IP Overview

This article refers to "Computer Network" (seventh edition) edited by Xie Xiren, the blog post is for learning use only, used to record notes

Overview

The first thing to note is the difference between the Internet and the Internet and the network (here is not deliberately pointed, but some proper nouns we need to master, in the subsequent statements will not be ambiguous with the basic concepts):
computer network ( network ) by a plurality of nodes and nodes connecting the link components. Nodes can be computers, hubs, switches, routers, etc., and a cloud can be used to represent a network. Computers connected to the network are often called hosts.
Networks can also be interconnected by routers, which constitutes a computer network with greater coverage. Such a network is called the internet, which is a network of networks.

The Internet (Internet) is a special term. It refers to the world's largest, open, and specific interconnection network that is interconnected by many networks. It uses the TCP / IP protocol as the communication rule. Its predecessor is the American ARPANET.
The network connects many computers together, and the Internet connects many networks together through routers. The computer connected to the network is often called the host.
Although the topology of the Internet is very complicated, it can be divided into two major blocks in terms of how it works: the edge part and the core part. The edge part is composed of all the hosts connected to the Internet. This part is directly used by users for communication and resource sharing. The core part is composed of a large number of networks and routers connecting these networks. This part provides services for the edge part (provides connectivity and switching).

The communication methods between the end systems at the edge of the network can usually be divided into: client-server (C / S) and peer-to-peer (P2P) and browser-server (B / S). Among them, the peer-to-peer connection mode requires the host to run the peer-to-peer connection software (P2P software).
The core part of the Internet. The router plays a special role in the core part of the network.The router is the key component to realize packet switching, its task is to forward the received packets, and the working mechanism is store and forward.
In order to clarify packet switching, the basic concepts of switching are introduced below. Switching is to dynamically allocate transmission line resources in a certain way.

• Circuit switching (switch used for telephone communication):
  Before using circuit-switched calls, you must first dial a request to establish a connection. After the called user hears the ringing tone from the switch and picks up the phone, from the calling end to the called end to establish a connection, it is a dedicated physical channel . This connection guarantees the communication resources required by the caller and the called party during the call. These resources will not be occupied by other users during the communication between the two parties. After the call is hung up, the switch will release the dedicated physical channel just used. . This exchange method that must go through the steps of establishing connection-> talking-> releasing connection is called circuit switching. The characteristic is: During the entire time of the call, the two users of the call always occupy the end-to-end communication resources.
• Packet switching
  When using circuit switching to transfer computer data, the transmission efficiency of its lines is often very low, because computer data appears on the transmission line in bursts. Therefore, packet switching is adopted in the computer network.
  Packet switching uses store-and-forward technology to divide a message (usually we call the entire block of data to be sent as a message) into several packets and then transmit them. Packets are also called " packets ", and the packet header Can be called " Baotou ". A packet is a unit of data transmitted on the Internet.
  The most important device in the core part of packet switching is a router. The router is used to forward packets, that is, packet switching . When a router receives a packet, it temporarily stores it, checks its header, looks up the forwarding table, finds the appropriate interface for forwarding according to the destination address in the header, and hands the packet to the next router. In this way, the packet is delivered to the final destination host in a store-and-forward manner. Routers must frequently exchange routing information between each other in order to create and dynamically maintain the forwarding table in the router, so that the forwarding table can be updated in a timely manner when the entire network topology changes.
  It should be noted that packet transmission does not need to occupy the communication resources of an end-to-end link before transmitting data. Only the link on which the packet is transmitted will occupy the communication resources of this link. After the packet arrives at a router, it is temporarily stored, the forwarding table is searched, and then forwarded from a suitable link. Packets are intermittently occupying communication resources section by section during transmission, and also save the overhead of establishing and releasing connections, so the data transmission efficiency is higher. Store-and-forward packet switching actually adopts a strategy of distributing (or dynamically) transmitting broadband in the process of data communication. This is very suitable for transmitting bursty computer data, which greatly improves the utilization rate of communication lines.
  In order to improve the reliability of the packet-switched network, the core of the Internet often uses a mesh topologySo that when there is network congestion or a few nodes, the link fails, the router can flexibly change the forwarding route without causing communication interruption or paralysis of the entire network. Packet switching will cause delay and certain overhead problems.
• Message exchange
  Message exchange is similar to packet exchange, except that the entire message is forwarded at once without dividing the message into groups, resulting in a large delay.
  
  The type of computer network.
• Classified according to the scope of the network. There are wide area network, metropolitan area network, local area network, personal area network and so on.
• Classified according to the users of the network. There are public network and private network.
  Computer network performance (performance index).
• Rate refers to the data transfer rate.
• Broadband, broadband has two different meanings: the bandwidth of the signal (analog signal), the unit is Hertz; in computer networks, bandwidth is used to indicate the ability of a channel in the network to transmit data (a channel in the network per unit time) "The highest data rate that can be passed ").
• Throughput. Throughput represents the actual amount of data that passes through a network in a unit of time. It is often used to measure a network in the real world in order to know how much data actually passes through the network.
• Delay. Refers to the time it takes for data to travel from one end of the network to the other. The delay consists of sending delay, propagation delay, processing delay, and queuing delay. In the total delay, which kind of delay dominates must be specifically analyzed.
• Delay broadband product, the product of propagation delay and bandwidth.
• Round trip time RRT refers to the time required for a two-way interaction, which includes the processing delay of the intermediate node, the queuing delay, and the transmission delay when forwarding data.
• There are two types of utilization rate: channel utilization rate and network utilization rate. Channel utilization indicates that a percentage of the channel is used, and network utilization is the weighted average of the channel utilization of the entire network. The channel utilization rate is not as high as possible.
  Computer network architecture is the focus of learning networks and the most basic. Computer networks use layered thinking to transform complex problems into several smaller local problems, and these smaller local problems are easier to study and deal with.
  In order to exchange data in an orderly manner in a computer network, you must abide by some pre-agreed rules. These rules clearly specify the format of the data exchanged and related synchronization issues . Synchronization in communication is often in a broad sense, with the meaning of timing. These rules, standards or conventions established for data exchange in the network are called network protocols.
  For a very complex computer network protocol, its structure should be hierarchical. We take the file transfer between two hosts as an example to illustrate the concept of hierarchical division.
  
  We can divide the work to be done into three categories. The first type of work is directly related to transferring files. For example, the file transfer application on the sending end should be confident that the file management program on the receiving end is ready to receive and store files. If the file formats used by the two hosts are different, at least one of the hosts should complete the file format conversion. These two tasks can be completed by a file transfer module. In this way, the two hosts can regard the file transfer module as the highest layer.
  However, we do not want the file transfer module to complete the details of all the work, which will make the file transfer module too complicated. A communication service module can be set up to ensure that files and file transfer commands are reliably exchanged between the two systems. In other words, let the file transfer module located above use the services provided by the communication service module below. The reliable service provided by the communication service module can also provide communication services for other modules. For example, the above file transfer module can be replaced with an e-mail module.
  In the same way, we construct a network access module, let this module be responsible for the work related to the details of the network interface, and provide services to the upper layer, so that the above communication service module can complete the task of reliable communication.
  The benefits of layering.
• Each layer is independent. A layer does not need to know how its next layer is implemented, but only needs to know the services provided by the layer through the interface between the layers. Because each layer only implements a relatively independent function, a complex problem that is difficult to handle can be decomposed into several smaller problems that are easier to handle.
• Good flexibility . When any layer changes (modify the service provided by a layer), as long as the interface relationship between the layers remains unchanged, all layers above or below this layer will not be affected.
• The structure can be separated , and each layer can be implemented with the most suitable technology
• Easy to implement and maintain.
• Can promote standardization .
  The various layers of the computer network and the set of protocols are the network architecture . The following briefly explains the architecture with five layers of protocols.
  
  -Application layer. The application layer is the highest layer in the architecture. The task of the application layer is to complete specific network applications through the interaction between application processes. The application layer protocol defines the rules for communication and interaction between application processes . There are HTTP, SMTP, DNS and other protocols. We refer to the data units interacted by the application layer as messages.
• Transport layer. The task of the transport layer is to provide communication between the processes in the two hostsGeneralData transmission service . The application process uses this service to transmit application layer messages. The transport layer has multiplexing and demultiplexing functions. Reuse means that multiple application layer processes can use the services of the lower transport layer at the same time. On the contrary, demultiplexing and multiplexing are to deliver the information received by the transport layer to the corresponding processes in the upper application layer. The transport layer has two protocols, TCP / UDP.
• Network layer. The network layer is responsible for providing communication services for different hosts on the packet-switched network. When sending data, the network layer encapsulates packets or user datagrams generated by the transport layer into packets or packets for transmission. Another task of the network layer is to select the appropriate route, so that the packets transmitted by the source host transport layer can find the destination host through the router in the network .
• data link layer. The data link layer is often referred to as the link layer. When transmitting data on the network, it is always transmitted on a section of a link, which requires the use of a special link layer protocol. The data unit in the link layer is a frame.
• Physical layer. The unit of data transmitted on the physical layer is bits. The physical layer needs to consider how much voltage represents "1" or "0" and how the receiver recognizes the bits sent by the sender. The physical layer also determines how many pins should be connected to the plug of the cable and how each pin should be connected. It should be noted that some physical media (twisted pair, coaxial cable, optical cable, wireless channel, etc.) used for transferring information does not belong to the physical layer.
  
  The architecture of network communication is described above. The TCP / IP structure is relatively simple (only four layers).
  
  It should be noted that the development of technology does not follow the strict OSI layering concept. In fact, the current TCP / IP architecture used by the Internet has sometimes evolved into the following figure, that is, some applications can directly use the IP layer or even the bottom network interface layer.