When we receive an ICMP unreach need fragmentation datagram, we take

proposed MTU value from it and update the TCP host cache. Then
tcp_mss_update() is called on the corresponding tcpcb. It finds the
just allocated entry in the TCP host cache and updates MSS on the
tcpcb. And then we do a fast retransmit of what we have in the tcp
send buffer.

This sequence gets broken if the TCP host cache is exausted. In this
case allocation fails, and later called tcp_mss_update() finds nothing
in cache. The fast retransmit is done with not reduced MSS and is
immidiately replied by remote host with new ICMP datagrams and the
cycle repeats. This ping-pong can go up to wirespeed.

To fix this:
- tcp_mss_update() gets new parameter - mtuoffer, that is like
  offer, but needs to have min_protoh subtracted.
- tcp_mtudisc() as notification method renamed to tcp_mtudisc_notify().
- tcp_mtudisc() now accepts not a useless error argument, but proposed
  MTU value, that is passed to tcp_mss_update() as mtuoffer.

Reported by:	az
Reported by:	Andrey Zonov <andrey zonov.org>
Reviewed by:	andre (previous version of patch)

1 files changed, 2 insertions, 1 deletions

diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index b602c27..3b8bdf7 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -675,7 +675,8 @@ void	 tcp_reass_destroy(void);
 void	 tcp_input(struct mbuf *, int);
 u_long	 tcp_maxmtu(struct in_conninfo *, int *);
 u_long	 tcp_maxmtu6(struct in_conninfo *, int *);
-void	 tcp_mss_update(struct tcpcb *, int, struct hc_metrics_lite *, int *);
+void	 tcp_mss_update(struct tcpcb *, int, int, struct hc_metrics_lite *,
+	    int *);
 void	 tcp_mss(struct tcpcb *, int);
 int	 tcp_mssopt(struct in_conninfo *);
 struct inpcb *