Java程序辅导

SSC - Communication and Networking
SSC - Communication and Networking
Java Socket Programming (II)
Shan He
School for Computational Science
University of Birmingham
Module 06-19321: SSC
SSC - Communication and Networking
Outline
Outline of Topics
UDP Socket Java Programming
Multicast in Java
SSC - Communication and Networking
UDP Socket Java Programming
User Datagram Protocol (UDP)
I Sends independent packets of data (called datagrams)
between computers without guarantees about arrival and
sequencing.
I Transaction-oriented: suitable for simple query-response
protocols,
I Examples: clock server, Domain Name System, ...
I Not connection-oriented (point-to-point) like TCP: suitable
for very large numbers of clients
I Examples: streaming media applications, e.g., IPTV
I Faster: no acknowledge of receiving packets, no flow control
and very simple error control (only check errors but no
correction).
I Examples: Voice over IP, online games, ...
SSC - Communication and Networking
UDP Socket Java Programming
Datagrams
I Datagrams: an independent, self-contained message sent over
the network whose arrival, arrival time, and content are not
guaranteed.
I UDP header consists of 4 fields, each of which is 16 bits
Table : UDB datagram
Bit 0-15 16-35
0 Source port Destination port
32 Length Checksum
... Data
SSC - Communication and Networking
UDP Socket Java Programming
Datagrams: checksum
I Used for error-checking of the UDP header and data
I Also include IP address to prevent misrouting
I Basic idea: the complement of a 16-bit sum calculated over
an IP ”pseudo-header” and the actual UDP data.
I A pseudo-header: the IP header and the UDP header without
checksum field
I At the receiver end: all the 16-bit words of the headers plus
data area are added together =⇒ Sum
I Sum + Checksum = 11111111 11111111
SSC - Communication and Networking
UDP Socket Java Programming
Datagrams: checksum calculation
Source IP address
Destination IP address
Protocol
UDP total length
Source port number
Destination port number
Data
source: San Diego University
SSC - Communication and Networking
UDP Socket Java Programming
UDP datagram communication in Java
I Two packages in java.net package:
I DatagramPacket : contains several constructors for
creating datagram packet object
I Example: DatagramPacket(byte[] buf, int length,
InetAddress address, int port);
I DatagramSocket : provides various methods for
transmitting or receiving datagrams over the network
I Example: void send(DatagramPacket p) or
void receive(DatagramPacket p)
SSC - Communication and Networking
UDP Socket Java Programming
Example 1: UDP word counting server
I We will create a simple UDP server waits for clients requests
and then accepts the message (datagram) and send back the
number of words in the message.
I Since DatagramPacket only accept byte array as its
argument, we need to convert String to byte[]
SSC - Communication and Networking
Multicast in Java
Multicast
I Imaging you need to send data to a group of 1,000 clients
I What’s wrong with TCP?
I Connection-based: you need 1,000 connections which consume
a lot of processing power on sender!
I Flow control: The arrival time is not the same for every clients
I Multicast is a special feature of UDP protocol that enable
programmer to send data to a group of receivers on a specific
multicast IP address and port.
I One-to-many and many-to-many real-time communication
over internet
I Send data only once to any number of any receivers
I The data is also called multicast packets
SSC - Communication and Networking
Multicast in Java
Multicast: Java example
I A multicast group is specified by a class-D IP address and by
a standard UDP port number.
I Class-D IP addresses are in the range 224.0.0.0 to
239.255.255.255, inclusive.
I Receivers must join multicast group to receive data
Multicast 
IP address
Sender
Joint to receive multicast 
message
Receivers
SSC - Communication and Networking
Multicast in Java
Multicast: Java example
I MulticastSocket class: sending and receiving multicast
packets
I MulticastSocket class is inherited from the
DatagramSocket class, with additional capabilities for
joining ”groups” of other multicast hosts on the internet.
I For Android App development, you can also use
MulticastSocket class
I More information can be found at Oracle’s webpage
SSC - Communication and Networking
Multicast in Java
Multicast: Java example
Steps of using MulticastSocket to receive data
I Step 1: create a MulticastSocket object with the desired
port
I Step 2: join a group using the
joinGroup(InetAddress groupAddr) method
I Step 3: leave the group using
leaveGroup(InetAddress addr) method.
SSC - Communication and Networking
Multicast in Java
Multicast: Java example
Steps of using MulticastSocket to send data
I Step 1: create a MulticastSocket
I Step 2: create a DatagramPacket object for data
I Step 3: set Time-To-Live (TTL) using setTimeToLive
method
I Step 4: send data using send() method
I Step 5: close the MulticastSocket using close()
method
Note: Time-To-Live (TTL): a value between zero and 255. Every
time a router forwards the packet, it decrements the TTL field in
the packet header. The packet will dropped if the value reaches
zero. Used to avoid packages being looped forever due to routing
errors.