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Network Working Group                                        E. LevinsonRequest for Comments: 1874            Accurate Information Systems, Inc.Category: Experimental                                     December 1995SGML Media TypesStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Abstract   This document proposes new media sub-types of Text/SGML and   Application/SGML.  These media types can be used in the exchange of   SGML documents and their entities.  Specific details for the exchange   or encapsulation of groups of related SGML entities using MIME are   currently being considered by the mimesgml Working Group <sgml-   internet@ebt.com>.1.      Introduction   A need exists for the transfer the elements of documents constructed   using the Standard Generalized Markup Language (SGML) [ISO-8879].   While the specific details of such transfers are being considered   general agreement exists on the need to register basic media types   for the SGML entities not covered by existing types.   The Standard Generalized Markup Language (SGML) is used to encode   document structure and a rigorous description of it is left to [ISO-   8879].  The terms used in the present document attempt to be   consistent with SGML terminology and usage.2.       The SGML Media-Types   There are two media-types for SGML parsable entities, Text/SGML and   Application/SGML.  Both have the same optional parameters.  Text/SGML   provides a fallback to Text/Plain for those without SGML capability.   Senders should base the choice between text and application media-   types on the entity's content.  Text is suggested for entities that   would be meaningful to a human being without SGML processing.   Application/SGML is recommended for all others.Levinson                      Experimental                      [Page 1]

RFC 1874                    SGML Media Types               December 19952.1.  Text/SGML         MIME type name:          Text         MIME subtype name:       SGML         Required parameters:     none         Optional parameters:     charset, SGML-bctf, SGML-boot         Encoding considerations: may be encoded         Security considerations: seesection 4 below         Published specification: ISO 8879:1986         Person and email address to contact for further information:                                  E. Levinson <ELevinson@Accurate.com>   The Text/SGML media-type can be employed when the contents of the   SGML entity is intended to be read by a human and is in a readily   comprehensible form.  That is the content can be easily discerned by   someone without SGML display software.  Each record in the SGML   entity, delimited by record start (RS) and record end (RE) codes,   must correspond to a line in the Text/SGML body part.   SGML entities that do not meet the above requirements should use the   Application/SGML media-type.   Seesection 2.3 for a description of the parameters.2.2.    Application/SGML         MIME type name:          Application         MIME subtype name:       SGML         Required parameters:     none         Optional parameters:     SGML-bctf, SGML-boot         Encoding considerations: may be encoded         Security considerations: seesection 4 below         Published specification: ISO-8879         Person and email address to contact for further information:                                  E. Levinson <ELevinson@Accurate.com>   Use the Application/SGML media-type for SGML text entities that are   not appropriate for Text/SGML.  When used, each record start (RS) and   record end (RE) character shall be explicitly represented by the bit   combination specified in the SGML declaration.   The parameters are described in the next section.Levinson                      Experimental                      [Page 2]

RFC 1874                    SGML Media Types               December 19952.3.    SGML Sub-type Parameters   The parameters for the Text/ and Application/SGML subtypes are   defined below.       charset     The charset parameter for Text/SGML is defined in                   [RFC-1521], the valid values and their meaning are                   registered by the Internet Assigned Numbers                   Authority (IANA) [RFC-1590].  The default charset                   value for all Text content-types is "us-ascii"                   [RFC-1521].                   The charset parameter is provided to permit non-                   SGML capable systems to provide reasonable                   behavior when Text/SGML defaults to Text/Plain.                   SGML capable systems will use the SGML-bctf param-                   eter.       SGML-bctf   The SGML-bctf (SGML bit combination transformation                   format) parameter describes the method used to                   transform the entity's sequence of constant width                   binary numbers (called "bit combinations" in [ISO                   8879, 4.24]) into the octet stream contained in                   the MIME body part.                   Valid values for SGML-bctf are the BCTF notation                   names defined in Annex C of [ISO-10744] and are                   reproduced for convenience in the Appendix.  The                   default value is "identity", i.e. perform no                   transformation.       SGML-boot   The SGML-boot parameter value is the content-ID of                   a MIME body part (Application/Octet-stream) that                   satisfies the requirements of the boot attribute                   in [ISO-10744].  The Appendix contains a summary                   of those requirements.  The SGML-boot parameter is                   only applicable if the SGML entity is a document                   entity.3.      Security Considerations   SGML entities contain information to be parsed and processed by the   recipient's SGML system.  Those entities may contain and such systems   may permit explicit system level commands to be execute while   processing the data.  To the extent that an SGML system will execute   arbitrary command strings recipients of SGML entities may be at risk.Levinson                      Experimental                      [Page 3]

RFC 1874                    SGML Media Types               December 1995   Parsable SGML entities may also contain explicit processing   instructions for a presentation or composition system; use of such   instructions present concerns similar to those of   Application/PostScript.4.      References       [ISO-8879]            Information processing -- 8-bit Single-Byte Coded Graphic            Character Sets -- Part 1: Latin Alphabet No. 1, ISO            8859-1:1987.       [ISO-8879]            ISO 8879:1986, Information processing -- Text and office            systems -- Standard Generalized Markup Language (SGML).       [ISO-10744]            ISO/IEC 10744:1992, Information technology --            Hypermedia/Time-based Structuring Language (HyTime) (as            modified by First Proposed Technical Corrigendum, ISO/IEC            JTC1/SC18 N5027)       [RFC-1521]            Borenstein, N., and N. Freed, "MIME (Multipurpose Internet            Mail Extensions) Part One:  Mechanisms for Specifying and            Describing the Format of Internet Message Bodies",RFC1521, Bellcore, Innosoft, September 1993.       [RFC-1590]            Postel, J., "Media Type Registration Procedure",RFC 1590,            USC/Information Sciences Institute, March 1994.       [RFC-1642]            Goldsmith, D., and M. Davis, "UTF-7, A Mail-Safe            Transformation Format of UNICODE",RFC 1642, Taligent,            Inc., July 1994.5.      Author's Address   Ed Levinson   Accurate Information Systems, Inc.   2 Industrial Way   Eatontown, NJ  07724   EMail: ELevinson@Accurate.comLevinson                      Experimental                      [Page 4]

RFC 1874                    SGML Media Types               December 1995APPENDIXISO-10744 BCTF Values and Boot AttributeA.1.    Bit Combination Transformation Format (BCTF) Values   The following list of Bit Combination Transformation Format (BCTF)   values is provided as a convenience.  The authoritative source is   [ISO-10744].       identity  Each bit combination is represented by a single                 octet; this BCTF can be used only for entities all                 of whose bit combinations have a value not exceeding                 255.       fixed-2   Each bit combination is represented by exactly 2                 octets, with the more significant octet first; this                 BCTF can be used only for entities all of whose bit                 combinations have a value not exceeding 65535.       fixed-3   Each bit combination is represented by exactly 3                 octets, with a more significant octet preceding any                 less significant octets; this BCTF can be used only                 for entities all of whose bit combinations have a                 value not exceeding 16777215.       fixed-4   Each bit combination is represented by exactly 4                 octets, with a more significant octet preceding any                 less significant octets.       utf-8     Each bit combination is represented by a variable                 number of octets according to UCS Transformation                 Format 8 defined in Annex P to be added by the first                 proposed drafted amendment (PDAM 1) to ISO/IEC                 10646-1:1993.       utf-7     Each bit combination is represented by a variable                 number of octets in the range 0 through 127 as                 described in [RFC-1642]; this BCTF can be used only                 for entities all of whose bit combinations have a                 value not exceeding 65535.       euc-jp    Each bit combination is treated as a pair of octets,                 most significant octet first, encoding a character                 using the                 Extended_UNIX_Code_Fixed_Width_for_Japanese charset,                 and is transformed into the variable length sequence                 of octets that would encode that character using theLevinson                      Experimental                      [Page 5]

RFC 1874                    SGML Media Types               December 1995                 Extended_UNIX_Code_Packed_Format_for_Japanese char-                 set.       sjis      Each bit combination is treated as a pair of octets,                 most significant octet first, encoding a character                 using the                 Extended_UNIX_Code_Fixed_Width_for_Japanese charset,                 and is transformed into the variable length sequence                 of octets that would encode that character using the                 Shift_JIS charset.A.2.    The Boot Attribute   The body part specified by the SGML-boot parameter contains a   sequence of triplets of positive integers separated by white space.   The triplets correspond to the described character set portion [IS0-   8879, 13.1.1.2] of the SGML declaration.  SGML-boot provides the   capability to identify the character set of the document's SGML   declaration when it uses significant SGML characters [ibid., 4.298]   in the SGML reference concrete syntax [ibid., 13.4] that have a   character number [ibid., 4.44] in the document's character set that   differs from us-ascii.  The default value is "0 128 0", all   characters are us-ascii.   Notes: (1) The triplet, <dscn noc bscn> has the following meaning.   Starting with character number dscn in the us-ascii character set,   renumber noc characters starting at bscn and incrementing by one.   Thus, 0 128 0, represents the identity mapping.  (2) The document's   declaration itself may also redefine the significant SGML characters;   the boot attribute is intended to bootstrap the SGML system's parse   of the declaration.Levinson                      Experimental                      [Page 6]