Java Network Programming

Chapter # 01
Basic Network Concepts

Network programming is no longer the province of a few specialists. It has become a core part of every developer’s toolbox. Today, more programs are network aware than aren’t. Besides classic applications like email, web browsers, and remote login, most major applications have some level of networking built in. For example:

• Text editors like BBEdit save and open files directly from FTP servers.

• IDEs like Eclipse and IntelliJ IDEA communicate with source code repositories like GitHub and Sourceforge.

• Word processors like Microsoft Word open files from URLs.

• Antivirus programs like Norton AntiVirus check for new virus definitions by con‐ necting to the vendor’s website every time the computer is started.

• Music players like Winamp and iTunes upload CD track lengths to CDDB and download the corresponding track titles.

• Gamers playing multiplayer first-person shooters like Halo gleefully frag each other in real time.

• Supermarket cash registers running IBM SurePOS ACE communicate with their store’s server in real time with each transaction. The server uploads its daily receipts to the chain’s central computers each night.

• Schedule applications like Microsoft Outlook automatically synchronize calendars among employees in a company.

Java was the first programming language designed from the ground up for network applications. Java was originally aimed at proprietary cable television networks rather than the Internet, but it’s always had the network foremost in mind. One of the first two real Java applications was a web browser. As the Internet continues to grow, Java is uniquely suited to build the next generation of network applications.

One of the biggest secrets about Java is that it makes writing network programs easy. In
fact, it is far easier to write network programs in Java than in almost any other language.
This book shows you dozens of complete programs that take advantage of the Internet.
Some are simple textbook examples, while others are completely functional applica‐
tions. One thing you’ll notice in the fully functional applications is just how little code
is devoted to networking. Even in network-intensive programs like web servers and
clients, almost all the code handles data manipulation or the user interface. The part of
the program that deals with the network is almost always the shortest and simplest. In
brief, it is easy for Java applications to send and receive data across the Internet.

This chapter covers the background networking concepts you need to understand be‐ fore writing networked programs in Java (or, for that matter, in any language). Moving from the most general to the most specific, it explains what you need to know about networks in general, IP and TCP/IP-based networks in particular, and the Internet. This chapter doesn’t try to teach you how to wire a network or configure a router, but you will learn what you need to know to write applications that communicate across the Internet. Topics covered in this chapter include the nature of networks; the TCP/IP layer model; the IP, TCP, and UDP protocols; firewalls and proxy servers; the Internet; and the Internet standardization process. Experienced network gurus may safely skip this chapter, and move on to the next chapter where you begin developing the tools needed to write your own network programs in Java.

Networks

A network is a collection of computers and other devices that can send data to and receive data from one another, more or less in real time. A network is often connected by wires, and the bits of data are turned into electromagnetic waves that move through the wires. However, wireless networks transmit data using radio waves; and most longdistance transmissions are now carried over fiber-optic cables that send light waves through glass filaments. There’s nothing sacred about any particular physical medium for the transmission of data. Theoretically, data could be transmitted by coal-powered computers that send smoke signals to one another. The response time (and environ‐ mental impact) of such a network would be rather poor.

Each machine on a network is called a node. Most nodes are computers, but printers, routers, bridges, gateways, dumb terminals, and Coca-Cola™ machines can also be no‐ des. You might use Java to interface with a Coke machine, but otherwise you’ll mostly talk to other computers. Nodes that are fully functional computers are also called hosts. I will use the word node to refer to any device on the network, and the word host to refer to a node that is a general-purpose computer.

Every network node has an address, a sequence of bytes that uniquely identifies it. You can think of this group of bytes as a number, but in general the number of bytes in an address or the ordering of those bytes (big endian or little endian) is not guaranteed tomatch any primitive numeric data type in Java. The more bytes there are in each address, the more addresses there are available and the more devices that can be connected to the network simultaneously.

Addresses are assigned differently on different kinds of networks. Ethernet addresses are attached to the physical Ethernet hardware. Manufacturers of Ethernet hardware use preassigned manufacturer codes to make sure there are no conflicts between the addresses in their hardware and the addresses of other manufacturers’ hardware. Each manufacturer is responsible for making sure it doesn’t ship two Ethernet cards with the same address. Internet addresses are normally assigned to a computer by the organization that is responsible for it. However, the addresses that an organization is allowed to choose for its computers are assigned by the organization’s Internet service provider (ISP). ISPs get their IP addresses from one of four regional Internet registries (the registry for North America is ARIN, the American Registry for Internet Numbers), which are in turn assigned IP addresses by the Internet Corporation for Assigned Names and Numbers (ICANN).

On some kinds of networks, nodes also have text names that help human beings identify them such as “www.elharo.com” or “Beth Harold’s Computer.” At a set moment in time, a particular name normally refers to exactly one address. However, names are not locked to addresses. Names can change while addresses stay the same; likewise, addresses can change while the names stay the same. One address can have several names and one name can refer to several different addresses.

All modern computer networks are packet-switched networks: data traveling on the network is broken into chunks called packets and each packet is handled separately. Each packet contains information about who sent it and where it’s going. The most important advantage of breaking data into individually addressed packets is that packets from many ongoing exchanges can travel on one wire, which makes it much cheaper to build a network: many computers can share the same wire without interfering. (In contrast, when you make a local telephone call within the same exchange on a traditional phone line, you have essentially reserved a wire from your phone to the phone of the person you’re calling. When all the wires are in use, as sometimes happens during a major emergency or holiday, not everyone who picks up a phone will get a dial tone. If you stay on the line, you’ll eventually get a dial tone when a line becomes free. In some countries with worse phone service than the United States, it’s not uncommon to have to wait half an hour or more for a dial tone.) Another advantage of packets is that checksums can be used to detect whether a packet was damaged in transit.

We’re still missing one important piece: some notion of what computers need to say to pass data back and forth. A protocol is a precise set of rules defining how computers communicate: the format of addresses, how data is split into packets, and so on. There are many different protocols defining different aspects of network communication. For example, the Hypertext Transfer Protocol (HTTP) defines how web browsers and servers communicate; at the other end of the spectrum, the IEEE 802.3 standard defines a protocol for how bits are encoded as electrical signals on a particular type of wire. Open, published protocol standards allow software and equipment from different ven‐ dors to communicate with one another. A web server doesn’t care whether the client is a Unix workstation, an Android phone, or an iPad, because all clients speak the same HTTP protocol regardless of platform.