Fairness Example: high priority for nearby stations Optimality Efficiency overhead

Transcription

1 Routing Requirements: Correctness Simplicity Robustness Under localized failures and overloads Stability React too slow or too fast Fairness Example: high priority for nearby stations Optimality Efficiency overhead A.29

2 Metric the smaller the value, the better the path Performance Criteria calculated based on one or several characteristics. Hop count the number of routers a pack must go through to reach a destination. 1 to 3? A.30

3 Performance Criteria Cost Bandwidth the data capacity of a link. Normally, the cost of a link with higher data capacity is smaller. Delay the length of time required to move a packet from source to destination Load the amount of activity on a network resource. Reliability A.31

4 Performance Criteria 1 to 3? A.32

5 Distributed and Centralized routing Distributed routing Each node has the responsibility of selecting an output link fro routing packets as they arrive Robust Centralized routing By a designated node Source Made by the source station A.33

6 Information Source Information for routing Topology of the network Traffic load Link cost Relate to routing method Flooding and random routing Distributed routing Collect from Local information: Adjacent nodes All nodes A.34

7 Information Source Information updating timing Depends on routing method Local information Update continuous More information update more frequently better decision However, it consumes more network resources. A.35

8 Routing Strategies Fixed Flooding Random Adaptive Distance vector routing Link state routing A.36

9 Static and Dynamic Routes Fixed routing use a programmed route for each source-destination pair of nodes in the network Based on expected traffic or capacity Only change when there is a change in the topology of the network Usually enter by a network administrator Adaptive routing the route is automatically adjusted for topology or traffic changes A.37

10 Fixed routing Example: usually stored at a network control center A.38

11 Fixed routing Each node need only store a single column of the routing directory A.39

12 Fixed routing A.40

13 Fixed routing Advantage Simplicity Work well in a reliable network with a stable load Disadvantage Lack of flexibility Do not react to network congestion or failtures A.41

14 Fixed routing Example: to avoid the overhead of dynamic routing if the network is accessible by only one path to hide parts of a network A.42

15 Flooding For sending a packet a source node to every one of its neighbors Example: From node 1 to node 6 A.43

16 Flooding At each node, an incoming packet is retransmitted on all outgoing links except for the link on which it arrived. A.44

17 Flooding End of transmission Approach 1 Retransmit a packet once Discard the packet if it was received before Have to remember the identity of all packets Approach 2 Use a hop count in each packet Count down from the maximum value A packet is discarded if the count reaches zero A.45

18 Flooding Example: Hop count = 3 Hop count =2 Hop count =1 Hop count =0 A.46

19 Flooding Advantage Highly robust All possible routes are tried Used for sending emergency messages At least one routes takes the minimum hopping route Used for setting up virtual circuit All nodes are visited Used for the dissemination (sending) of information to all nodes Example: sending routing information Disadvantage High traffic load A.47

20 Random Routing For sending a packet A node selects only one outgoing path Chosen at random Excluding the link on which the packet arrived Advantage Robust Simple Less traffic load (compare with flooding) Requires no network information Experiment: Random Routing (with return) Mini-project: Random Routing (without return) A.48

21 Adaptive Routing Routing decision change as conditions on the network change Example: Failure: When a node or trunk fails cannot use as part of a route Congestion: Choose other route Information must be exchanged among the nodes A.49

22 Adaptive Routing Example: enables routers to automatically use backup routes whenever necessary. A.50

23 Adaptive Routing Disadvantages Processing burden on nodes Increase network traffic Tradeoff between the quality of the information and the amount of overhead May react too quickly causing congestion-producing oscillation A.51

24 Adaptive Routing Two basic algorithms A.52

25 Distance Vector Routing In distance vector routing, each router periodically shares its knowledge about the entire network with its neighbors. Contains information about the entire network Routing only to neighbors Distance vector algorithms do not allow a router to know the exact topology of an internetwork. A.53

26 Distance Vector Routing (2) Example: Routers: A, B,, F Networks: Net:XX A.54

27 Distance Vector Routing (3) A.55

28 Distance Vector Routing (4) Initial routing table Network no cost next hop cost is based on hop count A.56

29 Distance Vector Routing (5) Updating the table When A receives a routing table from B, it uses the information to update its own table. A.57

30 Final routing tables Distance Vector Routing (6) A.58

31 Summary of updating algorithm The router first add one hop to the hop count field for each advertised route. Then, apply the following rules to the advertised route 1. If the advertised destination is not in the routing table, the router should add the advertised information to the table. A.59

32 Summary of updating algorithm (2) 2. If the advertised destination is in the routing table, a. If the next-hop field is the same, the router should replace the entry in the table with the advertised one. b. If the next-hop field is not the same, i. If the advertised hop count is smaller than the one in the table, the router should replace the entry in the table with the new one. ii. If the advertised hop count is not smaller, the router should do nothing. A.60

33 Summary of updating algorithm (3) Example, A.61

34 Distance vector topology changes A.62

35 Routing Loops Routing loops can occur if the network s slow convergence on a new configuration causes inconsistent routing entries. Example A.63

36 Routing loops (2) Assume Router C s preferred path to Network 1 is by way of Router B, and Router C has a distance of 3 to Network 1 in its routing table. When Network 1 fails, Router E sends an update to Router A stops routing packets to Network 1, but B, C, and D continue to route because they have not yet been informed about the failure. When Router A sends out its update, Routers B and D stop routing to Network 1. To Router C, Network 1 is still reachable via Router B. A.64

37 Routing loops (3) Now router C sends a periodic update to Router D, indicating a path to Network 1 by way of Router B. Router D changes its routing table to reflect this good, but incorrect, information, and propagates the information to Router A. Router A propagates the information to Routers B and E and so on. Any packet destined for Network 1 now loops from Router C to B to A to D, and back to C. A.65

38 Routing loops (4) To avoid this problem, the routing protocol permits the routing to loop until the metric exceeds its maximum allowed value. A.66

39 Line state routing Distance vector routing nonspecific information about distant networks and no knowledge of distant routers Line state routing Maintains full knowledge of distant routers and how they interconnect A.67

40 Construction of routing table Line state routing (2) Sharing of information A.68

41 Line state routing (3) Packet Cost In distance vector routing, cost refers to hop count. In link state routing, cost is a weighted value based on a variety of factors such as security levels, traffic, or the state of the link. Cost is applied when a packet leaves the router. A.69

42 Line state routing (4) A.70

43 Line state routing (5) Information sharing In link state routing, each router shares its knowledge of its neighborhood with all routers in the internetwork. Each router sends the information about its neighborhood to every other router by a process called flooding. A.71

44 Line state routing (6) A.72

45 Line state routing (7) When a router floods the network with information about its neighborhood, it is said to be advertising. The basis of this advertising is a short packet called a link state packet (LSP) or called link state advertisements packet (LSA) A.73

46 Example: Flooding of A s LSP Line state routing (6) A.74

47 Line state routing (7) Link state database Every router receives every LSP and puts the information into a link state database every router has exactly the same link state database A.75

48 Line state routing (8) Calculation of routing table calculate the shortest path between two points on a network using the Dijkstra algorithm Two terms node: networks and routers arcs: connections between a router and a network A.76

49 Line state routing (9) Shortest path tree 1. The algorithm begins from a router (called root). 2. Attached all nodes that can be reached from that router. Nodes and arcs are temporary at this step. A.77

50 Line state routing (10) 3. Identifies the arc with the lowest cumulative cost. This arc and the node to which it connects are now a permanent part of the shortest path tree. Cumulative cost A.78

51 Line state routing (11) 4. Attaches all nodes that can be reached from this chosen node. These nodes and their arcs are added temporarily to the tree. temporary A.79

52 Line state routing (12) 5. Step 3 and 4 are repeated until every node in the network has become a permanent part of the tree. The only permanent arcs are those that represent the shortest route to every node. Smallest cumulative cost A.80

53 Line state routing (13) (a) (b) A.81

54 Line state routing (14) (a) (b) A.82

55 Line state routing (15) (a) (b) A.83

56 Line state routing (16) (a) (b) A.84

57 Line state routing (17) (a) (b) Link state routing table for router A A.85

58 Line state routing (18) Distance vector vs link state routing A.86