util-linux/util-linux-warn-nfsudp.diff

Index: util-linux-2.12/mount/nfs.5
================================================================================
--- util-linux-2.12q/mount/nfs.5
+++ util-linux-2.12q/mount/nfs.5
@@ -201,9 +201,13 @@
 Mount the NFS filesystem using the TCP protocol instead of the
 UDP protocol. This is the default, but in case it fails (many NFS servers only
 support UDP) it will fallback and try UDP.
+.IP
+.B Before using NFS over UDP, please refer to the section WARNINGS below.
 .TP 1.5i
 .I udp
 Mount the NFS filesystem using the UDP protocol.
+.IP
+.B Before using NFS over UDP, please refer to the section WARNINGS below.
 .TP 1.5i
 .I noacl
 Assume no extended access control mechanisms like POSIX ACLs are used
@@ -217,6 +221,83 @@
 All of the non-value options have corresponding nooption forms.
 For example, nointr means don't allow file operations to be
 interrupted.
+.SH WARNINGS
+Using NFS over UDP on high-speed links such as Gigabit
+.BR "can cause silent data corruption" .
+.P
+The problem can be triggered at high loads, and is caused by problems in
+IP fragment reassembly. NFS read and writes typically transmit UDP packets
+of 4 Kilobytes or more, which have to be broken up into several fragments
+in order to be sent over the Ethernet link, which limits packets to 1500
+bytes by default. This process happens at the IP network layer and is
+called fragmentation.
+.P
+In order to identify fragments that belong together, IP assigns a 16bit
+.I IP ID
+value to each packet; fragments generated from the same UDP packet
+will have the same IP ID. The receiving system will collect these
+fragments and combine them to form the original UDP packet. This process
+is called reassembly. The default timeout for packet reassembly is
+30 seconds; if the network stack does not receive all fragments of
+a given packet within this interval, it assumes the missing fragment(s)
+got lost and discards those it already received.
+.P
+The problem this creates over high-speed links is that it is possible
+to send more than 65536 packets within 30 seconds. In fact, with
+heavy NFS traffic one can observe that the IP IDs repeat after about
+5 seconds.
+.P
+This has serious effects on reassembly: if one fragment gets lost,
+another fragment 
+.I from a different packet
+but with the
+.I same IP ID
+will arrive within the 30 second timeout, and the network stack will
+combine these fragments to form a new packet. Most of the time, network
+layers above IP will detect this mismatched reassembly - in the case
+of UDP, the UDP checksum, which is a 16 bit checksum over the entire
+packet payload, will usually not match, and UDP will discard the
+bad packet.
+.P
+However, the UDP checksum is 16 bit only, so there is a chance of 1 in
+65536 that it will match even if the packet payload is completely
+random (which very often isn't the case). If that is the case, 
+silent data corruption will occur.
+.P
+This potential should be taken seriously, at least on Gigabit
+Ethernet.
+Network speeds of 100Mbit/s should be considered less
+problematic, because with most traffic patterns IP ID wrap around
+will take much longer than 30 seconds.
+.P
+It is therefore strongly recommended to use 
+.BR "NFS over TCP where possible" ,
+since TCP does not perform fragmentation.
+.P
+If you absolutely have to use NFS over UDP over Gigabit Ethernet,
+some steps can be taken to mitigate the problem and reduce the
+probability of corruption:
+.TP +1.5i
+.I Jumbo frames:
+Many Gigabit network cards are capable of transmitting
+frames bigger than the 1500 byte limit of traditional Ethernet, typically
+9000 bytes. Using jumbo frames of 9000 bytes will allow you to run NFS over
+UDP at a page size of 8K without fragmentation. Of course, this is
+only feasible if all involved stations support jumbo frames.
+.IP
+To enable a machine to send jumbo frames on cards that support it,
+it is sufficient to configure the interface for a MTU value of 9000.
+.TP +1.5i
+.I Lower reassembly timeout:
+By lowering this timeout below the time it takes the IP ID counter
+to wrap around, incorrect reassembly of fragments can be prevented
+as well. To do so, simply write the new timeout value (in seconds)
+to the file
+.BR /proc/sys/net/ipv4/ipfrag_time .
+.IP
+A value of 2 seconds will greatly reduce the probability of IPID clashes on
+a single Gigabit link, while still allowing for a reasonable timeout
+when receiving fragmented traffic from distant peers.
 .SH FILES
 .I /etc/fstab
 .SH "SEE ALSO"
--- util-linux-2.12q/mount/nfsmount.c
+++ util-linux-2.12q/mount/nfsmount.c
@@ -737,8 +737,12 @@
 			goto fail;
 		}
 		fsock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
-	} else
+	} else {
 		fsock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+		fprintf(stderr,
+			"Using NFS over UDP can cause data corruption.\n"
+			"Please refer to the WARNINGS section of the nfs(5) manual page.\n");
+	}
 	if (fsock < 0) {
 		perror(_("nfs socket"));
 		goto fail;