<pre>Network Working Group                                           R. Clark
Request for Comments: 1683                                      M. Ammar
Category: Informational                                       K. Calvert
                                         Georgia Institute of Technology
                                                             August 1994


                 <span class="h1">Multiprotocol Interoperability In IPng</span>

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document was submitted to the IETF IPng area in response to <a href="./rfc1550">RFC</a>
   <a href="./rfc1550">1550</a>.  Publication of this document does not imply acceptance by the
   IPng area of any ideas expressed within.  Comments should be
   submitted to the big-internet@munnari.oz.au mailing list.

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Executive Summary</span>

   The two most commonly cited issues motivating the introduction of
   IPng are address depletion and routing table growth in IPv4.  Further
   motivation is the fact that the Internet is witnessing an increasing
   diversity in the protocols and services found in the network.  When
   evaluating alternatives for IPng, we should consider how well each
   alternative addresses the problems arising from this diversity.  In
   this document, we identify several features that affect a protocol's
   ability to operate in a multiprotocol environment and propose the
   incorporation of these features into IPng.

   Our thesis, succinctly stated, is:  The next generation Internet
   Protocol should have features that support its use with a variety of
   protocol architectures.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Introduction</span>

   The Internet is not a single protocol network [<a href="#ref-4" title="&quot;The Multiprotocol Internet&quot;">4</a>].  While TCP/IP
   remains the primary protocol suite, other protocols (e.g., IPX,
   AppleTalk, OSI) exist either natively or encapsulated as data within
   IP. As new protocols continue to be developed, we are likely to find
   that a significant portion of the traffic in future networks is not
   from single-protocol communications.  It is important to recognize
   that multiprotocol networking is not just a transition issue.  For
   instance, we will continue to see tunneling used to carry IPX traffic



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   over the Internet between two Novell networks.  Furthermore, the
   introduction of IPng is not going to result in a near term
   elimination of IPv4.  Even when IPng becomes the primary protocol
   used in the Internet, there will still be IPv4 systems in use.  We
   should consider such multiprotocol uses of the network as we design
   future protocols that can efficiently handle mixed protocol traffic.

   We have identified several issues related to the way in which
   protocols operate in a multiprotocol environment.  Many of these
   issues have traditionally been deemed "less important" by protocol
   designers since their goal was to optimize for the case where all
   systems supported the same protocol.  With the increasing diversity
   of network protocols, this approach is no longer practical.  By
   addressing the issues outlined in this paper, we can simplify the
   introduction of IPng to the Internet and reduce the risk for network
   managers faced with the prospect of supporting a new protocol.  This
   will result in a faster, wider acceptance of IPng and increased
   interoperability between Internet hosts.  In addition, by designing
   IPng to address these issues, we will make the introduction of future
   protocols (IPng2) even easier.

   The outline for this document is as follows.  In <a href="#section-3">Section 3</a> we
   motivate the issues of multiprotocol networking with a discussion of
   an example system.  In <a href="#section-4">Section 4</a> we describe three main techniques
   for dealing with multiple protocols.  This is followed in <a href="#section-5">Section 5</a>
   by a description of the various protocol features that are important
   for implementing these three techniques.  We conclude in <a href="#section-6">Section 6</a>
   with a summary of the issues raised.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Multiprotocol Systems</span>

   Consider the multiprotocol architecture depicted in Figure 1.  A
   system supporting this architecture provides a generic file-transfer
   service using either the Internet or OSI protocol stacks.  The
   generic service presents the user with a consistent interface,
   regardless of the actual protocols used.  The user can transfer files
   between this host and hosts supporting either of the single protocol
   stacks presented in Figures 2a and 2b.  To carry out this file
   transfer, the user is not required to decide which protocols to use
   or to adjust between different application interfaces.











<span class="grey">Clark, Ammar &amp; Calvert                                          [Page 2]</span></pre>
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             +-----------------------------------+
             |       File Transfer Service       |
             +-----------+-----------------------+
             |           |         FTAM          |
             |           +-----------------------+
             |   FTP     |       ISO 8823        |
             |           +-----------------------+
             |           |       ISO 8327        |
             |           +-----------+-----------+
             |           |TP0/RFC1006|   TP4     |
             +-----------+-----------+           |
             |          TCP          |           |
             +-----------+-----------+-----------+
             |    IP     |         CLNP          |
             +-----------+-----------------------+


 Figure 1:  Multiprotocol architecture providing file-transfer service


   +-----------+     +-----------+     +-----------+     +-----------+
   |   FTP     |     |   FTAM    |     |   FTAM    |     |   FTP     |
   +-----------+     +-----------+     +-----------+     +-----------+
   |   TCP     |     | ISO 8823  |     | ISO 8823  |     |   TCP     |
   +-----------+     +-----------+     +-----------+     +-----------+
   |    IP     |     | ISO 8327  |     | ISO 8327  |     |   CLNP    |
   +-----------+     +-----------+     +-----------+     +-----------+
                     |   TP4     |     |TP0/RFC1006|
                     +-----------+     +-----------+
                     |   CLNP    |     |   TCP     |
                     +-----------+     +-----------+
                                       |    IP     |
                                       +-----------+

    a) TCP/IP         b) OSI            c) <a href="./rfc1006">RFC 1006</a>       d) TUBA


      Figure 2:  Protocol stacks providing file-transfer service.

   Figure 2c depicts a mixed stack architecture that provides the upper
   layer OSI services using the Internet protocols.  This is an example
   of a "transition architecture" for providing OSI applications without
   requiring a full OSI implementation.  Figure 2d depicts a mixed stack
   architecture that provides the upper layer Internet applications
   using the OSI network protocol.  In addition to communicating with
   the two previous simple protocol stacks, the multiprotocol system of
   Figure 1 includes all the protocols necessary to communicate with
   these two new, mixed protocol stacks.



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   It is likely that many future network systems will be configured to
   support multiple protocols including IPng.  As the IPng protocol is
   deployed, it is unreasonable to expect that users will be willing to
   give up any aspect of their current connectivity for the promise of a
   better future.  In reality, most IPng installations will be made "in
   addition to" the current protocols.  The resulting systems will
   resemble Figure 1 in that they will be able to communicate with
   systems supporting several different protocols.

   Unfortunately, in most current examples, the architecture of Figure 1
   is implemented as independent protocol stacks.  This means that even
   though both TCP and CLNP exist on the system, there is no way to use
   TCP and CLNP in the same communication.  The problem with current
   implementations of architectures like Figure 1 is that they are
   designed as co-existence architectures and are not integrated
   interoperability systems.  We believe future systems should include
   mechanisms to overcome this traditional limitation.  By integrating
   the components of multiple protocol stacks in a systematic way, we
   can interoperate with hosts supporting any of the individual stacks
   as well as those supporting various combinations of the stacks.

   In order to effectively use multiple protocols, a system must
   identify which of the available protocols to use for a given
   communication task.  We call this the Protocol Determination [<a href="#ref-2" title="&quot;On the use of directory services to support multiprotocol interoperability&quot;">2</a>]
   task.  In performing this task, a system determines the combination
   of protocols necessary to provide the needed service.  For achieving
   interoperability, protocols are selected from the intersection of
   those supported on the systems that must communicate.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Multiprotocol Techniques</span>

   In this section we identify three main techniques to dealing with
   multiprotocol networks that are in use today and will continue to be
   used in the Internet.  The first two techniques, tunneling and
   conversion, are categorized as intermediate-system techniques in that
   they are designed to achieve multiprotocol support without changing
   the end-systems.  The third technique explicitly calls for the
   support of multiple protocols in end-systems.  By describing these
   techniques here, we can motivate the need for the specific protocol
   features described in <a href="#section-5">Section 5</a>.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>  Encapsulation/Tunneling</span>

   Encapsulation or tunneling is commonly used when two networks that
   support a common protocol must be connected using a third
   intermediate network running a different protocol.  Protocol packets
   from the two end networks are carried as data within the protocol of
   the intermediate network.  This technique is only appropriate when



<span class="grey">Clark, Ammar &amp; Calvert                                          [Page 4]</span></pre>
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   both end-systems support the same protocol stack.  It does not
   provide interoperability between these end systems and systems that
   only support the protocol stack in the intermediate network.  Some
   examples of this technique are:  a mechanism for providing the OSI
   transport services on top of the Internet protocols [<a href="#ref-13" title="&quot;ISO Transport Services on top of the TCP&quot;">13</a>],
   encapsulating IEEE 802.2 frames in IPX network packets [<a href="#ref-5" title="&quot;Standard for the Transmission of 802.2 Packets over IPX Networks&quot;">5</a>], tunneling
   IPX [<a href="#ref-10" title="&quot;Tunneling IPX Traffic Through IP Networks&quot;">10</a>] and AppleTalk traffic over the Internet backbone.  We expect
   IPng to be used for tunneling other network protocols over IPng and
   to be encapsulated.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>  Translation/Conversion</span>

   Despite their known limitations [<a href="#ref-8" title=" MITRE">8</a>], translation or conversion
   gateways are another technique for handling multiple protocols [11,
   12].  These gateways perform direct conversion of network traffic
   from one protocol to another.  The most common examples of conversion
   gateways are the many electronic mail gateways now in use in the
   Internet.  In certain cases it may also be feasible to perform
   conversion of lower layer protocols such as the network layer.  This
   technique has been suggested as part of the transition plan for some
   of the current IPng proposals [<a href="#ref-3" title="and B. Hinden">3</a>, <a href="#ref-15" title="&quot;CATNIP: Common architecture technology for next- generation internet protocol&quot;">15</a>].

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>  Multiprotocol End-Systems</span>

   We expect that IPng will be introduced as an additional protocol in
   many network systems.  This means that IPng should be able to coexist
   with other protocols on both end- and intermediate-systems.
   Specifically, IPng should be designed to support the Protocol
   Determination task described in <a href="#section-3">Section 3</a>.

   One technique that we consider for solving the Protocol Determination
   problem is to employ a directory service in distributing system
   protocol configuration information.  We have developed and
   implemented mechanism for using the Internet Domain Name System (DNS)
   [<a href="#ref-6" title="&quot;Domain Names - Concepts and Facilities&quot;">6</a>, <a href="#ref-7" title="P.">7</a>] to distribute this protocol information [<a href="#ref-2" title="&quot;On the use of directory services to support multiprotocol interoperability&quot;">2</a>].  Using this
   mechanism, a multiprotocol host can determine the protocol
   configuration of a desired host when it retrieves the network address
   for that host.  Then the multiprotocol host can match the
   configuration of the desired host to its own configuration and
   determine which protocols should be used to carry out the requested
   communication service.

   Another alternative to determining protocol information about another
   host is Protocol Discovery.  Using this approach, a host determines
   which protocols to use by trial-and-error with the protocols
   currently available.  The initiating host monitors successive
   attempts to communicate and uses the information gained from that
   monitoring to build a knowledge base of the possible protocols of the



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   remote system.

   This knowledge is used to determine whether or not a communication
   link can be established and if it can, which protocol should be used.

   An important aspect of the Protocol Discovery approach is that it
   requires an error and control feedback system similar to ICMP [<a href="#ref-9" title="&quot;Internet Control Message Protocol&quot;">9</a>],
   but with additional functionality (See <a href="#section-5">Section 5</a>).

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Protocol Features</span>

   In this section we identify features that affect a protocol's ability
   to support the multiprotocol techniques described in the previous
   section.  These features indicate specific areas that should be
   considered when comparing proposed protocols.  We present two
   different types of protocol features:  those that should be included
   as part of the IPng protocol standard, and those that should be
   considered as part of the implementation and deployment requirements
   for IPng.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>  Protocol Standard Features</span>

   o Addressing

      A significant problem in dealing with multiprotocol networks is
      that most of the popular network protocols use different
      addressing mechanisms.  The problem is not just with different
      lengths but also with different semantics (e.g., hierarchical vs.
      flat addresses).  In order to accommodate these multiple formats,
      IPng should have the flexibility to incorporate many address
      formats within its addressing mechanism.

      A specific example might be for IPng to have the ability to
      include an IPv4 or IPX address as a subfield of the IPng address.
      This would reduce the complexity of performing address conversion
      by limiting the number of external mechanisms (e.g., lookup
      tables) needed to convert an address.  This reduction in
      complexity would facilitate both tunneling and conversion.  It
      would also simplify the task of using IPng with legacy
      applications which rely on a particular address format.

   o Header Option Handling

      In any widely used protocol, it is advantageous to define option
      mechanisms for including header information that is not required
      in all packets or is not yet defined.  This is especially true in
      multiprotocol networks where there is wide variation in the
      requirements of protocol users.  IPng should provide efficient,



<span class="grey">Clark, Ammar &amp; Calvert                                          [Page 6]</span></pre>
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      flexible support for future header options.  This will better
      accommodate the different user needs and will facilitate
      conversion between IPng and other protocols with different
      standard features.

      As part of the support for protocol options, IPng should include a
      mechanism for specifying how a system should handle unsupported
      options.  If a network system adds an option header, it should be
      able to specify whether another system that does not support the
      option should drop the packet, drop the packet and return an
      error, forward it as is, or forward it without the option header.
      The ability to request the "forward as is" option is important
      when conversion is used.  When two protocols have different
      features, a converter may introduce an option header that is not
      understood by an intermediate node but may be required for
      interpretation of the packet at the ultimate destination.  On the
      other hand, consider the case where a source is using IPng with a
      critical option like encryption.  In this situation the user would
      not want a conversion to be performed where the option was not
      understood by the converter.  The "drop the packet" or "drop and
      return error" options would likely be used in this scenario.

   o Multiplexing

      The future Internet protocol should support the ability to
      distinguish between multiple users of the network.  This includes
      the ability to handle traditional "transport layer" protocols like
      TCP and UDP, as well as other payload types such as encapsulated
      AppleTalk packets or future real-time protocols.  This kind of
      protocol multiplexing can be supported with an explicit header
      field as in IPv4 or by reserving part of the address format as is
      done with OSI NSEL's.

      In a multiprotocol network there will likely be a large number of
      different protocols running atop IPng.  It should not be necessary
      to use a transport layer protocol for the sole purpose of
      providing multiplexing for the various network users.  The cost of
      this additional multiplexing is prohibitive for future high-speed
      networks [<a href="#ref-14" title="&quot;Layered multiplexing considered harmful&quot;">14</a>].  In order to avoid the need for an additional level
      of multiplexing, the IPng should either use a payload selector
      larger than the 8-bits used in IPv4 or provide an option for
      including additional payload type information within the header.

   o Status/Control Feedback

      With multiple protocols, the correct transmission of a packet
      might include encapsulation in another protocol and/or multiple
      conversions to different protocols before the packet finally



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      reaches its destination.  This means that there are many different
      places the transmission can fail and determining what went wrong
      will be a challenge.

      In order to handle this situation, a critical protocol feature in
      multiprotocol networks is a powerful error reporting mechanism.

      In addition to reporting traditional network level errors, such as
      those reported by ICMP [<a href="#ref-9" title="&quot;Internet Control Message Protocol&quot;">9</a>], the IPng error mechanism should
      include feedback on tunneling and conversion failures.  Also,
      since it is impossible to know exactly which part of a packet is
      an encapsulated header, it is important that the feedback
      mechanism include as much of the failed packet as possible in the
      returned error message.

      In addition to providing new types of feedback, this mechanism
      should support variable resolution such that a transmitting system
      can request limited feedback or complete information about the
      communication process.  This level of control would greatly
      facilitate the Protocol Discovery process described in <a href="#section-4.3">Section</a>
      <a href="#section-4.3">4.3</a>.  For example, a multiprotocol system could request maximal
      feedback when it sends packets to a destination it has not
      communicated with for some time.  After the first few packets to
      this "new" destination, the system would revert back to limited
      feedback, freeing up the resources used by the network feedback
      mechanisms.

      Finally, it is important that the information provided by the
      feedback mechanism be available outside the IPng implementation.
      In multiprotocol networks it is often the case that the solution
      to a communication problem requires an adjustment in one of the
      protocols outside the network layer.  In order for this to happen,
      the other protocols must be able to access and interpret these
      feedback messages.

   o MTU Discovery or Fragmentation

      A form of multiprotocol support that has long been a part of
      networking is the use of diverse data link and physical layers.
      One aspect of this support that affects the network layer is the
      different Maximum Transmission Units (MTU) used by various media
      formats.  For efficiency, many protocols will attempt to avoid
      fragmentation at intermediate nodes by using the largest packet
      size possible, without exceeding the minimum MTU along the route.
      To achieve this, a network protocol performs MTU discovery to find
      the smallest MTU on a path.





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      The choice of mechanism for dealing with differing MTUs is also
      important when doing conversion or tunneling with multiple
      protocols.  When tunneling is performed by an intermediate node,
      the resulting packets may be too large to meet the MTU
      requirements.  Similarly, if conversion at an intermediate node
      results in a larger protocol header, the new packets may also be
      too large.  In both cases, it may be desirable to have the source
      host reduce the transmission size used in order to prevent the
      need for additional fragmentation.  This information could be sent
      to the source host as part of the previously described feedback
      mechanism or as an additional MTU discovery message.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>  Implementation/Deployment Features</span>

   o Switching

      We define switching in a protocol as the capability to
      simultaneously use more than one different underlying protocol
      [<a href="#ref-1" title="&quot;Multi-protocol architectures as a paradigm for achieving inter-operability&quot;">1</a>].  In network layer protocols, this implies using different
      datalink layers.  For example, it may be necessary to select
      between the 802.3 LLC and traditional Ethernet interfaces when
      connecting a host to an "ethernet" network.  Additionally, in some
      systems IPng will not be used directly over a datalink layer but
      will be encapsulated within another network protocol before being
      transmitted.  It is important that IPng be designed to support
      different underlying datalink services and that it provide
      mechanisms allowing IPng users to specify which of the available
      services should be used.

   o Directory Service Requirements

      While not specifically a part of the IPng protocol, it is clear
      that the future Internet will include a directory service for
      obtaining address information for IPng.  In light of this, there
      are some features of the directory service that should be
      considered vis-a-vis their support for multiple protocols.

      First, the directory service should be able to distribute address
      formats for several different protocol families, not just IPng and
      IPv4.  This is necessary for the use of tunneling, conversion, and
      the support of multiprotocol systems.  Second, the directory
      service should include support for distributing protocol
      configuration information in addition to addressing information
      for the network hosts.  This feature will support the protocol
      determination task to be carried out by multiprotocol systems [<a href="#ref-2" title="&quot;On the use of directory services to support multiprotocol interoperability&quot;">2</a>].






<span class="grey">Clark, Ammar &amp; Calvert                                          [Page 9]</span></pre>
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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Conclusion</span>

   Future networks will incorporate multiple protocols to meet diverse
   user requirements.  Because of this, we are likely to find that a
   significant portion of the traffic in the Internet will not be from
   single-protocol communications (e.g., TCPng/IPng).  This will not
   just be true of near term, transitional networks but will remain as a
   reality for most of the Internet.  As we pursue the selection of
   IPng, we should consider the special needs of multiprotocol networks.
   In particular, IPng should include mechanisms to handle mixed
   protocol traffic that includes tunneling, conversion, and
   multiprotocol end-systems.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Acknowledgments</span>

   The authors would like to acknowledge the support for this work by a
   grant from the National Science Foundation (NCR-9305115) and the
   TRANSOPEN project of the Army Research Lab (formerly AIRMICS) under
   contract number DAKF11-91-D-0004.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  References</span>

   [<a id="ref-1">1</a>] Clark, R., Ammar, M., and K. Calvert, "Multi-protocol
       architectures as a paradigm for achieving inter-operability", In
       Proceedings of IEEE INFOCOM, April 1993.

   [<a id="ref-2">2</a>] Clark, R., Calvert, K. and M. Ammar, "On the use of directory
       services to support multiprotocol interoperability", To appear in
       proceedings of IEEE INFOCOM, 1994. Technical Report GIT-CC-93/56,
       College of Computing, Georgia Institute of Technology, ATLANTA,
       GA 30332-0280, August 1993.

   [<a id="ref-3">3</a>] Gilligan, R., Nordmark, E., and B. Hinden, "IPAE: the SIPP
       Interoperability and Transition Mechanism, Work in Progress,
       November 1993.

   [<a id="ref-4">4</a>] Leiner, B., and Y. Rekhter, "The Multiprotocol Internet", <a href="./rfc1560">RFC</a>
       <a href="./rfc1560">1560</a>, USRA, IBM, December 1993.

   [<a id="ref-5">5</a>] McLaughlin, L., "Standard for the Transmission of 802.2 Packets
       over IPX Networks", <a href="./rfc1132">RFC 1132</a>, The Wollongong Group, November
       1989.

   [<a id="ref-6">6</a>] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
       13, <a href="./rfc1034">RFC 1034</a>, USC/Information Sciences Institute, November 1987.






<span class="grey">Clark, Ammar &amp; Calvert                                         [Page 10]</span></pre>
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   [<a id="ref-7">7</a>] Mockapetris, P., "Domain Names - Implementation and
       Specification.  STD 13, <a href="./rfc1035">RFC 1035</a>, USC/Information Sciences
       Institute, November 1987.

   [<a id="ref-8">8</a>] Padlipsky, M., Gateways, Architectures, and Heffalumps", <a href="./rfc875">RFC 875</a>,
       MITRE, September 1982.

   [<a id="ref-9">9</a>] Postel, J., "Internet Control Message Protocol", STD 5, <a href="./rfc792">RFC 792</a>,
       USC/Information Sciences Institute, September 1981.

  [<a id="ref-10">10</a>] Provan, D., "Tunneling IPX Traffic Through IP Networks", <a href="./rfc1234">RFC</a>
       <a href="./rfc1234">1234</a>, Novell, Inc., June 1991.

  [<a id="ref-11">11</a>] Rose, M., "The Open Book", Prentice-Hall, Englewood Cliffs, New
       Jersey, 1990.

  [<a id="ref-12">12</a>] Rose, M., "The ISO Development Environment User's Manual -
       Version 7.0.", Performance Systems International, July 1991.

  [<a id="ref-13">13</a>] Rose, M., and D. Cass, "ISO Transport Services on top of the
       TCP", STD 35, <a href="./rfc1006">RFC 1006</a>, Northrop Research and Technology Center,
       May 1987.

  [<a id="ref-14">14</a>] Tennenhouse, D., "Layered multiplexing considered harmful", In
       IFIP Workshop on Protocols for High-Speed Networks. Elsevier, May
       1989.

  [<a id="ref-15">15</a>] Ullmann, R., "CATNIP: Common architecture technology for next-
       generation internet protocol", Work in Progress, October 1993.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Security Considerations</span>

   Security issues are not discussed in this memo.


















<span class="grey">Clark, Ammar &amp; Calvert                                         [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc1683">RFC 1683</a>         Multiprotocol Interoperability In IPng      August 1994</span>


<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Authors' Addresses</span>

   Russell J. Clark
   College of Computing Georgia Institute of Technology
   Atlanta, GA 30332-0280

   EMail: rjc@cc.gatech.edu


   Mostafa H. Ammar
   College of Computing Georgia Institute of Technology
   Atlanta, GA 30332-0280

   EMail: ammar@cc.gatech.edu


   Kenneth L. Calvert
   College of Computing Georgia Institute of Technology
   Atlanta, GA 30332-0280

   EMail: calvert@cc.gatech.edu






























Clark, Ammar &amp; Calvert                                         [Page 12]
</pre>