1. CSE524: Lecture 3
Application layer

2. Administrative CSE524 e-mail list Homework assignments
Homework #1
Access to machine resources for programming project and assignments
Get yourself an account
Use class laboratory

3. Where we're at.... Internet architecture and history
Internet in a nutshell
Packet trace example of protocols in action
Rest of the course
Application, Transport, Network, Data-link, Physical layers

4. This class Application layer Application layer overview and functions
Programming application protocols
BSD Socket API
Java Socket API
Java high-level networking API
Specific application protocols
HTTP
FTP
DNS
SMTP

5. Application layer overview
Application: communicating sets of distributed processes
e.g., , Web, P2P file sharing, instant messaging
running in end systems (hosts)
exchange messages to implement application
Application-layer protocols
one “piece” of an app
define messages exchanged by apps and actions taken
use communication services provided by lower layer protocols (TCP, UDP)
application
transport
network
data link
physical

6. Application layer functions
Applications
Implement desired functionality within application protocols when no underlying network service provides support
Mail, Web, News, P2P, etc….
Other functions
Security (S/MIME, PGP, S-HTTP)
Delivery semantics (multicast overlays, anycast)
Reliable data transfer (reliable multicast, reliable UDP)
Quality of service (QoS overlays, scheduling)
Congestion control (Non-TCP applications)
Flow control (Non-TCP applications)
Naming (DNS, URLs)
Routing (overlays)
Functionality that is common rolled into libraries and “middleware”

7. Application protocols define
How to implement functions
Types of messages exchanged, eg, request & response messages
Syntax of message types: what fields in messages & how fields are delineated
Semantics of the fields, ie, meaning of information in fields
Rules for when and how processes send & respond to messages
Public-domain protocols:
defined in RFCs
allows for interoperability
eg, HTTP, SMTP
Proprietary protocols:
eg, KaZaA

8. Client-server paradigm
initiates contact with server (“speaks first”)
typically requests service from server,
Web: client implemented in browser; in mail reader
Typical network app has two pieces: client and server
application
transport
network
data link
physical
reply
request
Server:
provides requested service to client
e.g., Web server sends requested Web page, mail server delivers

9. Processes communicating across network
process sends/receives messages to/from its socket
socket analogous to door
sending process shoves message out door
sending process asssumes transport infrastructure on other side of door which brings message to socket at receiving process
process
TCP with
buffers,
variables
socket
host or
server
process
TCP with
buffers,
variables
socket
host or
server
controlled by
app developer
Internet
controlled
by OS
API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later)

10. Addressing processes:
For a process to receive messages, it must have an identifier
Every host has a unique 32-bit IP address
Q: does the IP address of the host on which the process runs suffice for identifying the process?
Answer: No, many processes can be running on same host
Identifier includes both the IP address and port numbers associated with the process on the host.
Example port numbers:
HTTP server: 80
Mail server: 25

11. What transport service does an app need?
Data loss
some apps (e.g., audio) can tolerate some loss
other apps (e.g., file transfer, telnet) require 100% reliable data transfer
Bandwidth
some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective”
other apps (“elastic apps”) make use of whatever bandwidth they get
Timing
some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”

12. Transport service requirements of common apps
Application
file transfer
Web documents
real-time audio/video
stored audio/video
interactive games
instant messaging
Time Sensitive no
yes, 100’s msec
yes, few secs
yes and no
Data loss
no loss
loss-tolerant
Bandwidth
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up

13. Recall Internet transport protocol services
TCP service:
connection-oriented: setup required between client and server processes
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not provide: timing, minimum bandwidth guarantees
UDP service:
unreliable data transfer between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

14. Internet apps: application, transport protocols
layer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
proprietary
(e.g. RealNetworks)
(e.g., Dialpad)
Underlying
transport protocol
TCP
TCP or UDP
typically UDP
Application
remote terminal access
Web
file transfer
streaming multimedia
and games
Internet telephony

15. Programming application protocols
How do I program all of this?
Layering allows programmers to write programs without understanding underlying functions of TCP/IP
You will be evaluating this for yourselves...
API to transport layer
BSD socket API
java.net API

16. Programming application protocols
How can applications programmatically access TCP/UDP/IP?
Many possible interfaces
Too many to teach adequately in detail
Socket APIs
BSD socket API
Java socket API
Other APIs
Client-side Java URLconnections
Server-side Java servlets
RPC, CORBA, Java RMI (not covered)

17. Socket programming socket Socket API
a host-local, application-created/owned,
OS-controlled interface (a “door”) into which
application process can both send and
receive messages to/from another (remote or
local) application process
socket
Socket API
introduced in BSD4.1 UNIX, 1981
client/server paradigm
socket API supports three main usages
unreliable datagram (UDP)
reliable, byte stream (TCP)
raw IP datagrams (IP)

18. Socket-programming using TCP
Socket: a door between application process and end-end- transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one process to another
controlled by
application
developer
controlled by
application
developer
process
TCP with
buffers,
variables
socket
process
TCP with
buffers,
variables
socket
controlled by
operating
system
controlled by
operating
system
internet
host or
server
host or
server

19. Socket programming with TCP
Client must contact server
server process must first be running
server must have created a “listening” socket that welcomes client’s initial contact
Client contacts server by:
creating client-local TCP socket
specifying server IP address and port number of server process
When client creates socket: client TCP establishes connection to server TCP
When contacted by client, server TCP creates new socket for server process to communicate with client
allows server to talk with multiple clients (using multiple sockets)
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
application viewpoint

20. Sockets in action *,SIP:80 CIP:1099,SIP:80 *,SIP:80 CIP:1100,SIP:80

21. BSD sockets in C/Unix Socket API (socket.h)
socket(): create unnamed socket (data structure)
UDP (SOCK_DGRAM), TCP (SOCK_STREAM)
IP (SOCK_RAW)
bind(): name socket (bind local address to socket)
listen(): enable socket to accept connections
accept(): get connect() request from listen queue, allocate file descriptor for new socket
connect(): initiate connection on a socket (TCP handshake)
send(), sendto(), sendmsg(), writev(), write(): send data
recv(), recvfrom(), recvmsg(), readv(), read(): receive data
setsockopt(), getsockopt(): set socket options (such as buffer sizes, flag fields)
close(), shutdown(): teardown connection

22. BSD sockets in action UDP example TCP example client server client
bind()
bind()
socket()
bind()
connect()
listen()
sendto()
accept()
recvfrom()
write()
read()
sendto()
write()
recvfrom()
read()

23. BSD example For reference only...

24. Java network programming
Java network applications
java.net package
System-dependent
implementations

25. Java installation on church/state
J2SE
javac
java compiler
java
java interpreter

26. java.net classes Low-level networking classes (Sockets and Packets)
High-level URL networking classes
java.lang.Object
java.net.Socket
java.net.ServerSocket
java.net.DatagramSocket
java.net.MulticastSocket
java.net.DatagramPacket
java.net.URL
java.net.URLConnection
java.net.HttpURLConnection
java.net.URLencoder
java.net.InetAddress

27. java.net.Socket Constructors Some methods Socket(InetAddress, int)
Socket(String, int)
Socket(InetAddress, int, InetAddress, int)
Some methods
getInputStream()
getOutputStream
getInetAddress()
getPort()
getLocalAddress()
getLocalPort()
get/set individual socket options

28. java.net.ServerSocket Constructors Some methods ServerSocket(int)
ServerSocket(int, int) // backlog specified
ServerSocket(int, int, InetAddress) // local address and backlog specified
Some methods
accept()
getInetAddress()
getLocalPort()

29. Socket programming with TCP
Example client-server app:
client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream)
server reads line from socket
server converts line to uppercase, sends back to client
client reads, prints modified line from socket (inFromServer stream)
Input stream: sequence of bytes into process
Output stream: sequence of bytes out of process
outToServer
iinFromServer
inFromUser
client socket

30. Java client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
close
connectionSocket
read reply from
clientSocket
TCP
connection setup
create socket,
connect to hostid, port=x
clientSocket =
Socket()
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
send request using
clientSocket
read request from
connectionSocket
write reply to

31. Example: Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception {
String sentence;
String modifiedSentence;
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket

32. Example: Java client (TCP), cont.
Create
input stream
attached to socket
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close(); }
Send line
to server
Read line
from server

33. Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception {
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached
to socket

34. Example: Java server (TCP), cont
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
outToClient.writeBytes(capitalizedSentence); }
Create output
stream, attached
to socket
Read in line
from socket
Write out line
to socket
End of while loop,
loop back and wait for
another client connection

35. Socket programming with UDP
UDP: no “connection” between client and server
no handshaking
sender explicitly attaches IP address and port of destination
server must extract IP address, port of sender from received datagram
UDP: transmitted data may be received out of order, or lost
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server

36. java.net.DatagramSocket
Constructors
DatagramSocket()
DatagramSocket(int) // bind to specific port
DatagramSocket(int, InetAddress) // specify local address
Some methods
getLocalAddress()
getLocalPort()
receive(DatagramPacket)
send(DatagramPacket)
get/set individual socket options

37. java.net.DatagramPacket
Constructors
DatagramPacket(byte[], int) // receiving packets
DatagramPacket(byte[], int, InetAddress, int) // sending packets
Some methods
getAddress() // remote address
getPort() // remote port
getLength() // get packet length
getData() // return data received or to be sent
setAddress(InetAddress) // set remote address
setData(byte[]) // set packet data
setLength(int) // set packet length
setPort(int) // set remote port

38. Client/server socket interaction: UDP
Server (running on hostid)
create socket,
clientSocket =
DatagramSocket()
Client
Create, address (hostid, port=x,
send datagram request
using clientSocket
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
close
clientSocket
read reply from
clientSocket
write reply to
serverSocket
specifying client
host address,
port umber

39. Example: Java client (UDP)
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception {
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS

40. Example: Java client (UDP), cont.
Create datagram with data-to-send,
length, IP addr, port
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close(); }
Send datagram
to server
Read datagram
from server

41. Example: Java server (UDP)
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception {
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
Create
datagram socket
at port 9876
Create space for
received datagram
Receive
datagram

42. Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
serverSocket.send(sendPacket); }
Get IP addr
port #, of
sender
Create datagram
to send to client
Write out
datagram
to socket
End of while loop,
loop back and wait for
another datagram

43. High-level Java networking classes
Socket/Packet
Low level building blocks
Must implement all application-level logic
Many protocols based on URLs and/or tunneled in HTTP
Program at a higher-level to hide underlying protocol details
Do not re-implement HTTP, URL parsing, MIME handling for each application

44. High-level client-side Java networking classes
java.net.URL
Represent a URL object
java.net.URLConnection
Represent a connection to a URL which can be read and written from
java.net.HttpURLConnection
Subclass of URLConnection for URLs
Example

45. High-level server-side Java networking classes
Servlets
Dynamically generate content
Implement common protocol header logic
Example http servlets
Cookies
Content-type
Content-length
Servlet classes
javax.servlet.Servlet javax.servlet.HttpServlet
init()
service()
destroy()
javax.servlet.ServletRequest javax.servlet.ServletResponse
javax.servlet.HttpServletRequest javax.servlet.HttpServletResponse