RFC 993 (rfc993) - Page 2 of 28
PCMAIL: A distributed mail system for personal computers
Alternative Format: Original Text Document
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
resource-rich workstations. Finally, all workstations have some com-
mon characteristics that Pcmail should take advantage of. For in-
stance, 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
Clark & Lambert