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Open Shortest Path First Protocol (OSPF)

Open Shortest Path First Protocol (OSPF). Sudarshan Vasudevan svasu@cs.umass.edu. Overview. Introduction Motivation OSPF Basics Hierarchical Routing in OSPF Summary. Introduction. Development began in 1987 OSPF Working Group (part of IETF) OSPFv2 first established in 1991

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Open Shortest Path First Protocol (OSPF)

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  1. Open Shortest Path First Protocol(OSPF) SudarshanVasudevan svasu@cs.umass.edu

  2. Overview • Introduction • Motivation • OSPF Basics • Hierarchical Routing in OSPF • Summary

  3. Introduction • Development began in 1987 • OSPF Working Group (part of IETF) • OSPFv2 first established in 1991 • Many new features added since then • Updated OSPFv2 specification in RFC 2178

  4. Motivation • Original IGP used was RIP • Based on Bellman-Ford Algorithm • Worked well in small systems • Suffered from problems of Distance Vector Protocol • Count to Infinity Problem • Slow Convergence

  5. Motivation • Problems with Distance Vector Protocol • Large update packets • Slow response to topological changes • Need for a Link State Protocol • A long list of functional requirements follows

  6. Functional Requirements of OSPF • Faster Convergence and less consumption of network resources • A more descriptive routing metric • configurable • value ranges between 1 and 65,535 • no restriction on network diameters • Equal-cost multipath • a way to do load balancing

  7. Functional Requirements(contd.) • Routing Hierarchy • support large routing domains • Separate internal and external routes • Support of flexible subnetting schemes • route to arbitrary [address,mask] combinations using VLSMs • Security • Type of Service Routing

  8. OSPF Basicsthe essence • Distributed, replicated database model • describes complete routing topology • Link state advertisements • carry local piece of routing topology • Distribution of LSAs using reliable flooding • Link state database • identical for all the routers

  9. Link State Advertisements(LSAs) LS Age Options LS Type Link State ID Advertising Router LS Sequence Number LS Checksum Length 0 16 LSA Header

  10. LSAs contd. • Identifying LSAs • LS type field • Link State ID field • mostly carries addressing information • e.g. IP address of externally reachable network • Advertising Router field • originating router’s OSPF router ID

  11. LSAs contd. • Identifying LSA instances • needed to update self-originated LSAs • LS Sequence Number field • 32 bit values • monotonically increasing until some max value • 600 years to roll over! • LSA checksum and LS Age guard against potential problems

  12. LSAs contd. • Verifying LSA contents • LS Checksum field • computed by the originating router and left unchanged thereafter • LS age field not included in checksum • Removing LSAs from databases • LS Age field • ranges from 0 to 30 min. • Max Age LSAs used to delete outdated LSAs

  13. LSAs contd. • Other LSA Header fields • Options field • sometimes used to give special treatment during flooding or routing calculations • Length field • includes LSA header and contents • ranges from 20-65535 bytes

  14. Sample Router LSA 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 10.1.1.3 10.1.1.5

  15. Sample Router LSA contd.. LS Age 0 seconds Options LS Type E-bit,LS Type 1 10.1.1.1 Link State ID Advertising Router 10.1.1.1 LS Sequence Number 0x80000006 LS Checksum 0x9b47 Length 60 bytes Router Type 0 0 (ordinary) # of links 3 Link ID 10.1.1.3 Link Data Ifindex 2 (unnumbered link) Link 1 Link Type #TOS Metrics 1(point to point), 0 Metric 5

  16. Link State Database • Collection of all OSPF LSAs • databases exchanged between neighbors • synchronization thru reliable flooding • gives the complete routing topology • each OSPF router has identical link-state database

  17. Link State Database contd.. • Example of a link state database LS Type Link State ID Adv Router LS Checksum LS Seq No LS Age Router LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0 ….. …... ….. ….. …. …...

  18. Communication between OSPF Routers • OSPF packets encapsulated in IP packets • standard 24 byte header • OSPF packet type field • OSPF router ID of sender • Packet checksum • Authentication fields • OSPF Area ID

  19. Neighbor Discovery and Maintenance • OSPF Hello Protocol • Hello packets sent out every 10 seconds • helps to detect failed neighbors • RouterDeadInterval (default 40 seconds) • also ensures that link is bidirectional • neighboring routers agree on intervals • hello interval set so that a link is not accidentally brought down

  20. Database Synchronization • Crucial to ensure correct and loop free routing • must be done before 2 neighbors start communication • also whenever new LSAs are introduced • uses reliable flooding • each router sends LSA headers to its neighbor when connection comes up • requests only those LSAs which are recent

  21. Database Exchange • Neighboring routers first exchange hellos • a database description packet packet establishes the sequence number • the other router sends LSA headers • sequence number incremented for every pair od database description packets • implicit acknowledgement for the previous pair • after examining LSA headers explicit request sent for complete LSAs

  22. Reliable Flooding • Starts when a router wants to update self-originated LSAs • Link State Update packets • Neighbor installs more recent LSAs into its database • floods out on all interfaces except the one on which it arrived • reliability-retransmissions until acks received

  23. Reliable Flooding (contd..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 Time T1 u u u 10.1.1.3 10.1.1.5

  24. Reliable Flooding (contd..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 u u Time T2 u u u 10.1.1.3 10.1.1.5

  25. Reliable Flooding (contd..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 u Time T3 u 10.1.1.3 10.1.1.5

  26. Reliable Flooding (contd..) 10.1.1.1 10.1.1.2 10.1.1.4 10.1.1.6 Time T3+ ack ack ack ack ack 10.1.1.3 10.1.1.5

  27. Reliable Flooding(contd..) • Robustness • updates flooded over all the links , so failure of any link doesn’t affect database synchronization • LSAs refreshed every 30 minutes • LSA checksum field detects corruption • flooding loops avoided by LS Age field • MinLSInterval limits rate of LSA origination • Receivers can refuse to accept LSA updates if they received an update less than a second ago

  28. Routing Calculations • Link costs configurable by administrator • Smaller values for more preferred links • must make sense to add link costs • different costs for each link direction possible • Dijkstra’s shortest path algorithm • incrementally calculates tree of shortest paths • each link in the network examined once • computes multiple shortest paths (equal-cost multipath)

  29. Hierarchical Routing • Technique used to build large networks • minimizes consumption of network resources such as • router memory • router computing resources • link bandwidth • with flat routing linear increase in routing table size • with hierarchical, size increases logarithmically

  30. an example 10.3 10.3.3 10.0.0.0/8 10.3.2 10.3.1 10.1.3 10.2.3 10.1 10.2 10.1.1 10.1.2 10.2.2 10.2.1

  31. example contd.. • Consider a router in 10.1.1 • assume 16 entries in each of the first level partitions • with flat routing, 9*16 = 144 entries/router • with 3 level hierarchy, the router has 16 entries within 10.1.1.0/24 + entries for 10.1.2.0/24, 10.1.3.0/24,10.1.0.0/16 for a total of 19 entries. • Marked reduction in routing table size • but might lead to suboptimal routing

  32. OSPF Areas • Two-level hierarchical routing scheme through the use of areas • areas identified by 32-bit id • each area has its own link state database which is a collection of network-LSAs and router-LSAs • area’s topology hidden from all other areas • interconnection of areas through area border routers (ABRs) • ABR leaks IP addressing information to other areas through summary LSAs

  33. Sample Area Configuration Area 0.0.0.1 10.2.1.0/24 10.2.2.0/24 1 3 1 I J 3 3 B C Area 0.0.0.2 Area 0.0.0.3 10.1.2.0/24 1 1 1 E G 1 2 2 3 10.3.7.0/24 3 Area 0.0.0.0 1 A D A A 3 10.8.2.0/24 1 10.1.1.0/24 3 H F 3 1 1

  34. OSPF Areas contd.. • Example of Summary LSA(router B) LS Age 0 Options LS Type 0x2, Type 3(summary-LSA) Link State ID 10.2.0.0 Advertising Router Router B’s router ID LS Sequence Number 0x80000001 LS Checksum Length 28 bytes Network Mask 255.255.0.0 TOS TOS 0 (normal) Metric Cost of 7

  35. OSPF Areas contd.. • Reduction in link state databases of an area • reduction in amount of flooding traffic needed for synchronization • reduction in the cost of the shortest path calculations • increased robustness • routing protection • Hidden prefixes

  36. Area Organization • All the areas are connected to area 0.0.0.0 also called the backbone area • need not have a direct physical connection though • virtual links provide logical link to backbone • summary LSAs tunneled across non backbone areas • exchange of routing information between areas using Distance Vector Protocol • absence of redundant paths between areas • not subject to convergence problems

  37. Incorporating external routing information • Special routers called AS boundary routers at the edge of OSPF domain • ASBRs originate AS-External LSAs • only routes for which the choice of an ASBR makes sense are imported • otherwise default routes are used • AS external LSAs similar to Summary LSAs with 2 additional fields • Forwarding address • external route tag

  38. Interaction with areas • AS-External LSAs flooded across borders • ASBR summary LSAs used to know the location of the originator of AS-External LSA • Link State ID of ASBR Summary LSA set to the OSPF router ID of the ASBR whose location is advertised • similar to summary LSA in all other respects

  39. OSPF Area Types • Restrict the amount of external routing information within an area • used when resources especially router memory is very limited • two types of restricted areas • Stub Areas • NSSAs or Not-So-Stubby-Areas

  40. OSPF Area Types • Stub Areas • don’t support ASBRs and hence no AS-External-LSAs • routing to external destinations based on default routes originated by the area’s border routers • summary LSAs also made optional • must lie on the edge of OSPF routing domain • inter-area routing may also be based on default routes • improved scaling • but not preferred due to the possibility of suboptimal routes

  41. OSPF Area Types contd.. • NSSAs • import small amount of routing information • this information flooded to other areas by the NSSA Border router • Use Type-7 LSAs to import external routing information • translated into AS-External-LSA at the NSSA Border • one-way filter

  42. Summary • Why OSPF is needed in the Internet? • The basics of the protocol • The Link state Advertisements • Neighbor Discovery (Hello Protocol) • Database Synchronization and reliable flooding • Hierarchical Routing in OSPF • OSPF Areas and Area Organization • Interaction with External Routing Information • OSPF Area Types viz. Stub Areas and NSSAs

  43. Issues not covered • OSPF Network Types • Broadcast subnets • NBMA Subnets • OSPF Extensions • Multicast Routing using OSPF (MOSPF) • OSPF Management • and a whole lot of others!

  44. Further Reading • John T. Moy, OSPF - An Anatomy of an Internet Routing Protocol • Christian Huitema, Routing in the Internet • RFC 2178

  45. Concluding Remark Thank You!

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