Introduction to the Internet

Transcription

1 Introduction to the Internet CCST th Sep, 2010 Dr. Hayden Kwok-Hay So Department of Electrical and Electronic Engineering

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3 Internet in

4 Internet Basics The Internet is a network that connects millions of of computing devices throughout the world. The Internet is formed by connecting many different networks. Inter -Net Distributed management What s so special about the Internet? 4

5 Back in time Before the Internet, each University and military unit has their own network Different standards Some of them work only when the computers are physically close to each other Simple point-to-point connections Networking was a niche Src: DEC s PDP-10 5

6 ARPANET Advanced Research Projects Agency Network Project active during early 70s Commonly referred as the predecessor of the Internet A large-scale research effort involved multiple Universities on building networks that Connect computers remotely in long distance Scalable First operational packet-switch network 6

7 Today: 3 Big Ideas about Internet The end-to-end design principle The core network is dumb Layering Modular design Division of labor Packet switching Data are transmitted as packets Compared to circuit switching networks 7

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9 Circuit Switch vs Packet Switch Two important ways of network communication: Circuit switching dedicated circuit per connection e.g. telephone network (PSTN Public Switched Telephone Network) Packet switching Data sent through the network in discrete packets e.g. Internet 9

10 Circuit Switching End-to-end resources reserved for exclusive use by each connection Network resources (e.g., bandwidth) divided into pieces Pieces allocated to each connection Guaranteed performance Resource piece idle if not used by owning call (no sharing) 10

11 Packet Switching A 10 Mbs Ethernet statistical multiplexing C B queue of packets waiting for output link D 1.5 Mbs E Each end-end data stream divided into packets Different users share the same resource as needed Excessive queuing might cause packet drop 11

12 Packet Routing Getting packets from one computer to another Each data packet must be marked with the destination Just like the address on a postcard The header of a packet stores all the information about the content of the packet The payload contains the actual data The machine that decides how to route a packet in the network is called a router. 12

13 Internet Routers Surprisingly, the Internet routing protocol is relatively simple. Each stop between router is called a hop. Each router only knows the general direction on where to route a packet A hierarchal routing idea Example: Send a packet form the USA to Rm 516, Dept of EEE, HKU, HK 1. Send to HK 2. Postman (router) at HK sends to HKU 3. Router at HKU sends to EEE 4. Router at EEE sends to Rm

14 Internet structure: network of networks a packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP local ISP Tier-2 ISP local ISP Tier 1 ISP NAP local ISP Tier 1 ISP Tier-2 ISP local ISP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP 14

15 Visualizing Internet Routes Traceroute A standard tool to look for all the intermediate routers to a remote site Interesting website that provides visualization of routes 15

16 Is packet switch always better? Great for bursty data e.g. Web browsing resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem 16

17 Internet traffic going out of New York (2008) 17

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19 Layered Architecture (1) Divides the task of data communication into layers of smaller modules Each layer has a well defined sub-task Each module has restricted interaction with each other Each layer provides services to the layer(s) above utilizes services from the layer(s) below Uses services of Uses services of Layer 2 Layer 1 Layer 0 Provides services to Provides services to 19

20 Layered Architecture (2) The module in a particular layer only communicates with its counterpart at the other side of the network at the same layer. Any module can be replaced with another module at the same layer as long as it provides the exact same interface. This is the reason, e.g., your web browser works identically regardless of whether you are using WiFi network or a wired network, or 3G mobile network 20

21 Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: host-host data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits on the wire application transport network link physical 21

22 How Layering Works? Each layer hides the layers above from any detail about lower layers. Each layer focuses on its core functions, assuming other layers will handle the rest. data application transport network link physical data ack network link physical data e.g. A logical connection at transport layer application transport network link physical 22

23 Layering: physical communication data application transport network link physical application transport network link physical application transport network link physical network link physical data application transport network link physical 23

24 Abstracting Details Each computer thinks it is directly connected to another one through a magic cloud. 24

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26 End-to-End Design Principle In laymen s term: What happens at the end host is what really matters Slightly more technical: Some information are only known to the end hosts, so there should be a distinction in responsibility on what the end hosts should perform and what the network should perform. Jump to conclusion: The core network should be dumb. 26

27 End-to-End: 2 versions According to van Shewick s interpretation, there are two related version of end-to-end arguments as proposed by the original author: Narrow Version: The function in question can completely and correctly be implemented only with the knowledge and help of the application standing at the endpoints of the communication system. Therefore, providing that questioned function as a feature of the communication system itself is not possible [i.e. it is not possible to provide a complete and correct implementation of the function within the communication system only]. Broad Version: a function or service should be carried out within a network layer only if it is needed by all clients of that layer, and it can be completely implemented in that layer. Barbara van Schewick, Internet Architecture and Innovation, pp

28 Example: Reliable File Transfer While you are uploading your photo to Facebook on your mobile phone, you entered into an elevator and lost your mobile phone connection. Q: which of the following makes more sense? 1. The network is smart, so that when the connection comes back (after you exit the elevator), it should retry the upload of the photo. 2. The network id dumb, and terminate the connection to Facebook without uploading the picture. 28

29 File Upload (cont d) According to E2E principle, the Internet design picks (2) as the answer. Reason: the mobile phone network should not retry the upload because it does not, and should not understand the high-level requirement of the user. e.g. You may decide to redo the upload if it has failed, and if the photo upload was really that important. Or, you may just give up and upload another photo If the network (think your mobile phone carrier) retries the upload, AND you perform another upload at the same time, you will end up with two different versions of photo uploaded. 29

30 Quick Summary Internet is a collection of global networks End-to-End architecture allows the core of the network be constantly changing without affecting the end nodes Packet-switch network architecture allows many more nodes to share the same resource than circuit switch Standardized network protocols allow different computers to communicate Protocol stacks provide layer abstractions 30

31 Why do you care? Knowing how the Internet is structured and its history helps to understand the unprecedented social-economical impact it has brought to the modern world. It introduces brand new challenges in reasoning about: Security Privacy Business Government policy... More on that in Tutorial and some of next lecture 31

32 Questions to Ponder On 1. Think of 5 things that you get for free from the Internet. 2. Think of 5 things that you get for free in real life. 32