PCMAIL: A distributed mail system for personal computers
RFC 993

Document Type RFC - Unknown (December 1986; No errata)
Obsoleted by RFC 1056
Obsoletes RFC 984
Last updated 2013-03-02
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Network Working Group                              David D. Clark (MIT)
Request for Comments: 993                         Mark L. Lambert (MIT)
Obsoletes:  RFC-984                                       December 1986

        PCMAIL: A Distributed Mail System for Personal Computers

1. Status of this Document

   This document is a discussion of the Pcmail workstation-based distri-
   buted mail system.  It is a revision of the design published in NIC
   RFC-984.  The revision is based on discussion and comment from a
   variety of sources, as well as further research into the design of
   interactive Pcmail clients and the use of client code on machines
   other than IBM PCs.  As this design may change, implementation of
   this document is not advised.  Distribution of this memo is unlimit-
   ed.

2. Introduction

   Pcmail is a distributed mail system providing mail service to an ar-
   bitrary 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 per-
   manently connected to a network.  It is also designed to offer disk-
   less workstations full mail service.

   The system is divided into two halves.  The first consists of a sin-
   gle entity called the "repository".  The repository is a storage
   center for incoming mail.  Mail for a Pcmail user can arrive exter-
   nally 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

Clark & Lambert                                                 [Page 1]
RFC 993                                                    December 1986

   user's global mail state, called the "local mail state".  It is as-
   sumed 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 Sys-
   tem 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 mes-
   sage", "print a message", etc.).  DMSP also provides special opera-
   tions 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
   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 ade-
   quate mail service without hampering its performance on more
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