFC-993, save that a new, much simpler mail transport protocol is described. The new transport protocol is the result of continued research into ease of protocol implementation and use issues. Distribution of this memo is unlimited.
Pcmail is a distributed mail system providing mail service to an arbitrary number of users, each of whom owns one or more workstations. Pcmail's motivation is to provide very flexible mail service to a wide variety of different workstations, ranging in power from small, resource-limited machines like IBM PCs to resource-rich (where "resources" are primarily processor speed and disk space) machines like Suns or Microvaxes. It attempts to provide limited service to resource-limited workstations while still providing full service to resource-rich machines. It is intended to work well with machines only infrequently connected to a network as well as machines permanently connected to a network. It is also designed to offer diskless workstations full mail service.
The system is divided into two halves. The first consists of a single entity called the "repository". The repository is a storage center for incoming mail. Mail for a Pcmail user can arrive externally from the Internet or internally from other repository users. The repository also maintains a stable copy of each user's mail state (this will hereafter be referred to as the user's "global mail state"). The repository is therefore typically a computer with a large amount of disk storage.
The second half of Pcmail consists of one or more "clients". Each Pcmail user may have an arbitrary number of clients, typically single-user workstations. The clients provide a user with a friendly means of accessing the user's global mail state over a network. In order to make the interaction between the repository and a user's clients more efficient, each client maintains a local copy of its user's global mail state, called the "local mail state". It is assumed that clients, possibly being small personal computers, may not always have access to a network (and therefore to the global mail state in the repository). This means that the local and global mail states may not be identical all the time, making synchronization between local and global mail states necessary.
Clients communicate with the repository via the Distributed Mail System Protocol (DMSP); the specification for this protocol appears in appendix A. The repository is therefore a DMSP server in addition to a mail end-site and storage facility. DMSP provides a complete set of mail manipulation operations ("send a message", "delete a message", "print a message", etc.). DMSP also provides special operations to allow easy synchronization between a user's global mail state and his clients' local mail states. Particular attention has been paid to the way in which DMSP operations act on a user's mail state. All DMSP operations are failure-atomic (that is, they are guaranteed either to succeed completely, or leave the user's mail
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state unchanged ). A client can be abruptly disconnected from the repository without leaving inconsistent or damaged mail states.
Pcmail's design has been directed by the characteristics of currently available workstations. Some workstations are fairly portable, and can be packed up and moved in the back seat of an automobile. A few are truly portable--about the size of a briefcase--and battery- powered. Some workstations have constant access to a high-speed local-area network; pcmail should allow for "on-line" mail delivery for these machines while at the same time providing "batch" mail delivery for other workstations that are not always connected to a network. Portable and semi-portable workstations tend to be resource-poor. A typical IBM PC has a small amount (typically less than one megabyte) of main memory and little in the way of mass storage (floppy-disk drives that can access perhaps 360 kilobytes of data). Pcmail must be able to provide machines like this with adequate mail service without hampering its performance on more resource-rich workstations. Finally, all workstations have some common characteristics that Pcmail should take advantage of. For instance, workstations are fairly inexpensive compared to the various time-shared systems that most people use for mail service. This means that people may own more than one workstation, perhaps putting a Microvax in an office and an IBM PC at home.
Pcmail's design reflects the differing characteristics of the various workstations. Since one person can own several workstations, Pcmail allows users multiple access points to their mail state. Each Pcmail user can have several client workstations, each of which can access the user's mail by communicating with the repository over a network. The clients all maintain local copies of the user's global mail state, and synchronize the local and global states using DMSP.
It is also possible that some workstations will only infrequently be connected to a network (and thus be able to communicate with the repository). The Pcmail design therefore allows two modes of communication between repository and client. "Interactive mode" is used when the client is always connected to the network. Any changes to the client's local mail state are immediately also made to the repository's global mail state, and any incoming mail is immediately transmitted from repository to client. "Batch mode" is used by clients that have infrequent access to the repository. Users manipulate the client's local mail state, queueing the changes locally. When the client is next connected to the repository, the changes are executed, and the client's local mail state is synchronized with the repository's global mail state.
Finally, the Pcmail design minimizes the effect of using a resource- poor workstation as a client. Mail messages are split into two
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parts: a "descriptor" and a "body". The descriptor is a capsule message summary whose length (typically about 100 bytes) is independent of the actual message length. The body is the actual message text, including an RFC-822 standard message header. While the client may not have enough storage to hold a complete set of messages, it can usually hold a complete set of descriptors, thus providing the user with at least a summary of his mail state. For clients with extremely limited resources, Pcmail allows the storage of partial sets of descriptors. Although this means the user does not have a complete local mail state, he can at least look at summaries of some messages. In the cases where the client cannot immediately store message bodies, it can always pull them over from the repository as storage becomes available.
The remainder of this document is broken up into sections discussing the following:
- The repository architecture
- DMSP, its operations, and motivation for its design
- The client architecture
- A typical DMSP session between the repository and a client
- The current Pcmail implementation
- Appendices describing the DMSP protocol in detail
A typical machine running repository code has a relatively powerful processor and a large amount of disk storage. It must also be a permanent network site, for two reasons. First, clients communicate with the repository over a network, and rely on the repository's being available at any time. Second, people sending mail to repository users rely on the repository's being available to receive mail at any time.
The repository must perform several tasks. First, and most importantly, the repository must efficiently manage a potentially large number of users and their mail states. Mail must be reliably stored in a manner that makes it easy for multiple clients to access the global mail state and synchronize their local mail states with the global state. Since a large category of electronic mail is represented by bulletin boards (bboards), the repository should efficiently manage bboard mail, using a minimum of storage to store
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bboard messages in a manner that still allows any user subscribing to the bboard to read the mail. Second, the repository must be able to communicate efficiently with its clients. The protocol used to communicate between repository and client must be reliable and must provide operations that (1) allow typical mail manipulation, and (2) support Pcmail's distributed nature by allowing efficient synchronization between local and global mail states. Third, the repository must be able to process mail from sources outside the repository's own user community (a primary outside source is the Internet). Internet mail will arrive with a NIC RFC-822 standard message header; the recipient names in the message must be properly translated from the RFC-822 namespace into the repository's namespace.
Pcmail divides the world into a community of users. Each user is associated with a user object. A user object consists of a unique name, a password (which the user's clients use to authenticate themselves to the repository before manipulating a global mail state), a list of "client objects" describing those clients belonging to the user, a list of "subscription objects", and a list of "mailbox objects".
A client object consists of a unique name and a status. A user has one client object for every client he owns; a client cannot communicate with the repository unless it has a corresponding client object in a user's client list. Client objects therefore serve as a means of identifying valid clients to the repository. Client objects also allow the repository to manage local and global mail state synchronization; the repository associates with every client an "update list" of message state changes which have occurred since the client's last synchronization.
A client's status is either "active" or "inactive". The repository defines inactive clients as those clients which have not connected to the repository within a set time period (one week in the current repository implementation). When a previously-inactive client does connect to the repository, the repository notifies the client that it has been inactive for some time and should "reset" itself. Resetting a client has the effect of placing every message in every mailbox onto the client's update list. This allows the client to get a fresh global mail state from the repository when it next synchronizes (see synchronization discussion following). The reset is performed on the assumption that enough global state changes occur in a week that the client would spend too much time performing an ordinary local state- global state synchronization.
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Messages are stored in mailboxes. Users can have any number of mailboxes, which serve both to store and to categorize messages. A mailbox object both names a mailbox and describes its contents. Mailboxes are identified by a unique name; their contents are described by three numeric values. The first is the total number of messages in the mailbox, the second is the total number of unseen messages (messages that have never been seen by the user via any client) in the mailbox, and the third is the mailbox's next available message unique identifier (UID). The above information is stored in the mailbox object to allow clients to get a summary of a mailbox's contents without having to read all the messages within the mailbox.
Some mailboxes are special, in that other users may read the messages stored in them. These mailboxes are called "bulletin board mailboxes" or "bboard mailboxes". The repository uses bboard mailboxes to store bboard mail. Bboard mailboxes differ from ordinary mailboxes in the following ways:
- Their names are unique across the entire repository; for instance, only one bboard mailbox named "sf-lovers" may exist in the entire repository community. This does not preclude other users from having an ordinary mailbox named "sf-lovers".
- Subscribers to the bboard are granted read-only access to the messages in the bboard mailbox. The bboard mailbox's owner (typically the system manager) has read/update/delete access to the mailbox.
A bboard subscriber keeps track of the messages he has looked at via a subscription object. The subscription object contains the name of the bboard, its owner (the user who owns the bboard mailbox where all the messages are stored), and the UID of the first message not yet seen by the subscriber.
Users gain read-only access to a bboard by creating a subscription to it; they lose that access when they delete that subscription. A list of all bboard mailboxes available for subscription can be transmitted to the user on demand.
Associated with each mailbox are any number of message objects. Each message is broken into two parts--a "descriptor", which contains a summary of useful information about the message, and a "body", which is the message text itself, including its NIC RFC-822 message header. Each message is assigned a monotonically increasing UID based on the owning mailbox's next available UID. Each mailbox has its own set of UIDs which, together with the mailbox name and user name, uniquely identify the message within the repository. A descriptor holds the
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following information: the message UID, the message size in bytes and lines, four "useful" message header fields (the "date:", "to:", "from:", and "subject:" fields), and sixteen flags. These flags are given identifying numbers 0 through 15. Eight of these flags have well-known definitions and are reserved for the repository's use. The eight repository-defined flags mark:
- (#0) whether the message has been deleted
- (#1) whether it has been seen
- (#2) whether it has been forwarded to the user
- (#3) whether it has been forwarded by the user
- (#4) whether it has been filed (written to a text file outside the repository)
- (#5) whether it has been printed (locally or remotely)
- (#6) whether it has been replied to
- (#7) whether it has been copied to another mailbox
The remaining eight flags are availble for user use. Descriptors serve as an efficient means for clients to get message information without having to waste time retrieving the entire message from the repository.
"Address objects" provide the repository with a means for translating the RFC-822-style mail addresses in Internet messages into repository names. The repository provides its own namespace for message identification. Any message is uniquely identified by the triple (user-name, mailbox-name, message-UID). Any mailbox is uniquely identified by the pair (user-name, mailbox-name). In order to translate between RFC-822-style mail addresses and repository names, the repository maintains a list of address objects. Each address object is an association between an RFC-822-style address and a (user-name, mailbox-name) pair. When mail arrives from the Internet, the repository can use the address object list to translate the recipients into (user-name, mailbox-name) pairs and route the message correctly.
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4. Communication between repository and client: DMSP
The Distributed Mail System Protocol (DMSP) defines and manipulates the objects mentioned in the previous section. It has been designed to work with Pcmail's singlerepository/multiple-client model of the world. In addition to providing typical mail manipulation functions, DMSP provides functions that allow easy synchronization of global and local mail states.
DMSP has been completely re-specified in this version of Pcmail. Formerly, DMSP was implemented on top of the USP remote-procedure- call protocol. Since this protocol is not fully unofficially specified (let alone officially specified) anywhere, implementation of USP is difficult for sites wishing to implement Pcmail on different systems. We therefore have decided to completely redesign DMSP. It is now a very simple request/response protocol similar to SMTP or NNTP, running directly on a reliable bidirectional byte- stream such as TCP. The TCP contact port for DMSP has been designated 158. Requests and responses consist of ASCII characters; on octet-based transmission streams, each character is transmitted rightjustified in an octet with the high-order bit cleared to zero.
DMSP operations consist of an operation name, followed by zero or more tab or space characters, followed by zero or more arguments, each of which is separated from the operation name and other arguments by one or more space or tab characters. All operation requests, as well as all responses, must be terminated with a carriage-return plus line-feed (CR-LF) pair. All operation names and arguments must be taken from the set of alphanumeric characters plus the characters dash ("-"), underscore ("_"), and period (".").
DMSP operation names are case-insensitive; they may be transmitted in any combination of upper and lower case. DMSP arguments are case- insensitive but case-preserving; in other words a mailbox named "MarkL" may be referred to by an operation argument "markl", but will always be stored, and transmitted in a repository response, as "MarkL"; furthermore, any attempt to create a new mailbox "MaRkL" will not be permitted.
Each operation argument may contain no more than 64 characters. No single request or response line may contain more than 512 characters, including all white space and the terminating CR-LF.
A DMSP operation always results in a response, which may be followed
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in turn by a list, consisting of zero or more lines of CR-LF- terminated text terminated by a single period (".") plus a CR-LF. A response is always prefaced by a three-digit reply code; possible text following the response code can be in any format. The response code is sufficient to determine whether the operation succeeded or failed, or whether more text is forthcoming following the response line. Any text following the response code is for information only, and need not follow any particular format.
The first digit indicates whether the operation succeeded or failed, and if it succeeded whether or not more text should be presented to the repository. Definitions of the first digit are similar to those of NNTP:
1XX Informative message
2XX Operation completed successfully
3XX Operation completed successfully, present remainder of text and terminate with a single period plus CR-LF pair.
4XX Operation was performed and failed for some reason.
5XX Operation could not be performed because of a protocol syntax error of some sort.
The second digit indicates the type of object referred to by the response.
X0X Miscellaneous
X1X User operation
X2X Client operation
X3X Mailbox operation
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X4X Subscription operation
X5X Message operation
X6X Address operation
In an error response, the final digit can describe the type of error that occurred. Otherwise, it simply gives a response a unique number. Numbers 0 through 3 are significant in 4XX-class (error) responses only. Numbers 0-9 in all other responses serve only to differentiate responses dealing with the same type of object under different circumstances.
4X0 Operation failed because object exists
4X1 Operation failed because object does not exist
4X2 Operation failed because of an internal error
4X3 Operation failed because of an argument syntax error
Each operation generates one of a set of responses, detailed in the protocol specification appendix.
List termination is determined solely by a well-known character sequence (CR-LF, period, CR-LF). Since application data could well accidentally contain this termination sequence, the transmitting protocol module must modify application data so it contains no termination sequences. The receiving module must similarly undo the modification before presenting the data to the application at the receiving end.
The transmitting module modifies application data as follows: If a line of application data begins with a period, that period is duplicated. Since the termination sequence is a single period, accidental termination has now been prevented.
The receiving protocol checks incoming all incoming data lines for a leading period. A single period is a list terminator; a period followed by other text is removed before being presented to the
A DMSP session proceeds as follows: a client begins the session with the repository by opening a connection to the repository's machine. The client then authenticates both itself and its user to the repository with a "login" operation. If the authentication is successful, the user performs an arbitrary number of DMSP operations before ending the session with a "logout" operation, at which time the connection is closed by the repository.
Because DMSP can manipulate a pair of mail states (local and global) at once, it is extremely important that all DMSP operations are failure-atomic. Failure of any DMSP operation must leave both states in a consistent, known state. For this reason, a DMSP operation is defined to have failed unless an explicit acknowledgement is received by the operation initiator. This acknowledgement consists of a response code possibly followed by information, as described above.
Following is a general discussion of all the DMSP operations. The operations are broken down by type: general operations, user operations, client operations, mailbox operations, address operations, subscription operations, and message operations. Detailed operation specifications appear at the end of this document.
The first group of DMSP operations perform general functions that operate on no one particular class of object. DMSP has three general operations which provide the following services:
In order to prevent protocol version skew between clients and the repository, DMSP provides a "send-version" operation. The client supplies its DMSP version number as an argument; the operation succeeds if the supplied version number matches the repository's DMSP version number. It fails if the two version numbers do not match. The version number is a natural number like "100", "101", "200". The "send-version" operation should be the first that a client sends to the repository, since no other operation may work correctly if the client and repository are using different versions of DMSP.
Users can send mail to other users via the "send-message" operation. The message must have an Internet-style header as defined by NIC RFC-822. The repository takes the message and distributes it to the mailboxes specified by the message header's destination fields. If one or more of the mailboxes exists outside the repository's user community, the repository is responsible for handing the message to a
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local SMTP server. The message envelope is generated by the repository from the message contents since it may be difficult for some clients to perform envelope-generation functions such as address verification and syntax checking.
A success acknowledgement is sent from the repository only if (1) the entire message was successfully transmitted from client to repository, and (2) the message header was properly formatted. Once the repository has successfully received the message from the client, any subsequent errors in queueing or delivery must be noted via return mail to the user.
The last general operation is the "help" operation. The repository responds to "help" by printing an acknowledgement followed by a list of supported commands, terminated with a period plus CR-LF. The information is intended for display and can be in any format as long as the individual lines of text returned by the repository are CR- LF-terminated.
The next series of DMSP operations manipulates user objects. The most common of these operations are "login" and "logout". A client must perform a login operation before being able to access a user's mail state. A DMSP login operation takes five arguments: (1) the user's name, (2) the user's password, (3) the name of the client performing the login, (4) a flag set to 1 if the repository should create a client object for the client if one does not exist (0 else), and (5) a flag set to 1 if the client wishes to operate in "batch mode" and 0 if the client wishes to operate in "interactive" mode. The last flag value allows the repository to tune internal parameters for either mode of operation.
The repository can make one of three responses. First, it can make a success response, indicating successful authentication. Second, it can make one of several failure responses, indicating failed authentication. Finally, it can make a special response indicating that authentication was successful, but that the client has not been used in over a week. This last response serves as a hint that the client should consider erasing its local mail state and pulling over a complete version of the repository's mail state. This is done on the assumption that so many mail state changes have been made in a week that it would be inefficient to perform a normal synchronization.
When a client has completed a session with the repository, it performs a logout operation. This allows the repository to perform any necessary cleanup before closing the network connection.
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A user can change his password via the "set-password" operation. The operation works much the same as the UNIX change-password operation, taking as arguments the user's current password and a desired new password. If the current password given matches the user's current password, the user's current password is changed to the new password given. Because encryption can be difficult to perform on some resource-poor clients, passwords are transmitted in clear text. Clearly this is not an acceptable long-term solution, and alternatives are welcomed.
DMSP provides four operations to manipulate client objects. The first, "list-clients", tells the repository to send the user's client list to the requesting client. The list is a series of lines, one per client, containing the client's name, followed by whitespace, followed by a status string. The status is either "inactive" or "active". As with all text responses, the list is terminated with a period plus CR-LF.
The "create-client" operation allows a user to add a client object to his list of client objects. Although the login operation duplicates this functionality via the "create-this- client?" flag, the create- client operation is a useful means of creating a number of new client objects while logged into the repository via an existing client. The create-client operation requires as an argument the name of the client to create.
The "delete-client" operation removes an existing client object from a user's client list. The client being removed cannot be in use by anyone at the time. Delete-client also requires as an argument the name of the client to delete.
The last client operation, "reset-client", causes the repository to place all of the messages in all mailboxes onto the named client's update list. When a client next synchronizes with the repository, it will end up receiving a list of all descriptors when it requests a list of changed message descriptors for a particular mailbox. This is useful for two reasons. First, a client's local mail state could easily become lost or damaged, especially if it is stored on a floppy disk. Second, if a client has been marked as inactive by the repository, the reset-client operation provides a fast way of resynchronizing with the repository, assuming that so many differences exist between the local and global mail states that a normal synchronization would take far too much time.
DMSP supports seven operations that manipulate mailbox objects. First, "list-mailboxes" has the repository send to the requesting client information on each mailbox. The repository transmits one line of information per mailbox, terminating the list with a period plus CR-LF. Each line contains, in order and separated by whitespace, the mailbox name, "next available UID", total message count, and unseen message count. This operation is useful in synchronizing local and global mail states, since it allows a client to compare the user's global mailbox list with a client's local mailbox list. The list of mailboxes also provides a quick summary of each mailbox's contents without having the contents present.
The "create-mailbox" has the repository create a new mailbox and attach it to the user's list of mailboxes. It takes as an argument the name of the mailbox to create.
"Delete-mailbox" removes a mailbox from the user's list of mailboxes. All messages within the mailbox are also deleted and permanently removed from the system. Any address objects binding the mailbox name to RFC-822-style mailbox addresses are also removed from the system. Delete-mailbox takes as an argument the name of the mailbox to delete.
"Create-bboard-mailbox" allows a user to create a bboard mailbox. The name given as an argument must be unique across the entire repository user community. Once the bboard mailbox has been created, other users may subscribe to it, using subscription objects to keep track of which messages they have read on which bboard mailboxes.
"Delete-bboard-mailbox" allows a bboard's owner to delete a bboard mailbox. Subscribers who attempt to read from a bboard mailbox after it has been deleted are told that the bboard no longer exists. Again, the operation's argument is the name of the bboard mailbox to delete.
"Reset-mailbox" causes the repository to place all of the messages in a named mailbox onto the current client's update list. When the client next requests a list of changed message descriptors for this mailbox, it will receive a list of all message descriptors in the mailbox. This operation is merely a more specific version of the reset-client operation (which allows the client to pull over a complete copy of the user's global mail state). Its primary use is for mailboxes whose contents have accidentally been destroyed locally.
Finally, DMSP has an "expunge-mailbox" operation. Any message can be
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deleted and "undeleted" at will, since this simply changes the value of a flag attached to the message. Deletions are made permanent by performing an expunge-mailbox operation. The expunge operation causes the repository to look through a named mailbox, removing from the system any messages marked "deleted". Expunge-mailbox takes as an argument the name of the mailbox to expunge.
DMSP provides three operations that allow users to manipulate address objects. First, the "list-address" operation returns a list of address objects associated with a particular mailbox. Each address is transmitted on a separate line terminated by a CR-LF; the list is terminated with a period plus CR-LF.
The "create-address" operation adds a new address object that associates a (user-name, mailbox-name) pair with a given RFC-822- style mailbox address. It takes as arguments the mailbox name and the address name.
Finally, the "delete-address" operation destroys the address object binding the given RFC-822-style mail address and the given (user- name, mailbox-name) pair. Arguments are the address to delete and the mailbox it belongs to.
DMSP provides five subscription operations. The first, "list- subscriptions", gives the user a list of the bboards he is currently subscribing to. The list consists of one line of information per subscription. Each entry contains the following information, in order:
- The bulletin board's name
- The UID of the first message the subscriber has not yet seen
- The number of messages the subscriber has not yet seen
- The highest message UID in the bulletin board
"List-available-subscriptions" gives the user a list of all bboards he can subscribe to. The list consists of bboard names, one per line, terminated by a period plus CR-LF. "Createsubscription" adds a subscription to the user's list of subscriptions; it takes as an argument the name of the bboard to subscribe to. "Delete- subscription" removes a subscription from the list, and takes as an
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argument the name of the subscription to remove. Note that this does not delete the associated bboard mailbox (obviously only the bboard's owner can do that). It merely removes the user from the list of the bboard's subscribers. Finally DMSP allows the user to tell the repository which messages in a bboard he has seen. Every subscription object contains the UID of the first message the user has not yet seen; the "reset-subscription" operation updates that number, insuring that the user sees a given bboard message only once. Reset-subscription takes as arguments the name of the subscription and the new UID value.
The most commonly-manipulated Pcmail objects are messages; DMSP therefore provides special message operations to allow efficient synchronization, as well as a set of operations to perform standard message-manipulation functions.
A user may request a series of descriptors with the "fetch- descriptors" operation. The series is identified by a pair of message UIDs, representing the lower and upper bounds of the list. Since UIDs are defined to be monotonically increasing numbers, a pair of UIDs is sufficient to completely identify the series of descriptors. If the lower bound UID does not exist, the repository starts the series with the first message with UID greater than the lower bound. Similarly, if the upper bound does not exist, the repository ends the series with the last message with UID less than the upper bound. If certain UIDs within the series no longer exist, the repository obviously does not send them. The repository returns the descriptors in a list with the following format:
If a descriptor has been expunged, the repository transmits two consecutive lines of information: the word "expunged" on one line, followed by the message UID on the next line. "Expunged" notifications are only transmitted in response to a "fetch-changed- descriptors" command; they are an indication to the client that someone else has expunged the mailbox and that the client should remove the local copy of the expunged message.
If a descriptor has not been expunged, it is presented as six consecutive lines of information: the word "descriptor" on the first line, followed by a second line containing the message UID, flag states (see examples following), message length in bytes, and message length in lines, followed by four lines containing in order the message "from:" field, "to:" field, "date:" field, and "subject:" field. The entire list of descriptors is terminated by a period plus CR-LF; individual descriptors are not specially terminated since the first line ("expunged" or "descriptor") of a list entry determines
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the exact length of the entry (two lines or six lines).
The "fetch-changed-descriptors" operation is intended for use during state synchronization. Whenever a descriptor changes state (one of its flags is cleared, for example), the repository notes those clients which have not yet recorded the change locally. Fetch- changed-descriptors has the repository send to the client a maximum of the first N descriptors which have changed since the client's last synchronization, where N is a number sent by the client. The list sent begins with the descriptor with lowest UID. Note that the list of descriptors is only guaranteed to be monotonically increasing for a given call to "fetch-changed-descriptors"; messages with lower UIDs may be changed by other clients in between calls to "fetch- changeddescriptors". "Fetch-changed-descriptors" takes two arguments: the name of the mailbox to search, and the maximum number of descriptors for the repository to return.
Once the changed descriptors have been looked at, a user will want to inform the repository that the current client has recorded the change locally. The "reset-descriptors" command causes the repository to mark as "recorded by current client" a given series of descriptors. The series is identified by a low UID and a high UID. UIDs within the series that no longer exist are ignored. Arguments are: mailbox name, low UID in range, and high UID in range.
Whole messages are transmitted from repository to user with the "fetch-message" operation. The separation of "fetchdescriptors" and "fetch-message" operations allows clients with small amounts of disk storage to obtain a small message summary (via "fetch-descriptors" or "fetch-changed-descriptors") without having to pull over the entire message. Arguments are mailbox name, followed by message UID.
Frequently, a message may be too large for some clients to store locally. Users can still look at the message contents via the "print-message" operation. This operation has the repository send a copy of the message to a named printer. The printer name need only have meaning to the particular repository implementation; DMSP transmits the name only as a means of identification. Arguments are: mailbox name, followed by message UID, followed by printer identification.
Copying of one message into another mailbox is accomplished via the "copy-message" operation. A descriptor list of length one, containing a descriptor for the copied message, is returned if the copy operation is successful. This descriptor is required because the copied message acquires a UID different from the original message. The client cannot be expected to know which UID has been assigned the copy, hence the repository's sending a descriptor
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containing the UID. Arguments to copy-message are: source mailbox name, target mailbox name, and source message UID.
Each message has associated with it sixteen flags, as described earlier. These flags can be set and cleared using the "set-message- flag" operation. The first eight flags have special meaning to the repository as described above; the remaining eight are for user use. Set-message-flag takes four arguments: mailbox name, message UID, flag number (0 through 15), and desired flag state (0 or 1).
Clients can be any of a number of different workstations; Pcmail's architecture must therefore take into account the range of characteristics of these workstations. First, most workstations are much more affordable than the large computers currently used for mail service. It is therefore possible that a user may well have more than one. Second, some workstations are portable and they are not expected to be constantly tied into a network. Finally, many of the smaller workstations resource-poor, so they are not expected to be able to store a significant amount of state information locally. The following subsections describe the particular parts of Pcmail's client architecture that address these different characteristics.
The fact that Pcmail users may own more than one workstation forms the rationale for the multiple client model that Pcmail uses. A Pcmail user may have one client at home, another at an office, and maybe even a third portable client. Each client maintains a separate copy of the user's mail state, hence Pcmail's distributed nature. The notion of separate clients allows Pcmail users to access mail state from several different locations. Pcmail places no restrictions on a user's ability to communicate with the repository from several clients at the same time. Instead, the decision to allow several clients concurrent access to a user's mail state is made by the repository implementation.
Some workstations tend to be small and fairly portable; the likelihood of their always being connected to a network is relatively small. This is another reason for each client's maintaining a local copy of a user's mail state. The user can then manipulate the local mail state while not connected to the network (and the repository). This immediately brings up the problem of synchronization between local and global mail states. The repository is continually in a position to receive global mail state updates, either in the form of
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incoming mail, or in the form of changes from other clients. A client that is not always connected to the net cannot immediately receive the global changes. In addition, the client's user can make his own changes on the local mail state.
Pcmail's architecture allows fast synchronization between client local mail states and the repository's global mail state. Each client is identified in the repository by a client object attached to the user. This object forms the basis for synchronization between local and global mail states. Some of the less common state changes include the adding and deleting of user mailboxes and the adding and deleting of address objects. Synchronization of these changes is performed via DMSP list operations, which allow clients to compare their local versions of mailbox and address object lists with the repository's global version and make any appropriate changes. The majority of possible changes to a user's mail state are in the form of changed descriptors. Since most users will have a large number of messages, and message states will change relatively often, special attention needs to be paid to message synchronization.
An existing descriptor can be changed in one of three ways: first, one of its sixteen flag values can be changed (this encompasses the user's reading an unseen message, deleting a message, printing a message, etc). Second, a descriptor can be created, either by the delivery of a new message or by the copying of a message from one mailbox to another. Finally, a descriptor can be destroyed, via an "expunge-mailbox" operation.
In the above cases, synchronization is required between the repository and every client that has not previously noted the change. To keep track of which clients have noticed a global mail state change and changed their local states accordingly, each mailbox has associated with it a list of active clients. Each client has a (potentially empty) "update list" of messages which have changed since that client last synchronized.
When a client connects to the repository, it executes a DMSP "fetch- changed-descriptors" operation. This causes the repository to return a list of all descriptors on that client's update list. When the client receives the changed descriptors, it may do one of two things: if the descriptor is marked "expunged", it can remove the corresponding message from the local mailbox. If the descriptor is not expunged, the client can store the descriptor, thus updating the local mail state. After a changed descriptor has been recorded, the client uses the DMSP "reset-descriptors" operation to remove descriptors from its update list. Those descriptors will now not be sent to the client unless (1) it is explicitly requested via a "fetch-descriptors" operation, or (2) it changes again.
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In this manner, a client can run through its user's mailboxes, getting all changes, incorporating them into the local mail state, and marking the changes as recorded.
Because of the portable nature of some workstations, they may not always be connected to a network (and able to communicate with the repository). Since each client maintains a local mail state, Pcmail users can manipulate the local state while not connected to the repository. This is known as "batch" operation, since all changes are recorded by the client and made to the repository's global state in a batch, when the client next connects to the repository. Interactive operation occurs when a client is always connected to the repository. In interactive mode, changes made to the local mail state are also immediately made to the global state via DMSP operations.
In batch mode, interaction between client and repository takes the following form: the client connects to the repository and sends over all the changes made by the user to the local mail state. The repository changes its global mail state accordingly. When all changes have been processed, the client begins synchronization; this incorporates newly-arrived mail, as well as mail state changes by other clients, into the local state.
In interactive mode, since local changes are immediately propagated to the repository, the first part of batch-type operation is eliminated. The synchronization process also changes; although one synchronization is required when the client first opens a connection to the repository, subsequent synchronizations can be performed either at the user's request or automatically every so often by the client.
Smaller workstations may have little in the way of disk storage. Clients running on these workstations may never have enough room for a complete local copy of a user's global mail state. This means that Pcmail's client architecture must allow user's to obtain a clear picture of their mail state without having all their messages present.
Descriptors provide message information without taking up large amounts of storage. Each descriptor contains a summary of information on a message. This information includes the message UID, its length in bytes and lines, its status (contained in the eight system-defined and eight user-defined flags), and portions of its
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RFC-822 header (the "from:", "to:", "date:" and "subject:" fields). All of this information can be encoded in a small (around 100 bytes) data structure whose length is independent of the size of the message it describes.
Most clients should be able to store a complete list of message descriptors with little problem. This allows a user to get a complete picture of his mail state without having all his messages present locally. If a client has extremely limited amounts of disk storage, it is also possible to get a subset of the descriptors from the repository. Short messages can reside on the client, along with the descriptors, and long messages can either be printed via the DMSP print-message operation, or specially pulled over via the fetch- message operation.
The following example describes a typical communication session between the repository and a client mail reader. The client is one of three belonging to user "Fred". Its name is "office-client", and since Fred has used the client within the last week, it is marked as "active". Fred has two mailboxes: "fred" is where all of his current mail is stored; "archive" is where messages of lasting importance are kept. The example will run through a simple synchronization operation. Typically, the synchronization will be performed by a mail reader as part of a "get new mail" operation.
First Fred's mail reader connects to the repository and receives the following banner:
In order to access his global mail state, the mail reader must authenticate Fred to the repository; this is done via the DMSP login operation:
login fred fred-password office-client 0 0
This tells the repository that Fred is logging in via "office- client", and that "office-client" is identified by an existing client object in Fred's mail state. The first argument to the login operation is Fred's repository user name. The second argument is Fred's password. The third argument is the name of the client communicating with the repository. The fourth argument tells the repository not to create "office-client" even if it cannot find its client object. The final argument tells the repository that Fred's client is not operating in batch mode but rather in interactive mode.
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Fred's authentication checks out, so the repository logs him in.
200 command OK
Now that Fred is logged in, the mail reader performs an initial synchronization. This process starts with the mail reader's asking for an up-to-date list of mailboxes:
list-mailboxes
The repository replies with:
230 mailbox list follows: fred 2313 10 1 archive 101 100 0 .
This tells the mail reader that there are two mailboxes, "fred" and "archive". "Fred" has 10 messages, one of which is unseen. The next incoming message will be assigned a UID of 2313. "Archive", on the other hand, has 100 messages, none of which are unseen. The next message sent to "archive" will be assigned the UID 101. There are no new mailboxes in the list (if there were, the mail reader would create them. On the other hand, if some mailboxes in the mail reader's local list were not in the repository's list, the program would assume them deleted by another client and delete them locally as well).
To synchronize, the mail reader need only look at each mailbox's contents to see if (1) any new mail has arrived, or (2) if Fred changed any messages on one of his other two clients subsequent to "office-client"'s last connection to the repository.
The mail reader asks for any changed descriptors via the "fetch- changed-descriptors" operation. It requests at most ten changed descriptors since storage is very limited on Fred's workstation.
2101 expunged 2104 descriptor 2107 1100011100000010 1400 30 foo@bar.edu (Foo Jones) fred@PTT.LCS.MIT.EDU Wed, 9 Dec 87 10:43:52 EST A typical subject line descriptor 2312 0000000000000000 12232 320 joe@athena.mit.edu fred@PTT.LCS.MIT.EDU Thu, 17 Dec 87 18:24:09 PST Another typical subject line .
If a descriptor changed because it was expunged, it is transmitted as two lines: the word "expunged" on one line, followed by the message UID on the next line. If one of its flags changed state, or it is a new message, it is transmitted as six lines: the word "descriptor" on one line, followed by a line containing the message UID, flags, and length in bytes and lines, followed by the to, from, date, and subject fields, each on one line. The flags are transmitted as a single string of ones and zeroes, a one if the flag is on and a zero if the flag is off. All 16 flags are always transmitted. Flag zero's state is the first character in the flag string; flag fifteen's is the last character in the flag string.
The first two descriptors in the list have been expunged, presumably by Fred's expunging his mailbox on another client. The mail reader removes messages 2101 and 2104 from its local copy of mailbox "fred". The next descriptor in the list is one which Fred marked for deletion on another client yesterday. The mail reader marks the local version of the message as deleted. The last descriptor in the list is a new one. The mail reader adds the descriptor to its local list. Since all changes to mailbox "fred" have now been recorded locally, the update list can be reset:
reset-descriptors fred 1 2312
The repository responds with:
200 command OK
indicating that it has removed from "office-client"'s update list all
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messages in mailbox "fred" with UIDs between 1 and 2312 inclusive (in this case just two messages). "Fred" has now been synchronized. The mail reader now turns to Fred's "archive" mailbox and asks for the first ten changed descriptors.
fetch-changed-descriptors archive 10
The repository responds with:
250 descriptor list follows: .
The zero-length list tells the mail reader that no descriptors have been changed in "archive" since its last synchronization. No new synchronization needs to be performed.
Fred's mail reader is now ready to pull over the new message. The message is 320 lines long; there might not be sufficient storage on "office-client" to hold the new message. The mail reader tries anyway:
fetch-message fred 2312
The repository begins transmitting the message:
251 message follows: UID: 2312 From: joe@bar.mit.edu To: fred@PTT.LCS.MIT.EDU Date: Thu, 17 Dec 87 18:24:09 PST Subject: Another typical subject line
Fred,
...
Halfway through the message transmission, Fred's workstation runs out of disk space. Because all DMSP operations are defined to be failure-atomic, the portion of the message already transmitted is destroyed locally and the operation fails. The mail reader informs Fred that the message cannot be pulled over because of a lack of disk space. The synchronization process is now finished and Fred can start reading his mail. The new message that was too big to fit on "office-client" will be marked "off line"; Fred can use the mail
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reader to either remote-print it or delete and expunge other messages until he has enough space to store the new message.
Since Fred is running in interactive mode, changes he makes to any messages will immediately be transmitted into DMSP operations and sent to the repository. Depending on the mail reader implementation, Fred will either have to execute a "synchronize" command periodically or the client will synchronize for him automatically every so often.
The following section briefly describes a current Pcmail system that services a small community of users. The Pcmail repository runs under UNIX on a DEC Microvax-II connected to the Internet. The clients run on IBM PCs, XTs, and ATs, as well as Sun workstations, Microvaxes, and VAX-750s.
Client code for the IBM machines operates only in batch mode. Users make local state changes in a mail reader; the changes are queued until the user runs a network client program. The program connects to the repository, performs the queued changes, and synchronizes local and global mail states. The network client program then disconnects from the repository.
The IBM PC client code has gone through several revisions since the first Pcmail RFC was published. What was once a fairly primitive and cumbersome system has evolved into a system that makes excellent use of the PC's limited resources and provides a fairly powerful, easy- to-use mail reader.
Users access and modify their local mail state via a mail reader written in the Epsilon text editor's EEL extension language. Users are given a variety of commands to operate on individual messages and mailboxes, as well as to compose outgoing mail.
Synchronization and the processing of queued changes is performed by a separate program, which the user runs as desired. The program takes any actions queued while operating the mail reader, and converts them into DMSP operations. All queued changes are made before any synchronization is performed. The program can be invoked directly from the mail reader, without having to exit and restart.
The limitation of IBM PC client operation to batch mode was made because of development environment limitations. The mail reader cannot work with the network code inside it because of the network program architecture. The only solution was to provide a two-part
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client, one part of which read the mail and one part of which interacted with the repository. Although slightly cumbersome, the two-program setup works quite well.
Client code for the Suns, Microvaxes, and VAX-750s runs on 4.2/4.3BSD UNIX. It is fully interactive, with a powerful mail reader inside Richard Stallman's GNU-EMACS editor. Since UNIX-based workstations have a good deal of main memory and disk storage, no effort was made to lower local mail state size by keeping message descriptors rather than message text.
The local mail state consists of a number of BABYL-format mailboxes. The interface is very similar to the RMAIL mail reader already present in GNU-EMACS.
The mail reader communicates with the repository through network code implemented in EMACS-LISP. Changes to the local mail state are immediately made on the repository; although the repository is fast, there is a small noticeable delay in performing operations over the network.
There is no provision for automatic synchronization whenever new mail arrives or old mail is changed by another client. Instead, users must get any new mail explicitly. A simple "notification" program runs in the background and wakes up every minute to check for new mail; when mail arrives, the user executes a command to get the new mail, synchronizing the mailbox at the same time.
The repository is implemented in C on 4.2/4.3BSD UNIX. Currently it runs on DEC VAX-750s and Microvaxes, although other repositories will soon be running on IBM RT machines and Sun workstations. The repository code is designed to allow several clients belonging to a particular user to "concurrently" modify the user's state. A locking scheme prevents one client from modifying mail state while another client is modifying the same state.
Pcmail is now used by a small community of people at the MIT Laboratory for Computer Science. The repository design works well, providing an efficient means of storing and maintaining mail state for several users. Its performance is quite good when up to ten users are connected; it remains to be seen whether or not the repository will be efficient at managing the state of ten or a
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hundred times that many users. Given sufficient disk storage, it should be able to, since communication between different users' clients and the repository is likely to be very asynchronous and likely to occur in short bursts with long "quiet intervals" in between as users are busy doing other things.
Members of another research group at LCS are currently working on a replicated, scalable version of the repository designed to support a very large community of users with high availability. This repository also uses DMSP and has successfully communicated with clients that use the current repository implementation. DMSP therefore seems to be usable over several flavors of repository design.
The IBM PC clients are very limited in the way of resources. The mail reader/editor combination is quite powerful, making local mail state manipulation fairly easy. Obviously a big performance enhancement would be to provide a fully interactive client. As it is, batch-style synchronization is relatively time consuming due to the low performance of the PCs. The "batch-mode" that the PCs use tends to be good for those PCs that spend a large percentage of their time unplugged and away from a network. It is somewhat inconvenient for those PCs that are always connected to a network and could make good use of an "interactive-mode" state manipulation.
The UNIX-based clients are more powerful and easier to use than their PC counterparts. Synchronization is much faster, and there is far more functionality in the mail reader (having an interface that runs within GNU-EMACS helps a lot in this respect). Most of those people using the Pcmail system use the UNIX-based client code.
Following are a list of DMSP operations by object type, together with syntax, and possible responses. Some responses may be followed by zero or more lines of text, terminated by a single period plus CR-LF pair. Only success responses and common error responses are listed; a complete list of possible responses follows this appendix. Expressions in angle brackets (i.e. <mailbox-name>) are metalinguistic variables indicating a general request or response form. Operations with arguments have a sample invocation following the operation syntax and response.
General operations:
HELP 100 Repository version xxx. Following are supported: HELP SEND-VERSION SEND-MESSAGE LOGIN LOGOUT ... FETCH-MESSAGE COPY-MESSAGE .
SEND-VERSION <version-number> 200 Command OK 500 version skew!
i.e. SEND-VERSION 230
SEND-MESSAGE 350 enter message; end with "." To: markl From: markl Subject: a test message
this is a test message .
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Repository responds:
200 Command OK 403 message syntax error
User operations:
LOGIN <user> <password> <client> <create-p> <batch-p> 200 Command OK 221 Client out of date by > 1 week 404 Bad password 405 Client <client-name> is locked 411 No user named <user-name> 421 Client <client-name> not found
i.e. LOGIN markl foo random-client-name 1 0
LOGOUT 200 Command OK
SET-PASSWORD <old-password> <new-password> 200 Command OK 404 Incorrect old password
i.e. SET-PASSWORD foo bar
Client operations:
LIST-CLIENTS 220 Client list <name> <status> follows: client-1 active client-2 inactive client-3 active ... client-foobar active .
Each line of the list contains a client name, followed by whitespace, followed by the word "active" or the word "inactive", indicating whether or not the client has connected to the repository within the last week.
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CREATE-CLIENT <client-name> 200 Command OK 403 <client-name> is an illegal name 420 Client <client-name> exists
i.e. CREATE-CLIENT new-client
DELETE-CLIENT <client-name> 200 Command OK 421 Client <client-name> not found 405 Client <client-name> is locked
i.e. DELETE-CLIENT old-client
RESET-CLIENT <client-name> 200 Command OK 421 Client <client-name> not found 405 Client <client-name> is locked
Each line of the list contains a mailbox name, followed by the mailbox's next available unique identifier, followed by the number of messages in the mailbox, followed finally by the number of unseen messages in the mailbox. Unseen messages are those whose descriptors have flag #1 ("message has been seen") set to zero.
CREATE-MAILBOX <mailbox-name> 200 Command OK 403 <mailbox-name> is an illegal name 430 <mailbox-name> already exists 440 <mailbox-name> exists as a bboard subscription
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i.e. CREATE-MAILBOX current-events
DELETE-MAILBOX <mailbox-name> 200 Command OK 431 mailbox <mailbox-name> not found 440 <mailbox-name> is a bboard; use delete-bboard-mailbox
i.e. DELETE-MAILBOX income-tax-information
CREATE-BBOARD-MAILBOX <mailbox-name> 200 Command OK 430 a mailbox named <mailbox-name> already exists. 430 a bboard mailbox named <mailbox-name> already exists. 403 <mailbox-name> is an illegal name
i.e. CREATE-BBOARD-MAILBOX sf-lovers
DELETE-BBOARD-MAILBOX <mailbox-name> 200 Command OK 404 not owner of <mailbox-name> 431 no bboard mailbox named <mailbox-name>
i.e. DELETE-BBOARD-MAILBOX rec.autos
RESET-MAILBOX <mailbox-name> 200 Command OK 431 mailbox <mailbox-name> not found
i.e. RESET-MAILBOX british-cars
EXPUNGE-MAILBOX <mailbox-name> 200 Command OK 431 mailbox <mailbox-name> not found
EXPUNGE-MAILBOX british-cars
Address operations:
LIST-ADDRESSES <mailbox-name> 260 Address list for <mailbox-name> follows: address-1
Each line of the list consists solely of one address.
CREATE-ADDRESS <mailbox-name> <address-name> 200 Command OK 403 <mailbox-name> is an illegal name 431 mailbox <mailbox-name> not found 460 <address-name> already exists
i.e. CREATE-ADDRESS markl markl-bug-pcmail
DELETE-ADDRESS <mailbox-name> <address-name> 200 Command OK 431 mailbox <mailbox-name> not found 461 address <address-name> not found
Each line of the list consists of a bulletin-board name, followed by the UID of the first message which the user has not yet looked at, followed by the number of messages in the bulletin-board that the user has not yet looked at, followed by the bulletin-board's next available unique message identifier.
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CREATE-SUBSCRIPTION <bboard-name> 200 Command OK 403 <bboard-name> is an illegal name 430 A mailbox named <bboard-name> already exists 431 Bboard mailbox <bboard-name> not found 440 Already subscribing to <bboard-name>
i.e. CREATE-SUBSCRIPTION sf-lovers
DELETE-SUBSCRIPTION <bboard-name> 200 Command OK 441 Subscription <bboard-name> not found
i.e. DELETE-SUBSCRIPTION rec.music
RESET-SUBSCRIPTION <bboard-name> <new-UID> 200 Command OK 441 Subscription <bboard-name> not found
i.e. RESET-SUBSCRIPTION rec.music.gdead 1210
LIST-AVAILABLE-SUBSCRIPTIONS 241 All available bboards follow: mod.politics sfl tcp-ip forum ... comp.emacs .
Each line of the list consists solely of one bulletin-board name.
descriptor 2337 0001000001110000 481 14 croaker@ptt.lcs.mit.edu fred@anymachine.mit.edu Tue, 19 Jan 88 11:10:03 EST a typical subject line descriptor 2339 0000000000000000 1457 40 bob@lcs.mit.edu csr-people@ptt.lcs.mit.edu Mon, 18 Jan 88 13:08:17 +0000 another typical subject line expunged 2340 .
or
431 mailbox <mailbox-name> not found
i.e. FETCH-CHANGED-DESCRIPTORS markl 100
Each element of the descriptor list is either two or six lines long. Descriptors which have been expunged are transmitted as two lines: the word "expunged" on one line, followed by the message unique identifier on the next line. Descriptors which still exist are transmitted as six lines: the word "descriptor" on one line, followed by a line containing the message unique identifier, flag states (sixteen characters either one or zero depending on the associated flag value), followed by the message length in characters, followed by the message length in lines. The next four lines contain the message's "from:", "to:", "date:", and "subject:" fields, respectively. Flag zero's state is the first character in the flag string; flag fifteen's is the last character in the flag string.
FETCH-DESCRIPTORS <mailbox-name> <low-uid> <high-uid> 250 Descriptor list follows: descriptor 2337 0001000001110000 481 14 croaker@ptt.lcs.mit.edu fred@anymachine.mit.edu Tue, 19 Jan 88 11:10:03 EST a typical subject line descriptor 2339 0000000000000000 1457 40 bob@lcs.mit.edu csr-people@ptt.lcs.mit.edu
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Mon, 18 Jan 88 13:08:17 +0000 another typical subject line .
or
431 mailbox <mailbox-name> not found
i.e. FETCH-DESCRIPTORS british-cars 12 31
COPY-MESSAGE <src-mailbox> <target-mailbox> <source-UID> 250 Descriptor list follows: descriptor 2339 0000000000000000 1457 40 bob@lcs.mit.edu csr-people@ptt.lcs.mit.edu Mon, 18 Jan 88 13:08:17 +0000 another typical subject line .
or
400 cannot copy message onto itself 431 target mailbox <target-mailbox> not found 431 source mailbox <source-mailbox> not found 451 message <source-UID> not found
i.e. COPY-MESSAGE markl british-cars 2338
RESET-DESCRIPTORS <mailbox-name> <low-UID> <high-UID> 200 Command OK 431 mailbox <mailbox-name> not found
i.e. RESET-DESCRIPTORS markl 1 10000
PRINT-MESSAGE <mailbox-name> <UID> <printer-ID> 200 Command OK 401 printer <printer-name> not found 431 mailbox <mailbox-name> not found 451 message <UID> not found
i.e. PRINT-MESSAGE markl 2433 pravda
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SET-MESSAGE-FLAG <mailbox-name> <UID> <flagnum> <state> 200 Command OK 431 mailbox <mailbox-name> not found 451 message <UID> not found 500 flag number <flag-number> out of range
i.e. SET-MESSAGE-FLAG british-cars 23 0 1
FETCH-MESSAGE <mailbox-name> <UID> 251 message follows: From: markl@ptt.lcs.mit.edu To: markl@ptt.lcs.mit.edu Date: Sun, 17 Jan 88 11:11:11 EST Subject: anything
this is a sample of some message text .
or
431 Mailbox <mailbox-name> not found 451 message <UID> not found
100 Pcmail repository version XXX; following are supported 200 Command OK 220 Client list <name> <status> follows: 221 Client out of date by > 1 week 230 Mailbox list <name> <high UID> <#msgs> <#new> follows: 240 Subscription list follows: 250 Descriptor list follows: 251 Message follows: 260 Address list follows: 350 enter message; end with "." 400 cannot copy message onto itself 410 already logged in 420 client <name> already exists 430 mailbox <name> already exists 430 bboard mailbox <name> already exists 440 subscription <name> already exists 460 address <name> already exists 411 no user named <name> 421 client <name> not found 431 mailbox <name> not found 441 subscription <name> not found 451 message <UID> not found 461 address <name> not found 402 internal error message 403 syntax error in outbound message 404 bad password or permission denied 405 mail state is temporarily in use by another client 406 please log in 500 operation syntax error or illegal argument