Chapter 1 Overview - Section 1 - 1.3 composition of the Internet

One 、 The composition of the Internet

The Internet works in a way , It is divided into the following two parts ：
（1） Fringe part ： Consists of all hosts connected to the Internet , This part is directly used by users , For communication and Resource sharing .
（2） The core part of the ： It consists of a large number of networks and routers connecting these networks . This part provides services for the edge part （ Connectivity And switching ）

Two 、1.3.1 The edge of the Internet

1. Concept of communication

The edge of the Internet refers to all hosts connected to the Internet , Also known as End system , Small to personal PC, As big as the server .
host A With the host B Actually, the communication between them refers to running on the host A A program on and running on the host B To communicate with another program on the , So the communication between computers refers to the host A A process and host of B Communicate with another process .
The communication modes between end systems are usually divided into two categories ： Customer - Server mode （C/S The way ） and The equivalent way （P2P The way ）

2. Customer - Server mode

Customer （client） And the server （server） Both refer to the two application processes involved in communication （ In the application layer ）. Customer - The server mode describes the relationship between services and served processes .
Here's the picture , host A Run the client program , The host runs the server program , So the mainframe A It's the customer , host B Is the server ,A towards B Send request service ,B towards A Provide a request .

The customer is the service requester , Servers are service providers

Features of the client program ：
（1） After the client runs the client program , Automatically send service requests to the server , Therefore, you must know the server program IP Address .
（2） No special hardware or complex operating system is required
Characteristics of server program ：
（1） A program designed to provide a service , It can handle service requests from multiple remote or local customers at the same time .
（2） After system startup , It has been running , Passively wait and accept service requests from customers everywhere , Therefore, there is no need to know the address of the client program .
（3） It requires powerful hardware and complex operating system support .

After the client establishes contact with the server , The communication between the two is bidirectional .

3.P2P The way （ The equivalent way ）

Do not distinguish which is the server requestor , Which server provider . As long as both hosts are running peer-to-peer connection software （P2P Software ）, You can connect and communicate peer to peer . In fact, peer-to-peer connection is still using customers in essence - Server mode , Only each host in the peer-to-peer connection is both a server and a server .

1.3.2 The core of the Internet

The core of the Internet provides connectivity between hosts , The core part is the router , Its core task is packet switching , Forward received packets .
The concept of exchange ： Dynamically allocate transmission route resources in a certain way
Circuit switching ：
“ Establishing a connection （ Occupy communication resources ）”–“ conversation （ It always takes up communication resources ）”–“ Release the connection （ Return of communication resources ）”
characteristic ： For the whole time of the call , The two users on the call always occupy the end-to-end communication resources .
If circuit switching is used to transmit computer resources , The transmission efficiency is very low , Because computer data appears suddenly on the transmission line . So circuit switching is more suitable for making phone calls .
Packet switching ：
Packet switching generally adopts store and forward technology , Generally, the whole block of data to be sent is called message （message）, Before sending the message , Divide the message into smaller equal length data segments , Add a header before each data segment （ Necessary control information ）, Constitute the grouping （packet）, Also become a package . Packets are data units transmitted over the Internet . The first part of the packet contains the source address , Important control information such as destination address .

The core of the Internet consists of networks and routers that interconnect them , The host is at the edge of the Internet . Routers are connected by high-speed links , The host is connected with a relatively low rate link .

Although the host and router are computers , But their roles are different .
The role of the host ： It is convenient for users to process information .
What routers do ： Forward received packets , Perform packet switching .

According to the above figure, explain how the router performs packet switching .
Hypothetical host H1 Host computer H5 send data . The specific process is as follows ：
host H1 First group the data , Then send it to the router closest to it A, here , Only the link is occupied H1-A Communication resources .
Router A Put the received packets into the cache . Then search from the forwarding , Packets should be forwarded to the link A-C, So the packet is sent to the router C, Empathy , Only when the group is in A-C When transmitting on the link , Just occupied A-C Link resources , And it will not occupy other link resources in the network .
Router C Continue to look up the table , Finally, it is transmitted to the host H5.
Summarize the way the router forwards packets ：
1. Cache received packets
2. Look up the forwarding table , Find out which port to forward to the destination address
3. Forward the packet from the port

Packet switching will not occupy communication resources before sending data , And it will not occupy communication resources all the time , Packets occupy communication resources segment by segment during transmission , And it saves the cost of establishing connections and releasing connections , So the data transmission efficiency is higher .

The advantages of packet switching ：
1. Efficient ： The packet is in the process of transmission , Dynamically allocate transmission bandwidth , Occupy the communication link section by section .
2. flexible ： For each group , Choose the most appropriate forwarding route
3. rapid ： Send packets without establishing a connection
4. reliable ： Reliable network protocol , The core part of the Internet adopts a mesh topology .
The problem of packet switching ：
1. Packets need to be queued when they are stored and forwarded in the router , It will cause time delay .
2. Because there is no connection to ensure the resources required for communication , Therefore, the end-to-end bandwidth during communication cannot be guaranteed .
3. The control information carried by the header of each packet causes some overhead , Special management and control mechanisms are needed .

Message switching ： It also adopts the principle of store and forward , But this way is to forward the whole message , No grouping , Therefore, the delay is also greater .

Comparison of three exchange methods ：

summary

Tips ： Here is a summary of the article ：
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