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diff --git a/sys/netinet/icmp_var.h b/sys/netinet/icmp_var.h
--- a/sys/netinet/icmp_var.h
+++ b/sys/netinet/icmp_var.h
@@ -56,6 +56,7 @@
u_long icps_bmcasttstamp; /* b/mcast tstamp requests dropped */
u_long icps_badaddr; /* bad return address */
u_long icps_noroute; /* no route back */
+ u_long icps_pmtuchg; /* path MTU changes */
};
#ifdef _KERNEL
diff --git a/sys/netinet/ip.h b/sys/netinet/ip.h
--- a/sys/netinet/ip.h
+++ b/sys/netinet/ip.h
@@ -213,6 +213,7 @@
#define IPFRAGTTL 60 /* time to live for frags, slowhz */
#define IPTTLDEC 1 /* subtracted when forwarding */
#define IP_MSS 576 /* default maximum segment size */
+#define IP_MMTU 68 /* Minimal MTU from RFC 1191 */
/*
* This is the real IPv4 pseudo header, used for computing the TCP and UDP
diff --git a/sys/netinet/ip_icmp.c b/sys/netinet/ip_icmp.c
--- a/sys/netinet/ip_icmp.c
+++ b/sys/netinet/ip_icmp.c
@@ -179,6 +179,7 @@
static void icmp_send(struct mbuf *, struct mbuf *);
static int icmp_verify_redirect_gateway(struct sockaddr_in *,
struct sockaddr_in *, struct sockaddr_in *, u_int);
+static void icmp_pmtu_update(const struct icmp *, const int);
extern struct protosw inetsw[];
@@ -558,6 +559,9 @@
}
ip = mtod(m, struct ip *);
icp = (struct icmp *)(ip + 1);
+ /* Update PMTU to make it available for upper layers. */
+ if (V_rfc1191)
+ icmp_pmtu_update(icp, M_GETFIB(m));
/*
* The upper layer handler can rely on:
* - The outer IP header has no options.
@@ -1059,6 +1063,35 @@
}
return 0;
}
+
+/*
+ * Update path MTU from an ICMP message into the hostcache.
+ *
+ * Update only happens if the new MTU is smaller than the nexthop MTU. Values
+ * smaller than IP_MMTU are ignored.
+ */
+static void
+icmp_pmtu_update(const struct icmp *icmp, const int fibnum)
+{
+ uint32_t mtu = ntohs(icmp->icmp_nextmtu);
+ struct in_conninfo inc;
+
+ /* Pre-RFC1191 routers don't specify the mtu. */
+ if (mtu == 0)
+ mtu = ip_next_mtu(icmp->icmp_ip.ip_len, 1);
+ /* Ignore values less than minimum. */
+ if (mtu < IP_MMTU)
+ return;
+
+ bzero(&inc, sizeof(inc));
+ inc.inc_fibnum = fibnum;
+ inc.inc_faddr = icmp->icmp_ip.ip_dst;
+
+ if (mtu < tcp_maxmtu(&inc, NULL)) {
+ tcp_hc_updatemtu(&inc, mtu);
+ ICMPSTAT_INC(icps_pmtuchg);
+ }
+}
#endif /* INET */
/*
diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -85,6 +85,8 @@
#include <netinet/ip_options.h>
#include <netinet/ip_mroute.h>
+#include <netinet/tcp_var.h>
+
#include <netinet/udp.h>
#include <netinet/udp_var.h>
@@ -105,6 +107,10 @@
&mbuf_frag_size, 0, "Fragment outgoing mbufs to this size");
#endif
+VNET_DEFINE(int, rfc1191);
+SYSCTL_INT(_net_inet_ip, 0, rfc1191, CTLFLAG_VNET | CTLFLAG_RW,
+ &VNET_NAME(rfc1191), 0, "Enable RFC1191 PMTU support");
+
static void ip_mloopback(struct ifnet *, const struct mbuf *, int);
extern int in_mcast_loop;
@@ -304,6 +310,46 @@
ro->ro_flags |= (nh_flags & NHF_GATEWAY) ? RT_HAS_GW : 0;
}
+/*
+ * Calculates IPv4 path MTU for @dst based on the hostcache data and a fallback
+ * nexthop MTU value @nh_mtu.
+ *
+ * Note that @nh_mtu is a valid fall-back MTU if there is no PMTU entry in the
+ * hostcache. It does not need to necessarily come from nh_object, as is the
+ * case when this function is called from ip_output() for a broadcast
+ * destination.
+ *
+ * Returns the calculated MTU.
+ */
+static uint32_t
+ip_calcpmtu(const struct in_addr dst, uint32_t nh_mtu, u_int proto)
+{
+ uint32_t mtu = 0;
+ struct in_conninfo inc;
+
+ bzero(&inc, sizeof(inc));
+ inc.inc_faddr = dst;
+
+ /* TCP is known to react to pmtu changes so skip hc,
+ * XXX: Either add list of known good protocols or an inpcb flag filled
+ * by the PMTU aware protocol.
+ */
+ if (proto != IPPROTO_TCP)
+ mtu = tcp_hc_getmtu(&inc);
+
+ if (mtu)
+ mtu = min(mtu, nh_mtu);
+ else
+ mtu = nh_mtu;
+ /* XXX: below is an extremely unlikely case, because it is a minimal
+ * unfragmented packet size (RFC 791).
+ */
+ if (__predict_false(mtu < IP_MMTU))
+ mtu = IP_MMTU;
+
+ return mtu;
+}
+
/*
* IP output. The packet in mbuf chain m contains a skeletal IP
* header (with len, off, ttl, proto, tos, src, dst).
@@ -326,7 +372,6 @@
struct ifnet *ifp = NULL; /* keep compiler happy */
struct mbuf *m0;
int hlen = sizeof (struct ip);
- int mtu = 0;
int error = 0;
int vlan_pcp = -1;
struct sockaddr_in *dst, sin;
@@ -335,7 +380,7 @@
struct in_addr src;
int isbroadcast;
uint16_t ip_len, ip_off;
- uint32_t fibnum;
+ uint32_t fibnum, mtu = 0;
#if defined(IPSEC) || defined(IPSEC_SUPPORT)
int no_route_but_check_spd = 0;
#endif
@@ -550,6 +595,9 @@
in_ifaddr_broadcast(dst->sin_addr, ia)));
}
+ /* Determine the path MTU. */
+ if (V_rfc1191)
+ mtu = ip_calcpmtu(ip->ip_dst, mtu, ip->ip_p);
/* Catch a possible divide by zero later. */
KASSERT(mtu > 0, ("%s: mtu %d <= 0, ro=%p (nh_flags=0x%08x) ifp=%p",
__func__, mtu, ro,
diff --git a/sys/netinet/ip_var.h b/sys/netinet/ip_var.h
--- a/sys/netinet/ip_var.h
+++ b/sys/netinet/ip_var.h
@@ -180,6 +180,7 @@
struct sockopt;
struct inpcbinfo;
+VNET_DECLARE(int, rfc1191);
VNET_DECLARE(int, ip_defttl); /* default IP ttl */
VNET_DECLARE(int, ipforwarding); /* ip forwarding */
VNET_DECLARE(int, ipsendredirects);
@@ -195,6 +196,7 @@
VNET_DECLARE(int, drop_redirect);
extern struct pr_usrreqs rip_usrreqs;
+#define V_rfc1191 VNET(rfc1191)
#define V_ip_id VNET(ip_id)
#define V_ip_defttl VNET(ip_defttl)
#define V_ipforwarding VNET(ipforwarding)

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