1. LAN Bridge Spanning Tree Animation Simon Arlott

2. Broadcast Networks ● In a broadcast LAN, all packets are sent to all hosts – even if they are not the intended destination. ● This means that the bandwidth available has to be shared between all devices using the network at the same time.

3. Bridges ● Bridges link multiple LANs together, keeping the local traffic on each LAN separate avoiding unnecessary broadcasts.

4. Forwarding ● Packets received are forwarded by bridges to all other LANs if: – The destination is not on the current LAN. ● The bridge may not know that the destination is on the current LAN if it has never sent any packets. – The destination is all hosts (broadcast).

5. Host Cache ● A table of which LAN each host is on is maintained by bridges and updated when a packet arrives from an unknown host. ● Entries are removed after the host has not been seen for a certain period of time.

6. Loops ● When a loop of bridges is formed, packets that have an unknown destination or must be broadcast are repeatedly sent around the loop forever.

7. Spanning Tree ● The spanning tree algorithm discovers a subset of the topology that is loop-free but still has enough connectivity so that there is a path between every LAN.

8. Root Bridge Election ● The bridges elect a root bridge to act as the logical centre of the network. ● The root bridge influences configuration values of the protocol, overriding settings on other bridges. – Hello Time ● The time that elapses between generation of configuration messages. – Max Age ● The message age value at which a stored configuration message is considered too old. – Forward Delay ● The amount of time before a port is allowed to become part of the spanning tree.

9. Configuration Message ● Bridges send out configuration messages if: – They are the root bridge and the hello timer expires. – The message age timer expires. – Another bridge must be notified of the current root bridge. ● Configuration Messages contain: – The ID of the bridge and its port that sent the message. – The current root bridge and the path cost to reach it. – Configuration parameters. – Topology change flags.

10. Topology Change Notification ● When a topology change is detected by a bridge it notifies the root bridge using the shortest path to the root bridge – either directly or via designated bridges which are the closest to the root bridge on that LAN. ● Topology change notification messages are used for this function. ● The root bridge then sets the topology change flag when it sends out new configuration messages for a period of time.

11. Aims ● Create a program which can: – Set up a network configuration. – Run a simulation of the spanning tree algorithm. – Demonstrate what the algorithm does. – Show the status of the bridges and network when the algorithm is running.

12. Requirements ● Graphical interface allowing the configuration to be created, edited, and displayed. ● Save and load feature to store copies and allow re- use of networks created. ● Variable speed simulation. – Can be stopped and resumed at any time. – Interactive allowing the state of the network to be inspected. – Automated operation to run the simulation through predefined situations on its own.

13. Requirements (2) ● The current spanning tree that has been produced from running the algorithm should be visible on the display. ● Packets between one part of the network and another should be shown to indicate where traffic is flowing. ● The program should be easy to use, without being too simple and unable to show enough detailed information at the same time.

14. Requirements (3) ● Hosts can be configured to send out packets at random. ● Host and bridge parameters can be configured. – ID/Address – Hello Time – Maximum Age – Forward Delay ● Log of events and current status shown for hosts and bridges.

15. Program Overview ● The program is divided into three main modes: – Simulation view ● Viewing of the state of the simulation ● Simulation control (start, pause, etc.) – Network edit ● Creating and modifying network components – Script edit ● Creating and modifying scripting elements

16. Loading/Saving ● Networks can be loaded from and saved to files.

17. XML File Format

18. XML File Format (2)

19. Network Editing ● Graphical edit interface. ● Allows components to be: – Created – Moved – Configured ● Grid for easy placement and alignment.

20. Creating LANs ● Placed by clicking on the start and end points.

21. Creating Hosts ● Placed by clicking on a LAN and then where the host should extend to.

22. Creating Bridges ● Placed by clicking where the bridge should be created.

23. Bridge Ports ● Bridge ports are in the disabled state until connected to a LAN (or another bridge port). ● They are automatically assigned the next free ID when this happens.

24. Moving Components ● Hosts can be moved by right-clicking them and then moving them to their new position and clicking again. ● Moving hosts to another LAN requires right- clicking on the LAN the host should be moved to.

25. Moving Components (2) ● LANs and bridges can be moved in the same way:

26. Configuring Hosts ● Host parameters can be modified by selecting the host which brings up the current settings in the side panel – The ID and random packet sending parameters can be changed

27. Configuring Bridges ● Bridge parameters can be modified by selecting the bridge which brings up the current settings in the side panel. – The bridge ID and algorithm parameters can be changed. – For each port the ID and path cost parameters can be changed.

28. Simulation ● The simulation mode is always available from the menu or tool bar and is automatically paused when switching to another mode (e.g. Editing the network/script).

29. Control ● The simulation can be started, restarted and paused from the menu or tool bar. ● When there is no script specified the stepping mode is also available and the simulation can be changed between the two modes while running.

30. Speed ● The speed of the simulation can be adjusted from 0.1– 2.0 times real-time (based on bridge timers). ● The current speed is shown to the right of the speed selection slider, and is adjusted based on performance. ● When the simulation cannot be run at the requested speed (usually due to an excessive number of packets) it will be slowed down and then sped up again once the performance improves.

31. Packet Display ● Packets are shown as coloured blocks on the network. – Configuration Messages are shown in orange. – Topology Change Notifications are shown in red. – Host to host packets are shown in magenta.

32. Broadcast Network ● The simulated network acts in the same way as a broadcast network, packets are duplicated moving from one LAN to another and only removed when there is nowhere else for the packet to go. ● Only packets destined for a specific host (or all hosts) are shown moving to a host – otherwise having all the packets going to every host on the LAN would make it difficult to see when a packet destined for a host has reached it. Packet moving (left) towards connected LAN Packet continuing along original LAN and broadcast onto connected LAN

33. Bridge Ports ● The status of bridge ports are shown by changing their colour. ● Disabled ports are shown as light grey. ● Listening ports are shown as dark grey. ● Blocking ports are shown as red. ● Learning ports are shown as blue. ● Forwarding ports are shown as green.

34. Host Status ● The status of hosts can be viewed in the side panel after selecting them. ● A log of packets sent and received is shown as well as a function to send out a packet.

35. Bridge Status ● The status of bridges can be viewed in the side panel after selecting them. ● A log of events such as port states changing and the root ID changing is available. ● The current spanning tree protocol settings in effect and the host cache are also shown.

36. Sample Simulation (paused)

37. Scripting ● The scripting system has a useful interface for creating and modifying scripts. – A network can have at most one script. ● Scripts have a list of elements which wait for certain conditions and then respond in a predefined way.

38. Scripting (2) ● Script elements can be moved up and down, created and deleted using the four buttons at the right of each element.

39. Script States ● Script states wait for an event to occur before the script proceeds to the next element, optionally pausing. Waits for the spanning tree protocol to be complete on the network that the specified bridge is part of. Waits for the port state of a specific port to change to a certain state.

40. Script States (2) Waits for the root ID of a bridge to change the specified value. Waits for a given period of time. Waits for a host to receive a packet from another host.

41. Script Actions ● Script actions make changes to the settings of a component on the network or perform an action on a network component (e.g. Sending a packet from a host). Enables or disables a specific port on a bridge. Resets all the bridges on the network to their initial state.

42. Script Actions (2) Enables or disables the sending of packets at random on a specific host. Sends a packet from one host to another (or to all hosts).

43. Evaluation ● Almost all of the aims and objectives have been met except a couple which were too complicated to implement. ● The resulting is a sophisticated application that: – Allows the creation of a network. – Displays the network and simulation clearly. – Animates a correct simulation of the spanning tree protocol on it.

44. Limitations ● Maximum number of bridges is fixed at 100, and hosts at 1000 because higher IDs/addresses wouldn't fit on the displayed network. ● When running under Java Web Start the maximum file size is limited to 256KB.

45. Possible extensions and future work ● The program lacks the ability to undo/redo a change to the network configuration, this could make modifying the network simpler – if a mistake is made the change wouldn't have to manually be reverted. ● The ability to run the whole simulation in reverse, either running the simulation backwards or just returning to a previous state.