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How Internet Works

How Internet Works. EMC 165 Computer and Communication Networks Feb 3, 2004. Outline. How Internet Instrastructure Works How Routers Work How TCP/IP networks work How Routing Algorithms Work How NAT works. What is the Internet?. It is a global collection of networks, both big and small.

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How Internet Works

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  1. How Internet Works EMC 165 Computer and Communication Networks Feb 3, 2004

  2. Outline • How Internet Instrastructure Works • How Routers Work • How TCP/IP networks work • How Routing Algorithms Work • How NAT works

  3. What is the Internet? • It is a global collection of networks, both big and small. • Recall in Lecture 2, we mentioned that one of the greatest things about the Internet is that nobody really owns it. • These networks connect together in many different ways to form the single entity that we know as the Internet. In fact, the very name comes from this idea of interconnected networks.

  4. The Internet Concept

  5. The Internet Concept (Cont’d)

  6. Internet: Network of Networks • Every computer that is connected to the Internet is part of a network, even the one in your home. • For example, you may use a modem and dial a local number to connect to an Internet Service Provider (ISP). • At school/work, you may be part of a local area network (LAN), but you most likely still connect to the Internet using an ISP that your school/company has contracted with.

  7. Internet: Network of Networks (Cont’d) • When you connect to your ISP, you become part of their network. • The ISP may then connect to a larger network and become part of their network. • The Internet is simply a network of networks.

  8. Connecting Network of Networks • The amazing thing here is that there is no overall controlling network. • Instead, there are several high-level networks connecting to each other through Network Access Points or NAPs. • All the networks that make up the Internet rely on NAPs, backbones and routers to talk to each other.

  9. History of Internet • 1962, Paul Baran of the RAND Corporation was commissioned by the US Air Force to do a study on how it could maintain its command and control over its missiles and bombers, after a nuclear attack. Baran’s final proposal was a packet switched network. • 1968, Advanced Research Project Agency (ARPA) awarded the APRPANET contract to BBN. The physical network was constructed in 1969, linking 4 nodes: UCLA, SRI (Stanford), UCSB, University of Utah via 50 Kbps circuits. • The 1st email program was created by Ray Tomlison of BBN in 1972. • ARPA was later renamed the Defense Advanced Research Projects Agency (DARPA) in 1972. • In 1973, developments began on the protocol later to be called TCP/IP by a group headed by Vinton Cerf from Stanford and Bob Kahn from DARPA. • The term Internet was coined by Vint Cerf and Bob Kahn in their paper on TCP in 1974

  10. History of Internet - contd • Dr R. Metcalfe developed Ethernet in 1976, which allowed coaxiable cable to move data extremely fast. • Dept of Defense begain experimenting with the TCP/IP protocol in 1976 and soon decided to require it for use on ARPANET. • Total number of hosts on the backbones in 1976: 111+

  11. History of Internet - contd • National Science Foundation (NSF) created the 1st high-speed backbone in 1987 called the NSFNET. • NSFNET is a T1 line that connected 170 smaller networks together and operated at 1.544 Mbps. • IBM, MCI, and Merit worked with NSF to create the backbone and developed a T3 (45 Mbps) backbone the following year • Total number of hosts in the Internet: 56,000 in 1988. In 1990, this number has jumped up to 313,000 • In 1992, World-Wide Web was released by CERN. NSFNET backbone completely upgraded to T3. • Total number of hosts in the Internet in 1992 – 1.136 millions

  12. History of Internet - contd • In 1994, ATM (145 Mbps) backbone is installed on NSFNET. • Total number of hosts has increased to 3.864 millions in 1994 • Most Internet traffic is carried by backbones of independent ISPs including MCI, AT&T, Sprint, UUNet, BBN planet etc. • The total number of hosts in 1999 was around 15 millions and growing rapidly.

  13. Backbones • Backbones are typically fiber optic trunk lines • The trunk line has multiple fiber optic cables combined together to increase the capacity. • Fiber optic cables are designated OC for optical carrier such as OC-3, OC-12 or OC-48. • An OC-3 line is capable of transmitting 155 Mbps while an OC-48 can transmit 2,488 Mbps (2.488 Gbps).

  14. Logical addresses • Every piece of equipment that connects to a network has a physical address. This is an address unique to the piece of equipment. • The physical address is also called the Medium Access Control (MAC) address. It has 2 parts each 3 bytes long. The 1st 3 bytes identify the company that made the Network Interface Card (NIC), and the 2nd 3 bytes are the serial number of the NIC itself. • The interesting thing to note is a computer can have several logical addresses at the same time. • Logical addresses like IP address are assigned statically or dynamically.

  15. Internet Protocol: IP addresses • Every machine on the Internet has a unique underlying number, called an IP address. • The IP stands for Internet Protocol which is the language that computers use to communicate over the Internet. • A protocol is a pre-defined way that someone who wants to use a service talks with that service. That someone could be a person, but more often it is a computer program like a Web-browser. • A typical IP address looks like this 216.27.61.137 The four numbers in an IP address are called octets, because they each have eight bits. Each octet can contain any value between zero and 255. So, combining 4 octets give us 232 possible unique values. • Certain values are restricted from use as typical IP addresses e.g. 0.0.0.0 is reserved for the default network and 255.255.255.255 is reserved for broadcasts

  16. How TCP/IP network works.

  17. Version HLength Type of Service Total Length Identification Flags Fragment Offset Time-to-Live (Next) Protocol Header Checksum Source Address Destination Address IP Options Data Payload up to 65,535 bytes IPv4 Header 0 31

  18. IP Addresses - Motivation • Key aspect of a virtual network is a single, uniform address format • Can't use hardware addresses because different technologies have different address formats  Format must be independent of any particular hardware address format • Sending host puts destination internet address in packet • Destination address can be interpreted by any intermediate router  Routers examine address and forward packet on to the destination

  19. Classfull Addresses • Properties • 32-bit number • globally unique (with a few exceptions!) • hierarchical: network + host • Classes of addresses for specific types of networks

  20. Classfull Addresses • Generally assigned by authorities except from: • A-class net: 10.0.0.0 • B-class net 172.16.0.0 • C-class net 192.16.8.0 • Some college have a B-class net e.g.134.226.0.0 • Can arrange for Dept. of Comp. Science. to have a number of subnets in this domain e.g. 134.226.32.0, 134.226.51.0

  21. Summary • Virtual network needs uniform addressing scheme, independent of hardware • IP address is a 32-bit address • IP address is composed of a network address and a host address • Network addresses are divided into classes e.g. A, B and C • Dotted decimal notation is a standard format for Internet addresses: 134.226.32.57

  22. IP Address & Ethernet Address IP Address MAC Address

  23. Address Resolution Protocol (ARP) for Computer B

  24. ARP’s “Who has…?” Packet

  25. ARP’s Reply Packet

  26. Routed (Sub-)Networks Router

  27. Packet Size Matters!!! Packet Size= 7000 bytes 7000

  28. Network-specific MTU* *Maximum Transfer Unit

  29. Fragmentation • One technique - limit datagram size to smallest MTU of any network However: This approach requires knowledge about all networks involved in communication • IP uses fragmentation - datagrams can be split into pieces to fit in network with small MTU • Router detects datagram larger than network MTU • Splits into pieces • Each piece smaller than outbound network MTU

  30. Fragmentation (details) • Each fragment is an independent datagram • Includes all header fields • Bit in header indicates datagram is a fragment • Other fields have information for reconstructing original datagram • FRAGMENT OFFSET gives original location of fragment • Router uses local MTU to compute size of each fragment • Puts part of data from original datagram in each fragment • Puts other information into header

  31. Fields for Fragmentation 0 31 Version HLength Type of Service Total Length Identification Flags Fragment Offset Time-to-Live (Next) Protocol Header Checksum Source Address Destination Address IP Options Data Payload up to 65,535 bytes

  32. Ethernet to Tokenring

  33. Tokenring to Ethernet

  34. Tokenring to Ethernet

  35. Fragmentation & Reassembly • Each network has a Maximum Transmission Unit (MTU) • IP datagrams can be larger than most hardware MTUs • IP: 216 - 1 • Ethernet: 1500 • Token ring: 2048 or 4096 • Strategy • fragment when necessary (Datagram > MTU) • try to avoid fragmentation at source host • re-fragmentation is possible • fragments are self-contained datagrams • delay reassembly until destination host • do not recover from lost fragments

  36. Fragment Loss • IP may drop fragment • What happens to original datagram? • Destination drops entire original datagram • How does destination identify lost fragment? • Sets timer with each fragment • If timer expires before all fragments arrive, fragment assumed lost • Datagram dropped • Source (application layer protocol) assumed to retransmit Best Effort Delivery

  37. Internet Protocol: Domain Name System • If there are only a few hosts, then working with IP addresses is fine but with more and more hosts that came online, it becomes unwieldly. • The first solution is a simple text file maintained by the Network Information Center that mapped names to IP addresses. • But soon this text file became so large that it was too cumbersome to manage. • So, in 1983, University of Wisconsin created the Domain Name System which maps a hostname to an IP address automatically.

  38. Uniform Resource Locators • When you use the Web or send an email message, you use a domain name to do it. For example, the URL http://www.howstuffworks.com contains the domain name howstuffworks.com. So does the email address: jane@amazon.com. • Everytime we use a domain name, we use the Internet’s DNS servers to translate the human-readable domain name into the machine-readable IP address. • Top-level domain names include .com, .org, .net, .edu, .gov. Within every top-level domain, there is a huge list of 2nd-level domains. For example, in the .com 1st-level domain, there is • Yahoo • Microsoft • Amazon Every name in the .com top-level domain must be unique.

  39. Internet Naming Hierarchy The silent dot at the end of all addresses .ie .uk .com .net .tcd .co .ac www

  40. How to find www.cse.lehigh.edu? Domains edu Name server in Berkeley, CA lehigh 1. Ask top-level server for edu-server cse www 2. Ask .edu server for lehigh-server 3. Ask .lehigh server for cse-server DNS server 4. Ask .cse server for “www” machine 134.226.32.57

  41. DNS • DNS servers accept requests from programs, and other name servers, to convert domain names into IP addresses. When a request comes in, the DNS server can do one of the 4 things with it: • It can answer the request with an IP address because it already knows the IP address for the requested domain. • It can contact another DNS server and try to find the IP address for the name requested. It may have to do this multiple times • It can say, “I don’t know the IP address for the domain but here’s the IP address for a DNS server that knows more than I do” • It can return an error message because the requested domain name is invalid or does not exist.

  42. Name Server Architecture • Name agent (Resolver) • Interface with the local user programs • Identifies objects based on symbolic names • Name server • Converts symbolic names to addresses • Queries other name servers if the name is unknown

  43. Name server Name server Name server Name server Recursive Name Server Name agent

  44. Name server Name server Name server Name server Name agent Iterative Name Server

  45. Name server Name server Name server Name server Transitive Name Server Name agent

  46. Domain Name System (DNS) • Name server • Serves a hierarchical name space • Maps names to addresses • Stores auxiliary information • Authoritative name server • Mail exchanger • Round robin (load balancing)

  47. Putting it together uses a DNS query to find out what IP address www.cse.lehigh.edu has Computer B in Berkeley, CS wants to find a web page at “www.cse.lehigh.edu” Computer B knows that 134.226.0.0 is routed into direction of east coast knows about the Dept. of Comp. Sc. has an agreement with Lehigh has an agreement with AT&T www cse.lehigh.edu Router in CSE Router in Lehigh Router at AT&T Router in New York Router at Berkeley knows that 134.226.32.57 is on the local ethernet and uses ARP to get its ethernet address replies with 134.226.36.57 Berkeley DNS

  48. Best-Effort Delivery D1 D2 • Transfer of datagrams D1 & D2 • Possible deliveries: • D2 D1 • D1 D2 • D1 • D2 • nothing

  49. How Routers Work • Assume that there is a small company with 10 employees, each with a computer. 4 of the employees are animators, while the rest are in sales, accounting and management. • The animators send many very large files back and forth to one another. To do this, they will need a network. • When one animator sends a file to another, every one sees the traffic if the network used is Ethernet. Each computer checks to see if the packet is meant for its address. But since the file is big, this makes the network run very slowly for other users. • So, to keep the animators’ work from interfering with others, the company sets up 2 separate networks, one for the animators and one for the rest of the company. A router links the two networks and connects both networks to the Internet.

  50. How Routers Work - contd • Router is the only device that sees every message sent by any computer. • When the animator sends a huge file to another animator, the router looks at the recipient’s address and keeps the traffic on the animators’ network. • When the animator sends a message to the bookeeper, the router sees the recipient’s address and forwards the message between the two networks. • One of the tools a router uses to decide where to forward a packet is a configuration table. Such a table contains the following information • Information on which connections lead to particular groups of addresses • Priorities for connections to be used • Rules for handling both routine and special cases of traffic.

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