rfc787.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Request For Comments:  787                              A. Lyman Chapin
                                                             July 1981













Subject:  Connectionless Data Transmission Survey/Tutorial

From:     A. Lyman Chapin






The attached paper on connectionless  data  transmission  is  being
distributed to the members of a number of US organizations that are
involved or interested in the  development  of  international  data
communication standards.  Following a review period ending  Septem-
ber 1, 1981, a revised version of the paper  -  incorporating  com-
ments and suggestions received from reviewers - will be  considered
by the  American  National  Standards  Institute  (ANSI)  committee
responsible for Open Systems Interconnection (OSI) Reference  Model
issues (ANSC X3T5).  If approved, it will then be presented to  the
relevant  International  Organization  for  Standardization   (ISO)
groups as the foundation of a US position recommending  the  incor-
poration of connectionless data transmission by the Reference Model
and related OSI service and protocol standards.

Your comments on the paper, as well as an indication of the  extent
to which the concepts and services of connectionless data transmis-
sion are important to you and/or your organization,  will  help  to
ensure that the final version reflects a true  US  position.   They
should be directed to the author at the following address:




A. Lyman Chapin
Data General Corporation MS E111
4400 Computer Drive
Westborough, MA 01580

[phone] x3056

Connectionless Data Transmission, Rev. 1.00


                                ,---------------------------------,
X3S33/X3T56/81-85               |          WORKING PAPER          |
X3T5/81-171                     | This document has not been re-  |
X3T51/81-44                     | viewed or approved by the appro-|
X3S37/81-71R                    | priate Technical Committee and  |
                                | does not at this time represent |
                                | a USA consensus.                |
                                '---------------------------------'
















                  Connectionless Data Transmission


                          A. Lyman Chapin


                   22 May 1981     Revision  1.00

Connectionless Data Transmission, Rev. 1.00
















                      ABSTRACT

 The increasingly  familiar  and  ubiquitous  Re-
 ference Model of Open  Systems  Interconnection,
 currently being considered by the  International
 Organization  for  Standardization   (ISO)   for
 promotion to the status of a Draft International
 Standard, is based on  the  explicit  assumption
 that a "connection" - an association between two
 or  more  communicating   entities,   possessing
 certain characteristics  over  and  above  those
 possessed  by  the  entities  themselves  -   is
 required for the transfer of  data  in  an  Open
 Systems   Interconnection   (OSI)   environment.
 Although  the   connection-oriented   model   of
 communications behavior  has  proven  to  be  an
 extremely powerful concept, and has been applied
 successfully to the design and implementation of
 protocols and systems covering a wide  range  of
 applications, a growing  body  of  research  and
 experience suggests that a complementary concept
 -  connectionless  data  transmission  -  is  an
 essential part of the Open Systems  Interconnec-
 tion architecture, and  should  be  embraced  as
 such by the OSI  Reference  Model.   This  paper
 explores  the  concept  of  connectionless  data
 transmission and its relationship  to  the  more
 familiar concepts  of  connection-oriented  data
 transfer, developing a rationale for the  inclu-
 sion  of  the  connectionless  concept  in   the
 Reference Model  as  an  integral  part  of  the
 standard description of the OSI architecture.

Connectionless Data Transmission, Rev. 1.00





1  Introduction


 Over the past three years, a number  of  national  and  interna-
 tional  standards  organizations  have  expended  the  time  and
 efforts of a great many people to achieve a  description  of  an
 architectural  Reference  Model  for  interconnecting   computer
 systems considered to be "open" by virtue of their mutual use of
 standard  communication  protocols  and  formats.   The  current
 description, the Reference Model of Open Systems Interconnection
 (RM/OSI)[1], is generally accepted by the International  Organi-
 zation for Standardization (ISO),  the  International  Telephone
 and Telegraph Consultatitive  Committee  (CCITT),  the  European
 Computer Manufacturer's Association (ECMA),  and  many  national
 standards bodies,  including  the  American  National  Standards
 Institute (ANSI), and has progressed to the status  of  a  Draft
 Proposed Standard (DP7498) within ISO.  It  describes  the  con-
 cepts and principles of a communications architecture  organized
 hierarchically, by function, into  seven  discrete  layers,  and
 prescribes the services that each  layer  must  provide  to  the
 layer immediately above it (the  uppermost  layer  provides  its
 services to  user  applications,  which  are  considered  to  be
 outside  of  the  Open  Systems  Interconnection   environment).
 Building on the services available to  it  from  the  next-lower
 layer, each layer makes use  of  standard  OSI  protocols  which
 enable it to cooperate with other instances of  the  same  layer
 (its "peers") in other systems (see Figure 1).   This  technique
 of grouping related functions  into  distinct  layers,  each  of
 which implements a set of well-defined services that are used by
 the layer above, partitions a very complex, abstract  problem  -
 "how can the components of a distributed application,  operating
 in potentially  dissimilar  environments,  cooperate  with  each
 other?" - into a number of more manageable problems that enjoy a
 logical relationship to each other and can individually be  more
 readily understood.

 The Reference Model was developed to serve as  a  framework  for
 the coordination of existing and future  standards  designed  to
 facilitate the interconnection of data processing systems.   The
 purpose of OSI is to enable  an  end-user  application  activity
 (called an "application  process")  located  in  a  system  that
 employs OSI procedures  and  protocols  (an  "open"  system)  to
 communicate with any other appication  process  located  in  any
 other open system.  It is not  the  intent  of  OSI  to  specify
 either the functions or the implementation  details  of  systems
 that provide the OSI capabilities.  Communication is achieved by
 mutual adherence  to  agreed-upon  (standardized)  services  and
 protocols; the only thing that an OSI entity in a given layer in
 one system needs to know about an OSI entity in the  same  layer

User of (N)-services                       User of (N)-services
 [an (N+1)-entity]                           [an (N+1)-entity]
        \                                           /
         \                                         /
          \ /-----(N)-service-access-points-----\ /     (N+1)
-----------o-------------------------------------o------------
            \                                   /        (N)
             \<-----services provided to------>/
              \          (N+1)-layer          /
               \                             /
        ,------------,                 ,------------,
        |            |                 |            |
        | (N)-entity |<----"Peers"---->| (N)-entity |    (N)-LAYER
        |            |                 |            |
        '------------'                 '------------'
               \                             /
                \<----services required---->/
                 \     from (N-1)-layer    /
                  \                       /              (N)
-------------------o---------------------o--------------------
                    \                   /               (N-1)
                     \                 /
                      \               /
                       \             /
             ,--------------------------------,
             |                                |
             |                                |
             |           (N-1)-LAYER          |
             |                                |
             |                                |
             '--------------------------------'



         FIGURE 1 -  General Model of an OSI Layer



A Note on OSI Terminology
-------------------------

The construction of a formal system, such as the architecture of
Open Systems Interconnection, necessarily involves the introduc-
tion of unambiguous terminology (which also tends to be somewhat
impenetrable at first glance).   The terms found here and in the
text are all defined in an Appendix. The "(N)-" notation is used
to emphasize that the term  refers to an OSI characteristic that
applies to each layer individually.  The "(N)-" prefix stands in
generically  for the  name of a layer;  thus, "(N)-address", for
example, refers abstractly to the concept of an address associa-
ted with a specific  layer, while  "transport-address" refers to
the same concept applied to the transport layer.

Connectionless Data Transmission, Rev. 1.00



 of another system is how the other entity behaves, not how it is
 implemented.  In particular, OSI is not concerned with  how  the
 interfaces between adjacent layers are implemented  in  an  open
 system; any interface mechanism is acceptable,  as  long  as  it
 supports access to the appropriate standard OSI services.

 A major goal of the OSI standardization  effort  is  generality.
 Ideally, the Reference Model should serve as the  common  archi-
 tectural framework  for  many  different  types  of  distributed
 systems   employing   a   wide   range   of    telecommunication
 technologies, and certainly an important measure of the  success
 of OSI will be its ability to apply  the  standard  architecture
 across a broad spectrum of user applications.  The way in  which
 the Reference Model has  developed  over  the  past  four  years
 reflects an awareness of this goal (among others):  the  process
 began with the identification of the  essential  concepts  of  a
 layered  architecture,  including  the   general   architectural
 elements of protocols, and proceeded carefully from these  basic
 principles to a detailed description of each layer.  The organi-
 zation of the current Reference Model document [1] exhibits  the
 same top-down progression.  At the highest level, three elements
 are identified as basic to the architecture[1]:

      a) the application processes which exist  within  the  Open
         Systems Interconnection environment;

      b) the connections which join the application processes and
         permit them to exchange information; and

      c) systems.

 The assumption that a connection is a  fundamental  prerequisite
 for communication in the OSI environment permeates the Reference
 Model, and is in fact one  of  the  most  useful  and  important
 unifying concepts of the  architecture.   A  growing  number  of
 experts in the field, however, believe that  this  deeply-rooted