net.inet.tcp.rfc1323 removed from El Capitan (10.11). Or not?

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net.inet.tcp.rfc1323 removed from El Capitan (10.11). Or not?

Folks:

I have been using the sysctl command to disable RFC1323 (Window Scaling) on Mac OS X for years, but the 10.11 (El Capitan) update has disappeared this kernel variable.

In 10.10, you could do:

Yosemite:~ jms$ sudo sysctl net.inet.tcp.rfc1323=0

net.inet.tcp.rfc1323: 1 -> 0

However, in 10.11, the kernel variable has disappeared:

ElCapitan:~ jms$ sudo sysctl net.inet.tcp.rfc1323=0

sysctl: unknown oid 'net.inet.tcp.rfc1323'

I hunted down the BSD source and it's still in there, so the network kernel seems to have diverged at some point. Or, perhaps, someone just made a typo and this is a bug. The various Darwin projects don't seem to have 10.11 network source code, so I can't see what is going on there.

Apple is happy to confirm that this command has disappeared, but says that they won't provide any support for this unless I have an enterprise contract, which I don't have.

Anyone out there know what kernel variable replaces this? Or otherwise how to turn off RFC1323 Window Scaling in 10.11?

Thanks, Joel Snyder

MacBook Air (13-inch Mid 2012), OS X El Capitan (10.11)

Posted on Jan 5, 2016 2:52 PM

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Ara4n

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Apr 14, 2016 9:52 AM in response to JoelSnyder

Better late than never - I just got bitten by this after being forced to upgrade to El Capitan in order to use XCode 7.3. The removal of the RFC1323 sysctl is a huge issue on my network as Cisco Aironet access points apparently drop TCP flows whose windows get too large or whose throughput is too fast - see https://github.com/vector-im/vector-web/issues/227 for more detail. As a result, any fast uploads (e.g. HTTP file uploads) were wedged.

I'm guessing it's an error that the sysctl has been removed. The good news is that I just spotted in http://newosxbook.com/articles/9-10.11.html that

0x8040a940: 0x8045ae34.broken_peer_syn_rxmit_thres Description: Number of retransmitted SYNs before TCP disables rfc1323 and rfc1644 during the rest of attempts

is a thing - and indeed...

sudo sysctl net.inet.tcp.broken_peer_syn_rexmit_thres=0

...seems to re-disable TCP windowing and all is good again.

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