Routing subsystem
=================

The routing part of tinc reads/writes packets to/from the virtual network
device, sets up tunnels on demand and routes packets to/from the virtual
network device and the tunnels.  It can also exchange information about other
daemons using the tunnels.

The following datatypes are used in this subsystem:

struct node
-----------

This represents a tinc daemon. It keeps track of the best way to reach this
daemon (if it is reachable at all), which subnets are associated with this
daemon, and the connections it has with other daemons.

struct subnet
-------------

This represents a subnet, can have an expiry time and is associated with a node.

struct edge
-----------

This represents a connection from one tinc daemon to another. The edges have a
specific direction, and normally if there really is a connection between tinc
daemon A and B, there will be two edges, A -> B and B -> A.  This seems
redundant, but is necessary to allow disparities between information learned
from A and information learned from B.