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RFC 0926

 
 
 

Protocol for providing the connectionless mode network services

Part 2 of 3, p. 35 to 69
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prevText      Top       Page 35 
  Notes:

   1)  While the Padding, Error Reporting, and Header Error Detection
       functions must be provided, they are provided only when selected
       by the sending Network Service user.

   2)  The correct treatment of the Padding function involves no
       processing. Therefore, this could equally be described as a Type
       3 function.

   3)  The rationale for the inclusion of type 3 functions is that in
       the case of some functions it is more important to forward the
       PDUs between intermediate systems or deliver them to an
       end-system than it is to support the functions. Type 3 functions
       should be used in those cases where they are of an advisory
       nature and should not be the cause of the discarding of a PDU
       when not supported.

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7  STRUCTURE AND ENCODING OF PDUS

 7.1 Structure

  All Protocol Data Units shall contain an integral number of octets.
  The octets in a PDU are numbered starting from one (1) and increasing
  in the order in which they are put into an SNSDU. The bits in an octet
  are numbered from one (1) to eight (8), where bit one (1) is the
  low-order bit.

  When consecutive octets are used to represent a binary number, the
  lower octet number has the most significant value.

  Any subnetwork supporting this protocol is required to state in its
  specification the way octets are transferred, using the terms "most
  significant bit" and "least significant bit." The PDUs of this
  protocol are defined using the terms "most significant bit" and "least
  significant bit."

   Note:

    When the encoding of a PDU is represented using a diagram in this
    section, the following representation is used:

     a)  octets are shown with the lowest numbered octet to the left,
         higher number octets being further to the right;

     b)  within an octet, bits are shown with bit eight (8) to the left
         and bit one (1) to the right.

  PDUs shall contain, in the following order:

   1)  the header, comprising:

    a)  the fixed part;

    b)  the address part;

    c)  the segmentation part, if present;

    d)  the options part, if present

   and

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   2)  the data field, if present.

  This structure is illustrated below:

                       Part:                Described in:  

            +-------------------+                          
            |    Fixed Part     |            Section 7.2   
            +-------------------+                          

            +-------------------+                          
            |   Address Part    |            Section 7.3   
            +-------------------+                          

            +-------------------+                          
            | Segmentation Part |            Section 7.4   
            +-------------------+                          

            +-------------------+                          
            |   Options Part    |            Section 7.5   
            +-------------------+                          

            +-------------------+                          
            |       Data        |            Section 7.6   
            +-------------------+                          

                       Figure 7-1.  PDU Structure

Top       Page 38 
 7.2 Fixed Part

  7.2.1 General

   The fixed part contains frequently occuring parameters including the
   type code (DT or ER) of the protocol data unit. The length and the
   structure of the fixed part are defined by the PDU code.

   The fixed part has the following format:

                                                      Octet 
            +------------------------------------+          
            | Network Layer Protocol Identifier  |     1    
            |------------------------------------|          
            |         Length Indicator           |     2    
            |------------------------------------|          
            |   Version/Protocol Id Extension    |     3    
            |------------------------------------|          
            |            Lifetime                |     4    
            |------------------------------------|          
            |S |M |E/R|         Type             |     5    
            | P| S|   |                          |          
            |------------------------------------|          
            |          Segment Length            |    6,7   
            |------------------------------------|          
            |             Checksum               |    8,9   
            +------------------------------------+          

                  Figure 7-2.  PDU Header--Fixed Part

  7.2.2 Network Layer Protocol Identifier

   The value of this field shall be binary 1000 0001. This field
   identifies this Network Layer Protocol as ISO 8473, Protocol for
   Providing the Connectionless-mode Network Service.

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  7.2.3 Length Indicator

   The length is indicated by a binary number, with a maximum value of
   254 (1111 1110). The length indicated is the length in octets of the
   header, as described in Section 7.1, Structure. The value 255 (1111
   1111) is reserved for possible future extensions.

    Note:

     The rules for forwarding and segmentation ensure that the header
     length is the same for all segments (Derived PDUs) of the Initial
     PDU, and is the same as the header length of the Initial PDU.

  7.2.4 Version/Protocol Identifier Extension

   The value of this field is binary 0000 0001. This Identifies a
   standard version of ISO 8473, Protocol for Providing the
   Connectionless-mode Network Service.

  7.2.5 PDU Lifetime

   The Lifetime field is encoded as a binary number representing the
   remaining lifetime of the PDU, in units of 500 milliseconds.

   The Lifetime field is set by the originating network-entity, and is
   decremented by every network-entity which processes the PDU. The PDU
   shall be discarded if the value of the field reaches zero.

   When a network-entity processes a PDU, it decrements the Lifetime by
   at least one. The Lifetime shall be decremented by more than one if
   the sum of:

    1)  the transit delay in the subnetwork from which the PDU was
        received; and

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    2)  the delay within the system processing the PDU

   exceeds or is estimated to exceed 500 milliseconds. In this case, the
   lifetime field should be decremented by one for each additional 500
   milliseconds of delay. The determination of delay need not be
   precise, but where error exists the value used shall be an
   overestimate, not an underestimate.

   If the Lifetime reaches a value of zero before the PDU is delivered
   to the destination, the PDU shall be discarded. The Error Reporting
   function shall be invoked, as described in Section 6.10, Error
   Reporting Function, and may result in the generation of an ER PDU. It
   is a local matter whether the destination network-entity performs the
   Lifetime Control function.

   When the Segmentation function is applied to a PDU, the Lifetime
   field is copied into all of the Derived PDUs.

  7.2.6 Flags

   7.2.6.1 Segmentation Permitted and More Segments Flags

    The Segmentation Permitted flag determines whether segmentation is
    permitted. A value of one indicates that segmentation is permitted.

    A value of zero indicates that the non-segmenting protocol subset is
    employed. Where this is the case, the segmentation part of the PDU
    header is not present, and the Segment Length field serves as the
    Total Length field.

    The More Segments flag indicates whether the data segment in this
    PDU contains (as its last octet) the last octet of the User Data in
    the NSDU. When the More Segments flag is set to one (1) then
    segmentation has taken place and the last octet of the NSDU is not
    contained in this PDU. The More Segments flag cannot be set to one
    (1) if the Segmentation Permitted flag is not set to one (1).

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    When the More Segments flag is set to zero (0) the last octet of the
    Data Part of the PDU is the last octet of the NSDU.

   7.2.6.2 Error Report Flag

    When the Error Report flag is set to one, the rules in Section 6.10
    are used to determine whether to generate an Error Report PDU upon
    discard of the PDU.

    When the Error Report flag is set to zero, discard of the PDU will
    not cause the generation of an Error Report PDU.

  7.2.7 Type Code

   The Type code field identifies the type of the protocol data unit.
   Allowed values are given in Table 7-1:

                                Bits    5 4 3 2 1   
                    +-----------------------------+ 
                    |  DT PDU  |        1 1 1 0 0 | 
                    |-----------------------------| 
                    |  ER PDU  |        0 0 0 0 1 | 
                    +-----------------------------+ 

                      Table 7-1.  Valid PDU Types

  7.2.8 PDU Segment Length

   The Segment Length field specifies the entire length of the PDU
   segment including both header and data, if present. When the full
   protocol is employed and a PDU is not segmented, then the value of
   this field is identical to the value of the Total Length field
   located in the Segmentation Part of the header.

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   When the Non-segmenting protocol subset is employed, no segmentation
   part is present in the header. In this subset, the Segment Length
   field serves as the Total Length field of the header (see Section
   7.4.3).

  7.2.9 PDU Checksum

   The checksum is computed on the entire PDU header. This includes the
   segmentation and options parts, if present. A checksum value of zero
   is reserved to indicate that the checksum is to be ignored. The
   operation of the PDU Header Error Detection function ensures that the
   value zero does not represent a valid checksum. A non-zero value
   indicates that the checksum must be processed or the PDU must be
   discarded.

 7.3 Address Part

  7.3.1 General

   Address parameters are distinguished by their location, immediately
   following the fixed part of the PDU header. The address part is
   illustrated below:

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                                                      Octet  
          +--------------------------------------+           
          |                                      |           
          | Destination Address Length Indicator |      10   
          |                                      |           
          |--------------------------------------|           
          |                                      |      11   
          |         Destination Address          |           
          |                                      |      m-1  
          |--------------------------------------|           
          |                                      |           
          |   Source Address Length Indicator    |       m   
          |                                      |           
          |--------------------------------------|           
          |                                      |      m+1  
          |           Source Address             |           
          |                                      |      n-1  
          +--------------------------------------+           

                 Figure 7-3.  PDU header--Address Part

   7.3.1.1 Destination and Source Address Information

    The Destination and Source addresses are Network Service Access
    Point addresses as defined in ISO 8348/DAD2, Addendum to the Network
    Service Definition Covering Network Layer Addressing.

    The Destination and Source Address information is of variable
    length.

    The Destination Address Length Indicator field specifies the length
    of the Destination Address in number of octets. The Destination
    Address field follows the Destination Address Length Indicator
    field. The Source Address Length Indicator field specifies the
    length of the Source Address in number of octets. The Source Address
    Length Indicator field follows the Destination Address field. The
    Source Address field follows the Source Address Length Indicator
    field.

Top       Page 44 
    Each address parameter is encoded as follows:

                      Bits   8   7   6   5   4   3   2   1  
            +---------------------------------------------+ 
            | Octet  | Address parameter Length Indicator | 
            |   n    |           (e.g., 'm')              | 
            |---------------------------------------------| 
            | Octets |                                    | 
            |  n+1   |     Address Parameter Value        | 
            | thru   |                                    | 
            |  n+m   |                                    | 
            +---------------------------------------------+ 

                     Table 7-2.  Address Parameters

 7.4 Segmentation Part

  If the Segmentation Permitted Flag in the Fixed Part of the PDU Header
  (Octet 4, Bit 8) is set to one, the segmentation part of the header,
  illustrated below, must be present:

                                               Octet   
                +------------------------+             
                |  Data Unit Identifier  |     n,n+1   
                |------------------------|             
                |     Segment Offset     |    n+2,n+3  
                |------------------------|             
                |      Total Length      |    n+4,n+5  
                +------------------------+             

               Figure 7-4.  PDU Header--Segmentation Part

  Where the "Segmentation Permitted" flag is set to zero, the
  nonsegmenting protocol subset is in use.

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  7.4.1 Data Unit Identifier

   The Data Unit Identifier identifies an Initial PDU (and hence, its
   Derived PDUs) so that a segmented data unit may be correctly
   reassembled by the destination network-entity. The Data Unit
   Identifier size is two octets.

  7.4.2 Segment Offset

   For each segment the Segment Offset field specifies the relative
   position of the segment in the data part of the Initial PDU with
   respect to the start of the data field. The offset is measured in
   units of octets. The offset of the first segment is zero.

  7.4.3 PDU Total Length

   The Total Length field specifies the entire length of the Initial
   PDU, including both the header and data. This field is not changed in
   any segment (Derived PDU) for the lifetime of the PDU.

 7.5 Options Part

  7.5.1 General

   The options part is used to convey optional parameters. If the
   options part is present, it contains one or more parameters. The
   number of parameters that may be contained in the options part is
   indicated by the length of the options part which is:

    PDU Header Length - (length of fixed part +
                         length of address part +
                         length of segmentation part).

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   The options part of the PDU header is illustrated below:

                                               Octet 
                   +--------------------+            
                   |                    |       n+6  
                   |      Options       |            
                   |                    |       p    
                   +--------------------+            

                 Figure 7-5.  PDU Header--Options Part

   Each parameter contained within the options part of the PDU header is
   encoded as follows:

                          BITS    8  7  6  5  4  3  2  1  
             +------------------------------------------+ 
             |  Octets  |                               | 
             |    n     |  Parameter Code               | 
             |------------------------------------------| 
             |   n+1    |  Parameter Length (e.g., 'm') | 
             |------------------------------------------| 
             |   n+2    |  Parameter Value              | 
             |  n+m+1   |                               | 
             +------------------------------------------+ 

                   Table 7-3.  Encoding of Parameters

   The parameter code field is coded in binary and, without extensions,
   provides a maximum number of 255 different parameters. However, as
   noted below, bits 8 and 7 cannot take every possible value, so the
   practical maximum number of different parameters is less. A parameter
   code of 255 (binary 1111 1111) is reserved for possible extensions of
   the parameter code.

   The parameter length field indicates the length, in octets, of the
   parameter value field. The length is indicated by a binary number,
   'm', with a theoretical maximum value of 255. The practical maximum
   value of 'm' is lower. For example, in the case of a single parameter
   contained within the options part, two octets are required for the
   parameter code and the parameter length indication itself. Thus, the
   value of 'm' is limited to:

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     253 - (length of fixed part +
     length of address part +
     length of segmentation part).

   For each succeeding parameter the maximum value of 'm' decreases.

   The parameter value field contains the value of the parameter
   identified in the parameter code field.

   No parameter codes use bits 8 and 7 with the value 00.

   Implementations shall accept the parameters defined in the options
   part in any order. Duplication of options (where detected) is not
   permitted. Receipt of a PDU with an option duplicated should be
   treated as a protocol error. The rules governing the treatment of
   protocol errors are described in Section 6.10, Error Reporting
   Function.

   The following parameters are permitted in the options part.

  7.5.2 Padding

   The padding parameter is used to lengthen the PDU header to a
   convenient size (See Section 6.12).

    Parameter Code:       1100 1100
    Parameter Length:     variable
    Parameter Value:      any value is allowed

  7.5.3 Security

   This parameter is user defined.

    Parameter Code:       1100 0101
    Parameter Length:     variable
    Parameter Value:

     High order bit of first octet is Security Domain bit, S, to be
     interpreted as follows:

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      S=0

       +---------------------------
       | S | User Defined        ----
       +------------------------

      S=1

       +---------------------------
       | S | CODE | ORGANIZATION ----
       +------------------------

      where

       CODE = This field contains a geographic or non-geographic code to
              which the option applies.

       ORGANIZATION = This is a further subdivision of the CODE field
                      and is determined by an administrator of the
                      geographic or non-geographic domain identified by
                      the value of CODE.

  7.5.4 Source Routing

   The source routing parameter specifies, either completely or
   partially, the route to be taken from Source Network Address to
   Destination Network Address.

    Parameter Code:      1100 1000
    Parameter Length:    variable
    Parameter Value:     2 octet control information
                         succeeded by a concatenation
                         of ordered address fields
                         (ordered from source to destination)

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   The first octet of the parameter value is the type code. This has the
   following significance.

    0000 0001     complete source routing
    0000 0000     partial source routing

    <all other values reserved>

   The second octet indicates the octet offset of the next address to be
   processed in the list. A value of three (3) indicates that the next
   address begins immediately after this control octet. Successive
   octets are indicated by correspondingly larger values of this
   indicator.

   The third octet begins the intermediate-system address list. The
   address list consists of variable length address fields. The first
   octet of each address field identifies the length of the address
   which comprises the remainder of the address field.

  7.5.5 Recording of Route

   The recording of route parameter identifies the route of intermediate
   systems traversed by the PDU.

    Parameter Code:       1100 1011
    Parameter Length:     variable
    Parameter Value:      two octets control information
                          succeeded  by a concatenation of
                          ordered addresses

   The first octet is used to indicate that the recording of route has
   been terminated owing to lack of space in the option. It has the
   following significance:

    0000 0000     Recording of Route still in progress
    1111 1111     Recording of Route terminated

    <all other values reserved>

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   The second octet identifies the next octet which may be used to
   record an address. It is encoded relative to the start of the
   parameter, such that a value of three (3) indicates that the octet
   after this one is the next to be used.

   The third octet begins the address list. The address list consists of
   variable length address fields. The first octet of each address field
   identifies the length of the address which comprises the remainder of
   the field. Address fields are always added to the beginning of the
   address list; i.e., the most recently added field will begin in the
   third octet of the parameter value.

  7.5.6 Quality of Service Maintenance

   The Quality of Service parameter conveys information about the
   quality of service requested by the originating Network Service user.
   At intermediate systems, Network Layer relay entities may (but are
   not required to) make use of this information as an aid in selecting
   a route when more than one route satisfying other routing criteria is
   available and the available routes are known to differ with respect
   to Quality of Service (see Section 6.16).

    Parameter Code:       1100 0011
    Parameter Length:     one octet
    Parameter Value:      Bit 8:  transit delay vs. cost
                          Bit 7:  residual error probability vs.
                                  transit delay
                          Bit 6:  residual error probability vs.
                                  cost
                          Bits 5 thru 0 are not specified.

   Bit 8 is set to one indicates that where possible, routing decision
   should favor low transit delay over low cost. A value of 0 indicates
   that routing decisions should favor low cost over low transit delay.

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   Bit 7 set to one indicates that where possible, routing decisions
   should favor low residual error probability over low transit delay. A
   value of zero indicates that routing decisions should favor low
   transit delay over low residual error probability.

   Bit 6 is set to one indicates that where possible, routing decisions
   should favor low residual error probability over low cost. A value of
   0 indicates that routing decisions should favor low cost over low
   residual error probability.

 7.6 Priority

  The priority parameter carries the relative priority of the protocol
  data unit. Intermediate systems that support this option should make
  use of this information in routing and in ordering PDUs for
  transmission.

   Parameter Code:       1100 1100
   Parameter Length:     one octet
   Parameter Value:      0000 0000 - Normal (Default)
                         thru
                         0000 1111 - Highest

  The values 0000 0001 through 0000 1111 are to be used for higher
  priority protocol data units. If an intermediate system does not
  support this option then all PDUs shall be treated as if the field had
  the value 0000 0000.

 7.7 Data Part

  The Data part of the PDU is structured as an ordered multiple of
  octets, which is identical to the same ordered multiple of octets
  specified for the NS_Userdata parameter of the N_UNITDATA Request and
  Indication primitives. The data field is illustrated below:

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                                             Octet 
                  +------------------+                
                  |                  |      p+1       
                  |       Data       |                
                  |                  |       z        
                  +------------------+                

                  Figure 7-6.  PDU header--Data Field

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 7.8 Data (DT) PDU

  7.8.1 Structure

   The DT PDU has the following format:

                                                  Octet           
     +--------------------------------------+                     
     |  Network Layer Protocol Identifier   |      1              
     |--------------------------------------|                     
     |           Length Indicator           |      2              
     |--------------------------------------|                     
     |   Version/Protocol Id Extension      |      3              
     |--------------------------------------|                     
     |              Lifetime                |      4              
     |--------------------------------------|                     
     |SP|MS|E/R|      Type                  |      5              
     |--------------------------------------|                     
     |           Segment Length             |     6,7             
     |--------------------------------------|                     
     |              Checksum                |     8,9             
     |--------------------------------------|                     
     | Destination Address Length Indicator |     10              
     |--------------------------------------|                     
     |         Destination Address          |     11 through m-1  
     |--------------------------------------|                     
     |    Source Address Length Indicator   |      m              
     |--------------------------------------|                     
     |            Source Address            |     m+1 through n-1 
     |--------------------------------------|                     
     |         Data Unit Identifier         |     n,n+1           
     |--------------------------------------|                     
     |            Segment Offset            |     n+2,n+3         
     |--------------------------------------|                     
     |             Total Length             |     n+4,n+5         
     |--------------------------------------|                     
     |                Options               |     n+6 through p   
     |--------------------------------------|                     
     |                 Data                 |     p+1 through z   
     +--------------------------------------+                     

                     Figure 7-7.  PDU Header Format

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   7.8.1.1 Fixed Part

    1) Network Layer Protocol Identifier   See Section 7.2.2.
    2) Length Indicator                    See Section 7.2.3.
    3) Version/Protocol Id Extension       See Section 7.2.4.
    4) Lifetime                            See Section 7.2.5.
    5) SP, MS, E/R                         See Section 7.2.6.
    6) Type Code                           See Section 7.2.7.
    7) Segment Length                      See Section 7.2.8.
    8) Checksum                            See Section 7.2.9.

   7.8.1.2 Addresses

    See Section 7.3.

   7.8.1.3 Segmentation

    See Section 7.4.

   7.8.1.4 Options

    See Section 7.5.

   7.8.1.5 Data

    See Section 7.7.

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 7.9 Inactive Network Layer Protocol

                                              Octet         
            +-----------------------------+                 
            |  Network Layer Protocol Id  |     1           
            |-----------------------------|                 
            |           Data              |     2 through n 
            +-----------------------------+                 

              Figure 7-9.  Inactive Network Layer Protocol

  7.9.1 Network Layer Protocol Id

   The value of the Network Layer Protocol Identifier field is binary
   zero (0000 0000).

  7.9.2 Data Field

   See Section 7.7.

   The length of the NS_Userdata parameter is constrained to be less
   than or equal to the value of the length of the SN_Userdata parameter
   minus one.

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 7.10 Error Report PDU (ER)

  7.10.1 Structure

                                                  Octet           
     +--------------------------------------+                     
     |   Network Layer Protocol Identifier  |       1             
     |--------------------------------------|                     
     |           Length Indicator           |       2             
     |--------------------------------------|                     
     |     Version/Protocol Id Extension    |       3             
     |--------------------------------------|                     
     |               Lifetime               |       4             
     |--------------------------------------|                     
     |SP|MS|E/R|       Type                 |       5             
     |--------------------------------------|                     
     |             Segment Length           |      6,7            
     |--------------------------------------|                     
     |                Checksum              |      8,9            
     |--------------------------------------|                     
     | Destination Address Length Indicator |      10             
     |--------------------------------------|                     
     |         Destination Address          |     10 through m-1  
     |--------------------------------------|                     
     |     Source Address Length Indicator  |       m             
     |--------------------------------------|                     
     |             Source Address           |     m+1 through n-1 
     |--------------------------------------|                     
     |          Data Unit Identifier        |     n,n+1           
     |--------------------------------------|                     
     |             Segment Offset           |     n+2,n+3         
     |--------------------------------------|                     
     |              Total Length            |     n+4,n+5         
     |--------------------------------------|                     
     |                Options               |     n+6 through p-1 
     |--------------------------------------|                     
     |           Reason for Discard         |     p through q-1   
     |--------------------------------------|                     
     |       Error Report Data Field        |       z             
     +--------------------------------------+                     

                     Figure 7-10.  Error Report PDU

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   7.10.1.1 Fixed Part

    The fixed part of the Error Report Protocol Data Unit is set as
    though this is a new (Initial) PDU. Thus, references are provided to
    precious sections describing the composition of the fields
    comprising the fixed part:

    1) Network Layer Protocol Identifier   See Section 7.2.2.
    2) Length Indicator                    See Section 7.2.3.
    3) Version/Protocol Id Extension       See Section 7.2.4.
    4) Lifetime                            See Section 7.2.5.
    5) SP, MS, E/R                         See Section 7.2.6.
    6) Type Code                           See Section 7.2.7.
    7) Segment Length                      See Section 7.2.8.
    8) Checksum                            See Section 7.2.9.

   7.10.1.2 Addresses

    See Section 7.3.

    The Destination Address specifies the original source of the
    discarded PDU. The Source Address specifies the intermediate system
    or end system network-entity initiating the Error Report PDU.

   7.10.1.3 Segmentation

    See Section 7.4.

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   7.10.1.4 Options

    See Section 7.5.

   7.10.1.5 Reason for Discard

    This parameter is only valid for the Error Report PDU. It provides a
    report on the discarded protocol data unit.

    Parameter Code:

     1100 0001

    Parameter Length:

     two octets
     type of error encoded in binary:

      0000 0000:  Reason not specified.
      0000 0001:  Protocol Procedure Error.
                  other than below:
      0000 0010:  Incorrect checksum.
      0000 0011:  PDU discarded due to congestion.
      0000 0100:  Header syntax error (header cannot
                  be parsed).
      0000 0101:  Segmentation is needed but is not
                  permitted.

      1000 xxxx:  Addressing Error:
                  0000 0000:  Destination Address
                              Unreachable.
                  1000 0001:  Destination Address
                              Unknown.

      1001 xxxx:  Source Routing Error:
                  1001 0000:  Unspecified Source
                              Routing error.
                  1001 0001:  Syntax error in Source
                              Routing field.
                  1001 0010:  Unknown Address in
                              Source Routing field.
                  1001 0011:  Path not acceptable.

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      1010 xxxx:  Lifetime Expiration:
                  1010 0000:  Lifetime expired while
                              data unit in transit.
                  1010 0001:  Lifetime expired
                              during reassembly.

      1011 xxxx:  PDU discarded due to unsupported
                  option:
                  1011 0000:  unsupported option not
                              specified.
                  1011 0001:  unsupported padding
                              option.
                  1011 0010:  unsupported security
                              option.
                  1011 0011:  unsupported source
                              routing option.
                  1011 0100:  unsupported recording
                              of route option.
                  1011 0101:  unsupported QoS
                              Maintenance option.

     The second octet contains a pointer to the field in the associated
     discarded PDU which caused the error. If no one particular field
     can be associated with the error, then this field contains the
     value of zero.

   7.10.1.6 Error Report Data Field

    This field provides all or a portion of the discarded PDU. The
    octets comprising this field contain the rejected or discarded PDU
    up to and including the octet which caused the rejection/discard.

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8  FORMAL DESCRIPTION

 The operation of the protocol is modelled as a finite state automaton
 governed by a state variable with three values. The behavior of the
 automaton is defined with respect to individual independent Protocol
 Data Units. A transition of the automaton is prompted by the occurrence
 of an atomic event at one of three interfaces:

  1) an interface to the Transport Layer, defined by the service
      primitives of the Addendum to the Network Service Definition
      Covering Connectionless-mode Transmission;

  2) an interface to the subnetwork service provider, defined by the
      SN_UNITDATA primitive of Section 5.5 of this Standard;

  3) an interface to an implementation-dependent timer function defined
      by the TIMER primitives described in Section 5.6 of this Standard.

 In addition, a transition of the automaton may be prompted by the
 occurrence of a condition of the automaton.

 The atomic events are defined in Section 8.2. The occurrence of an
 atomic event is not in itself sufficient to cause a transition to take
 place; other conditions, called "enabling conditions" may also have to
 be met before a particular transition can take place. Enabling
 conditions are boolean expressions that depend on the values of
 parameters associated with the corresponding atomic event (that is, the
 parameters of some primitive), and on the values of locally maintained
 variables.

 More than one enabling condition -- and therefore, more than one
 possible transition -- may be associated with a single atomic event. In
 every such case, the enabling conditions are mutually exclusive, so
 that for any given combination of atomic event and parameter values,
 only one state transition can take place.

 Associated with each transition is an action, or "output." Actions
 consist of changes to the values of local variables and the sequential
 performance of zero or more functions. The operation of the finite
 state automaton is completely specified in Section 8.3 by defining the
 action associated with every possible transition.

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 8.1  Values of the State Variable

  The protocol state variable has three values:

  1)  INITIAL       The automaton is created in the INITIAL state.  No
                    transition may carry the automaton into the INITIAL
                    state.

  2)  REASSEMBLING  The automaton is in the REASSEMBLING state for the
                    period in which it is assembling PDU segments into a
                    complete PDU.

  3)  CLOSED        The final state of the automaton is the  CLOSED
                    state.  When the automaton enters the CLOSED state
                    it ceases to exist.

 8.2  Atomic Events

  An atomic event is the transfer of a unit of information across an
  interface.  The description of an atomic event specifies a primitive
  (such as an N_UNITDATA.Request), and the service boundary at which it
  is invoked (such as the Network Service boundary). The direction of
  information flow across the boundary is implied by the definition of
  each of the primitives.

  8.2.1  N.UNITDATA_request and N.UNITDATA_indication

   The N.UNITDATA_request and N.UNITDATA_indication atomic events occur
   at the Network Service boundary. They are defined by the Addendum to
   the Network Service Definition Covering Connectionless Data
   Transmission (ISO 8348/DAD1).

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   N.UNITDATA_request    (NS Source_Address,
                          NS_Destination_Address,
                          NS_Quality_of_Service,
                          NS_Userdata)

   N.UNITDATA_indication (NS_Source_Address,
                          NS_Destination_Address,
                          NS_Quality_of_Service, NS_Userdata)

   The     parameters     of     the     N.UNITDATA_request      and
   N.UNITDATA_indication  are  collectively  referred  to as Network
   Service Data Unit (NSDUs).

  8.2.2  SN.UNITDATA_request and SN.UNITDATA_indication

   The SN.UNITDATA_request and SN.UNITDATA_indication atomic events
   occur at the interface between the Protocol described herein and a
   subnetwork service provider. They are defined in Section 5.5 of this
   Standard.

   SN.UNITDATA_request    (SN_Source_Address,
                           SN_Destination_Address,
                           SN_Quality_of_Service,
                           SN_Userdata)

   SN.UNITDATA_indication (SN_Source_Address,
                           SN_Destination_Address,
                           SN_Quality_of_Service,
                           SN_Userdata)

   The parameters of the SN_UNITDATA request and SN_UNITDATA Indication
   are collectively referred to as Subnetwork Service Data Units
   (SNSDUs).

   The value of the SN_Userdata parameter may represent an Initial PDU
   or a Derived PDU.

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  8.2.3  TIMER Atomic Events

   The TIMER atomic events occur at the interface between the Protocol
   described herein and its local environment. They are defined in
   Section 5.6 of this Standard.

    S.TIMER_request  (Time,
                      Name,
                      Subscript)

    S.TIMER_cancel   (Name
                      Subscript)

    S.TIMER_response (Name,
                      Subscript)

 8.3  Operation of the Finite State Automation

  The operation of the automaton is defined by use of the formal
  description technique and notation specified in ISO/TC97/SC16 N1347.
  This technique is based on an extended finite state transition model
  and the Pascal programming language. The technique makes use of strong
  variable typing to reduce ambiguity in interpretation of the
  specification.

  This specification formally specifies an abstract machine which
  provides a single instance of the Connectionless-Mode Network Service
  by use of the Protocol For Providing the Connectionless-Mode Network
  Service. It should be emphasized that this formal specification does
  not in any way constrain the internal operation or design of any
  actual implementation. For example, it is not required that the
  program segments contained in the state transitions will actually
  appear as part of an actual implementation. A formal protocol
  specification is useful in that it goes as far as possible to
  eliminate any degree of ambiguity or vagueness in the specification of
  a protocol standard.

  The formal specification contained here specifies the behavior of a
  single finite-state machine, which provides the protocol

Top       Page 64 
  behavior corresponding to a single independent service request. It is
  expected that any actual implementation will be able to handle
  behavior corresponding to many simultaneous finite state machines.

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  8.3.1  Type and Constant Definitions

   const

    ZERO  = 0;
    max_user_data = 64512;

   type

    NSAP_addr_type  = ...;

     { NSAP_addr_type defines the data type for NSAP addresses, as
     passed across the Network Service Boundary. }

    NPAI_addr_type  = ...;

     { NPAI_addr_type defines the data type for the addresses carried in
     PDUs. }

    SN_addr_type    = ...;

     { SN_addr_type defines the data type for addresses in the
     underlying service used by this protocol. }

    quality_of_service_type = ...;

     { Quality_of_service_type defines the data type for the QOS
     parameter passed across the Network Service boundary. }

    SN_QOS_type     = ...;

     { SN_QOS_type defines the data type for the QOS parameter, if any,
     passed to the underlying service used by this protocol. }

    data_type       = ...;

     { Data_type defines the data type for user data. Conceptually this
     is equivalent to a variable length binary string. }

    buffer_type     = ...;

     { Buffer_type defines the data type for the memory resources used
     in sending and receiving of user data.  This provides capabilities
     required for segmentation and reassembly. }

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    timer_name_type = (lifetime_timer);
    timer_data_type = ...;

    network_layer_protocol_id_type = (ISO_8473_protocol_id);
    version_id_type  = (version1);
    pdu_tp_type      = (DT, ER);

    options_type    = ...;

     { Options_type defines the data type used to store the options part
     of the PDU header. }

    subnet_id_type  = ...;

     { The subnet_id_type defines the data type used to locally identify
     a particular underlying service used by this protocol.  In general
     there may be multiple underlying subnetwork (or data link)
     services. }

    error_type      = (NO_ERROR,
                       TOO_MUCH_USER_DATA,
                       PROTOCOL_PROCEDURE_ERROR,
                       INCORRECT_CHECKSUM, CONGESTION,
                       SYNTAX_ERROR,
                       SEG_NEEDED_AND_NOT_PERMITTED,
                       DESTINATION_UNREACHABLE,
                       DESTINATION_UNKNOWN,
                       UNSPECIFIED_SRC_ROUTING_ERROR,
                       SYNTAX_ERROR_IN_SRC_ROUTING,
                       UNKNOWN_ADDRESS_IN_SRC_ROUTING,
                       PATH_NOT_ACCEPTABLE_IN_SRC_ROUTING,
                       LIFETIME_EXPIRED_IN_TRANSIT,
                       LIFETIME_EXPIRED_IN_REASSEMBLY,
                       UNSUPPORTED_OPTION_NOT_SPECIFIED,
                       UNSUPPORTED_PADDING_OPTION,
                       UNSUPPORTED_SECURITY_OPTION,
                       UNSUPPORTED_SRC_ROUTING_OPTION,
                       UNSUPPORTED_RECORDING_OF_ROUTE_OPTION,
                       UNSUPPORTED_QOS_MAINTENANCE_OPTION);

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   nsdu_type = record
                   da   : NSAP_addr_type;
                   sa   : NSAP_addr_type;
                   qos  : quality_of_service_type;
                   data : data_type;
                end;

   pdu_type = record
                   nlp_id   : network_layer_protocol_id_type;
                   hli      : integer;
                   vp_id    : version_id_type; lifetime : integer;
                   sp       : boolean;
                   ms       : boolean;
                   er_flag  : boolean;
                   pdu_tp   : pdu_tp_type;
                   seg_len  : integer;
                   checksum : integer;
                   da_len   : integer;
                   da       : NPAI_addr_type;
                   sa_len   : integer;
                   sa       : NPAI_addr_type;
                   du_id    : optional integer;
                   so       : optional integer;
                   tot_len  : optional integer;
                      { du_id, so, and tot_len are present
                       only if sp has the value TRUE. }
                   options  : options_type;
                   data     : data_type;
                end;

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   route_result_type =
               record

            subnet_id    : subnet_id_type;
            sn_da        : SN_addr_type;
            sn_sa        : SN_addr_type;
            segment_size : integer;
         end;

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  8.3.2  Interface Definitions

   channel Network_access_point (User, Provider);

    by User:
        UNITDATA_request
           (NS_Destination_address : NSAP_addr_type;
            NS_Source_address      : NSAP_addr_type;
            NS_Quality_of_Service  : quality_of_service_type;
            NS_Userdata            : data_type);

    by Provider:
        UNITDATA_indication
           (NS_Destination_address : NSAP_addr_type;
            NS_Source_address      : NSAP_addr_type;
            NS_Quality_of_Service  : quality_of_service_type;
            NS_Userdata            : data_type);

   channel Subnetwork_access_point (User, Provider);

    by User:
        UNITDATA_request
           (SN_Destination_address : SN_addr_type;
            SN_Source_address      : SN_addr_type;
            SN_Quality_of_Service  : SN_QOS_type;
            SN_Userdata            : pdu_type);

    by Provider:
        UNITDATA_indication
           (SN_Destination_address : SN_addr_type;
            SN_Source_address      : SN_addr_type;
            SN_Quality_of_Service  : SN_QOS_type;
            SN_Userdata            : pdu_type);

   channel System_access_point (User, Provider);

    by User:
        TIMER_request
           (Time      : integer;
            Name      : timer_name_type;
            Subscript : integer);


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