System Calls and Control Commands

Here we sketch the mechanisms of establishing, switching and breaking a connection. As noted above, the NCP interacts with user processes via system calls and with other NCPs via control commands. We therefore begin with a partial description of system calls and control commands.

System calls will vary from one operating system to another, so the following description is only suggestive. We assume here that a process has several input-output paths which we will call ports. Each port may be connected to a sequential I/O device, and while connected, transmits information in only one direction. We further assume that the process is blocked (dismissed, slept) while transmission proceeds. The following is the list of system calls:

Init      <port>, <AEN 1>, <AEN 2>, <foreign socket>

where <port> is part of the process issuing the Init
               _
      <AEN 1>   |
and             +- are 8-bit AEN's (see Figure 2)
      <AEN 2>   |
               _|

The first AEN is used to initiate the connection; the second is used while the connection exists.

<foreign socket> is the 40-bit socket name of the distant end of the connection.

The lower-order bits of <AEN 1> and <AEN 2> must agree, and these must be the complement of the lower-order bit of <foreign socket>.

The NCP concatenates <AEN 1> and <AEN 2> each with the user number of the process and the HOST number to form 40-bit sockets. It then sends a Request for Connection (RFC) control command to the distant NCP. When the distant NCP responds positively, the connection is established and the process is unblocked. If the distant NCP responds negatively, the local NCP unblocks the requesting process, but informs it that the system call has failed.

Listen <port>, <AEN 1>

where <port> and <AEN 1> are as above. The NCP retains the ports and <AEN 1> and blocks the process. When an RFC control command arrives naming the local socket, the process is unblocked and notified that a foreign process is calling.

Accept <AEN 2>

After a Listen has been satisfied, the process may either refuse the call or accept it and switch it to another socket. To accept the call, the process issues the Accept system call. The NCP then sends back an RFC control command.

Close <port>

After establishing a connection, a process issues a Close to break the connection. The Close is also issued after a Listen to refuse a call.

Transmit <port>, <addr>

If <port> is attached to a send socket, <addr> points to a message to be sent. This message is preceded by its length in bits.

If <port> is attached to a receive socket, a message is stored at <addr>. The length of the message is stored first.

Control Commands

A vocabulary of control commands has been defined for communication between Network Control Programs. Each control command consists of an 8-bit operation code to indicate its function, followed by some parameters. The number and format of parameters is fixed for each operation code. A sequence of control commands destined for a particular HOST can be packed into a single control message.

RFC   <my socket 1>, <my socket 2>.
      <your socket>, (<link>)

This command is sent because a process has executed either an Init system call or an Accept system call. A link is assigned by the prospective receiver, so it is omitted if <my socket 1> is a send socket.

There is distinct advantage in using the same commands both to initiate a connection (Init) and to accept a call (Accept). If the responding command were different from the initiating command, then two processes could call each other and become blocked waiting for each other to respond. With this scheme, no deadlock occurs and it provides a more compact way to connect a set of processes.

CLS      <my socket>, <your socket>

The specified connection is terminated

CEASE    <link>

When the receiving process does not consume its input as fast as it arrives, the buffer space in the receiving HOST is used to queue the waiting messages. Since only limited space is generally available, the receiving HOST may need to inhibit the sending HOST from sending any more messages over the offending connection. When the sending HOST receives this command, it may block the process generating the messages.

RESUME   <link>

This command is also sent from the receiving HOST to the sending HOST and negates a previous CEASE.