1.3.2 Topology Discovery Tests
In topology discovery tests, the roles of stations are as the mapper and the responders. Topology discovery tests are an extension of quick discovery, and they can only be performed after quick discovery is complete. During quick discovery, a mapper temporarily fulfills the role of an enumerator while negotiating its intention to perform topology discovery tests with all responders involved.
Each responder that participates in quick discovery associates itself with a mapper if it does not already have an active association. It is only through this association that a responder accepts and responds to the associated mapper 's topology discovery test commands. This association is also reported by each responder in all quick discovery packet exchanges. While it is the ultimate goal to have only one mapper associated with all responders in a specific Ethernet broadcast domain, this mechanism puts the onus on the mapper to ensure that it stops itself completely (and releases any active associations) if it sees a quick discovery packet from any responder that is reporting an association to another mapper.
The mapping session makes assumptions about the behavior of the network infrastructure that interconnects the available responders, such as switches and hubs. Information about network interfaces and results from particular operations on responders provide the mapper with information to assess the network's topology. One key assumption made is that after a switch has learned a responder's segment, it does not forward traffic destined to that responder's Ethernet address to other segments.
After quick discovery, the mapper knows of available responders and the types of networks they are connected to (such as Ethernet or 802.11 wireless). If the application or higher-layer protocol sees two responders on Ethernet, it could direct this to request a responder to send Ethernet frames on the wire by using different source and destination MAC addresses and ask the other responder which of the Ethernet frames it received. The MAC addresses used are dedicated for use by this protocol.
The choice of which responder to use and the parameters of the topology discovery test are up to the application or higher-layer protocol. An implementation of this protocol merely allows applications to learn link details, with which they can construct topology maps using application-specific algorithms.
This protocol is used by such an application to request that a chosen responder send LLTD frames with a specified source and destination MAC address, where the source MAC address might or might not be the responder's own MAC address. To avoid interfering with other nodes' MAC addresses, this protocol defines a reserved range of MAC addresses that applications can use when they request that a responder use a source MAC address that it does not own.
This protocol is also used by such an application to ask other responders which test frames they have seen. This information allows the application to infer the existence of switches and hubs. For example, because a switch will remember a segment that it has seen, forwarding frames with the corresponding MAC address to that segment and flooding all segments for frames with previously unseen MAC addresses, applications can generate tests to determine whether a switch or a hub interconnects two responders.
For example, the application using this protocol might do the following. The application might direct one responder to use a specific LLTD MAC address and train a switch about the segment to which it is connected by sending a frame from that MAC address. The application might then invoke this protocol to request that a second responder send a frame to that MAC address. Finally, the application could ask a third responder for the test frames that it saw. If the third responder did not see the test frame (after multiple such tests to reduce the chance of packet loss), the application can conclude that the first and third responders are on different segments; that is, that a switch separates them.
A responder has to perform both quick discovery and topology discovery tests with different stations, where one is functioning as an enumerator and the other is functioning as a mapper.
In addition, this service also allows the mapper to ask a responder for additional property data that is too large to fit into the quick discovery responses.