4.1 Example 1: Mapping a Network

The following figure shows a typical network that interconnects two computers and a printer. The user might have connection problems between the two computers or between a computer and the printer for a variety of reasons, including a mismatch in IP addressing on the network. Application problems can motivate the user to generate a map of his or her network to help discover the problem.

The user designates one of the computers as a Mapper, and the printer (R1) and laptop PC (R2) function as responders. They are interconnected with an Ethernet hub.

Typical two-computers, one-printer network

Figure 5: Typical two-computers, one-printer network

The following figure shows the protocol exchange between the mapper and the two responders that are on the network.

Protocol exchange between networked mapper and two responders

Figure 6: Protocol exchange between networked mapper and two responders

The following list describes each step in the protocol exchange:

  1. The Mapper broadcasts a Discover frame with a Generation Number of zero to determine what responders are available on the network.

  2. Responder 1 (R1) broadcasts a Hello frame that indicates its current Generation Number and basic information, such as Host ID, Characteristics, and Physical Medium (Ethernet in this case) in its TLV_List.

  3. The Mapper broadcasts another Discover frame with the generation number given by R1, including R1's responder in the Station List.

  4. Responder 2 (R2) broadcasts a Hello frame that indicates its current Generation Number of zero, and basic information, such as Host ID, Characteristics, and Physical Medium (Ethernet in this case) in its TLV_List.

    Note R2 used its RepeatBAND load control mechanism (section 3.5.6.2) to not respond to the first Discover with a Hello response.

  5. The Mapper broadcasts another Discover frame with the generation number given by R1, including R2's MAC address in the Station List.

  6. The application now invokes this protocol with a series of tests for R1. to test the network topology. The LLTD Mapper sends a Charge frame to R1 to generate sufficient byte and frame credits in R1 for a request that will follow.

  7. The Mapper sends an Emit frame to R1, indicating that R1 is to send a Probe frame with a Source MAC Address of 00-0D-3A-D7-F2-01 and a Destination MAC Address of 00-0D-3A-D7-F1-41.

  8. R1 transmits the Probe frame.

    Note The Destination MAC Address does not address any machine in particular, so it traverses the network like a broadcast address.

  9. R1 sends an Ack frame to the Mapper to indicate that it has completed the Emit request. At this point, the Mapper indicates to the application that the series of tests has completed.

  10. The application asks this protocol to send a Query to R1 to get the list of MAC address seen by this responder.

  11. The application also asks this protocol to send a Query to R2 to get the list of MAC address that this responder has seen.

  12. R1 sends a QueryResp to the Mapper with no MAC address in the list, and the Mapper completes the application's request from step 10.

  13. R2 sends a QueryResp to the Mapper with an entry that indicates it saw a frame with a Source MAC Address of 00-0D-3A-D7-F2-01 and a Destination MAC Address of 00-0D-3A-D7-F1-41. The Mapper completes the application's request from step 11.

  14. The application decides to conduct another test and gives this protocol another set of commands for R1. The Mapper sends a Charge frame to R1 to generate sufficient byte and frame credits in R1 for a request that will follow.

  15. The Mapper sends an Emit frame to R1, indicating that R1 is to send a Probe frame with a Source MAC Address of 00-0D-3A-D7-F2-02 and R2's Destination MAC Address.

  16. R1 sends a Probe frame destined to R2 with a Source MAC Address of 00-0D-3A-D7-F2-02.

  17. R1 sends an Ack frame to the Mapper to indicate it has completed the Emit request. At this point, the Mapper indicates to the application that the latest test has completed.

  18. The application asks this protocol to send a Query to R2 to get the list of MAC addresses that this responder has seen.

  19. R2 sends a QueryResp to the Mapper with an entry that indicates that it saw a frame with a Source MAC Address of 00-0D-3A-D7-F2-02 and a R2's MAC address as the destination. This protocol indicates this information to the application.

    Note R2 did not return the MAC address pair that it reported in step 13 because after sending that information in step 13, it cleared that information from memory.

  20. The application asks this protocol to perform another test from R2, and the Mapper sends an Emit to R2 with a Sequence Number of zero and a request for R2 to send a Probe using R2's MAC address for the source and R1's MAC address for the destination.

    Note A zero sequence number indicates to R2 that it does not send an Ack frame to the Mapper when it has completed the Emit request. Hence, the Mapper completes the application's request immediately.

  21. R2 sends a Probe using R2's MAC address for the source and R1's MAC address for the destination.

  22. The application asks this protocol to perform another test from R1, and the Mapper sends a Charge frame to R1 to generate sufficient byte and frame credits in R1 for a request that will follow.

  23. The Mapper sends a second Charge frame to R1 to generate sufficient byte and frame credits in R1 for a request that will follow.

  24. The Mapper sends an Emit frame to R1 that requests R1 to send a Train frame using a Source MAC Address of 00-0D-3A-D7-F2-03 and R2's MAC address as the destination. The Mapper also sends a Probe frame using R1's MAC address as the Source and 00-0D-3A-D7-F2-03 as the Destination MAC Address.

  25. R1 sends a Train frame using a Source MAC Address of 00-0D-3A-D7-F2-03 and R2's MAC address as the destination.

  26. R1 sends a Probe using R1's MAC address for the Source and 00-0D-3A-D7-F2-03 as the Destination MAC Address.

  27. R1 sends an Ack frame to the Mapper to indicate that it has completed the Emit request, and the Mapper completes the application's request from step 22.

  28. The application asks this protocol to send a Query to R1 to get the list of MAC addresses that this responder has seen.

  29. The application also asks this protocol to send a Query to R2 to get the list of MAC addresses that this responder has seen.

  30. R1 sends a QueryResp to the Mapper with an entry that indicates that it saw a frame with R2's MAC address as the Source MAC address and R1's MAC address as the Destination MAC address. The Mapper completes the application's request from step 28.

  31. R2 sends a QueryResp to the Mapper with an entry that indicates it saw a frame with R1's MAC address as the Source MAC Address and Destination MAC Address of 00-0D-3A-D7-F2-03. The Mapper completes the application's request from step 29.

  32. The application finally directs this protocol to terminate the topology discovery session, and the Mapper broadcasts a Reset to indicate that the mapping session is complete.

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