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[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: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Stephen Hemminger 2007-07-07 23:03:44 -07:00 committed by David S. Miller
parent 1722933323
commit 1c8c7d6416
1 changed files with 25 additions and 0 deletions
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25
Documentation/networking/netdevices.txt
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@ -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: