rfc1006.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

M. Rose & D. Cass                                               [Page 1]




Network Working Group                    Marshall T. Rose, Dwight E. Cass
Request for Comments: RFC 1006    Northrop Research and Technology Center
Obsoletes: RFC 983                                               May 1987



                ISO Transport Service on top of the TCP
                               Version: 3


Status of this Memo

   This memo specifies a standard for the Internet community. Hosts
   on the Internet that choose to implement ISO transport services
   on top of the TCP are expected to adopt and implement this
   standard.  TCP port 102 is reserved for hosts which implement this
   standard.  Distribution of this memo is unlimited.

   This memo specifies version 3 of the protocol and supersedes
   [RFC983].  Changes between the protocol as described in Request for
   Comments 983 and this memo are minor, but are unfortunately
   incompatible.
































M. Rose & D. Cass                                               [Page 1]

RFC 1006                                                        May 1987


1.  Introduction and Philosophy


      The Internet community has a well-developed, mature set of
      transport and internetwork protocols (TCP/IP), which are quite
      successful in offering network and transport services to
      end-users. The CCITT and the ISO have defined various session,
      presentation, and application recommendations which have been
      adopted by the international community and numerous vendors.
      To the largest extent possible, it is desirable to offer these
      higher level directly in the ARPA Internet, without disrupting
      existing facilities.  This permits users to develop expertise
      with ISO and CCITT applications which previously were not
      available in the ARPA Internet.  It also permits a more
      graceful convergence and transition strategy from
      TCP/IP-based networks to ISO-based networks in the
      medium-and long-term.

      There are two basic approaches which can be taken when "porting"
      an ISO or CCITT application to a TCP/IP environment.  One
      approach is to port each individual application separately,
      developing local protocols on top of the TCP.  Although this is
      useful in the short-term (since special-purpose interfaces to the
      TCP can be developed quickly), it lacks generality.

      A second approach is based on the observation that both the ARPA
      Internet protocol suite and the ISO protocol suite are both
      layered systems (though the former uses layering from a more
      pragmatic perspective).  A key aspect of the layering principle
      is that of layer-independence.  Although this section is
      redundant for most readers, a slight bit of background material
      is necessary to introduce this concept.

      Externally, a layer is defined by two definitions:

         a service-offered definition, which describes the services
         provided by the layer and the interfaces it provides to
         access those services; and,

         a service-required definitions, which describes the services
         used by the layer and the interfaces it uses to access those
         services.

      Collectively, all of the entities in the network which co-operate
      to provide the service are known as the service-provider.
      Individually, each of these entities is known as a service-peer.

      Internally, a layer is defined by one definition:

          a protocol definition, which describes the rules which each
          service-peer uses when communicating with other service-peers.



M. Rose & D. Cass                                               [Page 2]

RFC 1006                                                        May 1987


      Putting all this together, the service-provider uses the protocol
      and services from the layer below to offer the its service to the
      layer above.  Protocol verification, for instance, deals with
      proving that this in fact happens (and is also a fertile field
      for many Ph.D. dissertations in computer science).

      The concept of layer-independence quite simply is:

          IF one preserves the services offered by the service-provider

          THEN the service-user is completely naive with respect to the
          protocol which the service-peers use


      For the purposes of this memo, we will use the layer-independence
      to define a Transport Service Access Point (TSAP) which appears
      to be identical to the services and interfaces offered by the
      ISO/CCITT TSAP (as defined in [ISO8072]), but we will in fact
      implement the ISO TP0 protocol on top of TCP/IP (as defined in
      [RFC793,RFC791]), not on top of the the ISO/CCITT network
      protocol.  Since the transport class 0 protocol is used over the
      TCP/IP connection, it achieves identical functionality as
      transport class 4.  Hence, ISO/CCITT higher level layers (all
      session, presentation, and application entities) can operate
      fully without knowledge of the fact that they are running on a
      TCP/IP internetwork.




























M. Rose & D. Cass                                               [Page 3]

RFC 1006                                                        May 1987


2.  Motivation


      In migrating from the use of TCP/IP to the ISO protocols, there
      are several strategies that one might undertake.  This memo was
      written with one particular strategy in mind.

      The particular migration strategy which this memo uses is based
      on the notion of gatewaying between the TCP/IP and ISO protocol
      suites at the transport layer.  There are two strong arguments
      for this approach:

      1.  Experience teaches us that it takes just as long to get good
      implementations of the lower level protocols as it takes to get
      implementations of the higher level ones.  In particular, it has
      been observed that there is still a lot of work being done at the
      ISO network and transport layers.  As a result, implementations
      of protocols above these layers are not being aggressively
      pursued. Thus, something must be done "now" to provide a medium
      in which the higher level protocols can be developed.  Since
      TCP/IP is mature, and essentially provides identical
      functionality, it is an ideal medium to support this development.

      2.  Implementation of gateways at the IP and ISO IP layers are
      probably not of general use in the long term.  In effect, this
      would require each Internet host to support both TP4 and TCP.
      As such, a better strategy is to implement a graceful migration
      path from TCP/IP to ISO protocols for the ARPA Internet when the
      ISO protocols have matured sufficiently.

      Both of these arguments indicate that gatewaying should occur at
      or above the transport layer service access point.  Further, the
      first argument suggests that the best approach is to perform the
      gatewaying exactly AT the transport service access point to
      maximize the number of ISO layers which can be developed.

        NOTE:     This memo does not intend to act as a migration or
                  intercept document.  It is intended ONLY to meet the
                  needs discussed above.  However, it would not be
                  unexpected that the protocol described in this memo
                  might form part of an overall transition plan.  The
                  description of such a plan however is COMPLETELY
                  beyond the scope of this memo.

      Finally, in general, building gateways between other layers in the
      TCP/IP and ISO protocol suites is problematic, at best.

      To summarize: the primary motivation for the standard described in
      this memo is to facilitate the process of gaining experience with
      higher-level ISO protocols (session, presentation, and
      application). The stability and maturity of TCP/IP are ideal for



M. Rose & D. Cass                                               [Page 4]

RFC 1006                                                        May 1987


      providing solid transport services independent of actual
      implementation.




















































M. Rose & D. Cass                                               [Page 5]

RFC 1006                                                        May 1987


3.  The Model


      The [ISO8072] standard describes the ISO transport service
      definition, henceforth called TP.

          ASIDE:    This memo references the ISO specifications rather
                    than the CCITT recommendations.  The differences
                    between these parallel standards are quite small,
                    and can be ignored, with respect to this memo,
                    without loss of generality.  To provide the reader
                    with the relationships:

                         Transport service    [ISO8072]       [X.214]
                         Transport protocol   [ISO8073]       [X.224]