Outline

Outline

Routing algorithm: routing packets from source to destination.
Optimality principle: finding the optimal path from source to destination.
Various routing algorithms
- Dijsktra’s algorithm
- Dynamic routing algorithms:
  - Distance vector algorithm
  - Link state routing
- Hierarchical routing

Routing Algorithms

Routing is the process of discovering network paths
- Decide what to optimize (e.g., fairness vs efficiency)
- Model the network as a graph of nodes and links
- Update routes for changes in topology (e.g., failures)

Optimality Principle

If router J is on the optimal path from I to K, then the optimal path from J to K is on the same route

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Identify the optimal path from source to destination.
Sink tree: optimal routes from all sources to a given destination.
Distance metrics:
- the number of hops, or
- time delay.

Optimality Principle

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Shortest Path Algorithm

Dijkstra’s algorithm computes a sink tree on the graph:

Each node is labelled with its distance from the source node to the best known path
Initially no paths are known
Each link is assigned a non-negative weight/distance
Shortest path is the one with lowest total weight

Shortest Path Algorithm

Algorithm:

Start with sink, set distance at other nodes to infinity
Labels tentative (○) or permanent (⚫), initially all tentative
Pick lowest distance non-permanent node, make perm.
Repeat from this node, until all nodes are permanent

Shortest Path Algorithm

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weights represent cost/distance

Shortest Path Algorithm

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A network and first five steps in finding the shortest paths from A to D. Pink arrows show the sink tree so far

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Distance Vector Routing

Two types of dynamic routing algorithm:

Distance vector, and
link state routing algorithms

Distance Vector Routing

Distance vector:

Each node maintains a table (vector of best distance to dest.)
Tables are updated by exchanging information between nodes
Tables have 2 entries: outgoing line and estimated distance (#hops or propagation delay)

Distance Vector Routing

Algorithm:

Each node knows distance of links to its immediate neighbours
Each node advertises a vector of the lowest known distances to all nodes
Each node uses received vectors to update its own
Repeat periodically

Distance Vector Routing

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Distance Vector Routing

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Black Holes

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The Count-to-Infinity Problem

Failures can cause DV to “count to infinity” while seeking a path to an unreachable node

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Distance Vector Routing

Link State Routing

Learn the network address of the neighbouring routers by sending HELLO packet, record name
Set the distance to each neighbour
Construct a packet telling all other routers what it has just learned
Send the packet to and receive packets from all other routers
Compute the shortest path by using Dijkstra’s algorithm

Link State Routing: LSPs

LSP (Link State Packet) for a node lists neighbours and weights of links to reach them

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Flooding

A simple method to send a packet to all network nodes
Each node floods a new packet received on an incoming link by sending it out on all of the other links
Nodes need to keep track of flooded packets to stop the flood
- E.g. by using sequence numbers
Does not rely on routing tables being built first

Hierarchical Routing

Hierarchical routing reduces the work of route computation but may result in slightly longer paths than flat routing

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Broadcast Routing

Broadcast sends a packet to all nodes
RPF (Reverse Path Forwarding): Arrived packets are checked to see if they arrived from a preferred link, which is the link that is normally used for sending packets towards the source of the broadcast.
1st hop: I sends packets to F, H, J, and N. Packets arrive on the same link that is used to send to I
2nd hop: 8 packets are generated, two by each router. 5 of them arrive on the preferred link
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Broadcast Routing

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Summary

Packet routing algorithms
Different routing algorithms
- Dijsktra’s algorithm
- Shortest path algorithm
- Distance vector routing
- Link state routing
- Hierarchical routing
Flooding
Broadcasting