ARP message header format and request flow

ARP Header format

Format specification ：

• Hardware type ：16 Bit fields , Used to define the operation ARP The type of network . Each LAN is assigned an integer based on its type . for example ： The type of Ethernet is 1.ARP Available on any physical network .
• Protocol type ：16 Bit fields , Used to define the protocol used . for example ： Yes IPv4 The protocol field is 0800.ARP Can be used for any high-level protocol
• Hardware address length ：8 Bit fields , Used to define the length of the physical address , In bytes . for example ： For Ethernet, the value is 6.
• Protocol address length ：8 Bit fields , Used to define the length of the logical address , In bytes . for example ： about IPv4 The value of the protocol is 4.
• opcode ：16 Bit fields , Used to define the type of message . There are two defined grouping types ：ARP The request is 1,ARP The response is 2,RARP The request is 3,RARP The answer is 4.
• Source hardware address ： This is a variable length field , Used to define the physical address of the sender . for example ： For Ethernet, the length of this field is 6 byte .
• Source logical address ： This is a variable length field , The logic used to define the sender （IP） Address . for example ： about IP The length of the protocol field is 4 byte .
• Destination hardware address ： This is a variable length field , Used to define the physical address of the target , for example , For Ethernet, this field is bit 6 byte . about ARP Request message , This field is full 0, Because the sender does not know the hardware address of the target .
• Destination logical address ： This is a variable length field , The logic used to define the goal （IP） Address , about IPv4 The length of the protocol field is 4 Bytes .

Send in Ethernet ARP On request , The source of etheric head MAC The address is local MAC, Purpose MAC The address is full f;ARP The source of MAC For the local MAC, Source IP It's local IP, Purpose MAC by 0, Purpose IP For the requested IP Address

ARP Request flow

A The host requests B host MAC Address time , Will judge first A、B Whether it belongs to the same network segment , Send in different ways ARP request .

• Same segment

1. A send out ARP radio broadcast , Switch learning A Of MAC Address , Concurrent flooding .
2. B received ARP Post packet transmission ARP Unicast to A, At the same time, I learned B Of MAC Address , And then A Acquire B Of MAC Address

• Cross network segment

1. A send out ARP radio broadcast , Request gateway's MAC
2. The packet is changed to ： Source ：A IP, Purpose ：B IP, Source MAC：A MAC, Purpose MAC： gateway MAC
3. A take ARP A packet is sent to A Gateway for
4. A Gateway query routing table , Modify the data package ： Source order IP unchanged , Source MAC Change to A The gateway MAC, Purpose MAC Change to the next jump MAC, Route recursion , Finally arrive at B Gateway for
5. B The gateway finds that this machine has B Of MAC Address , Please Return by the Way You Came B Of MAC Address to A

If AB Belong to the same network segment , however A To B There is no direct route , that A It will also follow the process of cross network segment , Send the packet to the gateway for processing .