[NET]: netdevice mtu assumptions documentation

Document the expectations about device MTU handling.
The documentation about oversize packet handling is probably too
loose.

IMHO devices should drop oversize packets for robustness,
but many devices allow it now. For example, if you set mtu to 1200
bytes, most ether devices will allow a 1500 byte frame in.
Signed-off-by: NStephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/netdevices.txt

@@ -20,6 +20,31 @@ private data which gets freed when the network device is freed. If
 separately allocated data is attached to the network device
 (dev->priv) then it is up to the module exit handler to free that.
 
+MTU
+===
+Each network device has a Maximum Transfer Unit. The MTU does not
+include any link layer protocol overhead. Upper layer protocols must
+not pass a socket buffer (skb) to a device to transmit with more data
+than the mtu. The MTU does not include link layer header overhead, so
+for example on Ethernet if the standard MTU is 1500 bytes used, the
+actual skb will contain up to 1514 bytes because of the Ethernet
+header. Devices should allow for the 4 byte VLAN header as well.
+
+Segmentation Offload (GSO, TSO) is an exception to this rule.  The
+upper layer protocol may pass a large socket buffer to the device
+transmit routine, and the device will break that up into separate
+packets based on the current MTU.
+
+MTU is symmetrical and applies both to receive and transmit. A device
+must be able to receive at least the maximum size packet allowed by
+the MTU. A network device may use the MTU as mechanism to size receive
+buffers, but the device should allow packets with VLAN header. With
+standard Ethernet mtu of 1500 bytes, the device should allow up to
+1518 byte packets (1500 + 14 header + 4 tag).  The device may either:
+drop, truncate, or pass up oversize packets, but dropping oversize
+packets is preferred.
+
+
 struct net_device synchronization rules
 =======================================
 dev->open: