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Data and Computer Communications Eighth & Ninth Edition by William Stallings Chapter 12 – Routing in Switched Networks.

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Presentation on theme: "Data and Computer Communications Eighth & Ninth Edition by William Stallings Chapter 12 – Routing in Switched Networks."— Presentation transcript:

1 Data and Computer Communications Eighth & Ninth Edition by William Stallings Chapter 12 – Routing in Switched Networks

2 Routing in Packet Switched Network  key design issue for (packet) switched networks  select route across network between end nodes  characteristics required: correctness correctness simplicity simplicity Robustness : route in the face of localized failures and overloads Robustness : 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 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 fairness fairness Optimality (universal) Optimality (universal) efficiency efficiency

3 Performance Criteria  used for selection of route  simplest is “minimum hop” can be generalized as “least cost” can be generalized as “least cost” a cost is associated with each link, and, for any pair of attached stations. a cost is associated with each link, and, for any pair of attached stations.  the least-cost criterion is more flexible, this is more common than the minimum-hop criterion.

4 Example Packet Switched Network The shortest path (fewest hops) The cost could be inversely related to the data rate numbers represent the current link cost in each direction. T he higher the data rate on a link, the lower the assigned cost of the link) or the current queuing delay on the link

5 Decision Time and Place  Two key characteristics of the decision are the time and place that the decision is made.  Decision time is determined by whether the routing decision is made on: packet or virtual circuit basis packet or virtual circuit basis datagram, a routing decision is made individually for each packetdatagram, a routing decision is made individually for each packet For internal virtual circuit operation, a routing decision is made at the time the virtual circuit is establishedFor internal virtual circuit operation, a routing decision is made at the time the virtual circuit is established fixed or dynamically changing fixed or dynamically changing  Place which node or nodes in the network are responsible for the routing decision Distributed - made by each node Distributed - made by each node Centralized ( by some designated node, such as a network control center) Centralized ( by some designated node, such as a network control center) Source Routing (allows the user to dictate a route through the network that meets criteria local to that user. Source Routing (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 (not always) distributed routing distributed routing using local knowledge, info from adjacent nodes, info from all nodes on a potential routeusing local knowledge, info from adjacent nodes, info from all nodes on a potential route central routing central routing collect info from all nodescollect info from all nodes  issue of update timing when is network info held by nodes updated when is network info held by nodes updated fixed - never updated fixed - never updated adaptive - regular updates adaptive - regular updates 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.

7 Routing Strategies  Fixed  Flooding  Random  Adaptive. The principal conditions that influence routing decisions are: Failure: When a node or link fails, it can no longer be used as part of a route. Congestion: When a particular portion of the network is heavily congested

8 Routing Strategies - Fixed Routing  use a single permanent route for each source to destination pair  determined using a least cost algorithm  route is fixed at least until a change in network topology at least until a change in network topology hence cannot respond to traffic changes hence cannot respond to traffic changes  advantage is simplicity  disadvantage is lack of flexibility

9 Fixed Routing Tables Stored at the Network control Center for each source- destination pair of nodes, the identity of the next node on the route. * it is not necessary to store the complete route for each possible pair of nodes.

10 Routing Strategies - Flooding  packet sent by node to every neighbor  eventually multiple copies arrive at destination  no network info required  each packet is uniquely numbered so duplicates can be discarded  need some way to limit incessant retransmission nodes can remember packets already forwarded to keep network load in bounds nodes can remember packets already forwarded to keep network load in bounds or include a hop count in packets or include a hop count in packets

11 Flooding Example

12 Properties of Flooding  all possible routes are tried very robust very robust  at least one packet will have taken minimum hop count route can be used to set up virtual circuit can be used to set up virtual circuit  all nodes are visited useful to distribute information (eg. routing) useful to distribute information (eg. routing)  disadvantage is high traffic load generated

13 Routing Strategies - Random Routing  Has the simplicity of flooding with much less load  node selects one outgoing path for retransmission of incoming packet excluding the link used by the incoming packet  selection can be random or round robin  a refinement is to select outgoing path based on probability calculation  no network info needed  but a random route is typically neither least cost nor minimum hop

14 Routing Strategies - Adaptive Routing  used by almost all packet switching networks  routing decisions change as conditions on the network change due to failure or congestion  requires info about network  disadvantages: decisions more complex decisions more complex tradeoff between quality of network info and overhead tradeoff between quality of network info and overhead reacting too quickly can cause oscillation reacting too quickly can cause oscillation reacting too slowly means info may be irrelevant reacting too slowly means info may be irrelevant

15 Adaptive Routing - Advantages  adaptive routing strategies are by far the most dominant, for two reasons: improved performance improved performance aid congestion control aid congestion control  but since is a complex system, may not realize theoretical benefits depending on the design and the nature of the load

16 Classification of Adaptive Routing Startegies  based on information sources local (isolated) local (isolated) route to outgoing link with shortest queueroute to outgoing link with shortest queue can include bias for each destinationcan include bias for each destination Rarely used - does not make use of available infoRarely used - does not make use of available info adjacent nodes adjacent nodes takes advantage on delay / outage infotakes advantage on delay / outage info distributed or centralizeddistributed or centralized all nodes all nodes like adjacentlike adjacent

17 Least Cost Algorithms  basis for routing decisions  If the criterion is to minimize the number of hops can minimize hop with each link cost 1 can minimize hop with each link cost 1 or have link value inversely proportional to capacity or have link value inversely proportional to capacity  defines cost of path between two nodes as sum of costs of links traversed in network of nodes connected by bi-directional links in network of nodes connected by bi-directional links where each link has a cost in each direction where each link has a cost in each direction  for each pair of nodes, find path with least cost link costs in different directions may be different link costs in different directions may be different  alternatives: Dijkstra or Bellman-Ford algorithms


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