rfc941.txt

RFC 相关的技术文档

ISO/TC-97/SC-6                                                 [Page 13]
RFC 941                                                       April 1985
Network Layer Addressing


                                    
                             **************                        
                        *****              *****                   
                     ***                        ***                
                  ***               Z              ***             
                     **                          **                
                       *                        *                  
                  ***   **                    **   ***             
                **   **   *                  *   **   **           
              **       *   **              **   *      .**         
             **         **   *            *   **    r .  **        
            *             *   *          *   *       .      *      
       X   *               *   *        *   * . ------------>*   Y 
          *                 *   *      *   *   /.  .     s   +*    
         *                   *   *    *   *   /  ..         +  *   
        *                    *   *    *   *  /     ..      +   *   
       **                     *   *  *   *  b        ..   +    **  
       * +                    *   *  *   *  |          ..+      *  
       *  +                   *   *  *   *  |    q      +       *  
      *    +                   *   **   * ..|          +         * 
      *     +                  *        *   |...      +     a    * 
      *      +                 *        *   | p .... +           * 
      *       +                 *      *    V       +            * 
      *        +  ************************************           * 
      *  *********                                    *********  * 
       **                                                      **  
                  ************************************             
         *********           +            +           *********    
       **                    +            +                    **  
       *                    +              +                    *  
       **                   +              +                   **  
        *                  +                +         c        *   
         *         a       +                +                 *    
          *               +                  +               *     
           *              +          b       +              *      
            *            +                    +            *       
             **          +                    +          **        
              **        +                      +        **         
                **      +                      +      **           
                  ***  +                        +  ***             
                     ***                        ***                
                        *****              *****                   
                             **************                        
                                    W                              

       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

    










ISO/TC-97/SC-6                                                 [Page 19]
RFC 941                                                       April 1985
Network Layer Addressing


  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: