rfc941.txt

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                         Table 8-2:  AFI Values

                    ___________________                        
                   |   DSP Syntax      |                       
                   |___________________|                       
                   |         |         |                       
         __________| Decimal | Binary  |                       
        |IDI format|         |         |                       
        |__________|_________|_________|                       
        |  X.121       36        37    |                       
        |______________________________|                       
        | ISO DCC      38        39    |                       
        |______________________________|                       
        |  F.69        40        41    |                       
        |______________________________|                       
        |  E.163       42        43    |                       
        |______________________________|                       
        |  E.164       44        45    |_____________________  
        |______________________________|Character | National | 
        |ISO 6523-ICD  46        47    |(ISO 646) |Character | 
        |______________________________|__________|__________| 
        | Local        48        49        50         51     | 
        |____________________________________________________| 

                                    

                The IDI formats are defined as follows:

   a) X.121

    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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RFC 941                                                       April 1985
Network Layer Addressing


    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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Network Layer Addressing


   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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RFC 941                                                       April 1985
Network Layer Addressing


  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














ISO/TC-97/SC-6                                                 [Page 23]
RFC 941                                                       April 1985
Network Layer Addressing


                                    

                     Table 8-3:  Maximum DSP Length

                    ___________________                        
                   |   DSP Syntax      |                       
                   |___________________|                       
                   |         |         |                       
         __________| Decimal | Binary  |                       
        |IDI format|         |         |                       
        |__________|_________|_________|                       
        |  X.121       24         9    |                       
        |______________________________|                       
        | ISO DCC      35        14    |                       
        |______________________________|                       
        |  F.69        30        12    |                       
        |______________________________|                       
        |  E.163       26        10    |                       
        |______________________________|                       
        |  E.164       23         9    |_____________________  
        |______________________________|Character | National | 
        |ISO 6523-ICD  34        13    |(ISO 646) |Character | 
        |______________________________|__________|__________| 
        | Local        38        15        19          7     | 
        |____________________________________________________| 

                                    

  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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Network Layer Addressing


 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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Network Layer Addressing


   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:

    

                        Table 8-4:  Values of g.

                      _____________________________
                    |  \  a  |      |      |      |
                    | d \    |  0   |   1  |  2   |
                    |____\___|______|______|______|
                    |   0       0       1     2   |
                    |_____________________________|
                    |   1       3       4     5   |
                    |_____________________________|
                    |   2       6       7     8   |
                    |_____________________________|