1. Intradomain Routing Outline Introduction to Routing
Distance Vector Algorithm
CS 640

2. Overview Forwarding vs Routing Network as a Graph
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 640

3. 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 640

4. 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 640

5. Example Destination Cost NextHop 5 3 B C 1 5 2 1 F A 1 2 1 1 E D G
B B
C D
D D
E D
F D
G G 3 B C 1 5 2 1 A F 1 2 1 1 E D G
CS 640

6. Routing Loops Example 1 Example 2 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 640

7. 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 640

8. 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 640

9. 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!
CS 640