Re: snort packet loss rate

[Matt Kettler <mkettler () evi-inc com>]

Jin Fang wrote:
> > How much bandwidth is on that adapter? Is it really a gigabit
> > connection, or is
> > it feeding a lower-speed line? Is the card on a PCI bus, or something
> > faster
> > such as PCI-X or PCI-E?
>
> It is having gigabit throughput and card is on a PCI-X.

Ok, what kind of CPU are you running? You mentioned you have lots of ram, which
doesn't hurt, but pure packet-pick-up rate when using simple tcpdump is largely
going to be limited by one of 6 things:

1) the packet size mix. Full-gigabit is hard to catch all of, and full-gigabit
with mostly small packets is even harder. What kind of data are you monitoring?
Normal TCP transfers, or a gigabit of tiny packets?

2) The design of the NIC. In this area you should be OK. the NC7781 is based on
a BCM570x type chip. While I dislike Broadcom for their secretive nature
regarding datasheets, their hardware is at least reasonably well designed.

3) the NIC driver in the OS.. again, you should be fine here, but you might want
to reasarch this avenue. There have been some reports of packet drops with the
FreeBSD bge driver:

http://unix.derkeiler.com/Mailing-Lists/FreeBSD/hackers/2004-05/0211.html

If you have any other gig nics laying around using a different chip/driver, you
could test one of those. If the packet-drops go away, it's likely a driver
issue. If not, it's likely something else.

4) the scheduling and design of the OS. FreeBSD does well here and shouldn't be
a problem. If you were running this on older families of windows you might have
a big problem.

5) CPU speed. Obviously, if the CPU isn't reasonably fast you can't keep up with
a gig stream.

6) the design of your PCAP library. ring buffered vs not can make a big
difference, but you still should be fine on FreeBSD regardless of the design.
You should at least be doing better than 50%, unless your CPU is slow.



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