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)
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This archive was generated by hypermail 2b29 : Mon Apr 23 2001 - 21:00:22 EST