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Example Network
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10.10.10.2 10.10.10.4 10.10.10.6
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Router IDs are
selected
independent of
interface addresses
3
4 2
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32
•Link costs are called Metric
• Metric is in the range [0 , 216
]
• Metric can be asymmetric](https://image.slidesharecdn.com/module11-ospf-150601074610-lva1-app6891/85/ospf-routing-protocol-5-320.jpg)
ospf routing protocol
- 2. 2 Link State Routing: Basic princples 1. Each router establishes a relationship (“adjacency”) with its neighbors 2.Each router generates link state advertisements (LSAs) which are distributed to all routers LSA = (link id, state of the link, cost, neighbors of the link) 3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges 4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network
- 3. 3 OSPF • OSPF = Open Shortest Path First • The OSPF routing protocol is the most important link state routing protocol on the Internet • History: – 1989: RFC 1131 OSPF Version 1 – 1991: RFC1247 OSPF Version 2 – 1994: RFC 1583 OSPF Version 2 (revised) – 1997: RFC 2178 OSPF Version 2 (revised) – 1998: RFC 2328 OSPF Version 2 (current version)
- 4. 4 Features of OSPF • Provides authentication of routing messages • Enables load balancing by allowing traffic to be split evenly across routes with equal cost • Type-of-Service routing allows to setup different routes dependent on the TOS field • Supports subnetting • Supports multicasting • Allows hierarchical routing
- 5. 5 Example Network 10.1.1.0/24 .1 .2 .2 10.10.10.1 10.1.4.0/24 10.1.2.0/24 .1 .4 10.1.7.0/24 10.1.6.0/24 10.1.3.0/24 10.1.5.0/24 10.1.8.0/24 .3 .3 .5 .2 .3 .5 .5 .4 .4 .6 .6 10.10.10.2 10.10.10.4 10.10.10.6 10.10.10.2 10.10.10.5 Router IDs are selected independent of interface addresses 3 4 2 5 1 1 32 •Link costs are called Metric • Metric is in the range [0 , 216 ] • Metric can be asymmetric
- 6. 6 10.1.1.0/24 .1 .2 .2 10.10.10.1 10.1.4.0/24 10.1.2.0/24 .1 10.1.3.0/24 10.1.5.0/24.3 .3 .2 .3 10.10.10.2 10.10.10.3 Link State Advertisement (LSA) • The LSA of router 10.10.10.1 is as follows: • Link State ID: 10.10.10.1 = Router ID • Advertising Router: 10.10.10.1 = Router ID • Number of links: 3 = 2 links plus router itself • Description of Link 1: Link ID = 10.1.1.1, Metric = 4 • Description of Link 2: Link ID = 10.1.2.1, Metric = 3 • Description of Link 3: Link ID = 10.10.10.1, Metric = 0 3 4 2 Each router sends its LSA to all routers in the network (using a method called reliable flooding)
- 7. 7 Network and Link State Database 10.1.1.0/24 .1 .2 .2 10.10.10.1 10.1.4.0/24 10.1.2.0/24 .1 .4 10.1.7.0/24 10.1.6.0/24 10.1.3.0/24 10.1.5.0/24 10.1.8.0/24 .3 .3 .5 .2 .3 .5 .5 .4 .4 .6 .6 10.10.10.2 10.10.10.4 10.10.10.6 10.10.10.2 10.10.10.5 LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age Router-LSA 10.1.10.1 10.1.10.1 0x9b47 0x80000006 0 Router-LSA 10.1.10.2 10.1.10.2 0x219e 0x80000007 1618 Router-LSA 10.1.10.3 10.1.10.3 0x6b53 0x80000003 1712 Router-LSA 10.1.10.4 10.1.10.4 0xe39a 0x8000003a 20 Router-LSA 10.1.10.5 10.1.10.5 0xd2a6 0x80000038 18 Router-LSA 10.1.10.6 10.1.10.6 0x05c3 0x80000005 1680 Each router has a database which contains the LSAs from all other routers
- 8. 8 Link State Database • The collection of all LSAs is called the link-state database • Each router has and identical link-state database – Useful for debugging: Each router has a complete description of the network • If neighboring routers discover each other for the first time, they will exchange their link-state databases • The link-state databases are synchronized using reliable flooding
- 9. 9 OSPF Packet Format OSPFMessageIPheader BodyofOSPFMessage OSPFMessage Header MessageType SpecificData LSA LSALSA ... LSA Header LSA Data ... Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters) OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89
- 10. 10 OSPF Packet Format sourcerouterIPaddress authentication authentication 32bits version type messagelength AreaID checksum authenticationtype BodyofOSPFMessage OSPFMessage Header 2: current version is OSPF V2 Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken over entire packet 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks
- 11. 11 OSPF LSA Format LinkStateID linksequencenumber advertisingrouter LinkAge LinkType checksum length LinkID LinkData LinkType Metric#TOSmetrics LSA LSA Header LSA Data LinkID LinkData LinkType Metric#TOSmetrics LSA Header Link 1 Link 2
- 12. 12 Discovery of Neighbors • Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces. • If two routers share a link, they can become neighbors, and establish an adjacency • After becoming a neighbor, routers exchange their link state databases OSPFHello OSPFHello:Iheard10.1.10.2 10.1.10.1 10.1.10.2 Scenario: Router 10.1.10.2 restarts
- 13. 13 Neighbor discovery and database synchronization OSPFHello OSPFHello:Iheard10.1.10.2 DatabaseDescription:Sequence=X 10.1.10.1 10.1.10.2 DatabaseDescription:Sequence=X, 5LSAheaders= Router-LSA,10.1.10.1,0x80000006 Router-LSA, 10.1.10.2,0x80000007 Router-LSA, 10.1.10.3,0x80000003 Router-LSA, 10.1.10.4,0x8000003a Router-LSA, 10.1.10.5,0x80000038 Router-LSA, 10.1.10.6,0x80000005 DatabaseDescription:Sequence=X+1,1 LSAheader= Router-LSA, 10.1.10.2,0x80000005 DatabaseDescription:Sequence=X+1 Sends empty database description Scenario: Router 10.1.10.2 restarts Discovery of adjacency Sends database description. (description only contains LSA headers) Database description of 10.1.10.2 Acknowledges receipt of description After neighbors are discovered the nodes exchange their databases
- 14. 14 Regular LSA exchanges 10.1.10.1 10.1.10.2 LinkStateRequestpackets, LSAs= Router-LSA, 10.1.10.1, Router-LSA, 10.1.10.2, Router-LSA, 10.1.10.3, Router-LSA, 10.1.10.4, Router-LSA, 10.1.10.5, Router-LSA, 10.1.10.6, LinkStateUpdatePacket, LSA = Router-LSA, 10.1.1.6,0x80000006 LinkStateUpdatePacket, LSAs = Router-LSA,10.1.10.1, 0x80000006 Router-LSA,10.1.10.2,0x80000007 Router-LSA,10.1.10.3,0x80000003 Router-LSA,10.1.10.4,0x8000003a Router-LSA,10.1.10.5,0x80000038 Router-LSA,10.1.10.6,0x80000005 10.1.10.2 explicitly requests each LSA from 10.1.10.1 10.1.10.1 sends requested LSAs 10.1.10.2 has more recent value for 10.0.1.6 and sends it to 10.1.10.1 (with higher sequence number)
- 15. 15 Routing Data Distribution • LSA-Updates are distributed to all other routers via Reliable Flooding • Example: Flooding of LSA from 10.10.10.1 10.10.10.1 10.10.10.2 10.10.10.4 10.10.10.6 10.10.10.2 10.10.10.5 LSA LSA Update database Update database ACK ACK LSA LSA LSA LSA ACK ACK ACK ACK LSA LSA LSA LSA Update database Update database ACK ACK ACK ACK Update database
- 16. 16 Dissemination of LSA-Update • A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet) • Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes. • Acknowledgements of LSA-updates: • explicit ACK, or • implicit via reception of an LSA-Update