Re: sFlow agent flow_sample->sequence_number question

From: the keralite <>
Date: 03/28/10
Message-ID: <>

Thanks for responding. That's useful info ie., understanding the intended
use behind this drops field.
So technically though my agent is aware of the exported packets being
dropped in the transmit buffer, I dont have to track such drops and report
them in subsequent flow samples of from the same data source. The collector
will assume that they were lost in the network and deal with it.

On Fri, Mar 26, 2010 at 9:08 PM, Neil McKee <> wrote:

> Hello,
> The drop-counter is mostly intended for the situation where you know that
> samples were lost before they even got as far as the agent software. For
> example, if you are reading from a hardware buffer queue and you discover
> that the buffer queue has overflowed since the last time you looked. In
> that scenario you know that some samples were lost, but you probably don't
> even know how many. All you can do is increment the drop counter (and if
> this is happening chronically then you might consider backing-off the
> hardware sampling rate).
> Once a sample makes it into the agent software then (and only then) the
> sequence number can be incremented, because it will definitely be encoded
> into a datagram and given to UDP to be sent to the collector. Of course,
> remembering what the 'U' stands for, the datagram may be dropped in any
> number of places. It might not even make it off the box! However that is
> not something you have to keep track of. As long as you incremented the
> sequence number (and the all-important sample_pool counter) then any
> non-systematic packet-loss can be accommodated by the collector, and will
> manifest as a small increase in the effective-sampling-rate (e.g. 1-in-400
> becomes 1-in-401).
> I hope this is clear.
> Regards,
> Neil
> On Mar 25, 2010, at 10:59 PM, the keralite wrote:
> > sFlow spec gurus,
> > If the sFlow agent drops a flow sample export datagram due to lack of
> > resources then the drops for that data source is incremented. What
> happens
> > to the squence_number that is reported in a flow sample? I assume that
> its
> > not incremented. OR does the agent increment both the sequence_number and
> > the drops fields?
> > TIA
> > ZC
Received on Sun Mar 28 13:38:59 2010

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