1 Network Layer Lecture 13 Imran Ahmed University of Management & Technology

2 Agenda Introduction & Network layer functions Routing principles Hierarchical routing The Internet protocol (IP) Routing in the Internet

3 RIP ( Routing Information Protocol) Distance vector algorithm. Distance metric: # of hops (max = 15 hops). Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement). RIP uses UDP port 520 & the broadcast IP address ( ) to advertise every route in its table once every 30 sec. Each advertisement: list of up to 25 destination nets within AS. RIP ver1 is considered as a classful protocol. RIPv1 is specified in RFC D C BA u v w x y z destination hops u 1 v 2 w 2 x 3 y 3 z 2

4 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead –routes via neighbor invalidated –new advertisements sent to neighbors –neighbors in turn send out new advertisements (if tables changed) –link failure info quickly propagates to entire net –poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

5 RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) Advertisements sent in UDP packets, periodically repeated physical link network forwarding (IP) table Transprt (UDP) routed physical link network (IP) Transprt (UDP) routed forwarding table

6 rtartb rtdrtc Network /24 rtd, 2 hops rta, 1 hop rtc, 3 hops rtb, 4 hops How RIP routers use the hop count

7 RIP Example In the previous slide; there are four routers A, B, C & D. (that all are running RIP) –Router A advertises its directly connected Ethernet network to router D with a hop count of 1. –Router D receives this route and realizes that from router D’s perspective, it is two hops away, then router D advertises this network to router C with a hop count of 2. –Router C in turn, advertises it to B and router B advertises it back to router A.

8 RIP Example When the route arrives back at router A, it believes that router B also has a connection to the same network. It has no way of knowing that router B is only advertising router A’s own route fourth-hand. So, router A must make a choice of which route to use. To make this choice, it compares the only metric it has, and determines that its directly connected route with a hop count of one is shorter than the route it receives from router B. At the same time, however, router A also sends an identical broadcast towards router B, which forwards it to router C, and then to router D which forwards it to router A, exactly like the first route. As routers B, C & D receive both routes, they compare the hop count and choose the shortest path.

9 RIPv1 Packet Format One RIP message can have up to 25 route entries 1: request 2: response 2: for IP Address of destination Cost (measured in hops) 1: RIPv1

10 RIP’s Implementation of DV RIP needs to deal with some of the shortcomings of Distance-Vector protocols: –Count-to-Infinity –Split Horizon & Poison Reverse –Triggered Updates –Timers

11 RIP’s Implementation of DV Count-to-Infinity: –This limits the diameter of the routing domain to 15, and also makes counting to infinity a little faster. Split Horizon: –If it is activated, a router omits sending routes back to the router it learned them from. –This helps in avoiding a process of mutual deception, where two routers tell each other that they can reach destination X via each other. Split Horizon with Poison Reverse: –If split horizon is activated with poison reverse, then a router behaves in the same way as in plain split horizon, but instead of not sending information back, it sends a route update with a metric of 16 (unreachable) to the router, it got the route from.

12 RIP’s Implementation of DV Triggered Updates: –It means that an incoming update message triggers the router to make its own update. –RIP does its updates by sending out its distance-vectors to all its neighbors. Flash Updates: –On a Cisco box, a router that cold-starts broadcasts a request packet to all its neighbors. Every neighbor responds by immediately unicasting a reply containing its distance-vector. –This functionality together with the normal triggered update functionality is called a flash update.

13 RIP’s Timers There are two sets of timers in RIP: –The RFC way –The Cisco way

14 RIP’s Timers – According to RFC Update – the time between each update (default is 30 sec.). Time-out – if no updates are received, wait this long after the last successful update before marking the route for deletion (default is 180 sec.). Garbage-collection – when a route has been marked for deletion, its metric is set to 16, and after this amount of time, it is removed from memory (default is 120 sec.).

15 RIP’s Timers – According to Cisco Update – the time between each update (default is 30 sec.). Invalid – if no updates are received, wait this long after the last successful update before marking the route invalid (default is 180 sec.). Holddown – when a route has become invalid, don’t accept updates about this route for this period of time (default is 180 sec.). Flush – if a route has been invalid for this time, flush (remove) it from memory (default is 240 sec.).

16 RIP Problems RIP takes a long time to stabilize –Even for a small network, it takes several minutes until the routing tables have settled after a change. RIP has all the problems of distance vector algorithms, e.g., routing loops etc. The maximum hop count in RIP is 15.

17 RIPv2 RIPv2 is an extends RIPv1: –Subnet masks are carried in the route information. –Authentication of routing messages. –Route information carries next-hop address. –Uses IP multicasting. Extensions of RIPv2 are carried in unused fields of RIPv1 messages.

18 RIPv2 Packet Format Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes Identifies a better next- hop address on the same subnet than the advertising router, if one exists (otherwise 0….0) 2: RIPv2 Subnet mask for IP address

19 RIP Security Issue: Sending bogus routing updates to a router. –RIPv1: No protection. –RIPv2: Simple authentication scheme. 2: plaintext password