[Oisf-users] tcp.segment_memcap_drop

Peter Manev petermanev at gmail.com
Thu Jun 26 07:56:20 UTC 2014

On Thu, Jun 26, 2014 at 9:22 AM, Victor Julien <lists at inliniac.net> wrote:
> On 06/26/2014 09:19 AM, Peter Manev wrote:
>> On Wed, Jun 25, 2014 at 3:37 PM, Kurzawa, Kevin
>> <kkurzawa at co.pinellas.fl.us> wrote:
>>> Using pcap because ... well, I don't know any better? I guess I don't
really know the alternatives. PF Ring is the other option right?
>> There is pcap, pf_ring and af_packet.
>> af_packet works "out of the box", just make sure your kernel is not
>> older than 3.2.
>> runmode: workers seems to be the best option for af_packet.
>> For pf_ring you need to compile and make a module, also make sure your
>> kernel is not older than 3.0 (2.6.32 being the bare minimum)
>> runmode: workers seems to be the best option for pf_ring as well.
>> Our wiki provides some guidance -
>> https://redmine.openinfosecfoundation.org/projects/suricata/wiki
>> and then there are a number of articles on the net and on our user
>> mail list archives regarding high perf tuning.
>>> Is this the potential source of the tcp.reassembly_gap?
>> No
> Uh, yes? Packet loss is certainly a big factor in tcp.reassembly_gap.
> Stats do show packet loss, so using a faster capture method may
> certainly help.

It may help.

Judging by the posted output ->
The number of tcp.reassembly_gap is 4 times higher than the number of
Based on that I drew the conclusion.
In my observations/experience in general most of the cases of big numbers
in the reassembly gaps (and much smaller number of kernel drops) counter
are due to ... well :) gaps in the traffic - either there were drops on the
mirror port or there was sudden peaks/fluctuations in the traffic and the
mirror port reached limits and similar things.

If we look at it from purely factual perspective in this case - how can one
dropped packet (and it may be any packet not just tcp) get to 4 reassembly


Peter Manev
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