1. Java Sockets Programming - Fundamentals
Goal: Make communication between computations possible
Interprocess Communication (IPC): mechanism for exchanging messages between two computing process (running on the same or different machines)
Examples: pipes or socket pairs first introduced in Berkeley Unix, remote procedure calls (RPC) and remote method invocation (RMI) of Java first introduced by Sun.
socket: Low level programming primitive that allows processes to exchange information through a communication channel
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2. CS 667: Socket Fundamentals Copyright Zlateva
Unix I/O and pipes
Main idea: Implement interprocess communication following the Unix I/O paradigm of Open Read/Write Close.
obtain permission
to connect
receive/send info
terminate connection
Background:
A Unix process has a set of I/O descriptors, to which the process writes to/reads from and that refer to files and devices.
A Unix pipe is a pseudo-file that connects two processes, provided the processes inherit file descriptors for that pipe from a common ancestor, e.g.
cat hello.java | lpr
Unix shell creates two processes with a pipe between them: cat reads hello.java, and instead of writing to the standard output (screen) sends it through the pipe to lpr which sends it to the printer
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Pipes and Sockets OS
(Unix)
Pipes:
A and B must be related
Unrelated processes cannot communicate through pipe
pipe
Process B
(lpr)
Process A
(cat)
Note: Unrelated processes can communicate through files - one process reading to the file, the other checking periodically; this still does not allow communication over network
Network communication channel
Process B
Process A
Sockets: Interface to protocol stack (TCP or UDP, IP) that is part of the OS kernel OS OS
History: The socket API was introduced in 4.2BSD, Computer System Research Group (CSRG) at Berkeley, The networking code (TCP/IP protocol stacks, socket interface, telnet, ftp client & server), was developed independently from the AT&T-derived Unix code. Merged in 4.4BSD-Lite, 1994 (Ste 1998,p.19)
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4. Socket in OSI and Internet Protocol Suit
Application
Presentation (data format, conversion)
Session (multi- threading)- message
Transport
segment/message
Network -
datagram/packet
DataLink – frames
Physical – bit
Application:
e.g. FTP, telnet, SMTP, , etc.
Network adapters & Device drivers
typically run in OS IP
Internet Protocol
TCP
Transmission
Control Protocol
UDP
User
Datagram Protocol
Raw Socket
Application Details:
Sockets
Communication Details:
Sending data, waiting for acknowledgement, sequencing data to arrive in order, calculating and verifying checksums, etc.
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5. End-to-End Communication Types
Connection-Oriented or Stream
Reliable ordered two-way connection service in two stages:
1. Connection setup: Establishes virtual connection from source to host (connection state established)
2. Transfer
Protocol: TCP/IP*
Connectionless or Datagram*
Unreliable ("transmit and pray") two-way connection service; simple extension of the underlying host-to-host IP delivery service to process-to process
Every packet (datagram) must contain full destination information (address of receiving socket and local descriptors)
Packets can be sent any time anywhere (no need to establish connection state);
Packets sent independently;
Protocol: UDP/IP
*Virtual Circuit technologies, such as X.25, Frame Relay, ATM, also provide a connection-oriented reliable/ordered delivery service (PeDa 2003). However they are network layer protocols, (as is IP), packets follow same route, and delivery is assumed reliable/ordered/hop-by-hop. As opposed to this TCP assumes unreliable best-effort delivery as provided by IP, datagrams may take different routes, and are assembled into a TCP segment at the ends of the communication channel, i.e. and end-to-end as opposed to hop-to-hop approach (PeDa 2003)
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Streams vs. Datagrams
TCP Stream
additional time for establishing the connection
less information transmitted as no need to include the address of receiving socket and local descriptors with each packet
no size limit on information size that can be sent to a specific location
good for services requiring transfer of data of indefinite length, e.g. remote login (rlogin, telnet), ftp
UDP Datagram
no connection set up time
more information transmitted than streams as all packets must include local descriptors and address of receiving socket
not known if packet delivered.
successive packets may have different routes and there is no guarantee they will arrive in order.
datagram size limited to 64 kB
appropriate if application protocol handles reliability, application requires multicast, or cannot tolerate TCP overhead
For more on TCP vs. UDP see (Ste 1998, p. 541)
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Socket Types
Socket Type Communication Type Protocol Stack
SOCK_STREAM stream, virtual circuit TCP/IP
SOCK_DGRAM datagram, connectionless UDP/IP
message-oriented
SOCK_RAW either- bypasses transmission /IP
layer; allows higher control of
packet transmission
not supported by Java
for security reasons
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8. Socket Types & Protocol Stacks - Examples
TCP socket
ICMP
IGMP IP
APR
Link
RARP
UDP
tftp
mrouted
TCP
ftp
ping
traceroute
UDP socket
RAW socket
Applications:
ping packet internet groper: sends ICMP echo request to some IP address and receives ICMP echo reply from that node
traceroute determines path of IP datagrams from host to destination
ftp file transmission protocol (here the application that uses it)
tftp trivial file transmission protocol used to bootstrap systems on LAN not on WAN
mrouted multicast routing
Protocols:
ICMP Internet Control Message Protocol: handles TCP/IP internally generated messages & control messages between routers and hosts; usually not used for application
IGMP Internet Group Management Protocol: used for multicasting
APR Address Resolution Protocol: maps IP addresses to hardware addresses
RARP Reverse Address Resolution Protocol: maps hardware addresses to IP
For more details see (Ste 1998)
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Naming and Routing
IP datagrams are delivered, routed, from host to destination based on a 32 bit identifier, called IP address, that contains the Network ID and the Host ID.
The address is often represented through 4 decimal numbers in the range of 0-255, (one for each 8 bit group), e.g.
128.xxx.xxx.xxx
or through a symbolic name, such as
metcs.bu.edu
The correspondence between symbolic and numerical identifiers is provided by a naming service, known as the Domain Name Service (DNS)
To meet the requirements of different types of organizations there are 5 IP address classes that provide address ranges with different lengths of the network and host IDs as appropriate for the corresponding organization type.
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Internet Addresses
Class A: large networks: more than 65,536 (>16 bits) hosts
Network ID Host ID
0… … … …255
Class B: medium size networks: ,536 (8-16 bits) hosts
Network ID Host ID
128… … … …255
Class C: small networks: (<=8 bits) hosts
Network ID Host ID
192… … … …255
Class D: Multicast
224… … … …255
Class E: Reserved for future use
240… … … …255
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11. Data Streams (review) -Some more important classes from the java.io
InputStream/ OutputStream: abstract class, provides read()/ write() method; created and tied to socket through the Socket method
getInputStream()/ getOuputStream() that returns an input/output stream for this socket.
Reader/ Writer: abstract class for writing to character (text) streams; the only methods a subclass must implement are write(char[], int, int), flush(), and close(); most subclasses override some of the Reader/ Writer methods in order to provide higher efficiency, additional functionality, or both.
InputStreamReader/ OutputStreamWriter extends Reader/ Writer: i/o stream reads/writes bytes and translates them into characters according to a specified character encoding.
BufferedReader/BufferedWriter extends Reader/ Writer class: reads/writes text from a character-input stream, buffering characters so as to provide more efficient reading of characters, arrays, and lines. Should be wrapped around any Reader/Writer with costly operation, such as stream and file readers/writers to reduce number of required accesses to the stream. Ha methods such as readLine not defined in InputStream or InputStreamReader
PrintWriter extends Writer class: prints formatted representations of objects to a text-output stream; creates necessary intermediate OuputStreamWriter; implements all of the print methods found in PrintStream but noy in OutputStream.
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Data Streams Examples
I-Stream bound to Socket object s
s.getInputStream()
BufferedReader in =
new BufferedReader(new InputStreamReader(<i-stream>);
O-Stream bound to Socket object s
s.getOutputStream()
PrintWriter out = new PrintWriter (<o-stream>, true);
autoFlush
println() flushes output buffer
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13. Data Streams (continued)
DataInputStream/DataOuputStream extends InputStream/ OutputStream; lets an application read primitive Java data types (as opposed to bytes) from an underlying input stream in a machine-independent way.
ObjectInputStream/ObjectOuputStream extends InputStream/ OutputStream: reads/writes serialized objects (as opposed to primitive data types, text, or bytes); when written to file provides persistent storage : only objects that implement the java.io.Serializable interface can be written to ObjectInputStream;
FilterInputStream/FilterOutputStream extends (non-buffered) InputStream/OutputStream by overriding their methods and providing additional functionality; improved performance by caching and flushing.
FileWriter/FileReader extends Writer/Reader class for reading character files
FileInputStream/FileOutputStream extends InputStream/ OutputStream for reading data from a File or from a FileDescriptor.
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14. Java Socket Types in java.net package
Class Description
Socket TCP endpoint, client socket
ServerSocket TCP endpoint, server socket
DatagramSocket UDP endpoint, for sending and receiving datagrams
DatagramPacket UDP endpoint, for sending datagrams
MulticastSocket extends DatagramSocket for sending and receiving IP multicast packets. A MulticastSocket is a (UDP) DatagramSocket, with additional capabilities for joining "groups" of other multicast hosts on the internet.
URL Uniform Resource Locator that points to an object on
the Internet such as a simple file, or more complicated
object such as query to a database or a search engine.
URLConnection abstract class; superclass of all classes that represent a
communications link between the application and a URL (e.g. CGI-bin script,DayTime service); connection is established in several steps;
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15. Opening a TCP Connection - Client Side
Opening a TCP client
Socket myClient = null;
try {
myClient = new Socket("host", PortNumber);
} catch (UnknownHostException uhe){
uhe.printStackTrace();
} catch(IOException e){
e.printStackTrace(); }
public Socket(String host, int port)
throws
UnknownHostException, IOException
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16. Opening a TCP Connection - Server Side
public ServerSocket(int port)
throws
IOException
Opening a TCP server socket
ServerSocket myService = null;
try {
myService = new SeverSocket(PortNumber);
} catch(IOException e){
e.printStackTrace(); }
Opening a TCP client type socket from server to listen and accept connection
Socket serviceClient = null; //NOTE: client socket type
serviceClient= myService.accept();
public Socket accept()
throws IOException
Listens for a connection to be made to this socket and accepts it. The method blocks until a connection is made.
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17. Creating Input Streams for TCP Connection
Creating an Input stream on the Client side
BufferedReader is = null;
try{
is = new BufferedReader(
new InputStreamReader(myClient.getInputStream()));
} catch(IOException ioe){
ioe.printStackTrace(); }
Creating an Input stream on the Server side
new InputStreamReader(serviceClient.getInputStream()));
Wrapping with BufferedReader for efficiency
Reading the input stream: bytes->char
Input stream to Client Socket
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18. Creating Output Streams for TCP Connection
Creating an Output stream on the Client side
DataOutputStream os = null;
try{
os = new DataOutputStream(myClient.getOutputStream());
} catch(IOException ioe){
ioe.printStackTrace(); }
Similarly on the Server side
os = new DataOutputStream(serviceClient.getOutputStream());
Output stream to client Socket
DataOtuputStream for writing primitive data types
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19. Closing a TCP Connection
Closing the TCP connection on the Client side
try{
os.close();
is.close();
myClient.close();
} catch(IOException ioe){
ioe.printStackTrace(); }
Closing the TCP connection on the Server side
serviceClient.close();
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20. CS 667: Socket Fundamentals Copyright Zlateva
Example1: "Greetings" "… and Salutations…" Files: SimpleServer.java, SimpleClient.java
Server Side
Client Side
checkServer
TESTPORT
clientSocket
is os c1 is os
SimpleServer started...
Please input password: Greetings
Greetings
we received: Greetings
....and Salutations...
Got from server ....and Salutations...
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21. One Server Should Handle Multiple Clients and Salutations
Server Side
Client Side c1 is os
checkServer
TESTPORT c1 is os
clientSocket
is os
Problem: In Example 1 the server can handle only one client at a time. The read() method blocks and waits for client input, i.e. if server attempts to read from a client it needs to get input from this client, before being able to read from another client
Solution: multiple threads on server side, each thread handles one client
Note: Another solution is to write a nonblocking server, which is usually the inferior solution. For more details see HSH 1999, p.315.
clientSocket
is os .
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22. Symbolic and Numerical Internet Addresses
Problem: Being able to transform symbolic in numerical Internet Ids and vice versa, i.e. having access to the correspondence
symbolic ID  numerical ID
Java class
InetAddress represents an Internet Protocol (IP) address.
Methods
getAddress() Returns the raw IP address (byte array) of this InetAddress object. The result is in network byte order: the highest order byte of the address is in getAddress()[0].
Ex:
InetAddress rawAddress;
byte [] rawAddress = address.getAddress();
// rawAddress[0] is the highest order byte
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23. Symbolic and Numerical Internet Addresses (continued).
Methods
getLocalHost() Returns the IP address of the local host. throws: UnknownHostException if no IP address for the host could be found (see SimpleServer code).
getByName( <string-host>) Determines the IP address of a host, given the name (as string) of the host.
isMulticastAddress() check if the InetAddress is a IP multicast address.
toString() Converts this IP address to a String; returns a string representation of this IP address.
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24. Creating Clients for Existing Services
Goal: Program a client to use existing Internet services, such as SMTP (Simple Mail Transfer Protocol), ping, finger
Main Idea:
Using the service means following the protocol (the language or the rules for exchanging information) that have been established for this service. In other words the client and server must speak the same language, or more precisely the client must know the commands the server understands.
The implementation of the client is based on the simple idea to send the commands through a socket to the server and receive the reply.
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25. CS 667: Socket Fundamentals Copyright Zlateva
Examples: SMTP, finger
From Unix shell From socket program
SMTP
% telnet metcs.bu.edu smptSocket = new Socket("metcs.bu.edu", 25);
HELO os.writeBytes("HELO\n");
MAIL os.writeBytes("MAIL From: ");
etc.
finger
% finger os.writeBytes(" ");
Output stream bound to socket
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26. Internet Port Services
The first 1024 ports (0…1023) are reserved for specific well known Internet services. Each service has a corresponding protocol. The Internet standards are established through and interactive process of development known as the "Request for Comment" (RFC), and the specifications are available as RFC or standards (STD)
Port Protocol RFC
7 Echo RFC 862
13 DayTime RFC 867
20 ftp RFC 959
25 SMPT RFC 821 and other extensions
80 HTTP RFC 2616
For further information see (HSH 1999, p.274ff)
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References:
(HoCo 2003) Cay S. Horstmann, Gary Cornell: Core Java 2. Volume 2: Advanced Features. Prentice Hall. ISBN: : 5th ed. A clear and detailed discussion of topics with good examples, but does not contain all topics covered in the course. Material covered in the last edition is essentially the same as in previous editions, but is presented more concisely, with J2SE version 1.3/1.4.
Ch.3 Networking
(PeDa 2003) Larry L. Peterson, Bruce S. Davie: Computer Networks - A Systems Approach, 3d ed, Morgan Kaufman: The standard text for a serious student of networks.
(HSH 1999) Merlin Hughes, Michael Shoffner, Derek Hamner: Java Network Programming : A Complete Guide to Networking, Streams, and Distributed Computing.  Manning Publications Company; ISBN: X: As the title suggests streams as the crux of network programming.
(Ste 1998) W.Richard Stevens: Unix Network Programminig, Vol.1, Networking APIs: Sockets and XTI , Prentice Hall: A classic of Unix programming.
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