So Mark, I got your sample pcap. Long story short, I think the
problem you're seeing is timing related and related to your box
kernel/CPU.
Here's what I got on my laptop running OS X:
atur...@macallan:~/Desktop> sudo tcpreplay -i en0 -M 20 Short_FTP.pcap
sending out en0
processing file: Short_FTP.pcap
Actual: 1000 packets (776854 bytes) sent in 0.36 seconds
Rated: 2157927.8 bps, 16.46 Mbps, 2777.78 pps
Statistics for network device: en0
Attempted packets: 1000
Successful packets: 1000
Failed packets: 0
Retried packets (ENOBUFS): 0
Retried packets (EAGAIN): 0
atur...@macallan:~/Desktop> sudo tcpreplay -i en0 -M 20
--timer=abstime Short_FTP.pcap
sending out en0
processing file: Short_FTP.pcap
Actual: 1000 packets (776854 bytes) sent in 0.30 seconds
Rated: 2589513.2 bps, 19.76 Mbps, 3333.33 pps
Statistics for network device: en0
Attempted packets: 1000
Successful packets: 1000
Failed packets: 0
Retried packets (ENOBUFS): 0
Retried packets (EAGAIN): 0
As you see, there are two runs using different timing mechanisms.
In my experience (and shown here) OSX's abstime is the hands down
winner for accuracy (unfortunately, it's not available on other
operating systems).
Under Linux, the default timing method is gettimeofday() which
depending on your CPU and kernel configuration may or may not use HPET
which is the next best thing. It wouldn't surprise me that your
system is not using the HPET timer which means calls to gettimeofday()
are pretty expensive and causes accuracy to suffer. I would try other
timing methods to see if any of them work better for you. If not,
determine if your CPU has the HPET and if so make sure your kernel is
configured to use it for gettimeofday() calls.
More information on timing is available in the tcpreplay online manual:
http://tcpreplay.synfin.net/trac/wiki/tcpreplay#ChoosingaTimingMethod
--
Aaron Turner
http://synfin.net/
http://tcpreplay.synfin.net/ - Pcap editing and replay tools for Unix & Windows
Those who would give up essential Liberty, to purchase a little temporary
Safety, deserve neither Liberty nor Safety.
-- Benjamin Franklin
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