Hands-on 5: Measuring Round Trip Times

Today's assignment is your fifth hands-on exercise. For this assignment, you need to do the following exercises and hand in your results during recitation on March 14th.

Measuring Round Trip Times With Ping

The ping utility is one of the more useful utilities for testing a network. The ping utility works by sending a short message of type echo-request to a host using a network protocol called ICMP, the Internet Control Message Protocol. A host that supports ICMP (and most do) and receives an echo-request message simply replies by sending a short ICMP message of type echo-response back to the originating host.

For more information, look at the man page on ping and the specifications for ICMP, located in RFC 792. To get to RFC 792 on Athena, type

    % attach rfc
    % more /mit/rfc/rfc792.txt

To get to the man page on ping, type

    % man ping

Exercises

Please record your answers to the two exercises below on a sheet of paper and turn it in at recitation. Please also include the amount of time that the assignment took to complete.

In this exercise, you will use the ping utility to send echo requests to a number of different hosts. To use the ping command on Athena, simply type the letters ping on the command line. A list of possible options will appear.

You may use any other form of the ping utility so long as it sends ICMP echo-request packets and can report the number of echo-response packets and the round trip time per packet. If you do use a non-Athena workstation, please make sure you use ping from some machine connected to the MIT network. Please also note that the TAs cannot guarantee tech support if you do not use an Athena Sun workstation.

On the Sun workstations on Athena, the ping utility is located in /usr/sbin/ping. On the SGI workstations on Athena, the ping utility is located in /usr/etc/ping. If you use the ping from a Sun workstation, you may have to use the -s option to get it to display the results that you want. Type machtype to determine the type of machine you are using. If you have any more questions, see the man pages for more details on how to use ping.

Exercise #1

a) For the following hosts, send 20 packets, each with a length of 56 data bytes. Indicate what percent of the packets resulted in a successful response. For the packets from which you received a response, write down the minimum, average, and maximum round trip times in milliseconds. Note that ping reports these times to you if you tell it how many packets to send on the command line.

The list of hosts are:

   www.lcs.mit.edu       
   www.stanford.edu 
   www.cs.vu.nl           (Vrije Univeristy, Prof. Kaashoek's alma mater)
   www.kyoto-u.ac.jp       (Kyoto University)

Note: You may find that the packet responses are 64 bytes instead of 56 bytes. Look at RFC 792 to find out the reason.

b) For 56-byte packets, what can you say about the differences in the minimum round-trip times for pings of www.lcs.mit.edu, www.stanford.edu, www.cca.vu.nl and www.kyoto-u.ac.jp? What reason can you give to explain why these differences occur?

c) For same hosts listed above, send 20 packets that have lengths of 512 data bytes and 1024 data bytes. Write down the minimum, average, and maximum round trip times in milliseconds.

d) When comparing 56-byte packets, 512-byte packets, and 1024-byte packets, what can you say about the difference in minimum round-trip times to the same host?

Exercise #2

a) For the following hosts, send 20 packets that have a length of 56 data bytes. Indicate what percent of the packets resulted in a successful response. For the packets that you received a response, write down the minimum, average, and maximum round trip times in milliseconds.

   www.pku.edu.cn          (Peking University)
   www.wits.ac.za          (University of the Witwatersrand, Johannesburg)
   atlas.lcs.mit.edu     
   www.microsoft.com 

b) For some of the hosts, you may not have gotten responses for all the packets you sent. Can you think of a few reasons as to why you might have not gotten a response?