📄 rfc941.txt
字号:
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.
ISO/TC-97/SC-6 [Page 7]
RFC 941 April 1985
Network Layer Addressing
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
ISO/TC-97/SC-6 [Page 8]
RFC 941 April 1985
Network Layer Addressing
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:
Network Service User
layer 4
______________________________ 0 _____________________________
\
layer 3 \____NSAP identified
by NSAP address
Network Service Provider
Figure 6-2 - NSAP Address
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
ISO/TC-97/SC-6 [Page 9]
RFC 941 April 1985
Network Layer Addressing
"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:
ISO/TC-97/SC-6 [Page 10]
RFC 941 April 1985
Network Layer Addressing
**************
***** *****
*** ***
*** ***
** ** ** ** <-- Global
** * * .** network
** ** ** . ** addressing
* * * . * domain
* * * . . *
* * * .. . *
* * * .. + *
* * * .. <-----------\
** * * .. + ** |
* + * * ..+ * |
* + * <------------------------------\|
* + * * ... + * |
* + * * ... + * |
* + * * .... + * |
* + * * + * |
* + ************************************ * |
* ********* + + ********* * |
** + + ** |
* + + * |
** + + ** |
* + + <-------------\|
* + + * |
* + + * |
* + + * |
* + + * |
** + + ** |
** + <--\ + ** |
** + \ + ** |
*** + \ + *** |
*** \ *** |
***** \**** |
***************\ Network
\------------- addressing
subdomains
Figure 6-3 - Domains and Subdomains
ISO/TC-97/SC-6 [Page 11]
RFC 941 April 1985
Network Layer Addressing
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.
ISO/TC-97/SC-6 [Page 12]
RFC 941 April 1985
Network Layer Addressing
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
|
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| | | |
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----- ----- ----- -----
| W | | X | | Y | | Z |
----- ----- ----- -----
| | |
| | |
--------------- @ --------
| | | | |
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----- ----- ----- ----- -----
| a | | b | | c | | a | | b |
----- ----- ----- ----- -----
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