ITI-510 Computer Networks ITI 510 – Computer Networks Meeting 3 Rutgers University Internet Institute Instructor: Chris Uriarte

ITI-510 Computer Networks Meeting 3 - Agenda Dynamic Routing Protocols Transport Layer Protocols: –UDP –TCP

ITI-510 Computer Networks Dynamic Routing Protocols Recall, the routing table on a host or router maintains the “next hop” information for destination hosts and networks. In simple networks, there is often only one path that packets can take – one default route. As networks become larger and more complex, a packet may have multiple paths to one destination. Dynamic Routing Protocols help keep routing tables up-to-date with accurate routing information.

ITI-510 Computer Networks Dynamic Routing Protocols: If Routers Could Talk… Dynamic Routing Protocols are “languages” that routers use to share route information with each other. Router A Router B Router C Network C Network B Network A “Hey everyone, I can reach network A!” “Hey everyone, I can reach network C!” “Hey everyone, I can reach network B!”

ITI-510 Computer Networks Dynamic vs. Static Routing Sometimes, network topologies don’t change very often and interconnected routers don’t need to share any route information with each other. In this case, we use static routing, where route information is manually entered into a router and requires a manual change when the network topology changes. In other cases, a network topology may change very often, or there can be multiple paths to a particular destination. In cases like this, Dynamic Routing Protocols may be used.

ITI-510 Computer Networks Simple Example of Dynamic Routing Protocols (If Routers Could Talk II…) –Routers exchange information with each other: Router A Router B Router C Network C Network B Network A I am directly attached to Network A. I Can reach network C in 1 hop. I can reach network B in 1 hop. I am directly attached to Network B. I Can reach network A in 1 hop. I can reach network C in 1 hop. I am directly attached to Network C. I Can reach network A in 1 hop. I can reach network B in 1 hop.

ITI-510 Computer Networks A Change In Network Topology… –A change in network topology will cause routers to change the routes they advertise. Router A Router B Router C Network C Network B Network A I am directly attached to Network A. I Can reach network C in 1 hop. I can reach network B in 1 hop. I am directly attached to Network B. I Can reach network A in 1 hop. I can reach network C in 1 hops. I am directly attached to Network C. I Can reach network A in 1 hop. I can reach network B in 2 hops. X Link between A&B goes down.

ITI-510 Computer Networks Classes of Routing Protocols: Measuring Routing Metrics How can we decide if one router is better than another? We can use different types of metrics (distance measurements) to compare two routes.

ITI-510 Computer Networks Distance Vector Protocols The most simple type of dynamic routing protocol. Routers keep simple routing metrics for each link. Metrics for each link may include: –The number of hops to a particular destination –The bandwidth, delay, reliability or cost-of-use for a link. Distance Vector algorithms base routing decisions on metric values alone.

ITI-510 Computer Networks Example: Distance Vector Protocol For this example, let’s base routing decisions solely on the least amount of hops needed to reach a destination. A B C D ENet B Net A Net E Net C In the above example, packets from Network A destined for Network E will chose the path A-B-E since that represents the least amount of hops - Even though packets have an alternate path to Network E (A-C-D-E)

ITI-510 Computer Networks Example 2: Distance Vector Protocols In this example, lets assign a “weight” to each link – the greater the weight, the faster the link. Routing decisions will be made based on the quickest path. A B C D ENet B Net A Net E Net C Here, Packets from Net A->E will choose the path A-C-D-E because the path is faster

ITI-510 Computer Networks RIP: Routing Information Protocol RIP is a simple distance vector protocol where each link is assigned a value of 1. The total metric of the path is the sum of all the hop costs. A B C D ENet B Net A Net E Net C SourceDestinationCost Next Hop Net ANet B1Rtr. B Net ANet E2Rtr. B Net A Net C1Rtr. C RIP Logic Table for Router A

ITI-510 Computer Networks Propagation of RIP Routes The sharing of route information between routers is known as route propagation. RIP packets are broadcast on the network by RIP- enabled routers. RIP routers “listen” for routes advertised by other RIP routers. RIP routers will ONLY advertise network routes to “neighbor” routers - routers they are directly attached to. When a peer router received a routing metric from another router, the route information is added to the routing table UNLESS the recipient router already has a BETTER route. Refer to Feit. Pg. 225 for a good description of the RIP propegation process.

ITI-510 Computer Networks RIP Routes, con’t. If a link is down between 2 routers, a router will set the metric for any relitive destinations to ‘16’, which means “I can’t get there”. When neighbors receive the advertised router of 16, they will attempt to recalculate routing paths. RIP is a very simple routing protocol that is useful in small networks.

ITI-510 Computer Networks RIP Version 1 RIP packets are sent to neighbor rourters every 30 seconds. If a neighbor router advertises a route of 16, other routers will begin to calculate new paths to the relevant destination networks. If a router does not receive RIP updates from its neighbor in 3 minutes, the router will assume the neighbor is down and will mark all routes as unreachable (metric = 16). The router will then search for another route for up to 2 minutes before totally deleting the entry from its routing tables.

ITI-510 Computer Networks Disadvantages of RIP The maximum metric for any path is 15 since 16 means “I can’t get there”. This is not effective for big networks. When there is a change of topology, RIP is often very slow to re-establish optimal routes – this is known as slow convergence. RIP does not have the ability to ‘split’ traffic across multiple links.