This article will share with you some of ByteDance’s favorite front-end interview questions about computer networks. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.
Note: The (xx) number appearing in front of each question represents the frequency of this question. This computer network foundation is based on 30 front-end Questions and corresponding answers, reference links, etc. compiled from the interview. The content of the article is compiled by the person who got the Offer.
HTTP cache is divided into strong cache and negotiation cache:
First verify whether the strong cache is available through Cache-Control. If the strong cache is available, then read the cache directly.
If not, then enter the negotiation cache phase and initiate an HTTP request. , the server checks whether the resource is updated by including the conditional request fields If-Modified-Since and If-None-Match in the request header:
If the resource is updated, then the resource and 200 are returned Status code
If the resource has not been updated, tell the browser to directly use the cache to obtain the resource
301 is similar, it will jump to a new website, but 301 represents that the resource of the accessed address has been permanently removed. This address should not be accessed in the future. Search engines will also use the new one when crawling. Replace this old one with the address. The returned address can be obtained from the location header of the returned response. The scenario of 301 is as follows:
(3 ) Q: What is HTTPS? Specific process
The browser transmits a client_random and encryption method list. After the server receives it, it passes it to the browser a server_random, encryption method list and digital certificate (including the public key), and then the browser performs legal verification on the digital certificate. , if the verification is passed, a pre_random is generated, and then encrypted with the public key and transmitted to the server. The server uses client_random, server_random and pre_random to encrypt the secret using the public key, and then uses this secret as the secret key for subsequent transmissions to encrypt and decrypt data. .
(4) Question: Three-way handshake and four-way waveThree-way handshakeThree-way handshake main process:
At the beginning, both parties are in CLOSED state, and then the server starts to listen to a certain port and enters the LISTEN state
Then the client actively initiates a connection, sends SYN, and then It becomes SYN-SENT, seq = x
After the server receives it, it returns SYN seq = y and ACK ack = x 1 (for the SYN sent by the client), it becomes After becoming SYN-REVD
, the client sends ACK seq = x 1, ack = y 1 to the server again, and becomes EASTABLISHED. When the server receives ACK, it also enters ESTABLISHED.
SYN requires peer confirmation, so the serialization of ACK needs to be increased by one. Anything that requires peer confirmation will consume one point of TCP message serialization
Why not twice?
Unable to confirm the client's receiving capabilities.
If the client sends a SYN message first, but it stays in the network, TCP will think that the packet has been lost, and then retransmit, and the connection will be established with two handshakes.
Wait until the client closes the connection. But if the packet arrives at the server later, the server will receive it, then send the corresponding data table, and the link will be established. However, the client has closed the connection at this time, resulting in a waste of link resources.
Why not four times?
More than four times are fine, but three times is enough
Waving four times
It is in the ESTABLISH state at the beginning, and then the client sends a FIN message with seq = p, and the state changes to FIN-WAIT-1
Server After receiving it, send ACK to confirm, ack = p 1, and then enter the CLOSE-WAIT state
After the client receives it, it enters the FIN-WAIT-2 state
After a while, wait for the data to be processed, send FIN and ACK again, seq = q, ack = p 1, enter the LAST-ACK stage
The client receives After FIN, the client enters TIME_WAIT (wait for 2MSL) after receiving it, and then sends ACK to the server ack = 1 1
After the server receives it, it enters the CLOSED state
The client still needs to wait for two MSLs at this time. If it does not receive a resend request from the server, it means that the ACK has arrived successfully. The wave ends and the client changes to the CLOSED state. Otherwise, the ACK is retransmitted.
Why do you need to wait for 2MSL (Maximum Segment Lifetime):
Because if you don’t wait, if the server still has many data packets to send to the client, and this When the client port is occupied by a new application, useless data packets will be received, causing data packet confusion. Therefore, the safest way is to wait until all data packets sent by the server are dead before starting the new application.
1 MSL guarantees that the last ACK message from the active closing party in four waves can finally reach the peer
1 MSL guarantees If the end does not receive ACK, then the retransmitted FIN message can reach
Why four times instead of three times?
**If it is three times, then the ACK and FIN of the server are combined into one wave. Such a long delay may prevent a TCP FIN from reaching the server, and then the client will continue to resend it. FIN
##References
send buffer area, and put the received data into Receive buffer area. There are often situations where the sender sends too much and the receiver cannot digest it, so flow control is needed, which is to control the sending of the sender through the size of the receive buffer. If the other party's receiving buffer is full, it cannot continue to send. This flow control process requires maintaining a sending window at the sending end and a receiving window at the receiving end.
TCP sliding windows are divided into two types:sending window and receiving window.
References
##Essence Different Ajax, which is asynchronous JavaScript and XML, is a web development technology for creating interactive web applications
websocket is a new protocol of HTML5 that implements Real-time communication between browser and server
Different life cycles: Websocket is a long connection, and the session is always maintained ajax will be disconnected after sending and receiving Scope of application: Initiator: For example, in an e-commerce scenario, the inventory of goods may change, so it needs to be reflected to the user in time, so the client will keep sending requests, and then the server will keep checking for changes, regardless of Regardless of the change, everything is returned. This is short polling. The performance of long polling is that if there is no change, it will not return, but wait for the change or timeout (usually more than ten seconds) before returning. If there is no return, the client does not need to keep sending requests. So the pressure on both sides is reduced. Reference link Reference link Reference resources The reason is that the entire network environment may be particularly poor and packet loss is easy, so the sender should pay attention. For congestion control, TCP Mainly maintains two core states: Then a relatively conservative slow start algorithm is used to slowly adapt to the network. During the initial transmission period, the sender and receiver will first establish a connection through a three-way handshake to determine the size of their respective receiving windows, and then Initialize the congestion window of both parties, and then double the size of the congestion window after each round of RTT (receiver and transmit delay) until the slow start threshold is reached. Then start congestion avoidance. The specific method of congestion avoidance is to double the congestion window in each previous round of RTT, and now add one in each round. Fast retransmission During the TCP transmission process, if packet loss occurs, the receiving end will send a repeated ACK, such as 5 packets are lost, 6 and 7 are reached, and then the receiving end will send the ACK of the fourth packet for 5, 6, and 7. At this time, the sending end receives 3 repeated ACKs. When it realizes that the packet is lost, it will Retransmit immediately without waiting for RTO (timeout retransmission time) Selective retransmission: optionally add the SACK attribute to the message header, mark those packets that have arrived through the left edge and right edge, and then Retransmit undelivered packets Quick recovery If the sender receives 3 duplicate ACKs and discovers packet loss, it feels like now The network condition has entered the congestion state, then it will enter the rapid recovery phase: will reduce the congestion threshold to half of the congestion window Then the congestion window size becomes the congestion threshold Then the congestion window increases linearly to adapt to the network conditions Aims to send a probe request to determine what constraints a request for a certain target address must have, and then send the real request according to the constraints. For example, pre-checking for cross-domain resources is sent first using the HTTP OPTIONS method. Used to handle cross-domain requests Flexible and scalable, except for the stipulations that spaces separate words and newlines separate fields, there are no other restrictions. It can not only transmit text, but also Transmit any resources such as pictures and videos Reliable transmission, based on TCP/IP, so it inherits this feature Request-response, there is There are responses Stateless, each HTTP request is independent, irrelevant, and does not need to save context information by default Disadvantages: Clear text transmission is not safe Reusing a TCP link will cause peer congestion None In a long connection scenario, a large amount of context needs to be saved to avoid transmitting a large amount of repeated information
OSI seven-layer model Application Layers Presentation layer Session layer Transport layer Network layer Data link layer Physical layer TCP/IP four-layer concept: Application layer: application layer, presentation layer, session layer: HTTP Transport layer: Transport layer: TCP/UDP Network layer: Network layer: IP Data link layer: Data link layer, physical layer TCP is a connection-oriented, reliable, transport layer communication protocol UDP is a connectionless transport layer Communication protocol, inherits IP characteristics, and is based on datagram Why is TCP reliable? The reliability of TCP is reflected in the stateful and controlled which will accurately record which data was sent, which data was received by the other party, and which was not received, and ensures that the data packets arrive in order. No mistakes are allowed, this is stateful When it realizes that a packet has been lost or the network environment is poor, TCP will adjust its behavior according to the specific situation, control its own sending speed or restart This is controllable On the contrary, UDP is stateless and uncontrollable Improved performance: Header compression Multiple channel multiplexing Server Push Programming Video! !
##AJAX client initiated
Do you know WebSocket? Long polling and short polling, WebSocket is long polling.
https://www.jianshu.com/p/3fc3646fad80
How to implement long connection in HTTP? At what point does it time out? By setting Connection: keep-alive in the header (request and response header), the HTTP1.0 protocol is supported, but it is turned off by default. From the HTTP1.1 protocol onwards, the connection defaults to Long connection
HTTP generally has the httpd daemon, in which you can set the keep-alive timeout. When the tcp link is idle for more than this time, it will be closed. You can also set the timeout in the HTTP header. Time
tcp_keepalive_intvl = 15
https://blog .csdn.net/weixin_37672169/article/details/80283935
Question: The difference between Fetch API and traditional Request
fetch conforms to the separation of concerns, uses Promise, the API is richer, and supports Async/Await
https://github.com/camsong/blog/issues/2
Text, pictures, videos, audio, etc. are all acceptable
tcp is a connection-oriented, reliable, transport layer communication protocol
Stateful means that TCP will confirm which messages have been sent, which messages have been received by the receiver, and which ones have not been received, ensuring that data packets arrive in order, and no packets are allowed. Error
Congestion Control Principle
Slow start threshold congestion avoidance
Question: What does OPTION do? Give an example of using OPTION?
Q: Do you know http? Which layer of agreement? (Application layer)
Q: OSI seven-layer model and TCP/IP four-layer model
(3) Question: How does the TCP protocol ensure reliability, and why is UDP unreliable?
HTTP 2 Improvement
References
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