Network and Communications

Ju Wang

Chapter 5

Routing Algorithm

Adopted from Choi’s notes

Virginia Commonwealth University

Design Issues

Store-and Forward packet Switching

Services Provided to the Transport layer

Implementation of Connectionless Service

Implementation of Connection oriented Service

Comparison of Virtual Circuit and Datagram Subnet

Store and Forward Packet Switching

Design Issues

Services Provided to the Transport Layer

Goals

Independent on the router technology

Router information shielding(number, type and topology)

Global and uniform network address

Datagram Switching

Implementation of Connection Oriented Service

Virtual circuit subnet

Every packet carry virtual circuit id and follows same route

Label switching (virtual circuit)

Design IssuesComparison of Virtual Circuits and Datagram subnet

Routing Algorithms

Routing algorithm is desirable if…

Correctness, simplicity, robustness, stability,fairness, optimality

NL Goal: Routing packets from the source to destination

Algorithm and data structure ?

Decision per packets

Why? Best route

What about VC?

Session routing

Correctness?

Simplicity

Stability

Need to converge to equilibrium

Fairness and optimality? Contradictory goals

Robustness

To topology and traffic change

What to optimize?

Mean packet delay? Network throughput?

Minimize the number of hop

Routing AlgorithmsPrinciple of optimality

If J is on the optimal path from I to K, then the optimal path from J to K also falls along the same route

Proof:

So what?

Sink tree

No cycles guarantee the bounded-hop delivery in real life?

Routing AlgorithmsShortest path Routing

Routing AlgorithmsFlooding

Send packet to every outgoing line except the one it arrives

– duplicated redundant packet

- hop count

Routing AlgorithmsDistance Vector Routing (Bellman Ford)

Problem?

Table giving the best known distance to each destination and line to use

Count to Infinity problem

Routing AlgorithmsLink State Routing

Each router must do the following:

1. Discover its neighbors, learn their network address.

2. Measure the delay or cost to each of its neighbors.

3. Construct a packet telling all it has just learned.

4. Send this packet to all other routers.

5. Compute the shortest path to every other router

Computing the New Route using Dijkstra

Building Link State Packets

Routing AlgorithmsHierarchical Routing

Save space! At the cost of path length (1A to 5C)

Routing AlgorithmsBroadcasting, Multicasting

Spanning tree

•Given–Graph G = (V,E)–Edge weights w: w(e) is weight for edge e.

•Find a spanning tree T which has minimum weight, i.e., minimum spanning tree

Recall, a spanning tree is a subgraph that is a tree that includes all the nodes.

Examples

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Min SpanningTree = 37

SpanningTree = 42

Kruskal’s Algorithm: Basic Idea

• Start with a collection of subtrees Actually, each node is a subtree

• Find the lightest edge joining two subtrees• Keep doing this until there is a single subtree

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Kruskal’s Algorithm: Basic Idea

Prim’s Algorithm: Basic Idea

• Start with a single subtree of ONE node• Grow this subtree by adding the lightest

edge coming out of it.• Keep doing this until the subtree is a

spanning tree.

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Prim’s Algorithm: Basic Idea