Wireshark-dev: Re: [Wireshark-dev] Netflow: How should Sequence Number field work?
From: Hadriel Kaplan <[email protected]>
Date: Sat, 4 Jul 2015 16:26:04 -0400
Since Netflow v9 is a Cisco-defined protocol, their own docs should arguably trump the IETF RFC for their protocol. (personally I would read that RFC to mean the number of packets/frames, not number of flows)

According to this:
http://www.cisco.com/en/US/technologies/tk648/tk362/technologies_white_paper09186a00800a3db9.html

... the sequence number in Netflow v9 represents the number of export packets (i.e., frames for Wireshark); whereas in previous Netflow versions it represented number of flows.

That is corroborated by this:
http://www.cisco.com/c/en/us/td/docs/net_mgmt/netflow_collection_engine/3-6/user/guide/format.html
which said for v5-v8 it was number of flows.

Looking at some captures of Netflow v9 from old bug submissions, it appears to be number of packets - including if the packet only contained a Template.

So perhaps you’re looking at captures with different Netflow versions?

Or maybe you’re looking at version 10 (i.e., IPFIX), which defines the sequence number as the number of Data Records, so neither packets/frames nor flows really. (though from our internal code's perspective, I guess they would be “flows")

-hadriel


> On Jul 4, 2015, at 9:27 AM, Martin Mathieson <[email protected]> wrote:
> 
> (I think my previous attempt to send this failed, so resending)
> 
> A few months ago I updated the Netflow dissector to do sequence
> analysis using the Sequence Number field within an Obvservation
> Domain, based upon RFC 3954 and a capture file I was looking at.
> 
> RFC 3954 describes the field as follows:
> 
> Sequence Number
>        Incremental sequence counter of all Export Packets sent from
>        the current Observation Domain by the Exporter.  This value
>        MUST be cumulative, and SHOULD be used by the Collector to
>        identify whether any Export Packets have been missed.
> 
> A Netflow frame has the Obvervation-domain + Sequence number at the
> top, then a number of flows below.  What is not clear to me is whether
> this field should advance by the number of flows in this frame, or the
> previous frame.
> 
> The capture included in this bug report
> https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=11047 increments
> according to the number found in the previous frame for this
> Obvervation domain (i.e. it represents the SN at the beginning of the
> current frame).
> 
> Whereas a different capture I was looking (when the sequence analysis
> was written) increments according to the number of flows in this frame
> (i.e. it represents the SN at/beyond the end of the current frame).
> 
> Could someone with a working knowledge of Netflow put me right?  Is
> there another spec where the usage of this field is made clear?  Are
> both approaches valid?
> 
> Thanks,
> 
> Martin
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