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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



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



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



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



CHAF’IN: CONNECTIONS

AND

CONNECTIONLESSDATA

TRANSMISSION

1369

1

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,



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



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.


\

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.

Connection-Oriented and Connectionless Good Document

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