What happens when I click on a URL?

1. Input URL, press Enter

2. Web browser translates URL

• If URL is incorrect, use Web browser’s default search engine to search the URL

3. If URL is correct, encode URL

• URL uses US-ASCII
• If there is a letter that is not US-ASCII, encode
• Domain name uses Punnycode encoding
• Use Percent encoding for path lower than domain name

2. Check HSTS list

Http Strict Transport Security

• HSTS list enforces use of HTTPS
• check HSTS list to see if HTTPS is needed
• if on HSTS list, need to request for HTTPS
• if not on HSTS list, request for HTTP

3. Browser checks for DNS entry corresponding IP address of website in cache

• Browser cache
• OS cache
• Router cache

• ISP cache

• 항상 DNS서버에 접속해서 찾으려면 시간이 오래걸리니
  
• 로컬 캐시 또는 DNS캐시에 저장해 두었다가
  
• 로컬 캐시 또는 DNS캐시에 1차적으로 접속해서 IP주소 확인
  

4. If not found in cache, ISP DNS server initiates DNS query to find IP address of server with the domain name

• 없으면 DNS서버 접속
• DNS서버 안에 도메인과 IP address가 매핑이 되어있는데
• 우리가 요청하면 도메인에 연결된 IP address를 가져와 접속하게 된다.

https://soheeparklee.github.io/posts/n-dns/

5. Get MAC address associated with IP address with ARP

ARP: Address Resolution Protocol

• devices use ARP to map an IP address to a MAC address

6. Browser initiates TCP connection

• 3-way-handshake to start connection
• SYN
• ACK
• 4-way-handstake to terminate connection

7. If HTTPS, add SSL(TLS) handshake

8. Browser sends HTTP request to the web server

• use HTTP protocol

9. Server sends HTTP response

• server sends HTTP response
• response in JSON, XML, HTML

10. Rendering by web browser

• When server responds with resource(HTML, CSS, JS)
• Redering by creating DOM Tree, CCSSOM Tree, Render Tree
  • DOM: HTML
  • CCSSOM(Cascading Style Sheets Object Model): CSS style
  • Render Tree: DOM ➕ CSSSOM