Networking with Networking with JavaJava

Introduction to NetworkingIntroduction to Networking

ProtocolsProtocols

Hi TCP connection request

HiTCP connectionreplyGot the

time? GET http://www.cs.huji.ac.il/~dbi

2:00<file>

time

Client

Server

Internet Architecture ModelInternet Architecture Model

Application (HTTP, FTP) DATA

Transport (TCP,UDP) HEADER DATA

Network (IP) HEADER HEADER DATA

Link (LINK) HEADER HEADER HEADER DATA

TCP (TCP (Transmission-Control Protocol)Transmission-Control Protocol)

• Enables symmetric byte-stream transmission between two

endpoints (applications)

• Reliable communication channel

• TCP perform these tasks:

- connection establishment by handshake (relatively slow)

- division to numbered packets (transferred by IP)

- error correction of packets (checksum)

- acknowledgement and retransmission of packets

- connection termination by handshake

UDP (User Datagram Protocol)UDP (User Datagram Protocol)

• Enables direct datagram (packet) transmission from one

endpoint to another

• No reliability (except for data correction)

- sender does not wait for acknowledgements

- arrival order is not guaranteed

- arrival is not guaranteed

• Used when speed is essential, even in cost of reliability

- e.g., streaming media, games, Internet telephony, etc.

PortsPorts

• A computer may have several applications that

communicate with applications on remote computers

through the same physical connection to the network

• When receiving a packet, how can the computer tell

which application is the destination?

• Solution: each channel endpoint is assigned a unique

port that is known to both the computer and the other

endpoint

Ports (cont)Ports (cont)

• Thus, an endpoint application on the Internet is

identified by

- A host name → 32 bits IP-address

- A 16 bits port

• Q: Why don’t we specify the port in a Web

browser?

Known PortsKnown Ports

• Some known ports are

- 20, 21: FTP

- 23: TELNET

- 25: SMTP

- 110: POP3

- 80: HTTP

- 119: NNTP

21 23 25 110 80 119

Client Application

web browsermail client

SocketsSockets

• A socket is a construct that represents one end-point of a

two-way communication channel between two programs

running on the network

• Using sockets, the OS provides processes a file-like

access to the channel

- i.e., sockets are allocated a file descriptor, and processes can

access (read/write) the socket by specifying that descriptor

• A specific socket is identified by the machine's IP and a

port within that machine

Sockets (cont)Sockets (cont)

• A socket stores the IP and port number of the other end-

point computer of the channel

• When writing to a socket, the written bytes are sent to

the other computer and port (e.g., over TCP/IP)

- That is, remote IP and port are attached to the packets

• When OS receives packets on the network, it uses their

destination port to decide which socket should get the

received bytes

Client-Server ModelClient-Server Model

• A common paradigm for distributed applications

• Asymmetry in connection establishment:

- Server waits for client requests (daemon) at a well known

address (IP+port)

- Connection is established upon client request

• Once the connection is made, it can be either symmetric

(TELNET) or asymmetric (HTTP)

• For example: Web servers and browsers

Client-Server InteractionClient-Server Interaction

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Server

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Client-Server InteractionClient-Server Interaction

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Client-Server InteractionClient-Server Interaction

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Client-Server InteractionClient-Server Interaction

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Client-Server InteractionClient-Server Interaction

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Java SocketsJava Sockets

Low-Level Networking

Java SocketsJava Sockets

• Java wraps OS sockets (over TCP) by the objects of

class java.net.Socket

• new Socket(String remoteHost, int remotePort) creates a

TCP socket and connects it to the remote host on the

remote port (hand shake)

• Write and read using streams:

- InputStream getInputStream()

- OutputStream getOutputStream()

A Socket ExampleA Socket Example

import java.net.*;import java.io.*;

public class SimpleSocket {

  public static void main(String[] args) throws IOException { ... next slide ...

 }}

 Socket socket = new Socket("www.cs.huji.ac.il", 80);    InputStream istream = socket.getInputStream();    OutputStream ostream = socket.getOutputStream();

    String request =       "GET /~dbi/admin.html HTTP/1.1\r\n"  +       "Host: www.cs.huji.ac.il\r\n" +       "Connection: close\r\n\r\n";            ostream.write(request.getBytes());

    byte[] response = new byte[4096]; int bytesRead = -1; 

    while ((bytesRead = istream.read(response)) >= 0) {      System.out.write(response, 0, bytesRead);    }    socket.close();

Java Java ServerSocketServerSocket

• ServerSocket represents a socket that listens and waits for

requests from clients

• Construction:

- new ServerSocket(int port)

- Why do want to specify the port?

• Listen and accept incoming connections

- Socket accept()

- returns a new socket (with a new port) for the new channel

- blocks until connection is made

public class EchoServer { public static void main(String[] args) throws IOException { ServerSocket serverSocket = new ServerSocket(8000); Socket socket = null;

while (true) { try { ... next slide ... } catch (IOException exp) { ... }

finally { try {if (!socket.isClosed()) socket.close(); } catch (IOException e) {} }}}}

socket = serverSocket.accept();

String clientName = socket.getInetAddress().getHostName();

BufferedReader reader = new BufferedReader( new InputStreamReader(socket.getInputStream())); PrintStream writer = new PrintStream(socket.getOutputStream());

writer.println("Hello " + clientName + "!"); writer.flush();

String lineRead = null;

while ((lineRead = reader.readLine()) != null) { writer.println("You wrote: " + lineRead); writer.flush(); }

Accepting ConnectionsAccepting Connections

• Usually, the accept() method is executed within

an infinite loop

- i.e., while(true){...}

• Whenever accept() returns, a new thread is

launched to handle that interaction

• Hence, the server can handle several requests

concurrently

TimeoutTimeout

• You can set timeout values to these blocking

methods:

- read() of Socket

- accept() of ServerSocket

• Use the method setSoTimeout(milliseconds)

• If timeout is reached before the method returns,

java.net.SocketTimeoutException is thrown

Java Sockets and HTTPJava Sockets and HTTP

HTTP Message StructureHTTP Message Structure

• A HTTP message has the following structure:

Request/Status-Line \r\nHeader1: value1 \r\nHeader2: value2 \r\n...HeaderN: valueN \r\n\r\n

Message-BodyMessage-Body

Reading HTTP MessagesReading HTTP Messages

• Several ways to interpret the bytes of the body

- Binary: images, compressed files, class files, ...

- Text: ASCII, Latin-1, UTF-8, ...

• Commonly, applications parse the headers of the

message, and process the body according to the

information supplied by the headers

- E.g., Content-Type, Content-Encoding, Transfer-Encoding

An ExampleAn Example

Parsing the HeadersParsing the Headers

• So how are the headers themselves are represented?

• Answer: The headers of a HTTP message must be in

US-ASCII format (1 byte per character)

• However, in order to parse the headers, it may be

impossible to use readLine() of BufferedReader,

since the body should be later read as binary bytes

• Hence, we convert bytes to characters

Socket socket = new Socket(argv[0], 80); InputStream istream = socket.getInputStream();OutputStream ostream = socket.getOutputStream();

 String request = "GET / HTTP/1.1\r\n" +                 "Host: " + argv[0] + "\r\n" +              "Connection: close\r\n\r\n"; ostream.write(request.getBytes());

StringBuffer headers = new StringBuffer(); int byteRead = 0;while ( !endOfHeaders(headers) && 

(byteRead = istream.read()) >= 0) {      headers.append((char) byteRead);}System.out.print(headers);socket.close();

Example: Extracting the HeadersExample: Extracting the Headers

public static boolean endOfHeaders(StringBuffer headers) {     int lastIndex = headers.length() - 1;

    if (lastIndex < 3 || headers.charAt(lastIndex) != '\n')      return false;

    return ( headers.substring(lastIndex - 3, lastIndex + 1)         .equals("\r\n\r\n"));  }

Example: Extracting the Headers Example: Extracting the Headers (cont)(cont)

Persistent ConnectionsPersistent Connections

• According to HTTP/1.1, a server does not have to close the

connection after fulfilling your request

• One connection (socket) can be used for several requests and

responses send more requests

- even while earlier responses are being transferred (pipelining)

• Question: how can the client know when one response ends

and a new one begins?

• To avoid persistency, require explicitly by the header:

Connection: close

URL and URLConnectionURL and URLConnection

High-Level Networking

ProtocolHost

NamePort

Number

File Nam

e

Reference

Working with URLsWorking with URLs

• URL (Uniform Resource Locator):

a reference (an address) to a resource on the Internet

http://www.cs.huji.ac.il:80/~dbi/main.html#notes

The Class URLThe Class URL

• The class URL is used for parsing URLs

• Constructing URLs:

- URL w3c1 = new URL("http://www.w3.org/TR/");

- URL w3c2 = new URL("http","www.w3.org",80,"TR/");

- URL w3c3 = new URL(w3c2, "xhtml1/");

• If the string is not an absolute URL, then it is considered

relative to the URL

Parsing URLsParsing URLs

• The following methods of URL can be used for

parsing URLs

getProtocol(), getHost(), getPort(), getPath(),

getFile(), getQuery(), getRef(), getQuery()

The class URLConnectionThe class URLConnection

• To establish the actual resource, we can use the object

URLConnection obtained by url.openConnection()

• If the protocol of the URL is HTTP, the returned object

is of class HttpURLConnection

• This class encapsulates all socket management and

HTTP directions required to obtain the resource

public class ContentExtractor {

  public static void main(String[] argv) throws Exception {    URL url = new URL(argv[0]);        System.out.println("Host: " + url.getHost());    System.out.println("Protocol: " + url.getProtocol());    System.out.println("----");            URLConnection con = url.openConnection();    InputStream stream = con.getInputStream();            byte[] data = new byte[4096]; int bytesRead = 0;     while((bytesRead=stream.read(data))>=0) {       System.out.write(data,0,bytesRead);}}}

About URLConnectionAbout URLConnection

• The life cycle of a URLConnection object has two

parts:

- Before actual connection establishment

• Connection configuration

- After actual connection establishment

• Content retrieval

• Passage from the first phase to the second is implicit

- A result of calling some committing methods, like getDate()

About HttpURLConnectionAbout HttpURLConnection

• The HttpURLConnection class encapsulates all HTTP

transaction over sockets, e.g.,

- Content decoding

- Redirection

- Proxy indirection

• However, you can control requests by its methods

- setRequestMethod, setFollowRedirects,

setRequestProperty, ...

Sending POST RequestsSending POST Requests

• In order to send POST requests with

HttpURLConnection, you have to do the following:

- Enable connection output: con.setDoOutput(true);

- Get the output stream: con.getOutputStream()

• This changes the method from GET to POST

- Write the message body into the stream

- close the stream (important!)

URL url = new URL("http://find.walla.co.il/");URLConnection connection = url.openConnection();connection.setDoOutput(true);

String query = "q=" + URLEncoder.encode(args[0], "UTF-8");    OutputStream out = connection.getOutputStream();  out.write(query.getBytes());out.close();

InputStream in = connection.getInputStream();int bytesRead = -1;  byte[] response = new byte[4096];while ((bytesRead = in.read(response)) >= 0)      System.out.write(response, 0, bytesRead);in.close();

POST / HTTP/1.1User-Agent: Java/1.5.0_01Host: find.walla.co.ilAccept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2Connection: keep-aliveContent-type: application/x-www-form-urlencodedContent-Length: 18

q=Java+networking

java SearchWalla "Java networking"

Sent HeadersSent Headers

URLEncoderURLEncoder

• Contains a utility method encode for converting a string

into an encoded format (used in URLs)

• To convert a string, each character is examined in turn:

- Space is converted into a plus sign +

- a-z, A-Z, 0-9, ., -, * and _ remain the same.

- The bytes of all special characters are replaced by

hexadecimal numbers, preceded with %

• To decode an encoded string, use decode() of the class

URLDecoder

Defining Default ProxyDefining Default Proxy

• For reading a URL using a proxy, we run java with the

environment variables for http.proxyHost and http.proxyPort set

properly: java -DproxyHost=wwwproxy.huji.ac.il –DproxyPort=8080 ...

• Another option is to set the environment variables in the

program itselfSystem.getProperties().put( "proxyHost", "wwwproxy.huji.ac.il" );

System.getProperties().put( "proxyPort", "8080" );

Not Covered: UDP ConnectionsNot Covered: UDP Connections

• The URL, URLConnection, Socket and

ServerSocket classes all use the TCP protocol to

communicate over the network

• Java programs can also use UDP protocol for

communication

• For that, use DatagramPacket, DatagramSocket,

and MulticastSocket classes