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Extending Networks

Extending Networks. Three Levels of Extension. Physical Layer Repeaters Link Layer Bridges Switches Network Routers: “Connecting networks”. (Physical Layer) 5-4-3 Rule. For IEEE 802.3 LANs, there is a limit on the length of a segment, how to extend the limit?

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Extending Networks

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  1. Extending Networks

  2. Three Levels of Extension • Physical Layer • Repeaters • Link Layer • Bridges • Switches • Network • Routers: “Connecting networks”

  3. (Physical Layer) 5-4-3 Rule • For IEEE 802.3 LANs, there is a limit on the length of a segment, how to extend the limit? • Adding repeaters (Hubs) while respecting the 5-4-3 rule: • Any path should traverse at most 5 segments • Any path should traverse at most 4 repeaters (hubs) • Any path should traverse at most 3 populated segments

  4. (Link Layer) Bridges • Filter network component • Back learning • Internetworking component (between LANs of different standards)

  5. DIX and 802.3 Frames DIX Preamble Dest. Source. Type Type CRC 4 8 6 6 2 Up to 1500 IEEE 802.3 Preamble Dest. Source. Length Type CRC 4 8 6 6 2 Up to 1500 How to make the difference?

  6. Connecting Networks • LANs are connected with point-to-point links • Packets are “routed” using another level of addresses other than MAC addresses • Paths may be multihop

  7. Network Layer Read Tanenbaum (Chapter 5)

  8. Where in the OSI Reference Model ? Session Layer Transport Layer Network Layer Link Layer Physical Layer

  9. Fundamental Functions of a Network Layer • Addressing • Routing • Congestion control • Note that not all network protocols provide congestion control

  10. Network Layer Protocols : two families • Connectionless • Each piece of information is sent as an independent entity. No state information is kept in hosts or routers • Connection oriented • There exists a virtual circuit over which all pieces of information will transit.

  11. Connectionless vs Connection-orientedTanenbaum Figure 5.4 • Circuit setup • Addressing • State information • Routing • Effect of routing failures • Congestion control

  12. Routing policy: updates the routing table Routing mechanism: decides how to route a packet depending on some policy (i.e, chooses the output line) Routing modifies Routing structure reads Quite elementary

  13. Routing Policy: Updating the Routing Tables • Manual • Using routing Daemons such as: • RIP • OSPF • EGP • BGP

  14. Routing Mechanism • Takes the decision how to route • For example, IP protocol implements in each Internet host a mechanism to route

  15. Desirable Properties of Routing • Correct • Simple • Fair • Robust • STABLE • Optimal

  16. Routing Mechanisms • Flooding • Hot-potatoe • Shortest path • Fixed routing • Dynamic routing

  17. Overview • Three big classes of routing policies • Distance vector routing (DVR) • Link state routing (LSR) • Hierarchical Routing (HR) • Application to Internet • RIP is a DVR • OSPF is an LSR • EGP, BGP (DVR)

  18. Distance Vector • A node • tells its neighbors only • its distance to EVERY NODE in the network • Example: • Initial A(0,1,4,inf), • B(1,0,1,1) • C(4,1,0,2) • D(inf,1,2,0) B 1 1 A 1 D 4 4 C A receives update from B ….

  19. Distance Vector (Problems!!!) • When ? • Links go down • What kind of problem ? • Count-to-infinity • Example • Solution • path vector A B C 1 1

  20. Link-State Routing • A node • tells its EVERY node • its distance to its NEIGHBORS • How ? • Send link-state packets (LSP) • using controlled flooding • Use Dijkstra’s algorithm B 1 1 A 1 D 4 4 C A receives update from B ….

  21. Internal and External Protocols Autonomous Systems Exterior Gateway Protocol Autonomous Systems LANs Internet Backbone Autonomous Systems Interior Gateway Protocol Autonomous Systems

  22. RIP (RFC 1058)(Routing Information Protocol) • Interior gateway protocol • Distance vector protocol • Uses split horizon to avoid count-to-infinity (Does not advertise a cost to a neighbor if it is a next hop for that destination) • Exchange each 30s • Time-out after 180 s.

  23. OSPF (RFC 1247)(Open Shortest Path First) • Interior gateway protocol • Link state protocol • Uses directly IP (while RIP uses UDP)

  24. BGP (RFC 1267)(Exterior Gateway Protocol) • Exterior gateway protocol • (Exception: BGP uses TCP !!!!) • Path Vector protocol (+ policy attributes) • Topology may be loop-free (BGP guarantees loop-freeness)

  25. The Internet Protocol (IP) RFC 791 Read 5.5

  26. A Connectionless Network Layer: the Internet Protocol (IP) • The Internet Protocol is found on every: • host that is connected to Internet • router on a LAN connected to the Internet • router on the backbone Application Transport Network IP Link Layer

  27. Fundamental Idea of IP • Routes between Networks, not between hosts • This allows shorter routing tables

  28. IP Header (Figure 3.1, p. 34) 0 31 15 16 V HL TOS Total Length 16-bit identification Flgs 13-bit frag. offset TTL Protocol 16-bit Hdr Checksum 32-bit source IP address 32-bit destination IP address Options (Variable 0 ---> ??) Data (TCP segment, or UDP Dtg, or ICMP ….)

  29. Internet Addresses 7bits 24 bits • Class A • Class B • Class C • Class D • Class E 0 netid hostid 14 bits 16 bits 1 netid hostid 0 21 bits 8 bits 1 netid hostid 1 0 28 bits 1 Multicast group ID 1 1 0 27 bits 1 Future use 1 1 1 0

  30. Special Internet Addresses 32 bits • This host • Local host • Local broadcast • Remote • Loopback 127.X.X.X 000000000000…..000000000000000000 00000….0000000 hostid 1111111111111…..11111111111111111 11111111111…..111111111 NetID

  31. Internet Addresses (Cont’d) • Unicast addresses (Classes A,B, and C) • Multicast addresses (Class D) • Dotted notation : the 32-address is divided in 4 groups of 8 bits (byte, octet). Each byte is expressed in base 10 separated by dots

  32. Subnetting • Why to do subnetting ? • How is it done ? • Notion of subnet mask

  33. Subnetting (RFC 950) 14 bits 16 bits Class B 1 netid hostid 0 n bits m bits netid hostid 5 bits 11 bits Example : netid hostid

  34. Subnetmask Idea : IP1 IP2 & (Bitwise AND) & Subnetmask Subnetmask = R1 = R2 R1 = R2 if IP1 and IP2 are on the same subnet R1 # R2 if IP1 and IP2 are on different subnets

  35. IP Routing • Based on a routing table with entries having • Destination IP addr (host ID or net ID) • Next hop router IP address • flags (hostid or netid, next hop or connected interface…) • network interface

  36. IP Routing Cont’d • 1) Try to find a complete IP address match in the routing table • 2) If 1) fails, then try to find a match with network id • if 1) and 2) fail search for default router

  37. Some IP Helpers • ARP (Address Resolution Protocol) : RFC 826 • RARP : Reverse Address Protocol (RFC 903) • BOOTP (RFC 951, 1048, 1084) replaced by • DHCP (RFC 1541) • ICMP (RFC 792)

  38. Extensions to IP • IPv6 • Mobile IP

  39. Conclusion • You must know: • The functions of the network layer • the difference between connectionless and connection oriented network protocols (what they can do and cannot do) • IP protocol and helpers (ICMP, DHCP, ARP) • the general features introduced by IPv6 • globally what is Mobile IP.

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