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1. Composition of the Internet
From the point of view of working methods, the Internet is divided into the following two parts:
(1) Edge part : It is composed of all hosts connected to the Internet. This part is directly used by users for communication and resource sharing .
(2) Core part : It consists of a large number of networks and routers connecting these networks. This part serves the edge part ( connectivity and switching)
2.1.3.1 The Edge of the Internet
1. The concept of communication
The edge of the Internet refers to all hosts connected to the Internet, also known as end systems , ranging from personal PCs to servers.
The communication between host A and host B actually refers to the communication between a program running on host A and another program running on host B, so the communication between computers refers to a certain process of host A and the host Another process of B communicates.
The communication methods between end systems are usually divided into two categories: client-server mode (C/S mode) and peer-to -peer mode (P2P mode)
2. Client-server approach
Both client (client) and server (server) refer to the two application processes ( in the application layer ) involved in the communication . The client-server approach describes the relationship between the service and the serviced process.
As shown in the figure below, host A runs a client program, and the host runs a server program, so host A is a client, host B is a server, A sends a request service to B, and B provides a request to A.
The client is the service requester and the server is the service provider
Features of the client program :
(1) After the client runs the client program, it will automatically send a service request to the server, so the IP address of the server program must be known. (2) Features of server programs
that do not require special hardware and complex operating systems : (1) Programs dedicated to providing certain services can handle multiple remote or local customer service requests at the same time. (2) After the system is started, it has been running, passively waiting and accepting service requests from customers from all over the world, so there is no need to know the address of the client program. (3) It requires powerful hardware and complex operating system support.
When the client establishes a connection with the server, the communication between the two is bidirectional.
3. P2P mode (peer-to-peer mode)
It does not distinguish which is the server requester and which server provider. As long as the two hosts are running peer-to-peer connection software (P2P software), peer-to-peer connection communication is possible. In fact, the peer-to-peer connection method still uses the client-server method in essence, but each host in the peer-to-peer connection is both a server and a server.
1.3.2 The Core Parts of the Internet
The core part of the Internet provides connectivity between hosts, and the core part is a router whose core task is packet switching and forwarding of received packets.
The concept of switching
: according to a certain way
to dynamically allocate the resources of the transmission route
. During the entire time of the call, the two users in the call always occupy end-to-end communication resources.
If circuit switching is used to transmit computer resources, the transmission efficiency is very low, because computer data appears on the transmission line in bursts. So circuit switching is better for making calls.
Packet switching :
Packet switching generally uses store-and-forward technology. Usually, the entire block of data to be sent is called a message. Before sending a message, the message is divided into smaller equal-length data segments. Each data segment A header (necessary control information) is added in front to form a packet and also become a packet. A packet is a unit of data transmitted on the Internet. The header in the packet contains important control information such as source address and destination address.
The core part of the Internet consists of networks and routers that interconnect them, and hosts are at the edge of the Internet. Routers are connected by high-speed links, while hosts are connected by relatively low-speed links.
Although hosts and routers are both computers, they function differently.
The role of the host: to facilitate the user to process information.
The role of the router: to forward the received packets and perform packet switching.
According to the above figure, explain how the router performs packet switching.
Suppose host H1 sends data to host H5. The specific process is as follows:
the host H1 first divides the data into packets, and then sends them to the router A closest to it. At this time, only the communication resources of the link H1-A are occupied.
Router A buffers the received packet. Then look up from the forwarding table, the packet should be forwarded to the link AC, so the packet is sent to the router C. Similarly, only when the packet is transmitted on the AC link, the link resource of AC is occupied, and it will not Occupy other link resources in the network.
Router C continues to look up the table, and finally sends it to host H5.
Summarize the way the router forwards the packet:
1. Cache the received packet
2. Look up the forwarding table to find out which port to forward the destination address from
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. When packets are transmitted, they will occupy communication resources one by one, and save the overhead of establishing and releasing connections, so the data transmission efficiency is higher. high.
Advantages of packet switching:
1. High efficiency: During the transmission of packets, the transmission bandwidth is dynamically allocated, and the communication link is occupied segment by segment.
2. Flexible: For each packet, select the most suitable forwarding route
3. Fast: Send packets without establishing a connection
4. Reliable: Ensure reliable network protocols, and the core part of the Internet adopts a mesh topology.
Problems of packet switching :
1. Packets need to be queued when they are stored and forwarded in the router, which will cause time delay.
2. Because there is no connection established to guarantee the resources required for communication, the end-to-end bandwidth for communication cannot be guaranteed.
3. The control information carried in the header of each packet causes a certain overhead and requires a special management and control mechanism.
Packet exchange : The principle of store and forward is also adopted, but this method is to forward the entire message without grouping, so the delay caused is also greater.
Comparison of the three exchange methods:
Summarize
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