IPv6 window scaling factor always 1 on initial SYN

[Bjoern A. Zeeb]

On Sun, 5 Apr 2009, Bjoern A. Zeeb wrote:

> On Sun, 5 Apr 2009, sthaug at nethelp.no wrote:
>
>> 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
>
> I think the answer to tthat is in sys/netinet/tcp_usrreq.c in the
> functuoins:
> tcp_connect
>
> 1106         /*
> 1107          * Compute window scaling to request:
> 1108          * Scale to fit into sweet spot.  See tcp_syncache.c.
> 1109          * XXX: This should move to tcp_output().
> 1110          */
> 1111         while (tp->request_r_scale < TCP_MAX_WINSHIFT &&
> 1112             (TCP_MAXWIN << tp->request_r_scale) < sb_max)
>
>                                                    ^^^^^^^^^^^
>
> 1113                 tp->request_r_scale++;
>
>
> and tcp6_connect
>
> 1174         /* Compute window scaling to request.  */
> 1175         while (tp->request_r_scale < TCP_MAX_WINSHIFT &&
> 1176             (TCP_MAXWIN << tp->request_r_scale) < so->so_rcv.sb_hiwat)
>
>                                                    ^^^^^^^^^^^
>
> 1177                 tp->request_r_scale++;
>
>
> I'll have to check why they are un-equal...


Ok, both versions had:	< so->so_rcv.sb_hiwat)

http://svn.freebsd.org/viewvc/base?view=revision&revision=166403

changed it for IPv4 the first time,

http://svn.freebsd.org/viewvc/base?view=revision&revision=172795

changed it a second time for IPv4.

Noone changed the IPv6 version.

The syncache already seems to do it for both v4/v6 (common code).

Can you try changing it to < sb_max) for IPv6 as well and see if
things work (better) for you?

/bz

-- 
Bjoern A. Zeeb                      The greatest risk is not taking one.