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The network in the eyes of application developers ： Cross host 、 Communication between two processes .
The background and history of the emergence of the network ： Communication discipline
Country -> Institutions / organization -> family -> personal

Overview of basic principles

This paper briefly introduces network communication from the perspective of hardware

The data is on the transmission medium （ Ethernet cable ) Spread on （ There may be signal decay 、 Signal error and other problems ), Incoming receiver .
thus , Only one network medium can be used to connect the two hosts , Data exchange can be realized from the perspective of hardware .

Networking of multiple hosts

Protocol and layering

To complete the whole communication , The problems to be dealt with in the middle are still very complex . Therefore, it is necessary to carry out division of labor and cooperation .

When there is layering , You need to target each layer , Specify some specifications , Let everyone abide by , So as to better complete the work . These norms have certain authority , Under the network discipline , Call it an agreement (protocol)

Protocol layer

For network protocols , It is often defined at several levels .

What is protocol layering
Protocol layering is similar to when making a phone call , Define different levels of protocols ：
In this case , The agreement has only two layers ; But the actual network communication will be more complex , It needs to be divided into more levels .

Network standard layering

layered
1. Academic school ：OSI (Open System Interconnect) Provides a total of 7 Layer solution .
2. Practical school ：TCP/IP 5 Layer protocol (4 Layer protocol , The physical layer is omitted in the expression ).

Network devices

3 An important network device (network deivce), Be careful ： These network devices are not hosts .
1. A hub (hub)
2. Switch (switch sw)
3. Router (router)

A hub （ The physical layer ）

LAN Internal communication problems
1. The simplest LAN : Two hosts are directly connected .
2. LAN with certain topology :
Use a hub for networking , Born in the same conflict domain . This is caused by the working principle of the hub .
Conflict domain
The teacher said :“ Zhang San , Please answer a question ”, Who can hear this sentence ? Everyone can hear .
however , Only Zhang San will respond . Because the data we transmit carries the destination address （“ Zhang San ”).

Self study scene , Everyone can take the initiative to speak . As a result, they can't hear each other .—— Conflict domain .
Already under the same conflict domain , How to solve ？
Through a certain conflict avoidance algorithm , Avoid conflicts （ Data link layer to solve ）.
The most common conflict avoidance algorithm ： After discovering the conflict , Silent random time , Send... Again .
Even if there is a conflict avoidance algorithm , The data in the same conflict domain can be effectively transmitted , But the efficiency of signal transmission will still have an impact . So we need to ：
1. Don't have too many hosts in the conflict domain .
2. Minimize the occurrence of conflict domains .

Switch （ Data link layer ）

Switch (switch) appear , To reduce the conflict domain 、 Divide the conflict domain .
A hub ： Only do tool copying , I don't know what the data is .
Switch ： You need to understand the target address put into the data link layer （ Need to be able to parse the data link layer protocol ), According to the destination address of the data , Decide who to give the data 、 Not to anyone .
The switch works at the data link layer .
According to the destination address in the data , Send the data only to the corresponding target . You can also set up a LAN through a switch .

Broadcast area

Address

Address ： The address of the data link layer refers to MAC Address ( Physical address )
MAC The length of the address is 48 position (6 Bytes ), Usually expressed as 12 individual 16 Hexadecimal number .
Such as ：00-16-EA-AE-3C-40 It's just one. MAC Address
The network card was fixed when it was produced , Network cards all over the world MAC The address will not conflict .

One LAN Inside , No matter what kind of equipment , Which topology , The host and the host can communicate normally —— Responsibilities of data link layer protocol .
The above is the responsibility of the data link ＋ Two devices (hub、switch) On which floor do you work , How it works .

domain name (domain)
www.baidu.com -> 182.193.33.232
localhost -> 127.0.0.1： Represents a domain name of this machine

Router （ The network layer ）

The network layer ： Through data routing , Solve cross LAN Communication problems between hosts . Routers work on the network layer .
The router is in at least two LANs （ There are at least two network cards ).
Data can only be in the same LAN The transmission router spans two LAN. therefore , Span LAN The transmission of needs to pass through the router
route (route)： Also known as pathfinding ; Router (router) ： A device used to complete the pathfinding function .

The network layer has its own address ：IP Address
IP The address is a 32 The binary number of bits , Usually divided into 4 individual “8 Bit binary number ”（ That is to say 4 Bytes ), Such as ：01100100.00000100.00000101.00000110.
Usually use “ dotted decimal ” In the way of , namely a.b.c.d In the form of (a,b,c,d All are 0~255 A decimal integer between ). Such as :100.4.5.6.
IP The address is the software address ： Within the same network IP It shouldn't be “ repeat ” Of .IP The address changes at any time .

Transport layer

Transport layer ： City comparison host . The host to host is all connected .
for example ： A person in the city ( process ） Want another person in another city ( process ) communicate .( Interprocess communication )
port (port) 0-65525 Unsigned number of two bytes .
process <-> port , Analogist <-> cell-phone number
Write the port , Corresponding to specific people ,
use express delivery , Write the recipient's phone number to correspond to that person .
A process can have multiple ports ( No conflict )
Only one person can be assigned to a port ( Can't repeat )

Five levels

Important agreement ：
HTTP/HTTPS/DNS         Belongs to application layer
TCP/UDP                       Belongs to the transport layer
IP                                   It belongs to the network layer

application layer

send out ： encapsulation （ You must carry the distribution and unpacking information )
receive ： Unpack / Divide up ( According to the control information of this layer )
send out
receive

Quintuples

stay TCP/IP Agreement , Five tuples are used to identify a network communication ：
1. Source IP： Identify the source host
2. Source port number ： Identify the process of sending data for this communication in the source host
3. Purpose IP： Identify the destination host
4. Destination port number ： Identify the process of receiving data for this communication in the destination host
5. Agreement No ： Identify the data format agreed by both sending process and receiving process