A system may take either an active role or a passive role in
session initialization. A system taking the active role must
send BFD control packets for a particular session, regardless
of whether it has received any BFD packets for that session.
A system taking the passive role must not begin sending BFD
packets for a particular session until it has received a BFD
packet for that session, and thus has learned the remote system's
discriminator value.