Date: Thu, 3 Feb 2000 18:02:30 -0800 (PST) From: Stephen Macmanus <stephenm@windriver.com> To: freebsd-net@freebsd.org Subject: Path MTU discovery Message-ID: <200002040202.SAA01528@brisbane.wrs.com>
next in thread | raw e-mail | index | archive | help
I have found some puzzling behavior in the path MTU discovery
implementation in Free BSD 3.4 which I hope someone can shed light
on.
The tcp_mtudisc routine adjusts the MSS for a connection in
response to the ICMP error which triggers adjustment of the path
MTU estimate to a new value. That routine includes the following
code:
struct tcpcb *tp = intotcpcb(inp);
struct rtentry *rt;
struct rmxp_tao *taop;
struct socket *so = inp->inp_socket;
int offered;
int mss;
if (tp) {
rt = tcp_rtlookup(inp);
if (!rt || !rt->rt_rmx.rmx_mtu) {
tp->t_maxopd = tp->t_maxseg = tcp_mssdflt;
return;
}
taop = rmx_taop(rt->rt_rmx);
offered = taop->tao_mssopt;
mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcpiphdr);
if (offered)
mss = min(mss, offered);
.
.
.
Both this routine and the tcp_mss routine examine the results of
tcp_rtlookup() to verify that a route to the destination address
exists. That test appears to be redundant. I can't see a situation
where the system is unable to find a route to the remote address
given that the routine can only execute after the socket assigns
local and remote address information, which includes the route lookup.
Similarly, I don't entirely understand the conditional execution
of the min() operation. It can only apply before the remote peer sends
the initial SYN since tao_mssopt is always non-zero after that point.
That could occur if the host generates the ICMP error by sending a TCP
segment which includes data along with the SYN flag. Examining the
tcp_output() routine, that behavior doesn't seem possible. Does anyone
know how it can occur?
Thanks,
Stephen
------------------
Stephen Macmanus #include <std_disclaimer.h>
Software Engineer
Wind River Systems
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-net" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200002040202.SAA01528>