Networking with Java 1: The Client Side. Introduction to Networking.

Networking with Java 1:Networking with Java 1:The Client SideThe Client Side

Introduction to NetworkingIntroduction to Networking

HUJI-CSHUJI-CSDBI 2008DBI 2008 3

ProtocolsProtocols

HiTCP connectionreplyGot the

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

2:00<file>

time

Client

Server

Hi TCP connection request


Networking as LayersNetworking as Layers

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

• Why don’t we specify the port in a Web browser?


Known PortsKnown Ports

• Some known ports are

– 20, 21: FTP

– 22: SSH

– 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

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();

Needed for forwarding for

example


TimeoutTimeout

• You can set timeout values to blocking method read() of Socket

• Use the method socket.setSoTimeout(milliseconds)

• If timeout is reached before the method returns, java.net.SocketTimeoutException is thrown

Read more about Socket Class

http://java.sun.com/j2se/1.5.0/docs/api/java/net/Socket.html

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 represented?

• Headers of a HTTP message must be in US-ASCII format (1 byte per character)


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

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")); }

• Why did we (inefficiently) read one byte at a time?• Is there any way to avoid this inefficiency?


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)

– saves “slow start” time

• How can the client know when one response ends and a new one begins?

• To avoid persistency, require explicitly by the header Connection: close

Parsing URLsParsing URLs


Query

ProtocolHost

NamePort

Number

File Nam

e

Reference

Working with URLsWorking with URLs

• URL (Uniform/Universal Resource Locator): a reference (address) to a resource on the Internet

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

http://www.google.com/search?hl=en&q=dbi+huji&btnG=Search


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()

Read more about URL Class

http://java.sun.com/j2se/1.5.0/docs/api/java/net/URL.html




URLEncoderURLEncoder

• Contains a utility method encode for converting a string into an encoded format (used in URLs, e.g. for searches)

• To convert a string, each char 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

Read more about URLEncoder Class

http://java.sun.com/j2se/1.5.0/docs/api/java/net/URLEncoder.html

Class URLConnectionClass URLConnection

High-Level Networking


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

Read more about URLConnection Class


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

• You can control requests by its methods

– setRequestMethod, setFollowRedirects, setRequestProperty, ...

Read more about HttpURLConnection Class

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Networking with Java 1: The Client Side. Introduction to Networking.

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