Movatterモバイル変換

Network Working Group                                  H. AlvestrandRequest for Comments: 2159                                   UNINETTCategory: Standards Track                               January 1998A MIME Body Part for FAXStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1998).  All Rights Reserved.1.  Introduction   This document contains the definitions, originally contained inRFC1494, on how to carry CCITT G3Fax in MIME, and how to translate it to   its X.400 representation.   NOTE: At the moment, this format does not seem appropriate for a   "general purpose image format for the Internet", if such a beast can   exist. It exists only to carry information that is already in G3 Fax   format, and may be usefully converted to other formats when used in   specific contexts.2.  The image/g3fax content-type   This content-type is defined to carry G3 Facsimile byte streams.   In general, a G3Fax image contains 3 pieces of information:     (1)   A set of flags indicating the particular coding scheme.           CCITT Recommendation T.30 defines how the flags are           transmitted over telephones.  In this medium, the flags are           carried as parameters in the MIME content-type header           field.     (2)   A structure that divides the bits into pages.  CCITT           recommendation T.4 describes a "return to command mode"           string; this is used here to indicate page breaks.Alvestrand                  Standards Track                     [Page 1]

RFC 2159                 MIME Body Part for FAX             January 1998     (3)   For each page, a sequence of bits that form the encoding of           the image.  CCITT recommendation T.4 defines the bit image           format.  This is used without change.  The highest bit of           the first byte is the first bit of the T.4 bitstream.2.1.  G3Fax Parameters   The following parameters are defined:      (1)   page-length - possible values: A4, B4 and Unlimited      (2)   page-width - possible values: A3, A4, B4      (3)   encoding - possible values: 1-dimensional, 2-dimensional,            Uncompressed      (4)   resolution - possible values: Fine, Coarse      (5)   DCS - a bit string, represented in Base64.      (6)   pages - an integer, giving the number of pages in the            document   If nothing is specified, the default parameter settings are:      page-length=A4      page-width=A4      encoding=1-dimensional      resolution=CoarseAlvestrand                  Standards Track                     [Page 2]

RFC 2159                 MIME Body Part for FAX             January 1998   It is possible (but misleading) to view the representation of these   values as single-bit flags. They correspond to the following bits of   the T.30 control string and X.400 G3FacsimileParameters:       Parameter               T.30 bit        X.400 bit       page-length=A4             no bit set       page-length=B4          19              21       page-length=Unlimited   20              20       page-width=A4              no bit set       page-width=A3           18              22       page-width=B4           17              23       encoding=1-dimensional     no bit set       encoding=2-dimensional  16              8       encoding=Uncompressed   26              30       resolution=Coarse          no bit set       resolution=Fine         15              9   The reason for the different bit numbers is that X.400 counts bits in   an octet from the MSB down to the LSB, while T.30 uses the opposite   numbering scheme.   If any bit but these are set in the Device Control String, the DCS   parameter should be supplied.2.2.  Content Encoding   X.400 defines the g3-facsimile data stream as a SEQUENCE of BIT   STRINGs. Each BIT STRING is a page of facsimile image data, encoded   as defined by Recommendation T.4.  The following content encoding is   reversible between MIME and X.400 and ensures that page breaks are   honored in the MIME representation.   An EOL is defined as a bit sequence of       000000000001 (eleven zeroes and a one).   Each page of the message is delimited by a sequence of six (6) EOLs   that MUST start on a byte boundary.  The image bit stream is padded   with zeroes as needed to achieve this alignment.   Searching for the boundary is a matter of searching for the byte   sequence (HEX) 00 10 01 00 10 01 00 10 01, which cannot occur inside   the image.Alvestrand                  Standards Track                     [Page 3]

RFC 2159                 MIME Body Part for FAX             January 1998   SeeSection 7.5 for the algorithm on conversion between this encoding   and the X.400 encoding.   The Base64 content-transfer-encoding is appropriate for carrying this   content-type.3.  g3-facsimile - image/g3fax   X.400 Body part: g3-facsimile   MIME Content-Type: image/g3fax   Conversion Type: nearly Byte copy   Comments:   The Parameters of the X.400 G3Fax body part are mapped to the   corresponding Parameters on the MIME Image/G3Fax body part and vice   versa.  Note that:      (1)   If fineResolution is not specified, pixels will be twice as            tall as they are wide      (2)   If any bit not corresponding to a specially named option is            set in the G3Fax NonBasicParameters, the "DCS" parameter            must be used.      (3)   Interworking is not guaranteed if any bit apart from those            specially named are used in the NonBasicParameters   From X.400 to G3Fax, the body is created in the following way:      (1)   Any trailing EOL markers on each bitstring is removed. The            bit order is changed to conform to the most common Internet            encoding (highest bit of first byte = first bit of the            G3Fax). The bitstring is padded to a byte boundary.      (2)   6 consecutive EOL markers are appended to each bitstring.      (3)   The padded bitstrings are concatenated together   An EOL marker is the bit sequence 000000000001 (11 zeroes and a   one).   From G3Fax to X.400, the body is created in the following way:      (1)   The body is split into bitstrings at each occurrence of 6            consecutive EOL markers. Trailing EOLs must NOT be removed,            since the X.400 Implementor Guide recommends that each page            should end with 6 consecutive EOLs.  (This is a change fromRFC 1494).Alvestrand                  Standards Track                     [Page 4]

RFC 2159                 MIME Body Part for FAX             January 1998      (2)   Each bitstring is made into an ASN.1 BITSTRING, reversing            the order of bits within each byte to conforom to the X.400            Implementors Guide recommendation for bit order in the            G3Fax body part.      (3)   The bitstrings are made into an ASN.1 SEQUENCE, which forms            the body of the G3Fax body part.4.  Usability of G3Fax body parts   This section is not part of the proposed standard, but is intended as   guidance for people implementing G3Fax handling, so that they know a   little about what to expect.   The DCS bitstring is a LONG thing; the T.30 Recommendation (1993)   gives 67 bits with specific functions, SG8 Report R33 extends this to   75 bits, and Report R41 (approved in 1995) extends it to 79 bits.   (For curiosity - bit 68 says that the coding is JPEG; bit 27 is   "error correcting mode). No sane implementor will send such things   without being able to negotiate them down if the recipient doesn't   support it, but there is no guarantee that messages with such bits   set in the DCS won't arrive through X.400.   The ISO P2 profile from 1995 [PROFILE] says that the profile makes   support for reception of two-dimensional and fine-resolution   mandatory if g3-facsimile is supported at all. Research by Andrew   Gordon of Net-Tel indicates that it is easy for an access unit to   support fine resolution, unlimited length and B4 length, while   support for B4 width is nearly impossible, and A3 width is hard.   Another interesting point is that some fax machines have trouble if   the scan lines do not contain exactly the declared number of pixels   on each scan line, so "omitting right-hand white space" is likely to   give trouble.5.  Security Considerations   There are no known security issues specific to the FAX body part.Alvestrand                  Standards Track                     [Page 5]

RFC 2159                 MIME Body Part for FAX             January 19986.  References   [MIME]       Freed, N., and N. Borenstein, "Multipurpose Internet Mail       Extensions (MIME) Part One: Format of Internet Message Bodies",RFC 2045, November 1996.   [GUIDE]       X.400 Implementor's Guide, version 8.   [PROFILE]       ISO/IEC ISP 12062-2: 1995:   [T.30]       ITU-T Recommendation T.30 (1993): Procedures for document       facsimile transmission in the general switched telephone network.7. Author's Address   Harald Tveit Alvestrand   UNINETT   P.O.box 6883 Elgeseter   N-7002 Trondheim   NORWAY   EMail: Harald.T.Alvestrand@uninett.noAlvestrand                  Standards Track                     [Page 6]

RFC 2159                 MIME Body Part for FAX             January 19988.  Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Alvestrand                  Standards Track                     [Page 7]