Subject: Echo Function and Asymmetry - Timer negotiation

From: "Tanya Shastri \(tanyas\)" <[email-masked]>
Date: Thu, 11 Aug 2005 13:58:27 -0700

For an Asynchronous BFD session that has Asymmetric Echo function the
following statements in the spec seem to be misleading, unless I am
missing something.
 
Section 6.4 says: "When a system is using the Echo function, it is
advantageous to choose a sedate transmission rate for Control packets
since liveness detection is being handled by the Echo packets."
Shouldn't it be the receive rate - or the transmission rate of Control
packets of the remote system - that should be sedate? Also the end that
does not have echo mode active relies on Control packets for liveness
detection and may still want to receive Control packets at a faster
rate.
 
Section 6.7.3 says: " When the Echo function is active, a system SHOULD
set bfd.DesiredMinTxInterval to a value of not less than one second"
>From the description in 6.7.4 this will force the remote end to have a
Detect Timer no faster than 1 second, since the Detect Timer is based on
the transmit rate of the remote system. Also this will not reduce the
rate at which it sends control packets to the end that has echo mode
active.
 
Wouldn't it be better to suggest that "RequiredMinRxInterval" in BFD
control packets should be set to a minimum of 1 second by the end that
has echo function active?
 
Thanks,
Tanya

---

From: Dave Katz <[email-masked]>
Date: Thu, 11 Aug 2005 14:52:10 -0700

On Aug 11, 2005, at 1:58 PM, Tanya Shastri ((tanyas)) wrote:

>
> For an Asynchronous BFD session that has Asymmetric Echo function  
> the following statements in the spec seem to be misleading, unless  
> I am missing something.
>
> advantageous to choose a sedate transmission rate for Control  
> packets since liveness detection is being handled by the Echo  
> packets."
> Shouldn't it be the receive rate - or the transmission rate of  
> Control packets of the remote system - that should be sedate? Also  
> the end that does not have echo mode active relies on Control  
> packets for liveness detection and may still want to receive  
> Control packets at a faster rate.

Yes, the language is a little slippery.  It dates from a time in BFD  
prehistory when echo mode had to be symmetric, and the use of  
"transmission rate" was meant to be generic, that is, the packet  
rate.  As you note, in the asymmetric case, the guy not running echo  
mode may like to be receiving control packets at a faster rate.

>
> Section 6.7.3 says: " When the Echo function is active, a system  
> SHOULD set bfd.DesiredMinTxInterval to a value of not less than one  
> second"
> From the description in 6.7.4 this will force the remote end to  
> have a Detect Timer no faster than 1 second, since the Detect Timer  
> is based on the transmit rate of the remote system. Also this will  
> not reduce the rate at which it sends control packets to the end  
> that has echo mode active.
>
> Wouldn't it be better to suggest that "RequiredMinRxInterval" in  
> BFD control packets should be set to a minimum of 1 second by the  
> end that has echo function active?

Same issue, but you're right.  I'll fix it in the next round.

--Dave

---

From: Reshad Rahman <[email-masked]>
Date: Thu, 11 Aug 2005 20:50:23 -0400

Dave Katz wrote:

>> the end that does not have echo mode active relies on Control packets 
>> for liveness detection and may still want to receive Control packets 
>> at a faster rate.
>
>
It's not clear to me what's the benefit of doing this if the asymmetric 
echo is being run at fast rate. Failure in any direction will be 
detected by the asymmetric echo and the other end (which isn't running 
echo) will be notified on echo failure. So it's not clear to me what's 
the benefit of the the guy not running echo to be receiving control 
packets at a faster rate. It would seem that with asymmetric echo we can 
still run control packets at a sedate rate in both directions. Or am I 
missing something?

Regards,
Reshad.

>> From the description in 6.7.4 this will force the remote end to have 
>> a Detect Timer no faster than 1 second, since the Detect Timer is 
>> based on the transmit rate of the remote system. Also this will not 
>> reduce the rate at which it sends control packets to the end that has 
>> echo mode active.
>>  
>> control packets should be set to a minimum of 1 second by the end 
>> that has echo function active?
>
>