foreword

The basics of computer networks are knowledge that all R&D/operation and maintenance engineers need to master, but they are often ignored.

Today, I will elaborate on the basic learning of computer network, covering TCP/UDP protocol, Http protocol, Socket, etc. I hope you will like it.

1. Computer network architecture

1.1 Introduction

definition

A collection of layers + their protocols for a computer network.

effect

Define the functions that the computer network can accomplish.

1.2 Structure Introduction

There are three types of computer network architecture: OSI architecture, TCP/IP architecture, and five-layer architecture.

OSI architecture: clear concept & complete concept, but complex & impractical;

TCP/IP architecture: Contains a series of network protocols that form the basis of the Internet, is the core protocol of the Internet & is widely used in LANs and WANs;

Five-layer architecture: a fusion of OSI and TCP/IP architecture, the purpose is to learn & explain the principles of computers.

The lower three layers are communication subnets, which are responsible for data transmission;

The upper three layers are resource subnets, which are equivalent to computer systems and complete data processing;

The transport layer connects the preceding and the following.

Detailed introduction to the architecture of TCP/IP

Because the TCP/IP architecture is relatively extensive, it is mainly explained.

OSI architecture in detail

2. TCP protocol _

Transmission Control Protocol, the Transmission Control Protocol

Belongs to the transport layer communication protocol

TCP-based application layer protocols include HTTP, SMTP, FTP, Telnet, and POP3.

3. UDP protocol _

3.1 Definition

User Datagram Protocol, the User Datagram Protocol

Belongs to the transport layer communication protocol

UDP-based application layer protocols include TFTP, SNMP and DNS

3.2 Features

Connectionless, unreliable, message-oriented, no congestion control, the details are as follows:

3.3 Advantages and disadvantages

Advantages: fast

Disadvantage: messages are easily lost (especially when the network is poor)

3.4 Application scenarios (corresponding to application layer protocols)

High communication speed is required

like:

Domain name conversion: DNS protocol

File transfer: FTP protocol

Network management: SNMP protocol

Remote file server: NFS protocol

3.5 Segment format

There are 2 fields in the UDP segment: data field & header field

The following mainly introduces the header (8 bytes, 4 fields)

3.6 Differences between TCP and UDP protocols

4. HTTP protocol _

Introduction

5、Socket

5.1 Introduction

That is, the socket is an intermediate software abstraction layer for communication between the application layer and the TCP/IP protocol family, and it is represented as a programming interface (API) that encapsulates the TCP/IP protocol family

Socket is not a protocol, but a programming call interface (API), which belongs to the transport layer (mainly solves how data is transmitted in the network)

That is: through Socket, we can develop through TCP/IP protocol on the Andorid platform

For users, they only need to call Socket to organize data to conform to the specified protocol, and then they can communicate.

Comes in pairs, a pair of sockets:

Socket ={(IP address 1:PORT port number ) , (IP address 2:PORT port number )}

A Socket instance uniquely represents the communication link of an application on a host

6. Other knowledge

6.1 Enter the url address in the browser ->> the process of displaying the homepage

Open a webpage, which protocols will be used in the whole process

6.2 IP address ( IPv4 address)

definition

A globally unique identifier for each host (or router) connected to the Internet

composition

IP address = 32 bits = network number + host number; that is, IP address::={<network number>, <host number>}

in:

Network number: indicates the network to which the host (or router) is connected. A network number must be unique across the entire Internet.

Host number: mark the host (or router). The network range specified by the network number preceding a host number must be unique.

Different types of IP addresses have different numbers of bytes occupied by the host number & network number; therefore: an IP address is unique within the entire network.

Classification

Traditional IP addresses are classified addresses, which are divided into five categories: A, B, C, D, and E.

The difference is that the number of bytes occupied by the network number & host number is different

Special attention: Among all kinds of IP addresses, some IP addresses are used for special purposes and cannot be used as host IP addresses

6.3 ICMP protocol

definition

Internet Control Message Protocol, the Internet Control Message Protocol

Belongs to the IP layer protocol

Note: The ICMP message is not a high-level protocol, but the data of the IP layer datagram, plus the header of the datagram, to form an IP datagram and send it out

effect

Forward IP packets more efficiently & improve chances of successful delivery

Also allows hosts / routers to report errors & exceptions

Classification

ICMP Error Report Message & ICMP Query Message

main application

PING (packet inter-network detection), Traceroute (track the path of a packet from source to destination, principle = send a series of IP datagrams from source host to destination host)

Below, the process of Ping will be mainly introduced.

6.4 Ping process

definition

Packet InterNet Groper , that is, packet internet detection

It is an important application of ICMP message : using IPCM echo request & echo reply message

It is an example where the application layer directly uses ICMP at the network layer , without TCP and UDP at the transport layer

effect

Test the connectivity of 2 hosts

principle

Send multiple ICMP echo request messages to the destination host

Calculate the round-trip time based on the timestamp in the ICMP echo reply message returned by the destination host

The final displayed results: the IP address sent to the destination host , the number of packets sent & received & lost, the minimum, maximum & average round-trip time