OSI、TCP/IP（A1）

1、 How computers work

Abstract language （ Such as the language we often use ）--- convert to --- code

code --- convert to --- Binary system

Binary system --- convert to --- Electrical signals >> The essence is to deal with high and low levels

Then process the electrical signal

（ Its operation mode generally follows certain methods >> agreement ）

In some scenes on the network ：

application = service = agreement = standard

OSI Reference model >> Open system interconnection reference model

Seven layer model , Also known as OSI（Open System Interconnection） Reference model , The reference model is the international organization for standardization （ISO） A standard system for interconnection between computers or communication systems . It's a seven story 、 Abstract model body , Not only a series of abstract terms or concepts , It also includes specific agreements .

From top to bottom, the seven layer model is ：

application layer ： The agreement has ：HTTP、 FTP、 TFTP、 SMTP、 SNMP、 DNS、TELNET、 HTTPS、 POP3 DHCP（ Provide various application services , Convert abstract language into code , Provide human-computer interaction interface ）

The presentation layer ： Presentation of data 、 Security 、 Compress . The format is ,JPEG、ASCll、DECOIC、 Encryption format, etc （ Convert the encoding to binary ）

The session layer ： establish 、 management 、 Terminate the conversation . Corresponding host process , Refers to the ongoing session between the local host and the remote host （ Maintain session links between web applications and web servers ）

Transport layer ： Define the protocol port number of data transmission , And flow control and error checking . The agreement has ：TCP UDP, Once the packet leaves the network card, it will enter the network transmission layer （ Realize end-to-end communication ）

The network layer ： Logical address addressing , Realize path selection between different networks . The agreement has ：ICMP、 IGMP、 IP（IPV4 IPV6） ARP、 RARP（ Use IP Address realizes logical addressing between hosts ）

obtain DIP Address method ：

1、 Know the address of the server directly

2、 Access the service area through the domain name

3、 Access the server through the application

4、 radio broadcast

Data link layer ： Establish logical connections 、 Address the hardware 、 Error checking and other functions . Combining bits into bytes and then into frames , use MAC Address access media , To discover but not correct （ Control physical hardware , Convert binary to electrical signal ）

MAC Address ：48 Bit binary structure >>1、 The only global     2、 use the same pattern

obtain DMAC Methods ——ARP agreement —— Address resolution protocol

positive ARP：

First , The host sends... In the form of broadcast ARP request , Based on the known IP Address , request MAC Address . All devices in the broadcast domain can receive request packets , All devices that receive packets will record the source in the packet IP Address and source MAC The correspondence of the address is related to the local ARP The cache table . after , Look at the request IP Address , If it's local IP Address , Will be performed ARP The reply , Local MAC Address inform . If it's not local IP Address , The packet will be discarded directly . after , When sending data again , Give priority to local ARP Cache table , If there are records in the table , Then the information will be sent directly according to the record , If there is no record , Then send ARP Request package acquisition MAC Address .

reverse ARP ：

adopt MAC The address for IP Address

free ARP：

1 Small self introduction     2、 Detect address conflicts

  The physical layer ： establish 、 maintain 、 Disconnect the physical connection （ Handle the transmission of electrical signals ）

TCP/IP Model

TCP/IP Protocol cluster

TCP/IP The standard model --- Four layer model             TCP/IP peer model --- Five layer model

PDU--- Protocol data unit

TCP/IP The model supports cross layer encapsulation , And in the OSI Not in the middle （ Cross layer encapsulation is generally used to send information between short-range direct connected devices , It can improve the forwarding efficiency ）

1、 Cross four layer packaging --- OSPF---89

2、 Span three , Four layer package --- STP