Lecture 10 IP 2

IP Routing

Generic Forwarding Architecture

Third Generation Routers

Graph Model

Represent each router as node
Direct link between routers represented by edge

Ways to Compute Shortest Paths

Centralized
- Use standard graph algorithm and collect graph structure in one place
Link-state
- Every node collects complete graph structure
- Each computs shortest paths from it
- Each generates its own routing table
Distance-vector
- Nodes construct their own tables iteratively
- Each sends information about its table to neighbors

Link State Protocol

Every node gets complete copy of graph
- Every node floods network with data about its outgoing links
Every node computes routes to every other node

Dijkstra's Algorithm

Shortest Path First Algorithm
- Traverse graph in order of least code from source
- Determine leaset cost path from source node s to every node v

OSPF Routing Protocol

Open Shortest-path First
OSPF has fast convergence when configuration changes
OSPF reliable flooding
- Transmit link state advertisements
- When should it be performed
  - Periodically
  - When status of link changes
- What happens when router goes down and comes back up
  - Router will send out LSAs with number 0

Distance-Vector Method

At any time, have cost/next hop of best known path to destination
Link cost changes:
- Node detects local link cost change and updates distance table
- Good news travels fast
- Bad news travels slow
Poison reverse
- Iterations don't converge
- Count to infinity problem
- Solution: make "infinite" smaller

Bellman-Ford Algorithm

Repeat
- For every node x
  - For every neighbor z
    - For every destination y
      - d(x,y) = Update(x, y, z)
Until converge

Comparison of Link-state and Distance-vector

Message complexity
- Link-state: with n nodes, E links, O(nE) messages
- Distance-vector: exchange between neighbors only
Speed of Convergence
- Link-state: Relatively fast
  - Complex computation, but can forward before computation
  - May have transient loops
- Distance-vector: convergence time varies
  - May have routing loops
  - Count-to-infinity problem
  - Faster with triggered updates
Space requirements
- Link-state: maintains entire topology
- Distance-vector: maintains only neighbor state
Robustness (router malfunctions)
- Link-state: node can advertise incorrect link cost
  - Each node computes its own table
- Distance-vector: node can advertise incorrect path cost
  - Each node's table used by others (error propagates)

Internet Control Message Protocol (ICMP)

Short messages used to send error & other control information
IP MTU discovery with ICMP
- When successful, no reply at IP level
Base-level protocol (IP) provides minimal service level
- Each step involves determining next hop
ICMP provides low-level error reporting