Socket Programming: exercises

SC250
Computer Networking I
Socket Programming:
Exercises
Prof. Matthias Grossglauser / Dr. Jörg Widmer
LCA/I&C
http://lcawww.epfl.ch
1
Exercise 1
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Assume you have our Upcase-client on one host
and the server on another host.
a) Suppose you run the TCP client without starting the
server. What happens exactly and why?
b) Suppose you run a UDP client without starting a UDP
server. What is different from the case of TCP and
why?
c) What happens with TCP and UDP if you start the server
after the client but before typing any input?
d) What happens if you connect the TCP client to a web
server running on port 80 instead of the Upcase server?
2
Solution to Exercise 1
a) If you run a TCP client first, the client will attempt to
make a TCP connection with a non-existent server
process. Creating the client socket will fail.
b) With UDP, the client doesn't establish a connection
with the server. Creating a socket works but sending a
datagram will fail.
c) With TCP, again creating the socket will fail. Since no
connection is established with UDP, everything should
work fine if you first run the client, then run the server,
and then type some input into the keyboard.
d) The client will establish a connection. If the user types
anything that is not HTTP, the server will respond with
an error message. If the user types GET / HTTP/1.0
the server delivers the corresponding web page.
3
Exercise 2
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TCP connection requests (SYN) are buffered at the server
socket. The server then performs the three-way
handshake. An accept() returns the next connection
request where the three-way handshake is completed.
Requests with and without completed three-way
handshake end up in the same buffer.
a)
What happens to a new request if the buffer is full?
b) What happens if a client sends a large number of bogus
SYNs to a server without ever completing the threeway handshake (SYN flooding)?
4
Solution to Exercise 2
a) The request cannot be served and the SYN packet is
dropped.
b) The buffer at the server is full of SYN packets where
the three-way handshake will never complete. Other
clients cannot access the server since the buffer for
connection requests is full.
A better implementation would be to have separate
buffers for unfinished connection requests and
connection requests with completed three-way
handshake.
5
Exercise 3
a) In the transport header of a packet you have the
source port and the destination port. The receiver
needs the destination port for demultiplexing but what
is the source port used for?
b) Assume a server has a server socket open on port X for
incoming TCP connections. What happens if on the
same server you open a UDP socket to receive
datagrams also using number X? Are there conflicts?
c) Is it possible to have multiple threads to handle clients
simultaneously with a UDP server? If yes, how? If no,
why not?
d) What about using multiple threads and multiple
sockets for receiving as with TCP?
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Solution to Exercise 3
a) The TCP receiver needs the source port to know where
to send the SYN/ACK to. For UDP, the source port is not
as important as for TCP but is usually used to
determine where to send responses to.
b) There are no conflicts. The OS has 2^16 ports for TCP
and a separate 2^16 ports for UDP. Remember, a
connection is uniquely identified by source IP and port,
destination IP and port, AND protocol type.
c) This is no problem. Create a new thread and hand over
the clients datagram. The thread can then process the
request in the background and then send the response.
d) It is possible, but there is no such thing as a server
socket for UDP. Thus, this has to be done "by hand".
Create a new socket upon first client contact and send
the new port number back to the client so it can send
subsequent datagrams there. This is rarely done!
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Exercise 4
Rewrite the UDP Java server using multiple threads.
(Simply annotate in the source code with what
changes have to be made.)

What do you put in a separate thread? Why?

Should you use a new socket for each client?
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Exercise 4: UDPServerMultiThreads
There are several correct ways to do this. This is one
example:
import java.io.*;
import java.net.*;
class UDPServerMultiThreads {
public static void main(String args[]) throws Exception {
DatagramSocket serverSocket = new
DatagramSocket(9876);
System.out.println("I am waiting on port 9876");
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Exercise 4: UDPServerMultiThreads
while(true) {
byte[] receiveData = new byte[1024];
DatagramPacket receivePacket = new
DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
ServerThread serverT = new ServerThread(
receivePacket);
serverT.startSocket();
}
}
}
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Exercise 4: ServerThread
import java.io.*;
import java.net.*;
public class ServerThread extends Thread {
private DatagramPacket receivePacket;
public ServerThread(DatagramPacket receivePacket)
throws SocketException {
this.receivePacket = receivePacket;
}
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Exercise 4: ServerThread
public void startSocket() throws Exception {
byte[] sendData = new byte[1024];
DatagramSocket responseSocket = new DatagramSocket();
String clientSentence = new String
(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
System.out.println(port);
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Exercise 4: ServerThread
String capitalizedSentence = clientSentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket = new DatagramPacket(
sendData, sendData.length, IPAddress, port);
responseSocket.send(sendPacket);
responseSocket.close();
}
}
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Exercise 5: using UDP vs TCP

Consider a file transfer application that transmits data
at a steady rate (for example, the sender generates a
N-bit unit of data every k time units, where k is small
and fixed). Also, when such an application starts, it
will stay on for a relatively long period of time.
Which transport protocol is more appropriate for this
application, TCP or UDP? Why?

Assume you have web transfers where each page fits
into one packet. What are the advantages and
disadvantages of using UDP instead of TCP?
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Solution to Exercise 5: UDP vs TCP
For such file transfers, TCP is preferable but what to use is not
always that clear (in theory, one could do HTTP with UDP).


TCP:
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useful if the application involves long sessions with
predictable smooth bandwidth requirements
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data needs to be transmitted in order and with no
losses; connection-oriented architecture helps with no
significant waste at application level
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overhead costs and delay of three-way handshake can
be amortized over the length sessions
UDP:

useful if the application involves short transactions with
only a few packets to be sent (no need for flow control,
avoid overhead of three-way handshake)

error recovery (e.g. retransmission/ordering) has to be
done at application layer if necessary
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