Question

In this problem, you will put together much of what you have learned about Internet protocols. Suppose you walk into a room, connect to Ethernet, and want to download a Web page. What are all the protocol steps that take place, starting from powering on your \(\mathrm{PC}\) to getting the Web page? Assume there is nothing in our DNS or browser caches when you power on your PC. (Hint: the steps include the use of Ethernet, DHCP, ARP, DNS, TCP, and HTTP protocols.) Explicitly indicate in your steps how you obtain the IP and MAC addresses of a gateway router.

Short answer

The protocol steps include connecting via Ethernet, obtaining an IP via DHCP, finding the gateway's MAC with ARP, resolving the web server's IP using DNS, establishing a TCP connection, sending an HTTP request, and receiving the web page.

Step-by-step solution

Powering On and Connecting via Ethernet

When you power on your PC, it physically connects to the Ethernet connection. Your network interface card (NIC) detects the active Ethernet link by listening to the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.

Obtaining an IP Address with DHCP

Since the computer lacks an IP address, it sends a DHCPDISCOVER message using the Dynamic Host Configuration Protocol (DHCP) to find a DHCP server. The DHCP server responds with a DHCPOFFER, offering an IP address and other network configuration details. Your PC then requests that IP address with a DHCPREQUEST, and finally, receives an acknowledgment (DHCPACK) with the IP address.

Using ARP to Find the Gateway Router's MAC

To reach outside networks, your PC needs the MAC address of the gateway router. The PC uses the Address Resolution Protocol (ARP) by broadcasting an ARP request for the router’s IP address. The gateway router responds with an ARP reply, including its own MAC address.

DNS Query to Resolve Web Page URL

Your web browser wants to connect to a web page, so it needs the IP address of the web server. It uses the Domain Name System (DNS) by sending a DNS query for the URL. This involves sending a DNS request to the local DNS server, which replies with the IP address of the web server you wish to visit.

Establishing a TCP Connection

With the web server's IP address known, the PC uses the Transmission Control Protocol (TCP) to establish a connection. It sends a TCP SYN packet to the web server, which responds with a SYN-ACK, and the PC responds with an ACK, completing the three-way handshake.

HTTP Request for the Web Page

Once the TCP connection is established, the PC sends an HTTP GET request over this connection to the web server, requesting the desired web page.

Receiving the Web Page

The web server processes the HTTP request and sends back the requested web page data, which your PC receives over the established TCP connection.

Key concepts

Ethernet

Ethernet is the backbone of wired networking that physically connects your PC to the internet. When you start your computer, it connects to Ethernet through a network interface card (NIC). The NIC uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol to detect if the network link is available.

Ethernet operates in layers, specifically at the data link layer (Layer 2) and the physical layer (Layer 1) of the OSI model. It facilitates devices in a local network to communicate by transmitting data packets called frames.

• Frames contain source and destination MAC addresses.
• Data is segmented into frames for efficient delivery.

Ethernet offers high speed and reliability, making it a common choice for local area networks (LANs). Its plug-and-play nature simplifies connecting and setting up new devices.

Dynamic Host Configuration Protocol (DHCP)

Dynamic Host Configuration Protocol (DHCP) simplifies the network setup by dynamically assigning IP addresses to devices. When your PC connects to a network and lacks an IP address, it sends a broadcast message known as DHCPDISCOVER to find a DHCP server.

Upon receiving the query, the server responds with a DHCPOFFER, providing an IP address and network details like subnet mask and gateway. The PC then sends a DHCPREQUEST to claim this offer, and finally, the server acknowledges with a DHCPACK.

• Ensures devices have unique IPs, preventing conflicts.
• Facilitates seamless IP management in growing networks.

Through DHCP, network configuration becomes automated, reducing manual input and human error, and ensuring efficient operation of networked environments.

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP) serves as the bridge between IP addresses and MAC addresses within a local network. When your computer needs to communicate with an external network, ARP helps in discovering the MAC address of the gateway router.

This process starts with an ARP request, a broadcast packet asking who owns a particular IP address. Once the gateway router receives the ARP request, it replies with an ARP reply, revealing its MAC address.

• Essential for MAC address lookups in an Ethernet network.
• Prevents unnecessary traffic by establishing direct communication paths.

ARP simplifies local area network operations by ensuring that data is correctly routed to its next physical destination.

Domain Name System (DNS)

The Domain Name System (DNS) translates human-readable domain names into IP addresses, which computers need to access websites. When your browser requests a URL, it first needs to determine the IP address of the target web server through a DNS query.

Your PC sends a DNS request to a local DNS server, which looks up the IP address associated with the requested domain and sends it back in a reply.

• Ensures users can easily access websites with names, not numbers.
• Maintains a hierarchical and distributed database of domain names.

Without DNS, navigating the internet would be cumbersome, as users would have to memorize numerical IP addresses.

Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) is fundamental for ensuring reliable data delivery over the network. It establishes and maintains a connection between your computer and a web server using a method known as the three-way handshake.

The three-way handshake involves:

• Your PC sending a TCP SYN packet to the server.
• The server replying with a SYN-ACK packet.
• Your PC sending an ACK to complete the connection setup.

This handshake process ensures both devices are ready to communicate, providing reliability by managing data packets' errors and ordering. TCP facilitates smooth browsing experiences and reliable file transfer over the web.

Hypertext Transfer Protocol (HTTP)

Hypertext Transfer Protocol (HTTP) is the foundation of any data exchange on the Web and a protocol used for transferring web pages. Once a TCP connection is established between your PC and a server, HTTP comes into play.

Your computer sends an HTTP GET request to retrieve a specific web page. This request travels over the established TCP connection. When the server receives the request, it processes it and sends back the requested web page data.

• Operates on top of TCP to ensure data integrity.
• Stateless protocol – each request-response pair is independent.

HTTP makes web browsing efficient and user-friendly, allowing seamless interactions with websites across the globe.