Application Layer

1
Application Layer

• http
• Socket programming
• Web server

2
Overview

• The world wide web http
• Socket programming with TCP
• Socket programming with UDP
• Building a simple web server in JAVA
• Reference
• Kuroses book 2.2, 2.6 - 2.8
• Kuroses CS653 class notes at UMASS

3
Application Layer Protocols

• Defines how an application's processes, running
  on different end systems, pass messages to each
  other.
• types of messages exchanged
• fields in the message, and meaning of the
  information in the fields
• when and how a process sends messages and
  responds to messages
• Examples http, telnet, ftp, internet telephony,
  video conference, audio/video on demand, and much
  more

4
Application Structure

• Client/Server Model
• Server offers service via well-defined interface.
  Client requests service.
• Client how to locate/request service
• Server whether/how to provide service
• example Web client (browser), server (web
  server)
• Peer/Peer Model
• Symmetric, both require transport of
  request/replies, sharing of data
• example chat, video conference

5
HTTP Hypertext Transfer Protocol

• Web implemented using client/server model
• client (browser) requests and displays received
  web page
• server receives requests, responds with requested
  html documents and other objects, such as image,
  JAVA applet, audio clip, etc.
• http protocol defines how web clients request web
  pages from servers and how servers transfer web
  pages to clients. (HTTP/1.1 is defined in
  RFC2068)
• http is stateless, transaction-oriented protocol
• client contacts server at port 80, then makes a
  request
• reply from server to client
• connection closed

6
HTTP Request Message

• Sent from client to server

request line (method, identifier, version) header
(additional info) body
7
Example http Request Message

• Sample GET command plus header
• GET /.html/main.html HTTP/1.0
• Connection Keep-Alive
• User-Agent Mozilla/4.61 en (Win95 I)
• Host hill.cs.ucr.edu8080
• Accept image/gif, image/x-xbitmap, image/jpeg,
  image/pjpeg, image/png, /
• Accept-Encoding gzip
• Accept-Language en
• Accept-Charset iso-8859-1,,utf-8
• Extension Security/Remote-Passphrase

8
HTTP Reply Message

• Sent from server to client
• Selected http reply codes

status line (status code, status text) header
(additional info) body
9
Example http Reply Message

• HTTP/1.1 200 OK
• Date Mon, 11 Oct 1999 185517 GMT
• Server Apache/1.2.6 Red Hat
• Last-Modified Sun, 10 Oct 1999 224513 GMT
• Content-Length 3713
• Accept-Ranges bytes
• Keep-Alive timeout15, max100
• Connection Keep-Alive
• Content-Type text/html
• a blank line
• html text of the web page

10
Authentication and Cookies

• HTTP server is stateless
• Its often desirable for a web site to identify
  users
• Authentication user name password. For example
• web email - mail.yahoo.com
• home banking - https//www.rcfcu.org/
• confidential info - UCR PAWS
• Cookies A cookie is a small amount of data that
  is sent from a web server to your browser.
  Cookies are generally used to help web sites
  determine the state of a client. For example
• my.yahoo.com - your Yahoo! ID
• www.amazon.com - your shopping cart

11
Authentication and Cookies

• After entering user name and password, while the
  browser remains open, the user name and password
  are cached. So the user is not prompted for each
  object it requests.
• HTTP performs a rather weak form of
  authentication, one that would not be difficult
  to break. Encryption, secure server.
• A cookie cannot be used to extract data from your
  computer.
• Cookies are never "executed" as code, so they
  can't contain programs or viruses.

12
Proxy Server

• Also called web cache. The proxy server has it
  own disk storage, and keeps copies of recently
  requested objects. It can substantially reduce
  network traffic.
• Users configure their browsers
• Preferences - Advanced - Proxies

13
Socket

• A socket is the interface between the application
  layer and the transport layer within a host. It
  is also referred to as the API between the
  application and the network.
• A processs socket can be thought of as the door
  a process sends messages into, and receives
  message from, the network through its socket.

14
TCP and UDP

• To run the application over TCP or over UDP ?
• TCP - Transmission Control Protocol.
• Connection-oriented service. Handshaking.
• Reliable transport service. No error, kept in
  proper order.
• Congestion control.
• No minimum transmission rate guarantee.
• No delay guarantee.
• UDP - User Datagram Protocol.
• Connectionless. Datagram, no handshake.
• Unreliable data transfer. Packet loss, out of
  order.
• No congestion control
• Good for real-time application (internet
  telephony).

15
Socket Programming with TCP

• Introduced in 1981 BSD 4.1 Unix
• A host-local, application created/owned,
  OS-controlled interface. An application process
  can both send and receive messages to/from
  another process.
• Two sockets on separate hosts connected by OS
  socket management routines. Application only see
  local socket.
• Sockets explicitly created, used, released by
  applications.
• Each socket has separate send/receive buffers,
  port id, parameters (get() and set()).
• Socket operations implemented as system calls.

16
Sockets Conceptual View
17
Socket, Client, Server

• A server runs on a specific computer and has a
  socket that is bound to a specific port number.
  The server just waits, listening to the socket
  for a client to make a connection request.

18
Socket, Client, Server

• The client knows the hostname and port number of
  the server process.
• Upon acceptance of a connection request, the
  server gets a new socket bound to a different
  port so that it can continue to listen to the
  original socket for connection requests.

19
An Example in Java

• Client
• Sends a message to server
• Reads the reply message from server and print it
• Server
• Wait for any message from client
• Once gets a message, converts the message to
  uppercase
• return it

20
Client Code

• import java.io.
• import java.net.
• class TCPClient
• public static void main(String argv) throws
  Exception
• Socket s new Socket(hill.ucr.edu, 3456)
• DataOutputStream snd
• new DataOutputStream(s.getOutputStream())
• BufferedReader rcv new BufferedReader(
• new InputStreamReader(s.getInputStream()))
• String msg Hello, World!
• snd.writeBytes(msg \n) // send message
• msg rcv.readLine() // receive
  message
• Sysmem.out.println(msg)
• s.close()

21
Server Code

• import java.io.
• import java.net.
• class TCPServer
• public static void main(String argv) throws
  Exception
• ServerSocket s new ServerSocket(3456) //
  welcome
• while(true)
• Socket c s.accept() // connection
• DataOutputStream snd
• new DataOutputStream(c.getOutputStream())
• BufferedReader rcv new BufferedReader(
• new InputStreamReader(c.getInputStream()))
• String msg rcv.readLine() //
  receive
• snd.writeBytes(msg.toUpperCase()\n) //
  send

22
IP Address and Port Number

• When sending a message, to identify the receiving
  process, one must specify two pieces of info
• the name or address (IP address) of the host
• an id (port number) that specifies the identity
  of the receiving process on the destination host
• Name - hill.ucr.edu
• IP address - 32 bit, globally unique.
• gethostbyname(hostname), 138.23.169.9,
  hill.ucr.edu
• Port Number - Popular application-layer protocols
  have been assigned specific port numbers. Avoid
  using those numbers. Web server - 80, POP3 - 110.

23
Port Numbers
24
Byte Ordering

• Different computers may store bytes of integer in
  different order in memory - big endian/little
  endian
• Network byte order is big-endian
• Integers should be explicitly converted to/from
  network byte order

25
UNIX Byte Order Conversion
26
Socket Programming with UDP

• UDP is connectionless. No handshaking.
• When a process wants to send a message to another
  process, it must attach the receiving processs
  IP address and port number. A new object -
  packet.
• Unreliable transport. UDP provides no guarantees
  that a packet will reach its destination.
• Rewrite the previous client/server example using
  UDP.

27
Client Code

• import java.io.
• import java.net.
• class UDPClient
• public static void main(String argv) throws
  Exception
• DatagramSocket s new DatagramSocket()
• InetAddress IP InetAddress.getByName(hill.u
  cr.edu)
• byte buf new byte1024
• String msg Hello, World!
• buf msg.getBytes()
• DatagramPacket sp
• new DatagramPacket(buf, buf.length, IP, 3456)
• s.send(sp) // send packet
• DatagramPacket rp new DatagramPacket(buf,
  buf.length)
• s.receive(rp) // receive packet
• Sysmem.out.println(new String(rp.getData()))
• s.close()

28
Server Code

• import java.io.
• import java.net.
• class UDPServer
• public static void main(String argv) throws
  Exception
• DatagramSocket s new DatagramSocket(3456)
• byte buf new byte1024
• while( true )
• DatagramPacket rp new DatagramPacket(buf,
  buf.length)
• s.receive(rp) // receive packet
• String msg new String(rp.getData())
• InetAddress IP rp.getAddress()
• int port rp.getPort()
• buf msg.toUpperCase().getBytes()
• DatagramPacket sp
• new DatagramPacket(buf, buf.length, IP,
  port)
• s.send(sp) // send packet

29
Building a Simple Web Server

• Lets combine the HTTP and socket programming in
  JAVA, and build a simple web server that
• accepts and parses the http request
• gets the requested file
• sends the response and the requested file
• Assume that the request file exists and the file
  name does not include path

30
Web Server (1)

• import java.io.
• import java.net.
• import java.util.
• class WebServer
• public static void main(String argv) throws
  Exceptions
• ServerSocket s new ServerSocket(3456) //
  welcome port
• while( true )
• Socket c s.accept() // connection port
• BufferedReader rcv new BufferedReader(
• new InputStreamReader(c.getInputStream()))
• DataOutputStream snd
• new DataOutputStream(c.getOutputStream())
• String rqst rcv.readLine() // http request
  message

31
Web Server (2)

• StringTokenizer tok new StringTokenizer(rqst)
  
• if(tok.nextToken().equals(GET)) // extract
  GET line
• String filen tok.nextToken() // get
  file name
• // Assume file exists and in current
  directory
• File file new File(filen)
• int len (int) file.length() // get num
  of bytes
• FileInputStream r new FileInputStream(filen
  )
• byte buf new bytelen
• r.read(buf)

32
Web Server (3)

• // response message
• snd.writeBytes("HTTP/1.0 200 Document
  Follows\r\n")
• if(filen.endsWith(".jpg"))
• snd.writeBytes("Content-Type
  image/jpeg\r\n")
• if (filen.endsWith(".gif"))
• snd.writeBytes("Content-Type
  image/gif\r\n")
• snd.writeBytes("Content-Length " len
  "\r\n")
• // blank line
• snd.writeBytes("\r\n")
• // requested file
• snd.write(buf, 0, len)

33
Web Server Comments

• Why \r\n? Defined end of line format in
  RFC2068.
• How to play it ?
• Server on hill.ucr.edu, port number 3456. In
  current directory, place an html file, e.g.,
  index.html
• javac WebServer.java
• java WebServer
• Client in your browser, open the following page
• http//hill.ucr.edu3456/index.html

34
Links

• Socket programming in JAVA is very
  straightforward. Here are some references
• JAVA Socket Programming Tutorial at Sun
• http//java.sun.com/docs/books/tutorial/networking
  /sockets/index.html
• E-book Developing Professional Java Applets by
  Hopson
• http//www.pbs.mcp.com/ebooks/1575210835/ch9.htm
• For Unix socket programming in C, see
• BSD Socket Interface at UW Madison
• http//www.cs.wisc.edu/cs640-1/cs640.htmlReferen
  ces