PCMAIL: A distributed mail system for personal computers
RFC 984

Document Type RFC - Unknown (May 1986; No errata)
Obsoleted by RFC 993
Last updated 2013-03-02
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Network Working Group                                     David D. Clark
Request for Comments: 984                                Mark L. Lambert
                                M. I. T. Laboratory for Computer Science
                                                                May 1986

        PCMAIL: A Distributed Mail System for Personal Computers

1. Status of this Document

   This document is a preliminary discussion of the design of a
   personal-computer-based distributed mail system.  It is published for
   discussion and comment, and does not constitute a standard.  As the
   proposal may change, implementation of this document is not advised.
   Distribution of this memo is unlimited.

2. Introduction

   Pcmail is a distributed mail system that provides mail service to an
   arbitrary number of users, each of which owns one or more personal
   computers (PCs).  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, which are
   typically PCs.  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". Since clients are PCs,
   they 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 atomic (that is, they are guaranteed

Clark & Lambert                                                 [Page 1]



RFC 984                                                         May 1986
PCMAIL

   either to succeed completely, or fail completely).  A client can be
   abruptly disconnected from the repository without leaving
   inconsistent or damaged mail states.

   Pcmail is a mail system for PCs.  Its design has therefore been
   heavily influenced by several characteristics unique to PCs. First,
   PCs are relatively inexpensive.  This means that people may own more
   than one PC, perhaps putting one in an office and one at home.
   Second, PCs are portable.  Most PCs can be packed up and moved in the
   back seat of an automobile, and a few are truly portable--about the
   size of a briefcase--and battery-powered.  Finally, PCs are
   resource-poor.  A typical 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).

   Because PCs are relatively inexpensive and people may own more than
   one, Pcmail has been designed to allow users multiple access points
   to their mail state.  Each Pcmail user can have several client PCs,
   each of which can access the user's mail by communicating with the
   repository over a network.  The client PCs all maintain local copies
   of the user's global mail state, and synchronize the local and global
   states using DMSP.

   It is possible, even likely, that many PCs 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 PC 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 as
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