Linux Network Gateway Services

Chapter 7: Network Gateway Services

A computer can communicate directly only with computers with which it shares a physical

connection. Given this fact, the computer on your desk should be able to communicate only with

computers that are electrically connected to the network cable that connects to your system. So,

how does it communicate with a computer on the other side of the world? There are two primary

techniques: circuit switching and packet switching.

Circuit switching

telephone, you hear a dial tone. At this point, you have an electrical connection to the telephone

switch at the local telephone company's central office. As you dial the telephone number, you

provide the switch with the information it needs to make additional connections. Using this

information, the switch connects your inbound port to an outbound port. If the number you are

calling is serviced by the local switch, it sets up a connection between the port your telephone is

attached to and the port connected to the phone you're calling. If the number you are calling is

remotely located, the local switch sets up a connection to the next switch down the line. Each switch

connects to the next switch in line until the switch servicing the remote phone is reached. This

creates a circuit from your phone to the remote phone, wherever it is located, which is dedicated to

your use until you hang up the phone. When your computer communicates over a modem, it uses

the telephone system to create a circuit between itself and the remote system, which is often the

server at an ISP that connects your system into the packet−switched Internet.

Packet switching is the technique used by most data networks. Every packet in the network

contains an address that tells the switch where the packet is bound. When the packet arrives at a

switch, the switch reads the address and determines how the packet should be forwarded. If the

switch has a physical connection to the destination node, it delivers the packet itself. Otherwise, it

forwards the packet to the next switch in the path toward the destination node. Each packet is

handled separately. No end−to−end connection is established

In the circuit−switched model, the connection is between your phone and the phone at the remote

end. In the packet−switched system, the connection is between your host and the local router.

Figure 7.1 illustrates that packet switches use hop−by−hop routes versus the end−to−end

connections used by circuit switchesis the technique used by the voice telephone network. When you pick up the