ndt-users list created

[Richard Carlson <>]

Matt;

I see a lot of NDT reports non-zero out-of-order stats.  Much more than I 
ever expected.  I've pretty well convinced myself that this is a host 
stack/driver issue instead of a network switch/router issue.  My working 
hypothesis is that the NIC driver caused this by somehow re-ordering the 
packets before they were handed up to the kernel.

As far as the NDT server goes, out-of-order = DupAcksIn/AckPktsIn  So the 
question is, why would the receiver generate DupAcks?  As far as I know, 
the only reason is because a gap appeared in the TCP stream.

In any case, a small amount of reordering, if it doesn't lead to timeout or 
fast retransmits should not cause that big of an impact on a TCP flow.  The 
receiver will just send a few more ACK packets.  The web100 variable list 
should tell you if this is happening.

How low is this throughput?  What does the BW*D product say using the 
send/receive buffers?  What OS are you using.  (I've had a bunch of 
problems with the Linux 2.6.13 and above NIC drivers.  The e100.c file was 
causing me a tremendous amount of problems (tcptrace reported an initial 
TCP window of 42 packets and then after receiving a single ACK the source 
dumped another 900+ packets into the network.  The tcpdump trace shows the 
rest of the 10 second test is consumed by re-transmitting everything from 
the beginning).  I solved my problem by replacing the drivers/net/e100.c 
file with a version from the 2.6.12 release tree.  I think it has something 
to do with the NAPI polling that was introduce in the 2.6.13 kernel tree.

Rich

At 12:33 PM 3/16/2006, Matt Crawford wrote:
> Has NDT been known to be in error about reporting packet reordering?
> Might there be bugs in web100 that mislead it?  We're getting low
> throughput on a 90ms RTT layer-2 path and NDT reports small amounts
> of reordering.  So far we are unable to find a switch to blame for it.

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Richard A. Carlson                              e-mail: 

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