ISO 7498:1984/Add 1:1987
(Main)Information processing systems — Open Systems Interconnection — Basic Reference Model — Addendum 1
Information processing systems — Open Systems Interconnection — Basic Reference Model — Addendum 1
Systèmes de traitement de l'information — Interconnexion des systèmes ouverts — Modèle de Référence de base — Additif 1
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Standards Content (Sample)
INTERNATIONAL STANDARD IS0 7498 : 1984/ADDENDUM 1
Published 1987-07-1 5
INTERNATIONAL ORGANIZATION FOR STANOAROIZATION*MEX~YHAPO~HAR OPrAHM3AUHR no CTAHflAPTH3AUIIM*ORGANlSATlON INTERNATIONALE DE NORMALISATION
Information processing systems - Open Systems
- Basic Reference Model
Interconnection
Addendum 1 : Connectionless-mode transmission
Systèmes de traitement de I'information - Interconnexion de systèmes ouverts - Modèle de référence de base
I : Transmission en mode sans connexion
Additif
Contents
Page
O Introduction . 1
1 Scope and field of application . 3
2 Definitions. . 3
3 Notation . 3
4 Introduction to Open Systems Interconnection (OS) . 3
5 Concepts of a layered architecture . 3
6 Introduction to the specific layers of the reference model . 7
7 Detailed description of the resulting OS1 architecture. . 9
0.1.2 The architectural objectives of the Reference Model do
O Introduction
not depend on the exclusive use of connections for all OS1
communications. It is the intention of this addendum to
0.1 About this addendum
introduce terms and define their use within the Reference
Model so that the two alternatives (connection-mode
0.1.1 IS0 7498 describes the Reference Model of Open
transmission and connectionless-mode transmission) can be
Systems Interconnection. It is the intention of IS0 7498 that
treated as complementary concepts which can be applied
the Reference Model should establish a framework for coor-
appropriately in the different circumstances for which each is
dinating the development of existing and future standards for
suited.
the interconnection of systems. The assumption that a con-
nection is a fundamental prerequisite for communication in the
0.2 What is connectionless-mode transmission
OS1 environment permeates the Reference Model and is one of
in the Reference Model ?
the most useful and important unifying concepts of the
architecture which it describes. However, since IS0 7498 was
0.2.1 General
produced it has been realized that this deeply-rooted con-
nection orientation unnecessarily limits the power and scope of
the Reference Model, since it excludes important classes of
0.2.1 .I The concept of connectionless-mode transmission in
applications and important classes of communication network one form or another\has always played an important role in the
technology which have a fundamentally connectionless nature.
specification of services and protocols for data communication.
Ref. No. IS0 7498 : 1984/Add.l : 1987 (E)
UDC 681.3.01
Descriptors : data processing, information, interchange, network interconnection, open systems interconnection, models, mode of data
transmission.
O International Organization for Standardization, 1987 0
Price based on 11 pages
Printed in Switzerland
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IS0 7498 : 19WAdd.l : 1987 (E)
The properties of connection-mode transmission are also rele-
The terms "message mode", "datagram", "transaction mode"
vant in a wide range of other applications.
and "connection-free" have been used in the literature to
describe variations on the same basic theme : the transmission
of a unit of data in a single self-contained operation without
0.2.3 Connectionless-mode transmission in the
establishing, maintaining, and releasing a connection.
Reference Model
0.2.3.1 Again in the formal terminology of the Reference
0.2.1.2 Since connectionless-mode transmission and
Model, connectionless-mode transmission is the transmission
connection-mode transmission are complementary concepts,
of a single unit of data from a source service-access-point to
they are best understood in juxtaposition, particularly since
one or more destination service-access-points without estab-
connectionless-mode transmission is defined most easily in
lishing a connection. A connectionless-mode service allows an
relationship to the concept of a connection.
entity to initiate such a transmission by the performance of a
single service access.
0.2.2 Connection-mode transmission in the
0.2.3.2 In contrast to a connection, an instance of the use
Reference Model
of a connectionless-mode service does not have a clearly
distinguishable lifetime. In addition it has the following fun-
damental characteristics :
0.2.2.1 In the formal terminology of the Reference Model, a
connection is an association established for the transfer of data
a) it requires only a pre-arranged association between the
between two or more peer-entities. This association is
peer-entities involved which determines the characteristics
established between the peer-entities themselves and between
of the data to be transmitted, and no dynamic agreement is
each entity and the next lower layer. The ability to establish and
involved in an instance of the use of the service;
release a connection and to transfer data over it is provided to
the entities in a given layer by the next lower layer as a
b) all the information required to deliver a unit of data,
connection-mode service. The use of a connection-mode ser-
destination address, quality of service selection, options,
vice by peer-entities proceeds through three distinct phases :
etc., is presented to the layer providing the connectionless-
mode service, together with the unit of data to be transmit-
a) connection establishment;
ted, in a single service access. The layer providing the con-
b) data transfer; and nectionless-mode service is not required to relate this access
to any other access.
c) connection release.
0.2.3.3 As a result of these fundamental characteristics it may
0.2.2.2 In addition to the clearly distinguishable lifetime ex-
also be true that
hibited by these phases a connection has the following fun-
damental characteristics :
a) each unit of data transmitted is routed independently by
the layer providing the connectionless-mode service;
a) it involves establishing and maintaining a three or more
party agreement concerning the transmission of data be-
b) copies of a unit of data can be transmitted to a number
tween the peer-entities concerned and the layer providing
of destination addresses.
the service;
0.2.3.4 These characteristics of connectionless-mode
b) it allows the negotiation between all the parties con-
transmission do not preclude making available to the service
cerned of the parameters and options that will govern the
user information on the nature and quality of service which may
transmission of data;
apply for a single invocation of the service or which may be
c) it provides connection identification by means of which observed over successive invocations of the service between
the overheads involved in address resolution and transmis-
pairs of service-access-points or among a set of service access
sion can be avoided on data transfers;
points.
d) it provides a context within which successive units of
data transmitted between the peer-entities are logically
0.3 Structure of this addendum
related, and makes it possible to maintain sequence and
provide flow control for those transmissions.
has a structure which is similar to that of
This addendum
IS0 74.98 in order to facilitate the eventual integration of the
0.2.2.3 The characteristics of connection-mode transmission
text of this addendum into a revision of IS0 74.98. There are
are particularly attractive in applications which call for relatively
two major clauses :
long-lived, stream-oriented interactions between entities in
stable configurations. Examples are provided by direct terminal
Clause 5 : this clause deals with general architectural principles
use of a remote computer, file transfer, and long-term attach-
and the sub-clauses correspond to sub-clauses in clause 5 of
ment of remote job entry stations. In these cases the entities in-
IS0 7498 under which different aspects of connectionless-
volved initially discuss their requirements and agree to the
mode transmission need definition.
terms of their interaction, reserving whatever resources they
Clause 7 : this clause deals with details which are specific to the
may need, transfer a series of related units of data to ac-
layers of the Reference Model of Open Systems Interconnec-
complish their mutual objective, and explicitly end their inter-
tion.
action, releasing the previously reserved resources.
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IS0 7498 : 1984/Add.l : 1987 (E)
1 Scope and field of application U) (NI-service;
This addendum
vi (Ni-service-access-point;
a) provides a general description of a connectionless-
w) (NI-service-data-unit;
mode service and of functions related to it, which may be
provided by layers of the Reference Model, and
xi (NI-user-data.
b) defines the positions within the Reference Model where
2.2 For the purpose of this addendum, the following defini-
the service and functions may be provided.
tions also apply :
This addendum adds to the concepts and principles defined in
IS0 7498; it does not modify them. This addendum
2.2.1 (Ni-connection : An association established by the
(N)-layer between two or more (N + 1)-entities for the transfer
- does not specify services and protocols for OSI;
of data, which provides explicit identification of a set of
(NI-data-transmissions and agreement concerning the (NI-data-
- is not an implementation specification;
transmission services to be provided for the set.
- is not a basis for appraising the conformance of actual
NOTE - This definition of (NI-connection is a refinement of the defini-
implementations.
tion given in IS0 7498; it does not change it.
2.2.2 (Ni-connection-mode transmission : (Nbdata-
transmission in the context of an (NI-connection.
2 Definitions
2.1 This addendum makes use of the following terms :
2.2.3 (NI-connectionless-mode transmission : (Nbdata-
transmission not in the context of an (NI-connection and not
a) (NI-address;
required to maintain any logical relationship between
(NI-service-data-units.
b) acknowledgement;
c) blocking;
3 Notation
di concatenation;
The layer notation is the same as that defined in IS0 7498.
e) (N )-data-transmission;
f) (Nbentity;
4 Introduction to Open Systems
Interconnection (OSI)
g) (NI-facility;
This clause makes no additions to clause 4 of IS0 7498.
h) flow control;
i) (NI-function;
5 Concepts of a layered architecture
j) (N )-interface-data-unit;
5.1 Introduction
k) (NI-layer;
This addendum makes no additions to 5.1 of IS0 7498.
I) open system;
5.1 Principles of layering
m) peer entities;
5.2.1 This sub-clause complements 5.2 of IS0 7498.
n) (NI-protocol;
5.2.2 In order for (N + 1 )-entities to be able to communicate
O) (Ni-protocol-control-information;
using an (NI-connection-mode service or an (Ni-connection-
less-mode service it is essential that a pre-arranged association
p) (NI-protocol-data-unit;
exists between them, constituted by the pre-knowledge which
it is essential that each (N + 1 )-entity has of the others in order
q) (NI-relay;
at least to initiate the use of the service. This association is
r) routing; established in ways which are not detailed in this addendum or
in IS0 7498 and comprises four elements :
s) segmenting;
a) knowledge of the addresses of the peer entities in-
volved;
t) sequencing;
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IS0 7498 : 1984/Add.l : 1987 (E)
those which are offered to the (NI-layer by the (N - 1 )-layer
b) knowledge of a protocol agreed by the peer entities for
use at least to initiate communication; and, if necessary, to convert between one mode of service and
another.
c) knowledge of the availability for communication of the
peer-entities;
5.3 Communication between peer-entities
d) knowledge of the quality of service available from the
5.3.1 This sub-clause complements 5.3 of IS0 7498.
(N )-service.
5.3.2 (N + 1 bentities can communicate using an (NI-connec-
NOTE - The pre-knowledge constituting a pre-arranged association
tionless-mode service provided that there is a pre-arranged
can be acquired in many ways; some examples are listed below :
association between them providing knowledge about each
a) from information acquired manually when contracts are ex-
other which allows them to do so. This knowledge should allow
changed with a service provider;
+ 1 bentities to be determined, it should
the locations of the (N
b) from information which a network administration may provide
determine the correct interpretation of (N)-service-data-units
in a directory or enquiry database;
by a receiving (N + 1)-entity, and it may define the rates of
c) from information that may be learned from previous instances
transfer, rates of response, and the protocol in use between the
of communication;
entities. The knowledge may result from prior agreement be-
d) from information that may be provided dynamically through tween the (N + l )-entities concerning the parameters, formats
the operation of management protocols.
and options to be used.
The total pre-knowledge constituting a pre-arranged association is like-
NOTE - Data transfer using an (NI-connection-mode service involves
ly to be acquired by a combination of the above.
the establishment of an (N)-connection prior to the data transfer, set-
ting up, dynamically, an association between the (N + 1 )-entities and
5.2.3 An (N + 1 )-entity provides no information to an (NI-con-
the (Ni-connection-mode service in addition to the association iden-
nectionless-mode service about the logical relationships be- tified in 5.1. This association involves elements which are not part of
the pre-arranged association described in 5.1, in particular :
tween (NI-service-data-units, apart from the source and
destination (Ni-service-access-point-addresses.
a) knowledge of the willingness of the peer entity or entities to
undertake a specific communication, and of the willingness of the
underlying service to support it; and
5.2.4 From the point of view of the (N + 1 )-entity this means
b) the ability for the peer entities to negotiate and renegotiate the
that it is not able to require the (NI-service to apply a particular
characteristics of the communication.
function to a sequence of (NI-service-data-units sent by it.
However, from the point of view of the (Ni-layer, this does not
5.3.3 (N + 1 bentities may require prior knowledge of the
imply any constraint on the functions which support the ser-
facilities offered by the service and the quality of service which
vice.
they can expect to receive from it in order to choose an
(N + 1 )-protocol to be used for communication over an (NI-con-
5.2.5 An (Nl-layer may offer a connection-mode service, a
nectionless-mode service.
+ 1 )-layer,
connectionless-mode service, or both, to the (N
using the service or services provided by the (N - 1 )-layer.
5.3.4 There are instances where the connectionless-mode
service provided by the (Ni-layer does not provide direct access
5.2.6 Both the (NI-connection-mode service and the
between all of the (Ni-service-access-points supported by the
(NI-connectionless-mode service are characterized by the layer. Connectionless-mode transmission can still occur be-
facilities which they offer to, and the quality of service seen by,
tween these service-access-points if one or more (N + l )-entities
the (N + 1 bentities. For both the (NI-connection-mode service provide a relay (see figure 1 which complements figure 6 in
and the (NI-connectionless-mode service, functions may be IS0 7498). The fact that an (NI-connectionless-mode transmis-
provided by the (NI-layer to enhance the facilities offered to,
sion is relayed by one or more (N + 1 bentities is known neither
and the quality of service seen by, the (N + 1 )-entities over by the (NI-layer nor by the (N +2)-layer.
Connectionless-mode transmission
# *
Figure 1 - Connectionless-mode transmission through a relay
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IS0 7498 : 1984/Add.l : 1987 (E)
5.4 Identifiers 5.7.2.5 If an (NI-service-data-unit cannot be accepted by an
(N + II-entity at the time of its arrival at an (NI-service-access-
This addendum makes no additions to 5.4 of IS0 7498.
point, the (N + 1 )-entity may apply interface flow control (see
5.7.3.2). This may result in the discarding of the (NI-service-
data-unit by the (NI-service provider or, where flow control
at the
is provided, in the exercise of interface flow control
5.5 Properties of service-access-points
sending (NI-service-access-point by the (Ni-service provider.
5.5.1 This sub-clause complements 5.5 of IS0 7498.
5.7.2.6 An (NI-connectionless-mode service may allow the
transmission of copies of an (NI-service-data-unit to a number
5.5.2 An (Ni-service-access-point may support :
of destination (NI-service-access-points. (NI-service-data-units
transmitted from a number of source (Ni-service-access-points
ai (Ni-connection-mode services only:
can be received at one destination (Ni-service-access-point.
The (Ni-layer does not assume any logical relationship between
bi (NI-connectionless-mode services only; or
these (NI-service-data-units.
ci (Ni-connection-mode services and (Nl-connectionless-
mode services concurrently.
5.7.2.7 No (NI-protocol-control-information is exchanged
between (NI-entities concerning the mutual willingness of the
(N + 1 i-entities to exchange data using an (Ni-connectionless-
5.5.3 A single (N+ 1)-entity may be using concurrently
mode service.
several (Ni-connections and an (NI-connectionless-mode ser-
1(1)
vice through one or more (Ni-service-access-points to which it
NOTES
is attached.
1 The specific interface mechanism employed by a particular im-
plementation of a connectionless-mode service may involve more than
one interface exchange to accomplish the single logical service access
5.5.4 (N + 1 i-entities distinguish between instances of the
necessary to initiate a connectionless-mode transmission. However,
(Ni-connectionless-mode services and the (Ni-connection-
this is a local implementation detail.
mode services offered concurrently through the same (Ni-ser-
2 The transmission of each (NI-service-data-unit by an (N)-connec-
vice-access-point by the uniqueness of the interactions
tionless-mode service should be entirely self-contained. All the ad-
prescribed for these services.
dressing and other information required by the (NI-layer to deliver the
(NI-service-data-unit to its destination should be included in the service
access for each transmission.
5.6 Data-units
3 It is a basic characteristic of a connectionless-mode service that no
negotiation of the parameters for a transmission takes place at the time
This addendum makes no additions to 5.6 of IS0 7498.
the service is accessed and no dynamic association is set up between
the parties involved. However, considerable freedom of choice can be
preserved by allowing most parameter values and options (such as
transfer rate, acceptable error rate, etc.) to be specified at the time the
5.7 Elements of layer operation
service is accessed. In a given implementation, if the local (N)-sub-
system determines immediately (from information available to it locally)
5.7.1 Introduction
that the requested transmission cannot be performed under the condi-
tions specified, it may abort the transmission, returning an implemen-
This sub-clause complements 5.7 and 5.8 of IS0 7498, to which tation specific error message. If the same determination is made later,
0
after the service access has been completed, the transmission is aban-
it relates.
doned, since the (Ni-layer is assumed not to have the information
necessary to take any other action.
5.7.2 Control of connectionless-mode transmission
5.7.3 Transfer of data
5.7.2.1 This sub-clause is specific to this addendum and has
no equivalent in IS0 7498.
5.7.3.1 General principles
5.7.2.2 All the information required by an (Ni-connectionless-
5.7.3.1.1 This sub-clause complements 5.7.6.1, 5.7.6.2 and
mode service to deliver an (NI-service-data-unit (destination ad-
5.7.6.3 of IS0 7498.
dress, quality of service required, options, etc.) is presented to
it with the (Ni-service-data-unit in a single logical service access
by the sending (N + 1)-entity.
5.7.3.1.2 Control information and user data are transferred
between (NI-entities in (NI-protocol-data-units. An (NI-
protocol-data-unit is a unit of data specified in an (Ni-protocol
5.7.2.3 All information related to an (NI-service-data-unit,
and contains (NI-protocol-control-information and possibly
together with the (Ni-service-data-unit itself, is received from
(Ni-user-data.
the (NI-service in a single logical service access by the receiving
(N + 1 )-entity.
5.7.3.1.3 (NI-protocol-control-information is transferred
between (Ni-entities using an (N - 1 i-service. (Ni-protocol-
5.7.2.4 To provide the (NI-connectionless-mode service, the
control-information is any information that supports the joint
(Ni-layer performs functions as described in 5.1. These func-
operation of (NI-entities. (NI-user-data is passed transparently
tions are supported by (NI-protocols.
between (Ni-entities using an (N - 1 i-service.
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IS0 7498 : 1984/Add.l : 1987 (E)
5.7.3.1.4 An (NI-protocol-data-unit has a finite size, which 5.7.4.2.2 An (NI-connectionless-mode service is character-
may be limited by the (N - II-service-data-unit size and by the
ized by two groups of quality of service parameters. The
capabilities of the (NI-protocol. (NI-protocol-data-units are parameters in the first group are defined entirely by the
mapped into (N - 1 1-service-data-units. The interpretation of an
behaviour of a single (NI-data-transmission and are the same as
(NI-protocol-data-unit is defined by the (NI-protocol in effect those defined for the (NI-connection-mode service. These
for the (NI-connectionless-mode service.
parameters are
a) expected transmission delay;
5.7.3.1.5 An (NI-service-data-unit is transferred between an
(N + 1 )-entity and an (NI-entity, through an (NI-service-access-
bi probability of corruption;
point, in the form of one or more (N)-interface-data-units. Each
(NI-service-data-unit is transferred as (NI-user-data in one or
c) probability of loss or duplication;
more (NI-protocol-data-units.
d) probability of wrong delivery;
5.7.3.2 Flow control
e) cost; and
fI protection from unauthorized access.
5.7.3.2.1 This sub-clause complements 5.7.6.4 of IS0 7498.
5.7.4.2.3 The parameters in the second group apply for multi-
5.7.3.2.2 If flow control functions are provided, they can
ple (N)-data-transmissions between pairs of (NI-service-access-
operate only on protocol-data-units and interface-data-units.
points. These parameters are
5.7.3.2.3 Two types of flow control are identified : a) expected throughput; and
b) probability of out-of-sequence delivery.
a) Peer flow control which regulates the rate at which
(NI-protocol-data-units are sent between (NI-entities sup-
porting (NI-connectionless-mode transmission between
5.7.4.2.4 The values of the parameters in both groups at a
pairs of (Ni-service-access-points or among a set of
given source (NI-service-access-point may vary with the
(NI-service-access-points. Peer flow control requires pro-
destination (N I-service-access-point.
tocol definitions and is based on protocol-data-unit size;
and
5.7.4.2.5 Sequencing, acknowledgement and error detection
and notification functions may be used in the (NI-layer in order
bI (NI-interface flow control, which regulates the rate at
to enhance the quality of service offered by the (NI-connection-
which (Nl-interface-data-units are passed between an
less-mode service over that offered by the (N - 1 )-connection-
(N+ 1)-entity and an (NI-entity that supports an (NI-con-
less-mode service.
nectionless-mode service. (Ni-interface flow control is
based on (NI-interface-data-unit size.
5.7.4.3 Sequencing
5.7.3:3 Segmenting, blocking and concatenation
5.7.4.3.1 This sub-clause complements 5.7.6.6 of IS0 7498.
5.7.3.3.1 This sub-clause complements 5.7.6.5 of IS0 7498.
5.7.4.3.2 The (N - 1 )-services provided by the (N - 1 I-layer
5.7.3.3.2 Data units in different layers of the Reference Model
of the OS1 architecture may not guarantee delivery of data
are not necessarily compatible in size. Segmenting, blocking or
between pairs of (N - 1)-service-access-points in the same
concatenation may be necessary.
order as it was submitted. If the (NI-layer provides a higher pro-
bability of in-sequence delivery than is offered by the
(N - 1 )-services, sequencing mechanisms must be present in
5.7.3.3.3 Segmenting in an (NI-layer requires the implemen-
the (NI-layer.
tation of a segmenting-reassembly protocol between
( NI-entities that support (N )-connectionless-mode transmission
between pairs of (NI-service-access-points.
5.7.4.4 Acknowledgement
5.7.4.4.1 This sub-clause complements 5.7.7.1 of IS0 7498.
5.7.4 Quality of service
5.7.4.4.2 An acknowledgement function may be used by peer
5.7.4.1 Introduction (NI-entities supporting (NI-connectionless-mode transmission
between pairs of (NI-service-access-points to obtain a higher
IS0 7498. loss than is provided
This sub-clause complements 5.7.7 of probability of detecting protocol-data-unit
by the (N - 1 I-layer.
5.7.4.2 Quality of service parameters
5.7.4.5 Error detection and notification
5.7.4.2.1 This sub-clause is specific to this addendum and has
5.7.4.5.1 This sub-clause complements 5.7.7.2 of IS0 7498.
no equivalent in IS0 7498.
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IS0 7498 : 1984/Add.l : 1987 (El
5.7.4.5.2 Error detection and notification functions may be 5.10.3 In order to allow combinations c) and d) two archi-
supported by protocols used between (Ni-entities supporting tectural elements are required :
(NI-connectionless-mode transmission between pairs of
a) a function to provide an (Ni-connection-mode service
(NI-service-access-points to provide a higher probability of
using an (N - 1 )-connectionless-mode service; and
both protocol-data-unit error detection and data corruption
detection than is provided by the (N - 1)-service.
b) a function to provide an (Nbconnectionless-mode ser-
vice using an (N - 1 i-connection-mode service.
5.7.4.5.3 The error detection or notification function is only
NOTE - Of these functions, function a) requires significant protocol-
as reliable as the (NI-connectionless-mode service itself. While
control-information. For example, there is a need to identify the con-
the (NI-service provider may attempt to provide a notification
nection which is constructed, control its state and provide sequencing
upon detection of data corruption or protocol-data-unit loss,
b) requires little or no additional
of service-data-units. Function
misdelivery, etc., it cannot be relied upon to be capable of
protocol-control-information, rather, it places constraints on the way in
doing SO for every instance of error detection.
which the connection-mode service is used.
5.8 Routing 5.10.4 An (NI-relay can either
a) join two connection-mode services to yield a connec-
5.8.1 This sub-clause complements 5.8 of IS0 7498.
tion-mode service; or
bi join two connectionless-mode services to yield a con-
5.8.2 An (NI-routing function enables an (N - 1 )-connection-
0
nectionless-mode service.
less-mode transmission to be relayed by a chain of (Ni-entities.
The fact that (NI-routing is being used is known by neither the
+ 1 )-layer nor the iN - 1 )-layer.
(N
5.10.5 Provision of a service of a given type using services of
different types requires that one of the services (either the
connection-mode or the connectionless-mode one, depending
5.9 Management
upon economic and technical factors) be first converted to the
other using the functions which have been identified, an
5.9.1 This sub-clause complements 5.9 of IS0 7498. (Nbrelay can then operate.
5.9.2 Systems and layer management provide for initialization 5.10.6 The basic cases of relaying are illustrated in figure 2.
action to establish support for connectionless-mode services
The use of conversions between services within a layer is not
between systems. explicitly constrained by the Reference Model but, where
several services are connected in tandem, the use of conver-
sions would be ordered to minimize the number of conversions
5.9.3 Management facilities may be provided to allow
necessary to arrive at a given composite service.
characteristics of the nature, quality and type of connection-
less-mode service provided by a layer to be conveyed to the
next higher layer prior to the invocation of that service. These 5.10.7 Where an (N - 1 )-connectionless-mode service is
facilities may provide this information either prior to any invoca-
enhanced to provide an (NI-connection-mode service, a
tion of the service or at any time during a period when it is number of (NI-connections may be supported by (N - 1)-con-
nectionless-mode transmission between the same (
...
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