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

   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.)

   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

   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.

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

   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

        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.

   [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