bfdd: fix broken FSM in passive mode

Problem:
One is with active mode, the other is with passive mode. Sometimes
the one with active mode is in `Down` stauts, but the other one
with passive mode is unluckily stuck in `Init` status:
It doesn't answer its peer with any packets, even receiving continuous
`Down` packets.

Root Cause:
bfdd with passive mode answers its peer only *one* packet in `Down` status,
then it enters into `Init` status and ignores subsequent `Down` packets.
Unluckily that *one* answered packet is lost, at that moment its peer
with active mode can only have to send `Down` packets.

Fix:
1) With passive mode, bfdd should start xmittimer after received `Down` packet.
Refer to RFC5880:
"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."

2) Currently this added xmittimer for passive mode can be safely removed
except receiving `AdminDown` packet:
    - `bfd_session_enable/bfd_set_passive_mode` doesn't start xmittimer
    - `ptm_bfd_sess_dn/bfd_set_shutdown` can remove xmittimer
Per RFC5880, receiving `AdminDown` packet should be also regarded as `Down`,
so just call `ptm_bfd_sess_dn`, which will safely remove the added xmittimer
for passive mode. In summary, call `ptm_bfd_sess_dn` for two status changes
on receiving `AdminDown`: `Init`->`Down` and `Up`->`Down`.

Signed-off-by: anlan_cs <vic.lan@pica8.com>

0 files changed, 0 insertions, 0 deletions