1. Data and Computer Communications
Chapter 12 – Routing in Switched Data Networks
“Data and Computer Communications”, 9/e, by William Stallings, Chapter 12 “Routing in Switched Networks”.
Ninth Edition
by William Stallings
Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson Education - Prentice Hall, 2011

2. Routing in Packet Switching Networks
Select route across network between end nodes
characteristics required:
correctness
simplicity
robustness
stability
fairness
optimality
efficiency
routing function seeks to design routes through the network for individual pairs of communicating end nodes such that the network is used efficiently.
generally, more than one route is possible.
Robustness has to do with the ability of the network to deliver packets via some route in the face of localized failures and overloads.
stability. Techniques that react to changing conditions have an unfortunate tendency to either react too slowly to events or to experience unstable swings from one extreme to another.
A tradeoff also exists between fairness and optimality. Some performance criteria may give higher priority to the exchange of packets between nearby stations compared to an exchange between distant stations. This policy may maximize average throughput but will appear unfair to the station that primarily needs to communicate with distant stations.
any routing technique involves some processing overhead at each node and often a transmission overhead as well, both of which impair network efficiency.

3. Performance Criteria used for selection of route
simplest is to choose “minimum hop”
can be generalized as “least cost” routing
because “least cost” is more flexible it is more common than “minimum hop”
a cost is associated with each link, and, for any pair of attached stations, the route through the network that accumulates the least cost is sought.

4. Example of Packet Switched Network
numbers represent the current link cost in each direction.
The shortest path (fewest hops) from node 1 to node 6 is 1-3-6, 2 hops (cost = = 10),
but the least-cost path is (cost = = 4).
cost could be inversely related to the data rate (i.e., the higher the data rate on a link, the lower the assigned cost of the link) or the current queuing delay on the link. In the first case, the least-cost route should provide the highest throughput. In the second case, the least-cost route should minimize delay.

5. Decision Time and Place
packet or virtual circuit
fixed or dynamically changing
decision place
distributed - each node makes decision, more complex, but more robust
centralized – made by a designated node
source – made by source station
For datagram, a routing decision is made individually for each packet.
For virtual circuit operation, a routing decision is made at the time the virtual circuit is established.
In the simplest case, all subsequent packets using that virtual circuit follow the same route. In more sophisticated network designs, the network may dynamically change the route assigned to a particular virtual circuit in response to changing conditions (e.g., overload or failure of a portion of the network).
For centralized routing, the decision is made by some designated node, such as a network control center. The danger of this latter approach is that the loss of the network control center may block operation of the network. The distributed approach is perhaps more complex but is also more robust. A third alternative, used in some networks, is source routing. In this case, the routing decision is actually made by the source station rather than by a network node and is then communicated to the network. This allows the user to dictate a route through the network that meets criteria local to that user.

6. Network Information Source and Update Timing
routing decisions usually based on knowledge of network, traffic load, and link cost
distributed routing
using local knowledge, information from adjacent nodes, information from all nodes on a potential route
central routing
collect information from all nodes
Most routing strategies require that decisions be based on knowledge of the topology of the network, traffic load, and link cost.
With distributed routing, in which the routing decision is made by each node, the individual node may make use of only local information, such as the cost of each outgoing link. Each node might also collect information from adjacent (directly connected) nodes, such as the amount of congestion experienced at that node. Finally, there are algorithms in common use that allow the node to gain information from all nodes on any potential route of interest. In the case of centralized routing, the central node typically makes use of information obtained from all nodes.
A related concept is that of information update timing, which is a function of both the information source and the routing strategy. Clearly, if no information is used (as in flooding), there is no information to update. If only local information is used, the update is essentially continuous. For all other information source categories (adjacent nodes, all nodes), update timing depends on the routing strategy. For a fixed strategy, the information is never updated. For an adaptive strategy, information is updated from time to time to enable the routing decision to adapt to changing conditions.
As you might expect, the more information available, and the more frequently it is updated, the more likely the network is to make good routing decisions. On the other hand, the transmission of that information consumes network resources.
issue of update timing
depends on routing strategy
fixed - never updated
adaptive - regular updates

7. Elements of Routing Techniques for Packet-Switching Networks
Throughput: packets per second

8. Routing Strategies - Fixed Routing
use a single permanent route for each source to destination pair of nodes
determined using a least cost algorithm
route is fixed
until a change in network topology
based on expected traffic or capacity
advantage is simplicity
disadvantage is lack of flexibility
does not react to network failure or congestion
A large number of routing strategies have evolved for dealing with the routing requirements of packet-switching networks, we survey four key strategies: fixed, flooding, random, and adaptive.
For fixed routing, a single, permanent route is configured for each source-destination pair of nodes in the network. Either of the least-cost routing algorithms described in Section 12.3 could be used. The routes are fixed, or at least only change when there is a change in the topology of the network. Thus, the link costs used in designing routes cannot be based on any dynamic variable such as traffic. They could, however, be based on expected traffic or capacity.
With fixed routing, there is no difference between routing for datagrams and virtual circuits. All packets from a given source to a given destination follow the same route. The advantage of fixed routing is its simplicity, and it should work well in a reliable network with a stable load. Its disadvantage is its lack of flexibility. It does not react to network congestion or failures.