An introduction to the specific process from inputting the url into the browser to initiating an http request.

The content of this article is about the specific process of inputting URL in the browser to initiating http request. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

User input url

When the user enters the url, the operating system will pass the input event to the browser. During this process, the browser may Do some preprocessing. For example, Chrome will estimate the website corresponding to the entered character based on historical statistics. For example, if you enter goog, it will be found that there is a 90% probability of visiting "www.google.com" based on the previous history, so it will be returned after the input. The TCP connection and even rendering started immediately in front of the car.

Then enter the URL and click Enter. At this time, the browser will check the URL. First determine the protocol. If it is http, it will be processed according to the Web. In addition, the URL will be checked for security.

After the security check is completed, the browser kernel will first check the cache, then set UA and other HTTP information, and then call the network request method under different platforms.

Note:
Browser and browser kernel are different concepts. Browser refers to Chrome and Firefox, while browser kernel refers to Blink and Gecko. Browser kernel It is only responsible for rendering. Cross-platform work such as GUI and network connection is implemented by the browser.

http network request

Query IP through DNS;
Send data through Socket

dns query ip

DNS, Domain Name System in English and Domain Name System in Chinese, is a service of the Internet. It is a distributed database that maps domain names and IP addresses to each other

Assumption The user enters www.google.com in the browser. The approximate process is:

If the input is a domain name, a dns query is required to resolve the domain name into an ip;

Conduct a DNS query The host or software is called a DNS resolver, and the workstation or computer used by the user is a resolver. Domain name resolution is the process of using a DNS resolver to obtain the corresponding IP. The resolver will query the domain name server for processing.

The main process is as follows:

1. Find the IP address of the domain name www.google.com from the browser cache

2. In If it is not found in the browser cache, search it in the operating system cache. In this step, you will also search the local hosts to see if there is a corresponding domain name mapping (of course it has been cached in the system DNS cache)

3. If it is not found in the system, go to your router to find it, because the router usually has its own DNS cache

If none of the above are found, then Continue to query the dns domain name server

• The resolver of the user's computer initiates a domain name resolution request to LDNS (that is, Local DNS, Internet Service Provider ISP) and queries www.google. com IP address, this is a recursive search process

• When the cache does not hit, LDNS queries the root domain name server for the IP address of www.google.com, LDNS query The process is an iterative query process

• The root tells LDNS that I don’t know the IP corresponding to www.google.com, but I know that you can ask the authorization server of the com domain. This domain belongs to He manages

• LDNS asks com’s authorization server for the IP address corresponding to www.google.com

• #com tells LDNS, I don’t know www .google.com corresponding IP, but I know you can ask the authorization server of the google.com domain. This domain is managed by him.

• LDNS asks the authorization server of google.com www. The IP address corresponding to google.com

• google.com queries its own ZONE file (also called zone file record), finds the IP address corresponding to www.google.com, and returns it to LDNS

• LDNS caches a record locally and returns the result to the parser of the user's computer

• After that, the parser of the user's computer gets After the result is obtained, it is cached in the DNS cache of your own operating system and returned to the browser. The browser will still cache it for a period of time.

Note,
The domain name query may have gone through the CDN scheduler (if there is a CDN storage function)

And , you need to know that dns resolution is very time-consuming, so if you resolve too many domain names, the first screen will load too slowly. You can consider dns-prefetch optimization

tcp/ip request

With the IP address, data can be sent through the Socket API. At this time, the TCP or UDP protocol can be selected.

http is essentially a tcp protocol.

TCP is a connection-oriented transport layer protocol. It can ensure that the communication between the communication hosts at both ends (sending end and receiving end) is reachable. It can handle abnormal situations such as packet loss and disordered transmission during transmission; in addition, it can effectively utilize broadband to alleviate network congestion.

Establishing a TCP connection requires three handshakes at the beginning:

The first handshake is to request the establishment of a connection, and the sender sends a connection request message
The second handshake is to receive the To the message sent from the sending end, it can be seen that the sending end now wants to establish a connection. Then the receiving end will send a message to the sending end.

The third handshake. After the sending end receives the sent message, it needs to check whether the returned content is correct; if it is correct, the sending end will do it again. Send confirmation packet

After the TCP connection is established, the HTTP request can be sent.

Note
Browsers have a limit on the number of connections for the same domain name, most of which are 6. In http1.0, one resource download often needs to correspond to a tcp/ip request, but like HTTP 2.0 Although the protocol uses only one TCP connection to transmit data, its performance is better and it can also implement request priority.

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