HTTP and the web

Next class reading is section 2.2 of the book.

The inner workings of the World Wide Web service will be presented.

Homework

Two end-systems (A& B) communicate through a packet switching network. There are two routers (R1 & R2) between the two end-systems. All links are 1Mbps. Host A sends 1 MByte of data to B using 520 bits packets. Each packet has a header of 20 bits.

1. How long does it take since transmission starts in Host A till the last bit has been received by Host B? (Consider propagation delay, processing and queueing delays negligible).
2. What is the end-to-end throughput?
3. Repeat questions 1&2 if propagation delay is 1 millisecond on each link.

Sample use of traceroute -T

With the required privileges, the traceroute command may give us some of the information that was missing before: http://pastebin.com/hU2siqwZ

You can find this tool useful too.

More work to do

Next week lecture will be based on Section 2.1 of the textbook. Let me know if I should focus on a particular concept you've found difficult to understand.

Please bring the following problems solved for the next class in written form:

1)Suppose users share a 2 Mbps link. Also suppose each user transmits continuosly at 1 Mbps when transmitting, but each user transmits only 20 percent of the time. (See the discussion of statistical multiplexing in Section 1.3.)
a. When circuit switching is used, how many users can be supported?
b. For the remainder of this problem, suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer users transmit at the same time? Why will there be a queuing delay if three users transmit at the same time?
c. Find the probability that a given user is transmitting.
d. Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.

2)Suppose Host A wants to send a large file to Host B. The path from Host A to
Host B has three links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps.
a. Assuming no other traffic in the network, what is the throughput for the file transfer.
b. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?
c. Repeat (a) and (b), but now with R2 reduced to 100 kbps.