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Computer Networks with Internet Technology William Stallings

Computer Networks with Internet Technology William Stallings. Chapter 11 Interior Routing Protocols. Introduction. Routing protocols essential to operation of an internet Routers forward IP datagrams from one router to another on path from source to destination

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Computer Networks with Internet Technology William Stallings

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  1. Computer Networks with Internet TechnologyWilliam Stallings Chapter 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 Chapter 11 Interior Routing Protocols

  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) Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  5. Figure 11.1 A Configuration of Routers and Networks Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  9. Figure 11.2Example Routing Tables Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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) Chapter 11 Interior Routing Protocols

  13. Figure 11.3 Example of Fluttering Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing 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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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) Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  21. Figure 11.4 Application of Exterior and Interior Routing Protocols Chapter 11 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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  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 Chapter 11 Interior Routing Protocols

  26. Figure 11.5 Dijkstra’s Algorithm Applied to Figure 11.1 Chapter 11 Interior Routing Protocols

  27. Figure 11.6 Bellman-Ford Algorithm Applied to Figure 11.1 Chapter 11 Interior Routing Protocols

  28. 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 Chapter 11 Interior Routing Protocols

  29. 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 Chapter 11 Interior Routing Protocols

  30. Figure 11.7 Distance Vector Algorithm Applied to Figure 11.1 Chapter 11 Interior Routing Protocols

  31. 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 Chapter 11 Interior Routing Protocols

  32. 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 Chapter 11 Interior Routing Protocols

  33. 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 Chapter 11 Interior Routing Protocols

  34. 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 Chapter 11 Interior Routing Protocols

  35. 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 Chapter 11 Interior Routing Protocols

  36. Figure 11.8Counting to Infinity Problem Chapter 11 Interior Routing Protocols

  37. 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) Chapter 11 Interior Routing Protocols

  38. 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 Chapter 11 Interior Routing Protocols

  39. Figure 11.9 RIP Packet Format Chapter 11 Interior Routing Protocols

  40. 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 Chapter 11 Interior Routing Protocols

  41. 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 Chapter 11 Interior Routing Protocols

  42. Open Shortest Path First (OSPF) • RIP limited in large internets • OSPF preferred interior routing protocol for TCP/IP based internets • Link state routing used Chapter 11 Interior Routing Protocols

  43. 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 Chapter 11 Interior Routing Protocols

  44. Flooding • Packet sent by source router to every neighbor • Incoming packet resent to all outgoing links except source link • Duplicate packets already transmitted are discarded • Prevent incessant retransmission • All possible routes tried so packet will get through if route exists • Highly robust • At least one packet follows minimum delay route • Reach all routers quickly • All nodes connected to source are visited • All routers get information to build routing table • High traffic load Chapter 11 Interior Routing Protocols

  45. Figure 11.10 Flooding Example Chapter 11 Interior Routing Protocols

  46. OSPF Overview • Router maintains descriptions of state of local links • Transmits updated state information to all routers it knows about • Router receiving update must acknowledge • Lots of traffic generated • Each router maintains database • Directed graph Chapter 11 Interior Routing Protocols

  47. Router Database Graph • Vertices • Router • Network • Transit • Stub • Edges • Connecting two routers • Connecting router to network • Built using link state information from other routers Chapter 11 Interior Routing Protocols

  48. Figure 11.11 Sample Autonomous System Chapter 11 Interior Routing Protocols

  49. Figure 11.12 Directed Graph of Autonomous System of Figure 19.7 Chapter 11 Interior Routing Protocols

  50. Link Costs • Cost of each hop in each direction is called routing metric • OSPF provides flexible metric scheme based on type of service (TOS) • Normal (TOS) 0 • Minimize monetary cost (TOS 2) • Maximize reliability (TOS 4) • Maximize throughput (TOS 8) • Minimize delay (TOS 16) • Each router generates 5 spanning trees (and 5 routing tables) Chapter 11 Interior Routing Protocols

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