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PROCEEDINGS
OF
THE IEEE,
VOL. 1,NO 2,DECEMBER 1983
1365
Connections and
Connectionless Data
A. LYMAN CHAPIN
Invited Paper
Abstract-The
complementary
concepts
of
connection-mode
ata
t rans-
fer and connectionless data transmission re the fundamental mode ls of
communication n
the
architectwe
of
Open Sys tem s Interconnection (OSI).
ment
and
maintenance
of
a connection,
which
represents a
dynamical ly
negotiated agreem ent
coIlcerning the ransfer
of a
seriesof
related u nits of
data; connectionless data transmission elies only on
the prior
knowledge
that peer entities
have of
e ch otber to transmit independent, unrelated
data units, and
does
not involve
the
establishment of a
connectioo
The wo
concepts together
dexlibe
all
of
the
pe er -tq ee r interact ions that take
place n the OS1 environment.The national and
international
organizations
concerned
with OS1
have
applied
these
concepts
succesdully
in the devel-
opment of
OS1
service
and pmtocd
standards.
AS the MIIWS
imply, conneetion-mode data e invdves the
edablish-
T
I.NTRODUCTION
HE REFERENCE MODELforOpenSystemsIntercon-
nection OSI), now an International Standard published by
the International Organization for Standardization ISO)
[l] and the Consultative Committee on International Telephone
and Telegraph CCITTJ [2], has evolved over the past five years
as an architectural framework for the development of communi-
cation service and protocol standards.
These
“OS1 standards” are
intended tofacilitate he nterconnection of computer systems
considered to be “open” by virtue of their mutual adherence
to
the standards. By now, the basic features
of this
architecture are
well known, and need not be restated here; they are discussed in
detail elsewhere in this issue of the PROCEEDINGS3].
In theearliestwork on OSI,communicationbetweenpeer
entities wasmodeledexclusively in terms of connection-based
interactions, which proceed through three distinct phasesn which
the entities initially discuss their requirements and agree on the
“ground rules” for their interaction; exchange a series of related
data units according to these rules; and finally erminate heir
interaction. This model was and is familiar to everyone working
in the field of computer communication. It provides a powerful
abstract description of the way in which many traditional tele-
communications ystemsanddistributedapplicationsoperate.
Consequently, the assumption that a connection is a basic prere-
quisite for communication in the OS1 environment quickly per-
meatedearly drafts of theReferenceModel,andcame to be
perceived
as
one of the most useful and unifymg concepts of the
OS1 architecture.
As people began to use the Reference Model to derive specifi-
cations for OS1 services and protocols, they discovered that the
deeply rooted connection orientation of the Model unnecessarily
limits the power and scope of OSI, since it excludes a large class
of applicationsandcommunication echnologies for which he
Manuscript
received June
15, 1983;
evised
August
25, 1983.
The author is with Data General Corporation,
Westborough, MA 01580.
most natural model of interaction
is
specifically connectionless.
Theproblemwasparticularly obvious to peopleworking
on
standards for local area networks
LAN’s):
connections are basi-
callypoint-to-point,unlikehemultipoint “ether” of many
LAN’s; and very high LAN ata rates demand very fast gateways
at points of network interconnection, suggesting a need for much
simplerprotocols and systems than thosedesigned o support
connection-mode data transfer. It became apparent that in order
for he Reference Model to serve
as
the common architectural
framework for the interconnection of open systems, the connec-
tion concept must be joined by the complementary concept of
connectionless data transmission. The process
of
extending he
Model in this way
will
be completed with the approval early in
1984 of an Addendum to the Reference Model covering connec-
tionless data transmission currently a Draft International Stan-
dard [4]). Atthis point, therefore, it is appropriate to examine the
relationships between the two concepts, particularly with respect
to heir mpact on theoutcome of currentandplannedOS1
standardization efforts.
11.
THE
OS1 ENVIRONMENT
In order for communication of any kind to take place among
peer entities in the OS1 environment, each peer must have some
“prior knowledge” of theenvironment hat ssharedby he
others, including at least the identityof each peer and a mutually
understoodprotocol or protocols)whichcanserve to initiate
peer-to-peercommunication. This sharedawarenessmay also
includeagreements on thedefaultvalues of parameters, he
presence or absence of optional services, the quality of service
that may be expected from service providers, the way in which
error conditions will be handled, and the observance of restric-
tions or constraints that follow rom hecharacteristics of a
particular mplementation.Collectively, hesepieces of “prior
knowledge” constitute an association between peer entities which
is due simply to theirexistence in theOS1environment, and
precedes any activity on their part.
The prior knowledge that characterizes these a priori associa-
tions is acquired in many ways, all of which are relevant to the
interconnection of real open systems but are not explicitly speci-
fied by OS1 standards primarily because they tend to be highly
implementation-specific). Some examples are information derived
fromystemsngineeringesignpecifications;nformation
acquired as a resultof executing a contract with the providerof a
specific communications service; information gathered over time
by observing or measuring the behavior or performance
of
vari-
ous components of theOS1environment; nformation nferred
from a statistical model of communication; and information that
0018-9219/83/1200-1365 01.00
01983 IEEE
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1366 PROCEEDINGS O
THE
I EEE
VOL.
71,
NO 2,
DECEMBER 1983
U s e r f 1 4 ) - s e r v i c e s
[ a n N + l ) - e n t l t y l
\
U s e r o f i ; ) - s e r v l c e s
[ a n i i + l ) - e n t i t y l
/
SUCCESSFUL
-
- UNSUCCESSFUL -
\ /
s e r v i c e s r o v i d e d to-
N c l ) - l a y e r
:: 1
\
s e r v i c e s
i r o n 14-1 ) - l a y e r
\
/
11-1
ig. 1. Generalmodel of an OS1 layer.
A Note
on
OS Terminology
Theconstructionofa formal system, uch as thearchitecture of Open
Systems Interconnection, necessarily involves the introduction
f
unambigu-
ous
terminology whichalso ends
to
be
somewhat mpenetrableat irst
glance). The “ N ) - ” notation is used to em phasize that the term refers to an
stands ngenerically
for thename
of
a ayer; hus, “(N)-ad dress,” for
OS
characteristic that
applies
to
each layer individually. The “ A’)-”prefix
example,refersabstractly o heconcept of an addressassociatedwitha
specific layer. while “ ransport-address” refers to the same concept app lied
to the Transport Layer.
may be provided in a directory or other database by a Network
Administrator or otherexternalauthority.Without this prior
knowledge, no meaningful communication between peer entities
can take place.
111. CONNECTIONS
A connection or
“
N)-connection,” in the formal terminology
of
OSI)
is a dynamic association established between twor more
entities “ N
+
1)-entities”) to controlheransfer of data
“ N)-servicedata units”)between hem.Strictly peaking,a
connectionactually joins the two or more N)-service access
points N)-SAP’S) o which the N
+
1)-entities are attached; in
the
OS1
model, all of the interactions between a service user and
a service provider take placeat a service access point see Fig.1 .
Theability to establish N) -co~ec tions, nd to convey data
units over them, is provided to N
+
1)-entities by the connec-
tion-mode N)-service.
A.
Characteristics
of
a Connection
Connection-mode data transfer displays the following funda-
mental characteristics:
I Clearh Distinguishable Lifetime: Connection-mode interac-
tions proceed hrough hree distinct sequential phases: connec-
tion
establishment; data transfer; and connection release. Fig.
2
illustratesschematically hesequence of operationsassociated
with connection-mode interactions. The three-phase lifetimeof a
connection may be spread out over a long period, and involve
many separate exchanges between the connectedN
+
1)-entities;
or it may be compressed nto a very short interaction, often called
“fast select,” in which all of the information necessary to estab-
lish heconnection, ransfer data, andclose heconnection s
conveyed in a small number of exchanges commonly one in each
direction).
2)
Three-PartyAgreement:Thesuccessfulestablishment of a
1 N ) - L A Y E R 1 ”;N ) -L A Y E R F O M
DISC NNECTISCONNECT
C O N F I R MN D I C A T I O N
REQUEST
-
- U S E R I N I T I A T E D - -
PROVIIJER INITIATED
Fig. 2 Connection-orientednteraction.
connection asserts and dynamicallymaintainshree-party
agreement concerning the transfer f data which goes far beyond
the participants’ “prior knowledge” of the OS1 environment. The
three parties-the two N
+
1)-entities that wish o communi-
cate, and the N)-service that provides them with the means to
do so-must first agree on their mutual willingness to participate
in the transfer
see
below). Thereafter, for
as
long
as
the connec-
tion persists, they must continue to agree on the acceptance of
each data unit ransferredover heconnection.There is no
possibility of data transfer hrough an unwillingservice to
an
unwilling partner, because hemutualwillingness of al three
parties must be established before the data transfer begins and
reaffirmed as each data unit is accepted by the receiver.
3
Negotiation and Renegotiation: In a connection-mode inter-
action,
no
connection is established-and no data are transferred
-until all parties agree on the 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 areunacceptable to the eceiver, and the eceivermay
suggest alternative parameter values and options along with
his
rejection.
If each party must reserve or allocate the resources such as
buffers and channels) that wi be required to carry out data-
transfer operations, negotiation provides an opportunity to scut-
tle the establishment of a connection if the resources that would
be equired to support it Cannot be obtained,
or
to explore
alternatives that could be supported with available resources. The
negotiation process also allows a varietyof access-control, secur-
ity, accounting, and identity-verification procedures o be carried
out to establish the willingness of the three parties involved to
undertake this instance of communication under these condi-
tions.
In
addition, whenmore than oneprotocol or class of
protocol s defined for a particular
OS1
layer, he negotiation
process provides an opportunity to select the one best suited to
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C W I N : C ON NEC TIO NS
AND
CONNECTIONLESSDATA
TRANSMISSION
1367
r l
SER A
r l
SER
N ) - S A P
QUEUE
FROM A TO B
\ QUEUE
FROM TO A
N ) - S E R V I C E R O V I D E
Fig. 3. Queue model of
a connection.
the urrent ircumstances.The greementshatesultrom
negotiation during theconnectionestablishmentphase
can
in
somecasesbemodified renegotiated) after aconnectionhas
been established and the data-transfer phase has begun.
4
Connection dentifiers:
Atconnectionestablishment ime,
each participating
N +
1)-entity is identified to the N)-service
by the address of the N)-service access point through which the
N + 1)-entity interacts with he
N)-serv ice .
The N)-service
uses these addresses to set up the requested connection. Subse-
quent requests to transfer data over the connection or to release
them) refer not to the addresses of the connected N)-SAP’S,but
to a connection dentifier supplied by he N)-service in OS1
parlance, an “ N)-connection endpoint dentifier”). This isa
locally significant “shorthand” reference that uniquely identifies
an established connectionduring its lifetime. Similarly, the proto-
col that supports the
N)-serv ice
typically employs a connection
identifier during he data-transfer phase rather than the actual
addresses of the corresponding service access points.
This
tech-
niquereduces heoverheadassociatedwith heresolutionand
transmission of addresses.
5 Data Unit Relationship:
Once a connection has been estab-
lished, it may be
used
to transfer successive data units, one after
another, until heconnection s eleasedbyone of the hree
parties.These data unitsare elated to eachothersimply by
virtue of being transferred in the context of a particular connec-
tion. Since data units transferred over a connection are related
ordinally as well, out-of-sequence, missing, and duplicated data
units
can
easily be detected and recovered. The data unit rela-
tionship maintained by a connection asoenables the
use
of flow
control techniques to ensure that the peer-to-peer data-transfer
rate does not exceed that which the correspondents are capable f
handling.
B.
Model of a Connection
A natural model for connection-mode data transfermaybe
constructed from the familiar concept of a quare. The successful
establishment of a connection between two service access points
is
represented in such a model by the creationf a pair of queues
that reside in the service provider see Fig.
3).
Onequeue s
created for each direction of information flow; every interaction
between the service provider and a
service
user then consists of
entering
an
object into one queue or removing an object from the
other queue.Theobjects that maybeplaced in aqueueare
service primitives and service data units, such as normal data,
expedited data, synchronizationmarks,contextselections,dis-
connects, and resets.
Such a queue effectively models the essential characteristics of
a connection: clearly distinguishable lifetime a pair of initially
empty queues is created by the connection establishment proce-
dure, and destroyed by connection release);data unit relationship
expressed as a set of rules governing the manipulation of objects
in hequeues);and hree-partyagreement
on
the ndividual
characteristics of eachqueuepair). By speclfylng he way
in
which service providers manage queues, he general model can
also be used to express more specific characteristics. Sequencing,
for example, can be included in the model by specifyrug that a
service provider may reverse the order of adjacent objects in a
queue if and only if the type of the following object is defined to
be “not sequenced”withrespect to the ypeof hepreceding
object. Similarly, the way in which the service provider is allowed
to constrain the ability of one service user to place objects in a
queue, based on the activity of the service user removing objects
from hequeue,providesarichlyvariabledescription of low
control.
An
elaboratemodel of connectionsnvariety of
individual contexts can be built up in
this
fashion.
It is important o note that thequeuemodeldescribes he
service of connection-mode data transfer as observedat two
connected service access pointsby the users of the service. It does
not describe the internal operation of the service provider.
IV.
CONNECTIONLESS
ATA
RANSMISSION
Connectionless data transmission is the transmission of inde-
pendent, unrelated data units sometimescalled “datagrams”)
from a source
service
access point to one or more destination
service access points in the absence of a connection. The ability
to convey N)-servicedata unitsbetween N)-service access
points without establishing, maintaining, and releasing an N)-
connection is provided to N + 1)-entities by the connectionless
N)-service.
A. Characteristics
of
Connectionless Data Transmission
Connectionless data transmission displays the following funda-
mental characteristics:
I
Two-p arty Agreement: Connection-mode ransfer equires
the establishment of a hree-party agreement between he par-
ticipating
N
+ 1)-entities and the N)-serv ice .A connectionless
service, however, involves only two-party agreements. There is an
a priori agreement between the corresponding N + 1)-entities,
unknown
to the N)-service, whichconsistsat east of their
“prior knowledge” of each other, and there are individual agree-
ments between each
N
+
1)-entity and the
N)-serv ice
provider;
but no N)-protocol information is exchanged between N)-enti-
ties concerning the mutual willingness of the
N
+ 1)-entities to
engage in a connectionless transmission or to accept a particular
data unit.
2)
Single-AccessService: Themostuser-visiblecharacteristic
of connectionless data transmission s hesingle service access
required to initiate the transmission of a data unit
see
Fig.
4 .
All
of the information required to deliver the data unit-destination
address, qualityof service selection, options, etc.-is presented to
the N)-service provider, along with the data, in a single service
primitiveoperation that isnot elated in any way to other
primitive operations, prioror subsequent.Once he service primi-
tive operation has taken place, no further communication occurs
between the provider and the user of the service concerning the
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1368
DATA
REQUEST
N)-LAYER
DATA
-
NDICATION
Fig. 4. C O M ~ C ~ ~ O ~ ~ S Sata
transmission.
D A T A
E Q U E S T
ATA
I
Fig. 5. Acknowledgeddatagram.
fate or subsequent disposition of that particular data unit. How-
ever, he wo-party agreements between he users and he pro-
vider of a connectionless service preserve considerable flexibility
by allowing a service user to specify parameter values and op-
tions-such as transferrate,acceptableerror rate, etc.-every
time the service is invoked. Dependmg
on
the way in which the
connectionless service is implemented, the service provider may
or maynotbeable to determinewhether he equestcanbe
carried out under the specified conditions.
A useful variantof connectionless data transmission commonly
referred toas “acknowledged datagram” displays the single-access
characteristic with he addition of a response from he service
provider to the service user confirming deliveryof the user’s data
unit to the destination service access point see Fig.
.
3
N o
Negotiation:
The
a priori
association between N + 1)-
entities described in Section II) establishes the protocol, parame-
ters, and other characteristics that determine the sigmficance of
data transmitted between them by a connectionless N)-service.
The users of such a service may, of course, employ their N
+
1 -
protocol to make. any further arrangements they wish concerning
their interpretation of the data transmittedand eceived; he
N)-service itself, however, s not a participant in any agreements
reached in this way, and does not provide support for them other
than by acting as a passive conveyor of data. This characteristic
contributes to the relative simplicity of connectionless protocols
by limiting the extent to which the N + 1)-layer interactions of
the service users impinge
on
the operation of the N)-protocol.
4 Data Unit Independence: From the standpoint of the service
provider, a data unit transmitted by a connectionless service is
completely unrelated to any other data unit. This does not mean
that an implementation of a connectionless
service
must actively
ensure that data units are unrelated, only that the service pro-
viderdoes not itselfperformanyfunctions to logicallyrelate
service data units in providing a connectionless service.
Data unit ndependence mplies hataseries of data units
handed one after another to a connectionless service for delivery
to the same destination will not necessarily be delivered to the
destination in that order; that is, “sequencing” isnot
an
intrinsic
property of connectionless data transmission. In spite of the fact
that aconnectionless ervicedoesnotexplicitly ecognizeor
establish any relationship between one data unit and another, at
least two circumstances may enable service users o expect a high
probability that data units will in fact be delivered in sequence:
PROCEEDINGS
OF THE
IFEE VOL. 1 NO
2
DECEMBER 1983
1) layer management may have access to information suggest-
ing that a very high probability of in-sequence delivery is
possible in a given situation; or
2)
the characteristics of the underlying
N
- 1)-senice may
include a high probability of in-sequence delivery of
N
-
1)-service data units, and the N)-service provider may be
able to make this characteristicavailable to users of the
connectionless N)-service.
Even when sequencing is provided by an underlying N - 1)-
service,however case 2) above), it will be possible or he
N)-serv ice
provider to make use of it only if al l requests for
connectionless N)-service are mapped onto N >service re-
questsatasingle
N
- 1)-serviceaccesspoint. This
wi
not
always be the case.
5 Sey-Contained Data Units: Data units transmitted using a
connectionless service, since they bear no relationship to other
data units, are entirely self-contained. All of the addressing and
other information neededby the service provider o deliver a data
unit to its destination must be included with each transmission.
This characteristic improves the robustness of a connectionless
serviceoperating in avolatileor ncompletelyunderstooden-
vironment, and reduces the amountf information other than the
data units hemselves hatmust be stored and/or distributed
within the service provider. On the other hand, the correspond-
ingly arger average data unit size usually represents a greater
overhead for each transmission than is incurred during the data
transfer phase of a connection.
B. Model of
Connectionless Data Transmission
The queue model introduced above Section 111-B) to describe
the basic properties of a connection contains at east wo ele-
ments that make it a oor model of connectionless data transmis-
sion:
1) Theconcept of aqueuestrongly mpliesa elationship
among he objects placed into and removed from it that runs
counter to the data unit independence propertyof connectionless
data transmission seeSectionIV-A). It also suggests that the
krvice provider allocates resources and processes requests
on
a
serviceaccesspointpairbasis.The minimal relationship that
existsamong data unitsduesimply to the act that theyare
transmitted between the same two
service
access points can be
expressed by a much less heavily freighted model.
2) It isifficult to clearlyescribeheroperties of
broadcast/multicast transmission in terms of queues that
link
pairs of service access points.
A more powerful model of connectionless data transmission
defines a
single
queue residing
in
the service provider to which
every service access point is implicitly ttached.
As
in the connec-
tion-orientedqueuemodel, erviceuser interacts withhe
service provider either y entering
an
object into the queue or by
removing an object from it. Only one type of object-a connec-
tionless service primitive-may be placed in the queue.
Such a queue effectively models the essential characteristics of
connectionless data transmission. In particular:
1
The existence and properties of the queue do not depend on
the behavior of service users. Awarenessof the queue’s character-
istics is part of the service users’ “prior knowledge” of the OS1
environment see Sections IV-A1and IV-M .
2) Any service user may place objectsnto the queue subject o
the constraints described in
3)
below).
Since
the queue is com-
monly accessible to al service users, the service provider operates
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CHAF’IN: CONNECTIONS
AND
CONNECTIONLESSDATA
TRANSMISSION
1369
1
A P P L I C A T I O N
A P P L I C A T I O N
P R E S E N T A T I O N
P R E S E M T A T I O N
S E S S I O N
S E S S I O N
T R A N S P O R T
T R A N S P O R T
llETWORK
NETWORK
D A T A L I N K
DATA LINK
P H Y S I C A L
P H Y S I C A L
P H Y S I C A L M E D I A F O R O P E N S Y S T E M S I N T E R C O N N E C T I O N
I
Fig. 6 . Layered OS1 architecture.
so
as to ensure that objects are removed from the queue only at
the service access point s) to which they are addressed. No other
relationshipamongserviceaccess points is mpliedby heex-
istence of the queue see Sections IV-A4 and IV-A5).
3) Thequeue has a inite but notnecessarilydeterminable
capacity.Theability ofserviceusers to placeobjects into the
queue, and the survival of objects after they have been placed in
the queue, are constrained by the activity of other service users
removing objects from the queue. his characteristic describes the
ability of a service provider to exercise congestion control, he
effects of which are distributed overall service users in ways that
are specified
as
fundamental properties
of
the queue.
In
contrast,
theeffects of flowcontrol in theconnection-orientedqueue
model are limited to individual service access point pairs.
V.
OS1
STANDARDS
As general architectural models for communication, both con-
nections and connectionless data transmission apply to each of
the
OS1
Reference Model’s even layers.Thedevelopment of
standard OS1 services and protocols, however, demands careful
analysis of the relative importance and utility
of
connectionless
and connection-mode operation at each layer, to avoid the pro-
liferation of incompatible or onlymarginallyusefulcombina-
tions.Such an analysis eeks to maintainabalancebetween
flexibility and stability, both
of
which are objectives of the OS1
standardization effort.
The OS1 standards that were well underway before the impor-
tance of the connectionless data transmission model was recog-
nized, ncluding he Network Service [5], the Transport Service
and Protocol [6] and the Session Service and Protocol [7], cur-
rently deal only with connections. Addenda to these connection-
oriented standards are being developed to describe connectionless
operation. After going hrough the sameapprovalprocess fol-
lowedbyhe standards themselves, ach ddendumwillbe
incorporated into the body
of
the corresponding standard at the
first revision of the standard usually five yearsafter the standard
is approved).
Standards projects begun more recently have been able o take
both models into account romheoutset.As esult,he
standards for LAN’s [8], the Data Link Service and Protocol [9],
the nternetworkProtocol [lo], thePresentationService nd
Protocol [ l l ] , and he Common ApplicationServices [12]
will
make appropriate use of both connectionless data transmission
and connections.
A .
Indioidual Layer Services and Protocols
Fig. 6 illustrates he ayered OS1 architecture
as
it is most
commonly drawn it shows two instances
of
the hierarchy, rep-
resentingheelationship etweenwo open systems).The
following ubsectionsdiscuss heuse of theconnectionand
connectionless models in the development
of
standards for each
of the seven layers.
1 Physical Layer:
Thedistinctionbetweenconnections and
connectionless data transmissions ifficult to demonstrate
satisfactorily at the Physical Layer, largely because the conceptf
aphysical “connection” is both ntuitiveandcolloquial.The
PhysicalLayer s esponsible orgenerating 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, o telecom-
munications media), is a signal pathway through a medium or a
combination of media. In this context, it is probably sufficient to
label“connectionless” hosephysical ransmission ystems in
which no explicit initial signaling procedure must be carried out
to set up a signal ath for data transmission. Differences between
the two models appear in any event to have little relevance for
the development of Physical Layer standards [13].
2)
Data Link Layer:
Data Link rocedures esigned for
transmission media that suffer relatively high it error rates such
as telephoneines nd other long-haul acilities) re lmost
universally connection;based, since it is generally more efficient
to recover point-tepoint bit-streamerrors at the Data Link
Layer with its comparatively short timeout intervals) than at a
higher layer. HDLC and ADCCP
[9]
are well-known examples of
Data Link ontrolswith an explicit onnectionorientation.
LAN’s, on the other hand, employ intrinsically reliable physical
transmissionsystems baseband and broad-band coaxialcable,
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1370
PROCEEDINGS OF
THE VOL. 71,
NO. 12, DECEMBER
983
fiber optics, etc.) in a restricted range generally
o
greater than 1
or 2 km), and are typically able o achieve extremely low it error
rates. In addition, themedia-accesscontentionmechanisms of
many LAN’s handle transmission errors as a matter of course.
Theusual approach to physical nterconnection iesallnodes
together on a common medium, creating an inherently broadcast
environment nwhichevery ransmission can be eceivedby
every station. Taking advantage of these characteristics virtually
demands a connectionless Data Link service.
The most significant LAN standardization effort, the IEEE’s
Project 802, has incorporated two Data Link procedures in the
Logical Link Control definition of the P802 Standard [8]. In one
procedure, information frames are unnumbered and may be sent
at any time by any tation without first establishing a connection.
The intended receiver may accept the frame and interpret it, but
isunder
no
obligation to do
so
and may insteaddiscard he
frame with
no
notice to the sender. Neitheris the sender notified
if no station recognizes he address coded into the frame, and
there s no receiver. This connectionlessprocedure, of course,
assumes the “friendly” environment and higher layer acceptance
of responsibility that are common characteristics of LAN’s. The
other procedure provides all of the sequencing, error recovery,
and otherpropertiesnormallyassociatedwithconnection-ori-
ented link procedures. It is in fact verysimilar to the HDLC
balanced asynchronous mode procedure.
3) Network Layer: Within theNetworkLayer,aconsistent
Network Service [5], with well-defined characteristics displayed at
service ccess points in end ystems,s onstructed rom
potpoum of point-to-point data
l inks
and subnetwork services,
each of which may have its own access method, address conven-
tions, native reliability, and administration. The existence f such
a variety of underlying communicationsservices complicates the
development of Network Layerstandards. Public network services
tend to be connection-oriented, because their providers must deal
with unpredictable and widely fluctuating loads, must limit in-
dividual variations in quality of service to arelativelynarrow
range speclfied in a contract hence must be very careful about
resource allocation),and must be able to charge for the service n
a fair and auditable) basis. Deterministic global resource alloca-
tion s of paramount importance.Privatenetworks, uch as
LAN’s, tend to be owned and used by the same organization, nd
their operating
costs
are generally recovered in wayshat are only
indirectly related to individual instances of
use.
Public network
administrators a so exercisegreaterglobalcontrolover heir
configurations than privatenetworkadministrators do, to the
extent that large public networks, even hough hey consist of
many ubunits,are eadilyoperated and perceivedby heir
users)
as
one “network”; large private networks, however, almost
always consist of a number of individual, interconnected smaller
networks, forming an “internet” in which boundaries related to
administration, ocal control, and mode of
use
persist. Private
internets, concerned with he flexible and reconfigurable inter-
connection of a variety
of
individual networks
and
correspond-
ingly reluctant to make too many assumptions about the nature
of individual underlyingservices tend to be connectionless.
These
differences have strongly influenced the development of
Network Layer standards. CCITT Recommendation X.25 is the
best known example of a connection-oriented network protocol;
it enjoys almost universal acceptance s the standard for access to
public data networks, and is evolving to serve as a standard for
connection-oriented operation over other long-haul communica-
tions facilities
as
well. The connectionless Internetwork Protocol
standard beingdeveloped ointlyby SO,ANSI,ECMA,and
NBS/ICST [lo] will provide hemuch-needed ramework or
network interconnection; approval by S0 is expected in 1984.A
single standard for heconnection-orientedNetworkService,
augmented by an addendum describing the connectionless Net-
work Service, will be approved by both IS0 and CCITT by the
endof1983 5],giving eason to hope that diversity in the
Network Layer
will
not lead to chaos.
4
Transport Layer: The Transport Layer is concernedwith
creating a uniform Transport
Service
[6] that
js
defined
on
an
end-system to end-systembasiswith espect
to
characteristics
such
as
error detection and recovery, multiplexing, addressing,
and quality of service. It is often described as a transition point:
the place in the OS1 hierarchy at which the application orienta-
tion of the upper three layers and the communications orienta-
tion of the lower three layers meet. When the Network Service is
connectionless, it is the Transport Layer that creates connections
for onnection-oriented pplications; and when
the
Network
Service is connection-oriented, the Transport Layer performs the
connection-management functions necessaryo provide a connec-
tionless service for connectionless applications. Consequently, the
standards for the Transport Service and Protocol augmented by
addenda for connectionless data transmission) include provisions
both for passing connection-oriented and connectionless services
through to higher layers, and for providing one kindf Transport
Service using an underlying Network Service of the other kind
see Section V-B below).
5
SessionLuyer: The concept of a session which binds pre-
sentation-entities into a structured relationship of some meaning-
fu
duration is inherently connection-oriented. Consequently,es-
sion Layer standards are concerned primarily with the character-
istics of session connections [7].Connectionless Session Service
and protocol standards are being developed simply to enable a
uniformly connectionless service to be passed efficiently through
the Session Layer to higher layers.
6)
Presentation Layer:
There re no special onsiderations
with respect to the
use
of connections and connectionless data
transmission in thePresentationLayer.Theoperation of the
Presentation Layer is connection-oriented when supporting Ap-
plication COM~C~~OI~S,nd ~ o ~ e c t i ~ n l e ~ shen supporting con-
nectionless Application data transmission.
7)
Application Layer:
ApplicationLayer standards provide
facilities for both connection-oriented and connectionless com-
municationamongapplicationprocesses [12].These facilities
support the requirements of user applications that are naturally
either connection-oriented or connectionless. Inherently connec-
tion-oriented applications include bulk file transfer particularly
when heckpoint/recovery eatures re mplemented);virtual
terminal usage long-term attachment of a terminal, workstation,
or other device to a remote host); and stream-oriented
access
to
distributed system components such as spoolers, print servers,
and remote-job-entry stations). Inherently co ~ect io nles s ppli-
cations include inward data collection periodic activeor passive
sampling of a argenumber of data sources);outward data
dissemination the distribution of a single pieceof information to
a large number of destinations); broadcast and multicast group
addressed) communication; and a variety of “request-response”
applications, in which a smgle request
is followed by a single
response.
A
more detailed discussion of connection-oriented and
connectionless application types may be found in [14].
B.
Luyer Service Combinations
The potential availability of twocomplementary services at
each layer of the architecture raises an obvious question-how o
choosebetween hem? It shouldbeclearat
this
point that
unilateral exclusion of one or the other, although it may simphfy
8/9/2019 Connection-Oriented and Connectionless Good Document
http://slidepdf.com/reader/full/connection-oriented-and-connectionless-good-document 7/7
CHAF’IN: CONNECTIONS A ND
CONNECTIONLESS
DATA TRANSMISSION 1371
OFFERS A CONNECTIONLESS
NI-SERVICE
N ) - L A Y E R
I
I
USES A CONNECTION-
O R I E N T E D N - l ) - S E R V I C E
OFFERS A CONNECTION-
O R I E N T E D N ) - S E R V I C E
N)-L AYER
I
I
N-~)-SERVICE
USES A CONNECTIONLESS
N - I ) - L A Y E R
Fig.7.Service type conversion.
N + ~ ) - L A Y E R
t
OFFERS A CONNECTIONLESS
N ) -SERV ICE
~ N ) - L A Y E R
N-~)-SERVICE
USES A CONNECTIONLESS
OFFERS A
CONNECTION-
O R I E N T E D
N ) - S E R V I C E
N)-LAYER
I
I
USES A CONNECTION-
O R l E l r T E DN - l ) - S E R V I C E
I i - I ) - L A Y E R
Fig. . ame-servicemapping.
A P P L I C A T I O N
\
P R E S E N T A T I O N
S E S S I O N
TRANSPORT
NETWORK
D A T A I N K
P H Y S I C A L
Fig.9. Layer ervicecombinations.
the situation for some applications, is not a general solution to
the problem. There are actuallywo parts to the question: how to
selectan appropriate set of cooperativeservices orallseven
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 ervices,how to provide or hedynamic
selection of one or the other in a given circumstance.
Both parts of the question turn out to be easier to deal with in
practice than in theory, since actual systems-as opposed to the
more abstract set of services and protocols collected under the
banner of OSI-will naturally be constructed n 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 combina-
tions of services for different applications
will
generally be as
sembled according to relatively simple rules established during
thedesign of thesystem.Thesechoices
will be
drivenby he
requirements of individual applications and by the characteristics
of the preferred or available) implementation technologies.
OS service and protocol standards, however, address the gen-
eral case, so
as
to accommodate a wide range of actual-system
configurations. The goal is to achieve a usefu balance between
power and simplicity. Clearly, the service definition for each layer
must ncludebothconnection-orientedandconnectionlessser-
vices;otherwise, heutility of aservice at one ayercouldbe
negated by the unavailability f a corresponding service elsewhere
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 primarily because the Applica-
tion Layer, which must provide a connectionless service to user
applications, cannot doso effectively if they do not. Recognizing
these olevariationsopensup hepossibility of restoringa
measure of the simplicity ost n he introduction of choice at
each ayerby imiting,
not
thechoices, but theplaces in the
hierarchy where conversion from one choice to the other-con-
nection to connectionless, or vice versa-is allowed see Fig.
7).
At this stage in the development of OSI, it appears that there are
excellent reasons for allowing such a conversion to take place in
the Transport and Network Layers and in the Data Link Layer,
if some physical transmission systems are considered to be con-
nectionless). In theother ayers, heprovision of onekind of
service to the next-higher layer must always be accomplished by
using hesamekind of service rom henext-lower ayer see
Figs. 8 and 9). This principle
of
like-to-likemapping snot
related to multiplexing; it refers to service
types
connection-ori-
ented and connectionless), not to actual services.) Such a restric-
tion, which hasbeen ncorporated in theAddendum to the
Reference Model covering connectionless data transmission [4],
contributes to the achievement of the balance mentioned above,
without excludmg those combinations of services that have dem-
onstrated their usefulness.
REFERENCES
[ l ] I S 0 Internationaltandard 7498,Informationrocessing
systems-Open Systems Interconnection-Basic reference model,” Oct.
1983.
[2]CCITTDraftRecommendation X .2 0 , “Referencemodel of opensys-
[3] J Dayand H. Zimmerman,“BasicreferencemodelofOpenSystems
tems interconnection for CCITT applications,” une 1983.
[4] IS 0 Draft International Standard D D D D , “Information processing sys-
Interconnection,” this issue, pp. 1334-1340.
tern-OpenSystems nterconnection-Addendum to thebasicrefer-
ence model covering connectionless data transmission,” Oct. 1983.
[5] C. Ware, “ Services and protocols of the Netwo rk Layer,” this issue, pp.
[6] K. Knightson,“Servicesandprotocolsof heTransportLayer,” this
[7] W. F.
Emmons
and A. Chandler, “Services and protocols of the Session
181 Draft IEEE Standard 802.2, “Logical link control,” Draft D , D e c 1982.
[9] J. Conard, “Services and protocols of the Data LinkLayer,” his issue ,
1384-1387.
issue, pp. 1394-13 .
Layer,” this issue, pp. 1397-1400.
[l o] R. Callon, “Internetwork protocol,” th is issue, pp. 1388-1393.
[ l l ] L. Hollis, “Se mc es and protocols of the Presentation Layer,” this ssue,
[12]
P.
Bartoli, “Application proce sses and the Application Layer,” this issue,
[13] F. M cClelland, “Services and protocols of the Physical Layer,” this issue,
[14]A.L.Chapin,“Connectionlessdata ransmission,” Comput Commun.
pp. 1378-1383.
pp. 1401-1403.
pp. 1404-1407.
pp. 1372-1377.
Reo., vol. 12, no. 2 , pp. 21-61, Apr. 1982.