2.  Structural view of networks ◦ Physical layer: wire, radio, etc. ◦ Data link layer: protocol principles, etc ◦ Network layer: routing, internetworking ◦ Transport layer: protocols (tcp), etc ◦ Application layer: email, web, class project  We will use one common method at the transport/network level (TCP/IP via sockets) application transport network link physical

3.  Internetwork: a network of networks  Uses packet switching to send data between machines on this “internet” Home network Institutional network Mobile network Global ISP Regional ISP

4.  Each machine has a unique address on the network (IP address)  Internet Protocol (IP): format of packets used to create these envelops of data ◦ Like mailing a multipage letter by putting each page into a separately mailed envelop ◦ May not arrive in correct order

5.  1. Hosts (e.g., computers and other devices) are mapped to a set of 32-bit* IP addresses. ◦ 147.129.181.14  2. The set of IP addresses is mapped to a set of identifiers called Internet domain names. ◦ 147.129.56.50 is mapped to www.ithaca.edu  3. A process on one Internet host can communicate with a process on another Internet host over a connection. * this is IPv4. There is a new IP standard iPv6 that uses 64-bit addresses. See http://en.wikipedia.org/wiki/IPv6http://en.wikipedia.org/wiki/IPv6

6.  Windows NT/2000/XP/Win 7 ◦ Click on start, run, type cmd and at the prompt type ipconfig ◦ Type exit to close the command prompt window.  You’ll see something like: Windows IP Configuration Ethernet adapter Local Area Connection: Connection-specific DNS Suffix. : IP Address............ : 192.168.1.101 Subnet Mask.......... : 255.255.255.0 Default Gateway....... : 192.168.1.1

7.  From the Finder, under Applications -> Utilities launch Network Utility. ◦ in OS X 9.x look in /System/Library/CoreServices/Applications ◦ Select the correct ethernet interface (normally Ethernet 1 (en0)). ◦ Information and utilities are available from the tabs above.  From the Finder, choose apple->System Preferences. ◦ Then click on “Network” and in the resulting window choose “ethernet”  Or, start a terminal window. ◦ Type “ifconfig –a” ◦ Look under “en0”

8.  For Linux or Unix users, to view their IP address or network information, users must have administrator or root privileges. 1.Open the Linux or Unix shell if you are utilizing a GUI interface for your Linux or Unix machine. 2.From the prompt, type ifconfig eth0 and press enter. You’ll see: eth0 Link encap:Ethernet HWaddr 00:A0:24:72:EB:0A inet addr:10.10.10.2 Bcast:10.0.0.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:5569799 errors:32 dropped:32 overruns:0 frame:6 TX packets:3548292 errors:0 dropped:0 overruns:0 carrier:3 Collisions:14 Interrupt:18 Base address:0xda00  As seen from the above example, users will commonly see the network settings for all their network devices when running the "ifconfig" command. First in the above example we have the network settings for the "lo" or "local loopback", next is the actual network settings of your network adapter.

9.  ping ◦ “a computer network administration utility used to test the reachability of a host on an Internet Protocol (IP) network and to measure the round-trip time for messages sent from the originating host to a destination computer”  Open a terminal window (Mac) or do run->cmd (Windows) A228536:~ barr$ ping www.ithaca.edu PING www.ithaca.edu (147.129.56.50): 56 data bytes 64 bytes from 147.129.56.50: icmp_seq=0 ttl=62 time=1.236 ms 64 bytes from 147.129.56.50: icmp_seq=1 ttl=62 time=1.269 ms 64 bytes from 147.129.56.50: icmp_seq=2 ttl=62 time=1.309 ms

10.  The whois utility looks up records in the databases maintained by several Network Information Centers (NICs). whois -h hostname domainName e.g. whois ithaca.edu Or, if you know of a whois server like whois.educause.net (stores edu names): whois -h whois.educause.net ithaca.edu

11. Barr$ whois ithaca.edu Domain Name: ITHACA.EDU Registrant: Ithaca College Network & Communication Services 110 Phillips Hall Ithaca, NY 14850-7028 UNITED STATES Administrative Contact: Bernie Rhoades Director, Network & Communication Services Ithaca College Information Technology Services 110 Phillips Hall Ithaca, NY 14850-7028 UNITED STATES (607) 274-1694 brhoades@ithaca.edu Technical Contact: William W. Weeks Network Services Manager Ithaca College Information Technology Services 110 Phillips Hall Ithaca, NY 14850-7028 UNITED STATES (607) 274-1311 wweeks@ithaca.edu Name Servers: RESOLVER1.ITHACA.EDU 147.129.1.2 RESOLVER2.ITHACA.EDU 147.129.94.4 RESOLVER3.ITHACA.EDU 208.81.240.2 Domain record activated: 18-Jun-1990 Domain record last updated: 24-Sep-2007 Domain expires: 31-Jul-2009

12.  Transmission Control Protocol (TCP): ◦ Used to create reliable byte-streaming communication ◦ Ensures that messages are passed error-free from one machine to another on an internet ◦ Divides data into discrete messages before passing them on to the IP layer ◦ At the receiving end, protocol used to “reconstruct” the packets correctly

13.  Applications utilize TCP/IP communication by having the sender and the receiver create end points called Sockets  Sockets are created and maintained by the Operating System (not the network stack)

14.  Created in the early 80’s as part of the original Berkeley distribution of Unix that contained an early version of the Internet protocols.  Provides a user-level interface to the network.  Underlying basis for all Internet applications.  Based on client/server programming model.

15.  What is a socket? ◦ To the kernel, a socket is an endpoint of communication. ◦ To an application, a socket is a file descriptor that lets the application read/write from/to the network.  In Unix all I/O devices, including networks, are modeled as files.  Clients and servers communicate with each other by reading from and writing to socket descriptors.  A socket is not a physical device! ◦ your computer may have one physical Ethernet connection ◦ and many sockets

16.  Each socket has a socket number (address) ◦ IP address of the host computer ◦ Port number local to that host to which the socket is attached  Port numbers below 1024 are reserved (e.g. for email and FTP)  Some addresses are “well known”,  Port 7: Echo server  Port 23: Telnet server  Port 25: Mail server  Port 80: Web server  You must choose a port not already in use

17.  To use TCP service a connection must be explicitly established between a socket on the sending machine and a socket on the receiving machine  A socket may be used for multiple connections at the same time.  A computer (and/or application) may have multiple open socket connections

18. Computer Physical connection (e.g. ethernet, RS232) OS application Port 80 Port 5678 socket IP: 147.129.30.2

19. Client / Server Session Client Server socket bind listen rio_readlineb rio_writenrio_readlineb rio_writen Connection request rio_readlineb close EOF Await connection request from next client open_listenfd open_clientfd acceptconnect

20. use netstat.exe program (ships with windows, mac, and linux).  Start ”Command Prompt" using run-> cmd on Windows  type netstat -na and then press Enter,  use command "more" to stop when your listing contains to much lines (netstat -na | more). Each line of netstat's listing contains these information: * Protocol type - it can be TCP or UDP. Two sockets can use the same port number when they use different protocols. * Local Address - local IP address which is used. In most cases it is 0.0.0.0 (any = not bound to the specific IP address. It can accept connection from any PC's interface). When IP address is specified then only connection from interface with the same IP address are accepted. Local Address field contains port number which is used to accept connection or receive UDP data. * Foreign Address - address of PC on remote side. It will be displayed when connection is established. * State - state of socket. You can see this states: LISTENING - socket waits for connection (or data in case of UDP socket) ESTABLISHED - connection to the remote side is established CLOSE_WAIT, TIME_WAIT - socket waits for connection close or other type of negotiation. To determine whether desired port number is already in use you must check Local Address field on all lines. On Windows:

21. Example line: TCP 0.0.0.0:25 0.0.0.0:0 LISTENING Some application which uses port 25 (SMTP server port) is running.

22. On Linux:  Read the /etc/services file.  If you want the list of open ports, try netstat -nlp ◦ Arguments: n = show network addresses as numbers, l = show listening sockets, p = show pid and name of program

23. On Macs:  Read the /etc/services file.  If you want the list of open ports, try netstat -n ◦ The port follows the IP address

24.  Networked applications may be structured any number of ways  One popular structure is client-server  Server: ◦ Usually unique ◦ Provides a service ◦ Runs continuously  Clients ◦ Usually many ◦ Send request for service, receive result from server ◦ Connect and disconnect as needed

25. Client process Server process 1. Client sends request 2. Server handles request 3. Server sends response 4. Client handles response Resource

26. Web server (port 80) Client host Server host 128.2.194.242 Echo server (port 7) Service request for 128.2.194.242:80 (i.e., the Web server) Web server (port 80) Echo server (port 7) Service request for 128.2.194.242:7 (i.e., the echo server) Kernel Client

27.  Servers are long-running processes (daemons in Unix). ◦ Created at boot-time (typically) by the init process (process 1) ◦ Run continuously until the machine is turned off.  Each server waits for requests to arrive on a well-known port associated with a particular service. ◦ Port 7: echo server ◦ Port 23: telnet server ◦ Port 25: mail server ◦ Port 80: HTTP server ◦ See http://www.answersthatwork.com/Download_Area/ATW_Library/Networking/Networ k__2-List_of_Common_TCPIP_port_numbers.pdf http://www.answersthatwork.com/Download_Area/ATW_Library/Networking/Networ k__2-List_of_Common_TCPIP_port_numbers.pdf  A machine that runs a server process is also often referred to as a “server.”

28.  Web server (port 80) ◦ Resource: files/compute cycles (CGI programs) ◦ Service: retrieves files and runs CGI programs on behalf of the client  FTP server (20, 21) ◦ Resource: files ◦ Service: stores and retrieve files  Telnet server (23) ◦ Resource: terminal ◦ Service: proxies a terminal on the server machine  Mail server (25) ◦ Resource: email “spool” file ◦ Service: stores mail messages in spool file See /etc/services for a comprehensive list of the services available on a Linux machine.

29.  Your web browser is the client  Computer containing the pages you are trying to access runs the web server  Your computer requests the page  Server sends the page  Many other people may be requesting the same page  Note: these are distinct applications. Your browser (client) never acts as a server

30.  As you develop networked applications: ◦ Is the client-server model a good model?  Minimizes network communication?  Minimizes computational redundancy?  If so: ◦ Which program will be on the server side (e.g. provide a service)? ◦ Which must be on the client side?

31.  python provides built-in support for TCP/IP network communication  There are methods for connecting a client and methods for connecting a server

32.  Within a Client application, create a socket and connect to a specific server: #Socket client example in python import socket #for sockets #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) Address Family : AF_INET (this is IP version 4 or IPv4) Type : SOCK_STREAM (this means connection oriented TCP protocol)

33.  Better to do error checking: #handling errors in python socket programs import socket #for sockets import sys #for exit try: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) except (socket.error, msg): print ('Failed to create socket. Error code: ' + str(msg[0]) + ', Error message : ' + msg[1]) sys.exit()

34.  To connect to a remote server need 2 things, IP address and port number. ◦ need to know the IP address of the remote server ◦ Here we used the ip address of google.com as a sample. host = 'www.google.com' try: remote_ip = socket.gethostbyname( host ) except socket.gaierror: #could not resolve print ('Hostname could not be resolved. Exiting’) sys.exit() print ('Ip address of ' + host + ' is ' + remote_ip) method to obtain an IP address from a host name

35.  Now connect to google on port 80 (the well known http port) #Connect to remote server port = 80 s.connect((remote_ip, port)) print ('Socket Connected to ' + host + ' on ip ' + remote_ip)

36.  Now we can send and receive messages ◦ sendall method sends bytes through the socket ◦ recv method receives a number of bytes #Send some data to remote server message = "GET / HTTP/1.1\r\n\r\n" try : #Set the whole string s.sendall(bytes(message, 'UTF-8')) except socket.error: #Send failed print ('Send failed') sys.exit() print ('Message send successfully’) #Now receive data reply = s.recv(4096) print (reply) must convert string to bytes

37.  Servers do the following actions: 1. Open a socket 2. Bind to a address(and port). 3. Listen for incoming connections. 4. Accept connections 5. Read/Send try: #create an AF_INET, STREAM socket (TCP) s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) except (socket.error, msg): print ('Failed to create socket. Error code: ' + str(msg[0]) + ', Error: ' + msg[1]) sys.exit(); open a socket for the server just as we did for the client

38.  Next must bind the socket to an address and a port  By doing this we ensure that all incoming data which is directed towards this port number on this machine is received by this application.  To bind the socket use the following method: ◦ s.bind(hostname, port number)

39.  Ways of specifying arguments to bind.  to find the name of the host that the program is running on use: ◦ hostname = socket.gethostname()  to create a “server” socket that is only visible within the same machine use ◦ s.bind(('localhost', 80)) or ◦ s.bind(('127.0.0.1', 80))  To specify that the socket is reachable by any address the machine happens to have use ◦ s.bind(('', 80))

40.  To bind the socket to an address and a port import socket import sys HOST = '' # Symbolic name meaning all available interfaces PORT = 8888 # Arbitrary non-privileged port try: s.bind((HOST, PORT)) except socket.error, msg: print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1] sys.exit() print 'Socket bind complete' the server socket will listen on port 8888 (not a well known port) on all interfaces

41.  The next step is to make the socket listen for incoming requests  We must put the socket into listening mode: ◦ s.listen(x)  The parameter x is called the backlog. ◦ It controls the number of incoming connections that are kept "waiting" if the program is already busy. ◦ x =10 means that if 10 connections are already waiting to be processed, then the 11th connection request shall be rejected.

42.  The final step is for the socket to accept incoming requests  When a request to this machine on the correct port comes in, the socket must accept the request: ◦ (conn, addr) = s.accept()  The result conn is a new socket with which we can communicate with the client.  The result addr is an array. ◦ first element is the IP of the client ◦ second element is the port the client is using

43. (conn, addr) = s.accept() #display client information print ('Connected with ' + addr[0] + ':' +str(addr[1]))  This displays the client addr

44.  Now can send & receive over the connection: #now keep talking with the client data = conn.recv(1024) print ("Server received: " + data) conn.sendall("Server says: Hello client!")

45.  When finished, close the connection & socket: conn.close() s.close()

46.  See Class web site: ◦ Course Resources -> Program Examples->find_ip.py ◦ Course Resources -> Program Examples->client.py ◦ Course Resources -> Program Examples->server.py

47.  Calculating area: ◦ Server calculates area given radius ◦ Client gets radius from user, requests area calculation

48. server radius client radius Area