change_mtu boundary checking error

["Hen, Shmulik" <shmulik.hen@intel.com>]

Hello,

Going through the change_mtu() code in the kernel, I came across the default
function supplied when calling ether_setup().
I could see that eth_change_mtu() (drivers/net/net_init.c) does the
following:

	if( (new_mtu < 68) || (new_mtu > 1500) )
		return -EINVAL;

Looking in include/linux/if_ether.h I found the following constants:
#define ETH_ALEN		6	/* Octets in one ethernet addr */
#define ETH_HLAN		14	/* Total octets in header. */
#define ETH_ZLEN		60	/* Min. octets in frame sans FCS */
#define ETH_DATA_LEN		1500	/* Max. octets in payload */
#define ETH_FRAME_LEN	1514	/* Max. octets in frame sans FCS */


Now, the high boundary seemed reasonable (ETH_FRAME_LEN - ETH_HLEN =
ETH_DATA_LEN) which gives 1500, but why is the low boundary set to 68 ?
According to my calculations, it should have been ETH_ZLEN - ETH_HLEN which
gives 46.

Doesn't mtu means only the payload size ?
Where did the 68 come from ?


	Thanks,
	Shmulik Hen
	Software Engineer
	Linux Advanced Networking Services
	Network Communications Group, Israel (NCGj)