Subject: BFD questions

From: "Phelan, Tom" <[email-masked]>
Date: Wed, 1 Oct 2003 10:57:46 -0400

Hi Dave,

I have some questions about BFD:

When you aren't in the UP state, bfd.DesiredMinTxInterval MUST be at least
one second.  To get sub-second failure detections, then, you need to change
bfd.DesiredMinTxInterval when you enter the UP state.  Does that change mean
you need to send a control packet with the P bit set (a la section 6.5.3)?
In other words, for a transition into UP, do you routinely send a control
packet with the P bit set?

The same thing seems to apply to transitions from UP.
bfd.DesiredMinTxInterval needs to be reset to one second.  Do you routinely
send a poll when exiting UP state?

About P/F operation - When you're polling in the UP state, it seems to me
that you want to timeout and go to FAILING if you don't receive an F bit
within the detection timeout.  However, in section 6.5.6, the detection
timer is restarted for each valid control packet, regardless of the setting
of the F bit.  Is this what you intend?  If so, does that imply an
additional, unmentioned, timer for poll response timeout?

When one side is active and the other passive:  Both initialize to the
FAILING state, the active side sends packets with H=0.  When the passive
side receives a packet, it can begin sending control packets of its own
(section 6.1), and moves to the DOWN state (section 6.5.6).  The natural
thing is for the passive side to set H=1 (after all, it started transmitting
because it heard the active side).  But the active side will only leave
FAILING if H=0 (section 6.5.6), and therefore the session won't come UP.
You seem to be in the same situation when both sides are active and one
side's initial packet is lost.  Have I missed something?

Tom Phelan