Maximizing Routing Efficiency in Computer Networks

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

2. 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

3. 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)

4. 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

5. Figure 11.4 Application of Exterior and Interior Routing Protocols

6. 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

7. 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

8. Exterior Router Protocols –Path-vector • 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

9. 11.2 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  • Two common algorithms • Dijkstra's algorithm • Bellman-Ford algorithm

10. 11.3Distance Vector Routing: RIP • 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

11. Figure 11.1 A Configuration of Routers and Networks (changed to 1) (changed to 1)

12. Figure 11.8 Distance Vector Algorithm Applied to Figure 11.1

13. 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

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

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

16. 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 ×

17. 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 • That is, A should not tell B “the distance to network 5”. • Erroneous route now eliminated within time out period (180 seconds)

18. 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 • B tells A and C “distance to network 5 is 16”

19. Figure 11.9 RIP Packet Format (v1) Command: 1: request, 2: response Address Family identifier: IP, IPX, … • Over UDP • Multicast: 224.0.0.9

20. RIP v2 Route Tag: 0 or AS#

22. 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

23. 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

24. 11.4Link-State Protocol: OSPF • RIP limited in large internets • Open Shortest Path First (OSPF) • OSPF preferred interior routing protocol for TCP/IP based internets • Link state routing used • Directly over IP

25. 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

26. 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

27. Figure 11.10 Flooding Example

28. 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

29. Router Database Graph • Vertices • Router • Network • Transit • Stub • Edges • Connecting two routers • Connecting router to network • Built using link state information from other routers

30. Figure 11.11 Sample Autonomous System

31. Figure 11.12 Directed Graph of Autonomous System of Figure 19.7

32. 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)

33. Figure 11.13 The SPF Tree for Router R6

34. Areas • Make large internets more manageable • Configure as backbone and multiple areas • Area – Collection of contiguous networks and hosts plus routers connected to any included network • Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas

35. Operation of Areas • Each area runs a separate copy of the link state algorithm • Topological database and graph of just that area • Link state information broadcast to other routers in area • Reduces traffic • Intra-area routing relies solely on local link state information

36. Inter-Area Routing • Path consists of three legs • Within source area • Intra-area • Through backbone • Has properties of an area • Uses link state routing algorithm for inter-area routing • Within destination area • Intra-area

37. Figure 11.14OSPF Packet Header * directly over IP

38. Packet Format Notes • Version number: 2 is current • Type: one of 5, see next slide • Packet length: in octets including header • Router id: this packet’s source, 32 bit • Area id: Area to which source router belongs • Authentication type: null, simple password or encryption • Authentication data: used by authentication procedure

39. OSPF Packet Types • Hello: used in neighbor discovery • Database description: Defines set of link state information present in each router’s database • Link state request • Link state update • Link state acknowledgement