IPv6 window scaling factor always 1 on initial SYN

[sthaug at nethelp.no]

On 7-STABLE, with kern.ipc.maxsockbuf=2621440, both sides set a window
scaling factor of 6 (i.e. SYN wscale 6, SYN-ACK wscale 6) using IPv4.

With the same value of kern.ipc.maxsockbuf, using IPv6, the side which
sends the initial SYN sets a window scaling factor of only 1, while
the other side sets a scaling factor of 6 in the SYN-ACK. This will
obviously limit throughput in many cases.

In both cases net.inet.tcp.rfc1323=1.

Anybody know why IPv6 behaves differently here?

tcpdump example:

22:20:37.282415 IP 193.75.4.50.53981 > 193.75.110.66.5555: S 1580765626:1580765626(0) win 65535 <mss 1460,nop,wscale 6,sackOK,timestamp 661320721 0>
22:20:37.282442 IP 193.75.110.66.5555 > 193.75.4.50.53981: S 1408884711:1408884711(0) ack 1580765627 win 65535 <mss 1460,nop,wscale 6,sackOK,timestamp 1581013561 661320721>

22:21:49.749586 IP6 2001:8c0:9a00:1::2.53983 > 2001:8c0:8500:1::2.5555: S 565631163:565631163(0) win 65535 <mss 1440,nop,wscale 1,sackOK,timestamp 661393190 0>
22:21:49.749633 IP6 2001:8c0:8500:1::2.5555 > 2001:8c0:9a00:1::2.53983: S 627173961:627173961(0) ack 565631164 win 65535 <mss 1440,nop,wscale 6,sackOK,timestamp 8

Steinar Haug, Nethelp consulting, sthaug at nethelp.no