What are the basic knowledge points of computer network

Ⅰ. Division of network levels

In order to establish computer networks on a larger scale, the International Organization for Standardization (ISO) In 1978, the "Open System Internet Reference Model" was proposed, the famous OSI (Open System Interconnection) model.

## In addition to the standard OSI seven-layer model, the common network layer divisions include the TCP/IP four-layer protocol. The corresponding relationship between them is as follows:

           

 

Ⅱ.OSI seven-layer network model

Whether it is the OSI seven-layer model or the TCP/IP four-layer model, each Each layer must have its own exclusive protocol to complete its corresponding work and communicate between the upper and lower layers. Let’s start with the detailed description of OSI’s seven-layer model:

⑴Physical layer

The physical layer is the most basic network structure, which is composed of various devices. This layer provides a reliable physical medium for transmitting data to the upper layer protocol. Simply put, the physical layer ensures that the original data is transmitted on various physical media.

⑵Data link layer

The data link layer provides services to the network layer on the basis of the physical layer. Its most basic service is to Data originating from the network is reliably transmitted to the target machine network layer of adjacent nodes.

The functions of this layer include: physical address addressing, data framing, flow control, data error detection and retransmission, etc.;

Main protocol : Ethernet protocol;

Important equipment: bridges, switches;

⑶Network layer

The purpose of the network layer is to realize data transmission between two end systems;

There are many protocols designed in the network layer: including the core protocol of the most important protocol TCP/IP ——IP protocol. The IP protocol only provides unreliable, connectionless transmission services. Also used in conjunction with the IP protocol to implement its functions are the Address Resolution Protocol ARP, the Reverse Address Resolution Protocol RARP, the Internet Message Protocol ICMP, and the Internet Group Management Protocol IGMP.

Important equipment: router

⑷Transport layer

The first end-to-end , host-to-host level; the transport layer is responsible for segmenting upper-layer data and providing end-to-end, reliable or unreliable transmission. In addition, the transport layer must also deal with end-to-end error control and flow control issues;

Mainly included protocols: TCP protocol, UDP protocol;

Important equipment: Gateway

⑸ Session layer

The session layer manages the session process between hosts, that is, it is responsible for establishing, Manage and terminate sessions between processes. The session layer also uses checkpoints inserted into the data to achieve data synchronization.

⑹Presentation layer

The presentation layer transforms the upper layer data or information to ensure that the application layer information of one host can be understood by the application program of another host. Data conversion at the presentation layer includes data encryption, compression, format conversion, etc.

⑺Application layer

Provides an interface for the operating system or network application to access network services.

Ⅲ.IP address

The IP address consists of two parts: the network bit and the host bit

Network part: used to describe the network range where the specified host is located

Host part: used to describe the specific location of the specified host in a specific network range;

⑴IP address classification: (This refers to the IPv4 address defined by the IPv4 protocol. The entire address is composed of 32-bit binary)

Class A: The first binary bit is 0, the network bit is 8 bits; 0.0.0.0~127.255.255.255

Class B: The first two binary bits are 10, the network bit is 16 bits; 128.0 .0.0~191.255.255.255

Class C: The first three binary bits are 110, and the network bit is 24 bits; 192.0.0.0~223.255.255.255

Class D: 224-239

Class E: 240-255

Class D and Class E do not divide network bits and host bits

⑵A, B, C Class private address

In order to solve the problem of IP address exhaustion, increase the number of times the IP address is taken; it can be reused, and the Internet cannot route it

Class A: 10.0.0.0/8, the range is: 10.0.0.0~10.255.255.255

Class B: 172.16.0.0/12, the range is: 172.16.0.0~172.31.255.255

Class C: 192.168.0.0/16, the range is 192.168.0.0~192.168.255.255

⑶ Reserved address (loopback address), cannot be Select the IP address used for configuration of other hosts,

0.0.0.0~0.255.255.255

127.0.0.0~127.255.255.255

⑷Automatically assign address segment:

169.254.0.0~169.254.255.255

⑸Network address: an address where the host bits are all 0 (All binary bits are 0)

For example: 1.0.0.0 represents the name of a range

⑹Broadcast address: An address with all host bits set to 1

For example: 1.255.255.255: As the destination address, it represents all IP addresses in the entire network segment

⑺Directed broadcast address ：

255.255.255.255: As the destination address, it represents all IP addresses in the entire IP address stack

IV. Division of subnet masks

With the continuous expansion of Internet applications, the disadvantages of IPv4 are gradually exposed, that is, too many network bits are occupied, and too few host bits are used, so the host addresses it can provide are becoming increasingly scarce. Currently, in addition to In addition to using NAT to allocate reserved addresses within the enterprise, a high-class IP address is usually subdivided to form multiple subnets for use by user groups of different sizes.

Subnetting is actually to increase the number of network bits in the IP address and reduce the number of host bits; in this way, the scope of the broadcast domain can be reduced, the number of hosts in the logical network segment can be reduced, and management can be facilitated and precise application of security policies;

Ⅴ.TCP/IP protocol

The TCP/IP protocol is the most basic protocol of the Internet, which is composed of the network layer The IP protocol is composed of the TCP protocol at the transport layer;

TCP is a connection-oriented communication protocol. A connection is established through a three-way handshake. When the communication is completed, the connection must be dismantled. Since TCP is connection-oriented, it can only be used for end-to-end communication. TCP provides a reliable data flow service. TCP uses a method called "sliding window" for flow control. The so-called window actually represents the receiving capability and is used to limit the sending speed of the sender.

TCP message header format:

16-bit source port

16-bit destination port number

32-bit sequence number: range (1 ~ 2^32-2) SEQ

The first data The sequence number of the segment is randomly selected;

The sequence number of the second data segment: The sequence number of the previous data segment The data part size of the previous data segment 1

3 2 confirmation number: range (2 ~ 2^32-1)

This receiver used to confirm the data that has been received , and requires the sender to continue transmitting the identification of subsequent data segments;

Generally speaking, the confirmation number is the sequence number of the next data segment to be sent by the sender;

4 digits The header length is 24 bytes to 60 bytes

3-digit reserved flag

3 Certification encryption logo seat

# 6 -bit TCP Features Bit:

URG: Emergency Poor Labeling Bit; if this logo position 1, the repost of the data The priority will be increased so that such data is forwarded first; if the URG flag bits of multiple data are set to 1 at the same time, the larger the 16-bit emergency pointer is, the higher the priority will be;

ACK: Confirmation flag bit related to connection management; used to respond to the party that actively initiates a connection establishment request or teardown request;

PSH: push bit, if the flag is set to 1, the receiving party can The data is not added to the cache queue and is directly handed over to the application process for processing;

RST: Reset the connection flag; when the TCP connection is exhausted or fails, the flag of the TCP connection is rebuilt;

SYN: Synchronization flag bit, used to signal the party that initiates a connection establishment request to initiate a connection during the process of establishing a TCP connection;

FIN: End the connection logo position. If the logo position 1, the other party will guide the TCP connection will be dismantled and confirm it; Implement traffic negotiation and control to prevent network congestion;

Sliding window:

Congestion window:

Emergency window:

Window size , meaning the number of data segments we can transmit at one time;

16-bit data segment checksum: verification information to ensure the integrity of the data;

16-bit emergency pointer: used to distinguish its priority when the URG flag bits are all set to 1;

Option: timestamp of data segment segmentation

Connection-oriented features of TCP protocol:

1. Connection establishment; three-way handshake

1) Generated by the sender A TCP header data. In the header, the source and destination ports are given by the application layer protocol; the sequence number is randomly selected, the confirmation number is 0, and the flag SYN is set to 1;

2) The receiver receives the After sending the SYN request data from the sender, it determines whether it can complete the data communication required by the other party; if it can, it returns a TCP header data generated by it; the source and destination ports are exactly the opposite of the previous data; the sequence number is random , the confirmation number is the sequence number of the other party's next data, and the SYN and ACK flags are set to 1 at the same time;

3) After the sender accepts the other party's response data, it checks whether its ACK flag is set to 1 ;If it is 1, check whether the SYN flag is set to 1. If it is also 1, confirm it; then generate the second TCP header data; the sequence number is the sequence number 1 of the previous data, and the confirmation number is the next data of the receiver. Serial number; ACK flag position 1;

2. Dismantle the connection and wave four times

1) When all data transmission is completed, one party will take the initiative to send the signal to the other party Send a TCP header data with the FIN flag set to 1;

2) After the other party receives such data with the FIN set to 1, it responds with a confirmation data with ACK set to 1

3) The other party Actively send a data with FIN set to 1, requesting to tear down the connection

4) The active end party confirms and sends TCP header data with ACK set to 1;

VI.UDP Protocol

UDP user data protocol is a connectionless communication protocol. UDP data includes destination port number and source port number information. Since communication does not require a connection, broadcast transmission can be achieved. UDP communication does not require confirmation from the receiver, it is an unreliable transmission, and packet loss may occur;

UDP and TCP are on the same layer;

UDP header:

16-bit source port

: The port number selected by the sender when encapsulating data. Generally speaking, the source port of the data sent by the client The number is a randomly selected idle port

1

6-digit destination port: The receiver of this data communication must transmit data from the transport layer to the application layer The port number used; generally speaking, the destination port number of the data is fixed; 16-bit UDP length

: The length of the entire datagram, including the header;

1

6-digit UDP checksum: The checksum of the entire UDP datagram, ensuring data integrity to a certain extent.

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