Route invalidation

[lyqyd]

Add an evaluation of incoming HT packets in routed to look for routes that use the receiving router as the gateway.

These should be dropped if a better route is present, removed if the current gateway is the router we received the HT from, and invalidated if there is no known route (or if the route is removed).

Invalidation:

The router which received the HT attempts to send an HQ packet directly to the host if and only if the gateway value it received was two (indicates that the sending router thinks there is a single hop--the receiving router)
The ID of the computer and a time stamp are written to a table. Each time a packet is received, the table is checked. Entries older than five seconds are invalid routes and an HQ packet is sent to all known routers. If the host responds, an HI packet is sent to all known routers to update routing information.
Any routers which have a zero cost for the host ID in question send it HQ packets directly using the same procedure, except that the entry is additionally marked for removal. If no HI is received within 7 seconds, each of these routers will send out HR packets, removing the host from the network.