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Populating the page: how browsers work

Resources: https://developer.mozilla.org/en-US/docs/Web/Performance/How_browsers_work

Summary:

Navigation is the first step in loading a web page. It occurs whenever a user requests a page by entering a URL into the address bar, clicking a link, submitting a form, as well as other actions.

DNS Lookup

  • The first step of navigating to a web page is finding where the assets for that page are located. If you navigate to https://example.com, the HTML page is located on the server with IP address of 93.184.216.34. If you've never visited this site, a DNS lookup must happen.
  • Your browser requests a DNS lookup, which is eventually fielded by a name server, which in turn responds with an IP address. After this initial request, the IP will likely be cached for a time, which speeds up subsequent requests by retrieving the IP address from the cache instead of contacting a name server again.
  • DNS lookups usually only need to be done once per hostname for a page load. However, DNS lookups must be done for each unique hostname the requested page references. If your fonts, images, scripts, ads, and metrics all have different hostnames, a DNS lookup will have to be made for each one.

TCP Handshake

  • Once the IP address is known, the browser sets up a connection to the server via a TCP three-way handshake. This mechanism is designed so that two entities attempting to communicate—in this case the browser and web server—can negotiate the parameters of the network TCP socket connection before transmitting data, often over HTTPS.
  • TCP's three way handshaking technique is often referred to as "SYN-SYN-ACK"—or more accurately SYN, SYN-ACK, ACK—because there are three messages transmitted by TCP to negotiate and start a TCP session between two computers. Yes, this means three more messages back and forth between each server, and the request has yet to be made.

TLS Negotiation

  • For secure connections established over HTTPS, another "handshake" is required. This handshake, or rather the TLS negotiation, determines which cipher will be used to encrypt the communication, verifies the server, and establishes that a secure connection is in place before beginning the actual transfer of data. This requires three more round trips to the server before the request for content is actually sent.

Response

  • Once we have an established connection to a web server, the browser sends an initial HTTP GET request on behalf of the user, which for websites is most often an HTML file. Once the server receives the request, it will reply with relevant response headers and the contents of the HTML.