Intra-domain Routing

1
Intra-domain Routing

• Outline
• Introduction to Routing
• Distance Vector Algorithm

2
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

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

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

5
Example
5

• Destination Cost NextHop
• B 1 B
• C 3 D
• D 1 D
• E 2 D
• F 4 D
• G 1 G

3
B
C
1
5
2
1
F
A
1
2
1
1
E
D
G
6
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

7
Loop-Breaking Heuristics

• Set infinity to 16
• Assume this is maximum number of hops in network
• Split horizon
• Dont 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?

8
Link State (Dijkstras 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

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 TTL0
• This is not easy!