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rfc:rfc1056

Network Working Group M. Lambert Request for Comments: 1056 MIT Obsoletes: RFC-993 June 1988

      PCMAIL: A Distributed Mail System for Personal Computers
                         Table of Contents
 1. Status of this Document                                      1
 2. Introduction                                                 2
 3. Repository architecture                                      4
      3.1. Management of user mail state                         5
      3.2. Repository-to-RFC-822 name translation                7
 4. Communication between repository and client: DMSP            8
      4.1. DMSP commands                                         8
      4.2. DMSP responses                                        8
      4.3. DMSP sessions                                        11
      4.4. General operations                                   11
      4.5. User operations                                      12
      4.6. Client operations                                    13
      4.7. Mailbox operations                                   14
      4.8. Address operations                                   15
      4.9. Subscription operations                              15
      4.10. Message operations                                  16
 5. Client Architecture                                         18
      5.1. Multiple clients                                     18
      5.2. Synchronization                                      18
      5.3. Batch operation versus interactive operation         20
      5.4. Message summaries                                    20
 6. Typical interactive-style client-repository interaction     21
 7. A current Pcmail implementation                             25
      7.1. IBM PC client code                                   25
      7.2. UNIX client code                                     26
      7.3. Repository code                                      26
 8. Conclusions                                                 26
 I. DMSP Protocol Specification                                 28
 II. Operations by name                                         37
 III. Responses by number                                       38

1. Status of this Memo

 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.

Lambert [Page 1] RFC 1056 PCMAIL June 1988

2. Introduction

 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

Lambert [Page 2] RFC 1056 PCMAIL June 1988

 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

Lambert [Page 3] RFC 1056 PCMAIL June 1988

 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:
  1. The repository architecture
  1. DMSP, its operations, and motivation for its design
  1. The client architecture
  1. A typical DMSP session between the repository and a

client

  1. The current Pcmail implementation
  1. Appendices describing the DMSP protocol in detail

3. Repository architecture

 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

Lambert [Page 4] RFC 1056 PCMAIL June 1988

 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.

3.1. Management of user mail state

 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.

Lambert [Page 5] RFC 1056 PCMAIL June 1988

 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:
  1. 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".
  1. 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

Lambert [Page 6] RFC 1056 PCMAIL June 1988

 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:
  1. (#0) whether the message has been deleted
  1. (#1) whether it has been seen
  1. (#2) whether it has been forwarded to the user
  1. (#3) whether it has been forwarded by the user
  1. (#4) whether it has been filed (written to a text file

outside the repository)

  1. (#5) whether it has been printed (locally or remotely)
  1. (#6) whether it has been replied to
  1. (#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.

3.2. Repository-to-RFC-822 name translation

 "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.

Lambert [Page 7] RFC 1056 PCMAIL June 1988

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.

4.1. DMSP commands

 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.

4.2. DMSP responses

 A DMSP operation always results in a response, which may be followed

Lambert [Page 8] RFC 1056 PCMAIL June 1988

 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

Lambert [Page 9] RFC 1056 PCMAIL June 1988

 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

Lambert [Page 10] RFC 1056 PCMAIL June 1988

 receiving application.

4.3. DMSP sessions

 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.

4.4. General operations

 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

Lambert [Page 11] RFC 1056 PCMAIL June 1988

 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.

4.5. User operations

 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.

Lambert [Page 12] RFC 1056 PCMAIL June 1988

 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.

4.6. Client operations

 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.

Lambert [Page 13] RFC 1056 PCMAIL June 1988

4.7. Mailbox operations

 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

Lambert [Page 14] RFC 1056 PCMAIL June 1988

 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.

4.8. Address operations

 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.

4.9. Subscription operations

 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:
  1. The bulletin board's name
  1. The UID of the first message the subscriber has not yet

seen

  1. The number of messages the subscriber has not yet seen
  1. 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

Lambert [Page 15] RFC 1056 PCMAIL June 1988

 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.

4.10. Message operations

 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

Lambert [Page 16] RFC 1056 PCMAIL June 1988

 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

Lambert [Page 17] RFC 1056 PCMAIL June 1988

 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).

5. Client Architecture

 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.

5.1. Multiple clients

 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.

5.2. Synchronization

 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

Lambert [Page 18] RFC 1056 PCMAIL June 1988

 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.

Lambert [Page 19] RFC 1056 PCMAIL June 1988

 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.

5.3. Batch operation versus interactive operation

 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.

5.4. Message summaries

 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

Lambert [Page 20] RFC 1056 PCMAIL June 1988

 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.

6. Typical interactive-style client-repository interaction

 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:
     200 Pcmail repository version 3.0.0 ready
 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.

Lambert [Page 21] RFC 1056 PCMAIL June 1988

 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.
     fetch-changed-descriptors fred 10
 The repository responds with:
     250 descriptor list follows:
     expunged

Lambert [Page 22] RFC 1056 PCMAIL June 1988

     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

Lambert [Page 23] RFC 1056 PCMAIL June 1988

 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

Lambert [Page 24] RFC 1056 PCMAIL June 1988

 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.

7. A current Pcmail implementation

 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.

7.1. IBM PC client code

 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

Lambert [Page 25] RFC 1056 PCMAIL June 1988

 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.

7.2. UNIX client code

 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.

7.3. Repository code

 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.

8. Conclusions

 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

Lambert [Page 26] RFC 1056 PCMAIL June 1988

 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.

Lambert [Page 27] RFC 1056 PCMAIL June 1988

I. DMSP Protocol Specification

 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
     .

Lambert [Page 28] RFC 1056 PCMAIL June 1988

 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.

Lambert [Page 29] RFC 1056 PCMAIL June 1988

     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
     i.e. RESET-CLIENT any-old-client
 Mailbox operations:
     LIST-MAILBOXES
     230 Mbox list <name> <high-UID> <#msgs> <#new> follows:
     mailbox-1 2338 8 1
     mailbox-2 59 44 0
     ...
     mailbox-foobar 19 9 0
     .
 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

Lambert [Page 30] RFC 1056 PCMAIL June 1988

     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

Lambert [Page 31] RFC 1056 PCMAIL June 1988

     address-2
     ...
     address-6
     .
     or
     431 mailbox <mailbox-name> not found
     i.e. LIST-ADDRESSES archive
     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
     i.e. DELETE-ADDRESS markl markl-info-cobol
 Subscription operations:
     LIST-SUBSCRIPTIONS
     240 subscription list follows:
     bboard-1 2573 33 2606
     bboard-2 541 4 545
     ...
     bboard-6 1530 43 1573
     .
 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.

Lambert [Page 32] RFC 1056 PCMAIL June 1988

     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.
 Message operations:
     FETCH-CHANGED-DESCRIPTORS <mailbox-name> <max-to-send>
     250 Descriptor list follows:
     expunged
     2333
     expunged
     2334

Lambert [Page 33] RFC 1056 PCMAIL June 1988

     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

Lambert [Page 34] RFC 1056 PCMAIL June 1988

     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

Lambert [Page 35] RFC 1056 PCMAIL June 1988

     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
     i.e. FETCH-MESSAGE current-events 495

Lambert [Page 36] RFC 1056 PCMAIL June 1988

II. Operations by name

 copy-message
 create-address
 create-bboard-mailbox
 create-client
 create-mailbox
 create-subscription
 delete-address
 delete-bboard-mailbox
 delete-client
 delete-mailbox
 delete-subscription
 expunge-mailbox
 fetch-changed-descriptors
 fetch-descriptors
 fetch-message
 help
 list-addresses
 list-available-subscriptions
 list-clients
 list-mailboxes
 list-subscriptions
 login
 logout
 print-message
 reset-client
 reset-descriptors
 reset-mailbox
 reset-subscription
 send-message
 send-version
 set-message-flag
 set-password

Lambert [Page 37] RFC 1056 PCMAIL June 1988

III. Responses by number

 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

Lambert [Page 38]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1056.txt · Last modified: 1988/06/16 17:53 by 127.0.0.1

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