📄 rfc941.txt
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----------------------
| | | |
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----- ----- ----- -----
| p | | q | | r | | s |
----- ----- ----- -----
Figure 7-1 (a) - Hierarchical Structure of NSAP Addresses
Inverted Tree Diagram
ISO/TC-97/SC-6 [Page 13]
RFC 941 April 1985
Network Layer Addressing
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** ** * * ** r . **
* * * * * . *
X * * * * * . ------------>* Y
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* * * * * / .. + *
* * * * * / .. + *
** * * * * b .. + **
* + * * * * | ..+ *
* + * * * * | q + *
* + * ** * ..| + *
* + * * |... + a *
* + * * | p .... + *
* + * * V + *
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********* + + *********
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Figure 7-1 (b) - Hierarchical Structure of NSAP Addresses
Domain Diagram
ISO/TC-97/SC-6 [Page 14]
RFC 941 April 1985
Network Layer Addressing
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
ISO/TC-97/SC-6 [Page 15]
RFC 941 April 1985
Network Layer Addressing
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:
NSAP Address Semantics------->Allocation by------->Abstract Syntax
|
|
|-->Representation in--->External
| Humanly-readable Reference
| Directories Syntax
|
|-->Encoding in--------->Concrete Syntax
Protocols
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).
ISO/TC-97/SC-6 [Page 16]
RFC 941 April 1985
Network Layer Addressing
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:
<----------------------NSAP ADDRESS------------------------->
___________________________________________________________
| | |
| IDP | DSP |
|___________|_______________________________________________|
:
:_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
:
___________________________________________________________:
| | |
| AFI | IDI |
|___________|_______________________________________________|
Figure 8-2 - NSAP Address Structure
ISO/TC-97/SC-6 [Page 17]
RFC 941 April 1985
Network Layer Addressing
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.
ISO/TC-97/SC-6 [Page 18]
RFC 941 April 1985
Network Layer Addressing
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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