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Versions: 00 01 02 03 04 05 06 07 08 rfc2306               Informational
 Network Working Group                                    Glenn Parsons
 Internet Draft                                          James Rafferty
 Expires in six months                                  January 2, 1997

        Tag Image File Format (TIFF) - F Profile for Facsimile

                     <draft-ietf-fax-tiff-08.txt>

  Status of this Memo

  This document is an Internet Draft.  Internet Drafts are working
  documents of the Internet Engineering Task Force (IETF), its Areas,
  and its Working Groups.  Note that other groups may also distribute
  working documents as Internet Drafts.

  Internet Drafts are valid for a maximum of six months and may be
  updated, replaced, or obsoleted by other documents at any time.  It
  is inappropriate to use Internet Drafts as reference material or to
  cite them other than as a "work in progress".

  To learn the current status of any Internet-Draft, please check the
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  munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
  ftp.isi.edu (US West Coast).

  Copyright Notice

  Copyright (C) The Internet Society (1997).  All Rights Reserved.

  Overview

  This document describes in detail the definition of TIFF-F that is
  used to store facsimile images.  The TIFF-F encoding has been
  folklore with no standard reference definition before this
  document.

  Internet Fax Working Group

  This document is a product of the IETF Internet Fax Working Group.
  All comments on this document should be forwarded to the email
  distribution list at <ietf-fax@imc.org>.





 Internet Draft                  TIFF-F                 January 2, 1997


 1.  Abstract

  This document references the Tag Image File Format (TIFF) to define
  the F profile of TIFF for facsimile (TIFF-F) as a file format that
  may be used for the storage and interchange of facsimile images.


 2.  TIFF Definition

  TIFF (Tag Image File Format) Revision 6.0 is defined in detail
  within [TIFF].

  A brief review of concepts used in TIFF is included in this
  document as background information, but the reader is directed to
  the original TIFF specification [TIFF] to obtain specific technical
  details.

 2.1  Baseline TIFF and Applications

  TIFF provides a method to describe and store raster image data.  A
  primary goal of TIFF is to provide a rich environment within which
  implementations can exchange image data.  [TIFF] characterizes
  Baseline TIFF as being the core of TIFF, the essentials that all
  mainstream TIFF developers should support in their products.
  Applications of TIFF are defined by using Baseline TIFF as a
  starting point and then defining "extensions" to TIFF that are
  used for the specific "application", as well as specifying any
  other differences from Baseline TIFF.


 3.  TIFF-F Definition

 3.1 Introduction

  Though it has been in common usage for many years, TIFF-F has
  previously never been documented in the form of a standard.  An
  informal TIFF-F document was originally created by a small group of
  fax experts led by Joe Campbell.  The existence of TIFF-F is noted
  in [TIFF] but it is not defined.  This document defines the F
  application of [TIFF]. For ease of reference, the term TIFF-F will
  be used throughout this document as a shorthand for "F Profile of
  TIFF for Facsimile".  TIFF-F files are intended for use with the
  image/tiff MIME media content-type which includes support for the
  "application" parameter (e.g, application=faxbw).

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
  NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  [REQ].


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 3.1.1 TIFF-F Historical Background

  Up until TIFF 6.0, TIFF supported various "Classes" which defined
  the use of TIFF for various applications. Classes were used to
  support specific applications and in this spirit, TIFF-F has been
  known historically as "TIFF Class F".  Previous informal TIFF-F
  documents used the "Class F" terminology.

  As of TIFF 6.0 [TIFF], the TIFF Class concept has been eliminated
  in favor of the concept of Baseline TIFF.  Therefore, this document
  updates the definition of TIFF-F as the F profile of TIFF for
  facsimile, by using Baseline TIFF as defined in [TIFF] as the
  starting point and then defining the differences from Baseline TIFF
  which apply for TIFF-F.   In almost all cases, the resulting
  definition of TIFF-F fields and values remains consistent with those
  used historically in earlier definitions of TIFF Class F.  Where
  some of the values for fields have been updated to provide more
  precise conformance with the ITU-T [T.4] and [T.30] fax
  recommendations, these differences are noted.

  3.1.2     Overview

  The intent of this specification is to document:

  1)  The fields and values which are applicable for this F profile
      of TIFF for facsimile.
  2)  A minimum set of TIFF-F fields and values which should be able
      to interwork with virtually all historic TIFF-F readers.
  3)  A broader range of values for the traditional TIFF-F fields
      that will provide support for the most widely used facsimile
      compressions, page sizes and resolutions, consistent with the
      ITU-T [T.4] and [T.30] recommendations.

  The structure of the TIFF-F definition will be as follows.  A brief
  review of the structure of TIFF files and practical guidelines for
  the writing and reading of multi-page TIFF-F files is provided in
  sections 3.1.3 and 3.1.4.

  A review of TIFF-F fields follows.  Section 3.2 reviews the fields
  from Baseline TIFF that are applicable for black and white (bi-
  level) images and are also used by TIFF-F.

  Section 3.3 reviews the other required TIFF-F fields. Several
  fields that are specific extensions for  TIFF-F  are reviewed in
  section 3.4.  There are also fields that may be helpful, but are
  not required.  These recommended fields are listed in the section
  3.5.  Section 3.6 defines the requirements for the minimum subset
  of TIFF-F fields and values to maximize interoperability.
  Several technical topics, including implementation issues and
  warnings are discussed in subsequent sections.  Finally, section
  3.9 introduces the TIFF-F Reader and Writer.  A table of the


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  required and recommended fields for a TIFF-F Reader is provided,
  along with details on the permitted set of values.

 3.1.3 Structure of TIFF Files

  The structure of TIFF files is specified within [TIFF].  In this
  section, a short summary of the TIFF structure is included for the
  informational purposes.   In addition, some practical guidelines
  for the use of this structure in reading and writing TIFF-F files
  are addressed in the following section 3.1.4.  The structure for
  writing "minimum subset" TIFF-F files is defined in section 3.6.2.

  A TIFF file begins with an 8-byte image file header that defines
  the byte order used within a file (see section 3.9.1), includes a
  magic number sequence that identifies the content as a TIFF file,
  and then uses an offset to point to the first Image File Directory
  (IFD).  An IFD is a sequence of tagged fields, sorted in ascending
  order (by tag value), that contains attributes of an image and
  pointers to the image data.   TIFF fields (also called entries)
  contain a tag, its type (e.g. short, long, rational, etc.), a count
  (which indicates the number of values/offsets) and a value/offset.
  However, the actual value for the field will only be present if it
  fits into 4 bytes; otherwise, an offset will be used to point to
  the location of the data associated with the field.  In turn, this
  offset may itself be used to point to an array of offsets.

  For the case of facsimile data, many documents consist of a series
  of multiple pages.   Within TIFF, these may be represented using
  more than one IFD within the TIFF file.   Each IFD defines a
  subfile whose type is given in the NewSubfileType field.  For the
  case of facsimile data that is placed in a TIFF-F file, each
  facsimile page in a multi-page document has its own IFD.   For bi-
  level facsimile files, multiple IFDs are organized as a linked
  list, with the last entry in each IFD pointing to the next IFD (the
  pointer in the last IFD is 0). (There is also another technique for
  organizing multiple IFDs as a tree, that uses the SubIFDs field,
  but this technique is not applicable for TIFF-F images.)  Within
  each IFD, the location of the related image data is defined by
  using fields that are associated with strips.  These fields
  identify the size of strips (in rows), the number of bytes per
  strip after compression and a strip offset, which is used to point
  to the actual location of the image strip.

  TIFF has a very flexible file structure, but the use of some
  practical guidelines for implementors when writing  multi-page TIFF-
  F files can produce TIFF structures which are easier for readers to
  process.   This is especially for implementations in environments
  such as facsimile terminals where a complex file structure is
  difficult to support.


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 3.1.4 Practical Guidelines for Writing/Reading Multi-Page TIFF-F Files

  Traditionally, historical TIFF-F has required readers and writers
  to be able to handle multi-page TIFF-F files.  Based on the
  experience of various TIFF-F implementors, it has been seen that
  the implementation of TIFF-F can be greatly simplified if certain
  practical guidelines are followed when writing multi-page TIFF-F
  files.  However, for interchange robustness, TIFF-F readers SHOULD
  be prepared to read TIFF files whose structure is consistent with
  [TIFF], which supports a more flexible file structure than is
  recommended here.

  The structure for a multi-page TIFF-F file will include one IFD per
  page of the document.   Therefore, each IFD will define the
  attributes for a single page.   For simplicity, the writer of TIFF-
  F files SHOULD present IFDs in the same order as the actual
  sequence of pages.  (The pages are numbered within TIFF-F beginning
  with page 0 as the first page and then ascending (i.e. 0, 1, 2,...).
  However, as noted in section 3.1.3, any field values over 4 bytes
  will be stored separately from the IFD. TIFF-F readers SHOULD expect
  IFDs to be presented in page order, but be able to handle exceptions.

  Per [TIFF], the exact placement of image data is not specified.
  However, the strip offsets for each strip of image are defined from
  within each IFD.   Where possible, a second simplifying assumption
  for the writing of TIFF-F files is to specify that the image data
  for each page of a multi-page document SHOULD be contained within a
  single strip (i.e. one image strip per fax page).   The use of a
  single image strip per page is very useful for implementations such
  as store and forward messaging, where the file is usually prepared
  in advance of the transmission, but other assumptions may apply for
  the size of the image strip for implementations which require the
  use of "streaming" techniques (see section 3.7.6).  In the event a
  different image strip size assumption has been used (e.g. constant
  size for image strips which may be less than the page size), this
  will immediately be evident from the values/offsets of the fields
  that are related to strips.   From the TIFF-F reader standpoint,
  one image strip per page permits the image data to be found through
  reference via a single offset, resulting in a much simplified image
  structure and faster processing.

  A third simplifying assumption is that each IFD SHOULD be placed in
  the TIFF-F file structure at a point which precedes the image which
  the IFD describes.  If any long field values are present (see
  section 3.1.3) then these SHOULD be placed after their referencing
  IFD and before the image data they describe.

  A fourth simplifying assumption for TIFF-F writers and readers is
  to place the actual image data in a physical order within the TIFF
  file structure which is consistent with the logical page order.  In
  practice, TIFF-F readers will need to use the strip offsets to find
  the exact physical location of the image data, whether or not it is
  presented in logical page order.

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  TIFF-F writers MAY make a fifth simplifying assumption, in which the
  IFD, the value data and the image data for which the IFD has offsets
  precede the next image IFD. These elements MUST precede the next
  image IFD in the minimum set TIFF-F files (see section 3.6.2).
  However, this principle has been relaxed in the case of TIFF-F to
  reflect past practices.

  So, a TIFF-F file which is structured using the guidelines of this
  section will essentially be composed of a linked list of IFDs,
  presented in ascending page order, which in turn each point to a
  single page of image data (one strip per page), where the pages of
  image data are also placed in a logical page order within the TIFF-F
  file structure.  (The pages of image data may themselves be
  stored in a contiguous manner, at the option of the implementor).


 3.2  Baseline TIFF  Required Fields for BiLevel Images

  Baseline TIFF per [TIFF] requires that the following fields be
  present for all BiLevel Images:  ImageWidth, ImageLength,
  Compression, PhotometricInterpretation, StripOffsets, RowsPerStrip,
  StripByteCounts, XResolution, YResolution and ResolutionUnit.
  TIFF-F uses all of these fields, but in some cases specifies a
  different range of acceptable values than Baseline TIFF.   Per
  [TIFF], if fields are omitted, the Baseline TIFF default value(if
  specified) will apply.

  In the field definitions which follow in this section and
  subsequent sections, the fields will be presented in the following
  form:

  Fieldname (tag-number) = values (if applicable). TYPE

  A brief summary of the Baseline TIFF fields and their use in TIFF-F
  follows:

  ImageWidth(256) = 1728, 2048, 2432, 2592, 3072, 3648, 3456, 4096,
                    4864.
      SHORT or LONG.  These are the fixed page widths in pixels.  The
      permissible values are dependent upon X and Y resolutions as
      shown in sections 2 and 3 of [T.4] and reproduced here for
      convenience:

       XResolution x Yresolution                  | ImageWidth
      --------------------------------------------|------------------
       204x98, 204x196, 204x391, 200x100, 200x200 | 1728, 2048, 2432
       300x300                                    | 2592, 3072, 3648
       408x391, 400x400                           | 3456, 4096, 4864
      --------------------------------------------|------------------


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      Historical TIFF-F did not include support for the following
      widths related to higher resolutions:  2592, 3072, 3648, 3456,
      4096 and 4864.   Historical TIFF-F documents also included the
      following values related to A5 and A6 widths:  816 and 1216.
      Per the most recent version of [T.4], A5 and A6 documents are
      no longer supported in Group 3 facsimile, so the related width
      values are now obsolete.  See section 3.8.2 for more
      information on inch/metric equivalencies and other
      implementation details.

  ImageLength (257).  SHORT or LONG. LONG recommended.
      The total number of scan lines in the image.

  Compression (259) = 3,4.  SHORT.
      This is a required TIFF-F field.  The permitted values for TIFF-
      F purposes are 3 and 4 as shown.   The default value per
      Baseline TIFF is 1 (Uncompressed), but this value is invalid
      for facsimile images.    Baseline TIFF also permits use of
      value 2 (Modified Huffman encoding), but the data is presented
      in a form which does not contain EOLs. Instead, TIFF-F specifies
      the value 3 for encoding one-dimensional T.4 Modified Huffman
      or 2-dimensional Modified READ data.   The detailed settings
      which apply for T.4 encoded data are specified using the
      T4Options field.  TIFF-F also permits use of the value 4 for
      the compression field, which indicates that the data is coded
      using a [T.6] compression method (i.e the Modified Modified
      READ two-dimensional method). The detailed settings which apply
      for T.6 encoded data are specified using the T6Options field.

      Please refer to the definitions of the T4Options and T6Options
      fields in section 3.3, and section 3.8 for more information on
      the encoding of images and conventions used within TIFF-F.

  PhotometricInterpretation (260) = 0,1.  SHORT.
      This field allows notation of an inverted ("negative") image:
              0 = normal
              1 = inverted

  StripOffsets (273).  SHORT or LONG.
      For each strip, the offset of that strip.  The offset is
      measured from the beginning of the file. If a page is expressed
      as one large strip, there is one such entry per page.

  RowsPerStrip (278).  SHORT or LONG.  LONG recommended.
      The number of scan lines per strip.  When a page is expressed
      as one large strip, this is the same as the ImageLength field.

  StripByteCounts (279).  LONG or SHORT.  LONG recommended.
      For each strip, the number of bytes in that strip. If a page is
      expressed as one large strip, this is the total number of bytes
      in the page after compression.  Note that the choice of LONG or
      SHORT depends upon the size of the strip.

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  ResolutionUnit (296) = 2,3.  SHORT.
      The units of measure for resolution:
              2 = Inch
              3 = Centimeter

      TIFF-F has traditionally used inch based measures.

  XResolution (282) = 204, 200, 300, 400, 408 (inches). RATIONAL.
      The horizontal resolution of the TIFF-F image expressed in
      pixels per resolution unit. The values of 200 and 408 have been
      added to the historical TIFF-F values, for consistency with
      [T.30].  Some existing TIFF-F implementations may also support
      values of 77 (cm).  See section 3.8.2 for more information on
      inch/metric equivalencies and other implementation details.

  YResolution (283) = 98, 196, 100, 200, 300, 391, 400  (inches).
                      RATIONAL.
      The vertical resolution of the TIFF-F image expressed in pixels
      per resolution unit. The values of 100, 200, and 391 have been
      added to the historical TIFF-F values, for consistency with
      [T.30].  Some existing TIFF-F implementations may also support
      values of 77, 38.5 (cm). See section 3.8.2 for more information
      on inch/metric equivalencies and other implementation details.


 3.3  TIFF-F Required Fields

  In addition to the Baseline TIFF fields, there are additional
  required fields for TIFF-F. A review of the additional required
  fields for TIFF-F follows:

  BitsPerSample (258) = 1.  SHORT.
      Since TIFF-F  is only used for black-and-white facsimile
      images, the value is  1 (the default) for all files.

  FillOrder (266) = 1, 2.  SHORT.
      TIFF  F readers must be able to read data in both bit orders,
      but the vast majority of facsimile products store data LSB
      first, exactly as it appears on the telephone line.
              1 = Most Significant Bit first.
              2 = Least Significant Bit first.

  NewSubFileType (254)= (Bit 1 = 1).  LONG.
      This field is made up of 32 flag bits.  Unused bits are
      expected to be 0 and bit 0 is the low order bit.   Bit 0 is set
      to 0 for TIFF-F.   Bit 1 is always set to 1 for TIFF-F,
      indicating a single page of a multi-page image. The same bit
      settings are used when TIFF-F is used for a one page fax image.
      See sections 3.1.1 and 3.1.2 for more details on the structure
      of multi-page TIFF-F image files.


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  PageNumber (285).  SHORT/SHORT.
      This field specifies the page numbers in the fax document.  The
      field comprises two SHORT values: the first value is the page
      number, the second is the total number of pages. Single-page
      documents therefore use 0000/0001 hex.  If the second value is
      0, the total number of pages in the document is not available.

  SamplesPerPixel (277) = 1.  SHORT.
      The value of 1 denotes a bi-level, grayscale, or palette color
      image.

There is also a requirement to include either the T4Options or the
T6Options field in a TIFF-F IFD, depending upon the setting of the
Compression field.  These fields are defined in the next section on
TIFF extensions.

3.4  TIFF-F Extensions

These are fields which are extensions beyond the required TIFF-F
fields.  The following fields have been defined as extensions in
[TIFF].

  T4Options (292) (Bit 0 = 0 or 1, Bit 1 = 0, Bit 2 = 0 or 1).  LONG.
      This field is required if the value for the compression field
      has been set to 3.   The values are set as shown below for TIFF-
      F.   For TIFF-F, uncompressed data is not allowed and EOLs MAY
      be byte aligned (see section 3.8.3).
              bit 0 = 0 for 1-Dimensional, 1 for 2-Dimensional (MR)
              bit 1 = must be 0 (uncompressed data not allowed)
              bit 2 = 0 for non-byte-aligned EOLs or 1 for byte-
                      aligned EOLs

      This field is made up of a set of 32 flag bits. Unused bits
      must be set to 0.  Bit 0 is the low order bit.  Please note
      that T4Options was known as G3Options in earlier versions of
      TIFF and TIFF-F.  The data in a TIFF-F image encoded using
      one of the T.4 methods is not terminated with an RTC (see
      section 3.8.5).

  T6Options (293) = (Bit 0 = 0, Bit 1 = 0)  LONG.
      This field is required for TIFF-F if value of the compression
      field has been set to 4. The value for this field is made up of
      a set of 32 flag bits.   Setting bit 0 to 0 indicates that the
      data is compressed using the Modified Modified READ (MMR) two-
      dimensional compression method.  MMR compressed Data is two-
      dimensional and does not use EOLs. Each MMR encoded image MUST
      include an "end-of-facsimile-block" (EOFB) code at the end of
      each coded strip (see section 3.8.6). Uncompressed data is not
      applicable for bi-level facsimile images, so that bit 1 must be
      set to 0.  Unused bits must be set to 0. Bit 0 is the low-order
      bit. The default value is 0 (all bits 0).


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              bit 0 = 0 for 2-Dimensional
              bit 1 = must be 0 (uncompressed data not allowed)

      In earlier versions of TIFF, this field was named
      Group4Options. The significance has not changed and the present
      definition is compatible.


  In addition, three new fields, defined as TIFF-F extensions,
  describe page quality.  The information contained in these fields
  is usually obtained from receiving facsimile hardware (if
  applicable).   These fields are optional.  They SHOULD NOT be used
  in writing TIFF-F files for facsimile image data that is error
  corrected or otherwise guaranteed not to have coding errors.

  Some implementations need to understand exactly the error content
  of the data.  For example, a CAD program might wish to verify that
  a file has a low error level before importing it into a high-
  accuracy document.  Because Group 3 facsimile devices do not
  necessarily perform error correction on the image data, the quality
  of a received page must be inferred from the pixel count of decoded
  scan lines. A "good" scan line is defined as a line that, when
  decoded, contains the correct number of pixels. Conversely, a "bad"
  scan line is defined as a line that, when decoded, comprises an
  incorrect number of pixels.

  BadFaxLines (326). SHORT or LONG
      This field reports the number of scan lines with an incorrect
      number of pixels encountered by the facsimile during reception
      (but not necessarily in the file).

      Note: PercentBad = (BadFaxLines/ImageLength) * 100

  CleanFaxData (327). SHORT
      N =
          0 = Data contains no lines with incorrect pixel counts or
             regenerated lines  (i.e., computer generated)
          1 = Lines with an incorrect pixel count were regenerated by
             receiving device
          2 = Lines with an incorrect pixel count are in the data  and
             were not regenerated by receiving device (i.e. data
             contains bad scan lines)

      Many facsimile devices do not actually output bad lines.
      Instead, the previous good line is repeated in place of a bad
      line. Although this substitution, known as line regeneration,
      results in a visual improvement to the image, the data is
      nevertheless corrupted.  The CleanFaxData field describes the
      error content of the data.  That is, when the BadFaxLines and
      ImageLength fields indicate that the facsimile device
      encountered lines with an incorrect number of pixels during


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      reception, the CleanFaxData field indicates whether these bad
      lines are actually still in the data or if the receiving
      facsimile device replaced them with regenerated lines.

  ConsecutiveBadFaxLines (328). LONG or SHORT.
      This field reports the maximum number of consecutive lines
      containing an incorrect number of pixels encountered by the
      facsimile device during reception (but not necessarily in the
      file).

      The BadFaxLines and ImageLength data indicate only the quantity
      of such lines.  The ConsecutiveBadFaxLines field is an
      indicator of their distribution and may therefore be a better
      general indicator of perceived image quality.

 3.5  Recommended Fields

 These are fields that MAY be used in encoding TIFF-F files, but are
 optional in nature and may be ignored by many TIFF readers.  These
 fields are called recommended consistent with historical TIFF-F
 practice.

  BadFaxLines (326) [defined in section 3.4]

  CleanFaxData (327) [defined in section 3.4]

  ConsecutiveBadFaxLines (328) [defined in section 3.4]

  DateTime (306).  ASCII.
      Date and time in the format YYYY:MM:DD HH:MM:SS, in 24-hour
      format. String length including NUL byte is 20 bytes. Space
      between DD and HH.

  DocumentName (269).  ASCII.
      This is the name of the document from which the document was
      scanned.

  ImageDescription (270).  ASCII.
      This is an ASCII string describing the contents of the image.

  Orientation (274).  SHORT.
      This field is designated as "Recommended" for consistency with
      historical TIFF-F, but is also a Baseline TIFF field with a
      default value of 1 per [TIFF]. The default value of 1 applies
      if the field is omitted, but for clarity, TIFF-F writers SHOULD
      include this field.  This field might be useful for displayers
      that always want to show the same orientation, regardless of
      the image.  The default value of 1 is "0th row is visual top of
      image, and 0th column is the visual left."  An 180-degree
      rotation is 3.  See [TIFF] for an explanation of other values.


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  Software (305).  ASCII.
      The optional name and release number of the software package
      that created the image.


 3.6   Requirements for TIFF-F Minimum Subset

 This section defines the requirements for a minimum subset of TIFF-F
 fields and values that all TIFF-F readers SHOULD support to maximize
 interoperability with current and historical TIFF-F implementations.
 The TIFF-F structure for writing minimum subset files is also
 defined.

 3.6.1   Summary of Minimum Subset Fields and Values

 A summary of the minimum subset TIFF-F fields and values is provided
 in the following table.  The required fields for the minimum subset
 are shown under the column labeled "Field".  The values for these
 fields in the minimum subset are shown under the column labeled
 "Minimum".

  Field             | Minimum      | Comment
  ------------------|--------------|-------------------------------
  BitsPerSample     | 1            |one bit per sample
  Compression       | 3            |3 for T.4 (MH)
  FillOrder         | 2            |LSB first
  ImageWidth        | 1728         |
  ImageLength       |              |required
  NewSubFileType    | Bit 1 = 1    |single page of multipage file
  PageNumber        | X/X          |pg/tot, 0 base, tot in 1st IFD
  PhotometricInterp | 0            |0 is white
  ResolutionUnit    | 2            |inches (default)
  RowsPerStrip      |=ImageLength  |
  SamplesPerPixel   | 1            |one sample per pixel
  StripByteCounts   |              |required
  StripOffsets      |              |required
  T4Options         | Bit 0 = 0    |MH
                    | Bit 1 = 0    |
                    | Bit 2 = 0,1  |Non-Byte-aligned,
                    |              | Byte-Aligned EOLs
  XResolution       | 204          |Units is per inch
  YResolution       | 196,98       |Units is per inch
  ------------------|--------------|------------------------------

 3.6.2     TIFF-F Minimum Subset File Structure

 For implementations which need to write minimum subset TIFF-F files,
 the file structure shown in Figure 3.1 MUST be used:


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                   +-----------------------+
                   |         Header        |------------+
                   +-----------------------+            | First IFD
                   |      IFD (page 0)     | <----------+ Offset
               +---|                       |------------+
               |   |                       |--+         |
         Value |   +-----------------------+  |         |
        Offset +-->|      Long Values      |  |         |
                   +-----------------------|  | Strip   |
                   |  Image Data (page 0)  |<-+ Offset  |
                   +-----------------------+            | Next IFD
                   |      IFD (page 1)     | <----------+ Offset
               +---|                       |------------+
               |   |                       |--+         |
         Value |   +-----------------------+  |         |
        Offset +-->|      Long Values      |  |         |
                   +-----------------------|  | Strip   |
                   |  Image Data (page 1)  |<-+ Offset  |
                   +-----------------------+            | Next IFD
                   |      IFD (page 2)     | <----------+ Offset
                   +-----------------------+
                   |          :            |
                   |          :            |

       Figure 3.1     TIFF-F Minimum Subset File Structure

 As depicted in Figure 3.1, the IFD of each page precedes the related
 Image Data for that page.  If present, any long field values appear
 between the IFD and the image data for that page.  For multiple page
 documents, each IFD/image pair is immediately followed by the next
 IFD/image pair in logical page order within the file structure,
 until all pages have been defined.

 The format for the TIFF Header is as defined in [TIFF].  When
 writing TIFF-F minimum subset files, the value for the byte order in
 the Header SHOULD be II (0x4949, denoting that the bytes in the TIFF
 file are in LSB first (little-endian) order.

 This results in a TIFF header whose content is as shown in Figure
 3.2.

     Offset |   Description     | Type   |     Value          |
   +--------+-------------------+--------+--------------------+
   |   0    |   Byte Order      | Short  |  0x4949 (II)       |
   +--------+-------------------+--------+--------------------+
   |   2    |   Version         | Short  |  42                |
   +--------+-------------------+--------+--------------------+
   |   4    | Offset of 0th IFD | Long   |  0x 0000 0008      |
   +--------+-------------------+--------+--------------------+

   Figure 3.2: Image File Header for Minimum Subset TIFF-F Files


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 3.7  Technical Implementation Issues

 3.7.1   Strips

  Those new to TIFF may not be familiar with the concept of "strips"
  embodied in the three fields RowsPerStrip, StripByteCount,
  StripOffsets.

  In general, third-party implementations that read and write TIFF
  files expect the image to be divided into "strips," also known as
  "bands."  Each strip contains a few lines of the image. By using
  strips, a TIFF reader need not load the entire image into memory,
  thus enabling it to fetch and decompress small random portions of
  the image as necessary.

  The dimensions of a strip are described by the RowsPerStrip and
  StripByteCount fields.  The location in the TIFF file of each strip
  is contained in the StripOffsets field.

  The size of TIFF-F strips is application dependent.  The
  recommended  approach for multi-page TIFF-F images is to represent
  each page as a single strip.

 3.7.2  Bit Order

  The default bit order in Baseline TIFF per [TIFF] is indicated by
  FillOrder=1, where bits are not reversed before being stored.
  However, TIFF-F typically utilizes the setting of FillOrder=2,
  where the bit order within bytes is reversed before storage (i.e.,
  bits are stored with the Least Significant Bit first).

  Facsimile data appears on the phone line in bit-reversed order
  relative to its description in CCITT Recommendation T.4.
  Therefore, a wide majority of facsimile implementations choose this
  natural order for storage. Nevertheless, TIFF-F readers must be
  able to read data in both bit orders.

 3.7.3  Multi-Page

  Many existing implementations already read TIFF-F like files, but
  do not support the multi- page field.  Since a multi-page format
  greatly simplifies file management in fax application software,
  TIFF-F specifies multi-page documents (NewSubfileType = 2) as the
  standard case.

 3.7.4 Compression

  In Group 3 facsimile, there are three compression methods which had
  been standardized as of 1994 and are in common use.  The ITU-T T.4
  recommendation defines a one-dimensional compression method known
  as Modified Huffman (MH) and a two-dimensional method known as
  Modified READ (MR) (READ is short for Relative Element Address
  Designate).  In 1984, a somewhat more efficient compression method
  known as Modified Modified READ (MMR) was defined in the T.6


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  recommendation.  It was originally defined for use with Group 4
  facsimile, so that this compression method has been commonly called
  Group 4 compression.  In 1991, the MMR method was approved for use
  in Group 3 facsimile and has since been widely utilized.

  TIFF-F permits three different compression methods.  In the most
  common practice, the one-dimensional compression method (Modified
  Huffman) is used.  This is specified by setting the value of the
  Compression field to 3 and then setting bit 0 of the T4Options
  field to 0.  Alternatively, the two dimensional Modified READ
  method (which is much less frequently used in historical TIFF-F
  implementations) may be selected by setting bit 0 to a value of 1.

  Optionally, depending upon the implementation requirements, the
  more efficient two-dimensional compression method from T.6 (i.e.
  MMR or "Group 4 compression") may be selected.  This method is
  selected by setting the value of the Compression field to 4 and
  then setting the value of the first two bits (and all unused bits)
  of T6options to 0.   More information to aid the implementer in
  making a compression selection is contained in section 3.8 on
  Implementation Warnings.

 3.7.5  Example Use of Page-quality Fields

  Here are examples for writing the CleanFaxData,  BadFaxLines, and
  ConsecutiveBadFaxLines fields:

      1.  Facsimile hardware does not provide page quality
          information: MUST NOT write page-quality fields.
      2.  Facsimile hardware provides page quality information, but
          reports no bad lines.  Write only BadFaxLines = 0.
      3.  Facsimile hardware provides page quality information, and
          reports bad lines.  Write both BadFaxLines and
          ConsecutiveBadFaxLines.  Also write CleanFaxData = 1 or 2 if
          the hardware's regeneration capability is known.
      4.  Source image data stream is error-corrected or otherwise
          guaranteed to be error-free such as for a computer generated
          file:  SHOULD NOT write page-quality fields.

 3.7.6   Use of TIFF-F for Streaming Applications

  TIFF-F has historically been used for handling fax image files in
  implementations such as store and forward messaging where the entire
  size of the file is known in advance.  While TIFF-F may also
  possibly be used as a file format for cases such as streaming
  applications, different assumptions may be required than those
  provided in this document (e.g., the entire size and number of pages
  within the image are not known in advance).  As a result, a
  definition for the streaming application of TIFF-F is beyond the
  scope of this document.


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 3.7.7  TIFF-F Export and Import

  Fax implementations that do not wish to support TIFF-F as a native
  format may elect to support it as import/export medium.

  Export

  It is recommended that implementations export multiple page TIFF-F
  files without manipulating fields and values.   Historically, some
  TIFF-F writers have attempted to produce individual single-page
  TIFF-F files with modified NewSubFileType and PageNumber (page one-
  of-one) values for export purposes.  However, there is no easy way
  to link such multiple single page files together into a logical
  multiple page document, so that this practice is not recommended.

  Import

  A TIFF-F reader MUST be able to handle a TIFF-F file containing
  multiple pages.

 3.8  Implementation Warnings

 3.8.1  Uncompressed data

  TIFF-F requires the ability to read and write at least one-
  dimensional T.4 Huffman ("compressed") data.  Uncompressed data is
  not allowed.  This means that the "Uncompressed" bit in T4Options
  or T6Options must be set to 0.

 3.8.2  Encoding and Resolution

  Since two-dimensional encoding is not required for Group 3
  compatibility, some historic TIFF-F readers have not been able to
  read such files.  The minimum subset of TIFF-F REQUIRES support for
  one dimensional (Modified Huffman) files, so this choice maximizes
  portability.  However, implementers seeking greater efficiency
  SHOULD use T.6 MMR compression when writing TIFF-F files.  Some
  TIFF-F readers will also support two-dimensional Modified READ
  files.  Implementers that wish to have the maximum flexibility in
  reading TIFF-F files SHOULD support all three of these compression
  methods (MH, MR and MMR).

  For the case of resolution, almost all facsimile products support
  both standard (98 dpi) vertical resolution  and "fine" (196 dpi)
  resolution.  Therefore, fine-resolution files are quite portable in
  the real world.

  In 1993, the ITU-T added support for higher resolutions in the T.30
  recommendation including 200 x 200, 300 x 300, 400 x 400 in dots
  per inch based units.  At the same time, support was added for


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  metric dimensions which are equivalent to the following inch based
  resolutions: 391v x 204h and 391v x 408h.  Therefore, the full set
  of inch-based equivalents of the new resolutions are supported in
  the TIFF-F writer, since they may appear in some image data streams
  received from Group 3 facsimile devices.  However, many facsimile
  terminals and older versions of  TIFF-F readers are likely to not
  support the use of these higher resolutions.

  Per [T.4], it is permissible for implementations to treat the
  following XResolution values as being equivalent: <204,200> and
  <400,408>.  In a similar respect, the following YResolution values
  may also be treated as being equivalent: <98, 100>, <196, 200>, and
  <391, 400>.   These equivalencies were allowed by [T.4] to permit
  conversions between inch and metric based facsimile terminals.

  In a similar respect, the optional support of metric based
  resolutions in the TIFF-F reader (i.e. 77 x 38.5 cm) is included
  for completeness, since they are used in some legacy TIFF-F
  implementations, but this use is not recommended for the creation
  of TIFF-F files by a writer.

 3.8.3  EOL byte-aligned

  The historical convention for TIFF-F has been that all EOLs in
  Modified Huffman or Modified READ data must be byte-aligned.
  However, Baseline TIFF has permitted use of non-byte-aligned EOLs
  by default, so that a large percentage of TIFF-F reader
  implementations support both conventions.   Therefore, the minimum
  subset of TIFF-F as defined in this document includes support for
  both byte-aligned and non-byte-aligned EOLs.

  An EOL is said to be byte-aligned when Fill bits have been added as
  necessary before EOL codes such that EOL always ends on a byte
  boundary, thus ensuring an  EOL-sequence of a one byte preceded by
  a zero nibble: xxxx0000 00000001.

  Modified Huffman encoding encodes bits, not bytes. This means that
  the end-of-line token may end in the middle of a byte. In byte
  alignment, extra zero bits (Fill) are added so that the first bit
  of data following an EOL begins on a byte boundary. In effect, byte
  alignment relieves application software of the burden of bit-
  shifting every byte while parsing scan lines for line-oriented
  image manipulation (such as writing a TIFF file).

  For Modified READ encoding, each line is terminated by an EOL and a
  one bit tag bit.  Per [T.4], the value of the tag bit is 0 if the
  next line contains two dimensional data and 1 if the next line is a
  reference line.   To maintain byte alignment, fill bits are added
  before the EOL/tag bit sequence, so that the first bit of data
  following an MR tag bit begins on a byte boundary.


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 3.8.4  EOL

  As illustrated in FIGURE 1/T.4 in [T.4], facsimile documents encoded
  with Modified Huffman begin with an EOL (which in TIFF-F may be
  byte-aligned). The last line of the image is not terminated by an
  EOL.  In a similar respect, images encoded with Modified READ two
  dimensional encoding begin with an EOL, followed by a tag bit.

 3.8.5  RTC Exclusion

  Aside from EOLs, TIFF-F files have historically only contained
  image data. This means that implementations which wish to maintain
  strict conformance with the rules in [TIFF] and compatibility with
  historical TIFF-F, SHOULD NOT include the Return To Control
  sequence (RTC) (consisting of 6 consecutive EOLs) when writing TIFF-
  F files.   However, implementations which need to support
  "transparency" of [T.4] image data MAY include RTCs when writing
  TIFF-F files if the flag settings of the T4Options field are set
  for non-byte aligned MH or MR image data.  Implementors of TIFF
  readers should also be aware that there are some existing TIFF-F
  implementations which include the RTC sequence in MH/MR image data.
  Therefore, TIFF-F readers MUST be able to process files which do
  not include RTCs and SHOULD be able to process files which do
  include RTCs.

 3.8.6  Use of EOFB for T.6 Compressed Images

  TIFF-F pages which are encoded with the T.6 Modified Modified READ
  compression method MUST include an "end-of-facsimile-block" (EOFB)
  code at the end of each coded strip. Per [TIFF], the EOFB code is
  followed by pad bits as needed to align on a byte boundary.   TIFF
  readers SHOULD ignore any bits other than pad bits beyond the EOFB.

 3.9  TIFF-F Fields Summary

  Implementations may choose to implement a TIFF-F Reader, TIFF-F
  Writer or both, depending upon application requirements.  The TIFF-
  F Reader is typically used to read an existing TIFF-F file which
  resides on a computer or peripheral device.  The TIFF-F Writer is
  typically used to convert a bi-level image bit stream into a TIFF-F
  compliant file. For many Internet applications, only the Reader
  needs to be implemented. The specific field support required for
  TIFF-F Readers and Writers is summarized below.

 3.9.1  TIFF Reader

  The fields in the following table are specified for a TIFF-F
  Reader.  The range of values for required and recommended fields
  are as shown.  The minimum subset of values are also shown. If


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  required fields are omitted in a TIFF-F file, the Baseline TIFF
  default value will apply.  Image data must not have any coding
  errors. In the table, certain fields have a value that is a
  sequence of flag bits (e.g. T4Options).  An implementation should
  test the setting of the relevant flag bits individually to allow
  extensions to the sequence of flag bits to be appropriately
  ignored.

  As noted within [TIFF], a TIFF file begins with an 8-byte image
  file header, of which the first two bytes (0-1) contain the byte
  order within the file.  The permissible values are:

      II- Byte order from least significant byte to the most
          significant byte (little-endian)
      MM - byte order is always from most significant to least
          significant (big-endian)

  For a TIFF-F Reader, the legal values are:

      ByteOrder: MM,II (Either byte order is allowed)

 3.9.1.1  Fields for TIFF-F Reader

  Recommended Fields in the table are shown with an asterisk (*).

  Other fields may be present, but they should be of an
  informational nature, so that a reader can elect to ignore them.

  Informational fields which are often present in TIFF-F images
  are:
     Software, Datetime, BadFaxLines, CleanFaxData and
     ConsecutiveBadFaxLines.


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  Field             | Values      | Minimum     | Comment
  ------------------|-------------|-------------|----------------------
  BitsPerSample     | 1           | 1           |one bit per sample
  Compression       | 3,4         | 3           |3 for T.4 (MH, MR)
                    |             |             |4 for T.6 - MMR
  FillOrder         | 2,1         | 2           |LSB first or MSB first
  ImageWidth        | 1728, 2048, | 1728        |depends on XResolution
                    | 2432, 2592, |             |
                    | 3072, 3648, |             |
                    | 3456, 4096, |             |
                    | 4864        |             |
  ImageLength       | >0          |             |required
  NewSubFileType    | Bit 1 = 1   | Bit 1 = 1   |single page of
                    |             |             |multipage file
  Orientation *     | 1           |             |1st row=top left,
                    |             |             | 1st col=top
  PageNumber        | X/X         | 0/1         |pg/tot, 0 base,
                    |             |             | tot in 1st IFD
  PhotometricInterp | 0,1         | 0           |0 is white
  ResolutionUnit    | 2,3         | 2           |inches (default)
  RowsPerStrip      |=ImageLength |=ImageLength |
                    | or other    |             |
  SamplesPerPixel   | 1           | 1           |one sample per pixel
  StripByteCounts   | >0          |             |required
  StripOffsets      | >0          |             |required
  T4Options         | Bit 0 = 0,1 | Bit 0 = 0   |MH,MR(incl if not MMR)
                    | Bit 1 = 0   | Bit 1 = 0   |
                    | Bit 2 = 0,1 | Bit 2 = 0,1 | Non-Byte-aligned and
                    |             |             | Byte-Aligned EOLs
  T6Options         | 0           |             |MMR (incl only if MMR)
  XResolution       | 204,200,300,| 204         | If unit is per inch
                    | 400,408,    |             |
                    | 77          |             | If unit is per cm
  YResolution       | 196,98,100, | 196,98      | If unit is per inch
                    | 200,300,391,|             |
                    | 400,        |             |
                    | 77,38.5     |             | If unit is per cm
  ------------------|-------------|-------------|----------------------


 3.9.2  TIFF-F Writer

  For the case of writing (creating) a TIFF-F file format from an
  image data stream or other raster data, implementations SHOULD
  write files which can be read by a TIFF-F Reader as defined in
  3.9.1.  It is recommended that all fields from the table in 3.9.1.1
  SHOULD be included when writing TIFF-F files in order to  minimize
  dependencies on default values. Image data must not have any coding
  errors.

  Other fields may be present, but they should be of an informational
  nature, so that a Reader may elect to ignore them.

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  For the case of writing "minimum subset" TIFF-F files, the rules
  defined in section 3.6 apply.

  Informational fields that may be useful for TIFF-F files are:
      Software, Datetime, BadFaxLines, ConsecutiveBadFaxLines

  TIFF Writers SHOULD only generate the fields that describe
  facsimile image quality when the image has been generated from a
  fax image data stream where error correction (e.g. Group 3 Error
  Correction Mode) was not used.  These fields are:  CleanFaxData,
  BadFaxLines and ConsecutiveBadFaxLines.


 4.  MIME sub-type image/tiff

  [TIFFREG] describes the registration of the MIME content-type image/
  tiff to refer to TIFF 6.0 encoded image data.   When transported by
  MIME, the TIFF content defined by this document must be encoded
  within an image/tiff content type. In addition, an optional
  "application" parameter is defined for image/tiff to identify a
  particular application's subset of TIFF and TIFF extensions for the
  encoded image data, if it is known. Typically, this would be used
  to assist the recipient in dispatching a suitable rendering package
  to handle the display or processing of the image file.

 4.1 Refinement of MIME sub-type image/tiff for Application F

  Since this document defines a facsimile specific profile of TIFF,
  it is useful to note an appropriate application parameter for the
  image/tiff MIME content-type.

  The "faxbw" application parameter is defined for black and white
  facsimile.  It is suitable for use by applications that can process
  one or more TIFF for facsimile profiles or subsets used for the
  encoding of black and white facsimile data.

  Since this document defines a profile of TIFF for facsimile which
  is suitable for use with black and white facsimile image data,
  applications which use this profile or its minimum subset should set
  the value of the application parameter to "faxbw".

  An example of the use of the image/tiff MIME Content-type with the
  application parameter set with the value "faxbw" follows:

   Example:

         Content-type: image/tiff; application=faxbw

  In this example, use of this parameter value will enable
  applications to identify the content as being within a profile or
  subset of TIFF for Facsimile that is suitable for encoding black
  and white image data, before attempting to process the image data.

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 5.  Implementation Usage

 5.1 Internet Fax Usage

  The usage of TIFF-F is envisioned as a component of Internet Fax.
  It is anticipated that Internet Fax may use both a TIFF-F Reader
  and TIFF-F Writer. The details of the Internet Fax services and
  their use of TIFF-F will be specified in other documents.

 5.2 VPIM Usage

  The Application F of TIFF (i.e. TIFF-F content) is a secondary
  component of the VPIM Message as defined in [VPIM2].  Voice
  messaging systems can often handle fax store-and-forward
  capabilities in addition to traditional voice message store-and-
  forward functions.  As a result, TIFF-F fax messages can optionally
  be sent between compliant VPIM systems, and may be rejected if the
  recipient system cannot deal with fax.

  Refer to the VPIM Specification for proper usage of this content.


 6.  Security Considerations

  This document describes the encoding for TIFF-F, which is a profile
  of the TIFF encoding for facsimile.  As such, it does not create any
  security issues not already identified in [TIFFREG], in its use of
  fields as defined in [TIFF]. There are also new TIFF fields defined
  within this specification, but they are of a purely descriptive
  nature, so that no new security risks are incurred.

  Further, the encoding specified in this document does not in any
  way preclude the use of any Internet security protocol to encrypt,
  authenticate, or non-repudiate TIFF-F encoded facsimile messages.


 7.  Authors' Addresses

  Glenn W. Parsons
  Northern Telecom
  P.O. Box 3511, Station C
  Ottawa, ON  K1Y 4H7
  Canada
  Phone: +1-613-763-7582
  Fax:   +1-613-763-2697
  Email: Glenn.Parsons@Nortel.ca


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  James Rafferty
  Human Communications
  12 Kevin Drive
  Danbury, CT 06811-2901
  USA
  Phone: +1-203-746-4367
  Fax:   +1-203-746-4367
  Email: Jrafferty@worldnet.att.net


 8. References

  [MIME1] N. Freed and N. Borenstein,  "Multipurpose Internet Mail
       Extensions (MIME) Part One: Format of Internet Message
       Bodies", RFC 2045, Innosoft, First Virtual, Nov 1996
  [MIME4] N. Freed and N. Borenstein,  "Multipurpose Internet Mail
       Extensions (MIME) Part Four: Registration Procedures", RFC
       2048, Innosoft, First Virtual, Nov 1996.
  [REQ] S. Bradner, "Key words for use in RFCs to Indicate
       Requirement Levels", RFC 2119, March 1997.
  [T.30] ITU-T Recommendation T.30 - "Procedures for Document
       Facsimile Transmission in the General Switched Telephone
       Network", June, 1996
  [T.4] ITU-T Recommendation T.4 - "Standardization of Group 3
       Facsimile Apparatus for Document Transmission", June, 1996
  [T.6] ITU-T Recommendation T.6 - "Facsimile Coding Schemes and
       Coding Control Functions for Group 4 Facsimile Apparatus",
       March, 1993
  [TIFF] Adobe Developers Association, TIFF (TM) Revision 6.0 -
       Final, June 3, 1992.
  [TIFFREG] G. Parsons, J. Rafferty and S. Zilles, "Tag Image File
       Format (TIFF) - image/tiff:  MIME Sub-type Registration ",Work
       In Progress, <draft-ietf-fax-tiff-reg-04.txt>, January 1997.
  [VPIM2] G. Vaudreuil and G. Parsons, "Voice Profile for Internet
       Mail - version 2", Work In Progress, <draft-ema-vpim-06.txt>,
       November 1997.


  9.  Full Copyright Statement

  Copyright (C) The Internet Society (1997).  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 implmentation 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


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  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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