Re: Detection time in demand mode

Dave Katz <dkatz@juniper.net> Sun, 20 March 2005 19:12 UTC

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From: Dave Katz <dkatz@juniper.net>
Date: Sun, 20 Mar 2005 12:13:50 -0700
To: roggie-bfd@sohu.com
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Subject: Re: Detection time in demand mode
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On Mar 20, 2005, at 5:12 AM, roggie-bfd@sohu.com wrote:

>
> Dave,
>   In section 6.7.4 , it said :
>    In Demand mode, the Detection Time calculated in the local system is
>    equal to bfd.DetectMult, multiplied by the agreed transmit interval
>    (the greater of bfd.RequiredMinRxInterval and the last received
>    Desired Min TX Interval.)  bfd.DetectMult is (roughly speaking, due
>    to jitter) the number of packets that have to be missed in a row to
>    declare the session to be down.
>   From the above sentence , i get the flowing conclosion , for example 
> :
>   A------------------B
> DM=3                DM=1
> MTI=100ms           MTI=10ms
> MRI=20ms            MRI=200ms
>   if A sends a poll sequence , the transmit interval is 
> 200ms[max(100ms,200ms)] ,
>   the detection time is 60ms[3*max(20ms,10ms)] , is it correct??

No, and thank you for catching this, I somehow managed to goof the 
language completely.  The casual text is right--the agreed transmit 
interval is 200 ms, so the detection time would be 600 ms.  I'll fix 
this ASAP;  I can't believe that none of us noticed this in two years 
as it's pretty fundamental!  Here's the proper language:

    In Demand mode, the Detection Time calculated in the local system is
    equal to bfd.DetectMult, multiplied by the agreed transmit interval
    of the local system (the greater of bfd.DesiredMinTxInterval and the
    last received Required Min RX Interval.)  bfd.DetectMult is (roughly
    speaking, due to jitter) the number of packets that have to be missed
    in a row to declare the session to be down.

>
>   another question : in the spec , it always said "If Demand mode is 
> not active" ,
>   i wants to ask is it the flowing meaning if one side set to demand 
> mode and
>   the other side dose not set , the side setted to demand mode also 
> MUST send
>   period control packet ? i am not sure !

 From the spec:

6.5. Demand Mode

   Demand mode is negotiated by virtue of both systems setting the
   Demand (D) bit in its BFD Control packets.  Both systems must request
   Demand mode for it to become active.

If this is not clear, please let me know.

--Dave

Detection time in demand mode roggie-bfd
Re: Detection time in demand mode Dave Katz