Introduction to TCP/IP Interfaces for TSX

The term "interface" is sprinkled throughout TCP/IP. An interface usually corresponds to a device through which the sends and receives packets. Hence, if you have an ethernet card which connects to a router, you can think of there being an interface to the ethernet card. Whether the designers of TCP/IP meant for us to understand this as the interface between TCP/IP and the card, or as the interface between the card and the rest of the world, is unknown to me.

In any case, there is always an ip address associated with an interface. The ethernet card interface has one, and only one, ip address.

As far as TCP/IP is concerned, there are two types of interfaces supported by TSX: ethernet and serial. The serial interfaces use either SLIP (Serial Line Interface Protocol) or PPP (Point to Point Protocol).

As far as you are concerned, there are THREE types of interfaces supported by TSX:

• Ethernet cards. These are used on a TSX system to connect to the internet, through some gizmo like a router or an ethernet based ISDN gizmo or a FRAD gizmo. They are also used to interconnect multiple TSX systems together in a LAN. If you do both these things, the same ethernet card serves both purposes.
  
• PTYs, or Pseudo Terminals. TCP/IP believes these are just serial lines. For a serial connection there are two IP addresses: the IP address associated with the serial interface itself -- the local IP address -- and the IP address associated with the interface on the other end of that serial cable -- the remote IP address. The reason that the TCP/IP interface in TSX is a PTY instead of a physical serial connection is to allow the PTY server to do useful things related to "hooking up" the PTY with a real TTY. Bear in mind that TCP/IP has no idea this happens; it thinks that a PTY is a direct, hard-wired serial connection to some other computer which is also running TCP/IP networking. There are two reasons for PTYs to exist on a TSX-Online system. First, you might use a PTY to connect your system to your internet provider. In this case, the useful thing that the PTY server does is execute the connection script to call your provider, and worry about reconnecting if carrier drops. Second, PTYs are used when incoming callers want to go into passthrough SLIP or PPP mode. In this case, the useful thing that the PTY server does is find an available interface and hook it up to the caller's modem line. In both cases, the PTY server also worries about all that PPP protocol stuff.
  
• In the case that you are doing multiple domain hosting, you will use a third type of interface, similar to a PTY, called a virtual terminal. Virtual terminals are not managed by the pty server; they exist soley for the purpose of creating an interface that can be associated with an ip address. There will be one virtual terminal, with one ip address, associated with each extra domain supported on that computer.