How the Internet Works

Transcription

1 How the Internet Works (in about an hour) Nick McKeown Professor of Electrical Engineering and Computer Science, Stanford University

2 Four nodes connected (UCLA, SRI, UCSB, Utah) A network to survive nuclear attack. US Government starts ARPANET project Paul Baran 1 st network connects two computers

3 The Internet in 1969 end host router Also in 1969 link

4 What did they use it for? end host router link 1. Sending files between scientists: Here is a big file of astronomy data! 2. Where shall we have lunch today? 3. Remote login to another computer.

5 1971 First typed here QWERTYUIOP and printed here

6 router end host ,000,000 end hosts end hosts link ,000 end hosts

7 Then in 1993 something even BIGGER happened!!!

8 1993: The first web browser (Mosaic) Marc Andreessen 1993 President Bill Clinton

9 The number of Internet users in the world 3.6B people ~ 40% of world Source:

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11 How does it all work?

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13 My Program Someone else s Program

14 Data

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20 Packets may be damaged Data?!%%*

21 Packets may arrive out of order 12

22 Packets may be duplicated Duplicate Data Data

23 They may not arrive at all! Data Full

24 Summary so far Applications send and receive data in packets. over an Internet that is unreliable.

25 What do Internet packets look like?

26 Internet addresses Data Internet IP Address Data

27 Internet Addresses ( IP address ) Data Internet IP Address All Internet packets carry a destination IP address. We usually write the IP address like this:

28 Internet IP Addresses The IP address tells each router where to send the packet next. A network in Stanford the CS department University at Stanford University The computer yuba.stanford.edu

29 Can we see the path our packets take? Yes! On your computer, try: traceroute q1 yuba.stanford.edu

30 yuba.stanford.edu 700km (6ms round-trip) cs.usc.edu

31 15,000km (120ms round-trip)

32 Try traceroute to. yuba.stanford.edu,

33 How packets find their way across the Internet

34 Routers forward packets one at a time. Routers look at IP addresses, then send packets to a router closer to the destination. Data

35 IP Addresses The IP address tells a router where to send the packet next. IP addresses have structure A network in The Stanford the computer CS department University yuba.stanford.edu at Stanford University An address managed An address The by RIPE computer at (European Koç University IP Networks)

36 Summary so far Applications send and receive data in packets. over an Internet that is unreliable. Packets are forwarded hop-by-hop based on the final destination address.

37 The Internet cannot be trusted!! The Internet doesn t promise to deliver packets in order. It doesn t promise to deliver packets quickly, or on time. It doesn t even promise to deliver them at all! It just makes a best-effort attempt. Data

38 Sending data reliably over an Internet that is unreliable

39 How Network Applications My Program Communicate Someone else s Program The most common method: Communication is in both directions bidirectional. Communication is reliable (if there is a working path between the two computers). It s like an unformatted pipe: You push data in at one end, and it pops out correctly at the other end. The applications decide how the data is formatted inside the pipe.

40 Byte Stream Model Setup connection A Internet B

41 Byte Stream Model Stream of bytes Stream of bytes Internet A B

42 Byte Stream Model Close connection A Internet B

43 World Wide Web (HTTP) Client Server Setup connection Internet

44 World Wide Web (HTTP) Client GET/HTTP/1.1 index.html Stream of bytes Server Stream of bytes Internet HTTP/ OK < contents of index.html >

45 World Wide Web (HTTP) Client Server Close connection Internet

46 Your Application program e.g. Chrome, Skype TCP IP Data TCP makes sure all the data is delivered The server e.g. Google, Facebook TCP IP

47 TCP s job Makes sure all data is delivered correctly. Delivers data to the application in the right order. How? Add sequence numbers to every packet (so the receiver can check if any are missing, and put them in right order) When a packet arrives, send an acknowledgment of receipt or ACK back to the sender If no acknowledgment is received, resend the data

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49 http client (e.g. Chrome) Chrome Application http request GET index.html TCP TCP (http) Deliver to the http server IP IP TCP (http) IP address (TCP) at this destination Ethernet Ethernet IP TCP (http) IP address (TCP) Ethernet MAC address (IP) starting with this link

50 Here it goes. Ethernet data IP data TCP data (http) IP Address (TCP) Ethernet MAC address (IP)

51 http server (e.g. http server http request Application GET file Deliver to the http server TCP TCP (http) Deliver to TCP IP Ethernet IP TCP (http) IP address (TCP) Ethernet MAC address (IP) Ethernet

52 My Program Summary of what we learned Applications send and receive data in packets. Someone else s Program over an Internet that is unreliable. Packets are forwarded hop-by-hop using the IP destination address. Our applications use TCP to make sure they are delivered and put back in the correct order.