rfc2041.txt

RFC 的详细文档！

Noble, et. al.               Informational                     [Page 21]

RFC 2041                 Mobile Network Tracing             October 1996


   The question of just how mobile such mobile hosts are can also be
   investigated through our traces.  Position data are provided by
   traces that either involved GPS or user-supplied positions with our
   trace collection tools.  This data is valuable for comparing and
   validating various mobility prediction algorithms.  Given adequate
   network infrastructure and good signal measurements, we can determine
   the mobile location within a region that is significantly smaller
   than the cell size.  We are developing a tool to combine position
   information and signal measurement from many traces to identify the
   "signal quality" signature for different regions inside a building.
   Once this signature database is completed and validated, it can be
   used to generate position information for other traces that contain
   only the signal quality information.

6. Related Work

   The previous work most relevant to mobile network tracing falls into
   two camps.  The first, chiefly exemplified by tcpdump [7] and the BSD
   Packet Filter, or BPF [8], collect network traffic data.  The second,
   notably Delayline [6], and the later Probe/Fault Injection Tool [4],
   and the University of Lancaster's netowrk emulator [3], provide
   network modulation similar to PaM.

   There are many systems that record network packet traffic; the de
   facto standard is tcpdump, which works in concert with a packet
   filter such as BPF. The packet filter is given a small piece of code
   that describes packets of interest, and the first several bytes of
   each packet found to be interesting is copied to a buffer for tcpdump
   to consume.  This architecture is efficient, flexible, and has
   rightly found great favor with the networking community.

   However, tcpdump cpatures only traffic data.  It records neither
   information concerning mobile networking devices nor mobile host
   location.  Rather than adding seperate software components to a host
   running tcpdump to capture this additional data, we have chosen to
   follow an integrative approach to ease trace file administration.  We
   have kept the lessons of tcpdump and BPF to heart; namely copying
   only the information necessary, and transferring data up to user
   level in batches.  It may well pay to investigate either
   incorporating device and location information directly into BPF, or
   taking the flexible filtering mechanism of BPF and including it in
   our trace collection software.  For the moment, we do not know
   exactly what data we will need to explore the properties of mobile
   networks, and therefore do not exclude any data.

   There are three notable systems that provide packet modulation
   similar to PaM. The earliest such work is Delayline, a system
   designed to emulate wide-area networks atop local-area ones; a goal



Noble, et. al.               Informational                     [Page 22]

RFC 2041                 Mobile Network Tracing             October 1996


   similar to PaM's.  The most striking difference between Delayline and
   PaM is that Delayline's emulation takes place entirely at the user-
   level, and requires applications to be recompiled against a library
   emulating the BSD socket system and library calls.  While this is a
   portable approach that works well in the absence of kernel-level
   source access, it has the disadvantage that not all network traffic
   passes through the emulation layer; such traffic may have a profound
   impact on the performance of the final system.  Delayline also
   differs from PaM in that the emulated network uses a single set of
   parameters for each emulated connection; performance remains fairly
   constant, and cannot change much over time.

   The Lancaster network emulator was designed explicitly to model
   mobile networks.  Rather than providing per-host modulation, it uses
   a single, central server through which all network traffic from
   instrumented applications passes.  While this system also does not
   capture all traffic into and out of a particular host, it does allow
   modulation based on multiple hosts sharing a single emulated medium.
   There is a mechanism to change the parameters of emulation between
   hosts, though it is fairly cumbersome.  The system uses a
   configuration file that can be changed and re-read while the system
   is running.

   The system closest in spirit to PaM is the Probe/Fault Injection
   Tool.  This system's design philosophy allows an arbitrary protocol
   layer -- including device drivers -- to be encapsulated by a layer
   below to modulate existing traffic, and a layer above to generate
   test traffic.  The parameters of modulation are provided by a script
   in an interpreted language, presently Tcl, providing considerable
   flexibility.  However, there is no mechanism to synthesize such
   scripts -- they must be explicitly designed.  Furthermore, the use of
   an interpreted language such as Tcl limits the use of PFI to user-
   level implementations of network drivers, and may have performance
   implications.

7. Future Work

   This work is very much in its infancy; we have only begun to explore
   the possible uses for mobile network traces.  We have uncovered
   several areas of further work.

   The trace format as it stands is very IP-centric.  While one could
   imagine using unknown IP addresses for non-IP hosts, while using
   header-only properties to encode other addressing schemes, this is
   cumbersome at best.  We are looking into ways to more conveniently
   encode other addressing schemes, but are content to focus on IP
   networks for the moment.




Noble, et. al.               Informational                     [Page 23]

RFC 2041                 Mobile Network Tracing             October 1996


   Two obvious questions concerning wireless media are the following.
   How does a group of machines perform when sharing the same bandwidth?
   How asymmetric is the performance of real-world wireless channels?
   While we do have tools for merging traces taken from multiple hosts
   into a single trace file, we've not yet begun to examine such
   multiple-host scenarios in depth.  We are also looking into
   instrumenting wireless base stations as well as end-point hosts.

   Much of our planned work involves the PaM testbed.  First and
   foremost, many wireless channels are known to be asymmetric;
   splitting the replay trace into incoming and outgoing modulation
   entries is of paramount importance.  We would like to extend PaM to
   handle multiple emulated interfaces as well as applying different
   modulation parameters to packets from or to different destinations.
   One could also imagine tracing performance from several different
   networking environments, and switching between such environments
   under application control.  For example, consider a set of traces
   showing radio performance at various altitudes; an airplane simulator
   in a dive would switch from high-altitude modulation traces to low-
   altitude ones.

   Finally, we are anxious to begin exploring the properties of real-
   world mobile networks, and subjecting our own mobile system designs
   to PaM to see how they perform.  We hope others can make use of our
   tools to do the same.

Acknowledgements

   The authors wish to thank Dave Johnson, who provided early pointers
   to related work and helped us immeasurably in RFC formatting.  We
   also wish to thank those who offered comments on early drafts of the
   document:  Mike Davis, Barbara Denny, Mark Lewis, and Hui Zhang.
   Finally, we would like to thank Bruce Maggs and Chris Hobbs, our
   first customers!

   This research was supported by the Air Force Materiel Command (AFMC)
   and ARPA under contract numbers F196828-93-C-0193 and DAAB07-95-C-
   D154, and the State of California MICRO Program.  Additional support
   was provided by AT&T, Hughes Aircraft, IBM Corp., Intel Corp., and
   Metricom.  The views and conclusions contained here are those of the
   authors and should not be interpreted as necessarily representing the
   official policies or endorsements, either express or implied, of
   AFMC, ARPA, AT&T, Hughes, IBM, Intel, Metricom, Carnegie Mellon
   University, the University of California, the State of California, or
   the U.S. Government.






Noble, et. al.               Informational                     [Page 24]

RFC 2041                 Mobile Network Tracing             October 1996


Security Considerations

   This RFC raises no security considerations.

Authors' Addresses

   Questions about this document can be directed to the authors:

   Brian D. Noble
   Computer Science Department
   Carnegie Mellon University
   5000 Forbes Avenue
   Pittsburgh, PA  15213-3891

   Phone:  [phone]
   Fax:    [phone]
   EMail: [e-mail]


   Giao T. Nguyen
   Room 473 Soda Hall #1776 (Research Office)
   University of California, Berkeley
   Berkeley, CA  94720-1776

   Phone:  [phone]
   Fax:    [phone]
   EMail: [e-mail]


   Mahadev Satyanarayanan
   Computer Science Department
   Carnegie Mellon University
   5000 Forbes Avenue
   Pittsburgh, PA  15213-3891

   Phone:  [phone]
   Fax:    [phone]
   EMail: [e-mail]


   Randy H. Katz
   Room 231 Soda Hall #1770 (Administrative Office)
   University of California, Berkeley
   Berkeley, CA  94720-1770

   Phone:  [phone]
   Fax:    [phone]
   EMail: [e-mail]



Noble, et. al.               Informational                     [Page 25]

RFC 2041                 Mobile Network Tracing             October 1996


References

    [1] Chen, J. B., and Bershad, B. N.  The Impact of Operating System
        Structure on Memory System Performance.  In Proceedings of the
        14th ACM Symposium on Operating System Principles (Asheville,
        NC, December 1993).

    [2] Dahlin, M., Mather, C., Wang, R., Anderson, T., and Patterson,
        D.  A Quantitative Analysis of Cache Policies for Scalable
        Network File Systems.  In Proceedings of the 1994 ACM SIGMETRICS
        Conference on Measurement and Modeling of Computer Systems
        (Nashville, TN, May 1994).

    [3] Davies, N., Blair, G. S., Cheverst, K., and Friday, A.  A
        Network Emulator to Support the Development of Adaptive
        Applications.  In Proceedings of the 2nd USENIX Symposium on
        Mobile and Location Independent Computing (April 10-11 1995).

    [4] Dawson, S., and Jahanian, F.  Probing and Fault Injection of
        Dependable Distributed Protocols.  The Computer Jouranl 38, 4
        (1995).

    [5] Gloy, N., Young, C., Chen, J. B., and Smith, M. D.  An Analysis
        of Dynamic Branch Prediction Schemes on System Workloads.  In
        The Proceedings of the 23rd Annual International Symposium on
        Computer Architecture (May 1996).

    [6] Ingham, D. B., and Parrington, G. D.  Delayline:  A Wide-Area
        Network Emulation Tool.  Computing Systems 7, 3 (1994).

    [7] Jacobson, V., Leres, C., and McCanne, S.  The Tcpdump Manual
        Page.  Lawrence Berkeley Laboratory, Berkeley, CA.

    [8] McCanne, S., and Jacobson, V.  The BSD Packet Filter:  A New
        Architecture for User-level Packet Capture.  In Proceedings of
        the 1993 Winter USENIX Technical Conference (San Deigo, CA,
        January 1993).

    [9] Mills, D. L.  Improved Algorithms for Synchronizing Computer
        Network Clocks.  IEEE/ACM Transactions on Networking 3, 3 (June
        1995).

   [10] Mummert, L. B., Ebling, M. R., and Satyanarayanan, M.
        Exploiting Weak Connectivity for Mobile File Access.  In
        Proceedings of the 15th Symposium on Operating System Prinicples
        (Copper Mountain, CO, December 1995).





Noble, et. al.               Informational                     [Page 26]

RFC 2041                 Mobile Network Tracing             October 1996


   [11] Nelson, M. N., Welch, B. B., and Ousterhout, J. K.  Caching in
        the Sprite Network File System.  ACM Transactions on Computer
        Systems 6, 1 (February 1988).

   [12] Schilit, B., Adams, N., Gold, R., Tso, M., and Want, R.  The
        PARCTAB Mobile Computing System.  In