Dynamic Route Discovery

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Dynamic Route Discovery (DRD) is an algorithm used to automatically discover the proper source route that reaches a remote station on either a token ring or a Fiber Distributed Data Interface (FDDI) network.

It relieves the DLS user from discovering and maintaining source routes. The algorithm implements the spanning tree, as recommended by 802.5.

When the DLS user issues a transmission request (for example, DL_CONNECT_REQ or DL_UNITDATA_REQ) on a medium supporting source routing, the DRD algorithm consults a local cache of source routes. If there is a hit, the cached source route is used immediately. Otherwise, the DRD queues the transmission request and starts the discovery algorithm. If the algorithm finds a source route, the new route is cached, and the queued requests are transmitted using this new route. If the algorithm times out with no replies (approximately 10 seconds), the queued requests are rejected.

The cache is periodically flushed of stale entries. An entry becomes stale after 5 minutes of no new requests.

Note: After a connection is established, the source route discovered during the connection setup is used to the exclusion of the DRD. This has two effects:
  • If the source route changes during a connection, the connection continues to use the original source route.
  • If the original source route becomes invalid, the connection breaks, and no rediscovery is attempted until a new connection is started.