Please explains what happens when a user on a client machine is visiting a webpage on an Internet server.

Write briefly to explain:

a) What happens on each of the four layers, namely the application layer, transport layer, network layer, and link layer.

b) What protocols are used on each individual layer, for what services or functionalities.

Synthesis: a day in the life of a web request * journey down protocol stack complete! - application, transport, network, link * putting-it-all-together: synthesis! - goal: identify, review, understand protocols (at all layers) involved in seemingly simple scenario: requesting www page - scenario: student attaches laptop to campus network, requests/receives www.google.com A day in the life: scenario A day in the life connecting to the Internet * connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 * Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server * Ethernet demuxed to IP demuxed, UDP demuxed to DHCP A day in the life Eonnecting to the Internet - DHCP server formulates DHCP ACK containing client's IP address, IP address of first-hop router for client, name \& IP address of DNS server * encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client * DHCP client receives DHCP ACK reply Client now has IP address, knows name \& addr of DNS server, IP address of its first-hop router A day in the life ARP (before DNS, before HTTP) * before sending HTTP request, need IP address of www.google.com: DNS * DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC - address offrouterinterface: received by router, which replies with ARP reply giving MAC address of router - inierffacew knows MAC address of first hop router, so can now send frame containing DNS query A day in the life using DNS A day in the life TCP connection carrying HTTP A day in the life HTTP request/reply