bfdd: Session state is not maintained until Detection Time after session goes down

[zmw12306]

The BFD implementation does not fully comply with RFC 5880 Section 6.8.18, which requires maintaining session state (including timing parameters) until a full Detection Time has passed without receiving BFD Control packets. Currently, session state is prematurely cleared in bfd_session_disable().

A system is REQUIRED to maintain session state (including timing parameters), even when a session is down, until a Detection Time has passed without the receipt of any BFD Control packets.

However,

frr/bfdd/bfd.c

Lines 413 to 435 in 259ffe1

	void bfd_session_disable(struct bfd_session *bs)
	{
	/* We are using data plane, we don't need software. */
	if (bs->bdc)
	return;
	/* Free up socket resources. */
	if (bs->sock != -1) {
	close(bs->sock);
	bs->sock = -1;
	}
	/* Disable all timers. */
	bfd_recvtimer_delete(bs);
	bfd_xmttimer_delete(bs);
	ptm_bfd_echo_stop(bs);
	bs->vrf = NULL;
	bs->ifp = NULL;
	/* Set session down so it doesn't report UP and disabled. */
	ptm_bfd_sess_dn(bs, BD_PATH_DOWN);
	}

This code:

Deletes receive/transmit timers immediately
Clears essential session state (e.g., vrf, ifp)
Does not retain state or timing information for the required Detection Time duration

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