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authorStephen Hemminger <shemminger@linux-foundation.org>2007-07-08 02:03:44 -0400
committerDavid S. Miller <davem@sunset.davemloft.net>2007-07-11 01:18:47 -0400
commit1c8c7d64169dc4b1ae3d8cd1bf35ea0a099b50ad (patch)
tree6a92042d97191913b8d72f700ede468f726fbbbb
parent1722933323b70f44b0548131604f1f3454c2aa8f (diff)
[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>
-rw-r--r--Documentation/networking/netdevices.txt25
1 files changed, 25 insertions, 0 deletions
diff --git a/Documentation/networking/netdevices.txt b/Documentation/networking/netdevices.txt
index a6cab674b676..37869295fc70 100644
--- a/Documentation/networking/netdevices.txt
+++ b/Documentation/networking/netdevices.txt
@@ -20,6 +20,31 @@ private data which gets freed when the network device is freed. If
20separately allocated data is attached to the network device 20separately allocated data is attached to the network device
21(dev->priv) then it is up to the module exit handler to free that. 21(dev->priv) then it is up to the module exit handler to free that.
22 22
23MTU
24===
25Each network device has a Maximum Transfer Unit. The MTU does not
26include any link layer protocol overhead. Upper layer protocols must
27not pass a socket buffer (skb) to a device to transmit with more data
28than the mtu. The MTU does not include link layer header overhead, so
29for example on Ethernet if the standard MTU is 1500 bytes used, the
30actual skb will contain up to 1514 bytes because of the Ethernet
31header. Devices should allow for the 4 byte VLAN header as well.
32
33Segmentation Offload (GSO, TSO) is an exception to this rule. The
34upper layer protocol may pass a large socket buffer to the device
35transmit routine, and the device will break that up into separate
36packets based on the current MTU.
37
38MTU is symmetrical and applies both to receive and transmit. A device
39must be able to receive at least the maximum size packet allowed by
40the MTU. A network device may use the MTU as mechanism to size receive
41buffers, but the device should allow packets with VLAN header. With
42standard Ethernet mtu of 1500 bytes, the device should allow up to
431518 byte packets (1500 + 14 header + 4 tag). The device may either:
44drop, truncate, or pass up oversize packets, but dropping oversize
45packets is preferred.
46
47
23struct net_device synchronization rules 48struct net_device synchronization rules
24======================================= 49=======================================
25dev->open: 50dev->open: