Username / Password : Request For Comments

RFC Number : 55

Title : Prototypical implementation of the NCP.

Network Working Group J. Newkirk
Request for Comments: 55 M. Kraley
J. Postel
S. Crocker
19 June 1970

A Prototypical Implementation of the NCP

While involved in attempting to specify the formal protocol, we also
attempted to formulate a prototypical NCP in an Algol-like language.
After some weeks of concentrated effort, the project was abandoned as
we realized that the code was becoming unreadable. We still,
however, felt the need to demonstrate our conception of how an NCP
might be implemented; we felt that this would help suggest solutions
for problems that might arise in trying to mold the formal
specifications into an existing system. This document is that
attempt to specify in a prose format what an NCP could look like.

There are obvious limitations on a project of this nature. We do
not, and cannot, know all of the quirks of the various systems that
must write an NCP. We are forced to make some assumptions about the
environment, system calls, and the like. We have tried to be as
general as possible, but no doubt many sites will have completely
different ways of conceptualizing the NCP. There is great difficulty
involved in conveying our concepts and the mechanisms that deal with
these concepts to people who have wholly different ways of looking at
things. We have, however, benefited greatly by trying to actually
code this program for our fictitious machine. Many unforeseen
problems surfaced during the coding, and we hope that by issuing this
document we can help to alleviate similar problems which may arise in
individual cases.

There is, of course, absolutely no requirement to implement anything
which is contained in this document. The only rigid rules which an
NCP _must_ conform to are stated in NWG/RFC#54. This description is
intended only as an example, _not_ as a model.

In the discussion which follows we first describe the environment to
be assumed and postulate a set of system calls. We discuss the
overall architecture of the NCP and the tables that will be used to
hold relevant information. Narratives of network operations follow.
A state diagram is then presented as a convenient method for
conceptualizing the cause-effect sequencing of events. The detailed
processing of each type of network event (system calls or incoming
network messages) is then discussed.

Newkirk, et al. [Page 1]

RFC 55 Prototypical Implementation of NCP June 1970

II. Environment

We assume that the host will have a time-sharing operating system in
which the CPU is shared by processes.

We envision that each process is tagged with a user number. There
may be more than one process with the same user number; if so, they
should all be cooperating with respect to using the network.

We envision that each process contains a set of ports which are
unique to the process. These ports are used for input to or output
from the process, from or to files, devices, or other processes.

We also envision that a process is not put to sleep (i.e., blocked or
dismissed) when it attempts to LISTEN or CONNECT. Instead it is
informed when some action is complete. Of course, a process may
dismiss itself so that it wakes up only on some external event.

To engage in network activity, a process attaches a local socket to
one of its ports. Sockets are identified by user number, host and
AEN; a socket is local to a process if the user numbers of the two
match and they are in the same host. Thus, a process need only
specify an AEN when it is referring to a local socket.

Each port has a status which is modified by system calls and
concurrent events outside the process (e.g., a 'close connection'
command from a foreign host). The process may look at a port's
status as any time (via the STATUS system call).

We assume a one-to-one correspondence between ports and sockets.

III. System Calls

These are typical system calls which a user process might execute.

We use the notation


SYSCALL is the name of the system call
ARGk, etc. are the parameters of the system call.

Newkirk, et al. [Page 2]

RFC 55 Prototypical Implementation of NCP June 1970


P specifies a port of the process
AEN specifies a local socket; the user number and host are
FS specifies a socket with any user number in any hose,
and with any AEN
CR the condition code returned

CONNECT attempts to attach the local socket specified by AEN to
the port P and to initiate a connection with a specific foreign
socket, FS. Possible values of CR are:

CR=OK The CONNECT was legal and the socket FS is being
contacted. When the connection is established
or refused the status will be updated.

CR = BUSY The local socket is in use (illegal command

CR = BADSKT The socket specification was illegal.

CR = NOROOM Local host's resources are exhausted.

CR = HOMOSEX Incorrect send/receive pair

CR = IMP DEAD Our imp has died

CR = LINK DEAD The link to the foreign host is dead because:
1. the foreign Imp is dead,
2. the foreign host is dead, or
3. the foreign NCP does not respond.


P specifies a port of the process
AEN specifies a local socket
CR the condition code returned

The local socket specified by AEN is attached to port P. If there
is a pending call, it is processed; otherwise, no action is taken.
When a call comes in, the user will be notified. After examining
the call, he may either accept or refuse it. Possible values of
CR are:

CR = OK Connection begun, listening


Newkirk, et al. [Page 3]

RFC 55 Prototypical Implementation of NCP June 1970





P specifies a port of the process
CR the condition code returned

Accept implies that the user process has inspected the foreign
socket to determine who is calling and will accept the call.
(Note: an interesting alternative defines ACCEPT as the implicit
default condition. Thus any incoming RFC automatically satisfies
a LISTEN.) Possible values of CR are:





CR = BADCOMM Illegal command sequence. (E.g., Accept issued
before a LISTEN.

CR = PREMCLS Foreign user aborted connection after RFC was
locally received but before Accept was executed.


P specifies a port of the process
BUFF specifies the text buffer for transmission
BITSRQST specifies the length to be transmitted in bits
BITSACC returns the number of bits actually transmitted
CR the condition code returned

Transmission takes place. Possible values for CR are:




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RFC 55 Prototypical Implementation of NCP June 1970

CR = NOT OPEN Connection is not open (illegal command

CR = BAD BOUND BITSRQST out of bounds (e.g., for a receive
socket BUFF was shorter than BITSRQST


P specifies the local socket of this process
CR the condition code returned

The process on the other (foreign) side of this port is to be
interrupted. Possible values of CR are:







P specifies a port of this process
RTAB the returned rendezvous table entry
CR the condition code returned

The relevant fields of the rendezvous table entry associated with
this port are returned in RTAB. This is the mechanism a user
process employs for monitoring the state of a connection.
Possible values of CR are:



Newkirk, et al. [Page 5]

RFC 55 Prototypical Implementation of NCP June 1970


P specifies a port of this process
CR the condition code returned

Activity on the connection attached to this port stops, the
connection is broken and the port becomes free for other use.
Possible values of CR are:






IV. The NCP - Gross Structure

We view the NCP as having five component programs, several
associative tables, and some queues and buffers.

The Component Programs (see Fig. 4.1)

1. The Input Handler

This is an interrupt-driven routine. It initiates Imp-to-Host
transmission into a resident buffer and wakes up the input
interpreter when transmission is complete.

2. The Output Handler

This is an interrupt-driven output routine. It initiates Host-
to-Imp transmission out of a resident buffer and wakes up the
output scheduler when transmission is complete.

3. The Input Interpreter

This program decides whether the input is a regular message
intended for a user, a network control message, an Imp-to Host
message, or an error. For each class of message this program
invokes a subroutine to take the appropriate action.

Newkirk, et al. [Page 6]

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4. The Output Scheduler

Three classes of messages are sent to the Imp

(a) Host-to-Imp messages
(b) Control messages
(c) Regular messages

We believe that a priority should be imposed among these
classes. The priority we suggest is the ordering above. The
output scheduler selects the highest priority message and
passes it to the output handler.

Host-to-Imp messages are processed first come first served.
Control messages are processed individually by host, each host
being taken in turn. A control message queue for each foreign
host is provided. When any particular host is scheduled for
output, as many control commands for that host as will fit are
concatenated into a single message. Regular messages are
processed in groups by host and link, each unique combination
being taken in turn.

5. The System Call Interpreter

This program interprets requests from the user. Each system
call has a corresponding routine which takes the appropriate

The two interesting components are the input interpreter and the
system call interpreter. These are similar in that the input
interpreter services foreign requests and the system call
interpreter services local requests.

The diagram in Figure 4.1 is our conception of the Network
Control Program. Squishy amoeba-like objects represent component
programs, cylinders represent queues, and the arrows represent
data paths. In this simplified diagram tables are not shown.
['Amoeba-like' objects in original hand drawing are now firm
rectangular boxes: Ed.]

The abbreviated labels in the figure have the following meanings:

HIQ - Host-to-Imp Queue
OCCQ - Output Control Command Queue
DQ - Data Queue
IHBUF - Input Handler Buffer
OHBUF - Output Handler Buffer

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RFC 55 Prototypical Implementation of NCP June 1970

^ | Fig. 4.1
| |
| System |
| Call |
| Interpreter |
|_________________| _____________
^ | | | |
| | | +---------------| Input |
| | | | +-----| Interpreter |
| | | | | | |
| V V V V -------------
|======| |=========| |=======| | ^
| D Q | | O C C Q | | H I Q | | |
|======| |=========| |=======| | |
| ^ | | | |
| | | | | |
| +--------)----------)---------+ |
| | | |
+-------+ | +------+ |
__V___V___V__ |
| | |
| Output | |
| Scheduler | |
|_____________| |
| |
V |
(===========) (===========)
( O H B U F ) ( I H B U F )
(===========) (===========)
| ^
______V______ ______|______
| | | |
| Output | | Input |
| Handler | | Handler |
| | | |
------------- -------------
| ^
| |
+----------+ +-----------+
| |
| |
| I M P |

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RFC 55 Prototypical Implementation of NCP June 1970

V. Tables in the NCP

We envision that the bulk of the NCP's data base is in associative
tables. By 'associative' we mean that there is some lookup routine
which is presented with a key and either returns successfully with a
pointer to the corresponding entry, or fails if no entry corresponds
to the key. The major tables are as follows:

1. The Rendezvous Table

This table holds the attributes of a connection. The table is
accessed by the local socket, but other tables may have
pointers to existing entries.

The components of an entry are:

(a) Local Socket
(b) Foreign Socket
(c) Link
(d) Connection State
(e) Flow State
(f) Data Queue
(g) Call Queue
(h) Port Pointer
(i) Their Buffer Size (only needed on the send side)
(j) Error State

An entry is created when either a CONNECT or a LISTEN system
call is executed or when a request for connection is received.
Various fields remain unused until after the connection is

2. The Input Link Table

The input interpreter uses the concatenation of the foreign
host and link as a key into the input table. The table is used
in processing a user-destined message on an incoming link by
providing a pointer into the rendezvous table.

3. The Output Link Table

The input interpreter uses the output link table to access the
flow state as RFNM's return from transmitted messages. The
output link table is keyed by host and link and provides a
pointer into the rendezvous table.

Newkirk, et al. [Page 9]

RFC 55 Prototypical Implementation of NCP June 1970

4. The Port Table

The system call interpreter uses the concatenation of the
process identification and the port identification as a key to
obtain a pointer into the rendezvous table.

5. The Output Control Command Table

The system call interpreter and the input interpreter use this
table to make entries in the appropriate output control command
queues. Commands are queued in separate table entries
corresponding to foreign hosts. Before output the contents of
the queue are concatenated into a large control message. The
components of an entry are:

(a) Host
(b) Output Control Command Queue

6. The Output Request Queue

This queue contains an entry for each connection which has data
requiring transmission to the net. There is only one entry per
connection, which is deleted when the last packet of data is
transmitted and is entered whenever a user makes a system
request for data transmission.

The entry is re-inserted if transmission is not completed
(message too long) or is prevented by the flow control
mechanism. The only component of an entry is a local socket.

7. The Host Live Table

This is a simple table listing the hosts which are alive. This
table is checked before establishing a connection and before
sending any data to ensure that the destination host actually
exists. At present the protocol does not define the procedure
to be followed for the Host up/Host down conditions. See

8. The Link Assignment Table

Link numbers are assigned by the receiver. This table records
which links are free and can, therefore, be assigned.

Newkirk, et al. [Page 10]

RFC 55 Prototypical Implementation of NCP June 1970

VI. Informal Description of Network Operations

We present here narratives describing the operation conducted during
the three major phases of network usage: opening, flow control, and

A. Opening

In order to establish a connection for data transmission, a pair
of RFC's must be exchanged. An RTS must go from the receive-side
to the send-side, and an STR must be issued by the send-side to
the receive-side. In addition, the receive-side, in its RTS, must
specify a link number. These RFC's (RFC is a generic term
encompassing RTS and STR) may be issued in any time sequence. A
provision must also be made for queuing pending calls (i.e., RFC's
which have not been dealt with by the user program). Thus, when a
user is finished with a connection, he may choose to examine the
next pending call from another process and decide to either accept
or refuse the request for connection. A problem develops because
the user may not choose to examine his pending calls; thus they
will merely serve to occupy queue space in the NCP. Several
alternative solutions to this problem will be mentioned later.

Utilizing the framework of the prototype system calls described
above, we envision at least four temporal sequences for obtaining
a successfully opened connection:

1. The user may issue a LISTEN, indicating he is willing to
consider connecting to anyone who sends him an RFC. When an
RFC comes in the user is notified. The user then decides
whether he wishes to connect to this socket and issues an
ACCEPT or a CLOSE on the basis of that decision. A CLOSE '
refuses' the connection, as discussed under 'Closing.' An
ACCEPT indicates he is willing to connect; an RFC is issued,
and the connection becomes fully opened.

2. Upon processing a user request for a LISTEN, the NCP
discovers that a pending call exists for that local socket.
The user is immediately notified, and he may ACCEPT or
CLOSE, as above.

3. The user issues a CONNECT, specifying a particular foreign
socket that he would like to connect to. An RFC is issued.
If the foreign process accepts the request, it answers by
returning an RFC. When this acknowledging RFC is received,
the connection is opened.

Newkirk, et al. [Page 11]

RFC 55 Prototypical Implementation of NCP June 1970

4. When presented with a CONNECT, the NCP may discover that a
pending call exists from the specified foreign socket to the
local socket in question. An acknowledging RFC is issued
and the connection is opened.

In all of the above cases the user is notified when the connection
is opened, but data flow cannot begin until buffer space is
allocated and an ALL command is transmitted.

Any of these connection scenarios will be interrupted if a CLS
comes in, as discussed under 'Closing.'

1. Pending Call Queues

It is essential that some form of queuing for pending RFC's
be implemented. A simple way to see this is to examine a
typical LISTEN-CONNECT sequence. One side issues a LISTEN,
the other a CONNECT. If the LISTEN is issued before the RFC
coming from the remote CONNECT arrives, all is fine.
However, due to the asynchronous nature of the net, we can
never guarantee that this sequence of events will occur. If
calls are not queued, and the RFC comes in before the LISTEN
is issued, it will be refused; if it arrives later, it will
be accepted. Thus we have an extremely ambiguous situation.

Unless one has infinite queue space, it is desirable that
some mechanism for purging the queues of old RFC's which the
user never bothered to examine. An obvious but informal
method is to note the time when each RFC is entered into the
queue, and then periodically refuse all RFC's which have
exceeded some arbitrary time limit. Another thought, which
probably should be included within the context of any
scheme, is for the NCP to send a CLS on all outstanding
connections or pending calls when a user logs out or blows

The scheme which is utilized in this description may seem at
first blush to be non-intuitive; but we feel it is more
realistic than other proposals. Basically, when a CONNECT
is issued, the NCP assumes that this socket wishes to talk
to the specified foreign socket and to that socket only. It
therefore purges from the pending call queue all non-
matching RFC's by sending back CLS's. Similarly, when the
connection is in the RFC-SEND state (a CONNECT has been
issued), all non-matching RFC's are refused. If a LISTEN-
ACCEPT or LISTEN- CLOSE sequence is executed, the remainder

Newkirk, et al. [Page 12]

RFC 55 Prototypical Implementation of NCP June 1970

of the pending calls are not removed from the queue, in the
expectation that the user may wish to accept these requests
in the future.

Although the latter method may seem to be arbitrary and/or
unnecessarily restrictive, we have not yet concocted a
scenario which would be prohibited by this method, assuming
that we are dealing with a competent programmer (i.e., one
who is wary of race conditions and the asynchronous nature
of the net). Of course whatever scheme or schemes a
particular site chooses is highly implementation dependent;
we suggest that some provision for the queuing of RFC's be
provided for a period of time at least of the order of
magnitude that they are retained in the CONNECT-clear scheme
mentioned above.

B. Flow Control

Meaningful data can only flow on a connection when it is fully
opened (i.e., two RFC's have been exchanged and closing has not
begun). We assume that the NCP's have a buffer for receiving
incoming data and that there is some meaningful quantity which
they can advertise (on a per connection basis) indicating the size
message they can handle. We further assume that the sending side
regulates its transmission according to the advertisements of that

When a connection is opened, a cell (called 'Their Size') is set
to zero. The receive-side will decide how much space it can
allocate and send an ALL message specifying that space. The
send-side will increment 'Their Size' by the allocated space and
will then be able to send messages of length less than or equal to
'Their Size' When messages are transmitted, the length of the
message is subtracted from 'Their Size'. When the receive-side
allocates more buffer space (e.g. when a message is taken by the
user, thus freeing some system buffer space), the number of bits
released is sent to the send-side via an ALL message.

Thus, 'Their Size' is never allowed to become negative and no
transmission can take place if 'Their Size' equals zero.

Notice that the lengths specified in ALL messages are increments
not the absolute size of the receiving buffer. This is
necessitated by the full duplex nature of the flow control
protocol. The length field of the ALL message can be 32 bits long
(note: this is an unsigned integer), thus providing the facility
for essentially an infinite 'bit sink', if that may ever be

Newkirk, et al. [Page 13]

RFC 55 Prototypical Implementation of NCP June 1970

C. Closing

Just as two RFC's are required to open a connection, two CLS's are
required to close a connection. Closing occurs under various
circumstances and serves several purposes. To simplify the
analysis of race conditions, we distinguish four cases: aborting,
refusing, termination by receiver, termination by sender.

A user 'aborts' a connection when he issues a CONNECT and then a
CLOSE before the CONNECT is acknowledged. Typically a user will
abort following an extended wait for the acknowledgment; his
system may also abort for him if he blows up.

A user 'refuses' a connection when he issues a LISTEN and, after
being notified of a prospective caller, issues a CLOSE. Any
requests for connection to a socket which is expecting a call from
a particular socket are also refused.

After a connection is established, either side may terminate. The
required sequence of events suggests that attempts to CLOSE by the
receive-side should be viewed as 'requests' which are always
honored as soon as possible by the send-side. Any data which has
not yet been passed to the user, or which continues over the
network, is discarded. Requests to CLOSE by the send-side are
honored as soon as all data transmission is complete.

1. Aborting

We may distinguish three cases:

a) In the simplest case, we send an RFC followed later by a
CLS. The other side responds with a CLS and the attempt
to connect ends.

b) The foreign process may accept the connection
concurrently with the local process aborting it. In this
case, the foreign process will believe the local process
is terminating an open connection.

c) The foreign process may refuse the connection
concurrently with the local process aborting it. In this
case, the foreign process will believe the local process
is acknowledging its refusal.

Newkirk, et al. [Page 14]

RFC 55 Prototypical Implementation of NCP June 1970

2. Refusing

After an RFC is received, the local host may respond with an
RFC or a CLS, or it may fail to respond. (The local host
may have already sent its own RFC, etc.) If the local host
sends a CLS, the local host is said to be 'refusing' the
request for connection.

We require that CLS commands be exchanged to close a
connection, so it is necessary for the local host to
maintain the rendezvous table entry until an acknowledging
CLS is returned.

3. Terminating by the Sender

When the user on the send side issues a CLOSE system call,
his NCP must accept it immediately, but may not send out a
CLS command until all the data in the local buffers has been
passed to the foreign host. It is thus necessary to test
for both 'buffer-empty' and
'RFNM-received' before sending the CLS command. As usual,
the CLS must be acknowledged before the entry may be

4. Terminating by the Receiver

When the user on the receive side issues a CLOSE system
call, his NCP accepts and sends the CLS command immediately.
Data may still arrive, however, and this data should be
discarded. The send side, upon receiving the CLS, should
immediately terminate the data flow.

VII. Connection Status

An excellent mechanism for describing the sequence of events required
to establish and terminate a connection involves a state diagram. We
may assume that each socket can be associated with a state machine,
and that this state machine may, at any time, be in one of ten
possible states. In any state, certain network events cause the
connection status to enter another state; other events are ignored;
still others are error. A transition may also involve the local NCP
performing some action. Figure 7.1 depicts the state machine.
Circles [now boxes: Ed] represent states (described below); arrows
show legal transitions between states. The labels on the arrows
identify the event which caused them (note that CLOSE is a system
call, CLS is a control command). Phrases after slashes denote the
action which should be performed while traveling over that arrow.
The arrow labeled '[E]RFC' (found between states 0 and 1) represents

Newkirk, et al. [Page 15]

RFC 55 Prototypical Implementation of NCP June 1970

the condition that whenever a connection enters the CLOSED state, the
pending call queue for that connection is checked [Original was
backwards 'E': Ed.]

If any pending calls exist in the queue, the connection moves to the
PENDING state. If an RFC is received for a socket in the CLOSED
state, it is also moved along this path to the PENDING state. Events
and the actions they cause are described in sections VIII and IX
below. Descriptions of the ten states follow:


The local socket is not attached to any port and no user has
requested a connection with it. (The table entry is non-


The socket is not attached to any port but one or more
requests for connection have been received. A LISTEN system
call will be satisfied immediately by the first entry in the
pending call queue for a matching request; all other pending
calls are deleted.


The socket is attached to a port. We are waiting for a user
to request connection with this socket.


We are listening and an RFC was received. The local user has
been informed of the pending call. He must respond with
either a CLOSE or an ACCEPT.


We have notified the user that his LISTEN has been satisfied
but he has not yet responded; if during this time the foreign
user aborts the connection by sending a CLS, we send a CLS to
acknowledge the abort and mark the fact with this state. When
the user accepts or refuses the call, we can inform him the
connection has been prematurely terminated.

Newkirk, et al. [Page 16]

RFC 55 Prototypical Implementation of NCP June 1970


This state is entered when:

a) The local user has attached this socket to a port by
issuing a CONNECT.
b) An RFC has been sent, and
c) No reply has been received.

When the user issues a CONNECT the pending call queue is

If a matching RFC is not found, the queue is deleted and this
state is entered. As new RFC's arrive they are compared with
our user's request. If they do not match, the RFC is
immediately refused. If the RFC matches, it completes the
initialization process and the connection enters the OPEN

(6) OPEN

RFC's have been exchanged and the connection is securely
established. Transmission may begin following receipt of an
ALL command from the receive side, and will then proceed
subject to flow control.


After the local user has executed a CLOSE, and we have issued
a CLS, we must wait for an acknowledging CLS before the
connection can be completely closed. If the appropriate CLS
has not already been received, this state is entered.


If we are on the send side and the local user executes a CLOSE
system call, a CLS cannot be issued if our data buffer is not
empty or if a RFNM for the last data message is outstanding.
The connection enters this state to wait for these conditions
to be fulfilled. Upon completion and acknowledgement of
output a CLS may be issued and the connection enters the CLS-
WAIT state, waiting for the acknowledging CLS. If a CLS
arrives while in the DATA-WAIT state we clear our buffer (the
CLS came from a receive socket, indicating it is no longer
interested in our data) and enter the RFNM-WAIT state to wait
for the network to clear.

Newkirk, et al. [Page 17]

RFC 55 Prototypical Implementation of NCP June 1970


If we are on the send side and a CLS command arrives, we
cannot issue an acknowledging CLS if we have not received the
RFNM for our last data message. We enter this state to await
the RFNM, and cease all further data transmission. When the
RFNM comes in, a CLS may then be issued, and the connection
will be closed.

Newkirk, et al. [Page 18]

RFC 55 Prototypical Implementation of NCP June 1970

CONN/ | CLOSED |<---------------------------+
send RFC | (0) | LISTEN |
+----------------| |-----------------------+ |
| |______________| | |
| | ^ | |
| [E]RFC | | CLS/send CLS | |
| ___V____|____ ___V____|____
| non-matching | | | |
| +-------------| (1) |----------+ +----| (2) |
| | |_____________| | | |_____________|
| | matching | | |
___V___V_____ CONN/send RFC| __V___V______
| | | ACCEPT/ | | CLS/
| RFC-SENT | RFC | send RFC | RFC-RECD | send CLS
| (5) |----------+ | +----------| (3) |---------+
|_____________| | | | |_____________| |
| | | | | | |
| | ___V___V___V___ SND&CLOSE | ____________ |
| | RCV&CLS/ | |-----------)->| | |
| | send CLS | OPEN | SND&CLS | | DATA-WAIT | |
| | +---------| (6) |--------+ | | (8) | |
| | | |_______________| | | |____________| |
| | | RCV&CLOSE/ | | | | |
| | | send CLS | | | | |
| | | | | | | CLS |
| | | ______V______ | | | |
| | | CLOSE/ | |CLOSE/ | | | |
| | | send CLS| CLS-WAIT |send CLS | | | |
| +---)--------->| (8) |<--------)--+ | |
| | |_____________| | | |
| | | ___V______V_ ______V___
| | | | | | |
| | | | RFNM-WAIT | | ABORT |
| | CLS | | (9) | | (4) |
| | | |____________| |__________|
| | | | |
| | ______V_______ RFNM/ | |
| | | | send CLS | |
| CLS/ +--------->| CLOSED |<----------+ |
| send CLS | (0) | ACCEPT|CLOSE |
+----------------->| |<----------------------------+

Figure 7.1
Connection State Diagram

Newkirk, et al. [Page 19]

RFC 55 Prototypical Implementation of NCP June 1970

VIII. Algorithms for the Input Interpreter

The following is a concise description of the NCP's responses to
incoming network commands. CS always indicates Connection State.
Note, CLOSE is a system call executed by the local user process, and
CLS is a network command.




If no entry exists, create one with status = PENDING CALL, and
queue the message.

If CS = LISTENING, then queue the entry, enter the RFC-RCVD state,
and inform the user of the request.

If CS = RFC-SENT but the new RFC does not match the request,
refuse the RFC.

In all other cases, check the RFC for a match. If none exists,
queue the RFC. If the RFC matches, then if:

CS = RFC-SENT, we enter the OPEN state.

CS = CLOSE-WAIT, the RFC is ignored.

otherwise, the request is illegal in all states which indicate
it has already been received (these states are 1,3,4,6,8,9).

In any case, if processing the RFC causes an overflow condition
(resources are exhausted), refuse the connection (send a CLS).


The pending call queue is searched. If the CLS doesn't match the
current request, but does match some other request, then delete
that request and issue a CLS. If there is no match, the CLS is

If the CLS matches the current request, and CS =

PENDING, then delete the current request. If the request queue
is empty, delete the entry; otherwise, leave the entry

Newkirk, et al. [Page 20]

RFC 55 Prototypical Implementation of NCP June 1970

RFC-RCVD, Issue a CLS and enter the ABORT state.
ABORT, ignore.

RFC-SENT, issue a CLS. If the pending call queue is empty
delete the entry, else enter the PENDING state.

OPEN, If we are on the receive side, response is identical to
the response for RFC-SENT. If we are on the send side,
clear the data queue, and if a RFNM is still pending enter
the RFNM-WAIT state. Otherwise response is identical to the
response for RFC-SENT.

CLS-WAIT, Issue a CLS and if the pending call queue is empty,
delete the entry, otherwise CS = PENDING.

DATA-WAIT, clear the data queue and enter the RFNM-WAIT state.
A matching CLS cannot occur in the CLOSED or LISTENING


Errors are queued for later attention by system programmers, and
are considered to be a system error in the host that originated
the exchange. (Not associated with any state).


The op code is changed to ERP and retransmitted (Not associated
with any state).


Upon receipt of an ERP, the system passes the text of the command
back to the process which issued the ECO.


These commands are enabled only in the OPEN state. Upon receiving
an INTERRUPT, the system causes an event to be sent to the
associated process. An INTERRUPT is ignored in the CLS-WAIT,
DATA-WAIT, and RFNM-WAIT states. In any other state it is an

Newkirk, et al. [Page 21]

RFC 55 Prototypical Implementation of NCP June 1970


ALLOCATE is valid only in the OPEN state, and may be sent only to
a send socket. The NCP increments the 'Their Size' field in the
associated rendezvous table entry by the size specified in the
ALLOCATE command.

In the CLS-WAIT and DATA-WAIT states this command is ignored; in
any other state it is an error.


If in the OPEN state, mark the Flow Control Status field in the
appropriate rendezvous table entry as RFNM-RECVD, and send more
data if required.

If in the DATA-WAIT state, maintenance the Flow Control Status.
If the data queue is empty issue a CLS and enter the CLS-WAIT
state; otherwise, transmit the next message.

If in the RFNM-WAIT state, maintenance the Flow Control Status and
issue a CLS. If the Pending Call queue is empty delete the
rendezvous table entry, otherwise CS = PENDING.

A Data-RFNM is an error in all other states.

IX. Algorithms for the System Call Interpreter

Each System Call is discussed, giving the state changes it may


If there is no entry, create one, issue an RFC, and enter the
RFC-SENT state.

If CS = PENDING, search the queue and reject all non-matching
requests. If no match is found issue an RFC and enter the
RFC-SENT state. If a match is found, issue an RFC and enter
the OPEN state. Transmission can commence as soon as buffer
space has been allocated.

In any other state this command is illegal.


If an entry doesn't exist, create one, and enter the LISTENING

Newkirk, et al. [Page 22]

RFC 55 Prototypical Implementation of NCP June 1970

If CS = PENDING, inform the user and enter the RFC-RCVD state.

In any other state this command is illegal.


If CS = RFC-RCVD, then issue an RFC and enter the OPEN state.
Data transmission can occur as soon as buffer space is

If CS = ABORT, inform the user of the premature termination of the
connection. If the pending call queue is empty, delete the
entry; otherwise, enter the PENDING state.

This command cannot be legally executed in any other state.


If CS =

LISTENING, then delete the entry.

RFC-RCVD, then issue a CLS and enter the CLS-WAIT state.

ABORT, inform the user of the premature termination of the
connection. If the pending call queue is empty, delete the
entry; otherwise, enter the PENDING state.

RFC-SENT, then issue a CLS and enter the CLS-WAIT state.

OPEN, if we are on the send side, and the data queue is not empty,
or if a Data-RFNM is still outstanding, enter the DATA-WAIT
state; otherwise, issue a CLS and enter the CLS-WAIT state.

CLS-WAIT, issuing a CLOSE in this state is a USER ERROR.

DATA-WAIT, issuing a CLOSE in this state is also an illegal

RFNM-WAIT, ignore the CLOSE.

A valid CLOSE cannot be issued if an entry does not exist, or if a
socket is in the PENDING state.

[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Anthony Anderberg 5/00 ]

Newkirk, et al. [Page 23]

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