1. Socket Programming with UDP

2. What is UDP? User Datagram Protocol UDP is
“Unreliable” Datagram Protocol?
UDP is
unreliable
no guarantee on delivery sequence
no connection (less overhead)‏

3. When would we use UDP? Quick short bursts of communication
Connection overhead would be inefficient
e.g. DNS queries
Real time applications
No congestion control, sending rate unregulated
e.g. audio/video streaming, loss of some packets is not critical
No connection state needed
More connections can be handled at endpoint

4. UDP – Packet Format 2 bytes for source/destination ports (0-65536)‏
MAC Header
IP Header
UDP Header
Data
Data Packet
32 bits
Source Port
Dest Port
Length
Checksum
Data
2 bytes for source/destination ports ( )‏
Length (bytes) = header + data
Checksum of header and data
Data = variable length
multiple of 4 bytes, padding done by kernel

5. Socket programming with UDP
UDP: no “connection” between client and server
no handshaking
sender explicitly attaches IP address and port of destination to each packet
server must extract IP address, port of sender from received packet
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
CPSC Application Layer

6. UDP Sockets in C
CPSC Application Layer

7. Socket programming with UDP
Socket creation with SOCK_DGRAM
int s = socket(PF_INET, SOCK_DGRAM,0);
No explicit connection
No listen()‏
No accept()‏

8. Client-Server Flow Client Server Connectionless!
No listen(), accept(), or connect()* calls
socket()
socket()
sendto()
bind()
recvfrom()
wait for next
packet from
arbitrary client
sendto()
recvfrom()
close()
close()

9. UDP socket methods - sendto()‏
ssize_t sendto( int sockfd,void *buff,
size_t nbytes, int flags,
const struct sockaddr* to,
socklen_t addrlen);
sendto() instead of send(); must specify client to send packet to
Returns bytes sent to the kernel network stack, not what was sent to the client!
Possible to send 0 bytes of data!
No error message to indicate non-receipt of data
Possible errors:
EBADF, ENOTSOCK: bad socket descriptor
EFAULT: bad buffer address
EMSGSIZE: message too large
ENOBUFS: system buffers are full

10. UDP socket methods - recvfrom()‏
ssize_t recvfrom( int sockfd, void *buff,
size_t nbytes,int flags,
struct sockaddr* from,
socklen_t *fromaddrlen);
recvfrom() instead of recv(); can receive from anyone
Buffer must be large enough, else data is lost forever
recvfrom() is blocking, returns only on packet receipt
Same errors as sendto, with addition of
EINTR: System call interrupted by signal
Useful if recvfrom() should timeout after a while

11. Issues with UDP packets
Datagram size
Should not be too large, else IP fragmentation occurs
increases the chance of lost packets
Connected UDP sockets
Calling connect on a socket specifies the destination
Can use send()/recv() instead of sendto()/recvfrom
If server calls connect, will not receive packets from anyone else
ICMP error messages
If server is not listening on port, it returns and ICMP error message to the sending host
sendto() may not return this error until next call to socket!
If ICMP error received, sendto() returns ECONNREFUSED

12. UDP Sockets in Java
CPSC Application Layer

13. Example: Java client (UDP)
process
Input: receives packet (recall thatTCP received “byte stream”)
Output: sends packet (recall
that TCP sent “byte stream”)
client UDP socket
CPSC Application Layer

14. 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
CPSC Application Layer

15. 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
CPSC Application Layer

16. 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
CPSC Application Layer

17. 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
CPSC Application Layer