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RFC 787 - Connectionless data transmission survey/tutorial


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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

 

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