Networking

Question 1

What are ports?

Answer 1

Communication endpoints between two devices; an integer in the range [0, 65535] (per device). Whenever an application tries to establish an Internet connection, it needs to claim a port number through which they can communicate with another device.

Question 2

What are the different type of ports?

Answer 2

• well-known/system ports [0, 1023]
• dynamic/private ports [49152, 65535]
• registered ports [1024, 49151]

Question 3

What are the protocols that implement a client-server model into an application?

Answer 3

Transmission Control Protocol(TCP) and User Datagram Protocol(UDP)

Question 4

What is an IP address?

Answer 4

An address that identifies a device on the Internet; each device connected to the internet has a unique IP address; IP = inernet protocol

Question 5

What version of IP is used right now?

Answer 5

IPv6, version 6: IP addresses consist of eight groups of four hexadecimal digits, separated by colons. In the day to day usage, however, IPv4 addresses are used.

Question 6

What is TCP? (Transmission Control Protocol)

Answer 6

A protocol over the Internet Protocol that provides reliable and ordered delivery of network packets.

Question 7

What does passive open refer to?

Answer 7

The server must first bind to a port and listen at it for incoming connections.

Question 8

What does active open refer to?

Answer 8

The client connecting to a started server that is awaiting connections.

Question 9

What does TCP use to establish a connection?

Answer 9

Three-way handshake:
1. SYN: client sends SYN to the server
2. SYN, ACK: the server sends back a SYN, together with ACK to acknowledge that it received the SYN from the client
3. ACK: the client sends back ACK

Question 10

What is duplex communication?

Answer 10

When the connection is guaranteed (after finishing the three-way handshake), the communication can be made in both directions(client <=> server)

Question 11

What does a socket(Socket/ServerSocket) do?

Answer 11

A server socket is used to bind to a port and listen for incoming messages; a socket requires the host/IP address and the port number)

Question 12

How can reading from the socket be done?

Answer 12

BufferedReader in = new BufferedReader(new InputStreamReader(s.getInputStream());
String input = in.readLine();

Question 13

How can writing to the socket be done?

Answer 13

PrintWriter out = new PrintWriter(s.getOutputStream(), true);
Scanner userInput;
out.println(userInput.nextLine())

Question 14

What ports can we use?

Answer 14

Generally, it is recommended to use only registered ports.

Question 15

What is UDP? (User Datagram Protocol)

Answer 15

A protocol that makes use of the Internet Protocol (IP) that does not provide reliable or ordered delivery of network packets. Instead, it provides faster delivery of network packets. Every packet is sent independently and there is no guarantee that the packets arrive in order, or at all even. This means that there is no concept of “state” in UDP.

Question 16

How does UDP initiate contact?

Answer 16

The client sends an initialisation packet to the server such that the server can register the client’s host name and port number. In a multi-threaded UDP server, the server will in turn send a packet back with information about the new port number that the client can connect to.

Question 17

What kind of socket does UDP use?

Answer 17

a DatagramSocket that can have either one parameter (the port number) or zero parameters (in which case it takes 0 and chooses a random available port number)

Question 18

How do you connect a socket?(TCP)

Answer 18

Using the method connect in the Socket class. Example:
s.connect(new InnetSocketAddress(“hostname”, 8189), 1000). 8189 is the port number and 1000 is the nr of milliseconds until it is considered timeout if the server hasn’t connected yet. On the server side, we do ServerSocket ss = new ServerSocket(8189); (passive open) Socket s = ss.accept() that accepts the connection and cause the active open.

Question 19

How can communication be enabled when using UDP?

Answer 19

Using DatagramPacket; Parameters: the data you want to transfer (as a byte array), the length of the array, the IP address of the recipient and the port number.