Network Working Group Edwin E. Meyer, Jr.
Request for Comments: 46 Massachusetts Institute of Technology
17 April 1970
ARPA Network Protocol Notes
The attached document contains comments and suggestions of the
Network Working Group at Project MAC. It is based upon the protocol
outlined in NWG/RFC 33, 36, and later documents.
This proposal is intended as a contribution to the dialog leading to
a protocol specification to be accepted by the entire Network Working
Group.
We solicit your comments.
I - INTRODUCTION
In this document the Network Working Group at MIT Project MAC suggest
modifications and extensions to the protocol specified by Carr,
Crocker, and Cerf in a preprint of their 1970 SJCC paper and extended
by Crocker in NWG/RFC 36. This document broadly outlines our
proposal but does not attempt to be a complete specification. It is
intended to be an indication of the type and extent of the protocol
we think should be initially implemented.
We agree with the basic concept of simplex communication between
sockets having unique identifiers. We propose the implementation of
a slightly modified subset of the network commands specified in
NWG/RFC36 plus the ERR command as specified by Harslem and Heafner in
NWG/RFC 40.
Given the basic objective of getting all ARPA contractors onto the
network and talking to each other at the earliest possible date, we
think that it is important to implement an initial protocol that is
reasonably simple yet extendable while providing for the major
initial uses of the network. It should be a simple protocol so as to
elicit the broadest possible support and to be easily implementable
at all installations with a minimum of added software.
While the protocol will evolve, the fundamentals of a protocol
accepted and implemented by all installations are likely to prove
very resistant to change. Thus it is very important to make the
initial protocol open-ended and flexible. A simple basic protocol is
more likely to succeed in this respect than a complicated one. This
[Page 1]RFC 46 ARPA Network Protocol Notes April 1970
does not preclude the existence of additional layers of protocol
between several installations so long as the basic protocol remains
supported.
We feel that three facilities must be provided for in the initial
protocol:
1. Multi-path communication between two existing processes which know
how to connect to each other.
2. A standard way for a process to connect to the logger (logging
process at a HOST) at a foreign HOST and request the creation of a
user process. (The login ritual may or may not be standardized.)
3. A standard way for a newly created process to initiate pseudo-
typewriter communication with the foreign process which requested
its creation.
The major differences between the protocol as proposed by Carr,
Crocker, and Cerf and this proposal are the following:
1. The dynamic reconnection strategy specified in Crocker's
NWG/RFC 36 is reserved for future implementation. We feel that
its inclusion would unduly complicate the initial implementation
of the protocol. We outline a strategy for foreign process
creation that does not require dynamic reconnection. Nothing in
this proposal precludes the implementation of dynamic reconnection
at a later date.
2. We propose that an "instance tag" be added to the socket
identifier so as to separate sockets belonging to different
processes of the same user coexisting at one HOST.
3. The following NCP commands have been added:
a. The ERR command specified in NWG/RFC 40 is included.
b. BLK and RSM commands are presented as possible alternatives to
the "cease on link" IMP command and SPD and RSM commands set
forth in NWG/RFC 36. Because these commands operate on socket
connections rather than link numbers, they do not impede the
implementation of socket connection multiplexing over a single
link number, should that later prove desirable.
c. An INT command that interrupts a process is specified. We feel
that it is highly important to be able to interrupt a process
that may be engaged in unwanted computation or output. To
implement the interrupt as a special format within a normal
[Page 2]RFC 46 ARPA Network Protocol Notes April 1970
message raises severe difficulties: the connection may be
blocked when the interrupt is needed, and the NCP must scan
each incoming message for an interrupt signal.
d. An ECO echoing command to test communications between NCPs is
included.
4. Sockets are conceptualized as having several states, and these are
related to conditions under which network requests may be queued.
This differs from the unlimited queuing feature, which presents
certain implementation difficulties.
5. The protocol regarding creation of a foreign process and
communication with it is removed to a separate User Control and
Communication (UCC) protocol level and is more fully specified.
II - A HIERARCHY OF PROTOCOLS
It seems convenient and useful to view the network as consisting of a
hierarchy of protocol and implementation levels. In addition to
aiding independent software and hardware development, provisions for
a layered protocol allow additions and substitution of certain levels
in experimental or special purpose systems.
We view the initial network communications system as a hierarchy of
three systems of increasing generality and decreasing privilege
level. These are:
1. IMP Network - The network of IMPs and physical communication lines
is the basic resource which higher level systems convert into more
generalized communication facilities. The IMP network acts as a
"wholesaler" of message transmission facilities to a highly
privileged module within each HOST.
2. Network Control Program - Each HOST contains a module called the
Network Control Program (NCP) which has sole control over
communications between its HOST and the IMP network. It acts as a
"retailer" of the wholesale communications facilities provided by
the IMP network. The network of NCPs can be viewed as a higher
level communications system surrounding the IMP network which
factors raw message transmission capabilities between HOSTs into
communication facilities between ordinary unprivileged processes.
[Page 3]RFC 46 ARPA Network Protocol Notes April 1970
H O S T A H O S T C
______________________________ ______________________
| | | |
| ____ ____ ____ ____ | | ____ ____ ____ |
| |Proc| |Proc| |Proc| | | | | |Proc| |Proc| | | |
| | A | | B | | C | |UCC | | | | D | | E | |UCC | |
| |____| |____| |____| |____| | | |____| |____| |____| |
| | | | | | | | | | |
- - - - - - |- - - |- - - |- - -|- - -|- - |- - -|- - - |- - - - - -
| | | | | NCP NETWORK | | | |
| | | | | | | | | | |
| _|_____|______|______|_ | | _|_____|______|_ |
| | | | | | | |
| | N C P A | | | | N C P C | |
| |_______________________| | | |________________| |
| || | | || |
|_____________________||_______| |_______||_____________|
|| ||
- - - - - - - - - - - -|| - - - - - - - - - - ||- - - - - - - - - -
|| IMP NETWORK ||
___||___ ____||__
| | | |
| IMP |------------| IMP |
| A | | C |
|________| |________|
| |
| ________ |
| | | |
+------| IMP |-----+
| B |
|________|
FIG 1. Modular View Of Network
3. User Control and Communication Module - The preceding two
communication systems are sufficient to permit communication
between unprivileged processes that already exist. However, one
of the primary initial uses of the network is thought to involve
the creation of a foreign user process through interaction with
the foreign HOST's logger. The User Control and Communication
Module (UCC) implements protocol sufficient for a process to
communicate with a foreign HOST's logger and to make initial
control communication with a created process. Such a process is
to have the same privileges (subject to administrative control) as
a local (to the foreign HOST) user process. The UCC module
communicates through the NCP in a manner similar to an ordinary
process. Except for the ability to close connections to a dead
[Page 4]RFC 46 ARPA Network Protocol Notes April 1970
process, the UCC module has no special network privileges. The
UCC protocol is only one of several third-level protocols that
could be implemented. For example, a set of batch processing
systems connected through the NCP system might implement a load-
sharing protocol, but not a UCC.
III - NETWORK CONTROL PROGRAM
Each HOST implements a module called the Network Control Program
(NCP) which controls all network communications involving that HOST.
The network of NCPs forms a distributed communication system that
implements communication paths between individual processes. The NCP
protocol issues involve: (i) the definition of these communication
paths, and (ii) a system for coordinating the distributed NCP system
in maintaining these communication paths. These are discussed below.
Sockets
Communication between two processes is made through a simplex
connection between two sockets: a send socket attached to one
process and a receive socket attached to another process. Sockets
have the following characteristics:
Socket Identifier - A socket identifier is used throughout the
network to uniquely identify a socket. It consists of 48 bits,
having the following components:
a. User Number (24 bits) - A socket attached to a process is
identified as belonging to that process by a user number
consisting of 8 bits of "home" HOST code plus 16 bits of user
code assigned by the home HOST. This user number is the same
for all sockets attached to any of his processes in any HOST.
b. Instance Tag (8 bits) - More than one process belonging to a
user may simultaneously exist within a single HOST. The
instance tag identifies the particular process to which a
socket belongs. A user's first process at a HOST to use the
network receives instance tag = 0 by convention.
c. HOST Number (8 bits) - This is the code of the HOST on which
the attached process exists.
d. Socket Code (8 bits) - This code provides for 128 send and 128
receive sockets in each process. The low order bit determines
whether this is a "send" (= 1) or "receive" (= 0) socket.
[Page 5]RFC 46 ARPA Network Protocol Notes April 1970
States of Sockets - Each socket has an associated state. The NCP may
implement more transitory states of a socket, but the three following
are of conceptual importance.
a. Inactive - there is no currently existing process which has
told the NCP that it wishes to listen to this socket. No other
process can successfully communicate with an inactive socket.
b. Open - Some process has agreed to listen to events concerning
this socket but it is not yet connected.
c. Connected - This socket is currently connected to another
socket.
Socket Event Queue - A queue of events to be disclosed to the owning
process is maintained for each open or connected socket. It consists
of a chronologically ordered list of certain events generated by the
action of one or more foreign processes trying to connect or
disconnect this socket. An entry in the event queue consists of the
event type plus the identifier of the foreign socket concerned. The
following event types are defined:
a. "request" - a foreign socket requests connection. (not queued
if local socket is already connected)
b. "accept" - a foreign socket accepts requested connection.
c. "reject" - a foreign socket rejects requested connection.
d. "close" - a foreign socket disconnects an existing connection.
A "request" event is removed from the queue when it is accepted or
rejected. The other events are removed from the queue as they are
disclosed to the owning process.
Some events are intended to be transparent to the process owning the
socket, and they do not generate entries in the event queue.
Although an event queue is conceptually unlimited, it seems necessary
to place some practical limit on its length. When an event queue for
a socket is full, any incoming event that would add to the queue
should be discarded and the sending NCP notified (via ERR command
described below).
[Page 6]RFC 46 ARPA Network Protocol Notes April 1970
NCP Control Communications
The NCP network coordinates its activities through control commands
passed between its individual components. These commands generally
concern the creation and manipulation of socket connections
controlled by the NCP receiving the command. A control command is
directed to a particular NCP by being sent to its HOST as a message
over link number 1 (designated as the control link), which is
reserved for that purpose. The IMP network does not distinguish
between these messages and regular data messages implementing
communication through a socket connection.
The following NCP control commands are defined:
a. Request for Connection
RFC <local socket> <foreign socket> [<link no.>]
An NCP directs this command to a foreign NCP to attempt to
initiate a connection between a local socket and a foreign socket.
If the foreign socket is open, the foreign NCP places a "request"
event into the socket's event queue for disclosure to the process
that owns it. If the foreign process accepts, the foreign NCP
returns a positive acknowledgement in the form of another RFC. It
rejects connection by issuing the CLS command (see below). An RFC
is automatically rejected without consulting the owning process if
the foreign socket is not open (inactive or connected). Multiple
RFCs to the same socket are placed into its event queue in order
of receipt. Any queued RFCs are automatically rejected by the NCP
once the owning process decides to accept a connection. The NCP
which has control of the "receive" socket of the potentially
connected pair designates a link number over which messages are to
flow.
b. Close a Connection
CLS <local socket> <foreign socket>
An NCP issues this network command to disconnect an existing
connection or to negatively acknowledge an RFC. There is a
potential race problem if an NCP closes a local send socket in
that the CLS command may reach the foreign NCP prior to the last
message over that socket connection. This race is prevented by
adhering to two standards: (i) A CLS command for a local send
socket is not transmitted until the RFNM for the last message to
the foreign socket comes back, and (ii) the foreign NCP processes
all incoming messages in the order received.
[Page 7]RFC 46 ARPA Network Protocol Notes April 1970
c. Block Output over a Connection
BLK <foreign send socket>
A process may read data through a receive socket slower than
messages are coming in and thus the NCP's buffers may tend to clog
up. The NCP issues this command to a foreign NCP to block further
transmission over the socket pair until the receiving process
catches up.
d. Resume Output over a Blocked Connection
RSM <foreign send socket>
An NCP issues this command to unblock a previously blocked
connection.
e. Interrupt the Process Attached to a Connection
INT <foreign socket>
Receipt of this message causes the foreign NCP to immediately
interrupt the foreign process attached to <foreign socket> if it
is connected to a local socket. Data already in transit within
the NCP network over the interrupted connection will be
transmitted to the destination socket. The meaning of "interrupt"
is that the process will immediately break off its current
execution and execute some standard procedure. That procedure is
not defined at this protocol level.
f. Report an Erroneous Command to a Foreign NCP
ERR <code> <command length> <command in error>
This command is used to report spurious network commands or
messages, or overload conditions that prevent processing of the
command. <code> specifies the error type. If <code> specifies an
erroneous network command, <command in error> is that command (not
including IMP header) and <command length> is an integer
specifying its length in bits. If <code> specifies an erroneous
message, <command in error> contains only the link number over
which the erroneous message was transmitted. (This is slightly
modified from the specification in NWG/RFC 40.)
[Page 8]RFC 46 ARPA Network Protocol Notes April 1970
g. Network Test Command
ECO <48 bit code> <echo switch>
An NCP may test the quality of communications between it and a
foreign NCP by directing to it an ECO command with an arbitrary
<48 bit code> (of the same length as a socket identifier) and
<echo switch> 'on'. An NCP receiving such a ECO command should
immediately send an acknowledging ECO with the same <48 bit code>
and <echo switch> 'off' to the originating NCP. An NCP does not
acknowledge an ECO with <echo switch> 'off'. We feel that this
command will be of considerable aid in the initial shakedown of
the entire network.
h. No Operation Command
NOP
An NCP discards this command upon receipt.
User Interface to the NCP
The NCP at each HOST has an interface through which a local process
can exercise the network, subject to the control of the NCP. The
exact specification of this interface is not a network protocol
issue, since each installation will have its own interface keyed to
its particular requirements. The protocol requirements for the user
interface to an NCP are that it provide all intended network
functions and no illegal privileges. Examples of such illegal
privileges include the ability to masquerade as another process,
eavesdrop on communications not intended for it, or to induce the NCP
to send out spurious network commands or messages.
We outline here an interface based on the Carr, Crocker, and Cerf
proposal that is sufficient to fully utilize the network. While this
particular set of calls is intended mainly for illustrative purposes,
it indicates the types of functions necessary.
The following calls to the NCP are available:
a. LISTEN <my 8 bit socket code>
A user opens this socket, creating an empty event queue for it.
This LISTEN call may block waiting for the first "request" event,
or it may return immediately.
[Page 9]RFC 46 ARPA Network Protocol Notes April 1970
b. INIT <my socket code> <foreign socket>
A user attempts to connect <my socket> to <foreign socket>. The
local NCP sends an RFC to the foreign NCP requesting that the
connection be created. The returned acknowledgemnet is either an
RFC (request accepted) or CLS (request rejected). At the caller's
option, the INIT call blocks on the expected "accept" or "reject"
event, or it can return immediately without waiting. In this case
the user must call STATUS (see below) at a later time to determine
the action by the foreign NCP. When a blocked INIT call returns,
the "accept" or "reject" event is removed from the event queue.
c. STATUS <my socket code>
This call reports out the earliest previously unreported event in
the queue of <my socket>. The STATUS call deletes the event from
the queue if that type of event is deleteable by disclosure.
d. ACCEPT <my socket code>
The user accepts connection with the foreign socket whose
"request" event is earliest in the event queue for <my socket>.
An acknowledging RFC is sent to the accepted foreign socket, and
the "request" event is deleted from the event queue. Should any
other "request" event exist in the queue, the NCP automatically
denies connection by sending out a CLS command and deleting the
event.
e. REJECT <my socket code>
The user rejects connection with the foreign socket whose
"request" event is earliest in the event queue for <my socket>.
The NCP sends out a CLS command and deletes the "request" event
from the queue.
f. CLOSE <my socket code>
The user directs the NCP to disconnect any active connection to
this socket and to deactivate the socket. The NCP sends out a CLS
command to the foreign socket if a connection has existed. The
status of the foreign socket also becomes closed once the "close"
event is disclosed to the foreign process.
g. INTERRUPT <my socket code>
The user directs the NCP to send out an INT command to the foreign
socket connected to <my socket>.
[Page 10]RFC 46 ARPA Network Protocol Notes April 1970
h. TRANSMIT <my socket code> <pointer> <nbits>
The user wishes to read (<my socket> is receive) or write (<my
socket> is send) <nbits> of data into or out of an area pointed to
by <pointer>. A call to write returns immediately after the NCP
has queued the data to send a message over the connection. The
call to write blocks only if the connection is blocked or if the
local NCP is too loaded to process the request immediately. Data
to be transmitted over a connection is formatted into one or more
IMP messages of maximum length 8095 bits and transmitted to the
foreign HOST over the link number specified in the RFC sent by the
NCP controlling the receiving connection. A "close" event in the
event queue for <my socket> is disclosed through the action of
TRANSMIT. A call to write discloses the "close" event
immediately. A read call discloses it when all data has been
read.
The History of a Connection From a User View
An Illustrative Example
Assume that process 'a' on HOST A wishes to establish connection with
process 'b' on HOST B. Before communication can take place, two
conditions must be fulfilled:
a. process 'a' must be able to specify to its NCP a socket in 'b's
socket space to which it wants to connect.
b. process 'b' must already be LISTENing to this socket.
1. Establishing the Connection
a. process 'b' LISTENs to socket 'Bb9'.
b. process 'a' INITs 'Bb9' to its 'Aa12'. The NCP at A generates
an RFC specifying link number = 47, which it chooses from its
available set of links. This is the link over which it will
receive messages if the connection is ACCEPTed by process 'b'.
c. process 'b' is informed of A's INIT request. He may REJECT
connection (NCP B sends back a CLS) or ACCEPT (NCP B sends back an
RFC).
d. If process 'b' ACCEPTs, the confirming RFC establishes the
connection, and messages can now flow.
[Page 11]RFC 46 ARPA Network Protocol Notes April 1970
HOST A | HOST B
INITIATOR | ACCEPTOR
PROCESS 'a' | PROCESS 'b'
|
|
| a. LISTEN 'socket code 9'
|
|
b. INIT 'socket code 12' 'Bb9' |
RFC 'AA12' 'Bb9' 'link 47' ==========>
|
| c. ACCEPT 'socket code 9'
| RFC 'Bb9' 'Aa12'
|
| d. TRANSMIT 'send buffer' 'len'
| 'socket 9'
<============== IMP message 'link 47' 'send buffer'
|
e. TRANSMIT 'rec buffer' 'length'
'socket 12' ============>
|
| f. CLOSE 'socket code 9'
|
last RFNM ===>
<============== CLS 'Bb9' 'Aa12'
closes socket 'Aa12' |
|
FIG 2. Establishing and Communicating over a Socket Connection
2. Sending Messages over a Connection.
a. Process 'b' issues a TRANSMIT call to send data through the
connection. NCP B formats this into an IMP message and sends it
to NCP A with link number = 47 as specified by A's RFC.
b. NCP A receives the raw message from NCP B with link number =
47. NCP A uses this link number in deciding who the intended
recipient is, and stores the message in a buffer for the recipient
process.
c. Process 'a' may issue a read (TRANSMIT) call for socket code 12
at any arbitrary time. The read call blocks if there is no data
pending for the socket. The read call picks up the specified
number of bits transmitted over socket code 12, perhaps across an
IMP message boundary. The boundaries of the IMP messages are
invisible to the read call.
[Page 12]RFC 46 ARPA Network Protocol Notes April 1970
d. Should process 'b' send data over the connection at a faster
rate than process 'a' picks it up, NCP A can issue a BLK command
to NCP B if A's buffers start filling. Later, when process 'a'
catches up NCP A can tell B to resume transmission via an RSM
command.
3. Process 'b' Closes the Connection
a. Process 'b' decides to close the connection, and it issues the
CLOSE call to NCP B. To avoid race problems B waits for the RFNM
from the previous message over this connection, then sends the CLS
command to NCP A. When the RFNM from the CLS command message
returns, NCP B flushes socket 'Bb9' from its tables, effecting the
close at its end and deactivating 'Bb9'.
b. Because of sequential processing within NCP A, the last message
to socket 'Aa12' is guaranteed to have been directed to a process
before the CLS from NCP B comes through. Upon receipt of the CLS
from B, NCP A marks socket 'Aa12' as "close pending" and places a
"close" event into the event queue of 'Aa12'.
c. Process 'a' can still issue read calls for socket 'Aa12' while
there is buffered data pending. When 'a' issues a read call after
the buffer has been emptied, the "close" event is disclosed to
inform 'a' of the closure, and socket 'Aa12' is flushed from the
tables of NCP A.
4. Process 'a' Closes the Connection
a. Let us return to step 2. and assume that process 'a' wants to
close the connection from its end. There is no race problem
because we assume that once 'a' issues a CLOSE call, it no longer
wants to read messages over that socket.
b. Assume that process 'a' issues a CLOSE call on socket 'Aa12'.
NCP A immediately sends out a CLS command to NCP B and marks
socket 'Aa12' as "close pending". Any data buffered for read on
'Aa12' is discarded. To allow remaining messages already in
transit from process 'b' to percolate through the IMP network to
NCP A and be discarded without error comments, NCP A retains
'Aa12' in its tables for a suitable period of time after receiving
the RFNM from the CLS command. During this period NCP A discards
all messages received over the closing connection. After allowing
a reasonable amount of time for these dead messages to come in,
NCP A flushes 'Aa12' from its tables, effectively closing the
connection and deactivating 'Aa12'. Further messages to socket
'Aa12' result in NCP A sending an ERR "erroneous command" to the
originating NCP.
[Page 13]RFC 46 ARPA Network Protocol Notes April 1970
c. When NCP B receives the CLS command, socket 'Bb9' is marked as
"close pending", and the CLS event is placed into the event queue
of 'Bb9'. The next time process 'b' wishes to write over that
socket, the CLS event is disclosed to inform him of the closure,
and socket 'Bb9' is removed from NCP B's tables.
IV - USER CONTROL AND COMMUNICATION PROTOCOL
Some process must exist which agrees to listen to anybody in the
network and create a process for him upon proper identification.
This process is called the logger and interacts through the NCP via
the network-related User Control and Communication (UCC) module,
which implements the necessary protocol. Except for one instance
(CLOSEing connections of dead processes), the process operating the
UCC module does not have special network privileges.
Under the UCC protocol a "requestor" process which has directed the
creation of a "foreign" process maintains two full-duplex pseudo-
typewriter connections: one to the foreign logger, and one to the
created process. The duplex connection to the foreign logger is used
to identify the requestor process to the logger, and after login to
return to the requestor process basic information concerning the
health of the created process. The duplex connection to the created
process is used for control communication to it.
Maintaining two full-duplex connections avoids reconnection problems
both when the logger transfers communication to the created process
and when it needs to regain control. This is at the modest expense
of requiring the requestor process to switch typewriter
communications between two sets of connections.
The way that communication is established is essentially as follows:
the requestor process first reserves four of its sockets having
contiguous socket codes. Then it "signals" the UCC, specifying one
of these sockets. From the "signal" the UCC knows which process is
calling, and by protocol, on which requestor socket pair the UCC is
to communicate with the requestor process, and which requestor socket
pair the created process is to use for its communications. This is
specified below in more detail.
Establishing and Operating a Remote Process
The UCC at each HOST always keeps a send socket with user number = 0,
instance tag = 0 open (active and unconnected) as a "signal" socket,
and periodically checks for INITs to this socket. Processes wishing
to create a process at this HOST must first signal the UCC by issuing
an INIT to this socket.
[Page 14]RFC 46 ARPA Network Protocol Notes April 1970
The requesting process must have four free sockets with contiguous
socket codes: <base_socket> (receive) through <base_socket+3>
(send). The high numbered send/receive set of sockets is used for
typewriter communication with the foreign UCC, the low numbered set
for typewriter communication with the created process.
1. The "requestor" process calls LISTEN twice to open the
<base_socket+2> and <base_socket+3> receive/send pair over which it
will talk to the foreign UCC. Then it sends out a "signalling" INIT
call on <base_socket> to the UCC "signal" socket. The only thing
that the UCC does with this "signalling" INIT call is to note down
the socket number <base_socket> from which it originated. The UCC
immediately rejects this request so as to keep its "signal" socket
open for other signals.
2. After receiving the expected REJECT on its initial INIT call to
the UCC's signal socket, the requestor process issues LISTENs for
<base_socket> and <base_socket+1>. (The created process will INIT
these sockets to establish control communication with the requestor
process.) The requestor process then blocks by calling STATUS
<base_socket+2> .
3. The UCC INITs a free send/receive socket pair to the requestor's
<base_socket+2> and <base_socket+3> on which the requestor process is
presumably LISTENing. The requestor process has called STATUS
<base_socket+2> with block option after LISTENing for the two
sockets, so that when the INIT from the foreign UCC reaches the
requestor process, STATUS returns with the INIT indication. The
requestor process verifies that the UCC is the process that is
calling, then it ACCEPTs the call. The requestor process then calls
STATUS <base_socket+3> and returns when the INIT for that socket
reaches it. It does a similar verify and ACCEPT. (There is an
arbitrary choice as for which socket the requestor process first
calls STATUS.) Two way communication is established when the
requestor process has ACCEPTed both INITs from the UCC. This
connection is maintained during the login ritual and throughout the
life of the created process. Should the requestor process fail to
respond properly within a limited amount of time to the INITs of the
UCC, the UCC abandons the connection attempt.
4. The requestor process must then perform the login ritual with the
UCC. (The initial protocol might standardize the login ritual.) If
the logger is not satisfied and wishes to cut off the requestor, the
UCC module CLOSEs both <base_socket+2> and <base_socket+3>, perhaps
after the logger has sent a suitable message.
[Page 15]RFC 46 ARPA Network Protocol Notes April 1970
5. If satisfied, the logger creates a process for the user. The UCC
maintains direct communication with the requestor, but this
connection is now used only to report basic information concerning
the created process.
6. The first task of a created process is to establish a dual
pseudo-typewriter control connection with its requestor process. The
created process INITs one of its send/receive socket pairs to the
requestor's <base_socket> and <base_socket+1>. If both requests are
ACCEPTed, the created process sends an initial message over this
connection. Then it goes to command level, in which it awaits a
typewriter command message over the connection. If the created
process is unable to establish duplex communication with the
requestor process, it should destroy itself. The UCC will either
CLOSE its own connections with the requestor or make arrangements for
another process to be created.
7. When a created process is logged-out, the UCC uses a privileged
entry to the NCP to CLOSE all connections between the dead process
and other processes, and to deactivate all open sockets of the dead
process. The UCC transmits a message back to the requestor process,
then CLOSEs the dual connections between it and the requestor
process.
8. The INTERRUPT call has a standard "quit" meaning when sent from a
requestor process to a created process over the requestor's receive
socket <base_socket>. All pending output from the created process is
aborted, and the it enters "command level" where it awaits a command
over the typewriter connection to the requestor process. The
interrupted processing is resumable by issuing a "start" command to
the created process. (Note that the rule about pending output is
more restrictive than that implemented by the INT NCP command.)
This document was prepared through the use of the MULTICS "runoff"
command. A source file consisting of intermixed text and "runoff"
requests was created using the "qed" text editor. This file was
then compiled by the "runoff" command to produce a finished copy.
The latest version of this document exists online in MULTICS in
the segment
>udd>Multics>Meyer>network_protocol.runoff
(END)
[Page 16]RFC 46 ARPA Network Protocol Notes April 1970
REQUESTOR FOREIGN
PROCESS LOGGER
-------------- -------------
a. LISTEN to sockets
<base_socket+2> and
<base_socket+3> to be
connected to foreign logger.
b. INIT <base_socket>
to "signal" socket of
foreign logger.
=======================================>
c. remember <base_socket>
and REJECT connection
to signal socket.
d. LISTEN to sockets e. INIT a logger socket
<base_socket> and pair to the requestor's
<base_socket_1> to be <base_socket+2> and
connected to the created process. <base_socket+3>.
/
<==========================/
f. ACCEPT connection
with sockets from
foreign logger.
PERFORM LOGIN RITUAL
CREATED
PROCESS
-------------
g. INIT any socket pair
to requestor's
<base_socket> and
<base_socket+1>
/
<===========================/
h. ACCEPT connection
with sockets from created
process.
FIG. 4 Establishing a Process at a Foreign HOST
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Miles McCredie 11/99 ]
[Page 17]
