    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

    (617) 366-8911 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
     connection orientation seriously and  unnecessarily  limits  the
     power and scope of the Reference  Model,  since  it  excludes  a
     large class of applications and implementation technologies that
     have an inherently connectionless nature.  They argue  that  the
     architectural objectives of the Reference Model do not depend on
     the  exclusive  use  of  connections  to  characterize  all  OSI
     interactions, and recommend that the two alternatives -  connec-
     tion oriented data transfer, and connectionless  data  transmis-
     sion - be  treated  as  complementary  concepts,  which  can  be
     applied in parallel to the different applications for which each
     is suited.

     At the November, 1980 meeting of the ISO subcommittee  responsi-
     ble for OSI (TC97/SC16), a working party laid a solid foundation
     for this argument in two documents: Report of the Ad  Hoc  Group

    Connectionless Data Transmission, Rev. 1.00

     on Connectionless Data Transmission[3], and Recommended  Changes
     to Section 3 of [the Reference Model] to Include  Connectionless
     Data Transmission[2];  and  the  importance  of  the  issue  was
     recognized by the full subcommittee in a resolution[25]  calling
     for comments on the two documents from all member organizations.
     The question of how the connectionless data transmission concept
     should be reflected in the OSI architecture - and in particular,
     whether or not it should become an  integral  part  of  the  Re-
     ference Model - will be debated  again  this  summer,  when  the
     current Draft Proposed Standard Reference Model becomes a  Draft
     International Standard.  The  remainder  of  this  article  will
     explore the issues that surround this question.

     2  What Is Connectionless Data Transmission?

     Connectionless data transmission (CDT), despite  the  unfamiliar
     name, is by no means a new concept.  In one form or another,  it
     has played an important role in the  specification  of  services
     and protocols for over a decade.  The terms "message  mode"[  ],
     "datagram"[35],      "transaction      mode"[22,23,24],      and
     "connection-free"[37,47] have been used  in  the  literature  to
     describe variations on the same basic theme: the transmission of
     a  data  unit  in  a  single  self-contained  operation  without
     establishing, maintaining, and terminating a connection.

     Since connectionless data transmission  and  connection-oriented
     data transfer are complementary concepts, they are  best  under-
     stood in juxtaposition, particularly since  CDT  is  most  often
     defined by its relationship to the more familiar  concept  of  a
     connection.

     2.1  Connection-Oriented Data Transfer

     A connection (or "(N)-connection", in the formal terminology  of
     OSI) is an association established between two or more  entities
     ("(N+1)-entities")          for          conveying          data
     ("(N)-service-data-units").    The    ability    to    establish
     (N)-connections, and to convey data units over them, is provided
     to (N+1)-entities by the (N)-layer as a set of services,  called
     connection-oriented (N)-services.  Connection-oriented  interac-
     tions proceed through three distinct sequential phases:  connec-
     tion  establishment;  data  transfer;  and  connection  release.
     Figure 2 illustrates schematically the  sequence  of  operations
     associated with connection-oriented interactions.   In  addition
     to this explicitly distinguishable duration,  or  "lifetime",  a
     connection exhibits the following fundamental characteristics:

                         Connection Establishment
                         ------------------------

           - Successful -                        - Unsuccessful -

      (N)-  |          |                     (N)-  |          |
    connect |          |(N)-connect        connect |          |  (N)-
    ------->|          |indication         ------->|          | connect
    request |          |                   request |          |indication
            |          |------->                   |          |------->
            |(N)-LAYER |                           |(N)-LAYER |
      (N)-  |          |<-------            (N)-   |          |<-------
    connect |          |                disconnect |          |  (N)-
    <-------|          |(N)-connect        <-------|          |disconnect
    confirm |          | response       indication |          | request
            |          |                           |          |

                              Data Transfer
                              -------------

      (N)-  |          |                     (N)-  |          |
      data  |          | (N)-data            data  |          |
    ------->|          |indication         ------->|          |  (N)-
    request |          |                   request |          |  data
            |          |------->                   |          |indication
            |(N)-LAYER |                           |(N)-LAYER |------->
            |          |                     (N)-  |          |
            |          |                     data  |          |
            |          |                   <-------|          |
            |          |                   confirm |          |
            |          |                           |          |

                            Connection Release
                            ------------------

         - User Initiated -                   - Provider Initiated -

    (N)-dis |          |                           |          |
    connect |          |                     (N)-  |          |  (N)-
    ------->|(N)-LAYER |(N)-disconnect   disconnect|(N)-LAYER |disconnect
    request |          |indication         <-------|          |------->
            |          |------->         indication|          |indication
            |          |                           |          |

                FIGURE 2 - Connection Oriented Interaction

    Connectionless Data Transmission, Rev. 1.00

             [Note: Much of the material in this  section  is
             derived from reference 3]

     1.  Prior negotiation.

     In a connection-oriented interaction,  no  connection  is  esta-
     blished - and no data are transferred - until all parties  agree
     on the set of parameters and options that will govern  the  data
     transfer.  An incoming connection establishment request  can  be
     rejected if it asserts parameter  values  or  options  that  are
     unacceptable to the receiver, and the receiver may in many cases
     suggest alternative parameter values and options along with  his
     rejection.

     The reason for negotiation during  connection  establishment  is
     the assumption that each party  must  reserve  or  allocate  the
     resources (such as buffers and channels) that will  be  required
     to carry out data transfer operations  on  the  new  connection.
     Negotiation provides an opportunity to scuttle the establishment
     of a connection when the resources that  would  be  required  to
     support it cannot be dedicated, or to propose alternatives  that
     could be supported by the available resources.

     2.  Three-party Agreement.

     The fundamental nature of a connection involves establishing and
     dynamically maintaining a three-party agreement  concerning  the
     transfer of data.  The three parties -  the  two  (N+1)-entities
     that wish to communicate, and the (N)-service that provides them
     with a connection - must first agree on their mutual willingness
     to participate  in  the  transfer  (see  above).   This  initial
     agreement establishes a connection.  Thereafter, for as long  as
     the connection persists, they must  continue  to  agree  on  the
     acceptance of each data unit transferred  over  the  connection.
     "With a connection, there is no  possibility  of  data  transfer
     through an unwilling service to an  unwilling  partner,  because
     the mutual willingness  must  be  established  before  the  data
     transfer can take place,  and  data  must  be  accepted  by  the
     destination partner; otherwise, no  data  [are]  transferred  on
     that connection."[3]

     3.  Connection Identifiers.

     At   connection   establishment   time,    each    participating
     (N+1)-entity is identified to the (N)-service by an (N)-address;
     the (N)-service uses these addresses to  set  up  the  requested
     connection.  Subsequent  requests  to  transfer  data  over  the
     connection (or to release it) refer not to  the  (N)-address(es)
     of the intended recipient(s), but  to  a  connection  identifier

    Connectionless Data Transmission, Rev. 1.00

     supplied   by   the   (N)-service   (in   OSI    parlance,    an
     "(N)-connection-endpoint-identifier").       This      is      a
     locally-significant "shorthand" reference that uniquely  identi-
     fies an established connection during its lifetime.   Similarly,
     the protocol units that carry  data  between  systems  typically
     include a mutually-understood logical identifier rather than the
     actual addresses of the correspondents.  This technique elimina-
     tes the overhead that would otherwise  be  associated  with  the
     resolution and transmission of addresses on every data transfer.
     In some  cases,  however  -  particularly  when  non-homogeneous
     networks are interconnected, and very location-sensitive addres-
     sing schemes are used - it can  make  dynamic  routing  of  data
     units extremely difficult, if not impossible.

     4.  Data Unit Relationship.

     Once a connection has  been  established,  it  may  be  used  to
     transfer one data unit after another, until  the  connection  is
     released by one of the three  parties.   These  data  units  are
     logically related to  each  other  simply  by  virtue  of  being
     transferred on  the  same  connection.   Since  data  units  are
     transferred over a connection  in  sequence,  they  are  related
     ordinally as well.  These data unit relationships are an  impor-
     tant characteristic of connections, since they create a  context
     for the interpretation of arriving data units that  is  indepen-
     dent of the data themselves.  Because a connection maintains the
     sequence  of  messages  associated  with  it,   out-of-sequence,
     missing, and duplicated messages  can  easily  be  detected  and
     recovered, and flow control techniques can be invoked to  ensure
     that the message transfer rate does not exceed  that  which  the
     correspondents are capable of handling.

     These  characteristics  make  connection-based   data   transfer
     attractive in applications that call for relatively  long-lived,
     stream-oriented interactions in stable configurations,  such  as
     direct terminal use of a remote  computer,  file  transfer,  and
     long-term attachments of remote job  entry  stations.   In  such
     applications, the interaction between communicating entities  is
     modelled very well  by  the  connection  concept:  the  entities
     initially discuss their requirements and agree to the  terms  of
     their interaction, reserving whatever resources they will  need;
     transfer a series of related  data  units  to  accomplish  their
     mutual objective; and explicitly end their interaction,  releas-
     ing the previously reserved resources.

     2.2  Connectionless Data Transmission

     In many other applications,  however,  the  interaction  between

    Connectionless Data Transmission, Rev. 1.00

     entities is more naturally modelled by the  connectionless  data
     transmission concept,  which  involves  the  transmission  of  a
     single self-contained data  unit  from  one  entity  to  another
     without prior negotiation or  agreement,  and  without  the  as-
     surance of delivery normally  associated  with  connection-based
     transfers.  The users of a connectionless  (N)-service  may,  of
     course, use their (N+1)-protocol to make any  prior  or  dynamic
     arrangements they wish concerning their  interpretation  of  the
     data transmitted and received; the (N)-service itself,  however,
     attaches no significance to individual data units, and does  not
     attempt to relate them in any way.  Two (N+1)-entities  communi-
     cating by means  of  a  connectionless  (N)-service  could,  for
     example, apply whatever techniques they  might  consider  appro-
     priate  in  the  execution  of  their  own   protocol   (timers,
     retransmission, positive or negative acknowledgements,  sequence
     numbers, etc.) to achieve the level of  error  detection  and/or
     recovery they desired.  Users of a connectionless, as opposed to
     connection-oriented, (N)-service are not restricted or inhibited
     in the performance of their (N+1)-protocol;  obviously,  though,
     the assumption is that CDT  will  be  used  in  situations  that
     either do not require the characteristics of  a  connection,  or
     actively benefit from the alternative characteristics of connec-
     tionless transmission.

     Figure 3 illustrates schematically the single operation  whereby
     a connectionless service may be employed to  transmit  a  single
     data unit.   Figure  4  shows  a  widely-implemented  variation,
     sometimes called  "reliable  datagram"  service,  in  which  the
     service  provider  undertakes  to  confirm   the   delivery   or
     non-delivery of each data unit.  It must be emphasized that this
     is not a true connectionless service, but is  in  some  sense  a
     hybrid, combining the delivery assurance of  connection-oriented
     service with the single-operation interface event of connection-
     less service.

     Many of those involved in OSI  standardization  activities  have
     agreed  on  a  pair  of  definitions  for  connectionless   data
     transmission, one for architectural and conceptual purposes, and
     one  for  service-definition  purposes[4].   The   architectural
     definition, which has been proposed for  inclusion  in  the  Re-
     ference Model, is:

     "Connectionless  Data  Transmission  is  the  transmission  (not
     transfer)   of   an   (N)-service-data-unit   from   a    source
     (N)-service-access-point   to   one    or    more    destination
     (N)-service-access-points without establishing an (N)-connection
     for the transmission."

     The service definition, which is intended to provide a  workable
     basis  for  incorporating  a  connectionless  service  into  the

                    |                       |
          (N)-data  |                       |
           request  |                       |
          --------->|                       |
                    |       (N)-LAYER       |
                    |                       |--------->
                    |                       |  (N)-data
                    |                       | indication
                    |                       |

           FIGURE 3 - Connectionless Data Transmission

          (N)-data  |                       |
           request  |                       |
          --------->|                       |
                    |                       |  (N)-data
                    |       (N)-LAYER       |--------->
                    |                       | indication
          <---------|                       |
          (N)-data  |                       |
           confirm  |                       |

            FIGURE 4 - "Reliable Datagram" Service

    Connectionless Data Transmission, Rev. 1.00

     service descriptions for  individual  layers  of  the  Reference
     Model, is:

     "A Connectionless  (N)-Service  is  one  that  accomplishes  the
     transmission of a  single  self-contained  (N)-service-data-unit
     between  (N+1)-entities  upon  the  performance  of   a   single
     (N)-service access."

     Both of these definitions  depend  heavily  on  the  distinction
     between the terms "transmit", "transfer", and "exchange":

     Transmit: "to cause to pass or be conveyed through  space  or  a
     medium."  This term refers to the act of conveying only, without
     implying anything about reception.

     Transfer: "to convey  from  one  place,  person,  or  thing,  to
     another."  A one-way peer-to-peer connotation restricts the  use
     of this term to cases in which the receiving peer  is  party  to
     and accepts the data transferred.

     Exchange: "to give and receive, or lose and take,  reciprocally,
     as things of the same kind."  A two-way peer-to-peer connotation
     restricts the use of this term to cases in which both  give  and
     receive directions are clearly evident.

     These  definitions  are  clearly  of   limited   usefulness   by
     themselves.  They do, however, provide a framework within  which
     to explore the following characteristics of CDT:

     1.  "One-shot" Operation.

     The most  user-visible  characteristic  of  connectionless  data
     transmission is the single service access required  to  initiate
     the transmission of a data unit.  All  of  the  information  re-
     quired to deliver the data unit - destination  address,  quality
     of service selection,  options,  etc.  -  is  presented  to  the
     connectionless (N)-service provider, along with the data,  in  a
     single logical service-access operation that is  not  considered
     by the (N)-service to be related in  any  way  to  other  access
     operations, prior or subsequent (note, however, that  since  OSI
     is not  concerned  with  implementation  details,  the  specific
     interface mechanism employed by a particular  implementation  of
     connectionless service might involve  more  than  one  interface
     exchange to accomplish what is, from  a  logical  standpoint,  a
     single operation).  Once the service  provider  has  accepted  a
     data unit for connectionless transmission, no further communica-
     tion occurs between the provider and the  user  of  the  service
     concerning the fate or disposition of the data.

    Connectionless Data Transmission, Rev. 1.00

     2.  Two-party Agreement.

     Connection-oriented data transfer requires the establishment  of
     a three-party agreement between the participating (N+1)-entities
     and the (N)-service.  A connectionless service, however,  invol-
     ves only two-party agreements: there may be an agreement between
     the corresponding (N+1)-entities, unknown  to  the  (N)-service,
     and there may be local agreements between each (N+1)-entity  and
     its local (N)-service provider, but no (N)-protocol  information
     is ever exchanged between  (N)-entities  concerning  the  mutual
     willingness of the (N+1)-entities to engage in a  connectionless
     transmission or to accept a particular data unit.

     In practice, some sort of a priori agreement (usually  a  system
     engineering design decision) is assumed  to  exist  between  the
     (N+1)-entities and the (N)-service concerning those  parameters,
     formats, and options that affect all three parties  as  a  unit.
     However, considerable freedom of choice is preserved by allowing
     the user of a connectionless service to specify  most  parameter
     values and options - such as  transfer  rate,  acceptable  error
     rate, etc. - at the time the service is  invoked.   In  a  given
     implementation, if the  local  (N)-service  provider  determines
     immediately (from information available to it locally) that  the
     requested operation cannot be  performed  under  the  conditions
     specified, it may abort  the  service  primitive,  returning  an
     implementation-specific error message across  the  interface  to
     the user.  If the same determination is made later on, after the
     service-primitive interface event has completed,  the  transmis-
     sion is  simply  abandoned,  since  users  of  a  connectionless
     service can be expected to recover lost data if it is  important
     for them to do so.

     3.  Self-contained Data Units.

     Data units transmitted via a connectionless service, since  they
     bear no relationship either to other data units or to a  "higher
     abstraction"   (such   as   a    connection),    are    entirely
     self-contained.  All of the  addressing  and  other  information
     needed by the service provider to deliver a  data  unit  to  its
     destination must be included in each transmission.  On  the  one
     hand, this represents a greater overhead than is incurred during
     the data transfer phase of a connection-oriented interaction; on
     the other, it greatly simplifies routing, since each  data  unit
     carries a complete destination address and can be routed without
     reference to connection-related information that  may  not,  for
     example, be readily available at intermediate nodes.

     4.  Data Unit Independence.

     The connectionless transmission of data creates no relationship,
     express or implied, between data units.  Each  invocation  of  a

    Connectionless Data Transmission, Rev. 1.00

     connectionless service begins the transmission of a single  data
     unit.  Nothing about the service invocation, the transmission of
     the data by the connectionless service, or the data unit  itself
     affects or is affected by any other  past,  present,  or  future
     operation, whether  connection-oriented  or  connectionless.   A
     series of data units handed one after the other to a connection-
     less service for delivery  to  the  same  destination  will  not
     necessarily be delivered to the destination in the  same  order;
     and the connectionless service will make no attempt to report or
     recover instances of non-delivery.

     Note:   A number of popular variations  on  CDT  include
             features that run  counter  to  those  described
             above.  These variations deserve to be discussed
             on their own merits, but should not be  confused
             with the architectural concept of connectionless
             data transmission.

     These characteristics make CDT attractive in  applications  that
     involve short-term request-response interactions, exhibit a high
     level of redundancy, must be flexibly reconfigurable, or  derive
     no benefit from guaranteed in-sequence delivery of data.

     3  The Rationale for Connectionless Data Transmission

     Because CDT is not as widely understood  as  connection-oriented
     data transfer, it has often been  difficult  in  the  course  of
     developing service and protocol definitions to adduce a  ration-
     ale for incorporating CDT, and even more difficult to  determine
     appropriate locations  for  connectionless  service  within  the
     layered hierarchy of OSI.   This  section  addresses  the  first
     concern; the next section will deal with the second.

     The most natural way to discover the power and  utility  of  the
     CDT  concept  is  to  examine  applications  and  implementation
     technologies that depend on it.  The following observations  are
     distilled from the specifications  and  descriptions  of  actual
     protocols and systems (many of which have been implemented), and
     from the work of individuals and organizations  engaged  in  the
     OSI standardization effort (quoted material is from reference 3,
     except where otherwise noted).   They  are  divided  into  seven
     (occassionally  overlapping)  categories  which   classify   the
     applications for which CDT is well suited.

     Inward data collection involves the periodic active  or  passive
     sampling of a large  number  of  data  sources.   A  sensor  net

    Connectionless Data Transmission, Rev. 1.00

     gathering data from dedicated measurement  stations;  a  network
     status monitor constantly refreshing its knowledge of a  network
     environment; and an automatic alarm or security system in  which
     each component regularly self-tests and reports the result,  are
     all engaged in this type  of  interaction,  in  which  a  "large
     number of sources may be reporting periodically  and  asynchron-
     ously to a single reporting point.   In  a  realtime  monitoring
     situation, these readings could normally be  lost  on  occassion
     without causing distress,  because  the  next  update  would  be
     arriving shortly.  Only  if  more  than  one  successive  update
     failed to arrive within a specified time limit would an alarm be
     warranted.   If,  say,  a  fast   connect/disconnect   three-way
     handshake cost twice as much as a one-way [connectionless]  data
     transmission which had  been  system  engineered  to  achieve  a
     certain acceptable statistical reliability figure, the  cost  of
     connection-oriented inward data collection for a  large  distri-
     buted  application  could  be  substantially  greater  than  for
     [connectionless collection], without a  correspondingly  greater
     benefit to the user."[3]

     Outward data dissemination is in a  sense  the  inverse  of  the
     first category; it concerns the distribution of  a  single  data
     unit to a large  number  of  destinations.   This  situation  is
     found,  for  example,  when  a  node  joins  a  network,  or   a
     commonly-accessible server  changes  its  location,  and  a  new
     address is sent to other nodes on the network; when a synchroni-
     zing message such as a real-time clock value must be sent to all
     participants in some distributed activity; and when an  operator
     broadcasts a nonspecific message (e.g., "Network coming down  in
     five minutes").  In such cases, the distribution cost (including
     time) may far exceed the cost of generating the  data;  control-
     ling the overall cost depends on keeping the cost of  dissemina-
     tion as low as possible.

     Request-response applications are those in which  a  service  is
     provided by a commonly accessible  server  process  to  a  large
     number of distributed request sources.  The typical  interaction
     consists of a single request followed by a single response,  and
     usually only the highest-level acknowledgement  -  the  response
     itself - is either necessary  or  meaningful.   Many  commercial
     applications (point of sale terminals, credit checking, reserva-
     tion systems, inventory control, and automated banking  systems)
     and some types of industrial process control, as  well  as  more
     general information retrieval systems (such as  videotex),  fall
     into this category.  In each case, the knowledge and expectation
     of each application component as to the nature of  the  interac-
     tion is represented in an application-process design and  imple-
     mentation that is known in advance, outside of OSI; lower  level
     negotiations,  acknowledgements,  and  other  connection-related
     functions are often unnecessary and cumbersome.

    Connectionless Data Transmission, Rev. 1.00

     An example of an application that combines  the  characteristics
     of inward  data  collection,  outward  data  dissemination,  and
     request-response interaction is described by the  Working  Group
     on Power System Control Centers of the  IEEE  Power  Engineering
     Society in a recent letter to the  chairman  of  ANSI  committee
     X3T51 concerning  the  use  of  data  communication  in  utility
     control centers[17].  They note that "a utility  control  center
     receives information from  remote  terminal  units  (located  at
     substations  and  generating  plants)  and  from  other  control
     centers, performs a variety of monitoring and control functions,
     and transmits commands to the remote terminals and  coordinating
     information to other control centers."   During  the  course  of
     these operations, the following conditions occur:

          1) Some measurements  are  transmitted  or  requested  from
             remote terminals or control centers every  few  seconds.
             No attempt is necessarily made to recover data lost  due
             to transmission error; the application programs  include
             provisions for  proper  operation  when  input  data  is
             occassionally missing.  [Inward data collection]

          2) Some data items are transferred from  commonly  accessed
             remote sites or multi-utility pool coordination  centers
             on   a   request-response   basis.     [Request-response
             interaction]

          3) In some cases, an application program may  require  that
             some measurements be  made  simultaneously  in  a  large
             number of locations.  In these cases, the control center
             will  broadcast  a   command   to   make   th   affected
             measurements.  [Outward data dissemination]

     In closing, they note that "utility control centers  around  the
     world use data communications in ways similar to  those  in  the
     United States."

     Broadcast and multicast (group  addressed)  communication  using
     connection-oriented services is awkward at best  and  impossible
     at   worst,   notwithstanding   the   occassional   mention   of
     "multi-endpoint  connections"  in  the  Reference  Model.   Some
     characteristics  of  connection-based  data  transfer,  such  as
     sequencing and error recovery, are very difficult to provide  in
     a  broadcast/multicast  environment,  and  may   not   even   be
     desirable; and it is not at  all  easy  to  formulate  a  useful
     definition of broadcast/multicast acknowledgement  that  can  be
     supported by a low-level protocol.  Where group addressing is an
     important application consideration, connectionless data  trans-
     mission is usually the only choice.

     Certain special applications,  such  as  digitized  voice,  data

    Connectionless Data Transmission, Rev. 1.00

     telemetry, and remote command  and  control,  involving  a  high
     level  of  data   redundancy   and/or   real-time   transmission
     requirements, may profit from the fact that CDT makes no  effort
     to detect or recover lost or corrupted data.  If the  time  span
     during which an individual datum  is  meaningful  is  relatively
     short, since it is quickly superceded by the next - or if, as in
     digitized voice transmission, the loss or corruption of  one  or
     even several data units is insignificant - the application might
     suffer far more from the delay that would  be  introduced  as  a
     connection-oriented service dealt with a lost or out-of-sequence
     data unit (even if retransmission or other  recovery  procedures
     were not invoked) than it would from the unreported  loss  of  a
     few data units in  the  course  of  a  connectionless  exchange.
     Other special considerations - such as the  undesirability,  for
     security reasons, of  maintaining  connection-state  information
     between data transfers in a military command and control  system
     - add force to the argument that CDT should be available  as  an
     alternative to connection-oriented data transfer.

     Local area networks (LANs) are probably the most fertile  ground
     for connectionless services, which find  useful  application  at
     several layers.  LANs  employ  intrinsically  reliable  physical
     transmission  media  and  techniques  (baseband  and   broadband
     coaxial  cable,  fiber  optics,  etc.)  in  a  restricted  range
     (generally no greater than 1 or 2 kilometers), and are typically
     able to achieve extremely low bit error rates.  In addition, the
     media-access contention  mechanisms  favored  by  LAN  designers
     handle transmission errors as a matter  of  course.   The  usual
     approach to physical interconnection ties all nodes together  on
     a common medium, creating an inherently broadcast environment in
     which every transmission  can  be  received  by  every  station.
     Taking advantage of these characteristics  virtually  demands  a
     connectionless data link service, and in fact most  current  and
     proposed LANs - the Xerox Ethernet[43], the  proposed  IEEE  802
     LAN standard[14,46], and many others - depend on such a service.
     As a bonus,  because  connectionless  services  are  simpler  to
     implement - requiring only two or  three  service  primitives  -
     inexpensive VLSI implementations are often possible.

     In addition, the applications for which LANs are often installed
     tend to be precisely those best handled by CDT.   Consider  this
     list of eight application classes identified  by  the  IEEE  802
     Interface Subcommittee as targets for the 802 LAN standard[46]:

     1.   Periodic   status   reporting   -   telemetry   data   from
     instrumentation, monitoring devices associated  with  static  or
     dynamic physical environments;

     2.  Special event reporting - fire alarms, overload or stressing
     conditions;

    Connectionless Data Transmission, Rev. 1.00

     3.  Security control - security door opening and closing, system
     recovery or initialization, access control;

     4.  File transfer;

     5.  Interactive transactions - reservation  systems,  electronic
     messaging and conferencing;

     6.  Interactive information exchange -  communicating  text  and
     word processors, electronic mail, remote job entry;

     7. Office information exchange - store and forward of  digitized
     voice messages, digitized graphic/image handling;

     8.  Real-time stimulus and response  -  universal  product  code
     checkout readers, distributed  point  of  sale  cash  registers,
     military  command  and  control,  and  other   closed-loop   and
     real-time applications.

     Of these, almost all have already  been  identified  as  classic
     examples of applications that have an essentially connectionless
     nature.  Consider this more detailed example  of  (8):  a  local
     area network with a large number of nodes and a large number  of
     services  (e.g.,  file  management,  printing,   plotting,   job
     execution,  etc.)  provided  at  various  nodes.   In   such   a
     configuration, it is impractical to maintain  a  table  at  each
     node giving the address of every  service,  since  changing  the
     location of a single service would require updating the  address
     table at every node.  An alternative is  to  maintain  a  single
     independent "server lookup" service, which performs the function
     of mapping the name of a given  service  to  the  address  of  a
     server providing that service.   The  server-lookup  server  re-
     ceives requests such as, "where is service X?", and returns  the
     address at which an instance of service X is currently  located.
     Communication  with  the  server-lookup  server  is   inherently
     self-contained,  consisting   of   a   single   request/response
     exchange.  Only the highest-level acknowledgement - the response
     from the lookup service giving the requested address - is at all
     significant.  The native reliability of the local  area  network
     ensures a low error rate; if a message should be lost,  no  harm
     is done, since the request will simply be re-sent  if  a  timely
     response does not arrive.  Such an interaction is poorly  model-
     led by the connection-oriented paradigm of opening a connection,
     transferring a stream of data, and closing the  connection.   It
     is perfectly suited to connectionless transmission techniques.

     Network interconnection (internetworking) can be  facilitated  -
     especially when networks of different types are involved, as  is
     often the case - if the internetwork service is  connectionless;

    Connectionless Data Transmission, Rev. 1.00

     and a number of related activities, such  as  gateway-to-gateway
     communication,  exhibit  the   request-response,   inward   data
     collection, and outward data dissemination characteristics  that
     are well supported by CDT.   One  of  the  best  examples  of  a
     connectionless internetwork service is described in  a  document
     published by the  National  Bureau  of  Standards  (Features  of
     Internetwork  Protocol[29],  which  includes  a  straightforward
     discussion of the merits of the connectionless approach:

             "The  greatest   advantage   of   connectionless
             service at the  internet  level  is  simplicity,
             particularly in  the  gateways.   Simplicity  is
             manifested in terms of smaller and less  compli-
             cated computer code and smaller computer storage
             requirements.  The gateways and  hosts  are  not
             required  to  maintain  state  information,  nor
             interpret call request and call clear  commands.
             Each     data-unit      can      be      treated
             independently...Connectionless service assumes a
             minim[al]   service    from    the    underlying
             subnetworks.   This  is  advantageous   if   the
             networks are diverse.  Existing internet  proto-
             cols which are intended for interconnection of a
             diverse variety  of  networks  are  based  on  a
             connectionless  service  [for  example  the  PUP
             Internetwork  protocol[44],  the  Department  of
             Defence Standard Internet Protocol[31], and  the
             Delta-t protocol developed at Lawrence Livermore
             Laboratory[45]]."

     The principle motivating the development of internetwork  servi-
     ces and protocols that make few assumptions about the nature  of
     the individual network services (the "lowest common denominator"
     approach) was formulated by Carl  Sunshine  as  the  "local  net
     independence principle"[39]: "Each local net  shall  retain  its
     individual address space, routing  algorithms,  packet  formats,
     protocols, traffic controls, fees, and other network  character-
     istics to the greatest extent  possible."   The  simplicity  and
     robustness of connectionless internetworking  systems  guarantee
     their widespread use as the number of different network types  -
     X.25 networks, LANs,  packet  radio  networks,  other  broadcast
     networks, and satellite networks - increases and  the  pressures
     to interconnect them grow.

     4  CDT and the OSI Reference Model

     As a concept, connectionless data transmission  complements  the
     concept of connection-oriented data transfer throughout the  OSI

    Connectionless Data Transmission, Rev. 1.00

     architecture.  As a basis for deriving standard OSI services and
     protocols, however, it has a greater impact on  some  layers  of
     the Reference Model than on others.   Careful  analysis  of  the
     relative  merits  of  connectionless   and   connection-oriented
     operation at each layer is necessary to control  the  prolifera-
     tion of incompatible or useless options and preserve  a  balance
     between the power of the complementary concepts and the stabili-
     zing objective of the OSI standardization effort.

     Figure 5 illustrates the layered OSI hierarchy  as  it  is  most
     commonly represented (it shows two instances of  the  hierarchy,
     representing the relationship between  two  OSI  systems).   The
     following sections discuss the CDT concept  in  the  context  of
     each of the seven layers.

     4.1  Physical Layer

     The duality of connections and connectionless service is  diffi-
     cult  to  demonstrate  satisfactorily  at  the  physical  layer,
     largely because the concept of a physical "connection"  is  both
     intuitive and colloquial.  The physical layer is responsible for
     generating and interpreting signals represented for the  purpose
     of transmission  by  some  form  of  physical  encoding  (be  it
     electrical, optical, acoustic, etc.), and a physical connection,
     in the most general sense (and restricting our consideration, as
     does the Reference Model itself, to  telecommunications  media),
     is a signal pathway through a medium or a combination of  media.
     Is  a  packet   radio   broadcast   network,   then,   using   a
     "connectionless" physical service?  No explicit  signal  pathway
     through a  medium  or  media  is  established  before  data  are
     transmitted.  On the other hand, it can easily be argued that  a
     physical connection is established with the introduction of  two
     antennae into the "ether"; and if the antennae are aimed at each
     other and designed to handle microwave transmission, the impres-
     sion that a physical connection exists is strengthened.  Whether
     or not one recognizes the possibility of connectionless physical
     services - other than purely  whimsical  ones  -  will  probably
     continue to depend on one's point of  view,  and  will  have  no
     effect on the development of actual telecommunication systems.

     4.2  Data Link Layer

     Many data link technologies -  particularly  those  coming  into
     popular use with the growth of local area networking -  are  far
     easier  to  understand  and  work  with  when  the   traditional
     connection-oriented concepts  (embodied,  for  example,  in  the
     widely-used HDLC, SDLC, and ADCCP standards) are replaced by the

             ,---------------------,            ,---------------------,
             |                     |            |                     |
    Level 7  |  Application Layer  |<---------->|  Application Layer  |
             |                     |            |                     |
             |----------|----------|            |----------|----------|
             |                     |            |                     |
    Level 6  | Presentation Layer  |<---------->| Presentation Layer  |
             |                     |            |                     |
             |----------|----------|            |----------|----------|
             |                     |            |                     |
    Level 5  |    Session Layer    |<---------->|     Session Layer   |
             |                     |            |                     |
             |----------|----------|            |----------|----------|
             |                     |            |                     |
    Level 4  |   Transport Layer   |<---------->|   Transport Layer   |
             |                     |            |                     |
             |----------|----------|            |----------|----------|
             |                     |            |                     |
    Level 3  |    Network Layer    |<---------->|    Network Layer    |
             |                     |            |                     |
             |----------|----------|            |----------|----------|
             |                     |            |                     |
    Level 2  |   Data Link Layer   |<---------->|   Data Link Layer   |
             |                     |            |                     |
             |----------|----------|            |----------|----------|
             |                     |            |                     |
    Level 1  |    Physical Layer   |<---------->|    Physical Layer   |
             |                     |            |                     |
             '---------------------'            '---------------------'

         FIGURE 5 - Layered Hierarchy of Open Systems Interconnection

    Connectionless Data Transmission, Rev. 1.00

     concept  of  connectionless  data  transmission.   The  previous
     discussion of local area networking has already made  the  point
     that the high-speed, short-range, intrinsically reliable  broad-
     cast transmission media used to interconnect stations  in  local
     area networks are complemented  both  functionally  and  concep-
     tually by connectionless data link techniques.

     One of the  organizations  currently  developing  a  local  area
     network data link layer standard  -  the  Data  Link  and  Media
     Access (DLMAC) subcommittee of IEEE 802 -  has  recognized  both
     the need to retain compatibility with existing long-haul techni-
     ques and the unique advantages of CDT for local area networks by
     proposing that two data link procedures be defined for the  IEEE
     802 standard.

     In one procedure, information frames are unnumbered and  may  be
     sent at any time by any station  without  first  establishing  a
     connection.  The intended receiver  may  accept  the  frame  and
     interpret it, but is under no  obligation  to  do  so,  and  may
     instead discard the frame with no notice to the sender.  Neither
     is the sender notified if  no  station  recognizes  the  address
     coded  into  the  frame,  and  there  is  no   receiver.    This
     "connectionless" procedure, of course,  assumes  the  "friendly"
     environment and higher-layer acceptance of  responsibility  that
     are   usually   characteristic    of    local    area    network
     implementations.

     The other procedure provides all of  the  sequencing,  recovery,
     and    other     guarantees     normally     associated     with
     connection-oriented link procedures.  It is in fact very similar
     to the ISO standard HDLC balanced asynchronous mode procedure.

     Data  link  procedures  designed  for  transmission  media  that
     (unlike those used in local area networks)  suffer  unacceptable
     error rates are almost universally connection-based, since it is
     generally  more  efficient   to   recover   the   point-to-point
     bit-stream errors detectable by  connection-oriented  data  link
     procedures at the data link layer (with its comparatively  short
     timeout intervals) than at a higher layer.

     4.3  Network Layer

     Connectionless network service is useful for many  of  the  same
     reasons that were  identified  in  the  previous  discussion  of
     network interconnection: it greatly simplifies  the  design  and
     implementation of systems; makes few assumptions about  underly-
     ing services; and is more efficient than  a  connection-oriented
     service when higher layers  perform  whatever  sequencing,  flow
     control, and error recovery is required by user applications (in

    Connectionless Data Transmission, Rev. 1.00

     fact, internetwork services are provided by the Network  Layer).
     CDT  also   facilitates   dynamic   routing   in   packet-   and
     message-switched networks,  since  each  data  unit  (packet  or
     message) can be directed along the most appropriate  "next  hop"
     unencumbered   by   connection-mandated   node   configurations.
     Examples of more or less connectionless  network  layer  designs
     and implementations abound: Zilog's  Z-net  (which  offers  both
     "reliable"   and   "unreliable"   service   options);   DECNET's
     "transport layer" (which corresponds to the OSI Network  layer);
     Livermore Lab's Delta-t protocol (although it  provides  only  a
     reliable   service,   performing   error   checking,   duplicate
     detection, and acknowledgement); the User Datagram protocol[48];
     and the  Cyclades  network  protocol[38].   In  fact,  even  the
     staunchly  connection-oriented   X.25   public   data   networks
     (Canada's Datapac is the  best  example)  generally  emply  what
     amounts to  a  connectionless  network-layer  service  in  their
     internal packet switches, which enables them to perform flexible
     dynamic routing on a packet-by-packet basis.

     4.4  Transport Layer

     The connectionless transport service is important  primarily  in
     systems that distinguish  the  Transport  layer  and  everything
     below it as providing something generically named the "Transport
     Service", and abandon or severely compromise  adherence  to  the
     OSI architecture above the Transport layer.  In such  systems  a
     connectionless transport service may  be  needed  for  the  same
     reasons that other (more OSI-respecting) systems need a  connec-
     tionless application service.  Otherwise, the purpose of  defin-
     ing a connectionless transport service is to enable a  uniformly
     connectionless service to  be  passed  efficiently  through  the
     Transport layer to higher layers.

     4.5  Session Layer

     The whole notion of a session which binds  presentation-entities
     into a relationship of  some  temporal  duration  is  inherently
     connection-oriented.  The purpose of defining  a  connectionless
     session service, therefore, is to enable a uniformly connection-
     less service to be passed efficiently through the session  layer
     to higher layers.  In this  sense,  the  connectionless  session
     service stands in precisely the same relationship to the connec-
     tionless transport service as a session-connection stands  to  a
     transport-connection.

    Connectionless Data Transmission, Rev. 1.00

     4.6  Presentation Layer

     Very much the same  considerations  apply  to  the  Presentation
     layer as apply to the Session layer.

     4.7  Application Layer

     The most obvious reason to define a  connectionless  application
     service - to give  user  application  processes  access  to  the
     connectionless services of the architecture - is  not  the  only
     one.  The application layer performs functions  that  help  user
     application processes to converse regarding the meaning  of  the
     information they exchange, and is also responsible  for  dealing
     with the overall system management aspects of the OSI operation.
     Over  and  above  the  many  user-application  requirements  for
     connectionless service, it may be profitably employed by  system
     management functions that monitor and report on  the  status  of
     resources in the local open system; by application layer manage-
     ment functions that need to interact in a request-response  mode
     with similar functions in  other  systems  to  perform  security
     access control; and by user application process  functions  that
     monitor the status of activities in progress.

     The potential availability of two complementary services at each
     layer of the architecture raises an obvious question  -  how  to
     choose between them?  It should be  clear  at  this  point  that
     unilateral exclusion of  one  or  the  other,  although  it  may
     simplify the situation for some applications, is not  a  general
     solution to the problem.  There are actually two  parts  to  the
     question: how  to  select  an  appropriate  set  of  cooperative
     services for all seven layers during the design of a  particular
     open system; and, if one or more layers of the system will offer
     both connection-oriented and  connectionless  services,  how  to
     provide for the dynamic selection of one or the other in a given
     circumstance.

     The second part is easiest to dispose of, since actual systems -
     as opposed to the more abstract set of  services  and  protocols
     collected under the banner of  OSI  -  will  generally  be  con-
     structed in such a way as  to  combine  services  cooperatively,
     with some attention paid to the way in which they will  interact
     to meet specific goals.  Although two services may  be  provided
     at a given layer, logical combinations of services for different
     applications will generally be assembled according to relatively
     simple rules established during the design of the system.

     Evaluating the requirements of the applications  a  system  must

    Connectionless Data Transmission, Rev. 1.00

     support and the characteristics of the preferred  implementation
     technologies will also answer  the  first  question.   A  system
     designed primarily to transport large  files  over  a  long-haul
     network would probably use  only  connection-oriented  services.
     One designed to collect data from widely scattered  sensors  for
     processing at a central  site  might  provide  a  connectionless
     application  service  but  use  a  connection-oriented   network
     service to achieve compatibility with  a  public  data  network.
     Another system, built around a local area network bus  or  ring,
     might use a connectionless data link service regardless  of  the
     applications   supported;   if   several   LANs   sere   to   be
     interconnected, perhaps with other network types, it might  also
     employ a connectionless internetwork service.

     The definition of OSI standard services and protocols,  however,
     must consider the general case, so as to accomodate a wide range
     of  actual-system  configurations.   The  motivating   principle
     should be to achieve a balance between the two  goals  of  power
     and simplicity.  The service  definition  for  each  layer  must
     include both connection-oriented  and  connectionless  services;
     otherwise, the utility of  a  service  at  one  layer  could  be
     negated by the unavailability of a corresponding  service  else-
     where in the  hierarchy.   However,  the  role  played  by  each
     service may be radically different from one layer to  the  next.
     The Presentation, Session, and Transport layers,  for  instance,
     need to support their respective  connectionless  services  only
     because the Application layer, which must provide a  connection-
     less service to user applications, cannot do so  effectively  if
     they do not.  Recognizing these role  variations  opens  up  the
     possibility of restoring a measure of the simplicity lost in the
     introduction of choice  at  each  layer  by  limiting,  not  the
     choices, but the places in the hierarchy where  conversion  from
     one choice to the other - connection to connectionless, or  vice
     versa - is allowed (see figure 6).  At this stage in the  devel-
     opment of the CDT concept, it appears that there are  exscellent
     reasons for allowing such a conversion  to  take  place  in  the
     Application, Transport, and Network layers (and in the Data Link
     layer, if some physical interconnection strategies are deemed to
     be connectionless).  In the other layers, the provision  of  one
     kind of service to the next-higher layer must always  be  accom-
     plished by using the same kind of service  from  the  next-lower
     layer (see figure 7).  (This principle of  like-to-like  mapping
     is not related to  multiplexing;  it  refers  to  service  types
     (connection-oriented  and   connectionless),   not   to   actual
     services.) Adopting such a restriction would contribute  to  the
     achievement of the balance mentioned  above,  without  excluding
     those combinations of  services  that  have  demonstrated  their
     usefulness.

                    ^                              ^   (N+1)-LAYER
                    |                              |
                    |                              |
    ----------------o------------------------------o----------------
                    |                              |
       ,-------------------------,    ,-------------------------,
       | Offers a connectionless |    |   Offers a connection-  |
       |       (N)-service       |    |   oriented (N)-service  |
       |            |            |    |            |            |
       |        (N)-LAYER        | OR |        (N)-LAYER        |
       |            |            |    |            |            |
       |   Uses a connection-    |    |  Uses a connectionless  |
       | oriented (N-1)-service  |    |      (N-1)-service      |
       '-------------------------'    '-------------------------'
                    |                              |
    ----------------o------------------------------o----------------
                    |                              |
                    |                              |
                    v                              v   (N-1)-LAYER

                   FIGURE 6 - Service Type Conversion

                    ^                              ^   (N+1)-LAYER
                    |                              |
                    |                              |
    ----------------o------------------------------o----------------
                    |                              |
       ,-------------------------,    ,-------------------------,
       | Offers a connectionless |    |   Offers a connection-  |
       |       (N)-service       |    |   oriented (N)-service  |
       |            |            |    |            |            |
       |        (N)-LAYER        | OR |        (N)-LAYER        |
       |            |            |    |            |            |
       |  Uses a connectionless  |    |   Uses a connection-    |
       |      (N-1)-service      |    | oriented (N-1)-service  |
       '-------------------------'    '-------------------------'
                    |                              |
    ----------------o------------------------------o----------------
                    |                              |
                    |                              |
                    v                              v   (N-1)-LAYER

                     FIGURE 7 - Same-Service Mapping

    Connectionless Data Transmission, Rev. 1.00

     5  Summary

     Support for incorporating connectionless data transmission as  a
     basic architectural element of the Reference Model has grown  as
     understanding of the concept has become  more  widespread.   The
     protocol development sponsored by various agencies of  the  U.S.
     Department of Defense, for example, have long recognized connec-
     tions and connectionless transmission as complementary concepts,
     and have employed both.  Similar work being  carried  out  by  a
     division of the Institute for Computer Science and Technology at
     the National Bureau of Standards, the result of which will be  a
     series of  Federal  Information  Processing  Standards,  depends
     heavily  on  connectionless  as  well   as   connection-oriented
     concepts.  The importance of CDT to some of these U.S.   efforts
     is reflected in comments received by ANSI committee X3T5  during
     the recent Reference Model ballot period, one  of  which  states
     that "Publication of this material [DP7498]  without  incorpora-
     tion  of  the  concerns  associated  with  Connectionless   Data
     Trans[mission] makes a mockery of U.S. interests."[18]  A  some-
     what less emotional expression of the same sentiment is embodied
     in  the  official   U.S.   Position   on   Connectionless   Data
     Transmission[9],   in   which   X3T5,   the   responsible   U.S.
     organization,  "endorses  SC16/N555  [Recommended   Changes   to
     Section 3 of [the  Reference  Model]  to  Include  CDT]  without
     exception and announces its intention to pursue  vigorously  the
     incorporation of CDT as the first major extension to  the  Basic
     Reference Model of OSI."  In the same document, X3T5 notes  that
     it "intends to issue and maintain a  version  of  DP7498  to  be
     referred to as DP7498-prime, incorporating the CDT  extensions."
     That there is also significant international support for the CDT
     concept is clear,  however,  from  the  membership  of  the  ISO
     SC16/WG1 Ad Hoc Group on Connectionless Data Transmission, which
     produced the N555 document last November; it includes  represen-
     tatives from France, Japan, Germany, and the United  Kingdom  as
     well as from the U.S.  Those who believe that the CDT concept is
     an essential part of the OSI architecture hope  that  eventually
     the DP7498-prime document, or its successor,  will  replace  the
     exclusively  connection-oriented  Reference  Model  before   the
     latter becomes an International Standard.

     6  Acknowledgements

                         [to be supplied]

    Connectionless Data Transmission, Rev. 1.00
    Appendix A: Vocabulary

                          APPENDIX A - Vocabulary

     OSI Terminology

     The following terms are  defined  in  either  the  text  or  the
     vocabulary annex (or both) of the Draft Proposed Reference Model
     of OSI (ISO/DP7498).  Some terms are given more than one defini-
     tion in different sections of the  Reference  Model;  these  are
     marked with an asterisk (*), to indicate that selection  of  the
     accompanying   definition   involved   the   author's   personal
     judgement.
                         [to be supplied]

     (N)-connection
     (N)-service-access-point
     (N)-service-access-point-address
     (N)-layer
     system
     (N)-entity
     (N)-connection-endpoint-identifier

     CDT Terminology

     The  following  terms,  not  yet  part  of  the   standard   OSI
     vocabulary,  relate  to  the  concept  of  connectionless   data
     transmission.

     "Connectionless  Data  Transmission  is  the  transmission  (not
     transfer)   of   an   (N)-service-data-unit   from   a    source
     (N)-service-access-point   to   one    or    more    destination
     (N)-service-access-points without establishing an (N)-connection
     for the transmission."

     "A Connectionless  (N)-Service  is  one  that  accomplishes  the

    Connectionless Data Transmission, Rev. 1.00
    Appendix A: Vocabulary

     transmission of a  single  self-contained  (N)-service-data-unit
     between  (N+1)-entities  upon  the  performance  of   a   single
     (N)-service access."

     Transmit: "to cause to pass or be conveyed through  space  or  a
     medium."  This term refers to the act of conveying only, without
     implying anything about reception.

     Transfer: "to convey  from  one  place,  person,  or  thing,  to
     another."  A one-way peer-to-peer connotation restricts the  use
     of this term to cases in which the receiving peer  is  party  to
     and accepts the data transferred.

     Exchange: "to give and receive, or lose and take,  reciprocally,
     as things of the same kind."  A two-way peer-to-peer connotation
     restricts the use of this term to cases in which both  give  and
     receive directions are clearly evident.

     datagram
     unit-data transfer/transmission
     transaction (from SC1/N688)
     data transmission (from DIS 2382 Section 9)

                       [End of Appendix A]

    Connectionless Data Transmission, Rev. 1.00
    Appendix B: References

                          APPENDIX B - References

     1.  Data Processing - Open  Systems  Interconnection  -  Basic
                     Reference Model.

             Source:         ISO/TC97/SC16
             Reference:      ISO/DP7498
                             X3T51/80-67
                             X3S33/X3T56/80-121
                             X3S37/80-115
             Date:           12/80

     2.  Recommended Changes to Section  3  of  97/16  N537,  Basic
                     Specifications of the Reference Model of  OSI,
                     to Include Connectionless Data Transmission.

             Source:         ISO/TC97/SC16/WG1  Ad  Hoc  Group   on
                                     Connectionless Data  Transmis-
                                     sion
             Reference:      ISO/TC97/SC16/N555
                             X3S37/81-9
                             X3T51/80-68
                             X3S33/X3T56/80-122
             Date:           11/80

     3.   Report  of  the  Ad  Hoc  Group  on  Connectionless  Data
                     Transmission.

             Source:         ISO/TC97/SC16/WG1  Ad  Hoc  Group   on
                                     Connectionless Data  Transmis-
                                     sion
             Reference:      ISO/TC97/SC16/N566
                             X3T51/80-69
                             X3S33/X3T56/81-13
                             X3S37/81-35
             Date:           11/80

     4.  Definitions of the Term "Connectionless Data Transmission"
                     (a letter to the chairman of ANSC  X3T51  from
                     the acting chairman of ANSC X3T56).

             Source:         ANSC X3S33/X3T56
             Reference:      X3S33/X3T56/81-22
                             X3T51/81-2
                             X3S37/81-6
             Date:           1/81

     5.  Connectionless Provisions for OSI Reference Model.

             Source:         ANSC X3S37
             Reference:      ISO/TC97/SC6/WG2/W12
                             X3S37/81-16R
             Date:           2/81

     6.  Comments on Recommended Changes  to  Section  3  of  97/16
                     N537, Basic  Specification  of  the  Reference
                     Model of OSI, to include  Connectionless  Data
                     Transmission, SC16/N555.

             Source:         DIN (FRG)
             Reference:      ISO/TC97/SC6/WG2/W10
             Date:           2/81

     7.  Connectionless Data Transmission.

             Source:         X3S33/X3T56 Ad Hoc  Group  on  Connec-
                                     tionless Data Transmission
             Reference:      X3S33/X3T56/81-26
             Date:           1/81

     8.  Contribution to Document ISO/TC97/SC16 N555 Concerning the
                     Extension of General Concepts from  the  Basic
                     Reference Model to Connectionless Data  Trans-
                     fer Mode.

             Source:         ISO/TC97/SC16/WG1 Ad Hoc Model  Exten-
                                     sion Group B
             Reference:
             Date:           3/81

     9.  US Position on Connectionless Data Transmission.

             Source:         ANSC X3T5
             Reference:      ISO/TC97/SC16/N605
                             X3T51/81-26
             Date:           3/81

     10. Revision  of  SC16/N551  to  Include  Connectionless  Data
                     Transmission.

             Source:         ANSC X3S33/X3T56
             Reference:      ISO/TC97/SC16/N602
                             X3S33/X3T56/81-67
                             X3T51/81-20
                             X3S37/81-17
             Date:           3/81

     11. Report of USA Vote and Comments on ISO DP7498.

             Source:         ANSC X3T5
             Reference:      ISO/TC97/SC16/N590
                             X3T51/81-29
             Date:           3/81

     12. USA Proposed  Revision  to  Draft  Basic  Session  Service
                     Specification,
                     ISO TC97/SC16 N553.

             Source:         ANSC X3S33/X3T56
             Reference:      ISO/TC97/SC16/N597
                             X3S33/X3T56/81-39R
                             X3T51/81-28
             Date:           3/81

     13.  USA  Proposed  Revision  to   Draft   Transport   Service
                     Specification,
                     ISO TC97/SC16 N563.

             Source:         ANSC X3S33/X3T56
             Reference:      ISO/TC97/SC16/N601
                             X3S33/X3T56/81-33R
                             X3T51/81-17
             Date:           3/81

     14. Comments on Connectionless Data Transmission.

             Source:         Robert F. Stover, Honeywell Inc.
             Reference:      Private communication
             Date:           4/81

     15. Proposed Changes to the OSI Transport Layer.

             Source:         Gregory Ennis, Sytek Inc.
             Reference:      X3T51 Reference  Model  Editing  Group
                             V3.B
             Date:           3/81

     16. Review of the ISO Draft Proposal (DP  7498),  Open  System
                     Interconnection   Reference   Model   (Project
                     IPSC-0168).

             Source:         National  Security   Agency,   Central
                                     Security  Service,  Department
                                     of Defense
             Reference:      NSA/CSS Serial T095/008/81
                             X3T51 Reference  Model  Editing  Group
                             V3.F
             Date:           3/81

     17. Comments on Draft Proposal ISO/DP7498.

             Source:         Working Group on Power System  Control
                                     Centers, IEEE Power  Engineer-
                                     ing Society
             Reference:      X3T51 Reference  Model  Editing  Group
                             V3.I, V4.4
             Date:           3/81

     18.  Review  of  ISO  Draft  Proposal   7498   (Open   Systems
                     Interconnection).

             Source:         Department of the Air Force
             Reference:      X3T51 Reference  Model  Editing  Group
                             V3.J, V4.5, V1.15, V2.H
             Date:           3/81

     19. Proposed Improvements to Section 6 of DP7498.

             Source:         A. Lyman Chapin, Data General Corpora-
                                     tion
             Reference:      X3T51 Reference  Model  Editing  Group
                             V3.M
             Date:           3/81

     20. Comments on Section 7.4 of DP7498.

             Source:         ANSC X3S33/X3T56
             Reference:      X3S33/X3T56/81-30
                             X3T51 Reference  Model  Editing  Group
                             V3.H
             Date:           3/81

     21. Comments on DP7498.

             Source:         ANSC X3S33/X3T56
             Reference:      X3S33/X3T56/81-60
                             X3T51 Reference  Model  Editing  Group
                             V3.N
             Date:           3/81

     22. USA Position Concerning Progression of the Reference Model
                     of Open Systems Interconnection (Parts  I  and
                     II of USA Comments on N309).

             Source:         ANSC X3T5
             Reference:      ISO/TC97/SC16/N405
                             X3T5/80-120
                             X3T51/80-43
             Date:           9/80

     23. Addenda to the USA Position Concerning Progression of  OSI
                     Reference Model (Parts I and II).

             Source:         ANSC X3T5
             Reference:      X3T5/80-143
                             X3T51/80-63
             Date:           9/80

     24. US Position on the  WG1  Rapporteur's  Report  of  October
                     1980.

             Source:         ANSC X3T5
             Reference:      X3T5/80-142
                             X3T51/80-62
             Date:           10/80

     25. Resolutions: ISO/TC97/SC16 - Open Systems Interconnection:
                     Berlin - November 12 - 14, 1980.

             Source:         ISO/TC97/SC16
             Reference:      ISO/TC97/SC16/N570
                             X3S33/X3T56/80-11
             Date:           11/80

     26. NBS  Analysis  of  Major  US  Government  Requirements  of
                     Transport Protocol Services.

             Source:         National  Bureau  of   Standards,   US
                                     Department of Commerce
             Reference:      ISO/TC97/SC16/N404
                             X3T51/80-32
                             X3S33/X3T56/80-82
             Date:           9/80

     27. Features of the Transport and Session Protocols.

             Source:         National  Bureau  of   Standards,   US
                                     Department of Commerce
             Reference:      X3S33/X3T56/80-30
             Date:           3/80

     28. Specification of the Transport Protocol.

             Source:         National  Bureau  of   Standards,   US
                                     Department of Commerce
             Reference:      X3S33/X3T56/81-59
             Date:           2/81

     29. Features of Internetwork Protocol.

             Source:         National  Bureau  of   Standards,   US
                                     Department of Commerce
             Reference:      X3T51/81-23
                             X3S33/X3T56/80-96
                             X3S37/81-31
             Date:           7/80

     30. Service Specification of an Internetwork Protocol.

             Source:         National  Bureau  of   Standards,   US
                                     Department of Commerce
             Reference:      X3T51/81-24
                             X3S33/X3T56/81-18
                             X3S37/81-32
             Date:           9/80

     31. DoD Standard Internet Protocol.

             Source:         US  Department  of  Defense   Advanced
                                     Research Projects Agency
             Reference:      X3S33/X3T56/80-17
                             X3S37/80-17
             Date:           1/80

     32. Connectionless Data Transfer (letter from the chairman  of
                     X3T51 to X3T55, X3T56, and X3S3).

             Source:         John Day, Digital Technology, Inc.
             Reference:      X3T51/80-76
             Date:           12/80

     33. Local Area Networks and the OSI Reference Model.

             Source:         Robert  R.  Shatzer,   Hewlett-Packard
                                     Corp.
             Reference:      X3T51/80-38
             Date:           8/80

     34. An Introduction to Local Area Networks.

             Source:         David D. Clark, et. al.
             Reference:      IEEE Proceedings 66:11
             Date:           11/78

     35. Issues in Packet-Network Interconnection.

             Source:         V.G. Cerf and P.T. Kirstein
             Reference:      IEEE Proceedings 66:11
             Date:           11/78

     36. Connectionless Data Transfer.

             Source:         John Neumann, Microdata Corp.
             Reference:      X3S33/X3T56/80-120
             Date:           12/80

     37. A Protocol for Packet Network Interconnection.

             Source:         V.G. Cerf and R.E. Kahn
             Reference:      IEEE  Transactions  on   Communication
                             COM-22 No. 5
             Date:           5/74

     38. The CYCLADES End-to-End Protocol.

             Source:         H. Zimmermann
             Reference:      Proceedings of the IEEE Vol. 66 No. 11
             Date:           11/78

     39.  Interprocess   Communication   Protocols   for   Computer
                     Networks.

             Source:         Carl Sunshine, USC/ISI
             Reference:      Stanford  Digital  Systems  Laboratory
                             TR105
             Date:           12/75

     40. CCITT Recommendation X.25 - Interface  Between  Data  Ter-
                     minal     Equipment     (DTE)     and     Data
                     Circuit-Terminating   Equipment   (DCE)    for
                     Terminals Operating  in  the  Packet  Mode  on
                     Public Data Networks.

             Source:         CCITT Study Group VII
             Reference:      COM VII/489
             Date:           11/80

     41. An Analysis of ARPAnet Protocols.

             Source:
             Reference:
             Date:

     42. ISO High-Level Data Link Control - Elements of Procedure.

             Source:         ISO
             Reference:      ISO/IS4335
             Date:           1977

     43. ETHERNET Specification (Version 1.0)

             Source:         Xerox Corporation
             Reference:      X3T51/80-50
             Date:           9/80

     44. PUP: An Internetwork Architecture.

             Source:         D.R. Boggs,  J.F.  Shoch,  E.A.  Taft,
                                     R.M. Metcalfe
             Reference:      IEEE  Transactions  on  Communications
                             COM-28 No. 4
             Date:           4/80

     45. Delta-t Protocol Preliminary Specification.

             Source:         R.W. Watson
             Reference:      Lawrence Livermore Laboratories
             Date:           11/79

     46. The Evolving IEEE 802 (Local Network) Standard.

             Source:         Bryan   R.   Hoover,   Hewlett-Packard
                                     Corporation
             Reference:
             Date:

     47. A System for  Interprocess  Communication  in  a  Resource
                     Sharing Computer Network.

             Source:         D. Walden
             Reference:      Communications of the ACM Vol. 15
             Date:           4/72