Julian Elischer wrote:
Eli Dart wrote:
see below...
Julian Elischer wrote:
Eli Dart wrote:
Stephen Clark wrote:
So was any decision reached on this issue - will FreeBSD changed
to accept a packet on an interface that is larger than the mtu on
that interface?
If possible, I'd like to see the ability to enforce interface MTU
for received packets preserved in a sysctl if it is removed for the
default config... In other words, something like:
net.link.mtu_limits_received_pktsize = 0|1
Then, default it to 0 to preserve 4.x behavior.
what would this achieve?
Answering himself.. it MAY allow a driver to optimise a bit by not
needing to cope with the posibility of receiving jubo packets? I can
not think of any other reason.. (except to break networks that are
apparently working fine).
The networks that are apparently working fine are most likely
misconfigured, IMHO.
Others have made a case for permitting an interface to accept as large
a packet as it can, regardless of configured MTU. That's fine for
theory.
My operational experience leads me to a different place. If an
interface receives a packet that is larger than its configured MTU, I
would prefer that the packet be dropped as a giant and a giants
counter incremented, regardless of whether the hardware can
theoretically receive the packet. In modern networks, an MTU mismatch
within a broadcast domain indicates a broken network, IMHO. If the
devices in the network are configured to enforce MTU for both tx and
rx, more problems get spotted during turnup, rather than surfacing
later on as difficult-to-diagnose problems that users only call about
after they are truly frustrated. And, if you have a giants counter
(or input error counter) you can look at, it makes it straightforward
to spot the problem.
(one could also stretch a bit and say that enforcing MTU on rx might
provide less surprise to code that consumes packets and has knowledge
of the MTU setting of an interface.....unfortunately I don't know
enough about the details of the network stack to know if this is a
real concern)
then we should have an MRU value.
mtu is mTu
note that if the following code is what is doing it, it is only enabled in
DIAGNOSTIC mode anyhow.
#ifdef DIAGNOSTIC
if (m->m_pkthdr.len >
ETHER_MAX_FRAME(ifp, etype, m->m_flags & M_HASFCS) &&
(ifp->if_capenable & IFCAP_LRO) == 0) {
if_printf(ifp, "discard oversize frame "
"(ether type %x flags %x len %u > max
%lu)\n",
etype, m->m_flags, m->m_pkthdr.len,
ETHER_MAX_FRAME(ifp, etype,
m->m_flags & M_HASFCS));
ifp->if_ierrors++; m_freem(m);
return;
}
in 6.1 that would become:
diff -u -r1.193.2.10 if_ethersubr.c
--- if_ethersubr.c 4 Mar 2006 09:23:34 -0000 1.193.2.10
+++ if_ethersubr.c 20 Jul 2007 22:52:43 -0000
@@ -99,6 +99,8 @@
extern u_char aarp_org_code[3];
#endif /* NETATALK */
+static int mtu_is_mru = 0;
+
/* netgraph node hooks for ng_ether(4) */
void (*ng_ether_input_p)(struct ifnet *ifp, struct mbuf **mp);
void (*ng_ether_input_orphan_p)(struct ifnet *ifp, struct mbuf *m);
@@ -536,16 +559,18 @@
}
eh = mtod(m, struct ether_header *);
etype = ntohs(eh->ether_type);
- if (m->m_pkthdr.len >
- ETHER_MAX_FRAME(ifp, etype, m->m_flags & M_HASFCS)) {
- if_printf(ifp, "discard oversize frame "
- "(ether type %x flags %x len %u > max %lu)\n",
- etype, m->m_flags, m->m_pkthdr.len,
- ETHER_MAX_FRAME(ifp, etype,
- m->m_flags & M_HASFCS));
- ifp->if_ierrors++;
- m_freem(m);
- return;
+ if (mtu_is_mru) {
+ if (m->m_pkthdr.len >
+ ETHER_MAX_FRAME(ifp, etype, m->m_flags & M_HASFCS)) {
+ if_printf(ifp, "discard oversize frame "
+ "(ether type %x flags %x len %u > max
%lu)\n",
+ etype, m->m_flags, m->m_pkthdr.len,
+ ETHER_MAX_FRAME(ifp, etype,
+ m->m_flags & M_HASFCS));
+ ifp->if_ierrors++;
+ m_freem(m);
+ return;
+ }
}
if (m->m_pkthdr.rcvif == NULL) {
if_printf(ifp, "discard frame w/o interface pointer\n");
@@ -931,9 +982,11 @@
SYSCTL_DECL(_net_link);
SYSCTL_NODE(_net_link, IFT_ETHER, ether, CTLFLAG_RW, 0, "Ethernet");
+SYSCTL_INT(_net_link_ether, OID_AUTO, MTUisMRU, CTLFLAG_RW,
+ &mtu_is_mru,0,"Allow MTU to limit recieved packet size");
#if defined(INET) || defined(INET6)
SYSCTL_INT(_net_link_ether, OID_AUTO, ipfw, CTLFLAG_RW,
ðer_ipfw,0,"Pass ether pkts through firewall");
#endif
#if 0
possibly wrapped in #ifdef MTUISMRU
so people could avoid the extra overhead.
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