LINK STATE PROTOCOLS (contents)

1. LINK STATE PROTOCOLS(contents) • Disadvantages of the distance vector protocols • Link state protocols • Why is a link state protocol better?

2. Disadvantages of the Distance Vector Protocols • Slow convergence • Counting to infinity problem • Lack of variety of metrics • No possibility of hierarchical routing • Bad performance in large networks

3. Link State Protocols • developed to overcome the disadvantages of the distance vector protocols • centralized database describes the topology of the whole network • calculation and routing are still distributed

4. Link State Routing Algorithm • Each router is responsible for meeting its neighbors and learning their names. • Each router constructs a link state packet (LSP)which consists of a list of names and cost for each of its neighbors. • The LSP is transmitted to all other routers. Each router stores the most recently generated LSPfrom each other router. • Each router uses complete information on the network topology to compute the shortest path route to each destination node.

5. Characteristics • information about adjacencies sent to all routers only when there is a change • each router maintains an identical database • a “shortest path” algorithm is used to find the best route • converge as quickly as databases can be updated

6. 2 1 A B C 4 3 5 6 D E LS seq. num. Link State Database From To Link Cost A B 1 1 2 A D 3 1 2 B A 1 1 2 B C 2 1 1 B E 4 1 2 C B 2 1 1 C E 5 1 1 D A 3 1 2 D E 6 1 1 E B 4 1 2 E C 5 1 1 E D 6 1 1 Link State Announcement (LSA) From A to B, link 1, distance = 1 number = 2

7. The Flooding Protocol • every node sends the message on every link except the one from where it received the message • very fast and very reliable, but wastes bandwidth • used for ordinary traffic in military networks • messages are sent only when there is a change or every 45 minutes

8. Securing the Map Updates • the flooding procedure includes hop-by-hop acknowledgments • the database description packets are transmitted in a secure fashion • each link state record is protected by a timer and is removed from the database if a refreshing packet does not arrive in due time • all records are protected by checksum • the messages can be authenticated, e. g. by passwords

9. 2 1 A B C 4 3 5 6 D E Dijkstra’s Algorithm Average time needed to compute the routing table is about 200ms for a 200 node network on a typical router. • Actual implementation in routers called “Forward Search” • Builds up a list of “Tentative” and “Confirmed” paths • Automatically prunes out longer paths

10. Why Is a Link State protocol Better? • fast loopless convergence • support of precise metrics and, if needed multiple metrics • support of a multiple paths to a destination • splitting very large networks in areas

11. Link State ProtocolsDisadvantages • more memory required • the link state database is needed in addition to the routing tables • much more complex procedure • higher probability for a bug in the program

12. Message complexity LS: with n nodes, E links, O(nE) msgs sent each DV: exchange between neighbors only convergence time varies Speed of Convergence LS: O(n2) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table DV: DV node can advertise incorrect path cost each node’s table used by others error propagate thru network Comparison of LS and DV algorithms

13. Path-vector paradigm • A variant of distance-vector • Each router maintains its path to a destination • Method of choosing the path • based on available paths through neighbors, choose the one of highest “preference” (not necessarily the shortest)