Web Protocols, P2P Networking, and Data Transmission

Client and Server Web Interaction

Steps for a Client (Browser)

• Determine the protocol (HTTP).
• Ask DNS for the IP address of the server.
• Make a TCP connection to the server.
• Send a request for the page; the server sends it back.
• Fetch other URLs as needed to display the page.
• Close idle TCP connections.

Steps for a Server

• Accept a TCP connection from the client.
• Get the page request and map it to a resource (e.g., file name).
• Get the resource (e.g., file from disk).
• Send the contents of the resource to the client.
• Release idle TCP connections.

Challenges in P2P Networking

1. How do peers find each other?
2. How do peers support rapid content downloads?
3. How do peers encourage each other to upload?

BitTorrent Protocol Download Steps

1. Start with the torrent description.
2. Contact the tracker to join and get a list of peers (with at least one seed peer), or use a DHT (Distributed Hash Table) index for peers.
3. Trade pieces with different peers.
4. Favor peers that upload to you rapidly, and “choke” peers that do not by slowing your upload to them.

Channel Utilization Analysis

Problem: The distance from Earth to a distant planet is approximately 9 × 10¹⁰ m. Calculate channel utilization for a stop-and-wait protocol on a 64 Mbps point-to-point link (Frame size: 32 KB, Speed of light: 3 × 10⁸ m/s).

a) Stop-and-Wait Utilization

• Delay (D) = (9 × 10¹⁰) / (3 × 10⁸) = 300 sec.
• Bandwidth-delay product (BD) = 64 Mbps × 300 sec = 19,200 Mbit = 19.2 Gbit.
• BD is equivalent to 75,000 frames.
• Utilization = (1 / (1 + 2 × 75,000)) × 100% ≈ 6.66 × 10⁻⁴%.

b) Sliding Window Utilization

For 100% utilization: (w / (1 + 2BD)) = 1. Given 2BD = 150,000, then w / (1 + 150,000) = 1. Therefore, the window size must be 150,001 frames.

Bit Stream Encoding Methods

For the provided bit stream, illustrate the following:

• a) Non-Return to Zero (NRZ)
• b) NRZ-Invert (NRZI)
• c) Manchester (with clock)
• d) Bipolar encoding