Java Reference
In-Depth Information
Name System (DNS) works. (DNS can also operate over TCP.) In fact, you could im‐
plement a reliable file transfer protocol using UDP, and many people have: Network
File System (NFS), Trivial FTP (TFTP), and FSP, a more distant relative of FTP, all use
UDP. (The latest version of NFS can use either UDP or TCP.) In these protocols, the
application is responsible for reliability; UDP doesn't take care of it (the application
must handle missing or out-of-order packets). This is a lot of work, but there's no reason
it can't be done—although if you find yourself writing this code, think carefully about
whether you might be better off with TCP.
The difference between TCP and UDP is often explained by analogy with the phone
system and the post office. TCP is like the phone system. When you dial a number, the
phone is answered and a connection is established between the two parties. As you talk,
you know that the other party hears your words in the order in which you say them. If
the phone is busy or no one answers, you find out right away. UDP, by contrast, is like
the postal system. You send packets of mail to an address. Most of the letters arrive, but
some may be lost on the way. The letters probably arrive in the order in which you sent
them, but that's not guaranteed. The farther away you are from your recipient, the more
likely it is that mail will be lost on the way or arrive out of order. If this is a problem,
you can write sequential numbers on the envelopes, then ask the recipients to arrange
them in the correct order and send you mail telling you which letters arrived so that
you can resend any that didn't get there the first time. However, you and your corre‐
spondent need to agree on this protocol in advance. The post office will not do it for
you.
Both the phone system and the post office have their uses. Although either one could
be used for almost any communication, in some cases one is definitely superior to the
other. The same is true of UDP and TCP. The past several chapters have all focused on
TCP applications, which are more common than UDP applications. However, UDP also
has its place; in this chapter, we'll look at what you can do with UDP. If you want to go
further, the next chapter describes multicasting over UDP. A multicast socket is a fairly
simple variation on a standard UDP socket.
Java's implementation of UDP is split into two classes:
DatagramPacket
and
Datagram
Socket
. The
DatagramPacket
class stuffs bytes of data into UDP packets called
data‐
grams
and lets you unstuff datagrams that you receive. A
DatagramSocket
sends as well
as receives UDP datagrams. To send data, you put the data in a
DatagramPacket
and
send the packet using a
DatagramSocket
. To receive data, you take a
DatagramPacket
object from a
DatagramSocket
and then inspect the contents of the packet. The sockets
themselves are very simple creatures. In UDP, everything about a datagram, including
the address to which it is directed, is included in the packet itself; the socket only needs
to know the local port on which to listen or send.
This division of labor contrasts with the
Socket
and
ServerSocket
classes used by TCP.
First, UDP doesn't have any notion of a unique connection between two hosts. One
