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Routing
This document provides an overview of routing concepts and protocols. It discusses the basic components of routing including algorithms, databases, and protocols. It describes different routing algorithm types such as static, distance vector, and link state. Specific routing protocols covered include RIP, OSPF, and BGP. It also discusses routing within autonomous systems and between autonomous systems on the internet.
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Routing
- 1. Routing Concept Sirak Kaewjamnong ComputerNetwork Tech and Security
- 2. 2 Agenda • Basic concepts •Routing components • Classes of routing protocol • Internet routing protocol
- 3. 3 What’s Routing Routing -path finding from one end to the other • Routing occurs at layer 3 • Bridging occurs at layer 2 Physical link Data link layer Network layer Transport layer Session layer Presentation layer Application layer Physical link Data link layer Network layer Transport layer Session layer Presentation layer Application layer Physical link Data link layer Network layer Network A Network B
- 4. 4 IP Routing • IPperforms: • search for a matching host address • search for a matching network address • search for a default entry • Routing done by IP router, when it searches the routing table and decide which interface to end a packet out. incoming which interface ?
- 5. 5 Routing Tables • Routingis carried out in a router by consulting routing table. • No unique format for routing tables, typically table contains: – address of a destination – IP address of next hop router – network interface to be used – subnet mask for the this interface – distance to the destination
- 6. 6 Routing Component • Threeimportant routing elements : – algorithm – database – protocol • Algorithm : can be differentiate based on several key characteristics • Database : table in routers or routing table • Protocol: the way information for routing to be gathered and distributed
- 7. 7 Routing Algorithm Design goals •Optimality - compute the best route • Simplicity/low overhead - efficient with a minimum software and utilization overhead • Robustness/stability- perform correctly in the face of unusual circumstances • Rapid convergence- responds quickly when the network changes • Flexibility- accurate adapt to a variety of network
- 8. 8 Routing Protocols • Routingprotocol : protocol to exchange of information between routers about the current state of the network • Routing protocol jobs – create routing table entries – keep routing table up-to-date – compute the best choice for the next hop router
- 9. 9 Routing Metrics • Howdo we decide that one route is better than another? • Solution : using a metric as a measurement to compare routes • Metrics may be distance, throughput, delay, error rate, and cost. • Today, IP supports Delay, Throughput, Reliability and Cost (DTRC)
- 10. 10 Hop Count • Ahop is defined as a passage through one router R1 R2 R3 1 hop 1 hop 1 hop 1 hop 2 hops
- 11. 11 Routing Algorithm Types •Static V.S. Dynamic • Source routing V.S. Hop-by-hop • Centralize V.S. Distributed • Distance vector V.S. Link state
- 12. 12 Routing Algorithm: StaticRoute • Manually configuration routing table • Can’t react dynamically to network change such as router’s crash • Work well with small network or simple topology • Unix hosts use command route to add an entry point to point connection route to this way only, no need for update
- 13. 13 Routing Algorithm: StaticTechnique Flooding • Every incoming packet is sent out every outgoing • Retransmit on all outgoing at each node • Simple technique, require no network information • Generate vast numbers of duplicate packet incoming flooding
- 14. 14 Routing Algorithm: DynamicRoute Dynamic route • Network protocol adjusts automatically for topology or traffic changes • Unix hosts run routing daemon routed or gated
- 15. 15 Routing Algorithm: DynamicRoute operation • Routing protocol maintains and distributes routing information Update Routing Information Routing Table Routing Table Routing Protocol Routing Protocol
- 16. 16 Routing Algorithm: SourceRouting • Source routing – Source will determine the entire route – Routers only act as store-forward devices • Hop-by-hop – Routers determine the path based on theirs own calculation
- 17. 17 Routing Algorithm: DistanceVector • Distance means routing metric • Vector means destination • Flood routing table only to its neighbors • RIP is an example • Also known as Bellmann-Ford algorithm or Ford- Fulkerson algorithm
- 18. 18 Distance Vector Algorithm •Using hop count as a metric • Each router periodically sends a copy of its routing table to neighbors • send <network X, hopcount Y> routing table W 0 X 0 Y 1 Z 2 routing table W 1 X 0 Y 0 Z 1 routing table W 2 X 1 Y 0 Z 0 W X Y Z R1 R2 R3
- 19. 19 Distance Vector RoutingUpdate • Step by step from router to router • Slow convergence R1 R2 R3 topology change Œ recompute R3’s routing table R3 sends out the updated table Ž recompute R2’s routing table R2 sends out the updated table recompute R1’s routing table
- 20. 20 Distance Vector: Broadcast(I) • The first round R1 R2 R3 R4 I J K L M I, 1 hop J, 1 hop I, 1 hop K, 1 hop L, 1 hop N J, 1 hop K, 1 hop M, 1 hop N, 1 hop L, 1 hop M, 1 hop O 1 hop O R5 N, 1 hop O, 1 hop
- 21. 21 Distance Vector: Broadcast(II) • The second round R1 R2 R3 R4 I J K L M I, 1 hop J, 1 hop K, 2 hops L, 2 hops M, 2 hops N, 2 hops I, 1 hop K, 1 hop L, 1 hop J, 2 hops M, 2 hops N, 2 hops O, 2 hops N J, 1 hop K, 1 hop M, 1 hop N, 1 hop I, 2 hops L, 2 hops O, 2 hops L, 1 hop M, 1 hop O, 1 hop I, 2 hops K, 2 hops J, 2 hops N, 2 hops O R5 N, 1 hop O, 1 hop J, 2 hops K, 2 hops M, 1 hop L, 2 hops
- 22. 22 Distance Vector: Broadcast(III) • The third round R1 R2 R3 R4 I J K L M I, 1 hop J, 1 hop K, 2 hops L, 2 hops M, 2 hops N, 2 hops O, 3 hops I, 1 hop K, 1 hop L, 1 hop J, 2 hops M, 2 hops N, 2 hops O, 2 hops N J, 1 hop K, 1 hop M, 1 hop N, 1 hop I, 2 hops L, 2 hops O, 2 hops L, 1 hop M, 1 hop O, 1 hop I, 2 hops K, 2 hops J, 2 hops N, 2 hops O R5 N, 1 hop O, 1 hop J, 2 hops K, 2 hops M, 1 hop L, 2 hops I, 3 hops
- 23. 23 Distance Vector: CrashedRecovery • R3 crashed • New complete route of R1 R1 R2 R3 R4 I J K L M N O R5 R1 routing table hop via 1 N/A 1 N/A 2 R2 2 R2 2 R3 2 R3 3 R5 net I J K L M N O hop via 1 N/A 1 N/A 2 R2 2 R2 3 R2 4 R2 3 R2 net I J K L M N O
- 24. 24 Count to Infinity •R2 does not hear any thing from R3 • R1 says : don’t worry, I can reach R3 in 2 hops, R2 update hop count to 3 • R1 sees R2’s update, then update itself to 4 and so on…… R1 R2 R3 hop count to R3 2 1 initial 2 3 1st round 4 3 2nd round 4 5 3rd round I J R3 crashed
- 25. 25 Split Horizon • Solveby set distance “16” as infinity • No destination can be more than 15 hops away from any other • Distance to X is not reported on the line that packet for X are sent • Actually, it reports with infinity R1 R2 R3 I J R3 crashed to R3to R3
- 26. 26 Dijkstra’s Shortest PathFirst Algorithm • Routers send out update messages whenever the state of a link changes. Hence the name: “Link State” algorithm. • Each router calculates lowest cost path to all others, starting from itself. • At each step of the algorithm, router adds the next shortest (i.e. lowest-cost) path to the tree. • Finds spanning tree routed on source router.
- 27. 27 Open Shortest PathFirst (OSPF) • RIP limited in large internets • OSPF preferred interior routing protocol for TCP/IP based internets • Link state routing used
- 28. 28 Routing Algorithm: LinkState • Flood routing information to all nodes • Each router finds who is up and flood this information to the entire routers • Use the link state to build a shortest path map to everybody • OSPF is an example • Also known as Shortest Path First (SPF) algorithm
- 29. 29 Flooding • Packet sentby 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
- 30. 30 Link State Overview •Using cost as a metric • Exchange its connection and cost to its neighbors • Each router compute the set of optimum path to all destination (Shortest Path First) link state W 0 X 0 link state X 0 Y 0 link state Y 0 Z 0 W X Y Z
- 31. 31 Link State Concept •Each router initially begins with directly connected network • Determine full knowledge of distant routers and theirs connection R2 R1 R3 R4 exchange link state packets Routing Table Œ build topological database Ž compute SPF update routing table
- 32. 32 Link State RoutingUpdate • Send information to other routers • Fast convergence R2 R1 R3 R4 topology change
- 33. 33 OSPF Overview • Routermaintains 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
- 34. 34 Link Costs • Costof 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)
- 35. 35 Areas • Make largeinternets 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
- 36. 36 Operation of Areas •Each are 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
- 37. 37 Comparison Distance Vector LinkState pass a copy of pass links state update whole routing table add metric from calculate the shortest path router to router to other routers frequent periodic update: event updated: fast slow convergence convergence
- 38. 38 Internet Routing Architecture Autonomous System Autonomous System Autonomous System IGP IGP IGP BGP4 BGP4 BGP4 IGP IGP IGPIGP IGP IGP EGP/BGP EGP/BGP EGP/BGP EGP/BGP EGP/BGP EGP/BGP Autonomous System Autonomous System
- 39. 39 Routing in theInternet The Internet uses hierarchical routing • The Internet is split into Autonomous Systems (AS’s) • Within an AS, the administrator chooses an Interior Gateway Protocol (IGP) • Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247). • Between AS’s, the Internet uses an Exterior Gateway Protocol • AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771)
- 40. 40 Autonomous System • ASis a collectionof LANs and WANs and the interconnectting routers which under the control of one management authority • The same AS runs the same Interior Gateway Protocol • Why setting up AS? - establish a direct link to each other rather than route through the core Internet • How to select AS? - register and get the AS number from IAB
- 41. 41 Gateway Protocol • Interiorgateway protocol – exchange routing information between routers within a single AS – RIP, RIP II, OSPF • Exterior gateway protocol – collect network-reachablity information for the AS – EGP, BGP
- 42. 42 Interior Routing Protocols •RIP • Uses distributed Bellman-Ford algorithm. • Updates sent every 30 seconds. • No authentication. • Originally in BSD UNIX. • OSPF • Link-state updates sent (using flooding) as and when required. • Every router runs Dijkstra’s algorithm. • Authenticated updates. • Autonomous system may be partitioned into “areas”.
- 43. 43 Exterior Routing Protocols Problems: •Topology: The Internet is a complex mesh of different AS’s with very little structure. • Autonomy of AS’s: Each AS defines link costs in different ways, so not possible to find lowest cost paths. • Trust: Some AS’s can’t trust others to advertise good routes (e.g. two competing backbone providers), or to protect the privacy of their traffic (e.g. two warring nations). • Policies: Different AS’s have different objectives (e.g. route over fewest hops; use one provider rather than another).
- 44. 44 Border Gateway Protocol(BGP-4) • BGP is not a link-state or distance-vector routing protocol. • BGP advertises complete paths (a list of AS’s). • Example of path advertisement: • “The network 171.64/16 can be reached via the path {AS1, AS5, AS13}”. • Paths with loops are detected locally and ignored. • Local policies pick the preferred path among options. • When a link/router fails, the path is “withdrawn”.
- 45. 45 References • http://www.cisco.com/en/US/products/hw/routers/p s274/index.html • http://www.cisco.com/en/US/products/hw/routers/p s274/products_data_sheet09186a008010fba1.html •Computer Networks with Internet Technology By William Stallings Chapter 11 Interior Routing Protocols