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RFC 62
Network Working Group D. C. Walden
Request for Comments: 62 BBN Inc.
Supercedes NWG/RFC #61 3 August 1970
A System for Interprocess Communication
in a
Resource Sharing Computer Network
1. Introduction
If you are working to develop methods of communications within a
computer network, you can engage in one of two activities. You can
work with others, actually constructing a computer network, being
influenced, perhaps influencing your colleagues. Or you can
construct an intellectual position of how things should be done in an
ideal network, one better than the one you are helping to construct,
and then present this position for the designers of future networks
to study. The author has spent the past two years engaged in the
first activity. This paper results from recent engagement in the
second activity.
"A resource sharing computer network is defined to be a set of
autonomous, independent computer systems, interconnected so as to
permit each computer system to utilize all of the resources of the
other computer systems much as it would normally call a subroutine."
This definition of a network and the desirability of such a network
is expounded upon by Roberts and Wessler in [9].
The actual act of resource sharing can be performed in two ways: in
an ad hoc manner between all pairs of computer systems in the
network; or according to a systematic network-wide standard. This
paper develops one possible network-wide system for resource sharing.
I believe it is natural to think of resources as being associated
with processes<1> and available only through communication with these
processes. Therefore, I view the fundamental problem of resource
sharing to be the problem of interprocess communication. I also
share with Carr, Crocker, and Cerf [2] the view that interprocess
communication over a network is a subcase of general interprocess
communication in a multi-programmed environment.
These views have led me to perform a two-part study. First, a set of
operations enabling interprocess communication within a single time-
sharing system is constructed. This set of operations eschews many
of the interprocess communications techniques currently in use within
time-sharing systems -- such as communication through shared memory
-- and relies instead on techniques that can be easily generalized to
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permit communication between remote processes. The second part of
the study presents such a generalization. The application of this
generalized system to the ARPA Computer Network [9] is also
discussed.
The ideas enlarged upon in this paper came from many sources.
Particularly influential were -- 1) an early sketch of a Host
protocol for the ARPA Network by S. Crocker of UCLA and W. Crowther
of Bolt Beranek and Newman Inc. (BBN); 2) Ackerman and Plummer's
paper on the MIT PDP-1 time-sharing system [1]; and 3) discussions
with W. Crowther and R. Kahn of BBN about Host protocol, flow
control, and message routing for the ARPA Network. Hopefully, there
are also some original ideas in this note. I alone am responsible
for the collection of all of these ideas into the system described
herein, and I am therefore responsible for any inconsistencies or
bugs in the system.
It must be emphasized that this paper does not represent an official
BBN position on Host protocol for the ARPA Computer Network.
2. A System for Interprocess Communication within a Time-Sharing System
This section describes a set of operations enabling interprocess
communication within a time-sharing system. Following the notation
of [10], I call this interprocess communication facility an IPC. As
an aid to the presentation of this IPC, a model for a time-sharing
system is described; this model is then used to illustrate the use of
the interprocess communication operations.
The model time-sharing has two pieces: the monitor and the processes.
The monitor performs such functions as switching control from one
process to another process when a process has used "enough" time,
fielding hardware interrupts, managing core and the swapping medium,
controlling the passing of control from one process to another (i.e.,
protection mechanisms), creating processes,caring for sleeping
processes, and providing to the processes a set of machine extending
operations (often called Supervisor or Monitor Calls). The processes
perform the normal user functions (user processes) as well as the
functions usually thought of as being supervisor functions in a
time-sharing system (systems processes) but not performed by the
monitor in the current model. A typical system process is the disc
handler or the file system. System processes is the disc handler or
the file system. System processes are probably allowed to execute in
supervisor mode, and they actually execute I/O instructions and
perform other privileged operations that user processes are not
allowed to perform. In all other ways, user and system processes are
identical. For reasons of efficiency, it may be useful to think of
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system processes as being locked in core.
Although they will be of concern later in this study, protection
considerations are not my concern here: instead I will assume that
all of the processes are "good" processes which never made any
mistakes. If the reader needs a protection structure to keep in mind
while he reads this note, the capability system developed in
[1][3][7][8] should be satisfying.
Of the operations a process can call on the monitor to perform, six
are of particular interest for providing a capability for
interprocess communication.
RECEIVE. This operation allows a specified process to send a message
to the process executing the RECEIVE. The operation has four
parameters: the port (defined below) awaiting the message -- the
RECEIVE port; the port a message will be accepted from -- the SEND
port; a specification of the buffer available to receive the message;
and a location to transfer to when the transmission is complete --
the restart location.
SEND. This operation sends a message from the process executing the
SEND to a specified process. It has four parameters: a port to send
the message to -- the RECEIVE port; the port the message is being
sent from -- the SEND port; a specification of the buffer containing
the message to be sent; and the restart location.
RECEIVE ANY. This operations allows any process to send a message to
the process executing the RECEIVE ANY. The operation has four
parameters: the port awaiting the message -- the RECEIVE port; a
specification of the buffer available to receive the message; a
restart location; and a location where the port which sent the
message may be noted.
SEND FROM ANY. This operation allows a process to send a message to
a process able to receive a message from any process. It has the
same four parameters as SEND. (The necessity for this operation will
be explained much later).
SLEEP. This operation allows the currently running process to put
itself to sleep pending the completion of an event. The operation
has one optional parameter, an event to be waited for. An example
event is the arrival of a hardware interrupt. The monitor never
unilaterally puts a process to sleep as a result of the process
executing one of the above four operations; however, if a process is
asleep when one of the above four operations is satisfied, the
process is awakened.
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UNIQUE. This operation obtains a unique number from the monitor.
A port is a particular data path to a process (a RECEIVE port) or
from a process (a SEND port), and all ports have an associated unique
port number which is used to identify the port. Ports are used in
transmitting messages from one process to another in the following
manner. Consider two processes, A and B, that wish to communicate.
Process A executes a RECEIVE to port N from port M. Process B
executes a SEND to port N from port M. The monitor matches up the
port numbers and transfers the message from process B to process A.
As soon as the buffer has been fully transmitted out of process B,
process B is restarted at the location specified in the SEND
operation. As soon as the message is fully received at process A,
process A is restarted at the location specified in the RECEIVE
operation. Just how the processes come by the correct port numbers
with which to communicate with other processes is not the concern of
the monitor -- this problem is left to the processes.
When a SEND is executed, nothing happens until a matching RECEIVE is
executed. Somewhere in the monitor there must be a table of port
numbers associated with processes and restart locations. The table
entries are cleared after each SEND/RECEIVE match is made. If a
proper RECEIVE is not executed for some time, the SEND is timed out
after a while and the SENDing process is notified. If a RECEIVE is
executed but the matching SEND does not happen for a long time, the
RECEIVE is timed out and the RECEIVing process is notified.
The mechanism of timing out "unused" table entries is of little
fundamental importance, merely providing a convenient method of
garbage collecting the table. There is no problem if an entry is
timed out prematurely, because the process can always re-execute the
operation. However, the timeout interval should be long enough so
that continual re-execution of an operation will cause little
overhead.
A RECEIVE ANY never times out, but may be taken back using a
supervisor call. A message resultant from a SEND FROM ANY is always
sent immediately and will be discarded if a proper receiver does not
exist. An error message is not returned and acknowledgment, if any,
is up to the processes. If the table where the SEND and RECEIVE are
matched up ever overflows, a process originating a further SEND and
RECEIVE is notified just as if the SEND or RECEIVE timed out.
The restart location is an interrupt entrance associated with a
pseudo interrupt local to the process executing the operation
specifying the restart location. If the process is running when then
event causing the pseudo interrupt occurs (for example, a message
arrives satisfying a pending RECEIVE), the effect is exactly as if
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the hardware interrupted the process and transferred control to the
restart location. Enough information is saved for the process to
continue execution at the point it was interrupted after the
interrupt is serviced. If the process is asleep, it is readied and
the pseudo interrupt is saved until the process runs again and the
interrupt is then allowed. Any RECEIVE or RECEIVE ANY message port
may thus be used to provide process interrupts, event channels,
process synchronization, message transfers, etc. The user programs
what he wants.
It is left as an exercise to the reader to convince himself that the
monitor he is saddled with can be made to provide the six operations
described above -- most monitors can since these are only additional
supervisor calls.
An example. Suppose that our model time-sharing system is
initialized to have several processes always running. Additionally,
these permanent processes have some universally known and permanently
assigned ports<2>. Suppose that two of the permanently running
processes are the logger-process and the teletype-scanner-process.
When the teletype-scanner-process first starts running, it puts
itself to sleep awaiting an interrupt from the hardware teletype
scanner. The logger-process initially puts itself to sleep awaiting
a message from the teletype-scanner-process via well-known permanent
SEND and RECEIVE ports. The teleype-scanner-process keeps a table
indexed by teletype number, containing in each entry a pair of port
numbers to use to send characters from that teletype to a process and
a pair of port numbers to use to receive characters for that teletype
from a process. If a character arrives (waking up the teletype-
scanner- process) and the process does not have any entry for that
teletype, it gets a pair of unique numbers from the monitor (via
UNIQUE) and sends a message containing this pair of numbers to the
logger-process using the ports for which the logger-process is known
to have a RECEIVE pending. The scanner-process also enters the pair
of numbers in the teletype table, and sends the character and all
future characters from this teletype to the port with the first
number from the port with the second number. The scanner-process
must also pass a second pair of unique numbers to the logger-process
for it to use for teletype output and do a RECEIVE using these port
numbers. When the logger-process receives the message from the
scanner-process, it starts up a copy of what SDS 940 TSS [6] users
call the executive<3>, and passes the port numbers to this copy of
the executive, so that this executive-process can also do its inputs
and outputs to the teletype using these ports. If the logger-process
wants to get a job number and password from the user, it can
temporarily use the port numbers to communicate with the user before
it passes them on to the executive. The scanner-process could always
use the same port numbers for a particular teletype as long as the
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numbers were passed on to only one copy of the executive at a time.
It is important to distinguish between the act of passing a port from
one process to another and the act of passing a port number from one
process to another. In the previous example, where characters from a
particular teletype are sent either to the logger-process or an
executive-process by the teletype-scanner-process, the SEND port
always remains in the teletype-scanner-process while the RECEIVE port
moves from the logger-process to the executive process. On the other
hand, the SEND port number is passed between the logger-process and
the executive-process to enable the RECEIVE process to do a RECEIVE
from the correct SEND port. It is crucial that, once a process
transfers a port to some other process, the first process no longer
use the port. We could add a mechanism that enforces this. The
protected object system of [9] is one such mechanism. Using this
mechanism, a process executing a SEND would need a capability for the
SEND port and only one capability for this SEND port would exist in
the system at any given time. A process executing a RECEIVE would be
required to have a capability for the RECEIVE port, and only one
capability for this RECEIVE port would exist at a given time.
Without such a protection mechanism, a port implicitly moves from one
process to another by the processes merely using the port at disjoint
times even if the port's number is never explicitly passed.
Of course, if the protected object system is available to us, there
is really no need for two port numbers to be specified before a
transmission can take place. The fact that a process knows an
existing RECEIVE port number could be considered prima facie evidence
of the process' right to send to that port. The difference between
RECEIVE and RECEIVE ANY ports then depends solely on the number of
copies of a particular port number that have been passed out. A
system based on this approach would clearly be preferable to the one
described here if it was possible to assume that all autonomous
time-sharing systems in a network would adopt this protection
mechanism. If this assumption cannot be made, it seems more
practical to require both port numbers.
Note that in the interprocess communication system (IPC) being
described here, when two processes wish to communicate they set up
the connection themselves, and they are free to do it in a mutually
convenient manner. For instance, they can exchange port numbers or
one process can pick all the port numbers and instruct the other
process which to use. However, in a particular implementation of a
time-sharing system, the builders of the system might choose to
restrict the processes' execution of SENDs and RECEIVEs and might
forbid arbitrary passing around of ports and port numbers, requiring
instead that the monitor be called (or some other special program) to
perform these functions.
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Flow control is provided in this IPC by the simple method of never
starting data transmission resultant from a SEND from one process
until a RECEIVE is executed by the receiver. Of course, interprocess
messages may also be sent back and forth suggesting that a process
stop sending or that space be allocated.
Generally, well-known permanently-assigned ports are used via RECEIVE
ANY and SEND FROM ANY. The permanent ports will most often be used
for starting processes and, consequently, little data will be sent
via them. If a process if running (perhaps asleep), and has a
RECEIVE ANY pending, then any process knowing the receive port number
can talk to that process without going through loggers. This is
obviously essential within a local time-sharing system and seems very
useful in a more general network if the ideal of resource sharing is
to be reached. For instance, in a resource sharing network, the
programs in the subroutine libraries at all sites might have RECEIVE
ANYs always pending over permanently assigned ports with well-known
port numbers. Thus, to use a particular network resource such as a
matrix manipulation hardware, a process running anywhere in the
network can send a message to the matrix inversion subroutine
containing the matrix to be inverted and the port numbers to be used
for returning the results.
An additional example demonstrates the use of the FORTRAN compiler.
We have already explained how a user sits down at his teletype and
gets connected to an executive. We go on from there. The user is
typing in and out of the executive which is doing SENDs and RECEIVEs.
Eventually the user types RUN FORTRAN, and executive asks the monitor
to start up a copy of the FORTRAN compiler and passes to FORTRAN as
start up parameters the port numbers the executive was using to talk
to the teletype. (This, at least conceptually, FORTRAN is passed a
port at which to RECEIVE characters from the teletype and a port from
which to SEND characters to the teletype.) FORTRAN is, of course,
expecting these parameters and does SENDs and RECEIVEs via the
indicated ports to discover from the user what input and output files
the user wants to use. FORTRAN types INPUT FILE? to the user, who
responds F001. FORTRAN then sends a message to the file-system-
process, which is asleep waiting for something to do. The message is
sent via well-known ports and it asks the file system to open F001
for input. The message also contains a pair of port numbers that the
file-system process can use to send its reply. The file-system looks
up F001, opens it for input, make some entries in its open file
tables, and sends back to FORTRAN a message containing the port
numbers that FORTRAN can use to read the file. The same procedure is
followed for the output file. When the compilation is complete,
FORTRAN returns the teletype port numbers (and the ports) back to the
executive that has been asleep waiting for a message from FORTRAN,
and then FORTRAN halts itself. The file-system-process goes back to
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sleep when it has nothing else to do<4>.
Again, the file-system process can keep a small collection of port
numbers which it uses over and over if it can get file system users
to return the port numbers when they have finished with them. Of
course, when this collection of port numbers has eventually dribbled
away, the file system can get some new unique numbers from the
monitor.
3. A System for Interprocess Communication Between Remote Processes
The IPC described in the previous section easily generalizes to allow
interprocess communication between processes at geographically
different locations as, for example, within a computer network.
Consider first a simple configuration of processes distributed around
the points of a star. At each point of the star there is an
autonomous operating system<5>. A rather large, smart computer
system, called the Network Controller, exists at the center of the
star. No processes can run in this center system, but rather it
should be thought of as an extension of the monitor of each of the
operating systems in the network.
If the Network Controller is able to perform the operations SEND,
RECEIVE, SEND FROM ANY, RECEIVE ANY, and UNIQUE and if all of the
monitors in all of the time-sharing systems in the network do not
perform these operations themselves but rather ask the Network
Controller to perform these operations for them, then the problem of
interprocess communication between remote processes if solved. No
further changes are necessary since the Network Controller can keep
track of which RECEIVEs have been executed and which SENDs have been
executed and match them up just as the monitor did in the model
time-sharing system. A networkwide port numbering scheme is also
possible with the Network Controller knowing where (i.e., at which
site) a particular port is at a particular time.
Next, consider a more complex network in which there is no common
center point, making it necessary to distribute the functions
performed by the Network Controller among the network nodes. In the
rest of this section I will show that it is possible to efficiently
and conveniently distribute the functions performed by the star
Network Controller among the many network sites and still enable
general interprocess communication between remote processes.
Some changes must be made to each of the four SEND/RECEIVE operations
described above to adapt them for use in a distributed Network
Controller. To RECEIVE is added a parameter specifying a site to
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which the RECEIVE is to be sent. To the SEND FROM ANY and SEND
messages is added a site to send the SEND to although this is
normally the local site. Both RECEIVE and RECEIVE ANY have added the
provision for obtaining the source site of any received message.
Thus, when a RECEIVE is executed, the RECEIVE is sent to the site
specified, possibly a remote site. Concurrently a SEND is sent to
the same site, normally the local site of the process executing the
SEND. At this site, called the rendezvous site, the RECEIVE is
matched with the proper SEND and the message transmission is allowed
to take place from the SEND site to the site from whence the RECEIVE
came.
A RECEIVE ANY never leaves its originating site and therein lies the
necessity for SEND FROM ANY, since it must be possible to send a
message to a RECEIVE ANY port and not have the message blocked
waiting for a RECEIVE at the sending site. It is possible to
construct a system so the SEND/RECEIVE rendezvous takes place at the
RECEIVE site and eliminates the SEND FROM ANY operation, but in my
judgment the ability to block a normal SEND transmission at the
source site more than makes up for the added complexity.
At each site a rendezvous table is kept. This table contains an
entry for each unmatched SEND or RECEIVE received at that site and
also an entry for all RECEIVE ANYs given at that site. A matching
SEND/RECEIVE pair is cleared from the table as soon as the match
takes place. As in the similar table kept in the model time-sharing,
SEND and RECEIVE entries are timed out if unmatched for too long and
the originator is notified. RECEIVE ANY entries are cleared from the
table when a fulfilling message arrives.
The final change necessary to distribute the Network Controller
functions is to give each site a portion of the unique numbers to
distribute via its UNIQUE operation. I'll discuss this topic further
below.
To make it clear to the reader how the distributed Network Controller
works, an example follows. The details of what process picks port
numbers, etc., are only exemplary and are not a standard specified as
part of the IPC.
Suppose that, for two sites in the network, K and L, process A at
site K wishes to communicate with process B at site L. Process B has
a RECEIVE ANY pending at port M.
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RFC 62 IPC for Resource Sharing 3 August 1970
SITE K SITE L
______ ______
/ \ / \
/ \ / \
/ \ / \
/ \ / \
| | | |
| Process A | | Process B |
| | | |
\ / \ /
\ / RECEIVE--> port M /
\ / ANY \ /
\______/ \______/
Process A, fortunately, knows of the existence of port M at site L and
sends a message using the SEND FROM ANY operation from port N to port
M. The message contains two port numbers and instructions for process
B to SEND messages for process A to port P from port Q. Site K's site
number is appended to this message along with the message's SEND port N.
SITE K SITE L
______ ______
/ \ / \
/ \ / \
/ \ / \
/ \ / \
| | | |
| Process A | | Process B |
| | | |
\ port N / \ port M /
\ /--->SEND FROM --->\ /
\ / ANY \ /
\______/ \______/
to port M, site L
containing K,N,P, & Q
Process A now executes a RECEIVE at port P from port Q. Process A
specifies the rendezvous site to be site L.
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SITE K SITE L
______ ______
/ \ / \
/ \ / \
/ \ Rendezvous/ \
/ \ table \
| | | |
| Process A | ^ | Process B |
| | | | |
\ port P / | \ /
\ / | \ /
\ / <--RECEIVE __/ \ /
\______/ MESSAGE \______/
to site L
containing P, Q, & K
A RECEIVE message is sent from site K to site L and is entered in the
rendezvous table at site L. At some other time, process B executes a
SEND to port P from port Q specifying site L as the rendezvous site.
SITE K SITE L
______ ______
/ \ / \
/ \ / \
/ \ Rendezvous/ \
/ \ table \
| | | |
| Process A | | Process B |
| | | |
\ port P / <--------- port Q /
\ / \ /
\ / SEND \ /
\______/ \______/
to site L
containing P & Q
A rendezvous is made, the rendezvous table is cleared, and the
transmission to port P at site K takes place. The SEND site number
(and conceivably the SEND port number) is appended to the messages of
the transmission for the edification of the receiving process.
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SITE K SITE L
______ ______
/ \ / \
/ \ / \
/ \ / \
/ \ / \
| | | |
| Process A | | Process B |
| | | |
\ port P / \ port Q /
\ /<--transmission<--\ /
\ / \ /
\______/ to port P, site K \______/
containing data and L
Process B may simultaneously wish to execute a RECEIVE from port N at
port M.
Note that there is only one important control message in this system
which moves between sites, the type of message that is called a
Host/Host protocol message in [2]. This control message is the
RECEIVE message. There are two other possible intersite control
messages: an error message to the originating site when a RECEIVE or
SEND is timed out, and the SEND message in the rare case when the
rendezvous site is not the SEND site. There must also be a standard
format for messages between ports. For example, the following:
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_________________ __________________ _____________
| rendezvous site | <6> | destination site | | source site |
|-----------------| |------------------| |-------------|
| RECEIVE port | | RECEIVE port | | RECEIVE port|
|-----------------| |------------------| |-------------|
| SEND port | | SEND port | | SEND port |
|-----------------| |------------------| |-------------|
| | | source site | | |
| | |------------------| | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| data | | data | | data |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
|_________________| |__________________| |_____________|
transmitted transmitted received
by SEND by Network by RECEIVE
process Controller process
In the model time-sharing system it was possible to pass a port form
process to process. This is still possible with a distributed Network
Controller.
Remember that, for a message to be sent from one process to another, a
SEND to port M from port N and a RECEIVE at port M from port N must
rendezvous, normally at the SEND site. Both processes keep track of
where they think the rendezvous site is and supply this site as a
parameter of appropriate operations. The RECEIVE process thinks it is
the SEND site also. Since once a SEND and a RECEIVE rendezvous the
transmission is sent to the source of the RECEIVE and the entry in the
rendezvous table is cleared and must be set up again for each further
transmission from N to M, it is easy for a RECEIVE port to be moved.
If a process sends both the port numbers and the rendezvous site
number to a new process at some other site which executes a RECEIVE
using these same old port numbers and rendezvous site specification,
the SENDer never knows the RECEIVEr has moved. It is slightly harder
for a send port to move. However, if it does, the pair of port
numbers that has been being used for a SEND and the original
rendezvous site number are passed to the new site. The process at the
new SEND site specifies the old rendezvous site with the first SEND
from the new site. The RECEIVE process will also still think the
rendezvous site is the old site, so the SEND and RECEIVE will meet at
the old site. When they meet, the entry in the table at that site is
cleared, and both the SEND and RECEIVE messages are sent to the new
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SEND site just as if they had been destined for there in the first
place. The SEND and RECEIVE then meet again at the new rendezvous
site and transmission may continue as if the port had never moved.
Since all transmissions contain the source site number, further
RECEIVEs will be sent to the new rendezvous site. It is possible to
discover that this special manipulation must take place because a SEND
message is received at a site that did not originate the SEND
message<7>. Note that the SEND port and the RECEIVE port can move
concurrently.
Of course, all of this could have also been done if the processes had
sent messages back and forth announcing any potential moves and the
new site numbers.
A problem that may have occurred to the reader is how the SEND and
RECEIVE buffers get matched for size. The easiest solution would be
to require that all buffers have a common size but this is
unacceptable since it does not easily extend to a situation where
processes in autonomous operating systems are attempting to
communicate. A second solution is for the processes to pass messages
specifying buffer sizes. If this solution is adopted, excessive data
sent from the SEND process and unable to fix into the RECEIVE buffer
is discarded and the RECEIVE process notified. The solution has great
appeal on account of its simplicity. A third solution would be for
the RECEIVE buffer size to be passed to the SEND site with RECEIVE
message and to notify the SEND process when too much data is sent or
even to pass the RECEIVE buffer size on to the SEND process. This
last method would also permit the Network Controller at the SEND site
to make two or more SENDs out of one, if that was necessary to match a
smaller RECEIVE buffer size.
The maintenance of unique numbers is also a problem when the processes
are geographically distributed. Three solutions to this problem are
presented here. The first possibility is for the autonomous operating
systems to ask the Network Controller for the unique numbers
originally and then guarantee the integrity of any unique numbers
currently owned by local processes and programs using whatever means
are at the operating system's disposal. In this case, the Network
Controller would provide a method for a unique number to be sent from
one site to another and would vouch for the number's identity at the
new site. The second method is simply to give the unique numbers to
the processes that are using them, depending on the non-malicious
behavior of the processes to preserve the unique numbers, or if an
accident should happen, the two passwords (SEND and RECEIVE port
numbers) that are required to initiate a transmission. If the unique
numbers are given out in a non-sequential manner and are reasonably
long (say 32 bits), there is little danger. In the final method, a
user identification is included in the port numbers and the individual
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operating systems guarantee the integrity of these identification
bits. Thus a process, while not able to be sure that the correct port
is transmitting to him, can be sure that some port of the correct user
is transmitting. This is the so-called virtual net concept suggested
by W. Crowther [2].<8>
A third difficult problem arises when remote processes wish to
communicate, the problem of maintaining high bandwidth connections
between the remote processes. The solution to this problem lies in
allowing the processes considerable information about the state of an
on-going transmission. First, we examine a SEND process in detail.
When a process executes a SEND, the local portion of the Network
Controller passes the SEND on to the rendezvous site, normally the
local site. When a RECEIVE arrives matching a pending SEND, the
Network Controller notifies the SEND process by causing an interrupt
to the specified restart location. Simultaneously the Network
Controller starts shipping the SEND buffer to the RECEIVE site. When
transmission is complete, a flag is set which the SEND process can
test. While a transmission is taking place, the process may ask the
Network Controller to perform other operations, including other SENDs.
A second SEND over a pair of ports already in the act of transmission
is noted and the SEND becomes active as soon as the first transmission
is complete. A third identical SEND results in an error message to
the SENDing process. Next, we examine a RECEIVE process in detail.
When a process executes a RECEIVE, the RECEIVE is sent to the
rendezvous site. When data resultant from this RECEIVE starts to
arrive at the RECEIVE site, the RECEIVE process is notified via an
interrupt to the specified restart location. When the transmission is
complete, a flag is set which the RECEIVE process can test. A second
RECEIVE over the same port pair is allowed. A third results in an
error message to the RECEIVE process. Thus, there is sufficient
machinery to allow a pair of processes always to have both a
transmission in progress and the next one pending. Therefore, no
efficiency is lost. On the other hand, each transmission must be
preceded by a RECEIVE into a specified buffer, thus continuing to
provide complete flow control.
4. A Potential Application
Only one resource sharing computer network currently exists, the
ARPA Computer Network. In this section, I discuss application of the
system described in this paper to the ARPA Network [2][5][9].
The ARPA Network currently incorporates ten sites spread across the
United States. Each site consists of one to three (potentially four)
independent computer systems called Hosts and one communications
computer system called an IMP. All of the Hosts at a site are
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directly connected to the IMP. The IMPs themselves are connected
together by 50-kilobit phone lines (much higher rate lines are a
potential), although each IMP is connected to only one to five other
IMPs. The IMPs provide a communications subnet through which the
Hosts communicate. Data is sent through the communications subnet in
messages of arbitrary size (currently about 8000 bits) called network
messages. When a network message is received by the IMP at the
destination site, that IMP sends an acknowledgment, called a RFNM, to
the source site.
A system for interprocess communication for the ARPA Network (let us
call this IPC for ARPA) is currently being designed by the Network
Working Group, under the chairmanship of S. Crocker of UCLA. Their
design is somewhat constrained by the communications subnet [5]<9>.
I would like to compare point-by-point IPC for ARPA with the one
developed in this paper; however, such a comparison would first
require description here, almost from scratch, of the current state
of IPC for ARPA since very little up-to-date information about IPC
for ARPA appears in the open literature [2]. Also, IPC for ARPA is
quite complex and the working documents describing it now run to many
hundred pages, making any description lengthy and inappropriate for
this paper.<10> Therefore, I shall make only a few scattered
comparisons of the two systems, the first of which are implicit in
this paragraph.
The interprocess communication system being developed for the ARPA
Network comes in several almost distinct pieces: The Host/IMP
protocol, IMP/IMP protocol, and the Host/Host protocol. The IMPs
have sole responsibility for correctly transmitting bits from one
site to another. The Hosts have sole responsibility for making
interprocess connections. Both the Host and IMP are concerned and
take a little responsibility for flow control and message sequencing.
Applications of the interprocess communication system described in
this paper leads me to make a different allocation of responsibility.
The IMP still continues to move bits from on site to another
correctly but the Network Controller also resides in the IMP, and
flow control is completely in the hands of the processes running in
the Hosts, although using the mechanisms provided by the IMPs.
The IMPs provide the SEND, RECEIVE, SEND FROM ANY, RECEIVE ANY, and
UNIQUE operations in slightly altered forms for the Hosts and also
maintain the rendezvous tables, including moving of SEND ports when
necessary. Putting these operations in the IMP requires the
Host/Host protocol program to be written only once, rather than many
times as is currently being done in the ARPA Network. It is perhaps
useful to step through the five operations again.
SEND. The Host gives the IMP a SEND port number, a RECEIVE port
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number, the rendezvous site, and a buffer specification (e.g., start
and end, or beginning and length). The SEND is sent to the
rendezvous site IMP, normally the local IMP. When a matching RECEIVE
arrives at the local IMP, the Host is notified of the RECEIVE port of
the just arrived message. This port number is sufficient to identify
the SENDing process, although a given operating system may have to
keep internal tables mapping this port number into a useful internal
process identifier. Simultaneously, the IMP begins to ask the Host
for specific pieces of the SEND buffer, sending these pieces as
network messages to the destination site. If a RFNM is not received
for too long, implying a network message has been lost in the
network, the Host is asked for the same data again and it is
retransmitted.<11> Except for the last piece of a buffer, the IMP
requests pieces from the Host which are common multiplies of the word
size of the source Host, IMP, and destination Host. This avoids
mid-transmission word alignment problems.
RECEIVE. The Host gives the IMP a SEND port, a RECEIVE port, a
rendezvous site, and a buffer description. The RECEIVE message is
sent to the rendezvous site. As the network messages making up a
transmission arrive for the RECEIVE port, they are passed to the Host
along with RECEIVE port number (and perhaps the SEND port number),
and an indication to the Host where to put this data in its input
buffer. When the last network message of the SEND buffer is passed
into the Host, it is marked accordingly and the Host can then detect
this. (It is conceivable that the RECEIVE message could also
allocate a piece of network bandwidth while making its network
traverse to the rendezvous site.)
RECEIVE ANY. The Host gives the IMP a RECEIVE port and a buffer
descriptor. This works the same as RECEIVE but assumes the local
site to be the rendezvous site.
SEND FROM ANY. The Host gives the IMP RECEIVE and SEND ports, the
destination site, and a buffer descriptor. The IMP requests and
transmits the buffer as fast as possible. A SEND FROM ANY for a
non-existent port is discarded at the destination site.
In the ARPA Network, the Hosts are required by the IMPs to physically
break their transmissions into network messages, and successive
messages of a single transmission must be delayed until the RFNM is
received for the previous message. In the system described here,
since RFNMs are tied to the transmission of a particular piece of
buffer and since the Hosts allow the IMPs to reassemble buffers in
the Hosts by the IMP telling the Host where to put each buffer piece
then pieces of a single buffer can be transmitted in parallel network
messages and several RFNMs can be outstanding simultaneously. This
enables The Hosts to deal with transmissions of more natural sizes
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and higher bandwidth for a single transmission.
For additional efficiency, the IMP might know the approximate time it
takes for a RECEIVE to get to a particular other site and warn the
Host to wake up a process shortly before the arrival of a message for
that process is imminent.
5. Conclusion
Since the system described in this paper has not been implemented, I
have no clearly demonstrable conclusions nor any performance reports.
Instead, I conclude with four openly subjective claims.
1) The interprocess communication system described in Section 2 is
simpler and more general than most existing systems of equivalent
power and is more powerful than most intra time-sharing system
communication systems currently available.
2) Time-sharing systems structured like the model in Section 2 should
be studied by designers of time-sharing systems who may see a
computer network in their future, as structure seems to enable
joining a computer network with a minimum of difficulty.
3) As computer networks become more common, remote interprocess
communication systems like the one described in Section 3 should be
studied. The system currently being developed for ARPA is a step in
the wrong direction, being addressed, in my opinion, more to
communication between monitors than to communication between
processes and consequently subverting convenient resource sharing.
4) The application of the system as described in Section 4 is much
simpler to implement and more powerful than the system currently
being constructed for the ARPA Network, and I suggest that
implementation of my method be seriously considered for adoption by
the ARPA Network.
<Footnotes>
1. Almost any of the common definitions of a process would suit the
needs of this paper.
2. Or perhaps there is only one permanently known port, which
belongs to a directory-process that keeps a table of
permanent-process/well-know-port associations.
3. That program which prints file directories, tells who is on other
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teletypes, runs subsystems, etc.
4. The reader should have noticed by now that I do not like to think
of a new process (consisting of a new conceptual copy of a
program) being started up each time another user wishes to use
the program. Rather, I like to think of the program as a single
process which knows it is being used simultaneously by many other
processes and consciously multiplexes among the users or delays
service to users until it can get around to them.
5. I use operating system rather than time-sharing system in this
section to point up the fact that the autonomous systems at the
network nodes may be either full blown time-sharing systems in
their own right, and individual process in a larger
geographically distributed time-sharing system, or merely
autonomous sites wishing to communicate.
6. For a SEND FROM ANY message, the rendezvous site is the
destination site.
7. For readers familiar with the once-proposed re-connection scheme
for the ARPA Network, the above system is simple, comparatively,
because there are no permanent connections to break and move;
that is, connections only exist fleetingly in the system
described here and can therefore be remade between any pair of
processes which at any time happen to know each other's port
numbers and have some clue where they each are.
8. Crowther says this is not the virtual net concept.
9. As one of the builders of the ARPA communications subnet, I am
partially responsible for these constraints.
10. The reader having access to the ARPA working documents may want
to read Specifications for the Interconnection of a Host to
an IMP, BBN Report No. 1822; and ARPA Network Working Group
Notes #36, 37, 38, 39, 42, 44, 46, 47, 48, 49, 50, 54, 55, 56,
57, 58, 59, 60.
11. This also allows messages to be completely thrown away by the IMP
subnet it that should ever be useful.
[REFERENCES]
1. Ackerman, W., and Plummer, W. An implementation of a
multi-processing computer system. Proc. ACM Symp. on
Operating System Principles, Gatlinsburg, Tenn.,
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RFC 62 IPC for Resource Sharing 3 August 1970
Oct. 1-4, 1967.
2. Carr, C. Crocker, S., and Cerf, V. Host/Host communication
protocol in the ARPA network. Proc. AFIPS 1970 Spring
Joint Comput. Conf., Vol. 36, AFIPS Press, Montvale, N.J.,
pp. 589-597.
3. Dennis, J., and VanHorn, E. Programming semantics for
multiprogrammed computations. Comm. ACM 9, 3 (March,
1966), 143-155.
4. Hansen, P.B. The nucleus of a multiprogramming system. Comm.
ACM 13, 4 (April, 1970), 238-241, 250.
5. Heart, F., Kahn, R., Ornstein, S., Crowther, W., and Walden, D.
The interface message processor for the ARPA computer
network. Proc. AFIPS 1970 Spring Joint Comput. Conf., Vol.
36, AFIPS Press, Montvale, N.J., pp. 551-567.
6. Lampson, B. SDS 940 Lectures, circulated informally.
7. _______. An overview of the CAL time-sharing system. Computer
Center, University of California, Berkeley, Calif.
8. _______. Dynamic protection structures. Proc. AFIPS 1969 Fall
Joint Comput. Conf., Vol. 35, AFIPS Press, Montvale, N.J.,
pp. 27-38.
9. Roberts, L., and Wessler, B. Computer network development to
achive resource sharing. Proc. AFIPS 1970 Spring Joint
Comput. Conf., Vol. 36, AFIPS Press, Monvale, N.J., pp.
543-549.
10. Spier, M., and Organick, E. The MULTICS interprocess
communication facility. Proc. ACM Second Symp. on Operating
Systems Principles, Princeton University, Oct. 20-22, 1969.
Author's Address
D. C. Walden
Bolt Bernakek and Newman, Inc.
Cambridge, Massachusetts
[ This RFC was put into machine readable form for entry ]
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