ttcp tool to measure
effective bandwidth, or throughput, on an emulated network.
This lab is based on experiments 11.1 and 11.2 from Hands-On Networking with Internet Technologies by Douglas Comer (2nd edition, 2005).
You may work individually or in pairs. If you work with a partner, please submit only one writeup. (One partner's lab notebook can link to the other.) You have one week to complete this assignment.
Connect to node A using
Read the manpage for the
On your node, test
ttcp in loopback, that is,
connecting back to the same computer. Start one copy in receive mode
and another in transmit mode. I suggest you use the
ttcp will automatically source (that
is, create) and sink
(discard) its own data. You can use the -n option to tell
many buffers of data to send; this should be sufficiently large that
the transfer takes tens of seconds.
> ttcp -r -s &
> ttcp -t -s -n 100000 localhost
You will have to wait while the data is transferred---be
patient! Note that
using tcp by default.
In your lab notebook, record the size of data sent (in bytes) and the time taken (in seconds). Record the throughput in Kbps or Mbps. (Note you will need to convert bytes to bits.)
Repeat the loopback test using UDP (the
Record the size, time, and throughput. How does UDP
throughput compare to TCP throughput?
Using TCP to obtain measurements, measure the throughput
for buffer sizes ranging from 16KB to 208KB in increments of 32KB. Use
-b option to specify the socket buffer
your measurements in a file or spreadsheet and plot the data. Paste
your plot into your lab notebook.
-Toption causes the sending
ttcpto "touch" each byte of data as it is read, to emulate processing on the receiver. Repeat the TCP experiment in exercise 3 with the
-Toption set for the receiver. How is throughput affected?
ssh. (You should already have an
sshconnection open to node A.)
Measure TCP throughput on the 100 Mpbs LAN as follows.
ttcp receiver on node A,
and the sender on node B. For the sender, instead of a
localhost give the short name
of the other machine (e.g.,
Transfer at least 100000 buffers, or enough data that the transfer takes tens of seconds. Be patient! However, it is more important that the connection take long enough time to reach steady state, than that you transfer all 819,200,000 bytes of data. If you wait more than about 60 seconds and it has not yet stopped, it is OK to hit CTRL-C on the sender. The throughput data collected will still be good.
Conduct a series of 5 such experiments, sending data from one node to the other. Record the throughput each time.
Compute and record three values:
Repeat your experiment with the 10 Mbps LAN by using nodes A and C. You can reduce the transfer size, as long as it still takes tens of seconds to complete the transfer. Compare the results.
Obtain the C language source code for
and write a paragraph or two giving a high-level explanation of how it
works. (Note: The code is not hugely long, but neither is it
well-documented. Do your best, but don't obsess!)
Create an experimental network topology in a "barbell" shape: Two "servers" should connect to a single router, two "clients" connect to a single router, and the two routers connect to each other.
Run two ttcp instances simultaneously across two client-server
pairs. How is TCP throughput affected by having two competing
flows? What if the bottleneck is the "core" link vs. the "edge"
Include your experimental setup, procedure, and results in your lab notebook.
You may wish to extend your experiment to more than two client-server pairs. What do you think will happen?
a tool for measuring bottleneck bandwidth. Revise your experimental
configuration from part A to use Linux
as the node OS, install pathrate (if it is not already installed), and
measure bottleneck bandwidths using pathrate. How do they compare to
the effective bandwidths measured using ttcp? How does the method used
for measuring bandwidth
differ? When might one approach or the other be preferable?
ttcpto measure TCP throughput across one or more "real" networks. To do this, try the following steps. You may want to work with a partner.
Identify two or more machines where you have login access
and can install (if necessary) and run
is installed by default on FreeBSD; there is a Darwin package to
install it on Mac OS; you can also get it for Linux.)
Locate, download, compile, and/or install
on at least two machines.
Record the hostname, IP address, and geographic location (if known) of each machine.
program to determine the RTT between each pair of machines. (You do not
need to determine the bandwidth. Why would RTT be easier to measure
than the "true" bandwidth of the network path?)
Conduct the experiment in part B, exercises 3 and 4, for one or more pairs of machines. You may want to try doing the same pair twice, reversing the role of sender and receiver. What do you learn?
If you do not have login access to two computers in different locations, you may still observe throughput over the Internet using ProtoGeni, a system of hosts in various locations that are dedicated to experiments on the wide-area Internet.
To earn a B, complete parts A, B, and C of the assignment, along with the discussion questions.
To earn an A, also complete parts D and E (one or more options).
I hope the Emulab tutorial will be good preparation for this experiment. You may need to refer back to the tutorial to remind you how to do some things.
ttcp manpage is very
useful. There are also some online tutorials; if you find one useful,
please cite it in your lab notebook and share it with others.
Feel free to ask for help (from me or your classmates) if you get stuck.
Answer the following questions to the best of your understanding.
ttcp? Why does it affect throughput?
Janet Davis (email@example.com)Created January 29, 2009