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Transcript
CSC 581 Communication Networks II Chapter 7b: Network Routing Dr. Cheer-Sun Yang Routing Algorithm Classifications • Static vs. dynamic(adaptive) • Centralized vs. distributed 2 Types of Routing Protocols • Centralized vs. Distributed protocols • Static vs. Adaptive • Link State(adaptive) vs. Distance Vector(Static) • Interior vs. Exterior routing protocols 3 Centralized vs. Distributed • Centralized – All interconnection information is generated and maintained at a single central location. See Partial Routing Tables in Figure 7.4 and Routing Matrix in Figure 7.5 • Distributed – There is no central control. Each node must maintain and determine its routing information independently. See Figure 7.6 4 Static vs. Adaptive • Static– Once a node determines its routing table, the node does not change it unless the network configuration is changed, e.g., a node is added or deleted. Distance Vector algorithm is the base of static routing protocols. • Adaptive – The routing table will be modified as network conditions (link status, congestion status, configuration) change. Any pitfalls? (See Figure 7.6 again.) Link State routing protocols are adaptive protocols. • Can you compare these two techniques? 5 Link State vs. Distance Vector • Link State – adaptive; based on link status information and Dijkstra’s Algorithm • Distance Vector – static; based on hop count and Bellman-Ford Algorithm 6 Interior vs. Exterior • Interior – only used within a domain • Exterior – between two domains via gateways, e.g., hierarchical routing architecture 7 Routing Tables • Each network node stores information about the network. • When a packet arrives, it uses the destination address and the routing table to decide the routine. 8 1 3 6 A 4 B Host 2 5 Switch or router Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 9 Figure 7.23 1 A 1 2 3 5 7 3 1 4 8 6 B 5 2 4 3 C 2 6 5 5 2 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton D 10 Figure 7.24 Node 3 Node 1 Incoming node VC A 1 A 5 3 2 3 3 Outgoing node VC 3 2 3 3 A 1 A 5 Incoming node VC 1 2 1 3 4 2 6 7 6 1 4 4 Outgoing node VC 6 7 4 4 6 1 1 2 4 2 1 3 Node 6 Incoming node VC 3 7 3 1 B 5 B 8 Outgoing node VC B 8 B 5 3 1 3 7 Node 4 Node 2 Incoming node VC C 6 4 3 Outgoing node VC 4 3 C 6 Incoming node VC 2 3 3 4 3 2 5 5 Outgoing node VC 3 2 5 5 2 3 3 4 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton Node 5 Incoming node VC 4 5 D 2 Outgoing node VC D 2 4 5 11 Figure 7.25 Link State vs. Distance Vector • Link State: uses propagation delay, transmission speed, etc., as cost factors; a central node collects the “cost” of all links and broadcast the information to all nodes; every node maintains the “whole picture” of the network and computes the shortest distance. • Distance Vector: uses hop count as a cost factor; every node only maintains the cost to its neighbors. 12 (a) 0000 0001 0010 0011 1 0100 0101 0110 0111 4 3 R2 R1 5 2 1000 1001 1010 1011 00 01 10 11 1 3 2 3 00 01 10 11 1100 1101 1110 1111 3 4 3 5 (b) 0000 0111 1010 1101 1 0001 0100 1011 1110 4 3 R1 R2 5 2 0011 0110 1001 1100 0001 4 0000 1 0100 4 0111 1 1011 4 1010 1 … … … … Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 0011 0101 1000 1111 13 Figure 7.27 Examples • Routing Information Protocol – an interior protocol; based on hop count (an example of distance vector). • Open Shortest Path First (OSPF) – an interior protocol; based on length, bit rate, delay, or dollar cost (examples of link state). • Border Gateway Protocol – an exterior protocol (an example of hierarchical routing). 14 15 Bellman-Ford Algorithm • Also called backward search algorithm • The theory on which Distance Vector routing protocols are based. 16 Bellman-Ford Algorithm B A C E D Cost (A,Z) = smallest of the cost of link from A to K + cost of cheapest route from K to Z ( K such that A to K has a link). 17 1 2 1 3 6 5 2 3 4 1 2 3 2 4 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 18 Figure 7.28 1 2 1 3 6 2 1 2 4 2 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 19 Figure 7.29 2 1 3 X 6 5 2 3 4 1 2 3 2 4 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 20 Figure 7.30 Bellman-Ford Algorithm A 2 D 7 1 2 E B 2 C 4 Network for Bellman-Ford Algorithm 21 Destination 22 23 (c) Third Iteration 24 Bellman-Ford Routing 25 Problems with Bellman-Ford Algorithm • Sometimes, a node may update its routing information slowly. • It reacts rapidly to good news, but leisurely to bad news. 26 Count-to-Infinity Problem A B C D E 1 1 2 1 2 3 1 2 3 4 A is down initially. Then, A comes back up. A B C D E 1 2 3 4 3 3 2 4 3 3 5 5 7 4 6 6 8 5 5 7 7 … 4 4 4 7 6 6 8 The link between A and B is cut.27 (a) 1 2 1 3 1 4 1 (b) 1 2 1 3 X 4 1 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 28 Figure 7.31 Split Horizon 1. Initially, all nodes are up. 2. Then, A goes down. 3. One the first change, B knows that A is down;C knows that C cannot reach A either. 3. C reports the distance to A as infinity. The distance to X is not reported backward to nodes incident with the link where packets sent to X must pass. In C, the distance to A is always reported to B as infinity. A B 1 C D 2 3 E 4 … 29 ARPANET Routing Strategies(1) • First Generation (1957, 1962) – Uses Bellman-Ford Algorithm (a distributed version) • Second Generation (1979) – – – – – Uses delay as performance criterion Delay measured directly Uses Dijkstra’s algorithm Good under light and medium loads Under heavy loads, little correlation between reported delays and those experienced 30 ARPANET Routing Strategies(2) • Third Generation (1987) – Link cost calculations changed – Measure average delay over last 10 seconds – Normalize based on current value and previous results – Link State Routing – Hierarchical Routing 31 Link State Algorithms • A node gathers information on the status of each link to each neighbor. • A node builds a link state packet for each link. • A node receiving link state packets forwards them to all of its neighbors except the one from which it receives the packet. • As link state packets are exchanged, each node eventually learns about the network topology, the cost, and the status of each link. 32 Dijkstra’s Algorithm – A Link State Algorithm • Finding a shortest path from a node. • A centralized static algorithm • Link state routing approaches are based on Dijkstra’s Algorithm. 33 1 2 1 3 6 2 3 4 2 2 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 34 Figure 7.32 Hierarchical Routing • All nodes are divided into groups called domains. • Routes between two nodes in a common domain are determined using the domain’s protocols. • Each domain has one or more specifically designated nodes called routers or gateways that determine routes between domains. • A domain can consist of subdomains. 35 36 Hierarchical Routing • Purpose: reduce the size of routing tables for routing in a large networking environment 37 Node 1 Destination Next node 2 2 3 3 4 4 5 2 6 3 Node 2 Destination Next node 1 1 3 1 4 4 5 5 6 5 Node 3 Destination Next node 1 2 4 5 6 Destination 1 2 3 5 6 1 4 4 6 6 Node 4 Next node 1 2 3 5 3 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton Node 6 Destination Next node 1 3 2 5 3 3 4 3 5 5 Destination 1 2 3 4 6 Node 5 Next node 4 2 4 4 6 38 Figure 7.26 39 40 Autonomous Systems (AS) • • • • Group of routers Exchange information Common routing protocol Set of routers and networks managed by single organization • A connected network – There is at least one route between any pair of nodes 41 Routing Information Protocol • Routing Information – About topology and delays in the internet • Routing Algorithm – Used to make routing decisions based on information 42 43 Open Shortest Path First (1) • • • • OSPF IGP of Internet Replaced Routing Information Protocol (RIP) Uses Link State Routing Algorithm – – – – Each router keeps list of state of local links to network Transmits update state info Little traffic as messages are small and not sent often RFC 2328 • Route computed on least cost based on user cost metric 44 Open Shortest Path First (2) • Topology stored as directed graph • Vertices or nodes – Router – Network • Transit • Stub • Edges – Graph edge • Connect two router • Connect router to network 45 Operation • Dijkstra’s algorithm used to find least cost path to all other networks • Next hop used in routing packets 46 Border Gateway Protocol (BGP) • For use with TCP/IP internets • Preferred EGP of the Internet • Messages sent over TCP connections – – – – Open Update Keep alive Notification • Procedures – Neighbor acquisition – Neighbor reachability – Network reachability 47 Other Routing Approaches • Flooding • Deflection Routing • Source Routing 48 (a) 1 3 6 4 2 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 49 Figure 7.33 - Part 1 of 3 (b) 1 3 6 4 2 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 50 Figure 7.33 - Part 2 of 3 (c) 1 3 6 4 2 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 51 Figure 7.33 - Part 3 of 3 Deflection Routing • Also known as hot potato routing • The network maintains multiple paths to a destination. • It tries all paths one at a time. • Advantage: a switch can be bufferless since packets do not have to wait for a specific port to become available. 52 0,0 0,1 0,2 0,3 1,0 1,1 1,2 1,3 2,0 2,1 2,2 2,3 3,0 3,1 3,2 3,3 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 53 Figure 7.34 busy 0,0 0,1 0,2 0,3 1,0 1,1 1,2 1,3 2,0 2,1 2,2 2,3 3,0 3,1 3,2 3,3 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton 54 Figure 7.35 Source Routing • Source hosts maintain the big picture 55 3,6,B 6,B 1,3,6,B 1 3 6 B A 4 B Source host 2 5 Copyright 2000 McGraw-Hill LeonGarcia and Widjaja Communicaiton Destination host 56 Figure 7.36 Required Reading • Chapter 7: section 7.4, 7.5, 7.6 57