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Transcript
Network Layer Fundamentals 3rd Tutorial Session for CEG3180B February 1st, 2005 Stejarel Veres <[email protected]> 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 Stejarel Veres <[email protected]> 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 Stejarel Veres <[email protected]> 3 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 Stejarel Veres <[email protected]> 4 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 Stejarel Veres <[email protected]> 5 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 Stejarel Veres <[email protected]> 6 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 Class Class Class Class A – large size networks B – medium size networks C – small size networks D – special (multicast) E – special (reserved) Stejarel Veres <[email protected]> 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 Class Class Class Class A – large size networks B – medium size networks C – small size networks D – special (multicast) E – special (reserved) Stejarel Veres <[email protected]> 8 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) Stejarel Veres <[email protected]> 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) Stejarel Veres <[email protected]> 10 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) Stejarel Veres <[email protected]> 11 Static vs. Dynamic Routing Static is: Simpler to configure, yet more difficult to maintain Very low CPU time-consuming and memoryconsuming 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 memoryconsuming Virtually a must for redundant topologies and larger networks Stejarel Veres <[email protected]> 12 Simple Routing Algorithm 1. Examine destination address to determine if class A, B or C 2. Extract the network part from the address 3. Search for the destination network in the routing table 4. If found, and next hop is reachable: route out the specified interface to the next hop 5. Otherwise, discard the packet and send ICMP Destination Host/Network Unreachable message to the sender Stejarel Veres <[email protected]> 13 Original IPv4 Addressing Issues 1. Inefficient address space allocation a large part of the address space is being wasted 2. Inefficient routing – large routing tables, routing processes very CPU intensive Stejarel Veres <[email protected]> 14 Solutions Devised 1. Subnetting 2. Default routing; Classless InterDomain Routing (CIDR), also known as “Supernetting” Stejarel Veres <[email protected]> 15 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 Stejarel Veres <[email protected]> 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 Stejarel Veres <[email protected]> 17 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 Stejarel Veres <[email protected]> 18 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) Stejarel Veres <[email protected]> 19 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 Stejarel Veres <[email protected]> 20 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 Stejarel Veres <[email protected]> 21 Modified Routing Algorithm 1. 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 2. If matched, and next hop is reachable: route out the specified interface to the next hop 3. Otherwise, discard the packet and send ICMP Destination Host/Network Unreachable message to the sender Stejarel Veres <[email protected]> 22 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 1. AND 192.168.0.5 and 255.255.255.240 (/28) = 192.168.0.0 2. AND 192.168.0.18 and 255.255.255.240 = 192.168.0.16 NO (192.168.0.0 != 192.168.0.16) Stejarel Veres <[email protected]> 23 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 1. AND 192.168.0.66 and 255.255.255.192 (/26) = 192.168.0.64 2. AND 192.168.0.90 and 255.255.255.192 = 192.168.0.64 YES (192.168.0.64 == 192.168.0.64) Stejarel Veres <[email protected]> 24 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 1. AND 192.168.32.115 and 255.255.255.248 (/29) = 192.168.32.112 (subnet address) 2. 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) Stejarel Veres <[email protected]> 25 Routing Table Example Given the following routing table: 192.168.1.0 192.168.1.0 0.0.0.0 255.255.255.0 255.255.255.240 0.0.0.0 Serial0 Serial1 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) Stejarel Veres <[email protected]> 26 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 Stejarel Veres <[email protected]> 27 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 Stejarel Veres <[email protected]> 28 Distance Vector vs. Link State Distance Vector are: Simpler Less CPU time-consuming and often less memoryconsuming Slower-converging More bandwidth-consuming Less scalable Link State are: More complicated CPU and memory intensive Faster-converging Less bandwidth-consuming Very scalable Stejarel Veres <[email protected]> 29 So, Which One To Choose? Distance vector: in small and simple networks, or in networks with slowerCPU 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) Stejarel Veres <[email protected]> 30 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 Stejarel Veres <[email protected]> 31 References 1. 2. 3. 4. 5. 6. 7. 8. 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/ios12 3/123cgcr/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/ios12 3/123cgcr/iprrp_r/ip2bookg.pdf Stejarel Veres <[email protected]> 32