ISO/IEC 8348:1993

I NTE R NAT I O NA L
ISO/IEC
STANDARD 8348
Second edition
1993-06-1 5
Information technology - Open Systems
Interconnection - Network Service
Defi n it i o n
Technologies de l'information - Interconnexion des systèmes
ouverts - Définition du service de réseau
Reference number
1-
ISOnEC 8348:1993(E)
Content Page
SECTION 1 . GENERAL .
1 Scope . . 1
2 Normative references . 1
2.1 Paired Recommendations I International Standards equivalent in
technical content . 2
2.2 Additional references . . 2
3 Definitions . 2 .
3.1 Basic reference model definitions . 2
3.2 Service conventions definilions . 3
3.3 Network Service definitions . = . 3
3.4 Network addressing definitions . 3
3.5 Network layer architecture definitions . 4
4 Abbreviations . 4
5 Conventions . 5
5.1 General conventions . 5
5.2 Parameters . 5
5.3 NC endpoint identification convention . 6
6 Overview and general characteristics . . 6
7 Types and classes of Network Service . . 6
SECTION 2 -DEFINITION OF THE CONNECTION-MODE SERVICE . 7
8 Features of the connection-mode Network Service . 7
9 Model of the connection-mode Network Service . 7
9.1 Model of the connection-mode Network Layer Service .
9.2 Model of a Network Connection .
9.2.1 Queue model concepts . 8
9.2.2 NC estahlishment . 9
9.2.3 Data transfer operations . 9
. 2
9.2.4 Reset operations .
9.2.5 NC release . .
O ISO/IEC 1993
All rights reserved . No part of this pubiiûition may be reproduced or utilized in my form or by
any means. electronic or mechanical. including photocopying and microfilm. without permission
in writing from the publisher .
ISOllEC Copyright Office O Case postale 56 0 CH-121 1 Genèvc 20 O Switzerland
Printed in Switzerland
ii
ISODEC 8348:1993(E)
10 Quality of the connection-mode Network Service . 1 . 11
10.1 Determination of QOS . . . 11
10.2 Definition of QOS-parameters . 12
10.2.1 NC establishment delay . 12
10.2.2 NC establishment failure probability . 13
10.2.3 ThrougEput . i . 13
10.2.4 Transit delay . 14
10.2.5 Residual error rate . 14
10.2.6 Transfer failure probability . 14
10.2.7 NC resilience . 15
10.2.8 NC release delay . . 15
10.2.9 NC release failure probability . 15
10.210 NC protection . 15
10.2.11 NCpriority . 16
10.2.12 Maximum acceptable cost . 16
11 Sequence of primitives . . 16
Relation of primitives at the two NC end points . 16
11.1
11.2 Sequence of primitives at one NC endpoint . 16
12 Connection establishment phase . . 19
12.1 Function . . 19
12.2 Types of primitives and parameters . 19
12.21 Addresses . 19
12.2.2 Called address parameter . 20
12.2.3 Calling address parameter . 20
12.2.4 Responding address parameter . 20
12.2.5 Receipt confirmation selection parameter . 21
12.2.6 Expedited data selection parameter . 21
12.2.7 QOS-parameter set . 21
12.27.1 Throughput . 22
12.2.7.2 Transit delay . 22
12.2.7.3 NC Protection . 24
12.2.7.4 NC Priority . 26
12.2.8 NS-user-data parameter . : . 27
12.3 Sequence of primitives . 27
13 Connection release phase . 29
13.1 Function . 29
13.2 Types of primitive and parameters . 30
13.2.1 Originator parameter . 30
13.2.2 Reason parameter . 30
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ISO/IEC 834&1993(E)
13.2.3 NS-user-data parameter . 31
13.2.4 Responding address parameter . 31
13.3 Sequence of primitives when releasing an established NC . 31
13.4 Sequence of primitives in an NS user rejection of an NC establishment
attempt . 32
13.5 Sequence of primitives in an NS provider rejection of an NC
establishment attempt . 33
14 Data transfer phase . 33
141 Data transfer . 33
141.1 Function . 33
141.2 Types of primitives and parameters . 34
14.1.2.1 NS-user-data parameter . 34
14.1.2.2 Confirmation request parameter .
14.1.3 Sequence of primitives .
14.2 Receipt confirmatio
14.2.1 Function .
14.2.2 Types of primitives and parameters .
14.2.3 Sequence of
14.3 Expedited data transfer service . . 36
14.3.1 Function .
14.3.2 Types of primi
14.3.2.1 NS-user-data parameter . . 36
14.3.3 Sequence of primitives .
14.4 Reset service .
14.4.1 Function .
14.4.2 Types of primitives and parameters . 37
14.4.2.1 Originator parameter . 37
14.4.2.2 Reason parameter .
14.4.3 Sequence of primitives .
SECTION 3 - DEFINITION OF THE CONNECTIONLESS-MODE SERVICE 41
15 Features of the connectionless-mode Network Service 41
16 Model of the connectionless-mode Network Service . 41
16.1 Model of the connectionless-mode Network Layer Service . 41
16.2 Model of a network connectionless-mode transmission . 41
17 Quality of the connectionless-mode Network Service .
17.1 Determination of QOS .
17.2 Definition of network connectionless-mode QOS-parameters . 43
17.2.1 Transit delay .
17.2.2 Protection from unauthorized access . 43
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ISOnEC 8348A993(E)
17.2.3 Cost determinants . 43
17.2.4 Residual Error Probability . 44
17.2.5 Priority . 44
17.3 Route selection considerations . 44
.............................................................................................................. 44
19 Data transfer. . 44
19.1 Function . 44
....................................................................... 45
19.2 Types of primitives and parameters
19.2.1 Addresses. . 46
19.2.2 Quality of Service .
19.2.3 NS-Userdata . . 46
19.3 Sequence of primitives . 46
Annexes
A Network Layer Addressing . 47
A.l General .
A.2 Scope . .
......................... 47
A.3 Concepts and terminology .
A.3.1 Network addresses. .
A.3.1.2 NSAP address .
A.3.1.3 Network protocol address information . 48
A.3.2 Domains . . 48
A.3.2.1 Global network addressing domain . 48
..................................... 49
A.3.2.2 Network addressing domain .
A.3.3 Authorities .
A.3.4 Network address all
A.4 Principles for creating the O
A.4.1 Hierarchical structure . . 50
A.4.2 Global identification of any NSAP .
A.4.3 Route independence.
AS Network address definition . .
-4.5.1 Network address
A.5.1.1 The IDP . . 52
A.5.1.1.1 TheAFI .
A.5.1.1.2 TheIDI .
A.5.1.2 TheDSP . 52
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ISOAEC 834&1993(E)
A.5.2 Network address abstract syntax . 52
A.5.2.1 Abstract syntax and allocation of the IDP . 53
A.5.2.1.1 Abstract syntax and allocation of the AFI . 53
A.5.2.1.2 Format and allocation of the ID1 . 53
A.5.2.2 Abstract syntax and allocation of the DSP . 55
A.5.2.3 Abstract syntax of the DSP . SS
A.5.3 Network address encodings . 56
A.5.4 Maximum Network address length . 57
A.6 Character based DSP allocation . 57
A.7 Reference publication formats . 58
A.8 Network entity titles . 58
B Rationales for the material in Annex A . 59
B.l ID1 formats (A.S.2.1.2) .
B.2 Reservation of AFI values 00-09 (Table A.2) . . 59
B.3 Derivation of the preferred encodings (A.S.3) .
C Facilities for conveying service characteristic ctionless-mode
: Network Services . . 61
C.l Introduction . . 61
C.2 Function .
C.3 Types of primitives and parameters .
C.4 Service characteristics . 62
C.4.1 Congestion Control . . 62
C.4.2 Sequence preservation probability . 62
C.4.3 Maximum NSDU lifetime . 63
C.5 Types of primitives and parameters . 63
C.5.1 Destination address . 63
C.5.2 Service characteristics/QOS parameter . 63
C.5.3 Reason for report . 63
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ISOAEC 8348:1993(E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for
worldwide standardization. National bodies that are members of IS0 or
I EC participate in the development of International Standards through
technical committees established by the respective organization to deal
with particular fields of technical activity. IS0 and IEC technical com-
mittees collaborate in fields of mutual interest. Other international organ-
izations, governmental and non-governmental, in liaison with IS0 and I EC,
also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, ISO/IEC JTC 1. Draft International Standards adopted
by the joint technical committee are circulated to national bodies for vot-
at least
ing. Publication as an International Standard requires approval by
75 % of the national bodies casting a vote.
International Standard ISO/IEC 8348 was prepared by Joint Technical
Committee ISO/IEC JTC 1, lnformation technology, Sub-Committee SC 6,
Telecommunications and information exchange between systems, in col-
laboration with CCITT. The identical text is published as CClTT Rec-
ommendation X.213.
This second edition cancels and replaces the first edition
(IS0 8348:1987), which has been technically revised.
Annex A forms an integral part of this International Standard. Annexes B
and C are for information only.
Vii
This Recommendation I International Standard is one of a set of CCITï Rec-
ommendations and International Standards produced to facilitate the inter-
connection of computer systems. It is related to other CCITï Recommendations
and International Standards in the set as defined by the Reference Model of Open
Systems Interconnection (OSI). The OS1 Reference Model (CCITï Ra. X.200 I
IS0 7498) subdivides the area of standardization for interconnection into a series of
layers of specification, each of a manageable size.
This CCITï Recommendation I International Standard defines the Service provided
by the Network Layer to the Transport Layer at the Boundary between the
Network and Transport Layers of the Reference Model. It provides for the de-
signers of Transport protocols a definition of the Network Service existing to sup-
port the Transport protocol and for the designers of Network protocols a definition
of the services to be made available through the action of the Network protocol
over the underlying service. This relationship is illustrated in the figure.
Transport i T~;;rt 1 uses Service
Proîocol
Network Service
Network
Network
Protocol
Figure - Relationship of the Network Service to OS1 Network
and Transport protocols
The use of the word "Network" to name the "Network" Layer of the OS1 Refer-
ence Model should be distinguished from the use of the word "network" to denote a
communications network as conventionally understood. To facilitate this distinction,
the term "subnetwork" is used for a collection of physical equipment, commonly
called a "network" (see CCIïT Rec. X.200 I IS0 7498). Subnetworks may be either
public networks or privately supplied networks. In the case of public networks, their
properties may be determined by separate CCïïï Recommendations such as
Recommendation X.21 for a circuit-switched network or Recommendation X.25 for
a packet-switched network.
Throughout the set of OS1 CCIïT Recommendations and International Standards
the term "Service" refers to the abstract capability provided by one layer of the OS1
Reference Model to the layer above it. Thus, the Network Service defined in this
Recommendation I International Standard is a conceptual architectural Service,
Independent of administrative divisions.
NOTE 1 - It is important to distinguish the specialized use of the term "Service" within the set
of OS1 CClTT Recommendations and International Standards from its use elsewhere to de-
scribe the provision of a service by an organization (such as the provision of a service, as
defined in other CCITT Recommendations, by an Administration).
Any particular subnetwork may or may not support the OS1 Network Service. The
OS1 Network Service may be provided by a combination of one or more sub-
networks and optional additional functions between or outside these subnetworks.
viii
I
ISO/IEC 8348 : 1993 (E)
INTERNATIONAL STANDARD
CCITT RECOMMENDATION
INFORMATION TECHNOLOGY - OPEN SYSTEMS INTERCONNECTION -
NETWORK SERVICE DEFINITION
SECTION 1 - GENERAL
1 Scope
This Recommendation I International Standard defines the OS1 Network Service in terms of
the primitive actions and events of the Service;
a)
the parameters associated with each primitive action and event, and the form which they take;
b)
the interrelationship between, and the valid sequences of, these actions and events.
c)
The principal objectives of this Recommendation I International Standard are
to specify the characteristics of a conceptual Network Service and thus, supplement the Reference
1)
Model in guiding the development of Network Layer protocols;
to encourage convergence of the capabilities offered by providers of subnetworks;
2)
to provide a basis for the individual enhancement of existing heterogeneous subnetworks to a common
3)
subnetwork-independent Network Service to enable them to be concatenated for the purpose of
providing global communication. (Such concatenation may involve optional additional functions which
are not defined in this Recommendation I International Standard.) A definition of the quality of service
is an important element of this Recommendation I International Standara
to provide a basis for the development and implementation of subnetwork-independent Transport Layer
4)
protocols decoupled from the variability of underlying public and private subnetworks and their specific
*
interface requirements.
This Recommendation I International Standard does not specify individual implementations or products nor does it
constrain the implementation of entities and interfaces within a system.
There is no conformance of equipment to this Recommendation I Intemational Standard. Instead, conformance is
achieved through implementation of conforming OS1 Network protocols which fulfil1 the Network Service defined in
this Recommendation I International Standard.
2 Normative references
The following CCITT Recommendations and International Standards contain provisions which, through reference in
this text, constitute provisions of this Recommendation I International Standard. At the time of publication, the editions
indicated were valid. All Recommendations and International Standards are subject to revision, and parties to
agreements based on this Recommendation I International Standard are encouraged to investigate the possibility of
applying the most recent edition of the CCITT Recommendations and International Standards listed below. Members
of the IEC and IS0 maintain registers of currently valid Internationai Standards. The CCITT Secretariat maintains a
list of the currently valid CCITT Recommendations.
CCITI' Rec. X.213 (1992 E)
ISO/DEC 8348 : 1993 (E)
2.1 Paired Recommendations I International Standards equivalent in technical conknt
-
CCI" Recommendation X.200 (1988), Reference model of Open Systems Interconnection for CCIïT
Applications.
IS0 7498: 1984, Information processing systems - Open Systems Interconnection - Basic Reference
Model.
- CCITT Recommendation X.210 (1988), Open Systems Interconnection layer service definition
conventions.
ISOfïR 8509: 1987, Information processing systems - Open Systems Interconnection - Service
conventions.
- CCITT Recommendation X.224 (1988), Transport protocol specification for Open Systems
Interconnection for CCIïT applications.
ISOlIEC 8073: 1992, Information technology - Telecommunications and information exchange between
systems - Open Systems Interconnection - Protocol for providing the connection-mode transport
services.
2.2 Additional references
-
CCITT Recommendation E. 163 (1998), Numbering plan for the international telephone service.
-
CCITT Recommendation E.164 (1991), The numberingplan for the ISDN era.
-
CCITT Recommendation F.69 (1988), Plan for telex destination codes.
-
CCITT Recommendation X.121 (1992), International numbering plan for public data networks.
-
CCITT Recommendation X.300, (1988) General principles for interworking between public networks
and between public networks and other networks for the provision of data transmission services.
-
ISOLEC 646 1991, Information technology - IS0 7-bit coded character set for information
interchange.
-
IS0 2375: 1985, Data processing - Procedure for registration of escape sequences.
-
IS0 3166: 1988, Codes for the representation of names of countries.
-
IS0 6523: 1984, Data interchange - Structures for the identification of organizations.
-
IS0 7498: 1984lAdd. 1: 1987, Information processing systems - Open Systems Interconnection - Basic
Reference Model - Addendum I: Connectionless-mode transmission.
-
IS0 8648: 1988, Information processing systems - Open Systems Interconnection - Intemal organization
of the Network Layer.
3 Definitions
For the purposes of this Recommendation I International Standard, the following definitions apply.
3.1 Basic reference model definitions
This Recommendation I International Standard is based on the concepts developed in the Basic Reference Model for
Open Systems Interconnection and makes use of the following terms defined in CCITT Rec. X.200 I IS0 7498:
a) expedited Network-Service-data-unit;
b) Network-address,
c) Network Connection;
d) Network-entity,
e) Network-protocol control information,
f) Network-protocol &ta unit,
2 CCI" Rec. X.213 (1992 E)
ISOAEC 8348 : 1993 (E)
g) Network Layer.
h) Network-relay.
i) Network-routing,
j) Network Service;
k) Network-Service-access-point;
1) Network-Service-access-point-address;
m) Network-Service-data-unit;
n) OS1 environment,
O) subnetwork;
p) title.
3.2 Service conventions definitions
This Recommendation I International Standard also makes use of the following terms defined in CCITT Rec. X.210 I
ISO/TR 8509, as they apply to the Network Layer:
a) confirm;
b) indication;
c) Network Service user;
d) Network Service provider;
e) primitive;
f) request;
g) response.
3.3 Network Service definitions
For the purpose of this Recommendation I International Standard, the following definitions also apply:
3.3.1 calling NS user: An NS user that initiates an NC establishment request.
3.3.2 called NS user: An NS user with whom a calling NS user wishes to establish an NC.
NOTE - Calling NS users and caüed NS users are defined with respect to a single NC. An NS user can be both a calling
and a called NS user simultaneously.
e
3.3.3 generic address: An address which identifies a set of NSAPs rather than a single specific NSAP.
3.3.4 Network Connection: An association established by a Network Layer between two NS users for the transfer
of data, which provides explicit identification of a set of Network data transmissions and agreement concerning the
services to be provided by the set.
NOTE - This definition clarifies that given in CCI" Rec. X.200 I IS0 7498.
I
3.3.5 Network connection-mode Data Transmission: The transfer of an NSDU from a source NSAP to a desti-
nation NSAP within the context of an NC that bas previously been established
3.3.6 Network connectioniess-mode Data Transmission: The transmission of an NSDU from a source NSAP to a
destination NSAP outside the context of an NC and without any requirement to maintain any logical relationship
among multiple invocations.
3.4 Network addressing definitions
Annex A, describing network addressing makes use of the following terms as defined below:
3.4.1
DTE address: Information used to identify a point of attachment to a public data network.
I CCFlT Rec. X.213 (1992 E) 3
ISO/IEC 8348 : 1993 (E)
3.4.2 subnetwork point of attachment: A point at which a real end system, interworking unit, or real subnetwork
is attached to a real subnetwork, and a conceptuai point at which a subnetwork service is offered within an end or
intermediate system.
3.4.3 subnetwork point of attachment address: Information used in the context of a particular real subnetwork to
identify a subnetwork point of attachment; or information used in the context of a particular subnetwork to identify the
conceptual point within an end or intermediate system at which the subnetwork service is offered. This term is used
interchangeably with the (equivalent) shortened form subnework address.
3.4.4 network protocol address information: Information encoded in a Network protocol data unit to carry the
semantics of a Network service access point address. (This is known as an “address signal” or as the “coding of an
address signal” in the public network environment.)
3.4.5 naming domain: A context within which a name allocated by a naming authority is unambiguous. Where the
name is an address, the context within which the name is allocated is called an addressing domain.
3.4.6 global network addressing domain: An addressing domain consisting of all of the Network service access
point addresses in the OS1 environment.
3.4.7 network addressing domain: A subset of the global nemork addressing domain consisting of all of the
Network service access point addresses allocated by one or more addressing authorities.
3.4.8 naming authority: That which allocated names from a specified naming domain, and which ensures that
so allocated are unambiguous. Where the naming authority allocates addresses, it is called an addressing
names
authority.
3.4.9 network addressing authority: An addressing authority that assigns and administers Network service access
point addresses within one or more network addressing domains.
3.4.10 abstract syntax: A notation which enables data types to be defined, and values of those types specified,
without determining the way in which they will be represented (encoded) for transfer by protocols.
3.5 Network layer architecture definitions
This Recommendation I Intemational Standard makes use of the following terms defined in CCITT Rec. X.300 and
IS0 8648.
a) subnetwork;
b) real subnetwork;
c) subnetwork service;
d) real end system;
e) interworking unit;
f) intermediate system;
g) relay entity.
4 Abbreviations
AFI Authority and format identifier
cc Country code
COR Confmtion of receipt
DCC Data country code
DSP Domain specific part
ENSDU Expedited Network-Service-data-unit
ICD International code designator
ID1 Initiai domain identifier
4 CCiTï Rec. X.213 (1992 E)
IsO/IEC 8348 : 1993 (E)
IDP Initial domain part
ISDN Integrated services digital network
N Network
Network Connection
NC
NL Network Layer
NPAI Network protocol addressing information
NPDU Network protocol data unit
Network Service
NS
NSAP Network-Service-access-point
NSDU Network-Service-data-unit
os1 Open Systems Interconnection
PSTN Public switched telephone network
m Postal, telephone and telegraph
Quality of Service
QOS
RPF Reference publication format
SNPA Subnetwork point of attachment
5 Conventions
5.1 General conventions
This Service Definition uses the descriptive conventions given by CCITT Rec. X.210 I ISO/TR 8509.
The layer service model, service primitives, and time-sequence diagrams taken from those conventions are entirely
abstract descriptions; they do not represent a specification for implementation.
5.2 Parameters
Service primitives, used to represent service-usedservice-provider interactions (see CCIïT Rec. X.210 I ISOA’R 8509),
convey parameters which indicate information available in the usedprovider interaction.
The parameters which apply to each group of Network Service primitives are set out in tables in clauses 12 to 14
and 19. Each “X’ in the tables indicates that the primitive labeling the column in which it falls may carry the
parameter labeling the row in which it falls.
Some entries are further qualified by items in brackets. These may be
an indication that the parameter is conditional in some way:
a)
indicates that the parameter is not present on the primitive for every NC; the parameter
(C)
definition describes the conditions under which the parameter is present or absent;
b) a parameter specific constraint:
indicates that the value supplied in an indication or confm primitive is always identical to
(=)
that supplied in the corresponding request or response primitive occurring at the peer NSAP;
an indication that some note applies to the entry:
c)
(Note x) indicates that the referenced note contains additional information pertaining to the parameter
and its use.
In any particular interface, not all parameters need be explicitly stated. Some may be implicitly associated with the
NSAP at which the primitive is issued.
CCïïï Rec. X.213 (1992 E) 5
ISO/IEC 8348 : 1993 (E)
5.3 NC endpoint identification convention
If an NS user needs to distinguish among several NCs at the same NSAP, then a local NC endpoint identification
mechanism must be provided. All primitives issued at such an NSAP would be required to use this mecbanism to
identify NCs. Such an implicit identification is not described as a parameter of the service primitives in this Service
Definition.
NOTE - The implicit NC endpoint identification must not be confused with the address parameters of the N-CONNECT
primitives (see 12.2).
6 Overview and general characteristics
The Network Service provides for the transparent transfer of data (i.e., NS-user-data) between NS users. It makes
invisible to these NS users the way in which supporthg communications resources are utilized to achieve this transfer.
In particular, the Network Service provides for the following:
Independence of underlying transmission media - The Network Service relieves NS users from all
concerns regarding how various subnetworks are used to provide the Network Service. The Network
Service hides from the NS user differences in the transfer of data over heterogeneous subnetworks, other
than quality of service.
End-to-end transfer - The Network Service provides for transfer of NS-user-data between NS users in
end systems. All routing and relaying functions are performed by the NS provider including the case
where several similar or dissimilar transmission resources are used in tandem or in parallel.
Transparency of transferred information - The Network Service provides for the transparent transfer of
octet-aligned NS-user-data andor control information. It does not restrict the content, format or coding
of the information, nor does it ever need to interpret its structure or meaning.
Quality of service selection- The Network Service makes available to NS users a means to request and
to agree to the quality of service for the transfer of NS-user-data. Quality of service is specifie
means of QOS-parameters representing characteristics such as throughput, transit delay, accuracy
reliability .
NS-user-addressing - The Network Service utilizes a system of addressing (NSAP addressing)
aliows NS users to refer unambiguously to one another.
Types and ciasses of Network Service
There are two types of Network Service:
a) a connection-mode service (defined in clause 2); and
b) a connectionless-mode service (defined in clause 3).
For a given instance of communication, the mode of service provided to both NS users is the same (i.e. connection-
mode or connectionless-mode). Choice of provision of the connectionless-mode Network Service or the connection-
mode Network Service is made in accordance with IS0 7498/Add. 1.
When referring to this Service Definition, an NS user or NS provider shall state which types(s) of service it expects to
use or provide.
There are no distinct classes of Network Service defined.
However, for the connection-mode service, two Network Layer Services, Receipt Confirmation and Expedited Data
Transfer, are NS provider-options.
A service which is an NS provider-option is one which an NS provider can choose either to provide or not to provide
for a pariicular NC. In circumstances where the NS provider chooses not to provide a provider-option service, it will
not be available in the Network Service. If the provider-option Receipt Confirmation or Expedited Data Transfer is
provided, it shall be as defined in 14.1 to 14.3.
CCï" Rec. X.213 (1992 E)
ISO/IEC 8348 : 1993 fE)
SECTION 2 - DEFINFITON OF THE CONNECTION-MODE SERVICE
8 Features of the connection-mode Network Service
The connection-mode Network Service offers the following features to an NS user:
the means to establish an NC with another NS user for the purpose of transferring NS-user-data in the
form of NSDUs. More than one NC may exist between the same pair of NS users;
the establishment of an agreement between the two NS users and the NS provider for a certain QOS
associated with each NC;
the means of transferring NSDUs in sequence on an NC. The transfer of NSDUs, which consist of an
integer number of octets, is transparent, in that the boundaries of NSDUs and the contents of NSDUs are
preserved unchanged by the Network Service, and there are no constraints on the NSDU content
imposed by the Network Service;
the means by which the receiving NS user may flow control the rate at which the sending NS user may
send NSDUs;
in some circumstances, the means of transferring separate expedited NSDUs in sequence (see clause 7).
Expedited NSDUs are limited in length and their transmission is subject to a different flow control from
normal data across the NSAP;
the means by which the NC can be returned to a defined state and the activities of the two NS users
synchronized by use of a reset service;
in some circumstances, the means for the NS user to confirm the receipt of an NSDU (see clause 7);
the unconditional, and therefore possibly destructive, release of an NC by either of the NS users or by
the NS provider.
9 Model of the connection-mode Network Service
9.1 Model of the connection-mode Network Layer Service
This Service Definition uses the abstract model for a layer service defined in clause 4 of CCITT Rec. X.210 I
ISOîTR 8509. The model defines the interactions between the NS users and the NS provider which take place at the
two NSAPs. Information is passed between the NS user and the NS provider by service primitives, which may convey
parameters.
9.2
Model of a Network Connection
Between the two endpoints of an NC, there exists a flow control function which relates the behaviour of the NS user at
one end receiving NS-user-data to the ability of the NS user at the other end to send NS-user-data. As a means of
specifying this flow control feature and its relationship with other capabilities provided by the Network Service, the
queue model of an NC, described in the following subclauses, is used.
This queue model of an NC is discussed only to aid in the understanding of the end-to-end service features perceived
by users of the Network Service. It is not intended to serve as a substitute for a precise, formal description of the
Network Service, nor as a complete specification of all allowable sequences of NS primitives, (Allowable primitive
sequences are specified in clause 11 - also, see Note below.) In addition, this model does not attempt to describe all the
functions or operations of Network Layer entities (including relay entities) which are used to provide the Network
Service. No attempt to specify or constrain Network Service implementations is implied.
In interpreting this service definition, statements in clauses 12 to 14 concerning the properties of individual primitives
have precedence over the general statements in this clause.
NOTE - In addition to the interaction between service primitives described by this model, there may be constraints
applied locally on the ability to invoke primitives, as well as service procedures defining particular sequencing constraints on some
primitives.
CCI" Rec. X.213 (1992 E) 7
ISO/IEC 8348 : 1993 (E)
9.2.1 Queue model concepts
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The queue model represents the operation of an NC in the abstract by a pair of queues linking the two NSAPs. There is
one queue for each direction of information flow (see Figure 1).
NS user B
NS user A
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Queue from B to A
NS provider
TIsoO3ûû-92
Figure 1 - Queue model of a Network Connection
Each queue represents a flow control function in one direction of transfer. The ability of an NS user to add objects to a
queue will be determined by the behaviour of the NS user removing objects from that queue and the state of the queue.
Objects are entered or removed from the queue, either as the result of interactions at the two NSAPs, or as the result of
NS provider initiatives.
The pair of queues is considered to be available for each potential NC.
The objects which may be placed in a queue as a result of interactions at an NSAP (see clauses 12 to 14) are
a) connect objects (associated with N-CONNECT primitives and all of their parameters);
b) octets of normai NS-user-data (associated with an N-DATA primitive);
c) indications of end-of-NSDU (associated with completion of an N-DATA primitive);
d) expedited NSDUs (associated with N-EXPEDITED-DATA pri
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