rfc971.txt

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   destination host to avoid passing around data that does not conform
   to the local word boundaries.  Another advantage is that it provides
   flexibility for future expansion.  Since the overall length is a part
   of the type definition, it allows a host to deal with or ignore data
   of types that it does not necessarily understand.  Since the
   interpretation of the data is not dependent on its position, message
   fields (or parameters) can be reordered, or optionally omitted.  The
   disadvantages of this approach are as follows.  Assuming that no
   field could be omitted, the external representation of the message
   may be longer than it would have been if an implicit representation
   had been used.  In addition, extra time may be consumed by the
   conversion between external format and local format, since the
   external format almost certainly will not match the local format for
   any of the participants.











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RFC 971                                                     January 1986
A Survey of Data Representation Standards


4. Data Representation Standards Scorecard

   The following table is a comparison of the data elements defined for
   the various standards being discussed.  It is provided in order to
   give a general idea of the types defined for each standard, but it
   should be noted that the grouping of these types does not indicate
   one type corresponds exactly to any other.  Where it is applicable,
   the identifier code appears in parantheses following the name of the
   data element.  Under "NUMBER", "S" stands for signed, "U" stands for
   unsigned, "V" stands for variable, and the number represents the
   number of bits.  For example, "Integer S16" means a "signed 16-bit
   integer".

   
 Type       CCITT        MMM         NBS         XEROX       Sun
 -----------------------------------------------------------------------
 END    | End-of-   | ENDLIST   | End-of-    |    --     |    --
        |  Contents |   (11)    | Constructor|           |
        |    (0)    |           |    (1)     |           |
        |           |           |            |           |
 PAD    | Null (5)  | NOP (0)   | No-Op (0)  |    --     |    --
        |           | PAD (1)   | Padding    |           |
        |           |           |   (33)     |           |
        |           |           |            |           |
 RECORD | Set (17)  | PROPLIST  | Set (11)   |    --     |    --
        |           |   (14)    |            |           |
        | Sequence  | LIST (9)  | Sequence   | Sequence  | Structure
        |   (16)    |           |   (10)     |           |
        |           |           |            | Record    |
        |           |           | Message    |           |
        |           |           |   (77)     |           |
        |    --     |    --     |     --     | Array     | Fixed Array
        |           |           |            |           | Counted Array
        | "Choice"  |    --     |     --     | Choice    |Discriminated-
        | "Any"     |           |            |           |   Union
        |           |           |            |           |
        | "Tagged"  | "name"    | Field (76) |    --     |    --
        |           |           |Unique-ID(9)|           |
        |    --     | SHARE-TAG |     --     |    --     |    --
        |           |   (12)    |            |           |
        |           | SHARE-REF |            |           |
        |           |   (13)    |            |           |
        |           |           |            |           |
        |    --     |    --     | Compressed |    --     |    --
        |           |           |   (70)     |           |
        |    --     | ENCRYPT   | Encrypted  |    --     |    --
        |           |   (14)    |    (71)    |           |


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RFC 971                                                     January 1986
A Survey of Data Representation Standards


 Type       CCITT        MMM         NBS         XEROX       Sun
 -----------------------------------------------------------------------
 BOOLEAN| Boolean(1)| BOOLEAN(2)| Boolean(8) | Boolean   | Boolean
        |           |           |            |           |
 NUMBER | Integer(2)| EPI (5)   | Integer(32)| Integer   | Integer
        |   SV      |   SV      |   SV       |   S16     |  S32
        |           | INDEX (3) |            | Cardinal  | Unsigned Int
        |           |   U16     |            |   U16     |  U32
        |           | INTEGER(4)|            |Unspecified|Enumeration
        |           |   S32     |            |   16      |  32
        |           |           |            | Long Int  |Hyper Integer
        |           |           |            |   S32     |  S64
        |           |           |            | Long Card |Uns Hyper Int
        |           |           |            |   U32     |  U64
        |           |           |            |           | Double Prec
        |           |           |            |           |   64
        |    --     | FLOAT (15)|     --     |    --     | Float Pt
        |           |   64      |            |           |   32
        |           |           |            |           |
 BIT-   | Bit String| BITSTR(6) | Bit-String |    --     |    --
  STRING|   (3)     |           |   (67)     |           |
        | Octet-    |    --     |     --     |    --     | Opaque
        |  String(4)|           |            |           |
        |           |           |            |           |
 STRING | IA5 (22)  | TEXT (8)  | ASCII-     | String    | Counted-
        |           |           |  String (2)|           |  Byte String
        |           | NAME (7)  |            |           |
        | Numeric   |           |            |           |
        |   (18)    |           |            |           |
        | Printable |           |            |           |
        |   (19)    |           |            |           |
        | T.61 (20) |           |            |           |
        | Videotex  |           |            |           |
        |   (21)    |           |            |           |















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RFC 971                                                     January 1986
A Survey of Data Representation Standards


 Type       CCITT        MMM         NBS         XEROX       Sun
 -----------------------------------------------------------------------
 OTHER  | UTC Time  |    --     | Date (40)  |    --     |    --
        |   (23)    |           |            |           |
        | Gen Time  |           |            |           |
        |   (24)    |           |            |           |
        |    --     |    --     | Property-  |    --     |    --
        |           |           |   List (36)|           |
        |    --     |    --     |Property(69)|    --     |    --
        |           |           |            |           |
        |    --     |    --     |    --      | Procedure |    --
        |           |           |            |           |
        |    --     |    --     | Vendor-    |    --     |    --
        |           |           |  Defined   |           |
        |           |           |   (127)    |           |
        |           |           | Extension  |           |
        |           |           |   (126)    |           |


5. Conclusions

   Of the standards discussed in this survey, the CCITT approach (X.409)
   has already gained wide acceptance.  For a system that will include a
   number of dissimilar hosts, as might be the case for an Internet
   application, a standard that employs explicit representation, such as
   the CCITT X.409, would probably work well.  Using the CCITT X.409
   standard it is possible to construct most of the data elements that
   are specified for the other standards, with the possible exception of
   the "floating point" type. However, some of the flexibility that has
   been built into this standard, such as the "private-use class" may
   lead to ambiguity and a lack of coordination between implementors at
   different sites.  If a standard such as the CCITT were to be used in
   an Internet experiment a fully defined (but large) subset would
   probably have to be selected.















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RFC 971                                                     January 1986
A Survey of Data Representation Standards


6. References

   [1]  "Message Handling Systems: Presentation Transfer Syntax and
        Notation", Recommendation X.409, Document AP VIII-66-E,
        International Telegraph and Telephone Consultative Committee
        (CCITT), Malaga-Torremolinos, June, 1984.

   [2]  J. Garcia-Luna, A. Poggio, and D. Elliot, "Research into
        Multimedia Message System Architecture", SRI International,
        February, 1984.

   [3]  "Specification for Message Format for Computer Based Message
        Systems", FIPS Pub 98 (also published as RFC 841), National
        Bureau of Standards, January, 1983.

   [4]  J. Postel, "Internet Multimedia Mail Transfer Protocol", USC
        Information Sciences Institute, MMM-11 (RFC-759 revised), March,
        1982.

   [5]  J. Postel, "Internet Multimedia Mail Document Format", USC
        Information Sciences Institute, MMM-12 (RFC-767 revised), March,
        1982.

   [6]  "Extended Data Representation Reference Manual", SUN
        Microsystems, September, 1984.

   [7]  "Courier: The Remote Procedure Call Protocol", XSIS-038112,
        XEROX Corporation, December, 1981.





















DeSchon                                                         [Page 9]