William Stallings Data and Computer Communications

1. William StallingsData and Computer Communications Chapter 10 Packet Switching

2. Why Packet Switching? • Using circuit switching for data • Inefficient • Resources dedicated to a particular connection • Much of the time a data connection is idle • Data rate is fixed • Both ends must operate at the same rate • Limits the utility of high-speed workstations

3. Use of Packets

4. Advantages • Line efficiency • Single node to node link can be shared by many packets over time • Packets queued and transmitted as fast as possible • Data rate conversion • Each station connects to the local node at its own speed • Nodes buffer data if required to equalize rates • Packets are accepted even when network is busy • Delivery may slow down

5. Switching Technique • Station breaks long message into packets • Packets are received, stored briefly (buffered) and past on to the next node on the network • Store and forward • Packets handled in two ways • Datagram • Virtual circuit

6. Datagram • Each packet treated independently • Packets can take any practical route • Packets may arrive out of order • Packets may go missing • Up to receiver to re-order packets and recover from missing packets

7. Virtual Circuit • Preplanned route established before any packets sent • Call request and call accept packets establish connection (handshake) • Each packet contains a virtual circuit identifier instead of destination address • No routing decisions required for each packet • Clear request to drop circuit • Not a dedicated path

8. Virtual Circuits vs. Datagram • Virtual circuits • Network can provide sequencing and error control • Packets are forwarded more quickly • No routing decisions to make • Less reliable • Loss of a node looses all circuits through that node • Datagram • No call setup phase • Better if few packets • More flexible • Routing can be used to avoid congested parts of the network

9. Packet Size

10. Circuit vs. Packet Switching

11. Routing • Finding a path through network nodes from source to destination • Characteristics of routing strategies • Correctness • Robustness • Stability • Fairness • Optimality • Effieciency (overhead for routing processing)

12. Performance Criteria • Criteria used for selection of route • Minimum hop • number of links • simplest • minimize the use of network resources used for routing decision • Least cost • minimize the total link cost • minimize delay • maximize throughput • minimize the monetary cost • more complex algorithms

13. Costing of Routes

14. Decision Time and Place • Routing decision time • Datagram (no virtual circuit) • for each packet • Virtual circuit • when the virtual circuit is established • route may change dynamically depending on current conditions

15. Decision Time and Place • Routing decision place • Which nodes are responsible for routing decision? • Distributed routing • each node decides the next one • most common • Centralized routing • designated central node (e.g. network control center) • not so robust since loss of this node may block the whole network • Source routing • routing decision is made by the originating station

16. Network Information Source and Update Timing • Routing decisions usually (not always) are based on knowledge of network (e.g. traffic load and link costs) • like flooding • Distributed routing • Nodes use local knowledge • cost of each outgoing link • May collect info from adjacent nodes • May collect info from all nodes on a potential route • Central routing • Collect info from all nodes

17. Network Information Source and Update Timing • Update timing • When is network info updated? • No information used (fixed, flooding) • no updates • Local information used only • update is continuous • Other nodes’ info used • regular updates to adapt the route changing conditions • Tradeoff between good adaptive routing and efficiency

18. Key Routing Strategies • Fixed • Flooding • Random • Adaptive

19. Fixed Routing • Single, permanent route for each source to destination pair • Determine routes using a least cost algorithm (Bellman-Ford, Dijkstra - appendix 10A) • Route fixed, unless there is a change in network topology • link costs should be static (e.g. cannot be current traffic or load) • Simple, works well with reliable and stable load networks • Disadvantage: lack of flexibility

20. Fixed RoutingTables

21. Flooding • No network info required • Packet sent by node to every neighbor • Incoming packets retransmitted on every link except incoming link • Eventually a number of copies will arrive at destination • Each packet is uniquely numbered so duplicates can be discarded at destination

22. Flooding Example

23. Flooding • Precautions against unlimited grow in circulation • Nodes can remember packets already forwarded to keep network load in bounds • Include a hop count in packets. • Set to a maximum value (e.g. diameter of the network) • Decrease one at each hop • Discard when 0

24. Properties of Flooding • All possible routes are tried • Very robust • can be used for emergency messaging • At least one packet will use minimum hop count route • Can be used to set up virtual circuit • All nodes are visited • Useful to distribute information (e.g. routing info)

25. Random Routing • Node selects one outgoing path for retransmission of incoming packet • Selection is at random • equally likely • all outgoing links are utilized in long-run • can select outgoing path based on a probability • e.g. probability based on data rate • good traffic distribution • No network info needed • Route is typically not least cost nor minimum hop

26. Adaptive Routing • Used by almost all packet switching networks • Routing decisions change as conditions on the network change • Failure • Congestion • Requires network info exchanged among nodes

27. Adaptive Routing - Drawbacks • Decisions more complex • Processing burden • Tradeoff between quality of network info and overhead • Overreaction may cause oscillations

28. Adaptive Routing - Advantages • Robustness • Improved performance • Aid congestion control • adaptive routing balances the loads

29. Classification • Based on information sources • Local • rarely used • Adjacent nodes • All nodes • Local • Route to outgoing link with shortest queue length • Good for load balancing, but may not be good for optimal routing • Can include bias for each destination (a value fornegative preferrence) • outgoing link is determined by minimizing queue length plus bias

30. Isolated Adaptive Routing

31. ARPANET Routing Strategies • First Generation • 1969 • Distributed, adaptive • Bellman-Ford algorithm • Node exchanges delay vector with neighbors (every 128 ms) • Update routing tables based on incoming info • Doesn't consider line speed, just queue length (delay) • Queueing delay is not the whole delay

32. ARPANET Routing Strategies • Second Generation • 1979 • distributed, adaptive • uses “measured” delay as performance criterion • computes delay every 10 secs • informs all others (using flooding) if there is a significant change • After all nodes change their routing tables, the routing tables may become obsolete • causes oscillations (under heavy loads) • result: not all routes can get the best path under heavy loads

33. ARPANET Routing Strategies • Third Generation • 1987 • Link cost calculations changed • Measure average delay over last 10 seconds, • Transformed into link utilization • Normalized based on current value and previous results • Resulting estimated link utilization is then transformed into delay

34. Bellman-Ford Algorithm • Minimum cost paths • Iterative • find the shortest paths from a source to all possible destinations using only link • then using max. two links by adding appropriate links to the paths of step 1 • then using max. 3 links on top of paths with two links • so on .. until no improvement is gained by adding more links • See example on board