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header length is the size of the ether_header data structure plus 8
(the size of the maximum LLC header).
The media headers are represented by the following data structures:
ether_header
The media header structure for Ethernet-related
media. The if_ether.h file defines the
ether_header structure.
fddi_header
The media header structure for FDDI-related
media. The if_fddi.h file defines the
fddi_header structure.
trn_header
The media header structure for Token Ring-related
media. The if_trn.h file defines the
trn_header structure.
6.3 Setting Up the Media
The following code shows how the el_attach( ) routine sets up
media-related information:
sc->is_ac.ac_bcastaddr = (u_char *)etherbroadcastaddr; 1
sc->is_ac.ac_arphrd = ARPHRD_ETHER; 2
ifp->if_mtu = ETHERMTU; 3
ifp->if_mediamtu = ETHERMTU; 4
ifp->if_type = IFT_ETHER; 5
((struct arpcom *)ifp)->ac_flag = 0; 6
sin = (struct sockaddr_in *)&ifp->if_addr; 7
sin->sin_family = AF_INET; 8
1 Sets the ac_bcastaddr member of the softc data structure for this
device to the Ethernet broadcast address. The system stores the
Ethernet broadcast address in the etherbroadcastaddr character
array. Tru64 UNIX defines the etherbroadcastaddr character array
in the if_ether.h file.
The name is_ac is an alternate name for the ess_ac member of the
ether_driver data structure. The ess_ac member is referred to as
the Ethernet common part and is actually an instance of the arpcom
data structure.
2 Sets the ac_arphrd member of the softc data structure for this
device to the constant ARPHRD_ETHER, which represents the Ethernet
hardware address. The if_arp.h file defines this constant.
For the Token Ring interface, set the ac_arphrd member to the
constant ARPHRD_802. The if_arp.h file also defines this constant.
For the FDDI interface, set the ac_arphrd member to the constant
ARPHRD_ETHER, which represents the Ethernet hardware address. See
RFC 826 for more details.
Implementing the Autoconfiguration Support Section (attach) 6–3