Re: Packet reordering in pcap capture file

From: Stephen Hemminger
Date: Sat Aug 05 2006 - 12:19:52 EST


On Sat, 05 Aug 2006 03:28:38 -0400
Alan Shieh <ashieh@xxxxxxxxxxxxxx> wrote:

> Hi everyone,
>
> I sometimes see packets stored out of order in pcap files that generated
> by "tcpdump -i any" on kernel 2.4.26 with all packets arriving and
> departing on an e1000 NIC. That is, the ordering by receive timestamp on
> the packets is not the same as the ordering of the packets within the file.
>
> In my precise scenario, packets of RX packets show up in the log 230 ms
> later than they ought to based on the receive timestamp. The kernel
> behavior (e.g., the packets that are sent by this node) seems to reflect
> the arrival of the Rx packet at the position in the logfile, rather than
> the arrival time according to the timestamp.
>
> What are some of the known causes of this behavior? I'd like to know
> what locks, etc. might be causing this processing / capture delay.

SMP or single CPU? What is the clock source being used?
If you had a CPU like dual-core AMD that doesn't sync TSC's and
that was the clock source, the timestamps could be wrong.
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