www.cs.amherst/~ccm/cs5/netdemo.html
Networking Protocols
Sit down at one of the computers and log in. This worksheet contains
a series of questions for you to answer, using the networking tools
on the system (and working on your own).
Please turn in your answers to the questions below on
Thursday, Sept 13th.
This worksheet will count for 10 points in your class participation score.
Some Interesting Network Addresses
You can use these IP numbers and names in the activities below. It is
fine to use others that you know of, but some sites may not cooperate.
| Domain Name |
IP Number |
DNS server |
What it is |
| www.cs.amherst.edu
| 148.85.77.95
| |
Math/CS department
|
| cs.umass.edu
| 128.119.240.41
| |
A mile away
|
| www.cpsr.org
|
| |
Computer Professionals for Social Responsibility.
|
| www2.usp.br
|
| |
University of Sao Paulo, Brazil
|
| www.dagstuhl.de
|
| |
Dagstühl Research Center, Germany
|
| www.gis.edu.gh
|
| |
Ghana International School, Accra, Ghana
|
| www.doshisha.ac.jp
|
| |
Doshisha University, Japan
|
Application Layer: Domain Name System
Sit down at any computer and login.
Open Finder/Applications/Utilities/Network Utility.
Click on Lookup. In the menu, select Any/All Information.
The Lookup server will accept either a domain name or an
IP number, and it tells you where to find the DNS server for that address.
The IP numbers of the lookup servers associated with the address will
appear in the Answer Section.
Task 1. For each address in the list above, find a host that
performs the DNS service. (You will also be asked to find IP
numbers for the actual hosts, below.)
Transport Layer: TCP
The TCP protocol keeps track of which packets go to which applications
using socket numbers (also called port numbers)
to identify categories of applications. The common
application protocols are on the List of Well Known Ports,
maintained by IANA, a subcommittee of ICANN.
For example, HTTP is always assigned to Port 80 or 81.
Task 2. Open your favorite browser. Surf the web to find
out answers to these questions:
- How many Well Known Ports are there in the IANA standard?
- What is the Well Known Port Number for SMTP, the email server transfer
protocol?
- What is the Well Known Port Number for TCP?
Network Layer: IP
The Network Utility program has several tools that can
tell you about IP.
Ping
Click on the Ping button on top.
Type cs.umass.edu in the window, and click the Ping
button on the right.
The ping program sends a packet to the destination host
requesting that it send you 10 test packets, one per second. What you see
in the window is the packet information as they arrive.
The rightmost column tells you how much time the test packet took to reach
you, in milliseconds (thousandths of seconds).
After all ten packets arrive, you can see the
min, max, and average round trip times.
Task 3. What is the farthest-away host you can find, in terms
of round trip time? (Write down the URL and the round trip time.)
Traceroute
The Traceroute program sends three probe packets to
the specified destination, and it asks every Internet router encountered
on the way to send back a reply message. What you see are the replies
from the routers, in order from nearest to farthest. You will see
both internal routers (working within a network) and gateway routers
(working at network borders). If a router fails to reply within a
time limit, a "*" is printed. Each row shows:
- A sequence number for the router in the path.
- A domain name for that router. You can usually spot the owner of
the router, and sometimes its location (like nyc or houston), from the
domain name. Some routers don't have names, just IP numbers.
- The IP number for the router, in parentheses. The first number
(sometimes the first two) tell you who owns this router.
- The round trip time for the three probes. If the probes travel
different paths, then you will see different routers responding,
with redundant sequence numbers.
Task 4:
Try running traceroute with a variety of network addresses.
- What are the IP numbers of the network addresses listed in
the table above?
- How many different networks, and how many different routers,
does a packet sent to the west coast USA typically travel over?
- Name two networking companies used in other countries.
netstat
Now try the netstat program.
Select Display routing table information to see the IP routing
table for this computer.
Select Display comprehensive network statistics to see usage
statistics for the protocols that use IP on this computer.
Task 5. Find out the answers to these questions.
- How many different routers does this computer know about?
- Is tcp the biggest network user on this machine, counting both
packets sent and received? If not, what is?
- How many tcp packets arrived out-of-order?
Data Link Layer: Ethernet LAN
Now open the program Applications/Utilities/X11. A little window
will pop up. In the window, type /usr/local/bin/wireshark. Ignore
the Permission Denied message and wait -- the program takes a little
while to start.
Wireshark can capture Ethernet frames that are sent to and from
computers on your local network. Note that IT has blocked Wireshark
so you can't see HTTP and SMTP frames that do not involve your
computer (but it is technically possible to see everything on the network).
- Select Capture/Interfaces, and click Start on the first
interface listed (en0). This will start the collection of Ethernet frames.
- Generate some network traffic -- open your web browser and
visit some sites outside the College; type in a bad URL; maybe IM somebody.
- Now stop the capture by clicking Capture/Stop
- A list of all the frames you captured appears in the window.
The top part shows all the frames, one per line. If you click on a line,
you can see more information about the frame: the middle section shows
the frame header (source, destination, etc), and the bottom section shows
the frame contents: you should be able to read some text on the right side.
Task 6.
- If you click on Protocol you can sort by protocol name.
Do that, then scroll down to find the HTTP frames you generated. Your
computer was either the Source or the Destination in all these frames.
What is the IP number of the computer you are using?
- Scroll to see the DNS and MDNS packets generate by your browser
to locate IP numbers for addresses you typed in. What was the destination
of these packets?
- Find some frames that do not involve your computer as either the
source or destination. Name two protocols used to send these frames.
When you are finished, close all windows and Log Out of this
computer.
Physical Layer
Visit my Museum of Transmission Media before you leave.