Routing: Cores, Peers and Algorithms

1. Routing: Cores, Peers and Algorithms Chapter 14

2. The Origin of Routing Tables • Routers form the base of an internet and handle all traffic except for direct delivery from one host to another • We have two questions: • What should be in routing tables? • How do routers get the information for the tables? • Initial routes may be determined: • from secondary storage into main memory at startup • from a set of addresses of connecting networks

3. The Origin of Routing Tables • In small internets, the routing tables may be established and modified by hand • In large internets, automated approaches are necessary

4. Routing with Partial Information • Hosts usually have two routes in their routing table • A route for the local network • A default route for nonlocal datagrams • Example of road signs with partial information, page 255 • Extreme Architectural Approaches • Star shaped topology with one road to each town, one central point • Arbitrary roads with signs for all towns at each intersection

5. Routing with Partial Information • The two extreme architectural approaches fail • Star - one machine would have to work as switch between all • Arbitrary - to keep all routing information at all sites is cumbersome and difficult to change • A third approach • Half of the cities lie in the east and half in the west • A bridge spans a river that separates the east and west • Road signs in the east list all destinations in the east, and only points to the west, and so on

6. Original Internet Architecture and Cores • When TCP/IP was developed, research sites were connected to the ARPANET (Internet backbone) • Routing tables were initialized and modified by hand • To begin automation for routing,: • asmall central set of routers kept complete information about all possible destinations • a larger set of outlying routers kept partial information • allows local administrators to make local changes • default routes at the outlying routers could indicate a central intersection

7. Core Routers • Those early routers were either: • Core routers controlled by Internet Network Operations Center • or Noncore routers controlled by inividual groups • Core routers communicated among themselves • information was consistent and the system was reliable • Sites assigned an Internet network address agreed to advertise that address to the core system

8. Core Routers • The early Internet core routing system consisted of routers connecting local area networks to the ARPANET • See Figure 14.1 • Hosts on the local networks passed nonlocal traffic to the core router • Routing information was exchanged between core routers • Default routes were not used; would be inefficient

9. Core Routers • This approach became impractical because: • the Internet outgrew a single, centrally managed long-haul backbone • protocols needed to maintain consistency among core routers became nontrivial • maintaining correct routing information became difficult

10. Peer Backbones • NSFNET attached to ARPANET through a single router in Pittsburgh • The core had explicit routes to all destinations in NSFNET • Routers inside NSFNET knew local destinations and used a default route (the Pittsburgh router) to send all non-NSFNET traffic to the core • Multiple connections were added between NSFNET and ARPANET as shown in Figure 14.4 and the two became peer backbone networks, or peers

11. Peer Backbones • With such an architecture, how do we route with peer backbones? • Routing loops are possible as in Figure 14.5 • Core systems work best for internets with a single, centrally managed backbone • Expanding the topology to multiple backbones makes routing complex

12. Automatic Route Propagation • The original Internet core system propagated complete routing information to all core routers • Today, many routers continue to communicate routing information • We need ways to automatically determine routes, and to continually update the information needed to determine the routes

13. Distance Vector Routing(aka Bellman-Ford) • A router keeps a list of all known routes in a table • When it boots, the routing table is initialized with an entry for each directly connected network • Each entry identifies a network and the distance to it, usually in hops - See Figure 14.6 • Periodically, all routers send a copy of their routing table to any other router it can reach directly • If a shorter path is found in one of these, the current path is replaced with the shorter one

14. Distance Vector Routing • See Figure 14.7 which shows an existing table for router K, and an update from router J • Is this a good choice? • Easy to implement • If routes change rapidly, routes may not stabilize • Some routers may have incorrect information • Message size is proportional to the number of networks in an internet

15. Gateway to Gateway Protocol • GGP was used by original core routers to exchange routing information - no longer used • Designed to travel in IP datagrams like TCP and UDP • Messages had a fixed format header identifying message type

16. Distance Factoring • Distance values were small, and the same values tended to be repeated often • To reduce the message size, avoid sending copies of the same distance number • The list of pairs (Network, Distance) is sorted by distance • each distance is represented once and all networks at that distance follow

17. Reliability • Most routing protocols use connectionless transport • Routing protocols that use UDP or IP must compensate for failures • by using checksums • sequence numbers for out of order delivery

18. Link-State Routing • Primary alternative to distance vector routing • Tests the status of all neighbor routers • Short message asking if neighbor is alive • If the neighbor responds, the link is up, else down • Routers periodically broadcast a message with the status of each of its links • Indicates whether communication is possible between pairs of routers • When link status information arrives at a router, it modifies its view of the internet

19. Shortest Path First • When link status changes, the router recomputes routes by applying Dijkstra’s Shortest Path algorithm • SPF computes the shortest paths to all destinations from a single source • Advantages • each router computes routes independently • messages are propagated unchanged • size does not depend on number of networks in internet

20. Summary • Hosts and routers usually contain partial routing information • The Internet used a core routing architecture in which cores had complete network information • Routing information is exchanged periodically • using distance-vector or link state algorithms

21. For Next Time • Read Chapter 15