Ping principle

I've read a lot of interview questions recently , Among them, I was also asked about ping And some principled things , So I read many people's blogs , Finally, it suddenly became clear , Decided to sum up , Also to prevent forgetting .

To understand ping Principle , Let's take a look first ARP The message format of

among op:

1： Express arp request 
2： Express arp The reply 
3： Express rarp request 
4： Express rarp The reply 

  
   

Next , Let's take a look at the same broadcast domain ping message ：

our host2 Want to go ping host1, So the mainframe 1 It is necessary to encapsulate the layer-2 message , First host2 Will check your MAC Address table , See if there is host1 The address of , without ,host2 It will send out ARP Broadcast packet
ARP The format of frame message is as follows ：（ omitted arp Hardware type of message , Upper protocol type ,MAC Address length ,IP Address length ）

Purpose MAC	Source MAC	The frame type	op	sender MAC	sender IP	The goal is MAC	The goal is IP
FF-FF-FF-FF-FF-FF	52-54-00-00-11-22	0806	1	52-54-00-00-11-22	192.168.1.2	00-00-00-00-00-00	192.168.1.1

First , After our switch receives the message , Will inquire about the switch MAC Address table （ With learning function ）, If there is host1 Of MAC Address , Direct return to host2; without , The switch will send to all ports ARP radio broadcast , After the host on other ports is found , If the goal is not yourself , Will discard the message ; until host1 After the host receives the message , Will respond host1 Of MAC What's the address , At the same time, learn the host host2 Of MAC Address , And follow the same format ARP The message is returned to host2.

host1 Back to ARP Frame message ：

Purpose MAC	Source MAC	The frame type	op	sender MAC	sender IP	The goal is MAC	The goal is IP
52-54-00-00-11-22	52-54-00-00-11-11	0806	2	52-54-00-00-11-11	192.168.1.1	52-54-00-00-11-22	192.168.1.2

This is the time , host host2 I've learned that host1 Of MAC Address , So send icmp message

Destination address	source address	…	Source IP	The goal is IP	…	ICMP message
52-54-00-00-11-11	52-54-00-00-11-22		192.168.1.2	192.168.1.1		Echo Request

host1 After receiving the message , Hui Fu

Destination address	source address	…	Source IP	The goal is IP	…	ICMP message
52-54-00-00-11-22	52-54-00-00-11-11		192.168.1.1	192.168.1.2		Echo Answer

Next , Let's take a look at the differences between different broadcast domains ping

First of all, ensure that the whole network can reach ！！！
If our host2 Still want to ping host ,host2 You have to go to the gateway to forward , But if you say , What if it doesn't know the gateway ？ It will look like ping host1 like that , Send... First ARP message , Determine the gateway mac And then send icmp Send the message to the router .
We omit arp message
host2 send out ICMP Message to gateway

Destination address	source address	…	Source IP	The goal is IP	…	ICMP message
52-54-00-00-11-33	52-54-00-00-11-22		192.168.1.2	192.168.2.1		Echo Request

When the router receives a message from host2 Of icmp After the message , Find the destination address IP by 192.168.2.1, Query route discovery , We have to find an interface to go out , So get rid of the original mac Address header , Add your own mac Address header and to host3 forward ,（ If there is no gateway host3 Of mac, Also have to send ARP Message inquiry , And the port of the router can also learn from the host mac Address , host host3 Can also learn router MAC Address ）, The specific message is as follows ：

Destination address	source address	…	Source IP	The goal is IP	…	ICMP message
52-54-00-00-11-55	52-54-00-00-11-44		192.168.1.2	192.168.2.1		Echo Request

host host3 I have learned about routers mac, This will return icmp message

Destination address	source address	…	Source IP	The goal is IP	…	ICMP message
52-54-00-00-11-44	52-54-00-00-11-55		192.168.2.1	192.168.1.2		Echo Answer