Network planning | [five transport layers and six application layers] knowledge points and examples

One 、 Knowledge point

 

Transport layer

1、 Transport layer  

The transport layer is also called the transport layer , be located application layer And network layer , Is the core of the layered network architecture . The role of the transport layer is based on the services provided by the communication subnet , Provide effective and reasonable transmission services for the application layer . So that high-level users do not have to care about the details of communication subnet implementation and specific quality of service when communicating with each other .

（1） Communication between two hosts is actually Application processes in both hosts communicate .

（2） The transport layer performs error checking and detection for the entire message segment , This is different from checking only the first checksum field IP.

（3） Depending on the application , The transport layer needs to execute different transport protocols to provide reasonable transport services ,TCP A full duplex reliable logical channel is established between transmission entities for applications ,UDP Provides unreliable connectionless services .

（4） The existence of transport layer makes transport service more reasonable and effective than network service .

（5） The transport layer uses a standard primitive set to provide transport services .

Transport layer port ：UDP and TCP Both use ports to communicate with the upper application . A port is a transport layer service access point , Is the interface of the user process . The port is the identification of the application layer process .

The port number has local meaning only , That is, the end slogan is only to identify the processes in the application layer of the computer .

Socket ：IP Combination of address and port number

Provide multiplexing / Split service .

• Multiplexing ( For sending host )︰ From different Socket Collect data blocks , And for each Data blocks Encapsulate the first message , Generate message segment , To the network layer .
• Deliver the data correctly to the target application process .
• Demultiplexing ( For the receiving host )︰ Deliver the data in the message segment to the correct socket .
• Different processes use the same transport layer protocol to transmit data .

 

A reliable agreement ： Use the confirmation mechanism to confirm the transmitted data

 

 

 

application layer

1、 Network application mode

2、 The domain name system DNS

• domain name (Domain Name)： A computer / The name of the computer group
• IP Address ： A network / The logical address of a computer
• IP The address can be understood as the receiving address , The domain name can be understood as the recipient ~

• The domain name system DNS ( Domain Name System ) It's the naming system used by the Internet , A host name with a specific meaning that is easy for people to remember ( Such as www.cskaoyan.com ) Convert to machine-friendly IP Address .
• IP Host name with specific meaning
• Ethernet card corresponds to ip. A host with multiple Ethernet cards , There are many. IP Address .

• 1. Local domain name server ︰ Every Internet service provider ISP, Or a university , Even a department in a University , Can have a local domain name server . When a host sends DNS When querying a request , This query request message is sent to the local domain name server of the host .
• ⒉. Root domain server ∶ At the highest level Domain name server . No matter which local domain name server , As long as you can't resolve a domain name on the Internet , First, help the root domain name server .
• 3. Authorized domain name server ︰ Each host must be at the authorized domain name server registration , It can always Converts the hostname it governs to the name of the host IP Address .
• 4. Top-level domain server ︰ Responsible for the management of all secondary domain names registered in the top-level domain name server . When I received DNS When querying a request , Give the corresponding answer .

• Single DNS name ~ Multiple IP Address /mac Address
• Single DNS name ~ Multiple hosts

• Internet The host that provides customer access on does not have to have a domain name .
• The same domain name may resolve different characters at different times IP Address .
• Multiple domain names can point to the same host IP Address .
• IP The host in the subnet can maintain its mapping by different domain name servers .

2、

• The server is dedicated to completing certain services , The client is the consumer of these services .
• The client is usually on the front end , Servers are usually located on the back end .
• The client and server realize the collaborative computing task through the network .
• The client is user oriented , The server is task oriented .

3、FTP A working model

The client port is optional , The server port is fixed .

When the connection ： Control first 、 Post data

When disconnected ： Data first 、 Rear control  

 

Whether to use TCP20 The establishment of data connection by port is related to the transmission mode ：

1. Use it in an active way TCP20 port
2. The passive mode is determined by the server and the client through negotiation ( port >1024)

 

FTP Provide the following functions ︰

1） Provide different kinds of host systems ( hard 、 Software architecture can be different ) File transfer capability between .2） Provide user access to remote control in the way of user rights management FTP File management capabilities on the server .3) Anonymity FTP To provide the ability of public file sharing .

notes ：FTP Not suitable for sharing read-write files between two computers .

 

3、“ anonymous ”FTP The server

A large part of the Internet FTP The server is called “ anonymous ”(Anonymous）FTP The server .

The purpose of this kind of server is to provide file copy services to the public , The user is not required to register with the server in advance , You don't have to get FTP Authorization of the server .

Anonymous（ anonymous FTP ） It enables users to establish a connection with the remote host and copy files from the remote host anonymously , Instead of being a registered user of the remote host .

The user uses a special user name “anonymous” Sign in FTP service , You can access the files exposed on the remote host .

  A service that uses a web browser to read or send e-mail （Gmail,Hotmail）——HTTP agreement

Between different mail servers ——SMTP agreement

 

 

 

 

 

 

Common port numbers

•  
• Port of 2 A role ： Judgment agreement 、 Determine whether it's a client or a server
• The client port is optional , The server port is fixed ( It can be determined whether the grouping is from client to server or vice versa )

  encapsulation —— From top to bottom

 

---

 

 

 

 

  Two 、 Example

 TCP Provisions of the agreement HTTP(  ) The port number of the process is 80.

A． Customer    B． analysis    C． The server    D． host

 【 analysis 】

Client access Web Server's HTTP The port number of the process is dynamically allocated by the operating system of the client . 

The range of hosts is too wide , It includes A and C.

 【 answer 】C

 

---

 

3、 ... and 、408 The real question

 

【2020 The true theme of the year 】 Suppose that the local domain name server in the network shown in the following figure only provides recursive query service , Other domain name servers only provide iterative query services ; Host access in LAN Internet Round trip time of each server on （RTT） Are all 10ms, Ignore other delays , If the host H Through hyperlinks http://www.abc.com/index.html, Request to browse plain text Web page index.html, From clicking the hyperlink to the browser receiving index.html Up to page , Minimum required 、 The longest time is ：

A、10ms,40ms B、10ms,50ms C、20ms,40ms D、20ms,50ms

 【 analysis 】UDP No connection needed .