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 Issues Comparison 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 Algorithms Principle 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 Algorithms Shortest path Routing

Routing Algorithms Flooding Send packet to every outgoing line except the one it arrives – duplicated redundant packet - hop count

Routing Algorithms Distance Vector Routing (Bellman Ford) Problem? Table giving the best known distance to each destination and line to use

Count to Infinity problem

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

Routing Algorithms Broadcasting, 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 Min Spanning Tree = 37 Spanning Tree = 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

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.

Prim’s Algorithm: Basic Idea