SGML: MIME, Encapsulating SGML

SGML: MIME, Encapsulating SGML Using the Multipart/Related Content-Type


       MIMESGML Working Group                           E. Levinson
       Internet Draft: MIME/SGML                ACCURATE Info. Sys.
       <draft-ietf-mimesgml-encap-00.txt>              June 1, 1995

                    Encapsulating SGML Documents Using
                    the Multipart/Related Content-Type

       This draft document is being circulated for comment.  Please
       send your comments to the authors or to the sgml-internet
       mail list <>.

       Archives of the email discussions are available at filed by date and

       Status of this Memo

       This document is an Internet Draft; Internet Drafts are
       working documents of the Internet Engineering Task Force
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       Please check the abstract listing in each Internet Draft
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       This draft describes the encapsulation of a Standard
       Generalized Markup Language (SGML) document withing a MIME
       message.  It proposes new content sub-types of Text/SGML,
       Application/SGML, and Application/SGML-notation, and a new
       header, Content-SGML-Entity.

       This specification uses the proposed Multipart/Related
       Content-Type  [RFC-REL] and access-type=content-id [RFC-
       ACTI] specifications.  Multipart/Related provides the
       mechanism for treating the entire document as a single
       object and access-type=content-type allows a single MIME
       entity to appear several times without replicating the body
       of that MIME entity.

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                            Table of Contents

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       1.      Introduction

       A need exists for the transfer of documents constructed
       using the Standard Generalized Markup Language (SGML) [ISO-
       8879].  Those documents consist of a set of inter-related
       components whose structural relationship must be preserved
       independently of the system on which the document exists.
       The components and their relationships are often represented
       as files with explicit internal references to other
       components (files).  The encapsulation described here
       permits such transfers using the Multipurpose Internet Mail
       Extensions (MIME) specification [RFC-1521].

       The goals for the MIME encapsulation of SGML is to permit
       the receiving system to display (or process) the SGML
       document with minimal effort and maximum flexibility.  In
       particular, multiple parses of the SGML document can be
       avoided by using the information from SGML entity and
       notation declarations.  The Content-SGML-Entity header makes
       that information available.

       Sections 2 and 3 define the basic elements for labelling the
       SGML entities.  Section 4 describes the encapsulation of the
       documents entities within a single Multipart MIME entity.
       The two sections following that describe the handling of
       incomplete or unparsable documents and the SGML Document
       Interchange Format (SDIF) [ISO-9069].

       1.1.    Terminology

       Both SGML and MIME use the term "entity" to refer to their
       basic components.  Here the use of "entity" generally
       connotes an SGML entity.  For MIME entities, body part, is
       used; in some contexts that proves awkward and "MIME entity"
       is used instead.  The context hopefully makes such usage

       Two SGML terms, SGML Document and SGML Document Entity, are
       used in this paper and the difference between them is
       significant.  An SGML Document [ISO-8879, 4.282] is the
       entire collection of objects or entities that make up a
       document.  Those objects include markup definitions, text
       with SGML markup, plain text, image data, etc.  An SGML
       document entity [ibid., 4.283], on the other hand, is the
       specific object with which an SGML system begins processing
       the SGML document.

       1.2.    Standard Generalized Markup Language (SGML)

       The Standard Generalized Markup Language (SGML) is used to
       encode document structure and layout.  A rigorous
       description of SGML is left to [ISO-8879].  The terms used
       in the present document attempt to be consistent with SGML
       terminology and usage.

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       An SGML document exists as a collection of one or more
       entities; entities are system independent analogues to
       files.  Those SGML entities are mapped to storage objects or
       files and the mapping may be one-to-one, many-to-one, or
       one-to-many.  The SGML document refers to the storage
       objects via entity declarations.  The declarations may
       define the name and type of the storage object or provide a
       name by which a SGML system can map the declared entity to a
       storage object.  Preservation of the structure of references
       from one entity to another, known in SGML as the entity
       structure, are key to the email exchange of SGML documents.

       For a person or application to receive and display a
       complete SGML document the mail message must carry a precise
       definition for each of the SGML entities.  In the sender's
       environment the entities may reference standard names,
       called formal public identifiers, or specific local files,
       or both.  Further, some SGML entities may refer to other
       entities, for example files containing text, images, or
       graphics.  The identity and content of each entity must be
       available to the recipients to enable them to transform the
       sender's entity references into an equivalent local
       reference and to instantiate the entities locally.  This
       document describes the MIME encapsulation of an SGML
       document that preserves the entity structure and permits the
       recipient of the encapsulated document to automatically
       instantiate it locally.

       1.3.    SGML Document Interchange Format (SDIF)

       SDIF [ISO-9069] defines a data stream structure for
       exchanging one or more SGML documents.  A multipart MIME
       message consisting of one or more documents as described
       below is a conforming SDIF data stream [N1781].

       2.      A Model for MIME/SGML

       Four issues must be addressed for the recipient's user agent
       to display a complete SGML document.  The various document
       parts must be specified and entity references on the
       sender's systems must be resolved to corresponding
       references on the receiver's system.  Similarly, notation
       declarations, that is, references to processors for non-SGML
       data, must be resolved into valid processes on the receiving
       system.  An appropriate application, called the unpacker,
       must be in control to present the MIME body parts and the
       entity and notation information to the display software.
       Finally, the MIME encapsulated SGML text entities must be
       independent of the sender's system representation

       The first three issues are addressed in the following
       manner.  Two MIME media-types (content-types) are defined
       for SGML text, Text/SGML and Application/SGML, and one for

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       conveying process associations, Application/SGML-notation.
       A new header, Content-SGML-Entity, provides the entity
       description for the body part containing the entity.
       Notation information, carried in an application/SGML-
       notation body part, associates notation declarations with
       MIME media-types.  The entities that constitute the SGML
       document are contained within the same Multipart/Related
       MIME entity.  These elements form the basis for the SGML
       MIME encapsulation.

       SGML defines an Entity Manager [ISO-8879, 4.123] that
       performs the mapping between SGML entities and the local
       file system.  The specification of that mapping is system
       dependent.  Consequently the each SGML entity shall be
       represented as one MIME entity.

       2.1.    The SGML Media-Types

       There are two media-types for SGML parsable data entities,
       Text/SGML and Application/SGML.  Both have the same optional
       parameters and produce the same results for recipients with
       SGML capability.  Text/SGML provides a fallback 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.

       A third media-type, Application/SGML-notation, applies to
       non-SGML data and provides the connection between an SGML
       declaration and a MIME media-type.

       2.1.1.  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: none
               Published specification: RFC-SGML
               Person and email address to contact for further information:
                                        E. Levinson <>

       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.

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       2.1.2.  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: none
               Published specification: RFC-SGML
               Person and email address to contact for further information:
                                        E. Levinson <>

       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 shal be
       explicitly represented by the bit combination specified in
       the SGML declaration.

       2.1.3.  Application/SGML-Notation

               MIME type name:          Application
               MIME subtype name:       SGML-Notation
               Required parameters:     none
               Optional parameters:     none
               Encoding considerations: none
               Security considerations: none
               Published specification: RFC-SGML
               Person and email address to contact for further information:
                                        E. Levinson <>

       The Application/SGML-Notation media-type provides the
       connection between the document's SGML notation declarations
       and MIME media-types.  The MIME entity must contain a
       Content-SGML-Entity.  The body of the SGML-Notation MIME
       entity contains a Content-Type header that specifies the
       media-type associated with the name parameter of the
       Content-SGML-Entity statement.

       Some SGML notation declarations may correspond to a script
       for an active media-type (e.g., safe-Tkl).  In those cases a
       MIME entity with the corresponding media-type should be
       used.  That MIME entity shall contain an appropriate
       Content-SGML-Entity header.

       2.2.    SGML Content-Type Parameters

       The parameters for the Text and Application SGML subtypes
       are defined below.

       charset     The charset parameter 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

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                   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-

       SGML-bctf   The SGML-bctf (SGML bit combination transformation
                   format) parameter describes the method used to
                   transform the sequence of constant width binary
                   numbers (called "bit combinations" in [ISO 8879,
                   4.24]) that constitute the entity 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 Appendix III.

       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].  Appendix III contains a summary
                   of those requirements.

       2.3.    ABNF Specification of the SGML Media-Types

               sgml-type := sgml-tora parm-list / sgml-nttn

               sgml-tora := ( "text" / "application" ) "/"  "SGML"

               sgml-nttn := "application" "/"  "SGML-notation"

               parm-list := *( ";" sgml-parm )

               sgml-parm := sgml-attr "=" value

               sgml-attr := "charset" / "SGML-bctf" / "SGML-boot"

       Note:  The sgml-tora, sgml-nttn, and sgml-attr strings are
       case independent.

       2.4.    Data Entities

       Data entities (those that contain data tha may not be pares-
       able as SGML) shall be included as MIME body parts whose
       media-types reflect the data content, i.e., Text/Plain,
       Image/JPEG, etc.

       3.      The Content-SGML-Entity Header

       The Content-SGML-Entity (cse) header is required when encapsu-
       lating an SGML document within a Multipart/Related MIME

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       entity.  The header contains information from the SGML entity
       declaration corresponding to the entity contained in the body
       of the body part.  A cse header is not required for an SGML
       entity that is not declared by any other entity in the MIME

       A catalog, defined in [TR9401], can be generated by the
       unpacker using the cse header data.

       When the same data is referred to by several SGML entity
       declarations, the data only need be present in the one MIME
       body part.  Subsequent body parts can use the
       Message/External-Body access-type=content-id media-type [RFC-
       ACTI].  Each of those body parts must have its own Content-
       SGML-Entity header.

       The Content-SGML-Entity header is defined as follows.

            entity-header := "Content-SGML-Entity" ":"
                            "decl-type" "=" decl-type
                            *( ";"  cse-parm )

            decl-type     := "doctype" / "linktype"  /
                             "general" / "parameter" /
                             "baseset" / "capacity"  / "syntax"

            cse-parm      := cse-attr "=" value /

            cse-attr      := "name" / "doctype" /
                            "linktype" / "public-id" / "system-id" /
                            "notation-name" / "content-type" /

            value         := token / quoted-string   ; c.f. [RFC-1521]

            extension-token := ( "X-" / "x-" ) token
                            ; no intervening white space

       decl-type   A string specifying the entity declaration type.
                   Decl-type is a token specifying how the entity was
                   declared.  Within an SGML document or subdocument
                   each entity type constitutes a unique name space.
                   The possible values for decl-type are:

                   doctype   An entity containing an external DTD
                             subset, declared by a doctype declara-
                             tion; the name in this case would be the
                             document type name.

                   baseset   An entity declared by a public identif-
                             ier in a base character set [production
                             174, ISO-8879,].

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                   capacity  An entity declared by a public identif-
                             ier in a capacity set [180, ISO-8879,

                   general   An entity declared in a entity declara-
                             tion as a general entity

                   linktype  An entity containing an external Link
                             Process Definition subset, declared in a
                             linktype declaration; the name parameter
                             is the link type name.

                   notation  The header describes a notation declara-
                             tion and, for Application/SGML-notation,
                             the body of the MIME body part contains
                             a content-type header giving the MIME
                             content type corresponding to the nota-
                             tion name or, for other media-types, a
                             description of the processing that the
                             notation specifies.

                   parameter An entity declared in an entity declara-
                             tion as parameter entity.

                   syntax    An entity declared by a public identif-
                             ier in a public concrete syntax [183,
                             ISO-8879, 13.4].

       doctype     A string specifying the document type name of the
                   DTD subset in which the entity was declared, if
                   the entity was declared in a DTD subset other than
                   the base DTD subset.  This parameter applies only
                   to entities with a decl-type of "general", "nota-
                   tion", or "parameter".

                   A parameter not defined in this document and
                   agreed upon by the parties using it, a group of
                   consenting adults.

       linktype    A string specifying the link type name of the Link
                   Process Definition (LPD) subset in which the
                   entity was declared, if the entity was declared in
                   an LPD subset.  This parameter is required only
                   for entities with a decl-type of "general" or

       name        A string giving the name of the entity and is
                   omitted if the entity has no name.

                   The notation name of an external entity.  Not
                   valid if the decl-type is "notation".  The value
                   of this parameter will be the value of the name

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                   parameter of a Content-SGML-Entity header.

       public-id   The public identifier in the entity's declaration.

                   The display version if a public text display ver-
                   sion was not present in the public id.  Use this
                   the parameter only if a device dependent display
                   version was used.

       system-id   The system identifier in the entity's declaration.

       MIME headers, including the cse header only contain US-ASCII
       characters.  SGML entity declarations, on the other hand, will
       contain characters from the SGML document's character set.
       Characters in that set that are not US-ASCII should be
       represented as an SGML numeric character reference in the
       reference concrete syntax (e.g., "&#nnn;", where "nnn" is the
       integer code position of the character).

       4.      Encapsulating SGML Documents

       SGML documents must be processed as a unit, handling the indi-
       vidula MIME body parts is not sufficient.  The MIME
       Multipart/Related provides the framework for handling the SGML
       composite structure.

       An SGML document can recursively contain subdocuments each of
       which has its own entity structure.  The name spaces for SGML
       entities are wholly contained within a subdocument.  Conse-
       quently the entity names specified on a Content-SGML-Entity
       header must exist in an environment that preserves those name
       spaces.  Recursively encapsulating each SGML subdocument
       within an Multipart/Related MIME entity accomplishes that.
       Thus a subdocument within the document occurs as its own
       Multipart/Related entity within the document's
       Mutltipart/Related entity.  The recursive MIME Multipart
       structure directly mirrors by the recursive subdocument struc-

       4.1.    The Multipart/Related Media-Type

       The Multipart/Related [RFC-REL] media-type contains a set of
       related body parts, in this case an SGML document, and its
       start parameter names the body part within the
       Multipart/Related MIME entity with which processing starts,
       the SGML document entity.  It must contain a Content-ID header
       whose value corresponds to the one in the start parameter.  If
       there is no start parameter then the first MIME entity in the
       Multipart/Related must be the SGML document entity.

       Below are sample excerpts of an encapsulated SGML document, An
       appendix contains the complete example.

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       4.2.    Examples

       The following examples point out some of the key features of
       the MIME/SGML encapsulation.  The examples cover a combined
       prolog and instance with an implied SGML declaration, the use
       of the Content-SGML-Entity, and Application/SGML-notation.

       4.2.1.  Implied SGML declaration

       Consider the following document instance which includes the
       SGML prolog, but which implies the SGML declaration.

            <!DOCTYPE book PUBLIC
                 "-//Acme//DTD Book//EN"
            <!ENTITY chap1 PUBLIC "-//Acme//TEXT chapt1//EN">
            <!ENTITY chap2 SYSTEM>
            <!ENTITY chap3 SYSTEM "chapt3.sgm">
                 <!NOTATION jxz SYSTEM "/usr/local/bin/jxz">
                 <!ENTITY fig1  SYSTEM "fig1.jxz" NDATA jxz>
            <book> &chap1; &chap2; &chap3; </book>

       The Multipart/Related MIME entity's start parameter's value is
       the content-id of the MIME body part containing the document.

            MIME-Version: 1.0
            Content-Type: Multipart/Related; boundary=tiger-lily
               start="<>"; type="application/SGML"

            Content-Type: Application/SGML
            Content-ID: <>

            <!DOCTYPE book PUBLIC
                 "-//Acme//DTD Book//EN"
            <!ENTITY chap1 PUBLIC "-//Acme//TEXT chapt1//EN">
            <!ENTITY chap2 SYSTEM>
            <!ENTITY chap3 SYSTEM "chapt3.sgm">
                 <!NOTATION jxz SYSTEM "/usr/local/bin/jxz">
                 <!ENTITY fig1  SYSTEM "fig1.jxz" NDATA jxz>
            <book> &chap1; &chap2; &chap3; </book>

       4.2.2   An SGML Text Entity

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       The entity "chap3" would be a MIME body part such as

            Content-Type: Text/SGML
            Content-SGML-Entity: decl-type=general; name=chap3;

            <chapt><H1>This is chapter THREE ...</chapt>

       Here, as in most other situations the cse header describes the
       entity contained in the body part.

       4.2.3   A Notation Declaration

       The notation declaration contained in the SGML prolog will be
       represented as a separate body part.

            Content-Type: Application/SGML-notation
            Content-SGML-Entity; decl-type=notation;
               name=jxz; system-id="/usr/local/bin/jxz";

            Content-type: Image/JPEG


       Note: It can be argued that a separate MIME body part associ-
       ating an SGML notation declaration name with a MIME media-type
       is redundant; the association exists in the body part contain-
       ing the actual data.  The content-type header gives the
       media-type and the Content-SGML-Entity header, the notation
       name.  That, however, does not suffice for entities that are
       not included directly.  There may be public entities that are
       not included in the encapsulation.

       4.2.4.  Script-based Notation

       Consider a notation declaration in which the non-SGML data is
       processed by a script interpreted by a local process.  In this
       case we use a x-safe-Tkl script.

               Content-Type: Application/X-safe-Tkl
               Content-SGML-Entity: decl-type=notation; name=stkl;

               [safe-Tkl script]

       5.      Partial or Incomplete Documents

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       Independent SGML and data entities included in MIME messages
       constitute independent MIME body parts and are not included
       within a Multipart/Related MIME entity.  The SGML entities
       shall have the SGML media-types appropriate to the data being
       sent; data entities shall use the media-type corresponding to
       their notation declaration.

       Content-SGML-Entity headers can be used with body parts that
       are not included within any Multipart/Related MIME entity.
       This allows, for example, an unpacker to add an entry to a
       catalog mapping the entity's public identifier to the file in
       which it stored the entity.

       6.      SGML Document Interchange Format (SDIF)

       A MIME encoding of SDIF [ISO-9069] is a conforming SDIF encod-
       ing [N-1781] and consists of one or more MIME encapsulated
       SGML documents.  When more than one document is present the
       documents must be contained in an appropriate Multipart MIME

       The following correspondence exists between MIME elements and
       SDIF ones.

            Data stream character set    SGML-boot parameter (see note)
            SDIF Name
              data-stream-name           Message-ID
              document-name              Content-ID
              explanatory comments       Content-Description
            Document descriptor          MIME body part indicated by
            Entity descriptor            MIME body part
            SDIF Identifer               Content-SGML-Entity

       Note: that the MIME encapsulation permits each document to
       have its own document character set.

       7.      Security

       SGML documents, like other compound documents, may contain
       entities whose media-types present security concerns, e.g.
       Application/PostScript.  Further SGML may contain explicit
       processing instructions for a presentation or composition sys-
       tem; use of such instructions present concerns similar to
       those of Application/PostScript.

       The use of active media-types with Notation declarations can
       provide an opportunity for the sender to execute a script or
       other code on the recipient's machine.  Unpacking software
       should alert the user when such situations arise.

       8.      References

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       [ISO-8824]  ISO 8824, Information processing systems -- Open
                   System Interconnection -- Specification of
                   Abstract Syntax Notation One (ASN.1).

       [ISO-8879]  ISO 8879:1986, Information processing -- Text and
                   office systems -- Standard Generalized Markup
                   Language (SGML).

       [ISO-9069]  ISO 9069:1988, Information Processing - SGML Sup-
                   port Facilities -- SGML Document Interchange For-
                   mat (SDIF).

       [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)

       [N-1781]    ISO/IEC JTC1/SC18/WG8 N1781, "Clarification of the
                   Requirements for Encoding the SGML Document Inter-
                   change Format (SDIF, ISO 9069).

       [RFC-822]   Crocker, D., Standard for the Format of ARPA
                   Internet Text Messages, August 1982, University of
                   Delaware, RFC 822.

       [RFC-1521]  N. Borenstein, N. Freed, "MIME  (Multipurpose
                   Internet Mail Extensions) Part One:  Mechanisms
                   for Specifying and Describing the Format of Inter-
                   net Message Bodies", 09/23/1993.

       [RFC-1522]  K. Moore, "MIME (Multipurpose Internet Mail Exten-
                   sions) Part Two: Message Header Extensions for
                   Non-ASCII Text", 09/23/1993.

       [RFC-1590]  J. Postel, "Media Type Registration Procedure",

       [RFC-1642]  D. Goldsmith, M. Davis, "UTF-7, A Mail-Safe
                   Transformation Format of UNICODE", 07/13/1994

       [RFC-REL]   H. Alvestrand, E. Levinson, "The MIME
                   Multipart/Related Content-type", Internet Draft,

       [RFC-ACTI]  E. Levinson, "The Message/External-Body Content-ID
                   Access Type", Internet Draft, draft-ietf-

       [TR9401]    SGML Open Consortium Technical Resolution
                   9401:1994, "Entity Management", 08/09/1994

       [US-ASCII]  Coded Character Set -- 7-Bit American Standard
                   Code for Information Interchange, ANSI X3.4-1986.

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       9.      Acknowledgements

       The editor has borrowed freely from the suggestions of others
       and in particular lifted text from James Clark and Charles F.
       Goldfarb (Information Management Consulting), and ideas from
       Roy Fielding (University of California, Irvine).  If any
       errors occurred in translating their words into this text,
       rest assure that the misinterpretation was mine.

       The editor also acknowledges Terry Allen (O'Reilly & Associ-
       ates, Inc.), Harald T. Alvestrand (UniNett), Nathaniel Boren-
       stein (First Virtual Holdings Incorporated), Daniel W. Con-
       nolly (W3O), Steven DeRose (EBT), Andy Gelsey (CSC), Paul
       Grosso (ArborText, Inc.), John Klensin (MCI), Einar Stefferud
       (Network Management Associates, Inc), and Erik Naggum (Naggum
       Software), for their suggestions, explanations, and encourage-
       ment.  No errors or faults in this document can be ascribed to
       them, they're all mine.

       10.     Author's Address

       Ed Levinson
       Accurate Information Systems, Inc.
       2 Industrial Way
       Eatontown, NJ  0772

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       Internet Draft                                                 MIME-SGML

                                APPENDIX A.

                             A Complete Example

       MIME-Version: 1.0
       Content-Type: Multipart/Related; boundary=tiger-lily
          start="<>"; type="application/SGML"

       Content-Type: Application/SGML
       Content-ID: <>

       <!DOCTYPE book PUBLIC
            "-//Acme//DTD Book//EN"
       <!ENTITY chap1 PUBLIC "-//Acme//TEXT chapt1//EN">
       <!ENTITY chap2 SYSTEM>
       <!ENTITY chap3 SYSTEM "chapt3.sgm">
            <!NOTATION jxz SYSTEM "/usr/local/bin/jxz">
            <!ENTITY fig1  SYSTEM "fig1.jxz" NDATA jxz>
       <book> &chap1; &chap2; &chap3; </book>
       Content-Type: Text/SGML
       Content-SGML-Entity: decl-type=general; name=chap1;
          public-id="-//Acme//TEXT chapt1//EN"

       <chapt><H1>This is chapter ONE ...</chapt>
       Content-Type: Text/SGML;
       Content-SGML-Entity: decl-type=general; name=chap2;

       <chapt><H1>This is chapter TWO ...</chapt>
       Content-Type: Text/SGML
       Content-SGML-Entity: decl-type=general; name=chap3;

       <chapt><H1>This is chapter THREE ...</chapt>
       Content-Type: Application/SGML
       Content-SGML-Entity: decl-type=doctype; name=book;
          public-id="-//Acme//DTD Book//EN";

       <--  Acme Widget Company  -->
       <-- Instruction Book DTD -->
       <!ELEMENT ...>

       Content-Type: image/jpeg
       Content-Transfer-Encoding: BASE64
       Content-SGML-Entity: decl-type=general; name=fig1;

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       Internet Draft                                                 MIME-SGML

          system-id="fig1.jxz"; notation-name=jxz

       [Base64 encoded binary image data]

       Levinson                     December 1995                     [Page 17]

       Internet Draft                                                 MIME-SGML

                                APPENDIX B.

                           Notes for Implementors

       An SGML document is encapsulated with the sender's references
       to her local storage objects intact.  The receiving system's
       SGML Entity Manager may be able to translate those references
       to to its local storage objects.  The recipient's storage
       objects must be provided by the MIME User Agent to the

       Other SGML systems, not capable of translating the sender's
       references, must depend on the packer to parse the SGML docu-
       ment and replace the sender's references with valid local

       Levinson                     December 1995                     [Page 18]

       Internet Draft                                                 MIME-SGML

                                APPENDIX C.

                  ISO-10744 BCTF Values and Boot Attribute

       C.1.    Bit Combination Transformation Format Values

       The following list Bit Combination Transformation Format
       (BCTF) values is provided as a convenience.  The authoritive
       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

       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

       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 the
                 Extended_UNIX_Code_Packed_Format_for_Japanese char-

       sjis      Each bit combination is treated as a pair of octets,
                 most significant octet first, encoding a character

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       Internet Draft                                                 MIME-SGML

                 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.

                 C.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,
       ] of the SGML declaration.  SGML-boot pro-
                 vides the capability to identify the character set
                 of the document's SGML declaration when it uses sig-
                 nificant 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-

                 Notes: (1) The triplet, <dscn noc bscn> has the fol-
                 lowing meaning.  Starting with character number dscn
                 in the us-ascii character set, renumber noc charac-
                 ters starting at bscn and incrementing by one.
                 Thus, 0 128 0, represents the identity mapping.  (2)
                 The document's declaration itself may also re-
                 define the significant SGML characters; the boot
                 attribute is intended to bootstrap the SGML system's
                 parse of the declaration.

       Levinson                     December 1995                     [Page 20]