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Network Layer Fundamentals

Network Layer Fundamentals. 3 rd Tutorial Session for CEG3180B February 1 st , 2005. The ISO OSI Model. A conceptual, layered model for designing networked systems (i.e., both the hardware and software components that relate to networking a certain system)

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Network Layer Fundamentals

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  1. Network Layer Fundamentals 3rd Tutorial Session for CEG3180B February 1st, 2005

  2. The ISO OSI Model • A conceptual, layered model for designing networked systems (i.e., both the hardware and software components that relate to networking a certain system) • 7 Layers (from top to bottom): Application, Presentation, Session, Transport, Network, Data Link, Physical • The higher the layer, the more abstract its functions are with respect to the actual physical transmission

  3. The ISO OSI Model • A conceptual, layered model for designing networked systems (i.e., both the hardware and software components that relate to networking a certain system) • 7 Layers (from top to bottom): Application, Presentation, Session, Transport, Network, Data Link, Physical • The higher the layer, the more abstract its functions are with respect to the actual physical transmission

  4. The ISO OSI Model (cont’d) • Allows for transparent peer-to-peer communication between same layers of two networked systems • Top four layers: “network layers”; the other three layers: “host layers” • Beginning with the upmost half of the Data Link Layer (the LLC Sub-Layer), operations are media-independent

  5. The Network Layer • Two fundamental functions: • Logical network topology and Addressing • Path determination (i.e., Datagram routing) • The rest of this discussion focuses on the IP (Internet Protocol), version 4 (IPv4) of the TCP/IP Protocol Stack

  6. Original IPv4 Addressing • 32-bit addresses (010010111…) • Most of the times written in the “dotted-decimal” format: 4 numbers between 0 and 255, separated by dots • E.g., 137.122.14.100 • Theoretically to yield 232 ~ 4.3 billion addresses

  7. Original IPv4 Addressing (cont’d) • Address space divided into “classes of addresses” based on the size of the networks it was supposed to be allocated to: • Class A – large size networks • Class B – medium size networks • Class C – small size networks • Class D – special (multicast) • Class E – special (reserved)

  8. Original IPv4 Addressing (cont’d) • Address space divided into “classes of addresses” based on the size of the networks it was supposed to be allocated to: • Class A – large size networks • Class B – medium size networks • Class C – small size networks • Class D – special (multicast) • Class E – special (reserved)

  9. Path Determination • I.e., datagram (packet) routing • The “hop-by-hop” routing paradigm: packet passes from router to router, each step bringing it closer to the destination • If a packet travels too many hops, it is discarded (in order to prevent routing loops)

  10. Path Determination • I.e., datagram (packet) routing • The “hop-by-hop” routing paradigm: packet passes from router to router, each step bringing it closer to the destination • If a packet travels too many hops, it is discarded (in order to prevent routing loops)

  11. Path Determination (cont’d) • Routers maintain “routing tables” containing, for each known destination network address: • The output interface for that destination • The next hop address for that destination • Routing tables updated statically (“by hand”) or dynamically (by using dynamic routing protocols)

  12. Static vs. Dynamic Routing • Static is: • Simpler to configure, yet more difficult to maintain • Very low CPU time-consuming and memory-consuming • Not at all suited for large networks and only marginally suited for redundant topologies • Dynamic is: • More difficult to configure, but need not be manually maintained up to date • Usually more CPU time-consuming and memory-consuming • Virtually a must for redundant topologies and larger networks

  13. Simple Routing Algorithm • Examine destination address to determine if class A, B or C • Extract the network part from the address • Search for the destination network in the routing table • If found, and next hop is reachable: route out the specified interface to the next hop • Otherwise, discard the packet and send ICMP Destination Host/Network Unreachable message to the sender

  14. Original IPv4 Addressing Issues • Inefficient address space allocation - a large part of the address space is being wasted • Inefficient routing – large routing tables, routing processes very CPU intensive

  15. Solutions Devised • Subnetting • Default routing; Classless Inter-Domain Routing (CIDR), also known as “Supernetting”

  16. Subnetting • “Borrowing” bits from the host portion for the network portion of the address • Network addresses expressed as pairs of “address” and “subnet mask” • The concept of “classes” becomes obsolete, yet designs have sometimes to accommodate older equipment with no knowledge of subnetting

  17. Subnetting • “Borrowing” bits from the host portion for the network portion of the address • Network addresses expressed as pairs of “address” and “subnet mask” • The concept of “classes” becomes obsolete, yet designs have sometimes to accommodate older equipment with no knowledge of subnetting

  18. Subnet Masks • 32-bit strings with a contiguous left side of 1’s and a contiguous right side of 0’s • The number of 1’s (the “length” of the subnet mask): how many bits of the address corresponds to the network part

  19. Subnet Masks (cont’d) • Written either in dotted-decimal format, or as /number_of_1’s (/length) • Original classes of addresses: • A – 255.0.0.0 (/8) • B – 255.255.0.0 (/16) • C – 255.255.255.0 (/24)

  20. Default Routing • Specifies a way to handle packets for which no specific entry exists in the routing table • “Fall-back”: the packed is routed via a “default gateway” that is supposed to know better what to do with it • Especially useful for “stub networks” • Helps keeping routing tables small • Default route entry: 0.0.0.0/0

  21. Classless Inter-Domain Routing • Grouping a number of contiguous network addresses into a larger routing table entry • E.g., 192.168.8.0/24 through 192.168.15.0/24 can be written as 192.168.8.0/21 • Helps keeping routing tables small

  22. Modified Routing Algorithm • For each routing table entry: perform AND between destination address and entry subnet mask; if result equals the entry network address and entry more specific (i.e., longer subnet mask) than the previous one, keep it and discard the other • If matched, and next hop is reachable: route out the specified interface to the next hop • Otherwise, discard the packet and send ICMP Destination Host/Network Unreachable message to the sender

  23. Subnetting Examples • Given the following two address/mask pairs, how can we tell whether they are on the same subnet or not? • 192.168.0.5/28 and 192.168.0.18/28 • AND 192.168.0.5 and 255.255.255.240 (/28) = 192.168.0.0 • AND 192.168.0.18 and 255.255.255.240 = 192.168.0.16 • NO (192.168.0.0 != 192.168.0.16)

  24. Subnetting Examples (cont’d) • Given the following two address/mask pairs, how can we tell whether they are on the same subnet or not? • 192.168.0.66/26 and 192.168.0.90/26 • AND 192.168.0.66 and 255.255.255.192 (/26) = 192.168.0.64 • AND 192.168.0.90 and 255.255.255.192 = 192.168.0.64 • YES (192.168.0.64 == 192.168.0.64)

  25. Subnetting Examples (cont’d) • Given the following address/mask pair, can you determine the subnet address and the address range for that subnet? • 192.168.32.115/29 • AND 192.168.32.115 and 255.255.255.248 (/29) = 192.168.32.112 (subnet address) • OR 192.168.32.112 and NOT 255.255.255.248 = 192.168.32.119 (broadcast address) • Address range: 192.168.32.112-119 (6 usable addresses, 113-118)

  26. Routing Table Example • Given the following routing table: 192.168.1.0 255.255.255.0 Serial0 192.168.1.0 255.255.255.240 Serial1 0.0.0.0 0.0.0.0 Serial2 • Address 192.168.1.20 will route by entry 1 • Address 192.168.1.5 will route by entry 2 • Address 192.168.3.35 will route by entry 3 (via the default gateway)

  27. Dynamic Routing Protocols • Can be classified from multiple points of view • By the algorithm they use for building routing tables: • Distance Vector: use “distance” metrics • Link State: use “cost” metrics and SPF algorithms • Hybrid • By the way they use and advertise subnet information: • Classless: they accept and advertise subnets • Classful: they ignore and don’t advertise subnets • By their intended use: • Exterior Gateway Protocols (EGP): inter-AS • Interior Gateway Protocols (IGP): intra-AS

  28. Examples of Routing Protocols • RIPv1: IGP, distance vector, classful • RIPv2: IGP, distance vector, classless • IGRP (Cisco): IGP, distance vector, classful • EIGRP (Cisco): IGP, advanced distance vector (sometimes called “hybrid”), classless • OSPF, IS-IS: IGP, link state, classless • BGP-4: EGP, hybrid, classless

  29. Distance Vector vs. Link State • Distance Vector are: • Simpler • Less CPU time-consuming and often less memory-consuming • Slower-converging • More bandwidth-consuming • Less scalable • Link State are: • More complicated • CPU and memory intensive • Faster-converging • Less bandwidth-consuming • Very scalable

  30. So, Which One To Choose? • Distance vector: in small and simple networks, or in networks with slower-CPU and small-sized memory routers • Link state: in large networks, and in networks requiring Shortest Path Tree calculation for the purpose of Traffic Engineering (i.e., MPLS-TE)

  31. Our Labs • Will consist of configuring Cisco routers for Static Routing (Lab 2), for OSPF routing within a single area (Lab 3), and for OSPF routing within a multi-area topology (Lab 4) • Technical documentation to be consulted listed in the References section of this presentation

  32. References • J. Postel, STD0005/RFC0791: Internet Protocol • J. Postel, STD0005/RFC0792: Internet Control Message Protocol • J. C. Mogul, J. Postel, STD0005/RFC0950: Internet Standard Subnetting Procedure • Y. Rekhter, T. Li, RFC1518: An Architecture for IP Address Allocation with CIDR • IANA, RFC3330: Special-Use IPv4 Addresses • Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot, E. Lear, RFC1918: Address Allocation for Private Internets • Cisco IOS IP Command Reference, Volume 1 of 4: Addressing and Services, Release 12.3 http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/ipras_r/ip1bookg.pdf • Cisco IOS IP Command Reference, Volume 2 of 4: Routing Protocols, Release 12.3 http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/iprrp_r/ip2bookg.pdf

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