📄 rfc941.txt
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h) IDP - Initial Domain Parti) AFI - Authority and Format Identifierj) IDI - Initial Domain Identifierk) DSP - Domain Specific Partl) NPDU - Network Protocol Data Unitm) SNPA - Subnetwork Point of Attachment5 CONVENTIONSNo particular standard conventions are invoked by this Addendum.ISO/TC-97/SC-6 [Page 7] RFC 941 April 1985Network Layer AddressingSECTION TWO - NETWORK LAYER ADDRESSING--------------------------------------6 CONCEPTS AND TERMINOLOGY FOR NETWORK LAYER ADDRESSING6.1 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 AddressISO/TC-97/SC-6 [Page 8] RFC 941 April 1985Network 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 termISO/TC-97/SC-6 [Page 9] RFC 941 April 1985Network 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 1985Network Layer Addressing ************** ***** ***** *** *** *** *** ** ** ** ** <-- Global ** * * .** network ** ** ** . ** addressing * * * . * domain * * * . . * * * * .. . * * * * .. + * * * * .. <-----------\ ** * * .. + ** | * + * * ..+ * | * + * <------------------------------\| * + * * ... + * | * + * * ... + * | * + * * .... + * | * + * * + * | * + ************************************ * | * ********* + + ********* * | ** + + ** | * + + * | ** + + ** | * + + <-------------\| * + + * | * + + * | * + + * | * + + * | ** + + ** | ** + <--\ + ** | ** + \ + ** | *** + \ + *** | *** \ *** | ***** \**** | ***************\ Network \------------- addressing subdomains Figure 6-3 - Domains and SubdomainsISO/TC-97/SC-6 [Page 11] RFC 941 April 1985Network Layer Addressing6.3 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 SCHEME7.1 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 1985Network 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 | | ------------------------------- | | | | | | | | ----- ----- ----- ----- | 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⌨️ 快捷键说明
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