Overview: IP routing

• Bellman-Ford
• Spanning Tree
• OSPF
• BGP
• Multicast Routing

Bellman-Ford routing

• Distance Vector:
  • router knows how far destination D is
  • router knows which on port to send packets for D
  • router only knows first hop to deliver D
• RIP: Routing Information Protocol
• Distributed algorithm: no router has a complete "map"
• overall idea: routers exchange routing tables, adding cost of reaching that router to see if route is better than what we have

Bellman-Ford data structures

• links table:

  port	neighbor	metric
  0	A	10
  1	C	2
  2	F	3
  3	A	1

• routing table:

  destination	port	metric
  C	1	2
  F	2	3
  A	3	1
  X	3	11
  Y	2	4

Bellman-Ford algorithm

• send my routing table to all my neighbors whenever my link table changes
• when I get a routing table from a neighbor on port P with link metric M:
  1. add L to each of the neighbor's metrics
  2. for each entry (D, P', M') in the updated neighbor's table:
     1. if I do not have an entry for D, add (D, P, M') to my routing table
     2. if I have an entry for D with metric M", add (D, P, M') to my routing table if M' < M"
  3. if my routing table has changed, I send all the new entries to all my neighbors

OSPF

• Open Shortest Path First
• within ASs
• Dijkstra shortest path algorithm
• flooding to distribute maps
• multiple metrics: shortest path algorithm applied with each metric

OSPF Flooding

• links table, as for distance vector (only neighbors and metrics, port information is not needed)
• each router keeps a counter i of messages sent
• messages from router R have format:
  (R, i, link table)
• each router keeps the most recent message received from every other router
• when I get a message (R, i, link table):
  • if I already have a message from R with sequence number i', and if i' >= i, I discard the message
  • otherwise I update my map from the message (discarding the old information from R), and forward the message to all my neighbors

BGP

• Boundary Gateway Protocol
• between ASs
• Distance Vector with route information:
  • instead of just sending you the metric to a destination, I send you the metric and the route used to get there
  • you can detect loops in the route and ignore it
• quickly catches loops, obsolete information
• also allows choices based on trust, cost, reliability
• an AS may have multiple border routers: one is selected as the BGP speaker

Routing Questions

• How fast do Link-State, Distance-Vector converge initially?
• How fast do Link-State, Distance-Vector converge after a change?
• How much traffic is needed to maintain routing information for Distance-Vector, Link-State, and BGP?