As per Relevance of the word reference, we have this rfc below:
Request For Comments: 787 A. Lyman
July 1981
Subject: Connectionless Data Transmission Survey/
From: A. Lyman
The attached paper on connectionless data transmission is
distributed to the members of a number of US organizations that
involved or interested in the development of international
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
by the American National Standards Institute (ANSI)
responsible for Open Systems Interconnection (OSI) Reference
issues (ANSC X3T5). If approved, it will then be presented to
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
and related OSI service and protocol standards
Your comments on the paper, as well as an indication of the
to which the concepts and services of connectionless data transmis
sion are important to you and/or your organization, will help
ensure that the final version reflects a true US position.
should be directed to the author at the following address
A. Lyman
Data General Corporation MS E111
4400 Computer
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
A. Lyman
22 May 1981 Revision 1.00
Connectionless Data Transmission, Rev. 1.00
The increasingly familiar and ubiquitous Re
ference Model of Open Systems Interconnection
currently being considered by the
Organization for Standardization (ISO)
promotion to the status of a Draft
Standard, is based on the explicit
that a "connection" - an association between
or more communicating entities,
certain characteristics over and above
possessed by the entities themselves -
required for the transfer of data in an
Systems Interconnection (OSI) environment
Although the connection-oriented model
communications behavior has proven to be
extremely powerful concept, and has been
successfully to the design and implementation
protocols and systems covering a wide range
applications, a growing body of research
experience suggests that a complementary
- connectionless data transmission - is
essential part of the Open Systems Interconnec
tion architecture, and should be embraced
such by the OSI Reference Model. This
explores the concept of connectionless
transmission and its relationship to the
familiar concepts of connection-oriented
transfer, developing a rationale for the inclu
sion of the connectionless concept in
Reference Model as an integral part of
standard description of the OSI architecture
Connectionless Data Transmission, Rev. 1.00
1
Over the past three years, a number of national and interna
tional standards organizations have expended the time
efforts of a great many people to achieve a description of
architectural Reference Model for interconnecting
systems considered to be "open" by virtue of their mutual use
standard communication protocols and formats. The
description, the Reference Model of Open Systems
(RM/OSI)[1], is generally accepted by the International Organi
zation for Standardization (ISO), the International
and Telegraph Consultatitive Committee (CCITT), the
Computer Manufacturer's Association (ECMA), and many
standards bodies, including the American National
Institute (ANSI), and has progressed to the status of a
Proposed Standard (DP7498) within ISO. It describes the con
cepts and principles of a communications architecture
hierarchically, by function, into seven discrete layers,
prescribes the services that each layer must provide to
layer immediately above it (the uppermost layer provides
services to user applications, which are considered to
outside of the Open Systems Interconnection environment).
Building on the services available to it from the next-
layer, each layer makes use of standard OSI protocols
enable it to cooperate with other instances of the same
(its "peers") in other systems (see Figure 1). This
of grouping related functions into distinct layers, each
which implements a set of well-defined services that are used
the layer above, partitions a very complex, abstract problem -
"how can the components of a distributed application,
in potentially dissimilar environments, cooperate with
other?" - into a number of more manageable problems that enjoy
logical relationship to each other and can individually be
readily understood
The Reference Model was developed to serve as a framework
the coordination of existing and future standards designed
facilitate the interconnection of data processing systems.
purpose of OSI is to enable an end-user application
(called an "application process") located in a system
employs OSI procedures and protocols (an "open" system)
communicate with any other appication process located in
other open system. It is not the intent of OSI to
either the functions or the implementation details of
that provide the OSI capabilities. Communication is achieved
mutual adherence to agreed-upon (standardized) services
protocols; the only thing that an OSI entity in a given layer
one system needs to know about an OSI entity in the same
User of (N)-services User of (N)-
[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)-
| | | |
'------------' '------------'
\ /
\<----services required---->/
\ from (N-1)-layer /
\ / (N
-------------------o---------------------o--------------------
\ / (N-1)
\ /
\ /
\ /
,--------------------------------,
| |
| |
| (N-1)-LAYER |
| |
| |
'--------------------------------'
FIGURE 1 - General Model of an OSI
A Note on OSI
-------------------------
The construction of a formal system, such as the architecture
Open Systems Interconnection, necessarily involves the introduc
tion of unambiguous terminology (which also tends to be
impenetrable at first glance). The terms found here and in
text are all defined in an Appendix. The "(N)-" notation is
to emphasize that the term refers to an OSI characteristic
applies to each layer individually. The "(N)-" prefix stands
generically for the name of a layer; thus, "(N)-address",
example, refers abstractly to the concept of an address associa
ted with a specific layer, while "transport-address" refers
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
implemented. In particular, OSI is not concerned with how
interfaces between adjacent layers are implemented in an
system; any interface mechanism is acceptable, as long as
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
systems employing a wide range of
technologies, and certainly an important measure of the
of OSI will be its ability to apply the standard
across a broad spectrum of user applications. The way in
the Reference Model has developed over the past four
reflects an awareness of this goal (among others): the
began with the identification of the essential concepts of
layered architecture, including the general
elements of protocols, and proceeded carefully from these
principles to a detailed description of each layer. The organi
zation of the current Reference Model document [1] exhibits
same top-down progression. At the highest level, three
are identified as basic to the architecture[1]:
a) the application processes which exist within the
Systems Interconnection environment
b) the connections which join the application processes
permit them to exchange information;
c) systems
The assumption that a connection is a fundamental
for communication in the OSI environment permeates the
Model, and is in fact one of the most useful and
unifying concepts of the architecture. A growing number
experts in the field, however, believe that this deeply-
connection orientation seriously and unnecessarily limits
power and scope of the Reference Model, since it excludes
large class of applications and implementation technologies
have an inherently connectionless nature. They argue that
architectural objectives of the Reference Model do not depend
the exclusive use of connections to characterize all
interactions, and recommend that the two alternatives - connec
tion oriented data transfer, and connectionless data transmis
sion - be treated as complementary concepts, which can
applied in parallel to the different applications for which
is suited
At the November, 1980 meeting of the ISO subcommittee responsi
ble for OSI (TC97/SC16), a working party laid a solid
for this argument in two documents: Report of the Ad Hoc
Connectionless Data Transmission, Rev. 1.00
on Connectionless Data Transmission[3], and Recommended
to Section 3 of [the Reference Model] to Include
Data Transmission[2]; and the importance of the issue
recognized by the full subcommittee in a resolution[25]
for comments on the two documents from all member organizations
The question of how the connectionless data transmission
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
current Draft Proposed Standard Reference Model becomes a
International Standard. The remainder of this article
explore the issues that surround this question
2 What Is Connectionless Data Transmission
Connectionless data transmission (CDT), despite the
name, is by no means a new concept. In one form or another,
has played an important role in the specification of
and protocols for over a decade. The terms "message mode"[ ],
"datagram"[35], "transaction mode"[22,23,24],
"connection-free"[37,47] have been used in the literature
describe variations on the same basic theme: the transmission
a data unit in a single self-contained operation
establishing, maintaining, and terminating a connection
Since connectionless data transmission and connection-
data transfer are complementary concepts, they are best under
stood in juxtaposition, particularly since CDT is most
defined by its relationship to the more familiar concept of
connection
2.1 Connection-Oriented Data
A connection (or "(N)-connection", in the formal terminology
OSI) is an association established between two or more
("(N+1)-entities") for conveying
("(N)-service-data-units"). The ability to
(N)-connections, and to convey data units over them, is
to (N+1)-entities by the (N)-layer as a set of services,
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
associated with connection-oriented interactions. In
to this explicitly distinguishable duration, or "lifetime",
connection exhibits the following fundamental characteristics
Connection
------------------------
- Successful - - Unsuccessful -
(N)- | | (N)- | |
connect | |(N)-connect connect | | (N)-
------->| |indication ------->| |
request | | request | |
| |-------> | |------->
|(N)-LAYER | |(N)-LAYER |
(N)- | |<------- (N)- | |<-------
connect | | disconnect | | (N)-
<-------| |(N)-connect <-------| |
confirm | | response indication | |
| | | |
Data
-------------
(N)- | | (N)- | |
data | | (N)-data data | |
------->| |indication ------->| | (N)-
request | | request | |
| |-------> | |
|(N)-LAYER | |(N)-LAYER |------->
| | (N)- | |
| | data | |
| | <-------| |
| | confirm | |
| | | |
Connection
------------------
- User Initiated - - Provider Initiated -
(N)-dis | | | |
connect | | (N)- | | (N)-
------->|(N)-LAYER |(N)-disconnect disconnect|(N)-LAYER |
request | |indication <-------| |------->
| |-------> indication| |
| | | |
FIGURE 2 - Connection Oriented
Connectionless Data Transmission, Rev. 1.00
[Note: Much of the material in this section
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
on the set of parameters and options that will govern the
transfer. An incoming connection establishment request can
rejected if it asserts parameter values or options that
unacceptable to the receiver, and the receiver may in many
suggest alternative parameter values and options along with
rejection
The reason for negotiation during connection establishment
the assumption that each party must reserve or allocate
resources (such as buffers and channels) that will be
to carry out data transfer operations on the new connection
Negotiation provides an opportunity to scuttle the
of a connection when the resources that would be required
support it cannot be dedicated, or to propose alternatives
could be supported by the available resources
2. Three-party Agreement
The fundamental nature of a connection involves establishing
dynamically maintaining a three-party agreement concerning
transfer of data. The three parties - the two (N+1)-
that wish to communicate, and the (N)-service that provides
with a connection - must first agree on their mutual
to participate in the transfer (see above). This
agreement establishes a connection. Thereafter, for as long
the connection persists, they must continue to agree on
acceptance of each data unit transferred over the connection
"With a connection, there is no possibility of data
through an unwilling service to an unwilling partner,
the mutual willingness must be established before the
transfer can take place, and data must be accepted by
destination partner; otherwise, no data [are] transferred
that connection."[3]
3. Connection Identifiers
At connection establishment time, each
(N+1)-entity is identified to the (N)-service by an (N)-address
the (N)-service uses these addresses to set up the
connection. Subsequent requests to transfer data over
connection (or to release it) refer not to the (N)-address(es
of the intended recipient(s), but to a connection
Connectionless Data Transmission, Rev. 1.00
supplied by the (N)-service (in OSI parlance,
"(N)-connection-endpoint-identifier"). This is
locally-significant "shorthand" reference that uniquely identi
fies an established connection during its lifetime. Similarly
the protocol units that carry data between systems
include a mutually-understood logical identifier rather than
actual addresses of the correspondents. This technique elimina
tes the overhead that would otherwise be associated with
resolution and transmission of addresses on every data transfer
In some cases, however - particularly when non-
networks are interconnected, and very location-sensitive addres
sing schemes are used - it can make dynamic routing of
units extremely difficult, if not impossible
4. Data Unit Relationship
Once a connection has been established, it may be used
transfer one data unit after another, until the connection
released by one of the three parties. These data units
logically related to each other simply by virtue of
transferred on the same connection. Since data units
transferred over a connection in sequence, they are
ordinally as well. These data unit relationships are an impor
tant characteristic of connections, since they create a
for the interpretation of arriving data units that is indepen
dent of the data themselves. Because a connection maintains
sequence of messages associated with it, out-of-sequence
missing, and duplicated messages can easily be detected
recovered, and flow control techniques can be invoked to
that the message transfer rate does not exceed that which
correspondents are capable of handling
These characteristics make connection-based data
attractive in applications that call for relatively long-lived
stream-oriented interactions in stable configurations, such
direct terminal use of a remote computer, file transfer,
long-term attachments of remote job entry stations. In
applications, the interaction between communicating entities
modelled very well by the connection concept: the
initially discuss their requirements and agree to the terms
their interaction, reserving whatever resources they will need
transfer a series of related data units to accomplish
mutual objective; and explicitly end their interaction, releas
ing the previously reserved resources
2.2 Connectionless Data
In many other applications, however, the interaction
Connectionless Data Transmission, Rev. 1.00
entities is more naturally modelled by the connectionless
transmission concept, which involves the transmission of
single self-contained data unit from one entity to
without prior negotiation or agreement, and without the as
surance of delivery normally associated with connection-
transfers. The users of a connectionless (N)-service may,
course, use their (N+1)-protocol to make any prior or
arrangements they wish concerning their interpretation of
data transmitted and received; the (N)-service itself, however
attaches no significance to individual data units, and does
attempt to relate them in any way. Two (N+1)-entities communi
cating by means of a connectionless (N)-service could,
example, apply whatever techniques they might consider appro
priate in the execution of their own protocol (timers
retransmission, positive or negative acknowledgements,
numbers, etc.) to achieve the level of error detection and/
recovery they desired. Users of a connectionless, as opposed
connection-oriented, (N)-service are not restricted or
in the performance of their (N+1)-protocol; obviously, though
the assumption is that CDT will be used in situations
either do not require the characteristics of a connection,
actively benefit from the alternative characteristics of connec
tionless transmission
Figure 3 illustrates schematically the single operation
a connectionless service may be employed to transmit a
data unit. Figure 4 shows a widely-implemented variation
sometimes called "reliable datagram" service, in which
service provider undertakes to confirm the delivery
non-delivery of each data unit. It must be emphasized that
is not a true connectionless service, but is in some sense
hybrid, combining the delivery assurance of connection-
service with the single-operation interface event of connection
less service
Many of those involved in OSI standardization activities
agreed on a pair of definitions for connectionless
transmission, one for architectural and conceptual purposes,
one for service-definition purposes[4]. The
definition, which has been proposed for inclusion in the Re
ference Model, is
"Connectionless Data Transmission is the transmission (
transfer) of an (N)-service-data-unit from a
(N)-service-access-point to one or more
(N)-service-access-points without establishing an (N)-
for the transmission."
The service definition, which is intended to provide a
basis for incorporating a connectionless service into
| |
(N)-data | |
request | |
--------->| |
| (N)-LAYER |
| |--------->
| | (N)-
| |
| |
FIGURE 3 - Connectionless Data
(N)-data | |
request | |
--------->| |
| | (N)-
| (N)-LAYER |--------->
| |
<---------| |
(N)-data | |
confirm | |
FIGURE 4 - "Reliable Datagram"
Connectionless Data Transmission, Rev. 1.00
service descriptions for individual layers of the
Model, is
"A Connectionless (N)-Service is one that accomplishes
transmission of a single self-contained (N)-service-data-
between (N+1)-entities upon the performance of a
(N)-service access."
Both of these definitions depend heavily on the
between the terms "transmit", "transfer", and "exchange":
Transmit: "to cause to pass or be conveyed through space or
medium." This term refers to the act of conveying only,
implying anything about reception
Transfer: "to convey from one place, person, or thing,
another." A one-way peer-to-peer connotation restricts the
of this term to cases in which the receiving peer is party
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
restricts the use of this term to cases in which both give
receive directions are clearly evident
These definitions are clearly of limited usefulness
themselves. They do, however, provide a framework within
to explore the following characteristics of CDT
1. "One-shot" Operation
The most user-visible characteristic of connectionless
transmission is the single service access required to
the transmission of a data unit. All of the information re
quired to deliver the data unit - destination address,
of service selection, options, etc. - is presented to
connectionless (N)-service provider, along with the data, in
single logical service-access operation that is not
by the (N)-service to be related in any way to other
operations, prior or subsequent (note, however, that since
is not concerned with implementation details, the
interface mechanism employed by a particular implementation
connectionless service might involve more than one
exchange to accomplish what is, from a logical standpoint,
single operation). Once the service provider has accepted
data unit for connectionless transmission, no further communica
tion occurs between the provider and the user of the
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
a three-party agreement between the participating (N+1)-
and the (N)-service. A connectionless service, however, invol
ves only two-party agreements: there may be an agreement
the corresponding (N+1)-entities, unknown to the (N)-service
and there may be local agreements between each (N+1)-entity
its local (N)-service provider, but no (N)-protocol
is ever exchanged between (N)-entities concerning the
willingness of the (N+1)-entities to engage in a
transmission or to accept a particular data unit
In practice, some sort of a priori agreement (usually a
engineering design decision) is assumed to exist between
(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
the user of a connectionless service to specify most
values and options - such as transfer rate, acceptable
rate, etc. - at the time the service is invoked. In a
implementation, if the local (N)-service provider
immediately (from information available to it locally) that
requested operation cannot be performed under the
specified, it may abort the service primitive, returning
implementation-specific error message across the interface
the user. If the same determination is made later on, after
service-primitive interface event has completed, the transmis
sion is simply abandoned, since users of a
service can be expected to recover lost data if it is
for them to do so
3. Self-contained Data Units
Data units transmitted via a connectionless service, since
bear no relationship either to other data units or to a "
abstraction" (such as a connection), are
self-contained. All of the addressing and other
needed by the service provider to deliver a data unit to
destination must be included in each transmission. On the
hand, this represents a greater overhead than is incurred
the data transfer phase of a connection-oriented interaction;
the other, it greatly simplifies routing, since each data
carries a complete destination address and can be routed
reference to connection-related information that may not,
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
Connectionless Data Transmission, Rev. 1.00
connectionless service begins the transmission of a single
unit. Nothing about the service invocation, the transmission
the data by the connectionless service, or the data unit
affects or is affected by any other past, present, or
operation, whether connection-oriented or connectionless.
series of data units handed one after the other to a connection
less service for delivery to the same destination will
necessarily be delivered to the destination in the same order
and the connectionless service will make no attempt to report
recover instances of non-delivery
Note: A number of popular variations on CDT
features that run counter to those
above. These variations deserve to be
on their own merits, but should not be
with the architectural concept of
data transmission
These characteristics make CDT attractive in applications
involve short-term request-response interactions, exhibit a
level of redundancy, must be flexibly reconfigurable, or
no benefit from guaranteed in-sequence delivery of data
3 The Rationale for Connectionless Data
Because CDT is not as widely understood as connection-
data transfer, it has often been difficult in the course
developing service and protocol definitions to adduce a ration
ale for incorporating CDT, and even more difficult to
appropriate locations for connectionless service within
layered hierarchy of OSI. This section addresses the
concern; the next section will deal with the second
The most natural way to discover the power and utility of
CDT concept is to examine applications and
technologies that depend on it. The following observations
distilled from the specifications and descriptions of
protocols and systems (many of which have been implemented),
from the work of individuals and organizations engaged in
OSI standardization effort (quoted material is from reference 3,
except where otherwise noted). They are divided into
(occassionally overlapping) categories which classify
applications for which CDT is well suited
Inward data collection involves the periodic active or
sampling of a large number of data sources. A sensor
Connectionless Data Transmission, Rev. 1.00
gathering data from dedicated measurement stations; a
status monitor constantly refreshing its knowledge of a
environment; and an automatic alarm or security system in
each component regularly self-tests and reports the result,
all engaged in this type of interaction, in which a "
number of sources may be reporting periodically and asynchron
ously to a single reporting point. In a realtime
situation, these readings could normally be lost on
without causing distress, because the next update would
arriving shortly. Only if more than one successive
failed to arrive within a specified time limit would an alarm
warranted. If, say, a fast connect/disconnect three-
handshake cost twice as much as a one-way [connectionless]
transmission which had been system engineered to achieve
certain acceptable statistical reliability figure, the cost
connection-oriented inward data collection for a large distri
buted application could be substantially greater than
[connectionless collection], without a correspondingly
benefit to the user."[3]
Outward data dissemination is in a sense the inverse of
first category; it concerns the distribution of a single
unit to a large number of destinations. This situation
found, for example, when a node joins a network, or
commonly-accessible server changes its location, and a
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
participants in some distributed activity; and when an
broadcasts a nonspecific message (e.g., "Network coming down
five minutes"). In such cases, the distribution cost (
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
provided by a commonly accessible server process to a
number of distributed request sources. The typical
consists of a single request followed by a single response,
usually only the highest-level acknowledgement - the
itself - is either necessary or meaningful. Many
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
general information retrieval systems (such as videotex),
into this category. In each case, the knowledge and
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
negotiations, acknowledgements, and other connection-
functions are often unnecessary and cumbersome
Connectionless Data Transmission, Rev. 1.00
An example of an application that combines the
of inward data collection, outward data dissemination,
request-response interaction is described by the Working
on Power System Control Centers of the IEEE Power
Society in a recent letter to the chairman of ANSI
X3T51 concerning the use of data communication in
control centers[17]. They note that "a utility control
receives information from remote terminal units (located
substations and generating plants) and from other
centers, performs a variety of monitoring and control functions
and transmits commands to the remote terminals and
information to other control centers." During the course
these operations, the following conditions occur
1) Some measurements are transmitted or requested
remote terminals or control centers every few seconds
No attempt is necessarily made to recover data lost
to transmission error; the application programs
provisions for proper operation when input data
occassionally missing. [Inward data collection
2) Some data items are transferred from commonly
remote sites or multi-utility pool coordination
on a request-response basis. [Request-
interaction
3) In some cases, an application program may require
some measurements be made simultaneously in a
number of locations. In these cases, the control
will broadcast a command to make th
measurements. [Outward data dissemination
In closing, they note that "utility control centers around
world use data communications in ways similar to those in
United States."
Broadcast and multicast (group addressed) communication
connection-oriented services is awkward at best and
at worst, notwithstanding the occassional mention
"multi-endpoint connections" in the Reference Model.
characteristics of connection-based data transfer, such
sequencing and error recovery, are very difficult to provide
a broadcast/multicast environment, and may not even
desirable; and it is not at all easy to formulate a
definition of broadcast/multicast acknowledgement that can
supported by a low-level protocol. Where group addressing is
important application consideration, connectionless data trans
mission is usually the only choice
Certain special applications, such as digitized voice,
Connectionless Data Transmission, Rev. 1.00
telemetry, and remote command and control, involving a
level of data redundancy and/or real-time
requirements, may profit from the fact that CDT makes no
to detect or recover lost or corrupted data. If the time
during which an individual datum is meaningful is
short, since it is quickly superceded by the next - or if, as
digitized voice transmission, the loss or corruption of one
even several data units is insignificant - the application
suffer far more from the delay that would be introduced as
connection-oriented service dealt with a lost or out-of-
data unit (even if retransmission or other recovery
were not invoked) than it would from the unreported loss of
few data units in the course of a connectionless exchange
Other special considerations - such as the undesirability,
security reasons, of maintaining connection-state
between data transfers in a military command and control
- add force to the argument that CDT should be available as
alternative to connection-oriented data transfer
Local area networks (LANs) are probably the most fertile
for connectionless services, which find useful application
several layers. LANs employ intrinsically reliable
transmission media and techniques (baseband and
coaxial cable, fiber optics, etc.) in a restricted
(generally no greater than 1 or 2 kilometers), and are
able to achieve extremely low bit error rates. In addition,
media-access contention mechanisms favored by LAN
handle transmission errors as a matter of course. The
approach to physical interconnection ties all nodes together
a common medium, creating an inherently broadcast environment
which every transmission can be received by every station
Taking advantage of these characteristics virtually demands
connectionless data link service, and in fact most current
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
implement - requiring only two or three service primitives -
inexpensive VLSI implementations are often possible
In addition, the applications for which LANs are often
tend to be precisely those best handled by CDT. Consider
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
instrumentation, monitoring devices associated with static
dynamic physical environments
2. Special event reporting - fire alarms, overload or
conditions
Connectionless Data Transmission, Rev. 1.00
3. Security control - security door opening and closing,
recovery or initialization, access control
4. File transfer
5. Interactive transactions - reservation systems,
messaging and conferencing
6. Interactive information exchange - communicating text
word processors, electronic mail, remote job entry
7. Office information exchange - store and forward of
voice messages, digitized graphic/image handling
8. Real-time stimulus and response - universal product
checkout readers, distributed point of sale cash registers
military command and control, and other closed-loop
real-time applications
Of these, almost all have already been identified as
examples of applications that have an essentially
nature. Consider this more detailed example of (8): a
area network with a large number of nodes and a large number
services (e.g., file management, printing, plotting,
execution, etc.) provided at various nodes. In such
configuration, it is impractical to maintain a table at
node giving the address of every service, since changing
location of a single service would require updating the
table at every node. An alternative is to maintain a
independent "server lookup" service, which performs the
of mapping the name of a given service to the address of
server providing that service. The server-lookup server re
ceives requests such as, "where is service X?", and returns
address at which an instance of service X is currently located
Communication with the server-lookup server is
self-contained, consisting of a single request/
exchange. Only the highest-level acknowledgement - the
from the lookup service giving the requested address - is at
significant. The native reliability of the local area
ensures a low error rate; if a message should be lost, no
is done, since the request will simply be re-sent if a
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.
is perfectly suited to connectionless transmission techniques
Network interconnection (internetworking) can be facilitated -
especially when networks of different types are involved, as
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-
communication, exhibit the request-response, inward
collection, and outward data dissemination characteristics
are well supported by CDT. One of the best examples of
connectionless internetwork service is described in a
published by the National Bureau of Standards (Features
Internetwork Protocol[29], which includes a
discussion of the merits of the connectionless approach
"The greatest advantage of
service at the internet level is simplicity
particularly in the gateways. Simplicity
manifested in terms of smaller and less compli
cated computer code and smaller computer
requirements. The gateways and hosts are
required to maintain state information,
interpret call request and call clear commands
Each data-unit can be
independently...Connectionless service assumes
minim[al] service from the
subnetworks. This is advantageous if
networks are diverse. Existing internet proto
cols which are intended for interconnection of
diverse variety of networks are based on
connectionless service [for example the
Internetwork protocol[44], the Department
Defence Standard Internet Protocol[31], and
Delta-t protocol developed at Lawrence
Laboratory[45]]."
The principle motivating the development of internetwork servi
ces and protocols that make few assumptions about the nature
the individual network services (the "lowest common denominator
approach) was formulated by Carl Sunshine as the "local
independence principle"[39]: "Each local net shall retain
individual address space, routing algorithms, packet formats
protocols, traffic controls, fees, and other network character
istics to the greatest extent possible." The simplicity
robustness of connectionless internetworking systems
their widespread use as the number of different network types -
X.25 networks, LANs, packet radio networks, other
networks, and satellite networks - increases and the
to interconnect them grow
4 CDT and the OSI Reference
As a concept, connectionless data transmission complements
concept of connection-oriented data transfer throughout the
Connectionless Data Transmission, Rev. 1.00
architecture. As a basis for deriving standard OSI services
protocols, however, it has a greater impact on some layers
the Reference Model than on others. Careful analysis of
relative merits of connectionless and connection-
operation at each layer is necessary to control the prolifera
tion of incompatible or useless options and preserve a
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
commonly represented (it shows two instances of the hierarchy
representing the relationship between two OSI systems).
following sections discuss the CDT concept in the context
each of the seven layers
4.1 Physical
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
intuitive and colloquial. The physical layer is responsible
generating and interpreting signals represented for the
of transmission by some form of physical encoding (be
electrical, optical, acoustic, etc.), and a physical connection
in the most general sense (and restricting our consideration,
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
"connectionless" physical service? No explicit signal
through a medium or media is established before data
transmitted. On the other hand, it can easily be argued that
physical connection is established with the introduction of
antennae into the "ether"; and if the antennae are aimed at
other and designed to handle microwave transmission, the impres
sion that a physical connection exists is strengthened.
or not one recognizes the possibility of connectionless
services - other than purely whimsical ones - will
continue to depend on one's point of view, and will have
effect on the development of actual telecommunication systems
4.2 Data Link
Many data link technologies - particularly those coming
popular use with the growth of local area networking - are
easier to understand and work with when the
connection-oriented concepts (embodied, for example, in
widely-used HDLC, SDLC, and ADCCP standards) are replaced by
,---------------------, ,---------------------,
| | | |
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
Connectionless Data Transmission, Rev. 1.00
concept of connectionless data transmission. The
discussion of local area networking has already made the
that the high-speed, short-range, intrinsically reliable broad
cast transmission media used to interconnect stations in
area networks are complemented both functionally and concep
tually by connectionless data link techniques
One of the organizations currently developing a local
network data link layer standard - the Data Link and
Access (DLMAC) subcommittee of IEEE 802 - has recognized
the need to retain compatibility with existing long-haul techni
ques and the unique advantages of CDT for local area networks
proposing that two data link procedures be defined for the
802 standard
In one procedure, information frames are unnumbered and may
sent at any time by any station without first establishing
connection. The intended receiver may accept the frame
interpret it, but is under no obligation to do so, and
instead discard the frame with no notice to the sender.
is the sender notified if no station recognizes the
coded into the frame, and there is no receiver.
"connectionless" procedure, of course, assumes the "friendly
environment and higher-layer acceptance of responsibility
are usually characteristic of local area
implementations
The other procedure provides all of the sequencing, recovery
and other guarantees normally associated
connection-oriented link procedures. It is in fact very
to the ISO standard HDLC balanced asynchronous mode procedure
Data link procedures designed for transmission media
(unlike those used in local area networks) suffer
error rates are almost universally connection-based, since it
generally more efficient to recover the point-to-
bit-stream errors detectable by connection-oriented data
procedures at the data link layer (with its comparatively
timeout intervals) than at a higher layer
4.3 Network
Connectionless network service is useful for many of the
reasons that were identified in the previous discussion
network interconnection: it greatly simplifies the design
implementation of systems; makes few assumptions about underly
ing services; and is more efficient than a connection-
service when higher layers perform whatever sequencing,
control, and error recovery is required by user applications (
Connectionless Data Transmission, Rev. 1.00
fact, internetwork services are provided by the Network Layer).
CDT also facilitates dynamic routing in packet-
message-switched networks, since each data unit (packet
message) can be directed along the most appropriate "next hop
unencumbered by connection-mandated node configurations
Examples of more or less connectionless network layer
and implementations abound: Zilog's Z-net (which offers
"reliable" and "unreliable" service options); DECNET'
"transport layer" (which corresponds to the OSI Network layer);
Livermore Lab's Delta-t protocol (although it provides only
reliable service, performing error checking,
detection, and acknowledgement); the User Datagram protocol[48];
and the Cyclades network protocol[38]. In fact, even
staunchly connection-oriented X.25 public data
(Canada's Datapac is the best example) generally emply
amounts to a connectionless network-layer service in
internal packet switches, which enables them to perform
dynamic routing on a packet-by-packet basis
4.4 Transport
The connectionless transport service is important primarily
systems that distinguish the Transport layer and
below it as providing something generically named the "
Service", and abandon or severely compromise adherence to
OSI architecture above the Transport layer. In such systems
connectionless transport service may be needed for the
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
connectionless service to be passed efficiently through
Transport layer to higher layers
4.5 Session
The whole notion of a session which binds presentation-
into a relationship of some temporal duration is
connection-oriented. The purpose of defining a
session service, therefore, is to enable a uniformly connection
less service to be passed efficiently through the session
to higher layers. In this sense, the connectionless
service stands in precisely the same relationship to the connec
tionless transport service as a session-connection stands to
transport-connection
Connectionless Data Transmission, Rev. 1.00
4.6 Presentation
Very much the same considerations apply to the
layer as apply to the Session layer
4.7 Application
The most obvious reason to define a connectionless
service - to give user application processes access to
connectionless services of the architecture - is not the
one. The application layer performs functions that help
application processes to converse regarding the meaning of
information they exchange, and is also responsible for
with the overall system management aspects of the OSI operation
Over and above the many user-application requirements
connectionless service, it may be profitably employed by
management functions that monitor and report on the status
resources in the local open system; by application layer manage
ment functions that need to interact in a request-response
with similar functions in other systems to perform
access control; and by user application process functions
monitor the status of activities in progress
The potential availability of two complementary services at
layer of the architecture raises an obvious question - how
choose between them? It should be clear at this point
unilateral exclusion of one or the other, although it
simplify the situation for some applications, is not a
solution to the problem. There are actually two parts to
question: how to select an appropriate set of
services for all seven layers during the design of a
open system; and, if one or more layers of the system will
both connection-oriented and connectionless services, how
provide for the dynamic selection of one or the other in a
circumstance
The second part is easiest to dispose of, since actual systems -
as opposed to the more abstract set of services and
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
to meet specific goals. Although two services may be
at a given layer, logical combinations of services for
applications will generally be assembled according to
simple rules established during the design of the system
Evaluating the requirements of the applications a system
Connectionless Data Transmission, Rev. 1.00
support and the characteristics of the preferred
technologies will also answer the first question. A
designed primarily to transport large files over a long-
network would probably use only connection-oriented services
One designed to collect data from widely scattered sensors
processing at a central site might provide a
application service but use a connection-oriented
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
applications supported; if several LANs sere to
interconnected, perhaps with other network types, it might
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
of actual-system configurations. The motivating
should be to achieve a balance between the two goals of
and simplicity. The service definition for each layer
include both connection-oriented and connectionless services
otherwise, the utility of a service at one layer could
negated by the unavailability of a corresponding service else
where in the hierarchy. However, the role played by
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
because the Application layer, which must provide a connection
less service to user applications, cannot do so effectively
they do not. Recognizing these role variations opens up
possibility of restoring a measure of the simplicity lost in
introduction of choice at each layer by limiting, not
choices, but the places in the hierarchy where conversion
one choice to the other - connection to connectionless, or
versa - is allowed (see figure 6). At this stage in the devel
opment of the CDT concept, it appears that there are
reasons for allowing such a conversion to take place in
Application, Transport, and Network layers (and in the Data
layer, if some physical interconnection strategies are deemed
be connectionless). In the other layers, the provision of
kind of service to the next-higher layer must always be accom
plished by using the same kind of service from the next-
layer (see figure 7). (This principle of like-to-like
is not related to multiplexing; it refers to service
(connection-oriented and connectionless), not to
services.) Adopting such a restriction would contribute to
achievement of the balance mentioned above, without
those combinations of services that have demonstrated
usefulness
^ ^ (N+1)-
| |
| |
----------------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)-
FIGURE 6 - Service Type
^ ^ (N+1)-
| |
| |
----------------o------------------------------o----------------
| |
,-------------------------, ,-------------------------,
| Offers a connectionless | | Offers a connection- |
| (N)-service | | oriented (N)-service |
| | | | | |
| (N)-LAYER | OR | (N)-LAYER |
| | | | | |
| Uses a connectionless | | Uses a 