Network Working Group ISO
Request for Comments: 941 April 1985
Addendum to the Network Service Definition Covering
Network Layer Addressing
ISO/DP8348/DAD2
(also TC 97/SC 6/N 3444)
Status of this RFC:
This document is distributed as an RFC for information only. It does
not specify a standard for the ARPA-Internet. Distribution of this
document is unlimited.
Note:
This document has been prepared by retyping the text of ISO/DP8348/DAD2
of October 1984 (also numbered SC 6/N 3444), which is currently
undergoing voting within ISO as a Draft Proposed Addendum to the
Network Service Definition. Although this RFC has been reviewed after
typing, and is believed to be substantially correct, it is possible
that typographic errors not present in the ISO document have been
overlooked.
Alex McKenzie
BBN Laboratories
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ISO Statement on the Status of this Document.
At its meeting in Zurich, April 2-11, 1984, SC 6/WG 2 produced document
SC 6 N 3134 and, in accordance with Resolution 49 of the SC 6 meeting in
Tianjin (September 19-30, 1983), forwarded it to the SC 6 Secretariat
for registration and ballot as a first Draft Proposed Addendum to the
Network Service Definition (ISO DP 8348/DAD2).
The letter ballot on SC 6/N 3134 closed on August 20, 1984. The results
of the ballot were 10-4-0-3 [approve-disapprove-abstain-no vote]; the
summary of voting is contained in document SC 6/N 3229 (late votes are
contained in documents SC 6/N 3333 and 3360). These ballot results were
reviewed at the SC 6/WG 2 meeting in Washington, October 15-25, 1984,
and document SC 6/N 3444 was produced as a progression of SC 6/N 3134,
taking into account as many of the ballot comments as possible. The
Editor's report, contained in document SC 6/N 3445, describes the
disposition of member body comments on the DP 8348/DAD2 letter ballot.
A resolution of the SC 6 meeting in Washington, October 22-26, 1984,
instructs the SC 6 Secretariat to register document SC 6/N 3444 as a
second Draft Proposed Addendum to ISO 8348, and to circulate it for a
two-month letter ballot.
Introduction
This Addendum to the Network Service Definition Standard, ISO 8348,
defines the abstract syntax and semantics of the Network Address
(Network Service Access Point Address). The Network Address defined in
this Addendum is the address that appears in the primitives of the
connection-mode Network Service as the calling address, called address,
and responding address parameters, and in the primitives of the
connectionless-mode Network Service as the source address and
destination address parameters.
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SCOPE AND FIELD OF APPLICATION
The scope of this Addendum is the definition of the abstract syntax and
semantics of the Network Address. This Addendum does not specify the
way in which the semantics of the NSAP address are encoded in Network
Layer protocols. The field of application of this Addendum is the same
as the field of application described in Clause 1 of the Network Service
Definition (ISO 8348).
2 REFERENCES
ISO 7498 Information Processing Systems - Open Systems
Interconnection - Basic Reference Model [Note: See also
CCITT Recommendation X.200]
DP 7498/DAD1 Information Processing Systems - Open Systems
Interconnection - Addendum to the Basic Reference Model
Covering Connectionless Data Transmission
DP 8509 Information Processing Systems - Open Systems
Interconnection - Service Conventions
ISO 8348 Information Processing Systems - Data Communications -
Network Service Definition [Note: See also CCITT
Recommendation X.213]
DIS 8348/DAD1 Information Processing Systems - Data Communications -
Addendum to the Network Service Definition Covering
Connectionless Data Transmission
DP 8648 Information Processing Systems - Data Communications -
Internal Organization of the Network Layer
ISO 6523 Data Interchange - Structure for the Identification of
Organizations
ISO 646 7-bit Coded Character Set for Information Processing
Interchange
ISO 2375 Procedure for the Registration of Escape Sequences
CCITT X.121 International Numbering Plan for Public Data Networks
CCITT E.163 Numbering Plan for the International Telephone Service
CCITT E.164 The Numbering Plan for the ISDN Era
CCITT F.69 Plan for Telex Destination Codes
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Temporary Note
The list of References in the published Addendum will contain
only approved ISO Standards and CCITT Recommendations; items may need
to be subtracted from, or added to, the current list.
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SECTION ONE - GENERAL
---------------------
3 DEFINITIONS
3.1 Reference Model Definitions
This Addendum makes use of the following terms defined in ISO 7498:
a) Network layer
b) Network service
c) Network service access point
d) Network service access point address
e) Network entity
f) Routing
g) Network address
h) Network protocol control information
i) Network protocol data unit
3.2 Service Conventions Definitions
This Addendum makes use of the following terms defined in ISO 8509:
j) Service user
k) Service provider
3.3 Network Layer Architecture Definitions
This Addendum makes use of the following terms defined in ISO 8648
(Internal Organization of the Network Layer):
l) Subnetwork
m) Real subnetwork
n) Subnetwork service
o) Real end system
p) Interworking unit
q) Intermediate system
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3.4 Network Addressing Definitions
This Addendum makes use of the following terms as defined below:
r) DTE address: information used to identify a point of attachment to
a public data network.
s) 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 conceptual point at which a subnetwork service is
offered within an end or intermediate system.
t) Subnetwork address (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 point at
which the subnetwork service is offered within an end or
intermediate system.
u) Network protocol address information: information encoded in a
network protocol data unit to carry the semantics of an NSAP
address. (This is known as an "address signal" or as the "coding of
an address signal" in the Public Data Network environment.)
v) Domain (of the OSI environment): a subset of the OSI environment
within which identifiers for OSI environment entities of the same
type are unambiguous.
w) Global network addressing domain: the set of all Network Service
Access Point addresses in the OSI environment.
x) Network addressing subdomain; a subset of the global network
addressing domain.
y) Authority (for a domain or subdomain): that which ensures that
identifiers within the corresponding domain or subdomain are
unambiguous.
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4 ABBREVIATIONS
This Addendum makes use of the following abbreviations:
a) NSAP - Network Service Access Point
b) NPAI - Network Protocol Addressing Information
c) DCC - Data Country Code
d) CC - Country Code
e) ICD - International Code Designator
f) PSTN - Public Switched Telephone Network
g) ISDN - Integrated Services Digital Network
h) IDP - Initial Domain Part
i) AFI - Authority and Format Identifier
j) IDI - Initial Domain Identifier
k) DSP - Domain Specific Part
l) NPDU - Network Protocol Data Unit
m) SNPA - Subnetwork Point of Attachment
5 CONVENTIONS
No particular standard conventions are invoked by this Addendum.
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SECTION TWO - NETWORK LAYER ADDRESSING
--------------------------------------
6 CONCEPTS AND TERMINOLOGY FOR NETWORK LAYER ADDRESSING
6.1 Network Addresses
This Addendum defines the Network Service Access Point (NSAP)
address. Since the term "network address" is commonly used in different
contexts to refer to different things a more specific description of
this concept is introduced below.
6.1.1 Subnetwork Address
In one context, the term "network address" may be used to refer to the
point at which a real end system, real subnetwork, or interworking
unit is attached to a real subnetwork, or to the point at which the
subnetwork service is offered within an end or intermediate system.
In the case of attachment to a public data network, this point is
called a DTE/DCE interface, and the term "DTE address" is used in
reference to it.
The specific term "subnetwork address" (or "subnetwork point of
attachment address") is used in this case, as illustrated in Figure
6-1:
subnetwork point of
attachment identified
________ by SNPA
________________ | | /\
| | |______|/ \_______
| Real End | ____________ Layer | * <-/ |\-> * | Layer
| system, real | | | 3 |______| |______| 3
|subnetwork, or|____| Real | | | | |
| interworking | |Subnetwork| | | | |
| unit | ^ |__________| |______| |______|
|______________| |
|
subnetwork point of End Intermediate
attachment identified System System
by subnetwork address
Figure 6-1 - Subnetwork Address
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The subnetwork address is the information that a real subnetwork needs
to identify a particular real end system, another real subnetwork, or
interworking unit that is attached to that real subnetwork.
In the public network environment, the subnetwork address is what the
public network operates on.
Note: The point identified by a subnetwork address is a point of
interconnection between a real end system or interworking unit and a
real subnetwork (in particular, in a public data network environment,
a DTE/DCE interface), and is not an OSI Service Access Point.
6.1.2 NSAP address
In another context, the term "network address" is used to refer to the
Network Service Access Point (NSAP) at which the OSI Network Service
is made available to a Network Service user by the Network Service
provider.
The specific term "NSAP address" is used in this case, as illustrated
in Figure 6-2:
The NSAP address is the information that the OSI Network Service
provider needs to identify a particular Network Service Access Point.
The values of the called address, calling address, and responding
address parameters in the N-CONNECT primitive, of the responding
address parameter in the N_DISCONNECT primitive, and of the source
address and destination address parameters in the N-UNIDATA primitive,
are NSAP addresses.
Note that since the Network Service primitives are conceptual, no
particular encoding of the NSAP address is specified by the Network
Service Definition.
In both CCITT and ISO usage, the terms "Network Address" (with both
the N and the A printed in capital letters) and "global network
address" are synonymous with the term "NSAP address". Use of the term
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"NSAP address" is preferred when it is essential to avoid confusion,
particularly in spoken references where "capitalization" is not
possible.
6.1.3 Network Protocol Address Information
In a third context, the term "network address" is used to refer to an
address that is carried as network protocol control information in a
network protocol data unit (NPDU).
The specific term "network protocol address information" (NPAI) is
used in this case.
In the public network environment, NPAI is also known as an "address
signal" or as the "coding of an address signal".
There is a relationship between the NSAP address that appears in
Network Service primitives and the NPAI that appears in a Network
Layer protocol, in that the semantics of the NSAP address is preserved
by the NPAI. The syntax and encoding of NPAI are defined by Network
layer Protocol standards, which also specify the relationship between
the NSAP address and the NPAI encoding employed by the protocol.
6.2 Domains
A domain is a subset of the Open Systems Interconnection environment
within which identifiers for OSI environment entities of the same type
are unambiguous.
6.2.1 Global Network Addressing Domain
The global network addressing domain is defined as the set of all
Network Service Access Point addresses in the OSI environment.
6.2.2 Network Addressing Subdomain
A network addressing subdomain is a set of Network Service access
Point addresses. It is a subset of the global network addressing
domain.
The relationship of the concepts of 6.2.1 and 6.2.2 is illustrated by
Figure 6-3:
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6.3 Authorities
The uniqueness of identifiers within a domain or subdomain is ensured
by an authority associated with that domain. The term "authority" does
not necessarily refer to an organization or administration: it is
intended to refer to whatever it is (in an abstract sense) that ensures
the uniqueness of identifiers in the associated domain.
Domains are characterized by the authority that administers the domain
and by the rules that are established by that authority for specifying
identifiers and identifying subdomains. The authority responsible for
each subdomain determines how identifiers will be assigned and
interpreted within that subdomain, and how any further subdomains will
be created.
The operation of an authority is independent of that of other
authorities on the same level of the hierarchy, subject only to any
common rules imposed by the parent authority.
6.4 Network Address Allocation
An addressing authority shall either allocate complete NSAP addresses,
or shall authorize one or more other authorities to allocate address.
Each address allocated by an addressing authority shall include a
domain identifier which identifies the allocating authority. An address
shall not be allocated to identify a domain or NSAP if the address has
previously been allocated to some other domain or NSAP, unless the
authority can ensure that all use of the previous allocation has
ceased.
The authority shall ensure that allocations are made in such a way that
efficient use is made of the address space.
7 PRINCIPLES FOR CREATING THE OSI NETWORK ADDRESSING SCHEME
7.1 Hierarchical Structure of NSAP Addresses
NSAP addresses are based on the concept of hierarchical addressing
domains, as explained in Clause 6. Each domain may be further
partitioned into subdomains. Accordingly, NSAP addresses have a
hierarchical structure.
The conceptual structure of NSAP addresses follows the principle that,
at any level of the hierarchy, an initial part of the address
unambiguously identifies a subdomain, and the rest is allocated by the
management of the subdomain to unambiguously identify either a lower
level subdomain or an NSAP within the subdomain. The part of the
address that identifies the subdomain depends on the level at which the
address is viewed.
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Note: This conceptual structure should not be considered as implying
any detailed administration of NSAP addresses.
Graphical representation of the hierarchical structure of NSAP
addresses may be made according to an inverted tree diagram, as in
Figure 7-1 (a), or a domain diagram, as in Figure 7-1 (b)
O
|
|
-------------------------------
| | | |
| | | |
----- ----- ----- -----
| W | | X | | Y | | Z |
----- ----- ----- -----
| | |
| | |
--------------- @ --------
| | | | |
| | | | |
----- ----- ----- ----- -----
| a | | b | | c | | a | | b |
----- ----- ----- ----- -----
|
|
----------------------
| | | |
| | | |
----- ----- ----- -----
| p | | q | | r | | s |
----- ----- ----- -----
Figure 7-1 (a) - Hierarchical Structure of NSAP Addresses
Inverted Tree Diagram
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7.2 Global Identification of any NSAP
In the context of Open Systems Interconnection, it is possible to
identify any NSAP within the global network addressing domain (see
Clause 6.2.1). Consequently,
a) At any Network Service Access Point, it is possible to identify
any other Network Service Access Point, within any OSI end system;
b) A global Network Address can therefore be defined to unambiguously
identify any Network Service Access Point;
c) The OSI protocols established between correspondent Network
entities convey the complete information contained in a Network
Address (see Clause 6.1.4);
d) An NSAP address identifies the same NSAP regardless of which
NS-user enunciated the address; and
e) An NS-user, when given an NSAP address of the NS-provider in a
primitive Indication, may subsequently use that NSAP address in
another instance of communication with the corresponding NSAP.
Some restrictions may be placed on communications in the context of
OSI, on the basis of: technical feasibility of an interconnection,
security, charging, etc. Such considerations are not related to Network
Layer addressing, and therefore are not discussed in this Addendum.
Note: The global identification of NSAPs should not be taken to imply
the universal availability of directory functions required to enable
communication among all NSAPs to which NSAP addresses have been
allocated.
7.3 Route Independence
Network Service users cannot derive routing information from an NSAP
address. They cannot influence the Network Service provider's choice of
route by means of the source and destination NSAP addresses. Similarly,
they cannot deduce from the source and destination NSAP addresses the
route that was used by the Network Service provider. This is not
intended to exclude the possibility that an OSI end system may need to
influence the route selected for a particular instance of communication
with another OSI end system. (In particular, it may need to influence
the selection of intermediate systems to be used, and the paths to be
taken between them.) The means whereby such an influence may be exerted
is, however, not the NSAP address. Elements of Network Layer protocol
may be required to control routing within intermediate systems; such
elements of protocol are distinct from the network protocol address
information (NPAI).
Notwithstanding the restrictions imposed on the use that a Network
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Service user may make of an NSAP address, it is recognized that NSAP
addresses should be constructed in such a way that routing through
interconnected subnetworks is facilitated. That is, the Network Service
provider and relay-entities in particular, may take advantage of the
address structure to achieve economical processing of routing aspects.
7.4 Service Type Independence
It may be necessary for Network Service users to distinguish Network
Layer services of different types (such as point-to-point versus
multipoint services, and connection-mode versus connectionless-mode
services). The nature of such service types is not explicitly contained
in the semantics of the NSAP address. Similarly, Network Layer quality
of service characteristics (such as throughput, transit delay, etc.)
are not explicitly specified by the NSAP address.
8 NETWORK ADDRESS DEFINITION
The intent of this document is best served by maintaining clear
distinctions among three concepts: the abstract semantics of the NSAP
address; the abstract syntax employed in this document as a means of
defining the abstract semantics of the NSAP address, and employed by
addressing authorities as a means of allocating and assigning addresses;
and the concrete syntax in which the NSAP address semantics are encoded
as NPAI in Network Layer protocols. These distinctions are illustrated
in Figure 8-1:
Figure 8-1 - Relationship of NSAP Address Semantics and Syntax
This Addendum does not specify the way in which the semantics of the
NSAP address are encoded in Network Layer protocols. Network Layer
protocol specifications define the way in which the NSAP address is
encoded as NPAI (see clause 6.1.4).
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8.1 Network Address Semantics
The NSAP address consists of two basic semantic parts. The first part
is the Initial Domain Part (IDP). The second part is the Domain
Specific Part (DSP). This is illustrated by Figure 8-2.
Following the conceptual structure of NSAP addresses described in
Clause 7.1, the IDP is a subdomain identifier: it specifies the
subdomain of the global network addressing domain (see Figure 7-1), and
identifies the authorities responsible for assigning addresses in each
of the subdomains created. The DSP is the corresponding subdomain
address. A further substructure of the DSP may or may not be defined by
the authority identified by the IDP.
8.1.1 The IDP
The Initial Domain Part of the NSAP address itself consists of two
parts. The first part is the Authority and Format Identifier (AFI).
The second part is the Initial Domain Identifier (IDI). This is
illustrated by Figure 8-2:
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8.1.1.1 The AFI
The Authority and Format Identifier specifies:
a) the format of the IDI (see clause 8.2.1.2);
b) the authority responsible for allocating values of the IDI (see
clause 8.2.1.2) and
c) the abstract syntax of the DSP (see clauses 8.2 and 8.2.3).
8.1.1.2 The IDI
The Initial Domain Identifier specifies:
a) the Network Addressing subdomain from which values of the DSP
are allocated; and
b) the authority responsible for allocating values of the DSP from
that subdomain.
8.1.2 The DSP
The semantics of the DSP is determined by the authority identified by
the IDI (see clause 8.1.1.2).
8.2 Network Address Abstract Syntax
The Network Address is defined in this Addendum in terms of an abstract
syntax which expresses the semantics of the Network Address. The use of
this abstract syntax as a descriptive device enables this Addendum to
convey, in written form, a complete definition of the Network Address
without restricting it to the specific encoding of the NPAI. It also
enables this Addendum to identify two alternative preferred concrete
synataxes of the Network Address, to which reference may be made by
Network Layer protocol specification standards so as to unambiguously
define the way in which the Network Address is encoded as NPAI.
8.2.1 Abstract Syntax and Allocation of the IDP
This clause defines the abstract syntax of the AFI, the currently
allocated values of the AFI, and the IDI formats corresponding to the
allocated AFI values. Among the currently allocated values of the
AFIsare values reserved for assignment to new IDI formats which may be
identified by ISO or CCITT. Assignment of these AFI values to new IDI
formats by either ISO or CCITT must be accompanied by appropriate
modification of this Addendum according to the rules established by
ISO for revising International Standards. Allocation of new AFI values
will be by joint agreement between ISO and CCITT, and will require an
appropriate modification of this Addendum.
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The abstract syntax of the IDP is decimal digits. The allocation of
the AFI (see Clause 8.1.1) ensures that the first decimal digit of the
IDP can never be zero. This provides a escape mechanism for use by
protocols that expect to hold incomplete NSAP addresses in a field
that normally carries a complete NSAP address. When the NSAP address
is represented as binary octets, the representation of the IDP is as
defined in Clause 8.3.1.
The length of the IDP depends on the IDI format specified by the value
of the AFI. The IDP length associated with each IDI format is given in
clause 8.2.1.2.
8.2.1.1 Abstract Syntax and Allocation of the AFI
The AFI consists of an integer with a value between 0 and 99 with an
abstract syntax of two decimal digits. The values of the AFI are
allocated or reserved as shown in Table 8-1:
Table 8-1: AFI ALLOCATIONS
00-09 Reserved - will not be allocated
10-35 Reserved for future allocation by joint agreement
of ISO and CCITT
36-51 Allocated and assigned to the IDI formats defined
in clause 8.2.1.2
52-59 Reserved for future allocation by joint agreement
of ISO and CCITT
60-69 Allocated for assignment to new IDI formats by
ISO
70-79 Allocated for assignment to new IDI formats by
CCITT
80-99 Reserved for future allocation by joint agreement
of ISO and CCITT
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8.2.1.2 Format and Allocation of the IDI
A specific combination of IDI format and DSP abstract syntax is
associated with each allocated AFI value, as summarized in Table 8-2:
The IDI consists of a sequence of up to 14 digits allocated
according to CCITT Recommendation X.121. The X.121 number
identifies an authority responsible for allocating and assigning
values of the DSP.
IDP length: Up to 16 digits.
b) ISO DCC
The IDI consists of a three-digit Data Country Code (DCC). ISO DCC
values are allocated by ISO and assigned to ISO member countries or
appropriately sponsored non-member countries or authorities. The
values of the ISO DCC are a subset of the DCC values allocated by
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CCITT in Recommendation X.121 to countries or geographical areas.
The DSP is allocated and assigned by the organization that
represents the country identified by the DCC.
IDP length: 5 digits.
c) F.69
The IDI consists of a telex number of up to 8 digits, allocated
according to CCITT Recommendation F.69, commencing with a 2- or
3-digit destination code. The telex number identifies an authority
responsible for allocating and assigning values of the DSP.
IDP length: Up to 10 digits.
d) E.163
The IDI consists of a public switched telephone network (PSTN)
number of up to 12 digits allocated according to CCITT
Recommendation E.163, commencing with the PSTN country code. The
PSTN number identifies an authority responsible for allocating and
assigning values of the DSP.
IDP length: Up to 14 digits.
e) E.164
The IDI consists of an ISDN number of up to 15 digits allocated
according to CCITT Recommendation E.164, commencing with the ISDN
country code. The ISDN number identifies an authority responsible
for allocating and assigning values of the DSP.
IDP length: Up to 17 digits
f) ISO 6523-ICD
The IDI consists of a 4-digit International Code Designator (ICD)
allocated according to ISO 6523. The ICD identifies an
organizational authority responsible for allocating and assigning
values of the DSP. The "structure of the code" required by ISO 6523,
clause 6.3(d), shall be registered as "According to ISO 8348
Addendum 2".
IDP length: 6 digits.
g) LOCAL
The IDI is null.
IDP length: 2 digits.
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Note 1:
In cases (a), (c), (d), and (e) above, when the IDP is followed by a
decimal-syntax DSP, no discernible boundary is identified in this
Addendum between the IDP digits and the DSP digits.
Note 2:
A figure illustrating the division of the global network addressing
domain according to these formats is contained in Annex B.
Note 3:
The use of a particular IDI format as the basis for allocating an
NSAP address does not constrain routing to that NSAP to go through
any particular subnetwork. For example, the use of the E.163 IDI
format as the basis for allocating an NSAP address does not mean
that access to the NSAP necessarily involves use of the telephony
subnetwork (see clause 7.3).
Note 4:
Formats a, c, d, and e are based on specific CCITT numbering plans,
and as such may be affected by any changes to those plans. It
should be understood that in identifying and describing these
formats, this Addendum observes the current status of CCITT work on
numbering plans, and does not establish any preference or position
whatsoever concerning the way in which CCITT may choose to modify
the plans, or their relationships with one another, in the future.
Changes to this may be necessary to take any such further work by
CCITT into account. For example, the CCITT numbering plans in some
cases may provide escape mechanisms (such as a zero, 8, or 9 prefix)
from one numbering plan to another. This results in the possibility
of a choice that must be made concerning which of formats a, c, d,
and e should be used for the allocation of NSAP addresses, and may
also lead to suggestions that it is not necessary to include all of
the formats a, c, d, and e in this Addendum. Such choices, however,
are made within the context and responsibility of CCITT, and no
preference for one choice or another is made or implied by this
Addendum.
8.2.2 Abstract Syntax and Allocation of the DSP
Values of the DSP are allocated by the authority identified by the IDI
in the syntax identified by the AFI (see clauses 8.1.1.2 and 8.2.1.2).
The allocating authority specifies the format and semantics of the
DSP. If the authority identified by the IDI authorizes one or more
authorities to allocate semantic parts of the DSP, then all those
authorities must allocate using the same abstract syntax used by the
parent authority.
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An authority may choose to allocate NSAP addresses with the DSP in a
decimal or binary abstract syntax for all IDI formats, and may choose
to allocate NSAP addresses with the DSP in a character (ISO 646) or
National Character abstract syntax when the IDI format is "Local" (see
Table 8-2). Clause 9 describes the latter case in detail.
8.2.3 Abstract Syntax of the DSP
The DSP may be allocated by the responsible authority in one of four
syntaxes, depending on the value of the AFI:
a) Binary: The DSP consists of zero or more binary octets, up to
the maximum specified in Table 8-3.
b) Decimal: The DSP consists of zero or more decimal digits, up
to the maximum specified in Table 8-3.
c) Character: The DSP consists of zero or more of those graphic,
characters with no national variant, plus the space
character, from ISO 646, up to the maximum specified
in Table 8-3.
d) National Character: The DSP consists of zero or more characters
from a character set determined by the allocating
authority, up to the maximum specified in Table 8-3.
Table 8-3 gives the maximum length of the DSP in its abstract syntax
for each of the IDI formats defined in clause 8.2.1.2. The
corresponding total NSAP address lengths are given in clause 8.4.
8.3 Network Address Concrete Syntax
As describe in Clause 8.1, the semantics of the NSAP address consists
of three fields in the following order:
a) the AFI, with an abstract syntax of two decimal digits;
b) the IDI, with an abstract syntax of a variable number of decimal
digits; and
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c) the DSP, with an abstract syntax of a variable number of one and
only one of the following types: binary octets, decimal digits,
characters, or national characters.
This Addendum does not specify the way in which the semantics of an
NSAP address are encoded in Network Layer protocols by a concrete
syntax in NPAI (see Note following this clause). These encodings are
specified in Network Layer protocol standards.
Note: Encoding implies more than a concrete syntax, such as the order
of bit transmission, representation as tones or other signals, etc.
Nevertheless, this Addendum identifies two alternative concrete
syntaxes (see clauses 8.3.1 and 8.3.2) of the Network Address.
Reference to these may be made by Network Layer protocol specification
standards. It is possible that the concrete syntax used to encode the
Network Address as NPAI in a Network Layer protocol may be chosen to be
identical to one of these concrete syntaxes. It is not required that
this be the case, however (see clause 9).
The entire NSAP address taken as a whole may be represented explicitly
as a string of either decimal digits (decimal concrete syntax) or
binary octets (binary concrete syntax) as defined below. Network Layer
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protocol specifications making reference to this Addendum shall specify
the way in which either the decimal concrete syntax or the binary
concrete syntax of the NSAP address (or both) is encoded as NPAI (see
clause 6.1.3).
8.3.1 Binary Concrete Syntax
The binary concrete syntax is generated by:
a) using two semi-octets to represent the two digits of the AFI,
yielding a value for each semi-octet in the rage 0000-1001;
b) padding the IDI with leading zero digits if necessary to obtain
the maximum IDI length (specified for each IDI format in clause
8.2.1.2), then using a semi-octet to represent the value of each
decimal digit (including leading padding digits, if preset),
yielding a value in the range 0000-1001; and, if the DSP syntax
is not decimal digits, using the semi-octet value 1111 as a pad
after the final semi-octet (if necessary) to obtain an integral
number of octets;
c) representing a decimal syntax DSP using the technique described in
(b);
d) representing a binary syntax DSP directly as binary octets;
e) when the IDI format is "Local", representing an ISO 646 character
syntax DSP by converting each character to a number in the range
32-127 using the ISO 646 encoding, with zero parity and the
parity bit in the most significant position, reducing the value
by 32, giving a number in the range 0-95, encoding this result as
a pair of decimal digits; and applying the technique described in
(b); and
f) when the IDI format is "Local", representing a National Character
syntax DSP by converting each national character to either one or
two octets according to the rules specified by the authority
responsible for allocating NSAP addresses including national
character DSP syntaxes.
8.3.2 Decimal Concrete Syntax
The decimal concrete syntax is generated by:
a) representing the two digits of the AFI directly as two decimal
digits;
b) padding the IDI with leading zero digits if necessary to obtain
the maximum IDI length (specified for each IDI format in Clause
8.2.1.2), representing the result directly as decimal digits;
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c) representing a decimal syntax DSP directly as decimal digits;
d) representing a binary syntax DSP as follows:
Taking the octets in pairs, convert each octet of the pair to a
number in the range 0-255; this generates six decimal digits,
abcdef, of which digits a and d may take on only the values o, 1, or
2. The pair of octets is represented by the sequence of five digits
gbcef, where the value of digit g is given in Table 8-4:
If the original binary field contained an odd number of octets, the
final octet is converted to a number in the range 0-255 and
represented as three decimal digits (000-255);
e) when the IDI format is "Local", representing an ISO 646
character syntax DSP using the technique described in Clause
8.3.1 (e); and
f) when the IDI format is "Local", representing a National
Character syntax DSP using the technique described in Clause
8.3.1 (f).
8.4 Maximum Network Address Length
The maximum length of the NSAP address for each of the combinations of
IDI abstract syntax is given in Table 8-5 both the decimal concrete
syntax and the binary concrete syntax.
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Note: These values assume a National Character representation of one
character as two binary octets (see clause 8.2.3).
From this table it is clear that:
a) the maximum length of an NSAP address in its binary concrete syntax
is 20 octets; and
b) the maximum length of an NSAP address in its decimal concrete
syntax is 40 digits.
A Network Layer protocol which is capable of conveying a string of
variable length with a maximum length of either 20 binary octets or 40
decimal digits is capable of encoding the full semantic content of any
Network Address.
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9 CHARACTER BASED DSP ALLOCATION
An authority may choose to allocate NSAP addresses with the DSP in a
National Character syntax. In such cases, the allocating authority must
define and publish the mapping of the National Character syntax to
either a binary abstract syntax or a decimal abstract syntax.
Note: It is recommended that this mapping be done by reference to the
ISO Register of Character Sets, which is maintained by the European
Computer Manufacturers Association (ECMA) acting as a registration
authority according to ISO 2375, "Procedure for the Registration of
Escape Sequences".
In the case where the authority defines and publishes the mapping of the
National Character set to a binary abstract syntax, the result must be
representable in either one or two octets per National Character. In
this case, the resulting DSP is considered to be based on the Binary
abstract syntax. AFI values from Table 8-2 and the mapping to binary and
decimal concrete syntaxes are based on the binary abstract syntax.
In the case where the authority defines and publishes the mapping of the
National Character set to a decimal abstract syntax, the result must be
representable in up to five decimal digits per National Character. In
this case, the resulting DSP is considered to be based on the decimal
abstract syntax. AFI values from Table 8-2 and the mapping to binary and
decimal concrete syntaxes are based on the decimal abstract syntax.
Note: The ability to base DSP allocation on National Character sets
allows DSP allocation based on international character sets. This may
simplify address assignment in some cases, and may facilitate
representation of NSAP address in humanly-readable form. Nevertheless,
NSAP addresses should not be confused with Application Layer entity
titles. NSAP addresses are not intended to provide the same degree of
human-readable, user-friendly naming and addressing capabilities as
may be expected in Application Layer entity titles.
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10 REFERENCE PUBLICATION FORMATS
Reference publication formats are defined to allow unambiguous
representation of NSAP addresses in both written and oral communication.
10.1 Decimal Reference Publication Format
The Decimal reference publication form (DRPF) consists of a string of
up to 40 decimal digits. The DRPF is the written inscription of the
decimal concrete syntax defined in clause 8.3.2.
10.2 Hexadecimal Reference Publication Format
The Hexadecimal reference publication format (HRPF) consists of the
symbol "/" (solidus) followed by a string of up to 40 hexadecimal
digits. The HRPF is the written inscription of the binary concrete
syntax defined in clause 8.3.1, using two hexadecimal digits ranging
from 00 through FF to represent each binary octet.
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ANNEX A - NETWORK ENTITY TITLES
This Annex is an integral part of the Addendum.
In order to perform routing functions and to distribute Network Layer
management information concerning routing among Network entities, it is
necessary to be able to unambiguously identify Network entities in end
systems and intermediate systems. The Reference Model (ISO 7498)
provides a definition of the concept of an (N)-entity title, which may
be used to permanently and unambiguously identify a Network entity in an
end system or intermediate system.
Any authority responsible for allocating addresses to NSAPs may choose
also to allocate Network entity titles. One of the ways in which this
can be done is to use the principles and mechanisms defined in this
Addendum for allocating Network addresses. When this approach is taken,
a Network entity title has the same abstract syntax as an NSAP address.
A value may be allocated as a Network entity tile only if it has not
been allocated as an NSAP address.
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ANNEX B - NSAP ADDRESS ALLOCATION
This Annex is not an integral part of the Addendum.
The division of the global Network addressing domain according to the
IDI formats described in clause 8.2.1.2 may be illustrated by the
following figure. The numbers adjacent to each line in the figure are
AFI values, as defined in Table 8-2 of clause 8.2.1.2.
Figure B-1 - NSAP Address Allocation on attached page.
00-09 Reserved - will not be allocated
10-35 Reserved for future allocation by joint agreement of ISO
and CCITT
36-37 X.121
38-39 ISO DCC
40-41 F.69
42-43 E.163
44-45 E.164
46-47 ISO ICD
48-51 Local
52-59 Reserved for future allocation by joint agreement of ISO
and CCITT
60-69 Allocated for assignment by ISO
70-79 Allocated for assignment by CCITT
80-99 Reserved for future allocation by joint agreement of ISO
and CCITT
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ANNEX C - RATIONALES
This annex contains tutorial and explanatory material, and is not an
integral part of the Addendum.
C.1 IDI FORMATS (Clause 8.2.1.2)
The rationale for the use of the specific IDI formats identified in
Clause 8.2.1.2 is to allow the allocation and assignment of NSAP
addresses to be based on existing, well-established network numbering
plans and organization-identification standards.
The CCITT numbering plans are included so as to allow for the
designation of the organization to which a number is assigned as an
authority for the assignment of NSAP addresses. If the organization
identified by a particular number from one of these plans chooses not
to define any further sub-addressing beyond that number, then the
number itself constitutes an NSAP address when it is used in the OSI
environment. This flexibility allows number allocated from the four
CCITT numbering plans identified in Clause 8.2.1.2 to be used directly
as NSAP addresses, with the addition of nothing more than the initial
AFI digits that identify the plan.
The ISO DCC format is included so as to allow for the designation,
where permitted by national regulations, of the organization that
represents a country in ISO (or an appropriately sponsored
organization) as an authority for the assignment of
geographically-based NSAP addresses. The way in which addresses are
allocated and assigned in the ISO DCC format is determined by the
designated organization, which might, for example, be the national
standards body that represents a country in ISO.
The ISO 6523-ICD format is included so as to allow for the designation,
where permitted by national regulations, of an organization that may or
may not be tied to a particular country as an authority for the
assignment of NSAP addresses according to the hierarchy appropriate for
that organization (which may not be based on geographical or national
boundaries). The way which addresses are allocated and assigned in the
ISO 6523-ICD format is determined by the designated organization, which
might, for example, be the United Nations World Health Organization.
The Local format is included so as to allow for proprietary or other
non-standard network addressing schemes to coexist with the standard
OSI network addressing scheme. Use of the Local format for these
non-standard address ensures that they cannot be confused with standard
OSI network addresses. This capability will be useful in the evolution
of existing networks to OSI, and for the accommodation of non-OSI
addressing schemes that may be used in proprietary network
architectures or for testing and other interim purposes. It should be
emphasized that
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the Local format is not intended to give non-OSI schemes a permanent
place in OSI, but rather to permit the OSI network addressing sheme to
be used wherever possible without risk of conflict with other schemes
(which can be encapsulated safely under the Local format).
C.2 RESERVATION OF AFI VALUES 00-09 (Table 8-2)
The reservation of AFI values beginning with the digit 0 is intended to
allow for the use of an initial 0 to handle special cases, such as:
a) as an escape to some other addressing scheme;
b) as a technique for the optimization of NSAP address encoding in
Network Layer protocols, when the different parts of the NSAP
address semantics are encoded in different fields of the protocol
header;
c) as a way to indicate, in a protocol header, that a field that
ordinarily contains a full NSAP address in fact contains something
less than a full address (for example, a shorthand form that omits
specification of the higher-order domains, which might be used for
communication within a particular subdomain environment).
There may be other cases in which the use of an initial 0 digit is
found to be useful. This Addendum merely reserves the AFI values 00-09,
and does not specify how they might be used; all such uses are outside
the scope of this Addendum.
C.3 DERIVATION OF THE CONCRETE SYNTAXES (Clause 8.3)
In describing the two "preferred" concrete syntaxes of the NSAP
address, Clauses 8.3.1 and 8.3.2 introduce two types of padding:
padding with zero digits at the beginning of an IDI, and padding with a
semi-octet with the value 1111 at the end of the binary encoding of an
IDI with an odd number of decimal digits.
The first type of padding is necessary because some of the IDI formats
allow the IDI to consist of a variable number of digits. Since there is
no explicit syntactic marker between the IDI and the DSP, the only way
to find the end of the IDI is to know how long it is. The AFI, which
identifies which IDI format is used, allows only the maximum length of
that IDI to be determined. Rather than introduce either a specific
syntactic marker or a new field containing the length of the IDI
(either of which would have greatly complicated the encoding and
parsing of NSAP addresses), the Addendum specifies that for encoding
purposes the IDI must first be padded out to its maximum length. Note
that this does not apply to the DSP; only to the IDI.
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The second type of padding is necessary to ensure that a binary
encoding of the IDI consists of an integral number of binary octets.
