Networking and socket communica2on

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Madison McCarthy
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2 Networking basics Overview Difference between: clients and servers Addressing IP addresses, hostnames, DNS Private addresses, localhost Port numbers Socket Byte- level between two hosts Java client: reading text Java server: clients, text 2

client Server program Provides service Web server

3 Client program Requests service Web browser Clients and servers Streaming audio player TwiMer client MMOG client Server program Provides service Web server Audio server at streaming Server at TwiMer MMOG server 3

4 Clients and servers Client program on" Doesn't talk to other clients Needs to know server's address Server program "always on" Handles requests from many clients Needs fixed address 4

5 Network components Transports data from source to host Uses IP address system Data is forwarded to a "silo" based on port # e.g. All port 80 requests go the web server Applica@on Actually reads and writes to socket Implements applica@on- specific "magic" e.g. Implemen@ng a mail reading/wri@ng protocol e.g. Implemen@ng a file retrieval (FTP) protocol e.g. Implemen@ng a par@cular online game 5

6 Naming computers Goal: Establish between A and B How do computer A and B refer to each other? The network needs an addressing system IP (Internet Protocol) address IPv4 address 32 bits = about 16 million hosts Usually expressed as four numbers (8 bits) e.g IP address uniquely iden@fies a network endpoint Devices in the network (e.g. switches, routers) use the IP address to get data to its des@na@on 6

7 from H5 to H

8 DNS Domain Name System Problem 1: Humans can't remember all the numbers in an IP address Domain Name System (DNS) Converts readable name to numeric IP address e.g. - > hmp://xkcd.com/302/ 8

Private IP addresses Problem 2: IPv4 only has 16 million addresses 7 billion people, all want a laptop, Xbox & iphone Private IP addresses Allow construc@on of a private network Route data between

9 Private IP addresses Problem 2: IPv4 only has 16 million addresses 7 billion people, all want a laptop, Xbox & iphone Private IP addresses Allow construc@on of a private network Route data between endpoints on the private network Addresses aren't valid outside network x.x, 10.x.x.x, /31.x.x Typically what you'll have: On home network On campus network (wired/wireless) (localhost) hmp://xkcd.com/742/ 9

10 Port numbers Problem 3: Many on same computer want to talk at Chrome process: Browser tab 1 wants Browser tab 2 wants Browser tab 3 wants Thunderbird process: client wants IMAP4 to techmail.mtech.edu Solu@on: Use IP address + port number A 16- bit number: Port number determines app message is routed to Just a "virtual" port, only exists in the OS 10

11 Port numbers Popular have known ports Ports : reserved for well- known services Only administrators can start servers on these ports Ports : available to any user- level Port Service 21 File transfer protocol (FTP) 22 Secure shell (SSH) 23 Telenet 25 Simple mail transfer protocol (SMTP) 53 Domain name system (DNS) 80 Hypertext transfer protocol (HTTP) 110 Post office protocol (POP) 143 Internet message access protocol (IMAP) 443 HTTP secure (HTTPS) 11

12 Use of port number Reques2ng a non- secure web page Reques2ng a secure web page :80 OS :443 OS web server mail server web server mail server Reques2ng new messages :143 OS web server mail server 12

13 Firewalls You can't always get there from here may by filtered by network e.g. by a firewall at the border of Tech's network e.g. by the wireless access point in Main Hall Onen by the port number 13

14 Sockets Socket API programming interface) Allows over IP (Internet Protocol) Originally in Berkeley Unix Thus: Berkeley sockets, BSD sockets De facto standard in all systems API in many programming languages: C/C++ Java C# 14

15 Java client: reading from a socket Step 1: Create a new Socket object Needs to know IP address of server + port number Socket socket = new Socket(" ", 5000); Step 2: Create an InputStreamReader Converts low- level socket data into characters stream InputStreamReader stream = new InputStreamReader(socket.getInputStream()); Step 3: Create a BufferedReader Provides buffered reading of character stream BufferedReader reader = new BufferedReader(stream); Step 4: Read some text String message = reader.readline(); 15

16 BufferedReader 16

17 Java client: to a socket Step 1: Create a new Socket object Or use an exis@ng one You can combine reads and writes to same socket Socket socket = new Socket(" ", 5000); Step 2: Create an PrintWriter Seen previously when wri@ng to a file PrintWriter writer = new PrintWriter(socket.getOutputStream()); Step 3: Write something writer.println("hello over there!"); 17

18 PrintWriter Just some of the methods in PrintWriter 18

19 Java socket server Client needs somebody to talk to! Server slightly different than client: Must be running before client connects Server decides port number to listen on But doesn't specify IP address Doesn't know who is going to connect Blocks, to accept an incoming client Then just as in client 19

20 Java socket server Step 1: Create a ServerSocket object Declares what port you are listening on Nobody else on the computer bemer be using it! ServerSocket serversock = new ServerSocket(5000); Step 2: Wait for a client to connect accept() method blocks un@l client arrives Returns a new Socket object for talking to client Socket sock = serversock.accept(); Step 3: Read/write same way as a client Create BufferedReader for reading strings Create PrintWriter for wri@ng strings 20

1. Server program starts up. 2. Starts listening on port 4242. 3.

Client program starts up 5. Requests connec@on to server IP address on port 4242. 6.

21 1. Server program starts up. 2. Starts listening on port OS sends all inbound requests to 4242 to the server program. process 4. Client program starts up 5. Requests to server IP address on port Server establishes a socket connec@on to client, using outgoing port number Server can listen for new clients on the 4242 port number. 21

22 Server: Magic 8 ball: Internet Edi@on keithv.com In co- lo facility somewhere in Texas Public IP address Running on port 5000 Delivers 1 of 20 messages at random Client(s): My laptop on the wireless network Your desktop on the wired network Both have a private IP address Displays message from the server 22

23 Basics of networking Summary Computer all have a numeric IP address Some computers have a friendly name (e.g. google.com) Port numbers iden@fy program to send request to Java socket communica@on Clients create: Socket object Servers create: ServerSocket, then a new Socket object per client that connects Reading via BufferedReader Wri@ng via PrintWriter 23

4. The transport layer

4. The transport layer 4.1 The port number One of the most important information contained in the header of a segment are the destination and the source port numbers. The port numbers are necessary to identify the application