1. 10/19/2015CST 415 - Computer Networks1 RIP - OSPF CST 415

2. 10/19/2015CST 415 - Computer Networks2 Topics Definitions RIP OSPF

3. 10/19/2015CST 415 - Computer Networks3 Definitions BGP – Boundary Gateway Protocol IGP – Interior Gateway Protocol RIP – Routing Information Protocol OSPF – Open Shortest Path First RIP and OSPF are both IGPs

4. 10/19/2015CST 415 - Computer Networks4 Definitions In the above diagram: BGP is used for inter-autonomous system communication. IGP is used for intra-autonomous system communication. IGP can be any number of different protocols (RIP, OSPF, etc.)

5. 10/19/2015CST 415 - Computer Networks5 Definitions There are many IGP protocols. The specific protocol a specific router depends on –The router manufacturer »e.g. Cisco may have a proprietary protocol that relies on a specific hardware implementation. –The generation of the router »IGPs continued to be refined from router generation to generation.

6. 10/19/2015CST 415 - Computer Networks6 RIP RIP (Routing Information Protocol) originated in a variant of UNIX –BSD standard UNIX –Original incarnation was called routed –RIP became widely used through the distribution of 4BSD –RIP was widely adopted as an IGP well before a standard existed

7. 10/19/2015CST 415 - Computer Networks7 RIP RIP is an application layer protocol. Therefore RIP uses well defined ports for communication. –Port 520 –This is a UDP port (e.g. send it and forget it) as opposed to BGP using a TCP port (guaranteed delivery)

8. 10/19/2015CST 415 - Computer Networks8 RIP RIP Operation Uses simple distance-vector routing Partitions participants into –Active: advertises routes to other participants –Silent: only listen to routes. Do not advertise route tables

9. 10/19/2015CST 415 - Computer Networks9 RIP Distance Vector Routing Each node knows the direct distance cost to each of it’s neighbor nodes. Every node sends it’s distance vector table to it’s neighbor nodes. When a node receives a distance vector update, it will update it’s own distance vector table with new information of cost and next hops for all nodes in network. Send new information along. Repeat until no new information is added.

10. 10/19/2015CST 415 - Computer Networks10 RIP Distance Vector Routing - Formalized –X : Source Node –Y : Destination Node –Z : Intermediate Node between X and Y –Dx(Y,Z) : Distance at X from Y to Z –c(X,Z) : Cost in Distance of the direct hop from Y to Z

11. 10/19/2015CST 415 - Computer Networks11 RIP – General Routing Idea Dx(Y,Z) = c(X,Z) + minw{Dz(Y,w)} The minw is taken over all the distances given to Y in Z's route update (e.g. Z could have multiple routes to Y). When a packet needs to be sent from X to Y, X simply sends it to the route is has computed as the shortest.

12. 10/19/2015CST 415 - Computer Networks12 RIP – Table Format Destination – The network address of the destination network. Next hop– IP address of the next packet destination. Distance – The distance in hops to the ultimate destination. Timers – Different times for route aging. Flags– Possible conditional flags associated with the route.

13. 10/19/2015CST 415 - Computer Networks13 RIP Dx(Y,Z) = c(X,Z) + minw{Dz(Y,w)} = 1 + min(2,4,8,3) = 3 Note: Z will really never have a list of routes to Y through the intermediary network. It will maintain “it’s” shortest route information. Backward propagation of shortest route information will select the “min”. What is the necessary topology of the Intermediary Network?

14. 10/19/2015CST 415 - Computer Networks14 RIP RIP uses “hop count” as the metric to measure distance. –Fewer hops may not result in the shortest latency. Active routers broadcast route update every 30 seconds. A Route will only stay active for 180 seconds. –Stale routes (e.g. haven’t been updated in 180 seconds) will be removed from the routing table.

15. 10/19/2015CST 415 - Computer Networks15 RIP RIP defines a maximum hop count for a valid route to be 16. –This helps avoid the propagation of circular routes.

16. 10/19/2015CST 415 - Computer Networks16 RIP Slow convergence occurs because route updates take a time to propagate across the network.

17. 10/19/2015CST 415 - Computer Networks17 RIP – Message Format

18. 10/19/2015CST 415 - Computer Networks18 RIP – Message Format CommandMeaning 1Request routing information 2Response containing network-distance pairs from senders routing tables. Version – 1 or 2 (2 handles subnet and supernetting) Family of Net – See BSD4 Network Family Mumbers (AF_INET for IP or 2).

19. 10/19/2015CST 415 - Computer Networks19 OSPF Open Shortest Path First OSPF uses a different algorithm to perform routing decisions. OSPF working group was organized in 1988 because RIP had several shortcomings when dealing with interior routing in a large heterogeneous network.

20. 10/19/2015CST 415 - Computer Networks20 OSPF Based on Bolt, Beranek, and Newman's (BBN's) SPF algorithm developed in 1978 for the ARPANET. Unlike RIP, OSPF can operate within a hierarchy. –The largest entity within the hierarchy is the autonomous system (AS). –OSPF is an intra-AS (interior gateway) routing protocol, capable of receiving routes from and sending routes to other ASs.

21. 10/19/2015CST 415 - Computer Networks21 OSPF Autonomous Systems are broken down into Areas. Areas communicate through Area Border Routers. The backbone network connects areas together. A Area Border Router maintains topological information about networks it is in charge of bridging.

22. 10/19/2015CST 415 - Computer Networks22 OSPF – Shortest Path Find the shortest path starting a vertex “x” and ending at vertex “y”. Example: Shortest path from R5 to R3.

23. 10/19/2015CST 415 - Computer Networks23 OSPF – Shortest Path Shortest path from R5 to R3. To do this, we will use the “greedy method” developed by Dijkstra. Basically: 1.Start at the destination. 2.Choose the shortest path back 3.traverse to that vertex. 4.Repeat until the source is reached.

24. 10/19/2015CST 415 - Computer Networks24 OSPF – Shortest Path Shortest path from R5 to R3. Basically: 1.Start at the destination. 2.Choose the shortest path back 3.traverse to that vertex. 4.Repeat until the source is reached. R3

25. 10/19/2015CST 415 - Computer Networks25 OSPF – Shortest Path Shortest path from R5 to R3. Basically: 1.Start at the destination. 2.Choose the shortest path back 3.traverse to that vertex. 4.Repeat until the source is reached. R3, R0

26. 10/19/2015CST 415 - Computer Networks26 OSPF – Shortest Path Shortest path from R5 to R3. Basically: 1.Start at the destination. 2.Choose the shortest path back 3.traverse to that vertex. 4.Repeat until the source is reached. R3, R0, R1

27. 10/19/2015CST 415 - Computer Networks27 OSPF – Shortest Path Shortest path from R5 to R3. Basically: 1.Start at the destination. 2.Choose the shortest path back 3.traverse to that vertex. 4.Repeat until the source is reached. R3, R0, R1, R4,

28. 10/19/2015CST 415 - Computer Networks28 OSPF – Shortest Path Shortest path from R5 to R3. Basically: 1.Start at the destination. 2.Choose the shortest path back 3.traverse to that vertex. 4.Repeat until the source is reached. R3, R0, R1, R4, R5 = 7

29. 10/19/2015CST 415 - Computer Networks29 OSPF – Shortest Path The algorithm described above is a simplified version of Dijkstras algorithm. The BBN algorithm is a further refinement dealing with path priority. BBN is based on a graph structure and a tree structure.

30. 10/19/2015CST 415 - Computer Networks30 OSPF – Shortest Path To effect this shortest path calculation –Each Area Border Router must maintain information related to it’s managed area topology. –As routers adjust routes, the new information is exchanged with neighbor routers. –When new information arrives, the routers must re-calculate their shortest path tables.

31. 10/19/2015CST 415 - Computer Networks31 OSPF – Message Format OSPF messages have two parts –The message header –The Payload »Hello Message »Database Description Message »Link Status Request Message »Link Status Update Message »Link Status Acknowledge Message

32. 10/19/2015CST 415 - Computer Networks32 OSPF – Message Format : Header Version number—Identifies the OSPF version used. Type—Identifies the OSPF packet type as one of the following: –Hello—Establishes and maintains neighbor relationships. Value - 1 –Database description—Describes the contents of the topological database. These messages are exchanged when an adjacency is initialized. Value - 2 –Link-state request—Requests pieces of the topological database from neighbor routers. These messages are exchanged after a router discovers (by examining database-description packets) that parts of its topological database are outdated. Value - 3 –Link-state update—Responds to a link-state request packet. These messages also are used for the regular dispersal of LSAs. Several LSAs can be included within a single link-state update packet. Value - 4 –Link-state acknowledgment—Acknowledges link-state update packets. Value - 5

33. 10/19/2015CST 415 - Computer Networks33 OSPF – Message Format : Header Message length—Specifies the packet length, including the OSPF header, in bytes. Source Router IP Address—Identifies the source of the packet. Area ID—Identifies the area to which the packet belongs. All OSPF packets are associated with a single area. Checksum—Checks the entire packet contents for any damage suffered in transit. Authentication type—Contains the authentication type. All OSPF protocol exchanges are authenticated. The authentication type is configurable on per-area basis. Authentication—Contains authentication information. Data—Contains encapsulated upper-layer information.