RE: DOWN state

[Satyam Sinha <satyamsinha at live.com>]

Hi Shahram,

Comments inline....

> Date: Fri, 14 Aug 2009 16:42:36 -0700
> Subject: Re: DOWN state
> From: vishwas.ietf at gmail.com
> To: davari at broadcom.com
> CC: rtg-bfd at ietf.org
>
> Hi Saharam,
>
> On Fri, Aug 14, 2009 at 3:49 PM, Shahram Davari<davari at broadcom.com> wrote:
> > Hi,
> >
> > The based draft sys that if BFD Control packet is not received during any
> > Detection Interval then the local system will go to Down state.
> > I have two questions:
> >
> > 1) When can the local system transition out of Down state? Is it after
> > receiving the first BFD packet with State=DOWN or INIT? or a number of such
> > BFD packets are required? in other words is there a Hysteresis?
> No hysteresis is required as such.

Any hysteresis would actually be a violation of the "Section 6.2 BFD state-machine".

Down state means that the session is down (or has just been created.)
   A session remains in Down state until the remote system indicates
   that it agrees that the session is down by sending a BFD Control
   packet with the State field set to anything other than Up.

>
> > 2) Should the remote system apply  a sliding window for Detection Time or
> > fixed slotted windows that are not overlapped are acceptable?
> What does the window contain? (I understand you are trying to do
> something similar to TCP). Are you using a mechanism like that for
> echo packets?

From a transmit perspective, if you take into account jitter, one should have the timers as sliding window based. One should schedule the next transmit at last-transmit-time + tx-timer (jittered) instead of expected-last-transmit-time + tx-timer (jittered). The variance in the jitter alongwith slotted window usage between two transmits could cause the tx-time between two packets to be more than the tx-timer. When the multiplier=1, such a variance could make your session go down.

From a receive perspective you could do either. If you used slotted windows, you might see more than one packet in a slot but you are still guaranteed to see atleast one packet every slot.

Regards,

Satyam

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