RIP OSPF CST 415 1232020 CST 415 Computer

RIP - OSPF CST 415 12/3/2020 CST 415 - Computer Networks 1

Topics • Definitions • RIP • OSPF 12/3/2020 CST 415 - Computer Networks 2

Definitions BGP – Boundary Gateway Protocol IGP – Interior Gateway Protocol RIP – Routing Information Protocol OSPF – Open Shortest Path First RIP and OSPF are both IGPs 12/3/2020 CST 415 - Computer Networks 3

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. ) 12/3/2020 CST 415 - Computer Networks 4

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. 12/3/2020 CST 415 - Computer Networks 5

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 4 BSD – RIP was widely adopted as an IGP well before a standard existed 12/3/2020 CST 415 - Computer Networks 6

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) 12/3/2020 CST 415 - Computer Networks 7

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 12/3/2020 CST 415 - Computer Networks 8

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. 12/3/2020 CST 415 - Computer Networks 9

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 12/3/2020 CST 415 - Computer Networks 10

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. 12/3/2020 CST 415 - Computer Networks 11

RIP – Table Format Destination Next hop Distance Timers Flags 12/3/2020 – The network address of the destination network. – IP address of the next packet destination. – The distance in hops to the ultimate destination. – Different times for route aging. – Possible conditional flags associated with the route. CST 415 - Computer Networks 12

RIP Dx(Y, Z) = c(X, Z) + minw{Dz(Y, w)} = 1 + min(2, 4, 8, 3) = 3 What is the necessary topology of the Intermediary Network? 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”. 12/3/2020 CST 415 - Computer Networks 13

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. 12/3/2020 CST 415 - Computer Networks 14

RIP • RIP defines a maximum hop count for a valid route to be 16. – This helps avoid the propagation of circular routes. 12/3/2020 CST 415 - Computer Networks 15

RIP Slow convergence occurs because route updates take a time to propagate across the network. 12/3/2020 CST 415 - Computer Networks 16

RIP – Message Format 12/3/2020 CST 415 - Computer Networks 17

RIP – Message Format Command Meaning 1 Request routing information 2 Response containing network-distance pairs from senders routing tables. Version – 1 or 2 (2 handles subnet and supernetting) Family of Net – See BSD 4 Network Family Mumbers (AF_INET for IP or 2). 12/3/2020 CST 415 - Computer Networks 18

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. 12/3/2020 CST 415 - Computer Networks 19

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. 12/3/2020 CST 415 - Computer Networks 20

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. 12/3/2020 CST 415 - Computer Networks 21

OSPF – Shortest Path Find the shortest path starting a vertex “x” and ending at vertex “y”. Example: Shortest path from R 5 to R 3. 12/3/2020 CST 415 - Computer Networks 22

OSPF – Shortest Path Shortest path from R 5 to R 3. 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. 12/3/2020 CST 415 - Computer Networks 23

OSPF – Shortest Path Shortest path from R 5 to R 3. Basically: 1. Start at the destination. 2. Choose the shortest path back 3. traverse to that vertex. 4. Repeat until the source is reached. R 3 12/3/2020 CST 415 - Computer Networks 24

OSPF – Shortest Path Shortest path from R 5 to R 3. Basically: 1. Start at the destination. 2. Choose the shortest path back 3. traverse to that vertex. 4. Repeat until the source is reached. R 3, R 0 12/3/2020 CST 415 - Computer Networks 25

OSPF – Shortest Path Shortest path from R 5 to R 3. Basically: 1. Start at the destination. 2. Choose the shortest path back 3. traverse to that vertex. 4. Repeat until the source is reached. R 3, R 0, R 1 12/3/2020 CST 415 - Computer Networks 26

OSPF – Shortest Path Shortest path from R 5 to R 3. Basically: 1. Start at the destination. 2. Choose the shortest path back 3. traverse to that vertex. 4. Repeat until the source is reached. R 3, R 0, R 1, R 4, 12/3/2020 CST 415 - Computer Networks 27

OSPF – Shortest Path Shortest path from R 5 to R 3. Basically: 1. Start at the destination. 2. Choose the shortest path back 3. traverse to that vertex. 4. Repeat until the source is reached. R 3, R 0, R 1, R 4, R 5 = 7 12/3/2020 CST 415 - Computer Networks 28

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. 12/3/2020 CST 415 - Computer Networks 29

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. 12/3/2020 CST 415 - Computer Networks 30

OSPF – Message Format • OSPF messages have two parts – The message header – The Payload 12/3/2020 » Hello Message » Database Description Message » Link Status Request Message » Link Status Update Message » Link Status Acknowledge Message CST 415 - Computer Networks 31

OSPF – Message Format : Header • • Version number—Identifies the OSPF version used. Type—Identifies the OSPF packet type as one of the following: – – – 12/3/2020 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 CST 415 - Computer Networks 32

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. 12/3/2020 CST 415 - Computer Networks 33