[network protocol] why learn network protocol

The three elements of the agreement ： grammar 、 semantics 、 The order

Why learn network protocols ：

Only through network protocol , To make a large number of machines cooperate with each other 、 Accomplish one thing together .

【 Take online ordering as an example 】

Enter an e-commerce website in the browser , The browser will go through DNS Or more precisely HTTPDNS Find specific IP.

After finding the target address , The browser starts to package its requests . For ordinary browsing requests , Often use HTTP agreement , But for shopping requests , Encrypted transmission is often required , So I will use HTTPS agreement .

DNS、HTTP、HTTPS The layer we are on becomes the application layer , After application layer encapsulation , The browser will give the package of the application layer to the next layer to complete , adopt socket Programming to achieve . The next layer is the transport layer . The transport layer has two protocols , One is connectionless protocol UDP, One is connection oriented TCP（ Connection oriented is ,TCP It will ensure that the package can reach the destination , If not , Send it all the time , Until you can ）. For payment , Often use TCP agreement .

TCP There are two ports in the protocol , One is the port on which the browser listens , One is the server monitoring port of e-commerce . Operating systems often judge by ports , Which process should it get the package for .

After the completion of transport layer encapsulation , The browser will deliver the package to the network layer of the operating system . The protocol of the network layer is IP agreement . stay IP There will be something in the agreement IP Address , That is, the browser's machine IP Address and destination IP Address .

When the operating system starts , Will be DHCP Protocol configuration IP Address , And the default gateway IP Address 192.168.1.1, Get the goal IP After the address , The operating system passes through ARP The protocol found the default gateway MAC Address , therefore , take IP Give the bag to the next floor , That is to say MAC layer , The network card will send the packet again , Get it to the gateway .

The communication protocol between gateway and gateway becomes routing protocol , Commonly used OSPF and BGP.

Thanks to the communication between gateways , Network packets can get the information of the next gateway from one gateway MAC Address , Until the target server is found .

Remove the target server MAC head , To the network layer of the operating system , Find out IP It's right , Just take it off IP head .IP What will be written on the head is TCP agreement , Then give it to the transport layer , namely TCP layer .

In this layer , For the package received , There is a reply package indicating that you have received （ just TCP A description of the layer , Not the result of the order request ）.

If after a while , The sender did not receive a reply , Sending end TCP The layer will resend the packet （ Resend means TCP End retry , It's not that the browser requests the order again , Unless it is TCP Something went wrong. , If the connection is broken ）.

The network packet arrived safely TCP After the layer ,TCP There is a target port number in the header , Through port number , You can find the port number that the e-commerce website is monitoring .

The process of e-commerce website has been HTTP Requested content , Will pass RPC call （ Remote procedure call ） To tell the relevant process what to do .

After the relevant processes are handled , Just reply to one HTTPS My bag , Inform the results of the order , The packet eventually enters the sender browser , Show payment success .

The network protocol to learn later ：

application layer ：DHCP  HTTP  HTTPS  RTMP  P2P  DNS  GTP RPC

Transport layer ：UDP  TCP

The network layer ：ICMP  IP  OSPF  BGP  IPSec  GRE

The link layer ：ARP  VLAN  STP

The physical layer ： Network jumpers