A more recent protocol commonly offered by telecommunications companies is called Frame Relay. The Frame Relay protocol shares a number of technical features with the X.25 protocol, but is much more like the IP protocol in behavior. Like X.25, Frame Relay requires special synchronous serial hardware. Because of their similarities, many cards support both of these protocols. An alternative is available that requires no special internal hardware, again relying on an external device called a Frame Relay Access Device (FRAD) to manage the encapsulation of Ethernet packets into Frame Relay packets for transmission across a network. Frame Relay is ideal for carrying TCP/IP between sites. Linux provides drivers that support some types of internal Frame Relay devices.
If you need higher speed networking that can carry many different types of data, such as digitized voice and video, alongside your usual data, ATM (Asynchronous Transfer Mode) is probably what you'll be interested in. ATM is a new network technology that has been specifically designed to provide a manageable, high-speed, low-latency means of carrying data, and provide control over the Quality of Service (Q.S.). Many telecommunications companies are deploying ATM network infrastructure because it allows the convergence of a number of different network services into one platform, in the hope of achieving savings in management and support costs. ATM is often used to carry TCP/IP. The Networking-HOWTO offers information on the Linux support available for ATM.
Frequently, radio amateurs use their radio equipment to network their computers; this is commonly called packet radio. One of the protocols used by amateur radio operators is called AX.25 and is loosely derived from X.25. Amateur radio operators use the AX.25 protocol to carry TCP/IP and other protocols, too. AX.25, like X.25, requires serial hardware capable of synchronous operation, or an external device called a "Terminal Node Controller" to convert packets transmitted via an asynchronous serial link into packets transmitted synchronously. There are a variety of different sorts of interface cards available to support packet radio operation; these cards are generally referred to as being "Z8530 SCC based," and are named after the most popular type of communications controller used in the designs. Two of the other protocols that are commonly carried by AX.25 are the NetRom and Rose protocols, which are network layer protocols. Since these protocols run over AX.25, they have the same hardware requirements. Linux supports a fully featured implementation of the AX.25, NetRom, and Rose protocols. The AX25-HOWTO is a good source of information on the Linux implementation of these protocols.
Other types of Internet access involve dialing up a central system over slow but cheap serial lines (telephone, ISDN, and so on). These require yet another protocol for transmission of packets, such as SLIP or PPP, which will be described later.
The Internet Protocol
Of course, you wouldn't want your networking to be limited to one Ethernet or one point-to-point data link. Ideally, you would want to be able to communicate with a host computer regardless of what type of physical network it is connected to. For example, in larger installations such as Groucho Marx University, you usually have a number of separate networks that have to be connected in some way. At GMU, the Math department runs two Ethernets: one with fast machines for professors and graduates, and another with slow machines for students. Both are linked to the FDDI campus backbone network.
This connection is handled by a dedicated host called a gateway that handles incoming and outgoing packets by copying them between the two Ethernets and the FDDI fiber optic cable. For example, if you are at the Math department and want to access quark on the Physics department's LAN from your Linux box, the networking software will not send packets to quark directly because it is not on the same Ethernet. Therefore, it has to rely on the gateway to act as a forwarder. The gateway (named sophus) then forwards these packets to its peer gateway niels at the Physics department, using the backbone network, with niels delivering it to the destination machine. Data flow between erdos and quark is shown in Figure 1.1.
Figure 1.1: The three steps of sending a datagram from erdos to quark
This scheme of directing data to a remote host is called routing, and packets are often referred to as datagrams in this context. To facilitate things, datagram exchange is governed by a single protocol that is independent of the hardware used: IP, or Internet Protocol. In Chapter 2, Issues of TCP/IP Networking, we will cover IP and the issues of routing in greater detail.
The main benefit of IP is that it turns physically dissimilar networks into one apparently homogeneous network. This is called internetworking, and the resulting "meta-network" is called an internet. Note the subtle difference here between an internet and the Internet. The latter is the official name of one particular global internet.
Of course, IP also requires a hardware-independent addressing scheme. This is achieved by assigning each host a unique 32-bit number called the IP address. An IP address is usually written as four decimal numbers, one for each 8-bit portion, separated by dots. For example, quark might have an IP address of 0x954C0C04, which would be written as 149.76.12.4. This format is also called dotted decimal notation and sometimes dotted quad notation. It is increasingly going under the name IPv4 (for Internet Protocol, Version 4) because a new standard called IPv6 offers much more flexible addressing, as well as other modern features. It will be at least a year after the release of this edition before IPv6 is in use.
You will notice that we now have three different types of addresses: first there is the host's name, like quark, then there are IP addresses, and finally, there are hardware addresses, like the 6-byte Ethernet address. All these addresses somehow have to match so that when you type rlogin quark, the networking software can be given quark's IP address; and when IP delivers any data to the Physics department's Ethernet, it somehow has to find out what Ethernet address corresponds to the IP address.
We will deal with these situations in Chapter 2. For now, it's enough to remember that these steps of finding addresses are called hostname resolution, for mapping hostnames onto IP addresses, and address resolution, for mapping the latter to hardware addresses.