OSI reference model summary

One 、OSI Reference model — OSI/RM

Open system interconnection reference model
1. The core idea ： layered
2. It is divided into seven floors ： From top to bottom application layer 、 The presentation layer 、 The session layer 、 Transport layer 、 The network layer 、 And the physical layer
Different functions of each layer , Responsible for different areas
① application layer — Provide various application services , Convert abstract language into code

② The presentation layer — Convert encoding to binary

③ The session layer — Maintain the session connection between the network application and the network server

④ Transport layer — Realize end-to-end transmission — Port number — 16 Bit binary structure
Port number ：0-65535 among 0 Reserved port for , therefore , The value of port number is 1-65535.
among ,1-1023 For well-known port number .
example ：http：80
https：443
FTP：20、21 （ File transfer protocol ）
DHCP：67、68
DNS：53
telnet：23
TCP：6
UDP：17
OSPF：89

⑤ The network layer — adopt Ip Address for logical addressing — IP Address is a logical address — SIP, DIP
Get the target IP Address method ：
1. Directly know each other's IP Address
2. Access through domain name ip Address （DNS）
3. Direct access through application sequences
4. radio broadcast — scanning

⑥ Data link layer — Control physical hardware — Binary conversion to electrical signal
（1） Ethernet — MAC Address — 48 Bit binary — Serial numbers burned when all chips leave the factory
1、 The only global
2、 use the same pattern — SMAC（ Source MAC）、DMAC（ The goal is MAC）— adopt MAC Address realizes physical addressing
（2） Get the target MAC Address method — ARP（ Address resolution protocol ）— Get another address from one address
1、ARP working principle ： First , The host sends through broadcast ARP Request package . adopt IP Address request MAC Address .
Because it is a broadcast frame , All devices in the broadcast domain can receive this request . After receiving the packet, the device sends the source of the packet IP
And the source MAC The correspondence of addresses is recorded in the local ARP The cache table , Then look at the request IP Address . If
The request is not local IP Address , Will be discarded directly ; If the request is local MAC Address ,
Is to ARP The reply . after , When sending data again , Priority will be given to local ARP Cache table , If there is something in the table
record , Will be sent directly according to the record . If there is no record , Then send ARP The request for MAC Address .
2、ARP species ：
（1） positive ARP： adopt IP The address for MAC Address .
（2） reverse ARP： adopt MAC The address for IP Address .
（3） free ARP：1、 Detect address conflicts ;
2、 Self introduction. （ On the radio ）
⑦ The physical layer — Process and transmit electrical signals

Two 、TCP/IP Standard model and OSI Seven layer model

TCP/IP The standard model ------ OSI Seven layer model
① application layer ------------- application layer 、 The presentation layer 、 The session layer
② Host to host layer ----- Transport layer
③ Internet layer ---------- The network layer
④ Network access layer ------- Data link layer 、 The physical layer

                      TCP/IP peer model 
                         application layer 
                         Transport layer 
                         The network layer 
                      Data link layer 、 The physical layer                         

①PDU — Protocol data unit

application layer — The data packet
Transport layer — Data segment
The network layer — Data packets
Data link layer — Data frame
The physical layer — Bit stream

② Encapsulation and de encapsulation

encapsulation — The process of adding data at each layer to achieve the purpose of this layer
application layer — The encapsulation of the application layer depends on the protocol working in the application layer
Transport layer — Port number — TCP/UDP
The network layer — IP Address — IP agreement
Link data layer — MAC Address — Ethernet protocol
The physical layer

TCP/IP It can support cross layer encapsulation ,OSI no way
And cross layer encapsulation only occurs when direct connected devices transmit in a short distance
1、 Cross four layer packaging — OSPF（ Routers are directly connected to each other ）
2、 Three or four layer packaging — STP（j Switches are directly connected to each other ）

③MTU — Maximum transmission unit

SOF — Frame header delimiter
DSAP — 1 Bytes — Indicates the upper layer protocol type of data frame reception
SSAP — 1 Bytes — Indicates the upper layer protocol of the data frame transmission source
Control（ It can complete the slicing operation ） — Two modes ：
（1） There is no connection
（2） Establish a reliable LLC conversation .