CIS 203

1. CIS 203 11 : Interior Routing Protocols

2. Introduction • Routing protocols essential to operation of an internet • Routers forward IP datagrams from one router to another on path from source to destination • Router must have idea of topology of internet • Routing protocols provide this information

3. Internet Routing Principles • Routers receive and forward datagrams • Make routing decisions based on knowledge of topology and conditions on internet • Decisions based on some least cost criterion (chapter 14)

4. Fixed Routing • Single permanent route configured for each source-destination pair • Routes fixed • May change when topology changes • Link cost not based on dynamic data • Based on estimated traffic volumes or capacity of link

5. Figure 11.1 A Configuration of Routers and Networks

6. Discussion of Example • 5 networks, 8 routers • Link cost for output side of each router for each network • Next slide shows how fixed cost routing may be implemented • Each router has routing table

7. Routing Table • One required for each router • Entry for each network • Not for each destination • Routing only needs network portion • Once datagram reaches router attached to destination network, that router can deliver to host • IP address typically has network and host portion • Each entry shows next node on route • Not whole route

8. Routing Tables in Hosts • May also exist in hosts • If attached to single network with single router then not needed • All traffic must go through that router (called the gateway) • If multiple routers attached to network, host needs table saying which to use

9. Figure 11.2Example Routing Tables

10. Adaptive Routing • As conditions on internet changes, routes may change • Failure • Can route round problems • Congestion • Can route round congestion • Avoid, or at least not add to further congestion

11. Drawbacks of Adaptive Routing • More complex routing decisions • Router processing increases • Depends on information collected in one place but used in another • More information exchanged improves routing decisions but increases overhead • May react two fast causing congestion through oscillation • May react to slow, being irrelevant • Can produce pathologies • Fluttering • Looping

12. Fluttering • Rapid oscillation in routing • Due to router attempting load balancing or splitting • Splitting traffic among a number of routes • May result in successive packets bound for same destination taking very different routes (see next slide)

13. Figure 11.3 Example of Fluttering

14. Problems with Fluttering • If in one direction only, route characteristics may differ in the two directions • Including timing and error characteristics • Confuses management and troubleshooting applications that measure these • Difficulty estimating round trip times • TCP packets arrive out of order • Spurious retransmission • Duplicate acknowledgements

15. Looping • Packet forwarded by router eventually returns to that router • Algorithms designed to prevent looping • May occur when changes in connectivity not propagated fast enough to all other routers

16. Adaptive Routing Advantages • Improve performance as seen by user • Can aid congestion control • Benefits depend on soundness of design • Adaptive routing very complex • Continual evolution of protocols

17. Classification of Adaptive Routing Strategies • Based on information sources • Local • E.g. route each datagram to network with shortest queue • Balance loads on networks • May not be heading in correct direction • Include preferred direction • Rarely used • Adjacent nodes • Distance vector algorithms • All nodes • Link-state algorithms • Both need routing protocol to exchange information

18. Autonomous Systems (AS) • Group of routers exchanging information via common routing protocol • Set of routers and networks managed by single organization • Connected • Except in time of failure

19. Interior Routing Protocol (IRP) • Passes routing information between routers within AS • Does not need to be implemented outside AS • Allows IRP to be tailored • May be different algorithms and routing information in different connected AS • Need minimum information from other connected AS • At least one router in each AS must talk • Use Exterior Routing Protocol (ERP)

20. Exterior Routing Protocol (ERP) • Pass less information than IRP • Router in first system determines route to target AS • Routers in target AS then co-operate to deliver datagram • ERP does not deal with details within target AS

21. Figure 11.4 Application of Exterior and Interior Routing Protocols

22. Approaches to Routing – Distance-vector • Each node (router or host) exchange information with neighboring nodes • Neighbors are both directly connected to same network • First generation routing algorithm for ARPANET • Node maintains vector of link costs for each directly attached network and distance and next-hop vectors for each destination • Used by Routing Information Protocol (RIP) • Requires transmission of lots of information by each router • Distance vector to all neighbors • Contains estimated path cost to all networks in configuration • Changes take long time to propagate

23. Approaches to Routing – Link-state • Designed to overcome drawbacks of distance-vector • When router initialized, it determines link cost on each interface • Advertises set of link costs to all other routers in topology • Not just neighboring routers • From then on, monitor link costs • If significant change, router advertises new set of link costs • Each router can construct topology of entire configuration • Can calculate shortest path to each destination network • Router constructs routing table, listing first hop to each destination • Router does not use distributed routing algorithm • Use any routing algorithm to determine shortest paths • In practice, Dijkstra's algorithm • Open shortest path first (OSPF) protocol uses link-state routing. • Also second generation routing algorithm for ARPANET

24. Exterior Router Protocols –Path-vector • Dispense with routing metrics • Provide information about which networks can be reached by a given router and ASs crossed to get there • Does not includedistance or cost estimate • Each block of information lists all ASs visited on this route • Enables router to perform policy routing • E.g. avoid path to avoid transiting particular AS • E.g. link speed, capacity, tendency to become congested, and overall quality of operation, security • E.g. minimizing number of transit Ass

25. Least Cost Algorithms • Least-cost criterion • If minimize number of hops, link value 1 • Link value may be inversely proportional to capacity, proportional to current load, or some combination • May differ in different two directions • E.g. if cost equaled length of queue • Cost of path between two nodes as sum of costs of links traversed • For each pair of nodes, find least cost path  • Dijkstra's algorithm • Bellman-Ford algorithm

26. Dijkstra's Algorithm • Find shortest paths from given node to all other nodes, by developing paths in order of increasing path length • Proceeds in stages • By kth stage, shortest paths to k nodes closest to (least cost away from) source have been determined • Set T • Stage (k + 1), node not in T with shortest path from source added to T • As each node added to T, path from source defined

27. Dijkstra's Algorithm – Formal (1) • N = set of nodes in the network • s = source node • T = set of nodes so far incorporated • w(i, j) = link cost from node i to node j • w(i, i) = 0 • w(i, j) =  if nodes not directly connected • w(i, j)  0 if nodes directly connected • L(n) = cost of least-cost path s to n currently known • At termination, cost of least-cost path in graph from s to n

28. Dijkstra's Algorithm – Formal (2) [Initialization] T = {s} i.e. set of nodes so far incorporated consists of only source node L(n) = w(s, n) for n ≠ s i.e. initial path costs to neighboring nodes are link costs [Get Next Node] Find neighboring node not in T with least-cost path from s Incorporate node into T Also incorporate edge incident on that node and node in T that contributes to the path. This can be expressed as: Find x Ï T such that Add x to T; add to T the edge that is incident on x and that contributes the least cost component to L(x), that is, the last hop in the path.

29. Dijkstra's Algorithm – Formal (3) [Update Least-Cost Paths] L(n) = min[L(n), L(x) + w(x, n)] for all n Ï T If the latter term is the minimum, the path from s to n is now the path from s to x concatenated with the edge from x to n. The algorithm terminates when all nodes have been added to T

30. Figure 11.5 Dijkstra’s Algorithm Applied to Figure 11.1

31. Bellman-Ford Algorithm • Find shortest paths from source node such that paths contain at most one link • Find shortest paths such that paths have at most two links • And so on

32. Figure 11.6 Bellman-Ford Algorithm Applied to Figure 11.1

33. Bellman-Ford Algorithm – Formal (1) • s = source node • w(i, j) = link cost from node i to node j • w(i, i) = 0 • w(i, j) =  if nodes are directly connected • w(i, j)  0 if nodes directly connected • h = maximum number of links in path at current stage • Lh(n) =cost of least-cost path from s to n such that no more than h links

34. Bellman-Ford Algorithm – Formal (2) [Initialization] L0(n) = , for all n  s Lh(s) = 0, for all h [Update] For each successive h  0: For each n ≠ s, compute Connect n with predecessor node j that achieves minimum Eliminate any connection of n with different predecessor node formed during an earlier iteration Path from s to n terminates with link from j to n

35. Comparison of Algorithms • Bellman-Ford • Link cost to all neighboring nodes to node n [i.e., w(j, n)] plus total path cost to those neighboring nodes from a particular source node s [i.e., Lh(j)] • Each node can maintain set of costs and associated paths for every other node and exchange information with direct neighbors • Each node can use Bellman-Ford based only on information from neighbors and knowledge of its link costs • Dijkstra • Each node must know link costs of all links • Information must be exchanged with all other nodes • Both converge under static conditions to same solution • If costs change algorithm will attempt to catch up • If cost depends on traffic • Depends on routes chosen • then feedback condition exists • Instabilities may result

36. Distance Vector Routing • Each node exchange information with neighbors • Directly connected by same network • Each node maintains three vectors • Link cost • Distance vector • Next hop vector • Every 30 seconds, exchange distance vector with neighbors • Use this to update distance and next hop vector

37. Figure 11.7 Distance Vector Algorithm Applied to Figure 11.1

38. Distributed Bellman-Ford • RIP is a distributed version of Bellman-Ford • Original routing algorithm in ARPANET • Each simultaneous exchange of vectors between routers is equivalent to one iteration of step 2 • In fact, asynchronous exchange used • At start-up, get vectors from neighbors • Gives initial routing • By own timer, update every 30 seconds • Changes are propagated across network • Routing converges within finite time • Proportional to number of routers

39. RIP Details – Incremental Update • Updates do not arrive from neighbors within small time window • RIP packets use UDP • Tables updated after receipt of individual distance vector • Add any new destination network • Replace existing routes with small delay ones • If update from router R, update all routes using R as next hop

40. RIP Details –Topology Change • If no updates received from a router within 180 seconds, mark route invalid • Assumes router crash or network connection unstable • Set distance value to infinity • Actually 16

41. Counting to Infinity Problem (1) • Slow convergence may cause: • All link costs 1 • B has distance to network 5 as 2, next hop D • A & C have distance 3 and next hop B

42. Counting to Infinity Problem (2) • Suppose router D fails: • B determines network 5 no longer reachable via D • Sets distance to 4 based on report from A or C • At next update, B tells A and C this • A and C receive this and increment their network 5 distance to 5 • 4 from B plus 1 to reach B • B receives distance count 5 and assumes network 5 is 6 away • Repeat until reach infinity (16) • Takes 8 to 16 minutes to resolve

43. Figure 11.8Counting to Infinity Problem

44. Split Horizon • Counting to infinity problem caused by misunderstanding between B and A, and B and C • Each thinks it can reach network 5 via the other • Split Horizon rule says do not send information about a route back in the direction it came from • Router sending information is nearer destination than you • Erroneous route now eliminated within time out period (180 seconds)

45. Poisoned Reverse • Send updates with hop count of 16 to neighbors for route learned from those neighbors • If two routers have routes pointing at each other advertising reverse route with metric 16 breaks loop immediately

46. Figure 11.9 RIP Packet Format

47. RIP Packet Format Notes • Command: 1=request 2=reply • Updates are replies whether asked for or not • Initializing node broadcasts request • Requests are replied to immediately • Version: 1 or 2 • Address family: 2 for IP • IP address: non-zero network portion, zero host portion • Identifies particular network • Metric • Path distance from this router to network • Typically 1, so metric is hop count

48. RIP Limitations • Destinations with metric more than 15 are unreachable • If larger metric allowed, convergence becomes lengthy • Simple metric leads to sub-optimal routing tables • Packets sent over slower links • Accept RIP updates from any device • Misconfigured device can disrupt entire configuration

49. Open Shortest Path First (OSPF) • RIP limited in large internets • OSPF preferred interior routing protocol for TCP/IP based internets • Link state routing used

50. Link State Routing • When initialized, router determines link cost on each interface • Router advertises these costs to all other routers in topology • Router monitors its costs • When changes occurs, costs are re-advertised • Each router constructs topology and calculates shortest path to each destination network • Not distributed version of routing algorithm • Can use any algorithm • Dijkstra