CS 6401 Intra-domain Routing Outline Introduction to Routing Distance Vector Algorithm

CS 6402 Overview Forwarding vs Routing –forwarding: to select an output port based on destination address and routing table –routing: process by which routing table is built Network as a Graph –Assume single admin. authority –Assume nodes are routers –Assume links have cost Problem: Find lowest cost path (sum of links) between two nodes

CS 6403 Routing Protocol Issues It may be simple to calculate least cost path if graph is static but… Links and routers go down Links and routers are added Traffic can cause links to overload How are costs calculated? Algorithm must be distributed in order to scale Rich area for research due to distributed, dynamic nature of the problem Different routers can have different routes at same time

CS 6404 Distance Vector (Bellman-Ford,RIP) Find SP for node such that paths contain at most 1 hop, then 2… Each node maintains a set of triples –(Destination, Cost, NextHop) –Assume each node knows cost of directly connected neighbors Exchange updates directly connected neighbors –periodically ( on the order of several seconds) – even if there are no changes –whenever its table changes (called triggered update) Each update is a list of pairs: –( Destination, Cost) Update local table if receive a “better” route –smaller cost –came from next-hop Refresh existing routes; delete if they time out

CS 6405 Example Destination Cost NextHop B 1 B C 3 D D 1 D E 2 D F 4 D G 1 G D G A B C E F

CS 6406 Routing Loops Example 1 –E detects that link to C has failed –E sets distance to C to infinity and sends update to D –D sets distance to C to infinity since it uses D to reach E –D receives periodic update from B with 2-hop path to C –D sets distance to C to 5 and sends update to E –E decides it can reach C in 3 hops via B Example 2 –link from A to G fails –A advertises distance of infinity to G –B and D advertise a distance of 2 to G –B decides it can reach G in 3 hops; advertises this to A –A decides it can read G in 4 hops; advertises this to D –D decides that it can reach G in 5 hops…

CS 6407 Loop- Breaking Heuristics Set infinity to 16 –Assume this is maximum number of hops in network Split horizon –Don’t send routes learned from a neighbor back to a neighbor Split horizon with poison reverse –Send route back to neighbor with negative information ie. Infinite What are the problems?

CS 6408 Link State (Dijkstra’s algorithm,OSPF) Find SP from a given node by sending path data to all nodes and developing paths in order of increasing length Strategy –send to all nodes (not just neighbors) information about directly connected links (not entire routing table) Link State Packet (LSP) –id of the node that created the LSP –cost of the link to each directly connected neighbor –sequence number (SEQNO) –time-to-live (TTL) for this packet

CS 6409 Link State (cont) Reliable flooding –store most recent LSP from each node –forward LSP to all nodes but one that sent it –generate new LSP periodically increment SEQNO –start SEQNO at 0 when reboot –decrement TTL of each stored LSP discard when TTL=0 –This is not easy!