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rfc:rfc971

Network Working Group Annette L. DeSchon Request for Comments: 971 ISI

                                                          January 1986
             A SURVEY OF DATA REPRESENTATION STANDARDS

Status of This Memo

 This RFC discusses data representation conventions in the
 ARPA-Internet and suggests possible resolutions.  No proposals in
 this document are intended as standards for the ARPA-Internet at this
 time.  Rather, it is hoped that a general consensus will emerge as to
 the appropriate approach to these issues, leading eventually to the
 adoption of ARPA-Internet standards.  Distribution of this memo is
 unlimited.

1. Introduction

 This report is a comparison of several data representation standards
 that are currently in use.  The standards, or system type
 definitions, that will be discussed are the CCITT X.409
 recommendation, the NBS Computer Based Message System (CBMS)
 standard, DARPA Multimedia Mail system, the Courier remote procedure
 call protocol, and the SUN Remote Procedure Call package.
 One purpose of this report is to determine how the CCITT standard,
 which is gaining wide acceptance internationally, compares with some
 of the other standards that have been developed in the areas of
 electronic mail, distributed interprocess communication, and remote
 procedure call.  The CCITT X.409 recommendation, which is entitled
 "Presentation Transfer Syntax and Notation" is an international
 standard which is a part of the X.400 series Message Handling Systems
 (MHS) specifications [1].  It has been adopted by both the NBS and
 the ISO standards organizations.  In addition, some commercial
 organizations have announced intentions to support a CCITT interface
 for electronic mail.  The NBS Computer Based Message System (CBMS)
 standard was developed previously and was published as a Federal
 Information Processing Standard (FIPS Publication 98) in 1983 [3].
 The DARPA Multimedia Mail system is an experimental electronic mail
 system which is in use in the DARPA Internet [2,4,5].  It is used to
 create and distribute messages that incorporate text, graphics,
 stored speech, and images and has been implemented on on several very
 different machines.  Courier is the XEROX network systems remote
 procedure call protocol [7].  The SUN Remote Procedure Call package
 implements "network pipes" between UNIX machines [6].

DeSchon [Page 1]

RFC 971 January 1986 A Survey of Data Representation Standards

2. Background

 This section presents a brief overview of the basic terminology and
 approach of each data representation standard.
 2.1. Interprocess Communication Standards
    The standards that are oriented towards distributed interprocess
    communication or remote procedure call, between like machines,
    generally favor the use of types that map easily into the types
    defined in the programming language in use on the system.  For
    example, the types defined for the XEROX Courier system resemble
    the types found in the Mesa programming language.  Similarly, the
    SUN Remote Procedure Call system types resemble the types found in
    the C programming language.  An advantage of a system implemented
    using like machines is that the external data representation can
    be defined in such a way that the conversion to and from the local
    format is minimal.
    2.1.1. Courier
       The Courier standard data types are used to define the data
       objects which are transported bi-directionally between system
       elements that are running the Courier remote procedure call
       protocol.  The "standard representation" of a type is the
       encoding of the data which is transmitted.  The "standard
       notation" refers to the conventions for the interpretation of
       the data by higher-level applications.  The standard
       representation of a data object encodes the value of the
       object, but the type of the object is determined by the
       software that generates or interprets the representation.
    2.1.2. SUN Remote Procedure Call Package
       The SUN Remote Procedure Call package includes routines which
       allow a process on one UNIX machine to consume data produced by
       a process on another UNIX machine.  This is called a "network
       pipe" and is an extension of the standard UNIX pipe.  The
       "eXternal Data Representation (XDR)" standard defines the
       routines that are used to encode or "serialize" data for
       transmission, or to decode or "deserialize" data for local
       interpretation. The syntax suggests that perhaps it should be
       called "remote interprocess communication" rather than "remote
       procedure call".

DeSchon [Page 2]

RFC 971 January 1986 A Survey of Data Representation Standards

 2.2. Message Standards
    The message oriented standards, including DARPA Multimedia Mail,
    NBS CBMS, and the CCITT X.409 standards, seem to favor more
    general, highly extensible type definitions.  This may have
    something to do with the expectation that a system will include
    many different machines, programmed using many different
    programming languages.
    2.2.1. DARPA Multimedia Mail
       The DARPA Multimedia Mail system was developed for use in DoD
       Internet community.  The set of data elements used in the
       Multimedia Message Handling Facility (MMHF) is referred to as
       its "presentation transfer syntax".  The encoding of these data
       elements varies with the data type being represented. Each
       begins with a one-octet "element-code".  Some data elements are
       of a pre-determined length.  For example, the INTEGER data
       element occupies five octets, one for the element-code, and
       four which contain the "value component".  Other data elements,
       however, may vary in length.  For example, the TEXT data
       element, is made up of a one-octet element-code, a three-octet
       count of the characters to follow, and a variable number of
       octets, each containing one right-justified seven bit ASCII
       character.  The element-code and the length constitute the "tag
       component".
       A "base data element" is self contained, while a "structured
       data element" is formed using other data elements.  The LIST
       data element is used to create structures composed of other
       elements.  The tag component of a LIST is made up of a
       one-octet element-code, a three-octet count of the number of
       octets to follow, and a two-octet count of the number of
       elements that follow.  The PROPLIST data element is used to
       create a structure that consists of a set of unordered
       name-value pairs.  The tag component of a PROPLIST is made up
       of a one-octet element-code, a three-octet count of the number
       of octets to follow, and a one-octet count of the number of
       name-value pairs in the PROPLIST.  Both the LIST and the
       PROPLIST elements are followed by an ENDLIST data element.
    2.2.2. NBS Computer Based Message System
       The NBS Computer Based Message System (CBMS) standard was
       developed to specify the format of a message at the interface
       between different computer-based message systems.  Each data
       element consists of a series of "components".  The five

DeSchon [Page 3]

RFC 971 January 1986 A Survey of Data Representation Standards

       possible types of component are the "identifier octet", the
       "length code", the "qualifier", the "property-list" component,
       and the "data element contents".  Every data element contains
       an identifier octet and a length code.  The identifier octet
       contains a one-bit flag that signifies whether the data element
       contains a property-list, and a code identifying the data
       element and signifying whether it contains a qualifier. In the
       NBS standard, the property-list is associated with a data
       element and contains properties such as a "printing-name" or a
       "comment".  The meaning of the qualifier depends on the data
       element code.  The length code indicates the number of octets
       following, and is between one and three octets in length.
       Each data element is inherently a "primitive data element",
       which contains a basic item of information, or a "constructor
       data element", which contains one or more data elements.  The
       "field" data element (itself a constructor) uses a qualifier
       component, which contains a "field identifier" to indicate
       which specific field is being represented within a message.
    2.2.3. CCITT Recommendation X.409
       The CCITT recommendation X.409 defines the notation and the
       representational technique used to specify and to encode the
       Message Handling System (MHS) protocols.  The following is a
       description of the CCITT approach to encoding type definitions.
       A data element consists of three components, the "identifier"
       (type), the "length", and the "contents".  An element and its
       components consist of a sequence of an integral number of
       octets.  An identifier consists of a "class" ("universal",
       "application-wide", "context-specific", or "private-use"), a
       "form" ("primitive" or "constructor"), and the "id code".
       There is a convention defined for both single-octet and
       multi-octet identifiers.  The length specifies the length of
       the contents in octets, and is itself variable in length.
       There is also an "indefinite" value defined for the length;
       this means that no length for the contents is specified, and
       the contents is terminated with the the "end-of-contents" (EOC)
       element.  In X.409 it is possible to determine whether a data
       element is a primitive or a constructor from the form part of
       the identifier.  In addition it is possible to "tag" the data
       by attaching meaning to an id code within the context of a
       specific application.

DeSchon [Page 4]

RFC 971 January 1986 A Survey of Data Representation Standards

3. Implicit Versus Explicit Representation

 In both the SUN Remote Procedure Call system and the XEROX Courier
 system the type definitions of external data are implicit.  This
 means that for a given type of call, or message, the type definitions
 which is to be used to interpret the data, are agreed upon by the
 sender and the receiver in advance.  In other words, parameters (or
 message fields) are assumed to be in a predefined order.  Each
 parameter is assumed to be of a predefined type.  This means the data
 cannot be reformated into the local form until it reaches a process
 that knows about the types of specific parameters.  At this point,
 the conversion can be accomplished using system routines that know
 how to convert from the external format to the local format.  If the
 system is homogeneous there may be very little conversion required.
 In addition, no extra overhead of sending the type definitions with
 the data is incurred.
 In the DARPA Multimedia Mail system, the NBS CBMS standard, and the
 CCITT X.409 recommendation, type definitions are explicit.  In this
 case the type definitions are encoded into the message.  There are
 several advantages to this approach.  One advantage is that it allows
 a low level receiver process in the destination host to convert the
 data from the standard form to a form appropriate for the local host,
 as it received.  This can increase efficiency if it allows the
 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.

DeSchon [Page 5]

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)    |           |

DeSchon [Page 6]

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)    |           |            |           |

DeSchon [Page 7]

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.

DeSchon [Page 8]

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]

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