rfc871.txt

RFC 相关的技术文档

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     RFC 871                                            September 1982
                                                                M82-47







               A PERSPECTIVE ON THE ARPANET REFERENCE MODEL
     



     
     

     













                              M.A. PADLIPSKY
                           THE MITRE CORPORATION
                          Bedford, Massachusetts
     
     



                                 Abstract
     

     

          The paper, by one of its developers, describes the
     conceptual framework in which the ARPANET intercomputer
     networking protocol suite, including the DoD standard
     Transmission Control Protocol (TCP) and Internet Protocol (IP),
     were designed.  It also compares and contrasts several aspects of
     the ARPANET Reference Model (ARM) with the more widely publicized
     International Standards Organization's Reference Model for Open
     System Interconnection (ISORM).








































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              "A PERSPECTIVE ON THE ARPANET REFERENCE MODEL"

                              M. A. Padlipsky
     
     
     

                               Introduction

          Despite the fact that "the ARPANET" stands as the
     proof-of-concept of intercomputer networking and, as discussed in
     more detail below, introduced such fundamental notions as
     Layering and Virtualizing to the literature, the wide
     availability of material which appeals to the International
     Standards Organization's Reference Model for Open System
     Interconnection (ISORM) has prompted many new- comers to the
     field to overlook the fact that, even though it was largely
     tacit, the designers of the ARPANET protocol suite have had a
     reference model of their own all the long.  That is, since well
     before ISO even took an interest in "networking", workers in the
     ARPA-sponsored research community have been going about their
     business of doing research and development in intercomputer
     networking with a particular frame of reference in mind.  They
     have, unfortunately, either been so busy with their work or were
     perhaps somehow unsuited temperamentally to do learned papers on
     abstract topics when there are interesting things to be said on
     specific topics, that it is only in very recent times that there
     has been much awareness in the research community of the impact
     of the ISORM on the lay mind.  When the author is asked to review
     solemn memoranda comparing such things as the ARPANET treatment
     of "internetting" with that of CCITT employing the ISORM "as the
     frame of reference," however, the time has clearly come to
     attempt to enunciate the ARPANET Reference Model (ARM)
     publicly--for such comparisons are painfully close to comparing
     an orange with an apple using redness and smoothness as the
     dominant criteria, given the philosophical closeness of the CCITT
     and ISO models and their mutual disparities from the ARPANET
     model.

          This paper, then, is primarily intended as a perspective on
     the ARM.  (Secondarily, it is intended to point out some of the
     differences between the ARM and the ISORM. For a perspective on
     this subtheme, please see Note [1])  It can't be "the official"
     version because the ARPANET Network Working Group (NWG), which
     was the collective source of the ARM, hasn't had an official
     general meeting since October, 1971, and can scarcely be
     resurrected to haggle over it.  It does, at least, represent with
     some degree of fidelity the views of a number of NWG members as
     those views were expressed in NWG general meetings, NWG protocol
     design committee meetings, and private conversations over the
     intervening years. (Members of the current ARPA Internet Working
     Group, which applied


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     RFC 871                                            September 1982


     and adapted the original model to a broader arena than had
     initially been contemplated, were also consulted.)  That might
     not sound so impressive as a pronunciamento from an international
     standards organization, but the reader should be somewhat
     consoled by the consideration that not only are the views
     expressed here purported to be those of the primary workers in
     the field, but also at least one Englishman helped out in the
     review process.

                     Historical/Philosophical Context

          Although rigorous historians of science might quibble as to
     whether they were "invented" by a particular group, it is  an
     historical fact that many now widely-accepted, fundamental
     concepts of intercomputer networking were original to the ARPANET
     Network Working Group. [2]  Before attempting to appreciate the
     implications of that assertion, let's attempt to define its two
     key terms and then cite the concepts it alludes to:

          By "intercomputer networking"  we mean the attachment of
     multiple, usually general-purpose computer systems--in the sense
     of Operating Systems of potentially different manufacture (i.e.,
     "Heterogeneous Operating Systems")--to some communications
     network, or communications networks somehow interconnected, for
     the purpose of achieving resource sharing amongst the
     participating operating systems, usually called Hosts.  (By
     "resource sharing" we mean the  potential ability for programs on
     each of the Hosts to interoperate with programs on the other
     Hosts and for data housed on each of the Hosts to be made
     available to the other Hosts in a more general and flexible
     fashion than merely enabling users on each of the Hosts to be
     able to login to the other Hosts as if they were local; that is,
     we expect to do more than mere "remote access" to intercomputer
     networked Hosts.)  By "the ARPANET Network Working Group," we
     mean those system programmers and computer scientists from
     numerous Defense Advanced Research Projects Agency-sponsored
     installations whose home operating systems were intended to
     become early Hosts on the ARPANET.  (By "the ARPANET" we mean,
     depending on context, either that communications network
     sponsored by DARPA which served as proof-of-concept for the
     communications technology known as "packet switching," or,
     consistent with common usage, the intercomputer network which was
     evolved by the NWG that uses that communications network--or
     "comm subnet"--as its inter-Host data transmission medium.)

          The concepts of particular interest are as follows:  By
     analogy to the use of the term in traditional communications, the
     NWG decided that the key to the mechanization of the
     resource-sharing goal (which in turn had been posited in their
     informal charter)





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     RFC 871                                            September 1982


     would be "protocols" that Hosts would interpret both in
     communicating with the comm subnet and in communicating with each
     other.  Because the active entities in Hosts (the programs in
     execution) were widely referred to in Computer Science as
     "processes," it seemed clear that the mechanization of resource
     sharing had to involve interprocess communication; protocols that
     enabled and employed interprocess communication became, almost
     axiomatically, the path to the goal.  Perhaps because the
     limitations of mere remote access were perceived early on, or
     perhaps simply by analogy to the similar usage with regard to
     distinguishing between physical tape drives and tape drives
     associated with some conventionally-defined function like the
     System Input stream or the System Output stream in batch
     operating systems, the discernible communications paths (or
     "channels") through the desired interprocess communication
     mechanism became known as "logical connections"--the intent of
     the term being to indicate that the physical path didn't matter
     but the designator (number) of the logical connection could have
     an assigned meaning, just like logical tape drive numbers.
     Because "modularity" was an important issue in Computer Science
     at the time, and because the separation of Hosts and Interface
     Message Processors (IMP's) was a given, the NWG realized that the
     protocols it designed should be "layered," in the sense that a
     given set of related functions (e.g., the interprocess
     communication mechanism, or "primitives," as realized in a
     Host-to-Host protocol) should not take special cognizance of the
     detailed internal mechanics of another set of related functions
     (e.g., the comm subnet attachment mechanism, as realized in a
     Host-Comm Subnet Processor protocol), and that, indeed, protocols
     may be viewed as existing in a hierarchy.

          With the notion of achieving resource sharing via layered
     protocols for interprocess communication over logical connections
     fairly firmly in place, the NWG turned to how best to achieve the
     first step of intercomputer networking:  allowing a distant user
     to login to a Host as if local--but with the clear understanding
     that the mechanisms employed were to be generalizable to other
     types of resource sharing.  Here we come to the final fundamental
     concept contributed by the NWG, for it was observed that if n
     different types of Host (i.e., different operating systems) had
     to be made aware of the physical characteristics of m different
     types of terminal in order to exercise physical control over
     them--or even if n different kinds of Host had to become aware of
     the native terminals supported by m other kinds of Hosts if
     physical control were to remain local--there would be an