Network Working Group M. Schwartz
Request for Comments: 1273 University of Colorado
November 1991
A Measurement Study of Changes in
Service-Level Reachability in the Global
TCP/IP Internet: Goals, Experimental Design,
Implementation, and Policy Considerations
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
In this report we discuss plans to carry out a longitudinal
measurement study of changes in service-level reachability in the
global TCP/IP Internet. We overview our experimental design,
considerations of network and remote site load, mechanisms used to
control the measurement collection process, and network appropriate
use and privacy issues, including our efforts to inform sites
measured by this study. A list of references and information on how
to contact the Principal Investigator are included.
Introduction
The global TCP/IP Internet interconnects millions of individuals at
thousands of institutions worldwide, offering the potential for
significant collaboration through network services and electronic
information exchange. At the same time, such powerful connectivity
offers many avenues for security violations, as evidenced by a number
of well publicized events over the past few years. In response, many
sites have imposed mechanisms to limit their exposure to security
intrusions, ranging from disabling certain inter-site services, to
using external gateways that only allow electronic mail delivery, to
gateways that limit remote interactions via access control lists, to
disconnection from the Internet. While these measures are preferable
to the damage that could occur from security violations, taken to an
extreme they could eventually reduce the Internet to little more than
a means of supporting certain pre-approved point-to-point data
transfers. Such diminished functionality could hinder or prevent the
deployment of important new types of network services, impeding both
research and commercial advancement.
To understand the evolution of this situation, we have designed a
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RFC 1273 A Measurement Study November 1991
study to measure changes in Internet service-level reachability over
a period of one year. The study considers upper layer service
reachability instead of basic IP connectivity because the former
indicates the willingness of organizations to participate in inter-
organizational computing, which will be an important component of
future wide area distributed applications.
The data we gather will contribute to Internet research and
engineering planning activities in a number of ways. The data will
indicate the mechanisms sites use to distance themselves from
Internet connectivity, the types of services that sites are willing
to run (and hence the type of distributed collaboration they are
willing to support), and variations in these characteristics as a
function of geographic location and type of institution (commercial,
educational, etc.). Understanding these trends will allow
application designers and network builders to more realistically plan
for how to support future wide area distributed applications such as
digital library systems, information services, wide area distributed
file systems, and conferencing and other collaboration-support
systems. The measurements will also be of general interest, as they
represent direct measurements of the evolution of a global electronic
society.
Clearly, a study of this nature and magnitude raises a number of
potential concerns. In this note we overview our experimental
design, considerations of network and remote site load, mechanisms
used to control the measurement collection process, and our efforts
to inform sites measured by this study, along with concomitant
network appropriate use and privacy issues.
A point we wish to stress from the outset is that this is not a study
of network security. The experiments do not attempt to probe the
security mechanisms of any machine on the network. The study is
concerned solely with the evolution of network connectivity and
service reachability.
Experimental Design
The study consists of a set of runs of a program over the span of one
to two days each month, repeated bimonthly for a period of one year
(in January 1992, March 1992, May 1992, July 1992, September 1992,
and November 1992). Each program run attempts to connect to 13
different TCP services at each of approximately 12,700 Internet
domains worldwide, recording the failure/success status of each
attempt. The program will attempt no data transfers in either
direction. If a connection is successful, it is simply closed and
counted. (Note in particular that this means that the security
mechanism behind individual network services will not be tested.)
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The machines on which connections are attempted will be selected at
random from a large list of machines in the Internet, constrained
such that at most 1 to 3 machines is contacted in any particular
domain.
The services to which connections will be attempted are:
__________________________________________________________________
Port Number Service Port Number Service
------------------------------------------------------------------
13 daytime 111 Sun portmap
15 netstat 513 rlogin
21 FTP 514 rsh
23 telnet 540 UUCP
25 SMTP 543 klogin
53 Domain Naming System 544 krcmd, kshell
79 finger
_________________________________________________________________
This list was chosen to span a representative range of service
types, each of which can be expected to be found on any machine in a
site (so that probing random machines is meaningful). The one
exception is the Domain Naming System, for which the machines
to probe are selected from information obtained from the Domain
system itself. Only TCP services are tested, since the TCP
connection mechanism allows one to determine if a server is
running in an application-independent fashion.
As an aside, it would be possible to retrieve "Well Known
Service" records from the Domain Naming System, as a somewhat less
"invasive" measurement approach. However, these records are not
required for proper network operation, and hence are far from
complete or consistent in the Domain Naming System. The only way
to collect the data we want is to measure them in the fashion
described above.
Network and Remote Site Load
The measurement software is quite careful to avoid generating
unnecessary internet packets, and to avoid congesting the internet
with too much concurrent activity. Once it has successfully
connected to a particular service in a domain, the software never
attempts to connect to that service on any machine in that domain
again, for the duration of the current measurement run (i.e., the
current 60 days). Once it has recorded 3 connection refusals at any
machines in that domain for a service, it does not try that service
at that domain again during the current measurement run. If it
experiences 3 timeouts on any machine in a domain, it gives up on the
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domain, possibly to be retried again a day later (to overcome
transient network problems). In the worst case there will be 3
connection failures for each service at 3 different machines, which
amounts to 37 connection requests per domain (3 for each of the 12
services other than the Domain Naming System, and one for the Domain
Naming System). However, the average will be much less than this.
To quantify the actual Internet load, we now present some
measurements from test runs of the measurement software that were
performed in August 1991. In total, 50,549 Domain Naming System
lookups were performed, and 73,760 connections were attempted. This
measurement run completed in approximately 10 hours, never initiating
more than 20 network operations (name lookups or connection attempts)
concurrently. The total NSFNET backbone load from all traffic
sources that month was approximately 5 billion packets. Therefore,
the traffic from our measurement study amounted to less than .5% of
this volume on the day that the measurements were collected. Since
the Internet contains several other backbones besides NSFNET, the
proportionate increase in total Internet traffic was significantly
less than .5%.
The cost to a remote site being measured is effectively zero. From
the above measurements, on average we attempted 5.7 connections per
remote domain. The cost of a connection open/close sequence is quite
small, particularly when compared to the cost of the many electronic
mail and news transmissions that most sites experience on a given
day.
Control Over Measurement Collection Process
The measurement software evolved from an earlier set of experiments
used to measure the reach of an experimental Internet white pages
tool called netfind [Schwartz & Tsirigotis 1991b], and has been
evolved and tested extensively over a period of two years. During
this time it has been used in a number of experiments of increasing
scale. The software uses several redundant checks and other
mechanisms to ensure that careful control is maintained over the
network operations that are performed [Schwartz & Tsirigotis 1991a].
In addition, we monitor the progress and network loading of the
measurements during the measurement runs, observing the log of
connection requests in progress as well as physical and transport
level network status (which indicate the amount of concurrent network
activity in progress). Finally, because the measurements are
controlled from a single centralized location, it is quite easy to
stop the measurements at any time.
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Network Appropriate Use and Privacy Issues
When we performed our initial test runs of this study, we attempted
to inform site administrators at each study site about this study, by
posting a message on the USENET newsgroup "alt.security" and by
sending individual electronic mail messages to site administrators.
We also informed the Computer Emergency Response Team (CERT) at CMU
of the study. As a practical matter, informing all sites turned out
to be quite difficult. Part of the problem was that no channels
exist to allow such information to be easily disseminated.
Approximately half of the messages we sent to site administrators
were returned by remote mail systems as undeliverable. Moreover, the
network traffic and remote site administrative load caused by the
study announcement messages far outstripped the network and
administrative load required by the study itself. Some sites felt
that the announcement was an unnecessary imposition of their time.
In addition to these practical problems, a broad announcement of this
study could affect the measurements it attempts to gather. Some
sites would likely react to the announcement by changing the
reachability of their services. Asking for explicit permission from
sites would yield even worse methodological problems, as this would
have provided a self-selected study group consisting of sites that
are less likely to disconnect from the Internet.
In contrast with our attempts to announce the study, running the
study without announcing it caused only a small number of site
administrators to notice the traffic and inquire about it to either
the CERT or to one of the responsible network contacts at the
University of Colorado. The remote site administrator and network
overhead of announcing the the study, coupled with the practical and
methodological problems of announcing the study, lead us to prefer to
run the study without further broad announcements. Yet, to avoid
causing alarm at a site detecting our network measurement activity,
it makes sense to announce the study.
To resolve this problem, we discussed the study with the Internet
Activities Board, Internet Engineering Steering Group, National
Science Foundation, representatives of several U.S. regional
networks, and a number of individuals involved with network security,
including the Computer Emergency Response Team, members of the
Internet Engineering Task Force Security and Advisory Group, and a
member of the Lawrence Livermore National Laboratory Computer
Incident Advisory Capability. The first part of our efforts resulted
in the production of Internet Request For Comments (RFC) number 1262
[Cerf 1991]. Beyond this, we have agreed that the appropriate action
at this point is to announce the study well ahead of running it via
the current RFC, augmented with an electronic posting that briefly
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describes the study goals and methodology and points to this RFC.
That announcement will be posted to the Internet Engineering Task
Force mailing list, the comp.protocols.tcp-ip USENET bulletin board,
and the Computer Emergency Response Team's cert-tools mailing list.
Moreover, in case a site misses these announcements, we will run the
measurement software in a fashion intended to minimize the effort a
site administrator might expend to determine the nature of the
activity after detecting it. In particular, we will run the program
from an account called "testnet" on a machine with few other users
logged in. "Fingering" [Zimmerman 1990] this machine will indicate
the testnet login. "Fingering" the testnet login will return
information about this study.
The data collected by this study is somewhat sensitive to privacy and
security concerns, in the sense that it might be used as a "road map"
of accessible network services. We will treat the raw data as
private information, publishing measurements only in global
statistical terms, divorced from the actual sites that make up the
underlying data points. We previously carried out a study with much
larger privacy implications than the current study [Schwartz & Wood
1991], and successfully masked the data to protect individual
privacy.
For Further Information
Information about the general research program within which this
study fit is available by anonymous FTP from latour.cs.colorado.edu,
in pub/RD.Papers. This directory contains a "README" file that
describes the overall research project (which focuses on resource
discovery), and includes a bibliography. Particularly relevant are:
o [Schwartz 1991b], a project overview;
o [Schwartz 1991a], about an earlier, simpler version of the
current study;
o [Schwartz & Tsirigotis 1991b], about the netfind white pages
tool;
o [Schwartz & Tsirigotis 1991a], which considers a number of
the techniques used in this experiment, including those for
controlling the progress of the measurements;
and
o [Schwartz & Wood 1991], about an earlier study we carried out
that raises significant potential privacy questions, for
which we carefully masked the underlying data, presenting the
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results without sacrificing individual privacy.
Also:
o [Cerf 1991], IAB guidelines for Internet measurement
activity.
Once the results of this study are complete, we will publish them in
a conference or journal, as well as by anonymous FTP.
Communication With Principal Investigator
If you would like to have your site removed from this study, or you
would like to be added to the list of people who receive results from
this study, or you would like to communicate with the Principal
Investigator for some other reason, please send electronic mail to
schwartz@cs.colorado.edu.
References
[Cerf 1991]
Cerf, V., Editor, "Guidelines for Internet Measurement
Activities", RFC 1262, IAB, October 1991.
[Schwartz & Tsirigotis 1991a]
Schwartz M., and P. Tsirigotis, "Techniques for
Supporting Wide Area Distributed Applications", Technical
Report CU-CS-519-91, Department of Computer Science,
University of Colorado, Boulder, Colorado, February 1991;
Revised August 1991. Submitted for publication.
[Schwartz & Tsirigotis 1991b]
Schwartz M., and P. Tsirigotis "Experience with a
Semantically Cognizant Internet White Pages Directory
Tool", Journal of Internetworking: Research and Experience,
2(1), pp. 23-50, March 1991.
[Schwartz 1991a]
Schwartz, M., "The Great Disconnection?", Technical Report
CU-CS-521-91, Department of Computer Science, University of
Colorado, Boulder, Colorado, February 1991.
[Schwartz & Wood 1991]
Schwartz M., and D. Wood, "A Measurement Study of
Organizational Properties in the Global Electronic Mail
Community", Technical Report CU-CS- 482-90, Department of
Computer Science, University of Colorado, Boulder, Colorado,
August 1990; Revised July 1991. Submitted for publication.
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[Schwartz 1991b]
Schwartz, M., "Resource Discovery in the Global Internet",
Technical Report CU-CS-555-91, Department of Computer
Science, University of Colorado, Boulder, Colorado,
November 1991. Submitted for publication.
[Zimmerman 1990]
Zimmerman, D., "The Finger User Information Protocol",
RFC 1194, Center for Discrete Mathematics and Theoretical
Computer Science, November 1990.
Security Considerations
Security issues are discussed in the "Network Appropriate Use and
Privacy Issues" section.
Author's Address
Michael F. Schwartz
Department of Computer Science
Campus Box 430
University of Colorado
Boulder, Colorado 80309-0430
Phone: (303) 492-3902
EMail: schwartz@cs.colorado.edu
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