Linux Network protocol stack (d)--Link layer (2) __linux

2. Agreement related
2.1, the 3rd layer of the Protocol management
In the Linux kernel, there are 3-tier protocols for two different purposes:
(1) The Ptype_all Management protocol is used primarily for analysis purposes, and it receives all packets arriving at Layer 3rd protocol.
(2) Ptype_base manages the normal 3-layer protocol, receiving only packets with the correct protocol identifier, such as the 0x0800 of the Internet.

Notice the difference between the Sb_buff and the Net_device in several fields:
Sb_buff:
Unsigned short protocol
High-level protocols seen from the perspective of the two-tier device, typical protocols include Ip,ipv6 and ARP, complete with a list of include/linux/if_ether.h.
unsigned char Pkt_type
The type of frame, and possible values are defined in include/linux/if_packet.h.

Net_device:
unsigned short type
Device type (Ethernet, Frame Relay, etc.). There is a complete list of types in Include/linux/if_arp.h.

2.2. Protocol Processing function Registration
When the protocol is registered, the kernel invokes Dev_add_pack to add a corresponding PACKET_TYPE data structure:
Include/linux/netdevice.h
struct Packet_type {
unsigned short type;     /* This is really htons (Ether_type). */
struct Net_device * DEV; /* NULL is wildcarded here/*
Int (* func) (struct Sk_buff *, struct net_device *,
struct Packet_type *);
void * AF_PACKET_PRIV;
struct List_head list;
};
Type: The protocol type, which can take some of the following values. Look at the types of protocols defined in If_ether.h:

#define ETH_P_LOOP 0x0060/* Ethernet Loopback Packet * *
#define ETH_P_PUP 0x0200/* Xerox PUP Packet * *
#define ETH_P_PUPAT 0x0201/* Xerox PUP Addr Trans Packet * *
#define ETH_P_IP 0x0800/* Internet PROTOCOL Packet * *
#define ETH_P_X25 0x0805/* CCITT X.25 * *
#define ETH_P_ARP 0x0806/* Address Resolution Packet * *
#define ETH_P_BPQ 0x08ff/* G8BPQ ax.25 Ethernet Packet [not a officially registered ID] * *