[c-nsp] Network performance question - TCP Window issue?

[John Neiberger]

On Sun, Apr 29, 2012 at 4:39 PM, Łukasz Bromirski <lukasz at bromirski.net> wrote:
> On 2012-04-29 19:57, John Neiberger wrote:
>>
>> The timing of this is coincidental. I've been helping to troubleshoot
>> a similar problem at work for days. Let's say we have three servers,
>> A, B and C. We transfer files between them and here is what we see:
>>
>> A to B: Fast (around 18 MB/s)
>> B to A: Slow (around 1 MB/s)
>> A to C: Slow (around 1MB/s)
>> C to A: Fast (around 18MB/s)
>>
>> In our case, Server A is fast when sending to B but not when sending
>> to C. C can send at a high speed when sending back to A, though.
>
>
> Typical problems with the different speed depending on the direction
> are caused either by duplex mismatch at access port (test, don't
> trust what one side tells you!) or problems with negotiating the
> TCP windows size (depending on the TCP/IP stack, application and tool
> you may get a number of different results).

We checked all the usual stuff and haven't found anything wrong
anywhere in the path despite looking at this from a number of
different angles for days. However, Drew emailed me off-list and
suggested that we look to see if apf is running on the linux servers.
This has a known "feature" that disables TCP window scaling, which
would certainly cause what we're seeing. I'm going to have a server
guy check it tomorrow.