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RFC2159 - A MIME Body Part for FAX

王朝other·作者佚名  2008-05-31
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Network Working Group H. Alvestrand

Request for Comments: 2159 UNINETT

Category: Standards Track January 1998

A MIME Body Part for FAX

Status 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 in RFC

1494, 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.

(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=Coarse

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.

See Section 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 from

RFC1494).

(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.

6. References

[MIME]

Freed, N., and N. Borenstein, "Multipurpose Internet Mail

Extensions (MIME) Part One: Format of Internet Message Bodies",

RFC2045, 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.no

8. 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.

 
 
 
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