Exercise 1a

ping is a program that sends an echo request packet once a second to a target computer. Upon receiving the echo request packet (also called a ping packet), the target computer returns the packet to the sender. Ping displays information about the packet sent and how long it took to receive the packet . This length of time is called the round trip time (RTT) and is representative of network latency.

First ssh to another instructional machine (other than nova.cs, quasar.cs, or pulsar.cs). Then, try to ping the computer you've ssh'ed from. To do this, give the command:

/usr/sbin/ping -s <local_computer_name> 56 5

Replace <local_computer_name> with the name of the computer you've connected from. For example, if you originally logged in on nova and ssh’d to po, you would type the following on po:

/usr/sbin/ping -s nova.cs.berkeley.edu 56 5

The "-s" says to send one ping request per second to the given site; the final 5 tells ping to send five packets before stopping and giving its summary. For more details about the various command line options, read the man pages.

Try pinging various sites and see what round trip times you get. Try some sites

• within the department (for example, rhombus.cs.berkeley.edu),
• within the campus (for example, mechatro2.me.berkeley.edu),
• elsewhere in the U.S. (for example, www.ee.vt.edu), and
• outside the U.S. (for example, www.cs.iitm.ernet.in).

For checkoff, determine if there is a significant difference between the average ping times to these different sites. A good question here would be "What is a significant difference?" After all, everything here seems to be measured in milliseconds! For now, you can approach the above question by just looking at the ratios of the ping times to different sites. Thus, a factor of two or more between ping times would be a significant difference.

Exercise 1b

Typically, on a local area network (LAN), every machine can directly contact every other machine. A larger network like the Internet is composed of many smaller networks, and it would be impossible for every machine to know how to reach every other machine. In order to connect two networks together, one might dedicate a computer (called a gateway) to routing traffic from one network to another. Suppose that a computer on the network A wanted to send a packet to a computer on the network B. Rather than storing the routing information for the network B, the computer on the network A would simply forward the packet to the gateway, and the gateway would forward it to its destination.

traceroute is a program that displays the routers between your computer and a destination computer. It also displays the RTTs from each router to the local computer. Read the man pages for more information on the command.

For checkoff, use traceroute on the above sites and see how the route varies. The third and fourth sites are outside U.C. Berkeley. Find out in general how many hops it takes to get out of the U.C. Berkeley campus.

Exercise 2

Find the routes from the instructional machines to different machines within Berkeley. In particular, run traceroute to the following machines:

• Soda Hall machines
  • bmrc.berkeley.edu
  • argus.cs.berkeley.edu
• Cory Hall machines
  • po.eecs.berkeley.edu
  • cory.eecs.berkeley.edu
• Servers for different departments within the College of Engineering
  • www.eecs.berkeley.edu
  • www.me.berkeley.edu
  • www.ce.berkeley.edu
  • www.ieor.berkeley.edu
  • www.millennium.berkeley.edu
• The different schools
  • haas.berkeley.edu
  • www.cchem.berkeley.edu
  • www-gse.berkeley.edu
  • www.cnr.berkeley.edu
• The residence halls
  • www.reshall.berkeley.edu
• I-House
  • ihouse.berkeley.edu

For checkoff, answer the following questions.

1. What is the network or IP address of your machine?
2. On which local subnet does your machine reside? (Hint: 128.32.local subnet.host)
3. Packets sent beyond your subnet must go through a gateway that links between your local area network and the network outside. Identify the IP address of the gateway for your machine.
4. After passing the gateway, packets are routed between the various departments and schools on campus. Identify one router address in the network that routes packets between the various departments and schools.
5. Draw a simple approximation of the Berkeley network hierarchy based on what you have obtained from your traceroute results.

Exercise 3 (Optional)

The directory ~cs61c/labs/15 contains four trace files (trace.1, trace.2, www.trace, and smtp.trace) of network traffic generated by the ethereal network analyzer. After copying these files to your directory, you can examine them by typing

ethereal tracefile

substituting the name of the trace file as the argument to ethereal. (If you’re close to your disk quota, be careful; three of the files are over 100K in size.)

For checkoff, answer the following questions:

1. What’s the IP address of the computer on which trace.1 and trace.2 were run?
2. What’s going on starting at frame 349 in trace.1?
3. The password of a user named "test" is sent in clear text in trace.2. What is it? (Hint: sort the frames by "protocol".)
4. A user in trace.2 initiated a telnet connection at frame 872. At what frame was the connection terminated? (Hint: sort the frames by "source".)
5. Find a file transfer (indicated by "continuation" in the traces) and estimate the relative percentages of header bytes transferred compared to actual data transferred.
6. www.trace was generated by one of the t.a.s doing some Web surfing. Where did he point his browser, and what page was he examining?

Further exploration

The various trace files contain a number of examples of different network protocols. They are fun to explore and perhaps imitate. For example, with the command

telnet www.google.com 80

and a command that was given in www.trace, you can connect to Google's web site and fetch their home page. smtp.trace resulted from one of the t.a.s sending mail to one of his other accounts; you might also try imitating that via the telnet program.