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Network Working Group                                        D. RobinsonRequest for Comments: 1154                                    R. Ullmann                                                    Prime Computer, Inc.                                                              April 1990Encoding Header Field for Internet Messages1. Status of the Memo   This RFC proposes an elective experimental Encoding header field to   permit the mailing of multi-part, multi-structured messages.   The use of Encoding updatesRFC 1049 (Content-Type), and is a   suggested update to RFCs 1113, 1114, and 1115 (Privacy Enhancement)   [4,7,8].   Distribution of this memo is unlimited.2. IntroductionRFC 822 [2] defines an electronic mail message to consist of two   parts, the message header and the message body, separated by an   apparently blank line.   The Encoding header field permits the message body itself to be   further broken up into parts, each part also separated from the next   by an apparently blank line.   Thus, conceptually, a message has a header part, followed by one or   more body parts, all separated by blank lines.   Each body part has an encoding type.  The default (no Encoding field   in the header) is a message body of one part of type "text".   3. The Encoding Field   The Encoding field consists of one or more subfields, separated by   commas.  Each subfield corresponds to a part of the message, in the   order of that part's appearance.  A subfield consists of a line   count, a keyword defining the encoding, and optional information   relevant only to the specific encoding.  The line count is optional   in the last subfield.3.1. Format of the Encoding Field   The format of the Encoding field is:Robinson & Ullmann                                              [Page 1]

RFC 1154      Encoding Header Field for Internet Messages     April 1990     [<count> <keyword> [<options>], ]* [<count>] <keyword> [<options>]     where:        <count>   := a decimal integer        <keyword> := a single alphanumeric token starting with an alpha        <options> := keyword-dependent options3.2. <count>   The line count is a decimal number specifying the number of text   lines in the part.  Parts are separated by a blank line, which is not   included in the count of either the proceeding or following part.   Because a count always begins with a digit and a keywords always   begins with an letter, it is always possible to determine if the   count is present.  (The count is first because it is the only   information of interest when skipping over the part.)   The count is not required on the last or only part.3.3. <keyword>   The keyword defines the encoding type.  The keyword is a common   single word name for the encoding type.  The keywords are not case-   sensitive.   The list of standard keywords is intended to be the same as the list   used for the Content-Type: header described in [6].  This RFC   proposes additions to the list.  Implementations can then treat   "Content-Type" as an alias of "Encoding", which will always have only   one body part.3.4. <options>   The optional information is used to specify additional keyword-   specific information needed for interpreting the contents of the   encoded part.  It is any sequence of tokens not containing a comma.3.5. Encoding Version Numbers   In general, version numbers for encodings, when not actually   available within the contents of the encoded information, will be   handled as options.3.6. Comments   Comments enclosed in parentheses may, of course, be inserted anywhere   in the Encoding field.  Mail reading systems may pass the comments toRobinson & Ullmann                                              [Page 2]

RFC 1154      Encoding Header Field for Internet Messages     April 1990   their clients.  Comments must not be used by mail reading systems for   content interpretation; that is the function of options.4. Encodings   This section describes some of the defined encodings used.   As with the other keyword-defined parts of the header format   standard, extensions in the form of new keywords are expected and   welcomed.  Several basic principles should be followed in adding   encodings:      - The keyword should be the most common single word name for the      encoding, including acronyms if appropriate.  The intent is that      different implementors will be likely to choose the same name for      the same encoding.      - Keywords not be too general: "binary" would have been a bad      choice for the "hex" encoding.      - The encoding should be as free from unnecessary idiosyncracies      as possible, except when conforming to an existing standard, in      which case there is nothing that can be done.      - The encoding should, if possible, use only the 7 bit ASCII      printing characters if it is a complete transformation of a source      document (e.g., "hex" or "uuencode").  If it is essentially a text      format, the full range may be used.  If there is an external      standard, the character set may already be defined.   Keywords beginning with "X-" are permanently reserved to   implementation-specific use.  No standard registered encoding keyword   will ever begin with "X-".4.1. Text   This indicates that the message is in no particular encoded format,   but is to be presented to the user as is.   The full range of the ASCII character set is used.  The message is   expected to consist of lines of reasonable length (less than 1000   characters).   On some transport services, only the 7 bit subset of ASCII can be   used.  Where full 8 bit transparency is available, the text is   assumed to be ISO 8859-1 [3] (ASCII-8).Robinson & Ullmann                                              [Page 3]

RFC 1154      Encoding Header Field for Internet Messages     April 19904.2. Message   This encoding indicates that the body part is itself in the format of   an Internet message, with its own header part and body part(s).  A   "message" body part's message header may be a full internet message   header or it may consist only of an Encoding field.   Using the message encoding on returned mail makes it practical for a   mail reading system to implement a reliable resending function, if   the mailer generates it when returning contents.  It is also useful   in a "copy append" MUA operation.   Message encoding is also used when mapping to X.400 to handle   recursively included X.400 P2 messages.4.3. Hex   The encoding indicates that the body part contains binary data,   encoded as 2 hexadecimal digits per byte, highest significant nibble   first.   Lines consist of an even number of hexadecimal digits.  Blank lines   are not permitted.  The decode process must accept lines with between   2 and 1000 characters, inclusive.4.4. EVFU   EVFU (Electronic Vertical Format Unit) specifies that each line   begins with a one-character "channel selector".  The original purpose   was to select a channel on a paper tape loop controlling the printer.   This encoding is sometimes called "FORTRAN" format. It is the default   output format of FORTRAN programs on a number of computer systems.   The legal characters are '0' to '9', '+', '-', and space.  These   correspond to the 12 rows (and absence of a punch) on a printer   control tape (used when the control unit was electromechanical).   The channels that have generally agreed definitions are:           1     advances to the first print line on the next page           0     skip a line, i.e., double-space           +     over-print the preceeding line           -     skip 2 lines, i.e., triple-space        (space)  print on the next line, single-spaceRobinson & Ullmann                                              [Page 4]

RFC 1154      Encoding Header Field for Internet Messages     April 19904.5. EDI   The EDI (Electronic Document Interchange) keyword indicates that the   message or part is a business document, formatted according to ANSI   X12 or related standards.   The first word after the EDI keyword indicates the particular   interchange standard.   A message containing a note and 2 X12 purchase orders might have an   encoding of:       Encoding: 17 TEXT, 146 EDI X12, 69 EDI X124.6. X.400   The Encoding header field provides a mechanism for mapping multi-part   messages between CCITT X.400 [1] andRFC 822.   The X.400 keyword specifies a section that is converted from an X.400   body part type not known to the gateway, or not corresponding to a   useful internet encoding.   If the message transits another gate, or if the receiving user has   the appropriate software, it can be decoded and used.   The X.400 keyword is followed by a second token indicating the method   used.  The simplest form is "X.400 HEX", with the complete X.409   encoding of the body part in hexadecimal.  More compact is "X.400   3/4", using the 3-byte to 4-character encoding as specified inRFC1113,section 4.3.2.4.4.7. uuencode   The uuencode keyword specifies a section consisting of the output of   the uuencode program supplied as part of uucp.4.8. encrypted   The encrypted keyword indicates that the section is encrypted with   the methods inRFC 1115 [8].  This replaces the possible use ofRFC934 [5] encapsulation.References  [1]  International Telegraph and Telephone Consultative Committee,       "Data Communication Networks: Message Handling Systems", In CCITT       Recommendations X.400 to X.430, VIIIth Plenary Assembly, Malaga-Robinson & Ullmann                                              [Page 5]

RFC 1154      Encoding Header Field for Internet Messages     April 1990       Torremolinos, 1984, Fascicle VIII.7 ("Red Book").  [2]  Crocker, D., "Standard for the Format of ARPA Internet Text       Messages",RFC 822, University of Delaware, August 1982.  [3]  International Organization for Standardization, "Information       processing - 8-bit single-byte coded graphic character sets -       Part 1: Latin alphabet No. 1", ISO 8859-1, ISO, 1987.  [4]  Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part       I -- Message Encipherment and Authentication Procedures",RFC1113, IAB Privacy Task Force, August 1989.  [5]  Rose, M., and E. Stefferud, "Proposed Standard for Message       Encapsulation",RFC 943, University of Delaware and NMA, January       1985.  [6]  Sirbu, M., "Content-type Header Field for Internet Messages",RFC1049, CMU, March 1988.  [7]  Kent, S., and J. Linn, "Privacy Enhancement for Internet       Electronic Mail: Part II -- Certificate-Based Key Management",RFC 1114, IAB Privacy Task Force, August 1989.  [8]  Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part       III -- Algorithms, Modes, and Identifiers",RFC 1115, IAB Privacy       Task Force, August 1989.Security Considerations   Security issues are not addressed in this memo.Authors' Addresses   David Robinson 10-30   Prime Computer, Inc.   500 Old Connecticut Path   Framingham, MA 01701   Phone: +1 508 879 2960 x1774   Email: DRB@Relay.Prime.COM   Robert Ullmann 10-30   Prime Computer, Inc.   500 Old Connecticut Path   Framingham, MA 01701Robinson & Ullmann                                              [Page 6]

RFC 1154      Encoding Header Field for Internet Messages     April 1990   Phone: +1 508 879 2960 x1736   Email: Ariel@Relay.Prime.COMRobinson & Ullmann                                              [Page 7]