tech-invite   World Map     

IETF     RFCs     Groups     SIP     ABNFs    |    3GPP     Specs     Glossaries     Architecture     IMS     UICC    |    search

RFC 0806

 
 
 

Proposed Federal Information Processing Standard: Specification for message format for computer based message systems

Part 2 of 3, p. 35 to 69
Prev RFC Part       Next RFC Part

 


prevText      Top       Page 35 
      4.  SYNTAX


           This section begins with an introduction to the concepts and
      elements  that  constitute  the  syntax for messages.  The second
      section presents an overview of the encoding scheme.   The  third
      section describes in detail the elements of the message syntax.



      4.1  Introduction


           This   specification   defines  syntactic  requirements  for
      messages when they are passed from one  CBMS  to  another.    The
      specification is designed to meet the following goals.


        o  Provide a concise flexible representation scheme.

        o  Simplify message parsing.

        o  Support non-textual components in messages (for example,
                                         3
           facsimile, graphics, or speech ).


      4.1.1  Message structure


           Messages   have   two  classes  of  components,  fields  and
      messages.  A field corresponds to one of the semantic  components
      defined  in  this  message  format  specification.   A message is
      simply another message.

           The type of a field in a message determines both its meaning
      and the form for its contents.  (See Section 4.3.2.)

           Fields in a  message  are  composed  of  syntactic  elements
      called  data  elements.    A  Message  data  element  is  used to
      represent messages; a Field data element  is  used  to  represent
      fields.    (The  term  "field"  is  simply  a semantic construct,
      distinct  from  "Field  Data  Element",  which  is  a   syntactic

      _______________

        3
         While  this message format specification is not intended to be
      used as a basis for the intnge of all facsimile information,
      it does  recognize  that  CBMS  messages  may  contain  facsimile
      components.

Top       Page 36 
      construct.)    Many  of the fields defined in this message format
      specification estricted to containing only one kind of  data
      element.  (See Section 4.3.2.)

           Each  field defined in this message format specification has
      been assigned  a  unique  numeric  identifier  that  is  used  in
      conjunction  with  the  Field data element.  Separate identifiers
      are provided for vendor-defined  fields  and  for  extending  the
      identifier  encoding  space.    A  list of fields and identifiers
      appears in Section 4.3.2 and in Appendix C.

           Throughout the  message  format  specification,  fields  are
      referred   to   by  label  name  rather  than  by  their  numeric
      identifiers.  Field labels are  names  like  "Sender",  "Warning-
      Date",  or  "Circulate-To".    The  field  labels  chosen for the
      specification are names that are in common use in current  CBMSs.
      The  specification  does  not  require  a CBMS to use these field
      labels in displaying fields to the user, although such  usage  is
      encouraged to provide a common user interface.  


      4.1.2  Data elements


           For  the  purpose  of determining compliance with the syntax
      defined in this specification, data elements are divided into two
      groups, basic and optional.


      BASIC     All  message  receiving  systems  must  process   these
                syntactic elements, interpreting their values according
                to the message format specification.

      OPTIONAL  Message   receiving  systems  need  not  process  these
                syntactic elements in order to be in compliance.


           In  addition,  complying  CBMSs   must   meet   requirements
      regarding  their  ability  to process the components found inside
      data elements.   These  requirements  are  discussed  in  Section
      4.2.2.  (Semantic compliance is defined in Section 3.1.2.)

           This  message  format  specification classifies data element
      types as  either  primitives  or  constructors.    (See  Sections
      4.1.2.1  and  4.1.2.2.)   Primitive data elements, such as ASCII-
      String, are basic building blocks.   Constructor  data  elements,
      such  as  Message  or  Sequence, contain one or more primitive or
      constructor data elements.  Some constructors, such as  Sequence,
      may  be  composed  of  any  other  data  element.   Some, such as
      Message, may contain only certain data elements.    (See  Section
      4.3.1.)

Top       Page 37 
      4.1.2.1  Primitive data elements

           A   primitive   data   element  contains  a  basic  item  of
      information; it is not composed  of  other  data  elements.    In
      current  CBMSs,  the most commonly used primitive data element is
      ASCII-String, a series of ASCII characters.  Other primitive data
      elements are Integer,  2's  complement  integers;  Bit-String,  a
      series of bits; and Boolean, either True or False.

           One primitive data element, End-Of-Constructor, is used only
      as  a structural element within constructor data elements and has
      no meaning by itself.  End-of-Constructor is used to  provide  an
      end  marker  for  constructor  data  elements that do not have an
      explicit length.  (See Section 4.2.2.1.)  Any other  use  is  not
      valid syntactically.


      4.1.2.2  Constructor data elements
         The  Data  Element  Contents  of  constructor  data elements
      contain one or more data elements.  The most general  form  of  a
      constructor is a Sequence or a Set, since both Sequences and Sets
      may contain any data element.  Other constructors are specialized
      forms of sequences.

           A  Message  data  element  is a constructor.  It may contain
      only  Field  data  elements,  other  Message  data  elements,  or
      encrypted  or  data  compressed forms of these elements.  A Field
      data element can contain any data element.    It  also  indicates
      which  specific field is being represented.  The contents of some
      fields are restricted to a single type of data element,  such  as
      ASCII-String or Date.


      4.1.3  Properties


           Any  data  element  may  have associated with it a Property-
      List, which contains properties such as a Printing-Name  (Section
      4.1.3.1)  or one or more Comments (Section 4.1.3.2).  A mechanism
      to support vendor-defined properties has been  supplied  by  this
      specification,  as  well  as  a  mechanism  to extend the list of
      property identifiers.


      4.1.3.1  Printing-names

           Printing-Names are  used  to  provide  labels  that  can  be
      displayed  along  with  their  respective  data  elements.    For
      example, a message originator may use a Printing-Name property to
      request that the To field of a message be labeled "Distribution:"
      when it is printed by its recipients.

Top       Page 38 
      4.1.3.2  Comments

           The  Comment  property  is  used  to  allow  comments  to be
      associated with any data element  without  affecting  its  actual
      contents.    For example, someone reviewing the text of a message
      could add the comment "This looks good" to the Text field without
      either altering the body itself  or  adding  a  separate  comment
      field.


      4.1.4  Data compression and encryption


           Two  constructor  data  elements,  Compressed and Encrypted,
      have  been  provided  for  use  by  a  CBMS  that  supports  data
      compression  or  encryption.    They  may  be  used  to  hold the
      compressed or encrypted contents of any data  element,  including
      Messages  and  Fields, and may occur wherever their compressed or
      encrypted contents may appear.  A mechanism is included to  allow
      the user to identify the encryption or compression algorithm used
      (Sections 4.3.4 and 4.3.5).


      4.1.5  Data sharing


           Data  sharing  is  the  multiple  use  of a data element via
      references to a single copy.  It is used in two situations.


        o  For economy when a large object appears more  than  once
           in  a  message.    Data  sharing  may  be  used  in this
           situation  to  economize  on  storage  and  transmission
           costs.

        o  For  consistency  when the same object appears more than
           once in a message.  If one instance of  that  object  is
           altered, all instances must reflect this alteration.  In
           this  case  several  copies  of the same object will not
           serve the purpose as well as data sharing.


           While there is a demonstrable need for facilities to support
      data  sharing,  this  specification  does  not  define   such   a
      mechanism.    At  this time there is insufficient experience with
      data  sharing  in  messages  to  allow  standardization.      The
      specification   is   sufficiently   flexible   however  to  allow
      extensions to the syntax for supporting data sharing at  a  later
      time.

Top       Page 39 
      4.2  Overview of Syntax Encoding


           This  section  provides  an  overview  of  the  notation and
      terminology  used  to  represent  the  syntactic  elements  (data
      elements) defined in this message format specification.

           All  data  elements consist of a series of components.  Each
      of the components is composed of a series of 8-bit groups  called
      octets.    In  this document, the bits are numbered starting from
      the low-order bit.  That is, the low-order (or least significant)
      bit is called "bit 0" and the high-order  (or  most  significant)
      bit is called "bit 7".  

           Five different components may appear in a data element.


        o  Identifier  octet  (identifying  particular type of data
           element)

        o  Length Code (specifying number  of  octets  that  appear
           following it in a data element)

        o  Qualifier (supplying additional identifying information)

        o  Property-List  component  (a  Property-List data element
           containing Property data elements)

        o  Data Element Contents (containing  actual  data  of  the
           data element)


      These components always appear in this order.  Not all components
      are  present  in  all  data  elements but the components that are
      present maintain this relative order.


      4.2.1  Identifier Octets


           The  identifier  octet  is   a   numeric   code   containing
      information  that  identifies  a  data element.  It is always the
      first component in a data element.  The Identifier octet contains
      a one-bit flag,  indicating  whether  or  not  the  data  element
      contains  a  Property-List, and a seven-bit unique identifier for
      the data element.  The value of the data element identifier  also
      indicates  whether  the data element has a Qualifier.  (See Table
      2.)

Top       Page 40 
          Bit Value     Meaning

           7    0   The data element does not have properties
                      associated.
                1   The data element has properties associated.

           6    0   The data element does not have a Qualifier.
                1   The data element has a Qualifier.



          TABLE 2.  TYPE BITS IN THE IDENTIFIER OCTET





           The  most significant bit (Bit 7) of the identifier octet is
      set to 1  if  there  are  properties  associated  with  the  data
      element;  it  is  set  to  0  if  there  are  none.   This bit is
      independent of the remaining seven bits in the identifier  octet,
      which   are   called   the   identifier,   and   provide   unique
      identification for data elements.  The associated properties  are
      specified in a Property-List component.

           The  second  most  significant bit (Bit 6) of the identifier
      octet  (the  most  significant  bit  of  the  identifier  itself)
      signifies  whether  or  not the data element has a Qualifier.  If
      the bit is set to 1, then the data element has a Qualifier; if it
      is a 0, the data element does not have a Qualifier.    The  seven
      bits  of the identifier uniquely identify the data element.  (See
      Figure 4.)

           Data elements all have a Length Code  component  immediately
      following the identifier octet.  (See 4.2.2.1.)


      4.2.2  Length code and Qualifier components


           The Length Code and the Qualifier are both usually one octet
      in  length.    They use an encoding scheme that permits extending
      the component to the size necessary to represent  the  length  of
      the data element or the value of the Qualifier component.

           The  most  significant  bit  of the Length Code or Qualifier
      components determines whether it is  one  or  several  octets  in
      length.  When the most significant bit is 0, the component is one

Top       Page 41 
      -----------------------------------------------------------------


           bit 7 6 5 4 3 2 1 0
              +---------------+
              |P 0 x x x x x x|     P0xxxxxx uniquely identifies a
              +---------------+     data element without a Qualifier.

              +---------------+
              |P 1 x x x x x x|     P1xxxxxx uniquely identifies a
              +---------------+     data element with a Qualifier.



      FIG. 4.  STRUCTURE OF IDENTIFIER OCTETS


      -----------------------------------------------------------------


      octet  in  length.  When the most significant bit is 1, the other
      seven bits of the first octet encode the number of octets in  the
      rest of the component.  The actual value begins in the next octet
      and is interpreted as an unsigned integer.

           A  single  octet  is  sufficient  for  most  Length Code and
      Qualifier components.  For those cases where  the  value  of  the
      Length  Code  or  the  Qualifier  must be greater than 127, extra
      octets can be added, up to a maximum of 127  octets.    Figure  5
      shows  the encoding scheme, as well as an example of a value less
      than 127 and one greater than 127.

           In order to comply with this message  format  specification,
      CBMSs  must  be able to determine the value of any length code or
      qualifier that is expressed  in  three  octets  or  less.    (The

       16
      2  -1).  This message format specification places  no  limitation
      on  the  value  of a length code or qualifier generated by a CBMS
      (except   for   the   absolute   limitation   inherent   in   the
      representation  scheme).    However,  the use of length codes and

       32
      2  -1)  should  be  avoided unless it is known that the receiving
      system can handle them.  

           Both Length Codes and Qualifiers have a  special  convention
      for  dealing  with  special situations.  Length Codes can specify
      that a data element had indeterminate  length;  a  Qualifier  can
      specify  that  a  data  element is implementation defined.  These
      cases are explained further in Sections 4.2.2.1 and 4.2.2.2.

Top       Page 42 
      -----------------------------------------------------------------


           bit 7 6 5 4 3 2 1 0
              +---------------+
              |0 x x x x x x x|                   xxxxxxx is the value.
              +---------------+

              +---------------+------//-------+
              |1 n n n n n n n|y y y y y y y y|          nnnnnnn is the
              +---------------+------//-------+        number of octets
                                                       that contain the
                                                        value yyyyyyyy.

              +---------------+
              |0 0 0 0 1 0 0 1|               This is an example with a
              +---------------+                   value of 9 (decimal).

              +---------------+---------------+
              |1 0 0 0 0 0 0 1|1 0 0 0 0 0 1 0|      This example has a
              +---------------+---------------+   value of 130 decimal.


      FIG. 5.  ENCODING MECHANISM FOR QUALIFIERS AND LENGTH CODES


      -----------------------------------------------------------------


      4.2.2.1  Length Codes

           The  Length Code indicates the number of octets following it
      in a data element (that is, excluding the  identifier  octet  and
      the  length  code  itself).   Length Codes appear in one of three
      formats, short, long, and indefinite.

           A short Length Code is one octet long.  Its most significant
      bit (Bit 7) is set to 0 and its value is in the range  0  through
      127.

           A  long  Length Code is at least two octets long.  The first
      octet always has its most significant bit (Bit 7) set to 1.   The
      other  seven  bits  of  this  octet  contain the number of octets
      making up the rest of the Length Code and  these  octets  contain

        1016
      (2     - 1) (that is, 127 octets to represent the value).

           An indefinite Length Code is  one  octet  long.    Its  most
      significant bit (Bit 7) is set to 1 and its other bits are all 0.
      (See  Figure  6.)    An indefinite Length Code may appear only as

Top       Page 43 
      -----------------------------------------------------------------


           bit 7 6 5 4 3 2 1 0
              +---------------+
              |0 x x x x x x x|             xxxxxxx is the value of the
              +---------------+                            length code.

              +---------------+------//-------+
              |1 n n n n n n n|y y y y y y y y|   nnnnnnn is the number
              +---------------+------//-------+  of octets that contain
                                                the value of the length
                                            code; these are represented
                                                            as yyyyyyy.
              +---------------+
              |1 0 0 0 0 0 0 0|            The "indefinite" length code
              +---------------+


      FIG. 6.  REPRESENTATION OF LENGTH CODES


      -----------------------------------------------------------------


      part  of  a  constructor  data  element;  it  may  not occur in a
                            4
      primitive data element .  A  constructor  data  element  with  an
      indefinite  length code has an End-Of-Constructor data element as
      the last data element in its Data Element Contents.  (The  length
      of  such  a  constructor data element is unrestricted although it
      must contain at least one data element -- the  End-of-Constructor
      that terminates it -- in its Data Element Contents.)

           Figure  7  shows  the Length Codes for three elements; their
      values are 38, 201, and 300.


      4.2.2.2  Qualifier

           The Qualifier component of a data element is used to provide
      information essential to the interpretation of the  data  element
      contents  that  is beyond that encoded in the identifier octet or
      length code.  For example, the identifier octet could contain the

      _______________

        4
         This is the result of most primitive elements  being  able  to
      contain  any  bit  pattern  (including the identifier for End-Of-
      Constructor).

Top       Page 44 
      -----------------------------------------------------------------


            +--------+
            |00100110|                          Length code for 38
            +--------+

            +--------+--------+
            |10000001|11001001|                 Length code for 201
            +--------+--------+

            +--------+--------+--------+
            |10000010|00000001 00101100|        Length code for 300
            +--------+--------+--------+


      FIG. 7.  EXAMPLES OF LENGTH CODES


      -----------------------------------------------------------------


      code  for  a field and the Qualifier component would specify what
      kind of field.

           The Qualifier component appears in only a few data elements.
      In the Bit-String data element, it indicates the number of unused
      bits in the final octet of the Data Element  Contents.    In  the
      Field  and  Property  data  elements, it indicates which field or
      property the data element represents.    In  the  Compressed  and
      Encrypted  data  elements,  it  indicates  which  compression  or
      encryption algorithm has been used.  In the Message data element,
      it indicates the type of message.

           In the sequence of data element  components,  the  Qualifier
      occurs  between the Length Code and the Property-List components.
      The length of the Qualifier component depends on the encoding  of
      the  Qualifier.   (See Figure 8.)  A short Qualifier is one octet
      long.  Its most significant bit is 0 and  its  value  is  in  the
      range  0 through 127.  A long Qualifier is at least two octets in
      length.  The most significant bit is always 1  and  the  other  7
      bits indicate the number of octets in the value of the Qualifier.

           This  message format specification allows implementations to
      define  their  own  values  for  Qualifiers.    A  vendor-defined
      Qualifier  is  any long Qualifier in which the first octet in the
      value is 0.  The value used to identify  this  Qualifier  is  not
      guaranteed  to  be  unique  and  the  same  value  may be used by
      different implementations to define different Qualifiers.

Top       Page 45 
      -----------------------------------------------------------------



               +--------+
               |00011011|            Qualifier with value 28 (decimal).
               +--------+


               +--------+--------+--------+
               |10000010|00000001 00001010|        Qualifier with value
               +--------+--------+--------+              266 (decimal).

               +--------+--------+--------+--------+
               |10000011|00000000|00000001 00001010|     Vendor-Defined
               +--------+--------+--------+--------+     Qualifier with
                                                             value 266.

               +--------+
               |10000000|              Undefined value for a Qualifier.
               +--------+



      FIG. 8.  EXAMPLES OF QUALIFIER VALUES


      -----------------------------------------------------------------


      4.2.3  Property-List


           A  Property  is  an  attribute  being associated with a data
      element.  The properties currently defined by this message format
      specification are Printing-Name and  Comment.    A  Property-List
      component  of  a  data  element is represented by a Property-List
      data element that in turn contains Property data elements.

           A data element contains at most one Property-List.  The most
      significant bit in the  identifier  octet  of  the  data  element
      indicates  whether  a  Property-List  is  present.   (See Section
      4.2.1.)


      4.2.4  Data Element Contents


           The Data Element Contents component of a data element is the
      actual data or information represented by a data element.    (The
      other  components  provide  the information necessary to identify
      and interpret the Data Element Contents.)

Top       Page 46 
           In  a primitive data element, the Data Element Contents is a
      series of octets interpreted according to  the  identifier  octet
      and any qualifier.

           In  a constructor data element, the Data Element Contents is
      a series of data elements.  When the Length Code component  of  a
      constructor  data  element is "indefinite", the last data element
      in the constructor's Data Element Contents is End-of-Constructor.

           The length of the Data Element Contents (in octets)  is  the
      difference  between  the  value of the Length Code and the sum of
      the following:


        o  the length of the Qualifier component  (depends  on  the
           data element)

        o  the length of the Property-List component



      4.3  Data Element Syntax


           This  message  format  specification  defines  nineteen (19)
      different data elements.  Section 4.3.1 defines the encoding form
      for data elements  in  general  and  the  syntax  for  each  data
      element.    Section  4.3.2  describes  the  use  of specific data
      elements as part of the Data Element Contents  of  a  Field  data
      element.   A summary of the syntactic form appears in Appendix F;
      summaries of the data element syntax appear in Appendix G.


      4.3.1  Data elements


           This section presents the general  syntactic  form  for  all
      data  elements  defined  by this message format specification and
      the detailed syntax for each data element.  The data elements are
      presented by syntactic class: primitive  data  elements  (Section
      4.3.1.1), and constructors (Section 4.3.1.2).

           For  convenience,  the following terminology is used in this
      section.

Top       Page 47 
                  Term            Meaning

              Primitive       a Primitive Data Element

              Constructor     a Constructor Data Element

              Element         any Data Element


           The  syntax  of  each  Element is presented in graphic form.
      The following conventions apply in the diagrams.  A single  octet
      is represented as follows.


          +--------+
          |        |
          +--------+


           Components that vary in length are represented as follows.


          +---//---+
          |        |
          +---//---+


           Each  Element  has  up to five components:  an Identifier, a
      Length Code, a Qualifier, a Property-List and  the  Data  Element
      Contents.  (See Section 4.2.)

           In  the  diagrams,  the  contents of the identifier octet is
      shown as a "P" followed by an identifier represented  in  binary.
      (See  Figure  4.)  The identifier itself is a seven bit quantity,
      right justified  in  the  identifier  octet.    Full  details  on
      identifier octets appear in Section 4.2.1.

           A length code is always represented in the following manner:


          +---//---+
          |Lxxxxxxx|
          +---//---+


           A qualifier is always represented in the following manner:


          +---//---+
          |Qxxxxxxx|
          +---//---+

Top       Page 48 
           A Property-List (if present) always immediately precedes any
      occurrence of Data Element Contents.

           The  Data Element Contents appears in diagrams as one of the
      following.


        o  "element(s)", which may be any data element(s)

        o  "anything",  which  is  undefined   and   may   be   any
           combination of bits

        o  a specific data element

        o  the  interpretation to be applied to the bits within the
           octets that constitute the element  (such  as  ASCII  or
           Integer)


           Two  data  elements have been reserved for special purposes.
      The Extension data  element  is  provided  to  allow  for  future
      expansion of the possible data elements.  The Vendor-Defined data
      element  allows  CBMS  vendors to define their own data elements.
      Vendor-Defined data elements are not  guaranteed  to  be  unique,
      since  two  implementations  could define different data elements
      using the same identifier.  Vendor-Defined data  elements  should
      be used and interpreted by prior agreement.

           In  the  following  sections, each element is presented with
      its name, compliance  classification  (BASIC  or  OPTIONAL),  its
      identifier   (both   in   hexadecimal  and  in  octal),  a  brief
      description of its use, and a graphic representation.  Each  data
      element description has the following form.

Top       Page 49 
      -----------------------------------------------------------------
      Data Element             (Compliance)   identifier   identifier
          Name                 ( Category )    octet         octet 
                                                    16            8

                     Description of the syntax of the data element.



                 +---//---+
                 |        |     Diagram representing data element
                 +---//---+




      -----------------------------------------------------------------


      4.3.1.1  Primitives

           The   data   elements   in  this  section  are  arranged  in
      alphabetical order by name.  (Appendix C presents the identifiers
      in numeric order.)
      ASCII-String             (BASIC)        02        002 
                                                16         8
                    This  data  element  contains  a  series  of  ASCII
                characters,   each  character  right-justified  in  one
                octet.    For  seven-bit  ASCII  characters,  the  most
                significant bit of each octet must be 0.


                 +--------+---//---+----//-----+
                 |P0000010|Lxxxxxxx|ASCII chars|
                 +--------+---//---+----//-----+

Top       Page 50 
      Bit-String               (OPTIONAL)     43        103 
                                                16         8
                This  data  element contains a series of bits.  It uses
                the Qualifier data  element  component  to  record  the
                number  of  bits  of  padding (as an eight bit unsigned
                integer) needed to fill the final  octet  of  the  Data
                Element  Contents  to  an  even  octet boundary.  These
                padding bits have no meaning and occur in the low order
                bits of the final octet.   The  valid  values  for  the
                Qualifier  component  are  0  through 7.  The number of
                bits in the Data Element Contents  is  calculated  from
                the following formula.


                8   *   number of octets   -   value of
                        in the Data            Qualifier component
                        Element Contents


                 +--------+---//---+---//---+---//---+
                 |P1000011|Lxxxxxxx|Qxxxxxxx|  bits  |
                 +--------+---//---+---//---+---//---+


      Boolean                  (OPTIONAL)     08        010 
                                                16         8
                This  data  element  contains  one octet whose value is
                either true or false.  False is represented by all bits
                being 0; true  is  represented  by  all  bits  being  1
                (although  any  non-zero value should be interpreted as
                true).


                 +--------+---//---+--------+
                 |P0001000|Lxxxxxxx| T or F |
                 +--------+---//---+--------+


      End-of-Constructor       (BASIC)        01        001 
                                                16         8
                This data element terminates the Data Element  Contents
                in  a  constructor  data  element  that  has indefinite
                length.  This data element has no  Contents  component.
                (Use of this element is described in Section 4.2.2.1.)


                 +--------+---//---+
                 |P0000001|Lxxxxxxx|
                 +--------+---//---+

Top       Page 51 
      Integer                  (OPTIONAL)     20        040 
                                                16         8
                This  data element contains a 2's complement integer of
                variable  length,  high  order  octet  first.    It  is
                recommended  that the data element contents be either 2
                or 4 octets long whenever possible.


                 +--------+---//---+---//---+
                 |P0100000|Lxxxxxxx| Integer|
                 +--------+---//---+---//---+


      No-Op                    (OPTIONAL)     00        000 
                                                16         8
                This data element does nothing.  No-Op is used whenever
                it is necessary to include a data  element  that  means
                "no operation".  It is a short placeholder.


                 +--------+---//---+
                 |P0000000|Lxxxxxxx|
                 +--------+---//---+


      Padding                  (OPTIONAL)     21        041 
                                                16         8
                This data element is used to fill any number of octets.
                The  contents  of  a  Padding element are undefined and
                convey no information.


                 +--------+---//---+---//---+
                 |P0100001|Lxxxxxxx|anything|
                 +--------+---//---+---//---+


      4.3.1.2  Constructors

           The  data  elements  in  this  section   are   arranged   in
      alphabetical order.

Top       Page 52 
      Compressed               (OPTIONAL)     46        106 
                                                16         8
                This  data  element  must  contain  a  Bit-String  data
                element.  It is used to represent  any  data  that  has
                been   compressed;   it   may   be  used  wherever  its
                uncompressed contents may appear.    A  Qualifier  data
                component  appears  in each Compressed data element; it
                contains a  compression identifier  (CID)  to  identify
                the  compression  algorithm used.  (See Section 4.3.5.)
                The Data Element Contents contains the product  of  the
                compression process.


                 +--------+---//---+---//---+--------//--------+
                 |P1000110|Lxxxxxxx|Qxxxxxxx|Bit-String Element|
                 +--------+---//---+---//---+--------//--------+



      Date                     (BASIC)        28        050 
                                                16         8
                This   data   element  contains  an  ASCII-String  data
                element, which is a representation of a date  and  time
                formatted   in   accordance   with   PUBS  4 [NatB-68],
                58 [NatB-79a] and 59 [NatB-79b].


                 +--------+---//---+------//------+
                 |P0101000|Lxxxxxxx| ASCII-String |
                 +--------+---//---+------//------+



      Encrypted                (OPTIONAL)     47        107 
                                                16         8
                This data element must contain a  Bit-String.    It  is
                used  to represent any data that has been encrypted; it
                may be  used  wherever  its  unencrypted  contents  may
                appear.    A  Qualifier  data component appears in each
                Encrypted  data  element;  it  contains  an  encryption
                identifier  (EID)  identifying the encryption algorithm
                used.  (See Section 4.3.4.)  The Data Element  Contents
                is the product of the encryption process.


                 +--------+---//---+---//---+--------//--------+
                 |P1000111|Lxxxxxxx|Qxxxxxxx|Bit-String Element|
                 +--------+---//---+---//---+--------//--------+

Top       Page 53 
      Extension                (OPTIONAL)     7E        176 
                                                16         8
                This  data  element  is  used  to  extend the number of
                available  data  elements  beyond  the  128  that   are
                possible   using  a  7-bit  identifier.    A  Qualifier
                component extends the encoding space  for  identifiers.
                (Extension and Vendor-Defined have the same syntax.)


                 +--------+---//---+---//---+---//---+
                 |P1111110|Lxxxxxxx|Qxxxxxxx|Anything|
                 +--------+---//---+---//---+---//---+


      Field                    (BASIC)        4C        114 
                                                16         8
                This   data  element  uses  a  Qualifier  data  element
                component.  The Qualifier component  contains  a  Field
                Identifier  (FID)  indicating  which  specific field is
                being represented.  (See Section 4.3.2.)


                 +--------+---//---+---//---+---//---+
                 |P1001100|Lxxxxxxx|Qxxxxxxx|elements|
                 +--------+---//---+---//---+---//---+


      Message                  (BASIC)        4D        115 
                                                16         8
                This data element may contain  Field  or  Message  data
                elements.    Its Qualifier component contains a Message
                type (MID) indicating the type of the  message.    (See
                Section  4.3.6.)  (The MID is completely different from
                the message identifier  in  the  Message-ID  field  and
                should not be confused with it.)


                 +--------+---//---+---//---+
                 |P1001101|Lxxxxxxx|Qxxxxxxx|
                 +--------+---//---+---//---+

                 +--------//---------//---------//---------//--------+
                 | Field, Message, Encrypted, or Compressed Elements |
                 +--------//---------//---------//---------//--------+

Top       Page 54 
      Property-List            (OPTIONAL)     24        044 
                                                16         8
                This  data  element  contains a series of Property data
                elements to be associated another data element.


                 +--------+---//---+-------//--------+
                 |P0100100|Lxxxxxxx|Property Elements|
                 +--------+---//---+-------//--------+


      Property                 (OPTIONAL)     45        105 
                                                16         8
                This  data  element  uses  a  Quali data   element
                component.       The   Qualifier   component   contains
                a  Property-Identifier (PID) to indicate which specific
                property is being represented.  (See Section 4.3.3.)


                 +--------+---//---+---//---+---//---+
                 |P1000101|Lxxxxxxx|Qxxxxxxx|elements|
                 +--------+---//---+---//---+---//---+


      Sequence                 (OPTIONAL)     0A        012 
                                                16         8
                This data element contains any series of data elements.
                Sequence differs from Set in  that  the  data  elements
                making  up the Data Element Contents must be considered
                as an ordered sequence (according  to  their  order  of
                appearance in the sequence.)


                 +--------+---//---+---//---+
                 |P0001010|Lxxxxxxx|elements|
                 +--------+---//---+---//---+


      Set                      (OPTIONAL)     0B        013 
                                                16         8
                This  data element contains any series of data elements
                with no ordering of the elements  implied.    (Sequence
                provides   an  ordered  series.)    Although  the  data
                elements  contained   in   a   Set   must   be   stored
                sequentially, the order in which they are stored is not
                defined and not processed.


                 +--------+---//---+---//---+
                 |P0001011|Lxxxxxxx|elements|
                 +--------+---//---+---//---+

Top       Page 55 
      Unique-ID                (OPTIONAL)     09        011 
                                                16         8
                This  data element is a unique identifier.  It need not
                be human-readable.  The Data Element Contents may be an
                ASCII-String, a Bit-String, or an Integer.


                 +--------+---//---+---//---+
                 |P0001001|Lxxxxxxx| element|
                 +--------+---//---+---//---+


      Vendor-Defined           (OPTIONAL)     7F        177 
                                                16         8
                This data element is  used  to  represent  vendor-  and
                user-defined  data  elements.    A  Qualifier component
                extends  the  encoding  space  for  identifiers.    The
                Qualifier  component  is  not  guaranteed  to be unique
                among all interconnected systems.  This data element is
                interpreted  according  to  prior   agreement   between
                systems.    (Extension and Vendor-Defined data elements
                have the same syntax.)


                 +--------+---//---+---//---+---//---+
                 |P1111111|Lxxxxxxx|Qxxxxxxx|Anything|
                 +--------+---//---+---//---+---//---+


      4.3.2  Using data elements within message fields


           The Data Element Contents of a particular field in a message
      must contain at least one  data  element.    The  types  of  data
      elements  that can appear in the Data Element Contents of a field
      are restricted according to what kind of field it is.  Appendix A
      (the master reference appendix for  fields) nes  which  data
      elements are valid as the Contents for each of the fields.

           Some  fields  have  a  Data  Element  Contents that contains
      "originators" or "recipients."  No data  element  represents  the
      identities of originators or recipients (because that encoding is
      not  within  the  scope  of  this  message format specification.)
      These descriptions simply  list  "originators"  or  "recipients",
      implying  no  restrictions on how the identifiers for originators
      or recipients are represented.

Top       Page 56 
      4.3.3  Properties and associated elements


           This message format specification defines two properties.
      Comment                                 01        001 
                                                16         8
                This  property may contain any series of data elements;
                it most commonly contains one or more ASCII-Strings.
      Printing-Name                           02        002 
                                                16         8
                This property contains one ASCII-String.  In this case,
                the ASCII-String may contain only  the  printing  ASCII
                characters plus the "space" character.


      4.3.4  Encryption identifiers


           This  message  format  specification  defines two encryption
      identification codes.
      Unspecified                             00        000 
                                                16         8
                Use of  this  encryption  identifier  as  part  of  the
                Encrypted  data  element  indicates that the encryption
                method being used was not specified  for  inclusion  as
                part of the data element.
      NBS-Standard                            01        001 
                                                16         8
                Use  of  this  encryption  identifier  as  part  of the
                Encrypted data element indicates that the NBS  standard
                method for data encryption [NatB-77] was used.


      4.3.5  Compression identifiers


           This  message  format  specification defines two compression
      identification codes for use with the Compressed data element.
      Unspecified                             00        000 
                                                16         8
                Use of this  compression  identifier  as  part  of  the
                Compressed  data element indicates that the compression
                method being used was not specified  for  inclusion  as
                part of the data element.
      NBS-Standard                            01        001 
                                                16         8
                Use  of  this  compression  identifier  as  part of the
                Compressed data element  is  reserved  at  the  present
                time.    It will be used in the future to indicate that
                the NBS standard method for data compression  was  used
                once the data compression standard is defined.

Top       Page 57 
      4.3.6  Message types


           This message format specification defines message type (MID)
      codes  for use in classifying the type of a message.  The message
      type could  be  confused  with  the  message  identifier  in  the
      Message-Id field; they are completely distinct concepts.
      NBS-Standard                            01        01 
                                                16        8
                This  message  type  marks  messages  defined  by  this
                message format specification.

Top       Page 58 
                            SUMMARY OF APPENDIXES




      Appendix A  Defines    the   fields   in   the   message   format
                  specification.  This  alphabetical  appendix  is  for
                  reference  use by implementors.  It contains semantic
                  definitions of fields from  Section  3.1.    It  also
                  defines  Field  Identifier values and specifies which
                  data elements are valid as the Contents for  each  of
                  the fields.

      Appendix B  Defines  the  data  elements  in  the  message format
                  specification.  This alphabetically ordered  appendix
                  is   for   reference   use   by   implementors.    It
                  consolidates information from Section 4.3.

      Appendix C  Provides a reference table listing the data  elements
                  in numerical order by their identifier octets.

      Appendix D  Provides a reference table summarizing the components
                  of messages according to whether they are required or
                  otional for CBMSs implementing the specification.

      Appendix E  Provides  a  reference  table  organizing the message
                  components according to the functional class  of  the
                  components.

      Appendix F  Provides   an  overview  of  the  syntactic  elements
                  defined by this message format specification.

      Appendix G  Summarizes syntactic elements  according  to  whether
                  they are required or optional for a CBMS implementing
                  the message format specification.

      Appendix H  Examples  of  each syntactic element displaying their
                  syntax and describing their associated semantics.

Top       Page 59 
                                 APPENDIX A
                  FIELDS -- IMPLEMENTORS' MASTER REFERENCE




           This  appendix  defines  all  of  the  fields in the message
      format specification for  reference  use  by  implementors.    It
      contains  semantics  definitions  of fields from Section 3.1.  It
      also defines Field Identifier values and which data elements  are
      valid  as  the  Contents  for  each  of  the  fields.   The field
      definitions appear alphabetically.

           Each field in the list has the following form:


      -----------------------------------------------------------------
      Field Name               Compliance   identifier  identifier
                                              value       value 
                                                   16          8

                   Description of the field semantics.   Names  of
              data  elements  that  are  valid in the Data Element
              Contents of this kind of field.

      -----------------------------------------------------------------


      Attachments              OPTIONAL       08        010 
                                                16         8
                This field  contains  additional  data  accompanying  a
                message.    It  is similar in intent to enclosures in a
                conventional mail system.  Contents of this  field  are
                unrestricted.
      Author                   OPTIONAL       0C        014 
                                                16         8
                This  field  identifies the individual(s) who wrote the
                primary contents of the message.   Use  of  the  Author
                field  is  discouraged  when the contents of the Author
                field and the From field would be completely redundant.
                This field contains one or more originator identities.
      Bcc                      OPTIONAL       0D        015 
                                                16         8
                This  field  identifies  additional  recipients  for  a
                message  (a  "blind carbon copies list").  The contents
                of this field are not to be included in copies  of  the
                message  sent  to the primary and secondary recipients.
                See section 3.2.1 for further discussion of the use  of
                blind  carbon  copies lists. This field contains one or
                more recipient identities.

Top       Page 60 
      Cc                       BASIC          06        006 
                                                16         8
                This   field  identifies  secondary  recipients  for  a
                message (a "carbon copies" list).  This field  contains
                one or more recipient identities.
      Circulate-Next           OPTIONAL       0E        016 
                                                16         8
                This field is used in conjunction with the Circulate-To
                field.    (See  Section  3.2.6.1.)    It identifies all
                recipients in a  circulation  list  who  have  not  yet
                received  the message.  This field contains one or more
                recipient identities.
      Circulate-To             OPTIONAL       0F        017 
                                                16         8
                This  field  identifies  recipients  for  a  circulated
                message.    (See  Section  3.2.6.1.)    It  is  used in
                conjunction with the Circulate-Next field.  This  field
                contains one or more recipient identities.
      Comments                 OPTIONAL       10        020 
                                                16         8
                This  field  permits  adding  comments onto the message
                without  disturbing  the  original  contents   of   the
                message.  While the Comments field will usually contain
                one or more ASCII-Strings, there are no restrictions on
                its contents.
      Date                     OPTIONAL       11        021 
                                                16         8
                This   field   contains   a  date  that  the  message's
                originator wishes to associate with  a  message.    The
                Date field is to the Posted-Date field as the date on a
                letter  is  to  the  postmark added by the post office.
                This field contains one Date.
      End-Date                 OPTIONAL       12        022 
                                                16         8
                This field contains the date on which a  message  loses
                effect.  (See also Section 3.2.5.)  This field contains
                one Date.
      From                     REQUIRED       01        001 
                                                16         8
                This  field  contains  the  identity of the originators
                taking formal responsibility for  this  message.    The
                contents  of  the  From field is to be used for replies
                when no Reply-to field appears  in  a  message.    This
                field contains one or more originator identities.
      In-Reply-To              OPTIONAL       13        023 
                                                16         8
                This  field designates previous correspondence to which
                this message is a reply.  The usual  contents  of  this
                field  would be the contents of the Message-ID field of
                the message(s) being replied to.  This  field  contains
                one or more Unique-IDs or ASCII-Strings.

Top       Page 61 
      Keywords                 OPTIONAL       14        024 
                                                16         8
                This  field  contains  keywords  or  phrases for use in
                retrieving a message.  This field contains one or  more
                ASCII-Strings.   (Each keyword or phrase is represented
                by a separate ASCII-String.)
      Message-Class            OPTIONAL       15        025 
                                                16         8
                This field indicates the purpose of  a  message.    For
                example,  it  might  contain values indicating that the
                message is a memorandum or a  data-base  entry.    This
                field contains one data element, an ASCII-String.
      Message-ID               OPTIONAL       16        026 
                                                16         8
                This  field contains a unique identifier for a message.
                This identifier is intended for machine generation  and
                processing.    Further  definition  appears  in Section
                3.2.4.1.  Only one Message-ID field is permitted  in  a
                message.    This  field  contains  one  data element, a
                Unique-ID.
      Obsoletes                OPTIONAL       26        046 
                                                16         8
                This field identifies one or more  messages  that  this
                one  supplants.    This  field  contains  at  least one
                Unique-ID and may contain more than one.
      Originator-Serial-Number OPTIONAL       17        027 
                                                16         8
                This field contains one or more serial numbers assigned
                by the message's originator.  (Messages  with  multiple
                recipients  should  all  have  the  same  value  in the
                Originator-Serial-Number field.   This  field  contains
                one  or  more ASCII-Strings.  (One ASCII-String is used
                for each serial number.)
      Posted-Date              REQUIRED       02        002 
                                                16         8
                This field contains the  posting  date,  which  is  the
                point  in  time  when  the  message  passes through the
                posting slot into a message transfer system.  Only  one
                Posted-Date  field  is  permitted  in  a message.  This
                field contains one Date.
      Precedence               OPTIONAL       18        030 
                                                16         8
                Ordinarily, message precedence or priority is a service
                request to  a  message  transfer  system.    A  message
                originator, however, can include precedence information
                in  a  message.  This field indicates the precedence at
                which the  message  was  posted.    One  example  of  a
                precedence   scheme   is  the  US  Military  categories
                "ROUTINE", "PRIORITY", "IMMEDIATE",  "FLASH  OVERRIDE",
                and   "EMERGENCY   COMMAND  PRECEDENCE".    This  field
                contains one ASCII-String.

Top       Page 62 
      Received-Date            OPTIONAL       19        031 
                                                16         8
                Delivery date.  This field may be added to a message by
                the   recipient's   message   receiving  program.    It
                indicates when the message left the delivery system and
                entered  the  recipient's  message  processing  domain.
                This field contains one Date.
      Received-From            OPTIONAL       1A        032 
                                                16         8
                This  field  contains  a  record  of  a  message's path
                through   a   message    transfer    system.        The
                recipient's  message  receiving  program  may store any
                such  information  that  it  obtains  from  a   message
                transfer  system  in  this field.  The contents of this
                field are unrestricted.
      References               OPTIONAL       20        040 
                                                16         8
                This field identifies other  correspondence  that  this
                message   references.    If  the  other  correspondence
                contains  a  Message-ID  field,  the  contents  of  the
                References  field must be the message identifier.  This
                field contains one or more Unique-IDs or ASCII-Strings.
      Reissue-Type             OPTIONAL       25        045 
                                                16         8
                This  field  is  used  in  conjunction   with   message
                encapsulating  (see  Section  3.2.2)  to  differentiate
                between messages being assigned or redistributed.  This
                field contains one  data  element,  usually  an  ASCII-
                String.
      Reply-To                 BASIC          03        003 
                                                16         8
                This field identifies any recipients for replies to the
                message.    This  field  contains one or more recipient
                identities.
      Sender                   OPTIONAL       22        042 
                                                16         8
                This field identifies the agent who sent  the  message.
                It  is  intended  either for when the sender is not the
                originator responsible for the message or  to  indicate
                who  among  a  group of originators responsible for the
                message actually sent it.  Use of the Sender  field  is
                discouraged  when  the contents of the Sender field and
                From field would be completely  redundant.    Only  one
                Sender  field  is  permitted  in a message.  This field
                contains one originator identity.
      Start-Date               OPTIONAL       23        043 
                                                16         8
                This field contains the date on which a  message  takes
                effect.  (See also Section 3.2.5.)  This field contains
                one Date.

Top       Page 63 
      Subject                  BASIC          07        007 
                                                16         8
                This field contains whatever information the originator
                provided  to  summarize  or  indicate the nature of the
                message.   This  field  contains  one  or  more  ASCII-
                Strings.
      Text                     BASIC          04        004 
                                                16         8
                This field contains the primary content of the message.
                Contents of this field are unrestricted.
      To                       REQUIRED       05        005 
                                                16         8
                This field identifies primary recipients for a message.
                This field contains one or more recipient identities.
      Warning-Date             OPTIONAL       24        044 
                                                16         8
                This  field is used either alone or in conjunction with
                an End-Date field.  It  contains  one  or  more  dates.
                These  dates  could  be  used  by  a message processing
                program as warnings of an impending end-date  or  other
                event.   (See also Section 3.2.5.)  This field contains
                one or more Dates.

Top       Page 64 
                                 APPENDIX B
               DATA ELEMENTS -- IMPLEMENTORS' MASTER REFERENCE




           The appendix defines all of the data elements in the message
      format  specification,  for  reference  use  by implementors.  It
      contains no new information but rather consolidates the syntactic
      information from Section 4.3.

           Each data element description has the following form.


      -----------------------------------------------------------------

      Data Element        (Compliance)   identifier   identifier
          Name            ( Category )    octet         octet 
                                               16            8

                Constructive class (primitive or constructor)

                Description of the syntax of the data element.



                +---//---+
                |        |     Diagram representing data element
                +---//---+



      -----------------------------------------------------------------


      ASCII-String             (BASIC)        02        002 
                                                16         8
                primitive

                This  data  element  contains   a   series   of   ASCII
                characters,   each  character  right-justified  in  one
                octet.    For  seven-bit  ASCII  characters,  the  most
                significant bit of each octet must be 0.


                 +--------+---//---+----//-----+
                 |P0000010|Lxxxxxxx|ASCII chars|
                 +--------+---//---+----//-----+

Top       Page 65 
      Bit-String               (OPTIONAL)     43        103 
                                                16         8
                primitive

                This  data  element contains a series of bits.  It uses
                the Qualifier data  element  component  to  record  the
                number  of  bits  of  padding (as an eight bit unsigned
                integer) needed to fill the final  octet  of  the  Data
                Element  Contents  to  an  even  octet boundary.  These
                padding bits have no meaning and occur in the low order
                bits of the final octet.   The  valid  values  for  the
                Qualifier  component  are  0  through 7.  The number of
                bits in the Data Element Contents  is  calculated  from
                the following formula.


                8   *   number of octets   -   value of
                        in the Data            Qualifier component
                        Element Contents


                 +--------+---//---+---//---+---//---+
                 |P1000011|Lxxxxxxx|Qxxxxxxx|  bits  |
                 +--------+---//---+---//---+---//---+


      Boolean                  (OPTIONAL)     08        010 
                                                16         8
                primitive

                This  data  element  contains  one octet whose value is
                either true or false.  False is represented by all bits
                being 0; true  is  represented  by  all  bits  being  1
                (although  any  non-zero value should be interpreted as
                true).


                 +--------+---//---+--------+
                 |P0001000|Lxxxxxxx| T or F |
                 +--------+---//---+--------+

Top       Page 66 
      Compressed               (OPTIONAL)     46        106 
                                                16         8
                constructor

                This  data  element  must  contain  a  Bit-String  data
                element.  It is used to represent  any  data  that  has
                been   compressed;   it   may   be  used  wherever  its
                uncompressed contents may appear.    A  Qualifier  data
                component  appears  in each Compressed data element; it
                contains a compression identifier (CID) to identify the
                compression algorithm used.  (See Section 4.3.5.)   The
                Data  Element  Contents  contains  the  product  of the
                compression process.


                 +--------+---//---+---//---+--------//--------+
                 |P1000110|Lxxxxxxx|Qxxxxxxx|Bit-String Element|
                 +--------+---//---+---//---+--------//--------+



      Date                     (BASIC)        28        050 
                                                16         8
                constructor

                This  data  element  contains  an   ASCII-String   data
                element,  which  is a representation of a date and time
                formatted in accordance with FIPS Publications 4 [NatB-
                68], 59 [NatB-79b], and 58 [NatB-79a].


                 +--------+---//---+------//------+
                 |P0101000|Lxxxxxxx| ASCII-String |
                 +--------+---//---+------//------+

Top       Page 67 
      Encrypted                (OPTIONAL)     47        107 
                                                16         8
                constructor

                This  data  element  must  contain a Bit-String.  It is
                used to represent any data that has been encrypted;  it
                may  be  used  wherever  its  unencrypted  contents may
                appear.  A Qualifier data  component  appears  in  each
                Encrypted  data  element;  it  contains  an  encryption
                identifier (EID) identifying the  encryption  algorithm
                used.   (See Section 4.3.4.)  The Data Element Contents
                is the product of the encryption process.


                 +--------+---//---+---//---+--------//--------+
                 |P1000111|Lxxxxxxx|Qxxxxxxx|Bit-String Element|
                 +--------+---//---+---//---+--------//--------+


      End-of-Constructor       (BASIC)        01        001 
                                                16         8
                primitive

                This data element terminates the Data Element  Contents
                in  a  constructor  data  element  that  has indefinite
                length.  This data element has no  Contents  component.
                (Use of this element is described in Section 4.2.2.1.)


                 +--------+---//---+
                 |P0000001|Lxxxxxxx|
                 +--------+---//---+


      Extension                (OPTIONAL)     7E        176 
                                                16         8
                constructor

                This  data  element  is  used  to  extend the number of
                available  data  elements  beyond  the  128  that   are
                possible   using  a  7-bit  identifier.    A  Qualifier
                component extends the encoding space  for  identifiers.
                (Extension and Vendor-Defined have the same syntax.)


                 +--------+---//---+---//---+---//---+
                 |P1111110|Lxxxxxxx|Qxxxxxxx|Anything|
                 +--------+---//---+---//---+---//---+

Top       Page 68 
      Field                    (BASIC)        4C        114 
                                                16         8
                constructor

                This   data  element  uses  a  Qualifier  data  element
                component.  The Qualifier component  contains  a  Field
                Identifier  (FID)  indicating  which  specific field is
                being represented.  (See Section 4.3.2.)


                 +--------+---//---+---//---+---//---+
                 |P1001100|Lxxxxxxx|Qxxxxxxx|elements|
                 +--------+---//---+---//---+---//---+


      Integer                  (OPTIONAL)     20        040 
                                                16         8
                primitive

                This data element contains a 2's complement integer  of
                variable  length,  high  order  octet  first.    It  is
                recommended that the data element contents be either  2
                or 4 octets long whenever possible.


                 +--------+---//---+---//---+
                 |P0100000|Lxxxxxxx| Integer|
                 +--------+---//---+---//---+


      Message                  (BASIC)        4D        115 
                                                16         8
                constructor

                This  data  element  may  contain Field or Message data
                elements.  Its Qualifier component contains  a  Message
                type  (MID)  indicating  the type of the message.  (See
                Section 4.3.6.)  (The MID is completely different  from
                the  message  identifier  in  the  Message-ID field and
                should not be confused with it.)


                 +--------+---//---+---//---+
                 |P1001101|Lxxxxxxx|Qxxxxxxx|
                 +--------+---//---+---//---+

                 +--------//---------//---------//---------//--------+
                 | Field, Message, Encrypted, or Compressed Elements |
                 +--------//---------//---------//---------//--------+

Top       Page 69 
      No-Op                    (OPTIONAL)     00        000 
                                                16         8
                primitive

                This data element does nothing.  No-Op is used whenever
                it  is  necessary  to include a data element that means
                "no operation".  It is a short placeholder.


                 +--------+---//---+
                 |P0000000|Lxxxxxxx|
                 +--------+---//---+


      Padding                  (OPTIONAL)     21        041 
                                                16         8
                primitive

                This data element is used to fill any number of octets.
                The contents of a Padding  element  are  undefined  and
                convey no information.


                 +--------+---//---+---//---+
                 |P0100001|Lxxxxxxx|anything|
                 +--------+---//---+---//---+


      Property-List            (OPTIONAL)     24        044 
                                                16         8
                constructor

                This  data  element  contains a series of Property data
                elements to be associated with another data element.


                 +--------+---//---+-------//--------+
                 |P0100100|Lxxxxxxx|Property Elements|
                 +--------+---//---+-------//--------+


Next RFC Part