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ECE3076 Homework 1 Answers - Prof. John Copeland, Assignments of Electrical and Electronics Engineering

Georgia Institute of Technology - Main CampusElectrical and Electronics Engineering
Prof. John Copeland

The answers to ece3076 homework 1, which covers the basics of networks, packet sniffers, and network protocol layers. It includes answers to questions about wireshark packet sniffer, tcp/ip and ethernet addresses, packet delay calculation, buffer overflow, and network design.

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Pre 2010

Uploaded on 08/05/2009

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ECE3076 Homework 1 - Answers
Due: Friday, Sept. 3, 2008 (in class)
The best way I found to learn about networks was to look at real packets and the details of the packet
structure. There is now a very good packet sniffer program, "wireshark", that can be easily installed on MS
Windows PCs. It is the successor to "ethereal" that has been around for many years on Linux and MacOS
(but was a bear to install on Windows).
Download the appropriate installer from http://www.wireshark.org/ and install on your PC, Mac , or Linux
system. For more information, look at http://en.wikipedia.org/wiki/Wireshark and my notes on filters and
installing wireshark on Mac OSX in http://www.csc.gatech.edu/copeland/jac/3076/info/. Also look at
http://www.csc.gatech.edu/copeland/jac/nowhere/ethereal_intro.pdf
1. After you get wireshark running, use your browser to look at www.umass.edu (see below*)
a. What is the IP address of www.umass.edu? __128_.__119__.__101__.__5___
b. What is a TCP port number of your PC for this exchange ? ____33568__ (different each time)
c. Is it the same for all TCP connections in this session (yes/no) ? ____ no __ (different each time)
d. What is a TCP port number of www.umass.edu for this exchange ? _______80__________
e. Is it the same for all TCP connections in this session (yes/no) ? _______yes________
f. Is it usually the same when you go to other Web sites (yes/no) ? ______yes_________
g. What is the Ethernet address of your PC? 00:11:24:95:66:b3 (different for every PC)
h. What is the apparent Ethernet address of www.umass.edu? 00:06:25:68:3a:78 f3 (different for nets)
i. Ping your local router (MSDOS window, type "ping <replace the last digit in your IP with "1">).
What is the apparent Ethernet address 00:06:25:68:3a:78 f3 (different for nets)
j. Is the answer in "h" the same as in "i" (yes/no) ? ____yes______(error in assignment, h & g = no)
k. Why should "h" and "i" be the same ? ____Ethernet carries outside packets only to the router____
l. What does the following capture filter do to clean up your wireshark display: "ip[16]<224 and
ip[19]<255" ___Stops capture of multicast (224+.x.x.x) and broadcast packets (x.x.x.255) ____
m. Can you see missing packets later ? (yes/no) __________
2. Review the five protocol layers in the textbook (sec. 1.7). The following should be one of those
layers. Show the name and number (1-5).
a. What layer has the TCP or UDP port number? _________Transport, layer 4 ______________
b. What layer has the IP address? ______Network, 3___________
c. What layer has the Web "GET" request? _______Application, 5____________
d. What layer has the Ethernet address? ________Link, 2______
e. What layer does not appear in the wireshark results above? _______Physical, 1__________
ECE3076 HW-1 p.1
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ECE3076 Homework 1 - Answers Due: Friday, Sept. 3, 2008 (in class) The best way I found to learn about networks was to look at real packets and the details of the packet structure. There is now a very good packet sniffer program, "wireshark", that can be easily installed on MS Windows PCs. It is the successor to "ethereal" that has been around for many years on Linux and MacOS (but was a bear to install on Windows). Download the appropriate installer from http://www.wireshark.org/ and install on your PC, Mac , or Linux system. For more information, look at http://en.wikipedia.org/wiki/Wireshark and my notes on filters and installing wireshark on Mac OSX in http://www.csc.gatech.edu/copeland/jac/3076/info/. Also look at http://www.csc.gatech.edu/copeland/jac/nowhere/ethereal_intro.pdf

1. After you get wireshark running , use your browser to look at www.umass.edu (see below*) a. What is the IP address of www.umass.edu? __ 128_.119.101.__5 ___ b. What is a TCP port number of your PC for this exchange? ____ 33568 __ (different each time) c. Is it the same for all TCP connections in this session (yes/no)? ____ no __ (different each time) d. What is a TCP port number of www.umass.edu for this exchange? _______ 80 __________ e. Is it the same for all TCP connections in this session (yes/no)? _______ yes ________ f. Is it usually the same when you go to other Web sites (yes/no)? ______ yes _________ g. What is the Ethernet address of your PC? 00:11:24:95:66:b3 (different for every PC) h. What is the apparent Ethernet address of www.umass.edu? 00:06:25:68:3a:78 f3 (different for nets) i. Ping your local router (MSDOS window, type "ping <replace the last digit in your IP with "1">). What is the apparent Ethernet address 00:06:25:68:3a:78 f3 (different for nets) j. Is the answer in "h" the same as in "i" (yes/no)? ____ yes ______(error in assignment, h & g = no) k. Why should "h" and "i" be the same? ____ Ethernet carries outside packets only to the router ____ l. What does the following capture filter do to clean up your wireshark display: "ip[16]<224 and ip[19]<255" ___ Stops capture of multicast (224+.x.x.x) and broadcast packets (x.x.x.255) ____ m. Can you see missing packets later? (yes/no) __________ 2. Review the five protocol layers in the textbook (sec. 1.7). The following should be one of those layers. Show the name and number (1-5). a. What layer has the TCP or UDP port number? _________ Transport, layer 4 ______________ b. What layer has the IP address? ______ Network, 3 ___________ c. What layer has the Web "GET" request? _______ Application, 5 ____________ d. What layer has the Ethernet address? ________ Link, 2 ______ e. What layer does not appear in the wireshark results above? _______ Physical, 1 __________ ECE3076 HW-1 p. 1

3. Packet Delay Calculation Two nodes(A and B) are connected by 2500 km optical fiber. At time zero the next packet is placed in A's. This packet has 1500 bytes. The link transmission rate is 50 Mbits/s for A to B. It takes negligible time for node a to decide where to send the packet, and copy it to the correct output buffer (processing delay = 10 us). a. If the output buffer of A had five 1200-byte packets when our packet arrives (t = 10 us), at what time does our packet start being transmitted to the next node? ___ 970 ____ us 10us + 512008/50e6 s b. At what time is the last bit in the packet transmitted by A (in microseconds)? ___ 1210 ____ us 970 us + 15008/50e6 s = 970 + 240 us c. At what time does the last bit arrive at B? _______ 2460 ______ us 1210 + 250,000m / (200m/us) d. What is the delay due to node A? _____ 2460 ______ us Names: a = processing delay + caching delay, b = a + transmission delay, c = b + propagation delay. 4. Buffer Overflow The network node A has a 1 Mbyte output buffer for the port going to B. At time zero it is empty. All connecting links operate at 50 Mbps. a. Suddenly 2 input links start running continuously with packets destined for node B. At what time does the output buffer for B overflow? _____ 160,000 ______ us (8b/B1MB) / (2*50Mb/s - 50Mb/s) * (1e us/s) b. If A is in a mesh network, how could it handle this surge without losing packets? **By forwarding packets on a different link

  1. Network Design** During the "busy hour," 10% of the hosts (PCs) at a company are active. When a host is active, it uses an average 150 kbits/s. a. If we use a design rule that we want our Internet connection to be 20% utilized (on the average), how many hosts can a T1 line (1.5 Mbps) serve? __ 20 ______ 0.20 * 1,500,000b/s / (0.10 150,000 b/s) b. If we have 2000 hosts, how much connectivity do we need? __ 150 M ___ bits/s, and c. If a T1 line is $100/month, and a T3 (34 Mbps) line is $500/month, what should you order (n T1's or m T3's)? _____________________ 5 T3's ________ (cheaper, and has room for growth) ECE3076 HW-1 p. 2