Unable to set socket size > 16MB

[Max Laier]

On Monday 20 August 2007, Kevin Oberman wrote:
> I am trying to tune a FreeBSD system for ~100 ms. RTT at 10 Gbps. (I
> posted another message about this back on 8/17). I am running current
> of late July 31.
>
> I am using iperf and I have confirmed (with gdb) that it is passing
> setsockopt a size of 67108864 and setsockopt is returning 0. When I
> capture the SYN packets, I am seeing a window of 64K and a scale
> factor of 8. For 64 MB, the scale factor should be 10.
>
> Is there some hidden limitation that would restrict this or is there a
> bug involved? I have set net.inet.tcp.(send|recv)space to
> 64m. kern.ipc.maxsockbuf=134217728.
>
> Here is the 3-way handshake:
> 13:57:45.571614 IP lbl.52460 > perf-bnl.commplex-link: S
> 4070670678:4070670678(0) win 65535 <mss 8960,nop,wscale
> 8,sackOK,timestamp 345761341 0> 13:57:45.665645 IP
> perf-bnl.commplex-link > lbl.52460: S 3909263475:3909263475(0) ack
> 4070670679 win 65535 <mss 8960,nop,wscale 8,nop,nop,timestamp
> 3623078172 345761341> 13:57:45.665683 IP lbl.52460 >
> perf-bnl.commplex-link: . ack 1 win 65535 <nop,nop,timestamp 345761435
> 3623078172>
>
> Any reason for this? Any workaround or fix? Or am I missing something?
 
http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/netinet/tcp_syncache.c#rev1.104 
seems to be the culprit:

                        while (wscale < TCP_MAX_WINSHIFT &&
                            (0x1 << wscale) < tcp_minmss)
					      /* 216 */
                                wscale++;

It's obvious that the above will bound wscale to 8 with the default of 216 
for minmss.  You should be able to set a higher minmss for a temporary 
work around, but this calculation really seems wrong to me.  Esp. given 
the following comment for tcp_minmss:

 ...
 * with packet generation and sending. Set to zero to disable MINMSS
 * checking. This setting prevents us from sending too small packets.
 */

-- 
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