Problem 16
Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice’s host to Bob’s host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.
Problem 17
Print out the header of an e-mail message you have recently received. How many Received: header lines are there? Analyze each of the header lines in the message.
Problem 18
From a user’s perspective, what is the difference between the download- anddelete mode and the download-and-keep mode in POP3?
Problem 19
Is it possible for an organization’s Web server and mail server to have exactly the same alias for a hostname (for example, foo.com)? What would be the type for the RR that contains the hostname of the mail server?
Problem 20
Suppose you can access the caches in the local DNS servers of your department. Can you propose a way to roughly determine the Web servers (outside your department) that are most popular among the users in your department? Explain.
Problem 21
Suppose that your department has a local DNS server for all computers in the department. You are an ordinary user (i.e., not a network/system administrator). Can you determine if an external Web site was likely accessed from a computer in your department a couple of seconds ago? Explain.
Problem 21
In BitTorrent, suppose Alice provides chunks to Bob throughout a 30-second interval. Will Bob necessarily return the favor and provide chunks to Alice in this same interval? Why or why not?
Problem 22
Consider a new peer Alice that joins BitTorrent without possessing any chunks. Without any chunks, she cannot become a top-four uploader for any of the other peers, since she has nothing to upload. How then will Alice get her first chunk?
Problem 22
Consider distributing a file of \(F=15\) Gbits to \(N\) peers. The server has an upload rate of \(u_{s}=30 \mathrm{Mbps}\), and each peer has a download rate of \(d_{i}=2 \mathrm{Mbps}\) and an upload rate of \(u\). For \(N=10,100\), and 1,000 and \(u=300 \mathrm{Kbps}, 700 \mathrm{Kbps}\), and 2 Mbps, prepare a chart giving the minimum distribution time for each of the combinations of \(N\) and \(u\) for both client-server distribution and P2P distribution.
Problem 23
What is an overlay network? Does it include routers? What are the edges in the overlay network?
