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