[Bloat] SO_SNDBUF and SO_RCVBUF

[Hal Murray]

> As I understand it (I thought) SO_SNDBUF and SO_RCVBUF are socket buffers
> for the application layer, they do not change the TCP window size either
> send or receive. Which is perhaps why they aren't used much. They don't do
> much good in iperf that's for sure! Might be wrong, but I agree with the
> premise - auto-tuning should work.

I sure expect them to do the obvious thing.

man 7 socket says:

       SO_SNDBUF
              Sets  or gets the maximum socket send buffer in bytes.

It doesn't actually say that turns into the TCP window size.

On Linux, there is a factor of 2 for overhead and whatever.

man tcp says:
      TCP uses the extra space for administrative purposes and inter-
       nal kernel structures, and the /proc file  values  reflect  the  larger
       sizes  compared  to the actual TCP windows.

So it looks like the number you feed it turns into the window size.

A few quick tests with netperf confirm that it is doing something close to 
what I expect but I haven't fired up tcpdump to verify that the window size 
is what I asked for.  netperf does print out values that are 2x what I asked 
for.

Yuck.  (That's Yuck at Linux, not netperf.)


-- 
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