The arp46_t struct is the structure of the arp-request and arp-reply packets. The information contained within the arp-reqests and arp-replies is used to build the arp table. The first three fields in the struct (a46_dstaddr, a46_srcaddr, and a46_ethtype) correspond to an ethernet header and the remaining fields are arp-specific.
"46" refers to the number of bytes in the packet - 16 for the ethernet header and 28 for the arp-specific fields.
typedef struct arp46
{
ether_addr_t a46_dstaddr;
ether_addr_t a46_srcaddr;
ether_type_t a46_ethtype;
union
{
struct
{
u16_t a_hdr, a_pro;
u8_t a_hln, a_pln;
u16_t a_op;
ether_addr_t a_sha;
u8_t a_spa[4];
ether_addr_t a_tha;
u8_t a_tpa[4];
} a46_data;
char a46_dummy[ETH_MIN_PACK_SIZE-ETH_HDR_SIZE];
} a46_data;
} arp46_t;
From RFC 2625:
+-------------------------+
| HW Type | 2 bytes
+-------------------------+
| Protocol | 2 bytes
+-------------------------+
| HW Addr Length | 1 byte
+-------------------------+
| Protocol Addr Length | 1 byte
+-------------------------+
| Op Code | 2 bytes
+-------------------------+
| HW Addr of Sender | 6 bytes
+-------------------------+
| Protocol Addr of Sender | 4 bytes
+-------------------------+
| HW Addr of Target | 6 bytes
+-------------------------+
| Protocol Addr of Target | 4 bytes
+-------------------------+
Total 28 bytes
ARP Packet Format
+---------+-------------------------------------------------------+
|Ethernet |For an ARP request, source MAC address is the MAC |
|Header |address of the host sending the ARP request, |
| |destination MAC address is the Ethernet broadcast |
| |address (FF:FF:FF:FF:FF:FF), frame type field is 0x806.|
| |For ARP reply, source MAC address is the MAC address of|
| |the host replying to the ARP request, destination MAC |
| |address is the MAC address of the host that sent the |
| |ARP request, and the frame type field is 0x806. |
+---------+-------------------------------------------------------+
|Hardware |Type of the hardware MAC address which is being mapped.|
|Address |For Ethernet the value of this field is 1. |
|Type | |
+---------+-------------------------------------------------------+
|Protocol |Type of the protocol address to which the MAC address |
|Address |is mapped. For IP address the value of this field is |
|Type |0x800. |
+---------+-------------------------------------------------------+
|Hardware |Size of the hardware MAC address. For Ethernet, the |
|Address |value of this field is 6. |
|Size | |
+---------+-------------------------------------------------------+
|Protocol |Size of the protocol address. For IP, the value of |
|Address |this field is 4. |
|Size | |
+---------+-------------------------------------------------------+
|Operation|Type of operation being performed. The value of this |
| |field can be 1 (ARP request), 2 (ARP reply) |
+---------+-------------------------------------------------------+
|Source |The hardware MAC address of the host sending the ARP |
|MAC |request or reply. This is same as the source MAC |
|address |address present in the Ethernet header. |
+---------+-------------------------------------------------------+
|Source |The IP address of the host sending the ARP request or |
|IP |reply. |
|address | |
+---------+-------------------------------------------------------+
|Target |The hardware MAC address of the host receiving the ARP |
|MAC |request or reply. This is same as the destination MAC |
|address |address present in the Ethernet header. |
+---------+-------------------------------------------------------+
|Target |The IP address of the host receiving the ARP request |
|IP |or reply. |
|address | |
+---------+-------------------------------------------------------+
