K 10
svn:author
V 8
gallatin
K 8
svn:date
V 27
2016-08-01T17:02:21.365381Z
K 7
svn:log
V 973
Rework IPV6 TCP path MTU discovery to match IPv4

- Re-write tcp_ctlinput6() to closely mimic the IPv4 tcp_ctlinput()

- Now that tcp_ctlinput6() updates t_maxseg, we can allow ip6_output()
  to send TCP packets without looking at the tcp host cache for every
  single transmit.

- Make the icmp6 code mimic the IPv4 code & avoid returning
  PRC_HOSTDEAD because it is so expensive.

Without these changes in place, every TCP6 pmtu discovery or host
unreachable ICMP resulted in a call to in6_pcbnotify() which walks the
tcbinfo table with the write lock held.  Because the tcbinfo table is
shared between IPv4 and IPv6, this causes huge scalabilty issues on
servers with lots of (~100K) TCP connections, to the point where even
a small percent of IPv6 traffic had a disproportionate impact on
overall throughput.

Reviewed by:	bz, rrs, ae (all earlier versions), lstewart (in Netflix's tree)
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D7272

END