On Sun, 5 Apr 2009, sth...@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...
/bz
--
Bjoern A. Zeeb The greatest risk is not taking one.
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