Ch. 9 Routing

Data Communications

By

Stallings

19.1 Routing in Packet Switched Networks

• Routing Algorithm Requirements – Correctness– Simplicity– Robustness--the ability of the network to deliver packets via some route in the face of

localized failures and overloads.– Stability--does not “over react” to network changes.– Fairness--as related to all other users.– Optimality--as related to some criterion.– Efficiency--as related to processing overhead.

12.1 Elements of Routing Techniques

– Performance Criteria• Number of hops, cost, delay, & throughput.• See Table 19.1

– Decision Time• Virtual Circuit--at connection establishment.• Datagram--before packet is placed in outgoing

buffer.– Decision Place

• Each node, central node, originating node.

19.1 Elements of Routing Techniques (cont.)

– Network Information Source

• None, local, adjacent nodes, nodes along the route, or all nodes.

– Network Information Update Timing

• Continuous, periodic, major load change, topology change.

19.1 Routing Strategies• Fixed Routing

– A route is selected for each source-destination pair of nodes.

– A central routing directory can then be distributed to the various nodes.

– Routes are not changed unless topology changes.

– Simple (advantage) but inflexible (disadvantage.)

19.1 Routing Strategies

• Fixed Routing , Example19.2

• (Fig. 19.2)– Refer back to the network in Fig. 19.1.– Central directory lists all the routing

information.– Each column of the central directory

becomes the Next Node columns in the nodal directories.

12.1 Routing Strategies (p.2)

• Flooding (Fig. 19.3)

– A packet is sent out on every outgoing link except the link that it arrived on.

– Duplicates must be discarded.• Hop counter could be used.

– Very robust (advantage.)

– High traffic loads are generated (disadvantage.)

12.2 Routing Strategies (p.3)

• Random Routing– An outgoing link is selected at random (based

on a probability distribution.)– Requires no use of network information

(advantage.)– Actual route will not be least-cost or minimum-

hop route (disadvantage.)

12.2 Routing Strategies(p.4)

• Adaptive Routing– These algorithms react to changing conditions

of the network, for example failures and congestion.

– Advantages--can improve performance and aid in congestion control.

– Disadvantages--complex, requires extra "overhead" traffic to collect information, and may react too quickly (unstable.)

12.2 Routing Strategies (p.5)

• Adaptive Routing(cont.)– Schemes can be characterized by

• Source of Network Information– Local--Fig. 19.4 Isolated Adaptive Routing

» Minimize Queue Length + Bias– Adjacent Nodes– All Nodes

• Distributed or Centralized Control

19.2 Examples: Routing in Arpanet

• First Generation –Distant Vector Routing– Distributed adaptive algorithm (1969)– Performance criteria--estimated delay (from queue

length).– Version of the Bellman-Ford Algorithm.– Problems: did not consider line speed, queue length

is not an accurate measure of delay, and the algorithm responded slowly to congestion and delay increases.

– See Fig. 19.5, 19.6 and discussion—page 601.

19.2 Internet Routing Examples (p.2)

• Second Generation (Link-State Routing)

– Distributed adaptive algorithm (1979).

– Performance criteria--delay (direct measurements).

– Version of Dijkstra's Algorithm.– Problem: did not work well for heavy loads.

19.2 Routing Strategy Examples (p.3)

• Third Generation ARPANET (1987)– The average delay is measured and transformed

into estimates of utilization.– The link "costs" were calculated as a function of

utilization--this helped to prevent oscillations.– Fig. 19.7--traffic could oscillate from link A to

link B and back.

19.4 Least-Cost Algorithms

• The Problem– Given a network of nodes connected by bi-directional

links, where each link has a cost associated with it in each direction, define the cost of a path between two nodes as the sum of the costs of the links traversed. For each pair of nodes find the path with least cost.

• Solutions– Dijkstra's Algorithm (1959)– Bellman-Ford Algorithm (1962)

Dijkstra's Algorithm

• Define:– N=set of nodes in the network.– s=source node.– T=set of nodes so far incorporated by the

algorithm.– w(i,j)=link cost from node i to node j; w(i,i)=0

and w(i,j)= if the nodes are not directly connected.

– L(n)= cost of the least-cost path from node s to node n that is currently known to the algorithm.

Dijkstra's Algorithm (p.2)• Three Steps (repeated until M=N)

– Step 1: Initialize Variables• T= {s}.

• L(n)=w(s,n) for n s.

– Step 2: Get Next Node• Find the neighboring node (x) which has the least-

cost path from node s and incorporate that node into T.

– Step 3: Update the least cost-paths.• L(n)= min[ L(n), L(x) + w(x,n)] for all n T.

• See Table 19.4a and Fig. 19.14.

Bellman-Ford Algorithm

• Define:– s = the source node.– w(i,j)=link cost from node i to node j.– h=maximum number of links in a path at the

current stage of the algorithm.

– Lh(n) = cost of the least-cost path from node s to node n under the constraint of no more than h links.

Bellman-Ford Algorithm (p.2)

• Step 1: Initialize– L0(n)=, for all n not equal to s.

– Lh(s) =0, for all h.

• Step 2: For each successive h,– L h+1(n) = Minj [Lh(j) + w(j,n)].

Comparison of the Algorithms

• Dijkstra’s– Complete topology information is needed.

• Bellman-Ford – Knowledge of link costs to each neighbor, and

the current “distance-vector” of each neighbor is required.