Short Answer Problems

• (3.1) Stream and message.
• (3.2 and 3.3) On the Internet:

  Stream	Message
  Connection-oriented	Connectionless
  1-1	1-1, 1-Many, or Many-1
  Bytes	Messages
  Arbitrary length	Limited-size messages
  Used for just about anything	Best for multimedia
  Built on TCP	Built on UDP

• (3.4) Whatever size it wants.
• (3.5) Message (because it is better for broadcast).
• (3.6) Messages can be (1) lost, (2) delivered out of order, (3) duplicated.
• (3.7) The algorithm is:
  1. A pair of apps requests a connection;
  2. The apps use the connection to exchange data;
  3. The apps request that the connection be terminated.
• (3.9) Applications:

  Server	Client
  Doesn’t need to know who the clients are	Must know the server
  Passively waits for connection	Actively initiates the connection
  Stays running	Generally terminates connection when done with server
  Special-purpose program providing the same service to multiple clients	Can be an arbitary program, contacting different servers it needs whenever it needs something
  Communicates by sending and receiving data

• (3.10) Hardware:

  Server-class Machine	Client Machine
  Sophisticated hardware with powerful operating system providing concurrency so many clients can be handled at the same time	End-user device (laptop, phone, etc.)
  Multiple CPUs, lots of memory, multi-user, multi-processing, multi-threaded, etc.	Can be simple

• (3.11) Yes, of course. Like, why not? How else would it make a request?
• (3.12) Straight from the book:
  • A single client
  • A single server
  • Multiple copies of a client that contact a given server
  • Multiple clients that each contact a particular server
  • Multiple servers, each for a particular service
• (3.13) Of course not, some computers can't do multiprocessing or multitasking, which are required to allow clients to access services concurrently.
• (3.14) IP address and port number.
• (3.15) Again, from the book:
  1. Use DNS to translate name to IP address;
  2. Specify that the service uses port N;
  3. Contact server and interact.
• (3.16) Threads of control. (Can be implemented with real threads on different cores or processors, green threads, or events.)
• (3.17) Bottlenecks.
• (3.18) Windows, macOS.
• (3.19) In C, through its integer descriptor, the same way you reference any O.S. resource. In other languages, libraries provide a socket object.
• (3.20) socket, bind, listen, accept, getpeername, getsockopt, setsockopt, connect, recv, recvmsg, recvfrom, send, sendmsg, sendto, close, shutdown.
• (3.21) Client: socket → conenct → send/recv → close. Server: socket → bind → listen → accept → send/recv → close or go back to listen. (The server part is not totally accurate because listen and accept are generally done on a main thread and worker threads do the send/recv part.
• (3.22) recv and send.
• (3.23) It can but it shouldn’t—the O.S. should pick the port for the client.

Scripts

Random Number Server and Client

const net = require('net');

const server = net.createServer((socket) => {
  socket.end(`${Math.random()}\n`);
});

server.listen(53211);

const net = require('net');

const client = new net.Socket();
client.connect({ port: 53211, host: process.argv[2] });
client.on('data', (data) => {
  console.log(data.toString('utf-8'));
});

Random Number Web Server

const http = require('http');

const PORT = 53211;

http.createServer((request, response) => {
  if (request.method === 'GET' && request.url === '/random') {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.end(`${Math.random()}`, 'utf-8');
  } else if (request.method === 'GET' && request.url === '/') {
    response.writeHead(200, { 'Content-Type': 'text/html' });
    response.end('<a href="/random">Get random number from server</a>');
  } else {
    response.writeHead(404, { 'Content-Type': 'text/plain' });
    response.end('Sorry, that’s not there');
  }
}).listen(PORT);

console.log(`Random number web server running at port ${PORT}`);

Chat Web Server and Client

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