II. The Protocol
The notion of a connection as explained in NWG/RFC #33 pervades the protocol. A connection is a simplex communication path, intended to be between two processes.
The primary function of the protocol is to provide for (1) establishment of connections, (2) regulation of flow over connections, and (3) termination of connections.
In addition, the protocol provides some ancillary functions such as sending simulated interrupt pulses and echoing test messages.
To provide a path for exchanging information about connections, we designate specific links, i.e. link one between each pair of hosts to be control links. Traffic on control links consists only of control commands, defined below.
Connections are named by a pair of sockets. Sockets are 40 bit names which are known throughout the network. Each host is assigned a private subset of these names, and a command which requests a connection names one socket which is local to the requesting host and one local to the receiver of the request.
Sockets are polarized; even numbered sockets are receive sockets; odd numbered ones are send sockets. One of each is required to make a connection.
To facilitate transmission of information over a connection, a unique link is assigned to each connection. One of the steps in establishing a connection, therefore, is the assignment of a link. Of the non-control links, zero is reserved for intra-network use, and links 32 to 255 are reserved for experiment and expansion. Thus only links 2 through 31 are available for regular use. Link assignment must either always be done by the receiver or always by the sender. We have (almost) arbitrarily chosen this to be the receiver's responsibility.
All regular messages consist of a 32 bit leader, marking, text, and padding. Marking is a (possibly null) sequence of zeroes followed by a 1; padding is a 1 followed by a (possibly null) sequence of zeroes.
A regular message sent over the control link (link 1) is called a control message. Its text is an integral (possibly zero) number of control commands in the form described below, and this text must end on a command boundary.
The commands used to establish a connection are STR and RTS. The STR command is sent from a prospective sender to a prospective receiver. Its <my socket> field contains a send socket local to the prospective sender; its <your socket> field contains a receive socket local to the prospective receiver. The RTS command is the dual, but is also contains a <link> field for link assignment. These two commands are referred to as requests-for-connection (RFC). A STR and an RTS match if the <my socket> field of one is identical to the <your socket> field of the other and vice versa. A connection is established where a matching pair of RFC's have been exchanged.
Hosts are prohibited from establishing more than one connection to any local socket. Therefore, a host may not use a socket for the <my socket> field of an RFC if that socket is mentioned in a previous RFC and the connection is not yet terminated.
The command used to terminate a connection is CLS. Each side must send and receive a CLS command before a connection is completely terminated and the sockets are free to participate in other connections. It is not necessary that both RFC's be exchanged before a connection is terminated. More details on termination are given below.
After a connection is established, the receiving host sends a ALL command which allocates space for the connection. The sender keeps track of how much space is available in the receiving host and does not transmit more text than the receiving host can accept, as explained above. A sender is also constrained by the local IMP from sending a message over a connection until the RFNM from the previous message is received.
After a connection is established, CLS commands sent by the receiver and sender have slightly different effects. CLS command sent by the sender indicate that no more messages will be sent over the connection. This command must not be sent if there is a message in transit over the connection.
CLS commands sent by the receiver act as demands on the sender to terminate transmission. However, since there is a delay in getting the CLS command to the sender, the receiver must expect its buffers to fill to the limit provided in ALL commands.
While a connection is established, either side may send INR or INS commands. The interpretation of these commands is a local matter, but in general they will provide and escape function.
Note that the ALL, INR and INS commands may be sent only after the connection is established and before a CLS command is sent.
A very simple test facility is provided by the ECO and ERP commands. Upon receiving a ECO command, a host must change the first eight bits to ERP and return it. These commands have no relationship to connections.
A NOP command is included for convenience. It is coded as zero to facilitate command message construction.
Finally, an ERR command is included for notifying a foreign host it has (apparently) made an error. At present, no specific list of errors is defined, and no action is defined for the receipt of ERR commands. Hosts should log ERR commands upon receipt so that system programmers can diagnose the trouble. A host may generate an ERR command at any time and for any reason, but it is advised that each host publish an exhaustive list of the ERR commands it may sent and their interpretations.
Network Control Commands
The following is a detailed description of the structure and format of each of the control commands.
To facilitate and clarify socket descriptions, the following conventions have been adopted:
<my socket> and <your socket> are used in the command descriptions.
<my socket> is local to the originator of the command.
<your socket> is local to the receiver of the command.
Control Command Formats
No Operation
_______
| |
| NOP |
|_______|
Request Connection, Receiver to Sender
______________________________________________
| | | | |
| RTS | my socket | your socket | link |
|_______|_____________|_______________|________|
Request Connection, Sender to Receiver
_____________________________________
| | | |
| STR | my socket | your socket |
|_______|_____________|_______________|
Close
_____________________________________
| | | |
| CLS | my socket | your socket |
|_______|_____________|_______________|
Allocate
__________________________
| | | |
| ALL | link | space |
|_______|________|_________|
Interrupt Sent by Receiving Process
_______________
| | |
| INR | link |
|______|________|
Interrupt Sent by Sending Process
_______________
| | |
| INS | link |
|______|________|
Echo Request
____________________________ _________
| | \ \ |
| ECO | length / / text |
|_______|____________________\ \________|
Echo Reply
____________________________ _________
| | \ \ |
| ERP | length / / text |
|_______|____________________\ \________|
Error Detected
____________________________ _________
| | \ \ |
| ERR | length / / text |
|_______|____________________\ \________|
The host is specified in the leader.
<link> is 8 bits
<space> is 32 bits long and is an unsigned integer.
<length> is an unsigned 16 bit integer.
<text> is as long as the length. The command is therefore 24 bits longer that the length. Maximum length is one message, to facilitate command decoding and manipulation.
All control command codes are 8 bit long:
NOP = 0
RTS = 1
STR = 2
CLS = 3
ALL = 4
INR = 5
INS = 6
ECO = 7
ERP = 8
ERR = 9
<my socket> and <your socket> are 32 bits long,
_______________________
| | |
| User number | AEN |
|_______________|_______|
24 bits for user number and 8 bits for AEN.