Networking with Java Introduction to Networking Protocols Hi

Networking with Java

Introduction to Networking

Protocols Hi TCP connection request Hi TCP connection reply Got the time? GET http: //www. cs. huji. ac. il/~dbi 2: 00 <file> time

Internet Architecture Model Application (HTTP, FTP) DATA Transport (TCP, UDP) Network (IP) Link (LINK) HEADER DATA HEADER DATA

TCP (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) • 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.

Ports • 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) • 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 Ports Client Application • Some known ports are - 20, 21: FTP mail client - 23: TELNET web browser - 25: SMTP - 110: POP 3 - 80: HTTP - 119: NNTP 21 23 25 110 80 119

Sockets • 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) • A socket stores the IP and port number of the other endpoint 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 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 Interaction

Client-Server Interaction

Client-Server Interaction

Client-Server Interaction

Client-Server Interaction

Java Sockets Low-Level Networking

Java Sockets • Java wraps OS sockets (over TCP) by the objects of class java. net. Socket • new Socket(String remote. Host, int remote. Port) creates a TCP socket and connects it to the remote host on the remote port (hand shake) • Write and read using streams: - Input. Stream get. Input. Stream() - Output. Stream get. Output. Stream()

A Socket Example import java. net. *; import java. io. *; public class Simple. Socket { public static void main(String[] args) throws IOException { . . . next slide. . . } }

Socket socket = new Socket("www. cs. huji. ac. il", 80); Input. Stream istream = socket. get. Input. Stream(); Output. Stream ostream = socket. get. Output. Stream(); String request = "GET /~dbi/admin. html HTTP/1. 1rn" + "Host: www. cs. huji. ac. ilrn" + "Connection: closern"; ostream. write(request. get. Bytes()); byte[] response = new byte[4096]; int bytes. Read = -1; while ((bytes. Read = istream. read(response)) >= 0) { System. out. write(response, 0, bytes. Read); } socket. close();

Java Server. Socket • Server. Socket represents a socket that listens and waits for requests from clients • Construction: - new Server. Socket(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 Echo. Server { public static void main(String[] args) throws IOException { Server. Socket server. Socket = new Server. Socket(8000); Socket socket = null; while (true) { try {. . . next slide. . . } catch (IOException exp) {. . . } finally { try {if (!socket. is. Closed()) socket. close(); } catch (IOException e) {} } }}}

socket = server. Socket. accept(); String client. Name = socket. get. Inet. Address(). get. Host. Name(); Buffered. Reader reader = new Buffered. Reader( new Input. Stream. Reader(socket. get. Input. Stream())); Print. Stream writer = new Print. Stream(socket. get. Output. Stream()); writer. println("Hello " + client. Name + "!"); writer. flush(); String line. Read = null; while ((line. Read = reader. read. Line()) != null) { writer. println("You wrote: " + line. Read); writer. flush(); }

Accepting 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

Timeout • You can set timeout values to these blocking methods: - read() of Socket - accept() of Server. Socket • Use the method set. So. Timeout(milliseconds) • If timeout is reached before the method returns, java. net. Socket. Timeout. Exception is thrown

Java Sockets and HTTP

HTTP Message Structure • A HTTP message has the following structure: Request/Status-Line rn Header 1: value 1 rn Header 2: value 2 rn. . . Header. N: value. N rn Message-Body

Reading 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 Example

Parsing 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 read. Line() of Buffered. Reader, since the body should be later read as binary bytes • Hence, we convert bytes to characters

Example: Extracting the Headers Socket socket = new Socket(argv[0], 80); Input. Stream istream = socket. get. Input. Stream(); Output. Stream ostream = socket. get. Output. Stream(); String request = "GET / HTTP/1. 1rn" + "Host: " + argv[0] + "rn" + "Connection: closern"; ostream. write(request. get. Bytes()); String. Buffer headers = new String. Buffer(); int byte. Read = 0; while ( !end. Of. Headers(headers) && (byte. Read = istream. read()) >= 0) { headers. append((char) byte. Read); } System. out. print(headers); socket. close();

Example: Extracting the Headers (cont) public static boolean end. Of. Headers(String. Buffer headers) { int last. Index = headers. length() - 1; if (last. Index < 3 || headers. char. At(last. Index) != 'n') return false; return ( headers. substring(last. Index - 3, last. Index + 1). equals("rn")); }

Persistent 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 URLConnection High-Level Networking

Working 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 Protocol Host Name Port Number File Name Reference

The Class URL • The class URL is used for parsing URLs • Constructing URLs: - URL w 3 c 1 = new URL("http: //www. w 3. org/TR/"); - URL w 3 c 2 = new URL("http", "www. w 3. org", 80, "TR/"); - URL w 3 c 3 = new URL(w 3 c 2, "xhtml 1/"); • If the string is not an absolute URL, then it is considered relative to the URL

Parsing URLs • The following methods of URL can be used for parsing URLs get. Protocol(), get. Host(), get. Port(), get. Path(), get. File(), get. Query(), get. Ref(), get. Query()

The class URLConnection • To establish the actual resource, we can use the object URLConnection obtained by url. open. Connection() • If the protocol of the URL is HTTP, the returned object is of class Http. URLConnection • This class encapsulates all socket management and HTTP directions required to obtain the resource

public class Content. Extractor { public static void main(String[] argv) throws Exception { URL url = new URL(argv[0]); System. out. println("Host: " + url. get. Host()); System. out. println("Protocol: " + url. get. Protocol()); System. out. println("----"); URLConnection con = url. open. Connection(); Input. Stream stream = con. get. Input. Stream(); byte[] data = new byte[4096]; int bytes. Read = 0; while((bytes. Read=stream. read(data))>=0) { System. out. write(data, 0, bytes. Read); }}}

About 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 get. Date()

About Http. URLConnection • The Http. URLConnection class encapsulates all HTTP transaction over sockets, e. g. , - Content decoding - Redirection - Proxy indirection • However, you can control requests by its methods - set. Request. Method, set. Follow. Redirects, set. Request. Property, . . .

Sending POST Requests • In order to send POST requests with Http. URLConnection, you have to do the following: - Enable connection output: con. set. Do. Output(true); - Get the output stream: con. get. Output. Stream() • 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. open. Connection(); connection. set. Do. Output(true); String query = "q=" + URLEncoder. encode(args[0], "UTF-8"); Output. Stream out = connection. get. Output. Stream(); out. write(query. get. Bytes()); out. close(); Input. Stream in = connection. get. Input. Stream(); int bytes. Read = -1; byte[] response = new byte[4096]; while ((bytes. Read = in. read(response)) >= 0) System. out. write(response, 0, bytes. Read); in. close();

Sent Headers java Search. Walla "Java networking" POST / HTTP/1. 1 User-Agent: Java/1. 5. 0_01 Host: find. walla. co. il Accept: text/html, image/gif, image/jpeg, *; q=. 2, */*; q=. 2 Connection: keep-alive Content-type: application/x-www-form-urlencoded Content-Length: 18 q=Java+networking

URLEncoder • 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 Proxy • For reading a URL using a proxy, we run java with the environment variables for http. proxy. Host and http. proxy. Port set properly: java -Dproxy. Host=wwwproxy. huji. ac. il –Dproxy. Port=8080. . . • Another option is to set the environment variables in the program itself System. get. Properties(). put( "proxy. Host", "wwwproxy. huji. ac. il" ); System. get. Properties(). put( "proxy. Port", "8080" );

Not Covered: UDP Connections • The URL, URLConnection, Socket and Server. Socket classes all use the TCP protocol to communicate over the network • Java programs can also use UDP protocol for communication • For that, use Datagram. Packet, Datagram. Socket, and Multicast. Socket classes