One . Common network programming interview questions

1. sketch OSI Seven tier protocol .

OSI Is an open communication system interconnection reference model .OSI The model has 7 The layer structure , Each layer can have several sub layers . OSI Of 7 The layers from top to bottom are 7 application layer 6 The presentation layer 5 The session layer 4 Transport layer 3 The network layer 2 Data link layer 1 The physical layer ; Among them, the top （ namely 7、6、5、4 layer ） Defines the capabilities of the application , below 3 layer （ namely 3、2、1 layer ） Mainly for end-to-end data flow through the network .

2. sketch TCP/IP Four layer protocol .

application layer ：HTTP
Transport layer ：TCP
The network layer ：IP
The link layer ： The Internet

application layer ： The type of communication this time determines , for instance FTP、DNS、SMTP wait , meanwhile HTTP Protocols also fall into the scope of the application layer . Popular speaking , The application layer decides what to do with this communication .

Transport layer ： The transport layer provides data transfer between two computers , There are two very popular protocols in the transport layer , Namely TCP and UDP agreement . Three handshakes often mentioned in an interview , Four waves are TCP Part of the agreement

The network layer ： The network layer is used to process these flowing packets , That is, if the corresponding packets are routed to the specified location , Select transmission route for network transmission during communication

Data link layer ： The data link layer contains the interface between software and hardware , And the hardware of various network devices , That is, the infrastructure at the bottom of the whole network communication process

3. TCP and UDP What's the difference ?

4. TCP When the connection is established 3 The specific process of handshake , And why every step of it is ?

Three handshakes
TCP The connection is made by three handshakes .

The first handshake

When a client initiates a connection request to the server , The client will send the synchronization sequence label SYN To the server , Here we set up SYN by x, Wait for server to confirm , At this time, the status of the client is SYN_SENT.

The second handshake

When the server receives the client sent SYN after , What the server needs to do is to confirm that the client sent it SYN, Here the server sends a confirmation packet ACK, there ACK by x+1, mean “ I received your message SYN 了 ”, meanwhile , The server will also send a SYN package , Here we set SYN by y. At this time, the status of the server is SYN_RECV.
In a word , Server side send SYN and ACK Two bags .

The third handshake

The client receives... From the server SYN and ACK After package , A confirmation packet needs to be sent to the server ACK,“ I also received your message SYN 了 , I'll send you a confirmation , And then we can fit together ”, there ACK by y+1, After sending , The client and server status is ESTABLISH, namely TCP Successful connection .

In three handshakes , Both the client and server send two packets SYN and ACK, But the two packages on the server side are sent at one time , The two packets of the client are sent in two times .

5. TCP The specific process of disconnection , Each of these steps is why you do that ?

Disconnect requests can be made by the client , It can also be sent by the server , Here we call A To the end B End request to disconnect .

First wave

A To the end B When a client requests to disconnect, it will ask B The client sends a message with FIN Marked segment , there FIN yes Finish It means .

Second wave

B End receipt A Sent FIN after ,B Now, maybe there is still data to be transmitted , therefore B The end doesn't go to A End send FIN, Instead, send a confirmation number first ACK, mean “ I received your request for disconnection , But I still have data to send out , Just a moment, please ”.

Third wave

When B The end of the business , So at this time B The end can be disconnected , here B To the end A End send FIN Serial number , It means you can disconnect this time .

Fourth wave

A End receipt B Sent by FIN after , Will send to B End send confirmation ACK, And then through two MSL Disconnect after a long time .

MSL yes Maximum Segment Lifetime, Maximum segment lifetime ,2 individual MSL It is the longest time that a message segment is sent and received

6. TCP Eleven states of ?

state ：
service / client ：CLOSED（ In the beginning , Both the server and the client are down ）
Customer ：SYN_SENT（ Send a connection request ）,
service ：LISTEN（ monitor , Waiting for the connection ）,
service ： SYN_RECV（ Upon receipt of the request , reply ）,
service / Customer : ESTABLISHED（ Establishing a connection ）,
Customer ： FIN_WAIT_1（ Disconnect waiting 1）,
service ：CLOSE_WAIT（ Turn off waiting , Transfer the data that has not been transmitted ）,
client ： FIN_WAIT_2（ received ACK, Wait for the data transfer to complete ）,
service ： LAST——ACK（ Send a shutdown request ）,
Customer ： TIME_WAIT（ Reply to disconnect request ）,
Customer ： CLOSING（ Something happened to the client FIN, But I didn't receive the server's ACK, But I received the server's FIN. This happens when the server sends a packet loss , Because sometimes there are accidents in network transmission ）

7. What is? socket? The brief is based on tcp Protocol socket communication process .

socket( Socket for short ) It's interprocess communication ⼀ Kind of ⽅ type , It can realize inter process communication between different hosts , We ⽹ All kinds of services are available ⼤ Most of them are based on Socket To complete the communication .
Communication process ：

1. Server first socket Function to create a socket , Use this socket to monitor the communication .
2. use bind Function to bind a port number and IP Address . Because the local computer may have multiple URLs and IP, every last IP And ports have multiple ports . One needs to be specified IP Monitor with port .
3. Server calls listen function , Make this port of the server and IP In a listening state , Wait for the client to connect .
4. For the client socket Function to create a socket , Set remote IP And port .
5. The client calls connect Function to connect to the port specified by the remote computer .
6. The service is used accept Function to accept connections from remote computers , Establish communication with clients .
7. After establishing the connection , For the client write Functional direction socket Middle write data . It can also be used. read Function to read the data sent by the server .
8. The service is used read Function to read the data sent by the client , It can also be used. write Function to send data .
9. After completing the communication , use close Function off socket Connect .

8. The brief is based on udp Protocol socket communication process .

Communication process ：
udp There is no need to establish a connection between the client and the server , So there is no process of establishing a connection in the program . Before you communicate , Need to set up a network socket .
The server needs to bind a port , Listen for received messages on this port .
The client needs to set up remote IP And port , The information that needs to be delivered needs to be sent to this IP And ports .

9. TCP Why not two connections ? It's three handshakes ?

For reliable data transmission , TCP The parties to the agreement , You have to maintain a serial number , To identify the packets sent , What has been received by the other party .
The process of three handshakes is that the communication parties inform each other of the starting value of the serial number , And confirm that the other party has received the start value of the serial number .
If it's just two handshakes , At most, only the starting serial number of the connection initiator can be confirmed , The serial number chosen by the other party is not confirmed .

10. Why based on tcp Protocol communication is based on udp Protocol communication is more reliable ?

TCP A reliable guarantee of , It's a three way handshake mechanism , This mechanism ensures that the data , Guaranteed his reliability . and UDP There is no the ,udp When the message is sent , Don't verify whether it reaches the other party , So it's not reliable .

11. Design concurrent server in network programming , Using multiprocessing and multithreading , What's the difference, please ?

The fundamental difference is ： With multiple processes, each process has its own address space (address space), Threads share address space . All the other differences come from this ：

1. Speed ： Fast thread generation , The communication between threads is fast 、 Switch quickly and so on , Because they're in the same address space .
2. Resource utilization ： The resource utilization of threads is better because they are in the same address space .
3. Synchronization problem ： Threads use common variables / Do I need to use synchronization mechanism when I use memory or because they are in the same address space .

Advantages of multithreading ：

1. The cost of creating and environment switching is low
2. Synchronization costs less （ Synchronization between processes usually involves OS kernel ）
3. Data replication can be done by “ Process local memory ”

The limitations of multithreading ：

1. Performance loss
   （1） Single processor 、“ Computing is heavy ” Applications that don't benefit from multithreading
   （2） High precision locking strategy will bring high synchronization cost
2. Less robust
   Received between threads “MMU Protect ” Little or no
3. Lack of high precision access control

12. Connect on a large scale , How to design concurrency model ?

Large scale connectivity , First, exclude multiprocessing implementations , Large scale connections using multiple processes require a lot of memory space
Consider using a multithreaded server or this coprocessor to implement a concurrent server .

• Coprocessor to achieve server advantages :
  1. No thread context switching overhead
  2. There is no need for atomic locking and synchronization
  3. Easy to switch control flow , Simplify the programming model
  4. High concurrency + High scalability + Low cost ： One CPU Supporting tens of thousands of projects is not a problem . So it is very suitable for high concurrency processing .

• shortcoming :
  1. Can't use multi-core resources ： The essence of a process is a single thread , It cannot at the same time Single CPU For multiple cores , The process needs to work with the process to run in multiple CPU On . Of course, most of the applications we write in our daily life don't have to , Unless it is cpu Intensive application .
  2. To block （Blocking） operation （ Such as IO when ） It will block the whole program

Two . Common network programming written test questions .

1. A
   
   analysis ：TCP/IP The transport layer of the agreement is TCP and UDP Two agreements .

2. D
   analysis ： Convert a domain name into something that the network can recognize ip Address , And then the actual network communication .

3. B
   
   analysis ： The connectionless system is UDP Service delivery , Just throw out the byte stream

4. C
   
   analysis ：IP The address is expressed in dotted decimal system

5. D
   
   analysis ：OSI The data units exchanged between layers are called service data units SDU.

6. B
   
   analysis ：TCP It's a unicast two-way service

7. A
   
   analysis ：TCP The protocol uses three handshakes to connect

8. C
   
   analysis ： The so-called port , It's like a house number , The client can use the ip Address to find the corresponding server , But there are many ports on the server side , Each application corresponds to a port number .

9. E
   analysis ： Fight for the TCP The handshake process is ：SYN,SYN+ACK,ACK

10. C
    
    analysis ：OSI The application layer is from the top to the bottom of the seven layers , The presentation layer , The session layer , Transport layer , The network layer , Data link layer , The physical layer .

11. C
    
    analysis ：ACK It should be transmitted from SYN+1 namely ACK=x+1,SYN unchanged ,Seq by y

12. D
    
    analysis ： The network is classified as follows
    

13. D
    
    analysis ：RARP agreement : The physical address can be converted to IP Address
    ARP agreement : Can be IP Address translation to physical address

14. C
    
    analysis ：Netstat: Shows which network connections are working . Default display TCP Connected information .
    ping: Check network connectivity , It can help us analyze and judge the network fault
    Ipconfig: Show IP Address 、 Subnet mask and default gateway values .
    Nslookup: Inquire about DNS The record of , Check whether domain name resolution is normal , It is used to diagnose network problems when the network fails .

15. C
    
    analysis ： The main elements are generally understood : How to say it , What to say , When to talk