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

2. ITI-510 Computer Networks Meeting 3 - Agenda Concepts related to Dynamic Routing Dynamic Routing Protocols

3. 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.

4. 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!”

5. 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.

6. 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.

7. 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.

8. ITI-510 Computer Networks Classes of Routing Protocols: Measuring Routing Metrics Q: How can we decide if one router is better than another? –A: We can use different types of metrics (distance measurements) to compare two routes. Route metrics can be shared between routers using dynamic routing protocols. There are two basic classes of dynamic routing protocols: Distance Vector protocols and Link State protocols.

9. 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.

10. 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 path represents the least amount of hops - Even though packets have an alternate path to Network E (A-C-D-E)

11. 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. 15 10 5 2 3

12. 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

13. 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 receives a routing metric from another router, the route information is added to the routing table UNLESS the recipient router already has a BETTER route.

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

15. ITI-510 Computer Networks RIP Version 1 RIP packets are sent to neighbor routers 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.

16. ITI-510 Computer Networks RIP Version 2 RIP version 2 adds some additional information to the RIP messages exchanged between routers: –Subnet Mask: a network address AND subnet mask is sent with each message. –More specific route information: including more detailed “next hop” route details (gets to Network X via Router X). –Authentication: RIP messages can be authenticated before information is added to the routing table. Now the preferred RIP implementation.

17. ITI-510 Computer Networks Downsides to Using RIP Version 1 No Subnet Mask – subnet masks are not included in RIP v1 messages. LAN Broadcasts – RIP v1 messages are broadcast to every host on the entire network. No Authentication – False RIP messages can be introduced into the network since there are no methods used to authenticate messages. Can’t Distinguish Fast from Slow Links Creates Excessive Network Traffic – A lot of information is broadcast to every host on a network every 30 seconds.

18. 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’ or ‘load balance’, traffic across multiple links.

19. ITI-510 Computer Networks Cisco’s IGRP A distance vector routing protocol that is more sophisticated than RIP. Developed by Cisco, but available on many platforms. IGRP’s routing metric can be calculated using many factors including a link’s latency and total bandwidth. IGRP can split traffic across links that have the same cost, or very similar costs.

20. ITI-510 Computer Networks IGRP Routing Messages Messages exchanged between peer routers, like RIP. Instead of only containing the number of hops to a destination, RIP messages can update several types of metric information: –Link Delay –Bandwidth –Load –Reliability –Hop Count –Path MTU (maximum size of a packet allowed on a particular path) Routing Decisions can be made on any of the message fields.

21. ITI-510 Computer Networks Advantages of Using IGRP More robust routing metrics supported Less network traffic since updates are only sent every 90 seconds

22. ITI-510 Computer Networks Link State Router Protocols Routers that use Link State protocols discover the paths to other networks by building a map of the network and using the map to build a “shortest path” roadmap to each network. Link state routers know the topology of the network upon startup. Routing updates are ONLY sent when the network topology changes, so traffic is kept to a minimum.

23. ITI-510 Computer Networks OSPF – Open Shortest Path First A very popular link state routing protocol that uses distance metrics to make routing decisions. Designed to scale well and enable fast convergence of network topology changes. A routing standard, developed by the Internet Engineering Task Force – a public technology that can be implemented by router vendors without fee.

24. ITI-510 Computer Networks Advantages of OSPF Routers in an OSPF network detect topology changes and re-establish loop- free routes quickly. OSPF supports traffic splitting (load balancing) across multiple equivalent paths. Supports subnet masks Supports message authentication Very low network traffic overhead – updates are only shared between routers when changes are made to the network topology.

25. ITI-510 Computer Networks OSPF Area Routing An network that runs OSPF is comprised of several OSPF Areas, each of which is comprised of several subnets, routers and/or hosts. Each area is assigned a number. OSPF routes traffic from one area to another. Every router in a particular OSPF area only knows the detailed topology about the area it belongs to. Each router in a single area contains an identical topology map of that area. When packet needs to be routed from one area to another, packets are passed to backbone routers, which are responsible for routing between areas.

26. ITI-510 Computer Networks The OSPF Backbone The OSPF Backbone is know as Area 0. The OSPF Backbone contains all routers that belong to multiple areas, known as backbone routers. The backbone “glues together” all other other OSPF areas.

27. ITI-510 Computer Networks Example: OSPF Topology

28. ITI-510 Computer Networks Autonomous Systems Since the Internet is comprised of many very large networks that are owned by major telcos and ISPs, we need some standard to identify and define each of these large networks. An Autonomous System (AS) is a connected group of IP networks that have a single and clearly defined routing policy. An Autonomous System is usually comprised of all the networks and subnetworks that belong to an Internet Service Provider – e.g. UUNET’s network, MCI’s network, etc.

29. ITI-510 Computer Networks Routing within Autonomous Systems Since an Autonomous system is controlled by one entity, routing within an Autonomous System can use any type of routing protocol. A service provider can choose to use RIP, OSPF, IGRP or any other routing protocol within its own Autonomous System. The service provider is responsible for routing traffic within it own network AND is responsible for routing data between its customers and the rest of the Internet (other Autonomous Systems)

30. ITI-510 Computer Networks Routing Between Autonomous Systems When a service provider needs to route traffic to another AS (e.g. other major service provider), it will route transactions through a border router. A border router sits at the edge of an AS and connects one AS to another. The routing protocol that is run between border routers is usually referred to as an exterior routing protocol.

31. ITI-510 Computer Networks Identifying Autonomous Systems Service providers will apply for an Autonomous System Number (ASN) from the American Registry for Assigned Numbers (ARIN). ARIN will assign the service provider a unique 16-bit ASN that is then used as the general description, or name, for a provider’s Autonomous System. (e.g. Rutgers’s ASN is 46) Internet routing across Autonomous Systems is based on selecting a chain of Autonomous Systems that connect a source to a destination.

32. ITI-510 Computer Networks BGP – Border Gateway Protocol BGP is an exterior routing protocol that is used for large-scale routing. It is the most popular exterior routing protocol on the Internet today. Autonomous Systems often have very well-defined entry and exit point (border routers). BGP allows border routers to exchange routing information about paths, or chains, to other Autonomous Systems.

33. ITI-510 Computer Networks Sharing BGP Routing Information If AS 50 contains networks A and B, it will report to a peer BGP router, belonging to another AS, that is it contains networks A and B. –E.g. If networks 128.6.0.0 and 128.7.0.0 are part of AS 46, the border router for AS 46 will tell the border router for AS 1245 (an peer BGP router) that it contains those networks. The border router for AS 1245 will tell other BGP routers that they can reach 128.6.0.0 and 128.7.0.0 through itself AND through AS 46 and so on…