>Number: 146628
>Category: kern
>Synopsis: [patch] TCP does not clear DF when MTU is below a threshold
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Sun May 16 10:00:14 UTC 2010
>Closed-Date:
>Last-Modified:
>Originator: Matthew Luckie
>Release: FreeBSD 8.0-STABLE i386
>Organization:
>Environment:
System: FreeBSD mylar.luckie.org.nz 8.0-STABLE FreeBSD 8.0-STABLE #3: Sun May
16 21:31:15 NZST 2010 [email protected]:/usr/src/sys/i386/compile/mylar
i386
>Description:
FreeBSD, like most operating systems, will refuse to lower TCP's
segment size in response to an ICMP needfrag below a threshold. In
FreeBSD's case, this is 512 bytes. If a needfrag next-hop MTU 256 is
received, FreeBSD will reduce the connection's segment size to 512
bytes, and will then resend the presumed missing packet, but without
first clearing the DF bit. If the Path MTU is in fact less than 512
bytes FreeBSD will get another needfrag, which it will ignore. The
patch below will cause subsequent segments to be sent without the DF
bit set, and does not change FreeBSD's default behaviour of refusing
to reduce its segment size below a defined threshold.
>How-To-Repeat:
install net/scamper
scamper -F ipfw -I "tbit -M 256 -u '<url on webserver>' -i <ip address>"
>Fix:
--- patch-pmtud begins here ---
--- tcp_var.h.orig 2009-08-03 20:13:06.000000000 +1200
+++ tcp_var.h 2010-05-14 21:03:42.000000000 +1200
@@ -234,6 +234,7 @@
#define TF_ECN_PERMIT 0x4000000 /* connection ECN-ready */
#define TF_ECN_SND_CWR 0x8000000 /* ECN CWR in queue */
#define TF_ECN_SND_ECE 0x10000000 /* ECN ECE in queue */
+#define TF_IPDF 0x20000000 /* set the DF bit */
#define IN_FASTRECOVERY(tp) (tp->t_flags & TF_FASTRECOVERY)
#define ENTER_FASTRECOVERY(tp) tp->t_flags |= TF_FASTRECOVERY
--- tcp_subr.c.orig 2009-08-03 20:13:06.000000000 +1200
+++ tcp_subr.c 2010-05-16 21:26:50.000000000 +1200
@@ -656,7 +656,9 @@
tlen += sizeof (struct tcpiphdr);
ip->ip_len = tlen;
ip->ip_ttl = V_ip_defttl;
- if (V_path_mtu_discovery)
+ if (tp != NULL && tp->t_flags & TF_IPDF)
+ ip->ip_off |= IP_DF;
+ else if (tp == NULL && V_path_mtu_discovery)
ip->ip_off |= IP_DF;
}
m->m_len = tlen;
@@ -757,6 +759,9 @@
tp->t_flags = (TF_REQ_SCALE|TF_REQ_TSTMP);
if (V_tcp_do_sack)
tp->t_flags |= TF_SACK_PERMIT;
+ if (V_path_mtu_discovery)
+ tp->t_flags |= TF_IPDF;
+
TAILQ_INIT(&tp->snd_holes);
tp->t_inpcb = inp; /* XXX */
/*
@@ -1361,9 +1366,11 @@
if (mtu < max(296, V_tcp_minmss
+ sizeof(struct tcpiphdr)))
mtu = 0;
- if (!mtu)
+ if (!mtu) {
mtu = V_tcp_mssdflt
+ sizeof(struct tcpiphdr);
+ tp->t_flags &= ~TF_IPDF;
+ }
/*
* Only cache the the MTU if it
* is smaller than the interface
--- tcp_syncache.c.orig 2010-05-16 21:30:21.000000000 +1200
+++ tcp_syncache.c 2010-05-16 21:31:00.000000000 +1200
@@ -779,6 +779,9 @@
if (sc->sc_flags & SCF_ECN)
tp->t_flags |= TF_ECN_PERMIT;
+ if (V_path_mtu_discovery)
+ tp->t_flags |= TF_IPDF;
+
/*
* Set up MSS and get cached values from tcp_hostcache.
* This might overwrite some of the defaults we just set.
--- tcp_output.c.orig 2009-11-18 05:17:11.000000000 +1300
+++ tcp_output.c 2010-05-16 20:38:25.000000000 +1200
@@ -1181,7 +1181,7 @@
* Section 2. However the tcp hostcache migitates the problem
* so it affects only the first tcp connection with a host.
*/
- if (V_path_mtu_discovery)
+ if (tp->t_flags & TF_IPDF)
ip->ip_off |= IP_DF;
error = ip_output(m, tp->t_inpcb->inp_options, NULL,
--- patch-pmtud ends here ---
>Release-Note:
>Audit-Trail:
>Unformatted:
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