Subject: RESEND: Re: Questions on RFC5880

From: HongBo Wei <[email-masked]>
Date: Tue, 23 Nov 2010 22:30:11 +0800

Hi Dave,
Thanks for your clarification.

Additional question about the Poll Sequence: (eg. bfd.SessionState is Up, Tx
interval is 100msec, Now we want to change the Tx interval to 50msec)

-- Option 1: the local system send BFD Control Packets with the Poll bit set
every 50msec; when recieved BFD Control Packets with the Final bit set, the
local system send BFD Control Packets with the Poll bit clear every 50msec.

-- Option 2: the local system send BFD Control Packets with the Poll bit
clear every 50msec; At the same time, the local system send BFD Control
Packets with the Poll bit set every 1sec; when we recieved BFD Control
Packets with the Final bit set from remote, the local system terminate BFD
Control Packets with the Poll bit set, and calculate the Detection time.

I prefer to Option 1. What do you think about this?

Best regards,
steven

On Nov 22, 2010, at 5:05 PM, wei.hongbo at
zte.com.cn<wei.hongbo%20at%20zte.com.cn>wrote:

Hello Daves and All,

I have some questions regarding the BFD protocol, In RFC5880 "section 6.8.3
Timer Manipulation" written:

>>> When bfd.SessionState is not Up, the system MUST set
>>> bfd.DesiredMinTxInterval to a value of not less than one second
>>> (1,000,000 microseconds).

Q1: When the bfd.SessionState changed from INIT to UP(or DOWN->UP), the
DisiredMinTxInterval field in the Control packets will change to the
configured parameter (eg, 50ms). So, the Tx Value changed. Do we need a Poll
Sequence here?

Yes.

>>> In any case other than those explicitly called out above, timing
>>> parameter changes MUST be effected immediately (changing the
>>> transmission rate and/or the Detection Time).

Q2: If the local system reduces it transmit interval (eg. Tx: 100ms -> 50ms)
and initiate a Poll Sequence (send Control packet with P bit set), Can we
immediately change
the local system's Tx interval before we receive the Control packet with F
bit set from the remote?

Yes, you must do so, because the receive side will be cranking down its
detection time;  if, for instance, you were going from 1 sec to 50 msec, the
remote end will change its detection time to 150 msec and the session would
time out.

Q3: Regard to the Poll Sequence, one system will receive the Control packet
with P bit set and then start to detect; the remote system will receive the
Control packet with
F bit set and then start to detect. The differece between the two system's
start point is the transmit time of Control packet with F bit set.
    But, The Tx interval becomes smaller and smaller(eg. some Vendors
support 10ms), the transmit time of Control packet with F bit set MAY larger
than the calculated Detection time. Do we need some private trick to fix
that?

No;  the round-trip time of the poll sequence can be longer than a detection
time.  The important point, as you sensed in the second question, is that if
the remote system's detection time is going to drop as a result of the local
system reducing its TX interval, the local system must start sending at the
faster rate at the same time.  This will ensure that the remote end doesn't
time out.

The only case where the timing of the poll sequence matters is in Demand
Mode, because there otherwise is no control traffic;  if the round-trip time
is greater than the detection time, the session will always time out (see
section 6.6).

--Dave

Best regards,
steven

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