Understanding Linux Network Internals

Ethernet Versus IEEE 802.3 Frames

A number of protocols go under the loose term Ethernet. The 802.2 and 802.3 standards are represented by the protocols ETH_P_802_2 and ETH_P_802_3, respectively, but there are many other Ethernet protocols, listed in Table 13-2, as well as the LLC and SNAP extensions. The standards institute a couple of hacks to support all of these variations (h_proto isdiscussed in the following section).

Table 13-2. Valid Ethernet types (when h_proto > 1536)

Protocol	Ethernet type	Function handler
a The reason why IP has two handlers has to do with the possibility for the kernel to retrieve the IP configuration by means of protocols like RARP/BOOTP. The ic_bootp_recv handler is used only at boot time to take care of the dynamic IP configuration, and it is uninstalled once the configuration has been retrieved. See net/ipv4/ipconfig.c.
ETH_P_IP	0x0800	ip_rcv ic_bootp_recv a
ETH_P_X25	0x0805	X25_lap_receive_frame
ETH_P_ARP	0x0806	arp_rcv
ETH_P_BPQ	0x08FF	bpq_rcv
ETH_P_DNA_RT	0x6003	dn_route_rcv
ETH_P_RARP	0x8035	ic_rarp_recv
ETH_P_8021Q	0x8100	vlan_skb_rcv
ETH_P_IPX	0x8137	ipx_rcv
ETH_P_IPV6	0x86DD	ipv6_rcv
ETH_P_PPP_DISC ETH_P_PPP_SES	0x8863 0x8864	pppoe_disc_rcv pppoe_rcv

Ethernet was designed before the IEEE created its 802.2 and 802.3 standards. The latter are not pure Ethernet, even though they are commonly called Ethernet standards. Fortunately, the ...