The Internet Protocol
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.
IP Over Serial Lines
Serial Line IP Compressed SLIP Point-to-Point ProtocolThe Transmission Control Protocol
Sending datagrams from one host to another is not the whole story. If you log in to quark , you want to have a reliable connection between your rlogin process on erdos and the shell process on quark . Thus, the information sent to and fro must be split up into packets by the sender and reassembled into a character stream by the receiver. Trivial as it seems, this involves a number of complicated tasks.
A very important thing to know about IP is that, by intent, it is not reliable. Assume that ten people on your Ethernet started downloading the latest release of Netscape's web browser source code from GMU's FTP server. The amount of traffic generated might be too much for the gateway to handle, because it's too slow and it's tight on memory. Now if you happen to send a packet to quark , sophus might be out of buffer space for a moment and therefore unable to forward it. IP solves this problem by simply discarding it. The packet is irrevocably lost. It is therefore the responsibility of the communicating hosts to check the integrity and completeness of the data and retransmit it in case of error.
This process is performed by yet another protocol, Transmission Control Protocol (TCP), which builds a reliable service on top of IP. The essential property of TCP is that it uses IP to give you the illusion of a simple connection between the two processes on your host and the remote machine, so you don't have to care about how and along which route your data actually travels. A TCP connection works essentially like a two-way pipe that both processes may write to and read from. Think of it as a telephone conversation.