TCP Performance

Blake Dunlap ikiris at
Tue Aug 27 15:42:17 UTC 2013

This really sounds like you aren't testing the correct flow type in
i/jperf, or you have some QoS queues for http traffic but not the perf
traffic that are filled.

Regardless, your problem looks like either tail drops or packet loss, which
you showed originally. The task is to find out where this is occurring, and
which of the two it is. If you want to confirm what is going on, there are
some great bandwidth calculators on the internet which will show you what
bandwidth you can get with a given ms delay and % packet loss.

As far as flow control, its really outside the scope. If you ever need flow
control, there is usually a specific reason like FCoE, and if not, it's
generally better to just fix the backplane congestion issue if you can,
than ever worry about using FC. The problem with FC isn't node to node, its
when you have node to node to node with additional devices, it isn't smart
enough to discriminate, and can crater your network 3 devices over when it
would be much better to just lose a few packets.


On Tue, Aug 27, 2013 at 9:49 AM, Nick Olsen <nick at> wrote:

> Duplex mismatch has been checked across the board. On every device.
> Nick Olsen
> Network Operations (855) FLSPEED  x106
> ----------------------------------------
> From: "Chad Dailey" <nanog at>
> Sent: Tuesday, August 27, 2013 10:48 AM
> To: nick at
> Subject: Re: TCP Performance
> Check for duplex mismatch at the server.
> On Mon, Aug 26, 2013 at 2:07 PM, Nick Olsen <nick at> wrote:
> Greetings all, I've got an issue I was hoping to put a few more eyes on.
>  Here's the scenario. Downloading a file at our Border is multiple orders
> of magnitude faster then a few hops out. Using the same 128MB test file, I
> tested at two different locations. As well as between them. Using multiple
> connections improves throughput, However it's the single stream issue
> we're
> looking at right now. All testing servers in question are Centos Linux.
>  Orlando Datapath: Cogent>Orlando Border Router (Mikrotik)>HP Procurve
> Switch> Server Results: 2013-08-29 05:04:09 (52.6 MB/s) - `128mbfile.tgz'
> saved [127926272/127926272]
>  Cocoa NOC Datapath: Cogent>Orlando Border Router (Mikrotik)>Licensed
> Microwave Link (300+Mb/s Capacity)>East Orange Router (Mikrotik)> Licensed
> Microwave Link (300+Mb/s Capacity)>Cocoa Router (Mikrotik)>Licensed
> Microwave Link (300+Mb/s Capacity)>Colo Router (Mikrotik)>NOC Router
> (Mikrotik)>HP Procurve Switch>Server Results: 2013-08-26 13:42:25 (398
> KB/s) - `128mbfile.tgz' saved [127926272/127926272]
>  Orlando-Cocoa NOC Datapath: Orlando Server>HP Procurve Switch>Orlando
> Border Router (Mikrotik)>Licensed Microwave Link (300+Mb/s Capacity)>East
> Orange Router (Mikrotik)> Licensed Microwave Link (300+Mb/s
> Capacity)>Cocoa
> Router (Mikrotik)>Licensed Microwave Link (300+Mb/s Capacity)>Colo Router
> (Mikrotik)>NOC Router(Mikrotik)>HP Procurve Switch>ServerResults:
> 2013-08-26 13:56:25 (3.31 MB/s) - `128mbfile.tgz' saved
> [134217728/134217728]
>  Now, For the fun of it. I ran Iperf single TCP between our Cocoa and
> Orlando POP's. Just like the HTTP test above. (Server has a 100Mb/s port).
> It maxes out the port, Unlike the HTTP test.
>  [root at ded01 ~]# iperf -c
> ------------------------------------------------------------Cli
> ent connecting to, TCP port 5001TCP window size: 16.0 KByte
> (default)------------------------------------------------------------[  3]
> local port 47281 connected with port 5001[
> ID]
> Interval       Transfer     Bandwidth[  3]  0.0-10.0 sec   114 MBytes
> 95.7
> Mbits/sec
> Here's associated packet captures for each transfer. As well as full wget
> output and traceroutes for each test. As you can see, The tests crossing
> the wireless links show about 3x more TCP re-transmits/dup ACK's. But I'm
> not sure I'm sold this could show such a huge drop in throughput. Other
> then that, nothing really stands out to me as to why these transfers are
> so
> much slower. Intra-network iperf testing shows full throughput the whole
> way with single connection. As well as UDP testing. One thing to note is
> the Iperf testing has far less TCP re-transmit/dup acks then any of the
> HTTP testing, Crossing the same Microwave Links and routers.
> I appreciate any insight anyone might have. Thanks!
>  Nick Olsen
> Network Operations (855) FLSPEED  x106

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