UDP - Java Network Programming - page 420

broadcast address for the network with the local address 192.168.254.*. UDP broad‐
casting is often used for protocols such as DHCP that need to communicate with servers
on the local net whose addresses are not known in advance. This option is controlled
with these two methods:
public void setBroadcast ( boolean on ) throws SocketException
public boolean getBroadcast () throws SocketException
Routers and gateways do not normally forward broadcast messages, but they can still
kick up a lot of traffic on the local network. This option is turned on by default, but if
you like you can disable it thusly:
socket . setBroadcast ( false );
This option can be changed after the socket has been bound.
On some implementations, sockets bound to a specific address do not
receive broadcast packets. In other words, you should use the Data
gramPacket(int port) constructor, not the DatagramPacket(InetAd
dress address, int port) constructor to listen to broadcasts. This
is necessary in addition to setting the SO_BROADCAST option to true.
IP_TOS
Because the traffic class is determined by the value of the IP_TOS field in each IP packet
header, it is essentially the same for UDP as it is for TCP. After all, packets are actually
routed and prioritized according to IP, which both TCP and UDP sit on top of. There's
really no difference between the setTrafficClass() and getTrafficClass() methods
in DatagramSocket and those in Socket . They just have to be repeated here because
DatagramSocket and Socket don't have a common superclass. These two methods let
you inspect and set the class of service for a socket using these two methods:
public int getTrafficClass () throws SocketException
public void setTrafficClass ( int trafficClass ) throws SocketException
The traffic class is given as an int between 0 and 255. Because this value is copied to an
eight-bit field in the TCP header, only the low-order byte of this int is used; and values
outside this range cause IllegalArgumentException s.
The JavaDoc for these options is severely out of date, and describes a
quality of service scheme based on bit fields for four traffic classes: low
cost, high reliability, maximum throughput, and minimum delay. This
scheme was never widely implemented and probably hasn't been used
in this century.