URI Fragment Identifiers for the text/csv Media Type

Abstract

This memo defines URI fragment identifiers for text/csv MIME entities. These fragment identifiers make it possible to refer to parts of a text/csv MIME entity identified by row, column, or cell. Fragment identification can use single items or ranges.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7111.

IESG Note

The change to the text/csv media type registration requires IESG approval, as the IESG is the change controller for that registration. The IESG has, after consultation with the IETF community, approved the change, which is specified in Section 5 of this document.

Copyright Notice

Copyright © 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  What is text/csv? . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Why text/csv Fragment Identifiers?  . . . . . . . . . . .   3
       1.2.1.  Motivation  . . . . . . . . . . . . . . . . . . . . .   3
       1.2.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Incremental Deployment  . . . . . . . . . . . . . . . . .   4
     1.4.  Notation Used in this Memo  . . . . . . . . . . . . . . .   4
   2.  Fragment Identification Methods . . . . . . . . . . . . . . .   5
     2.1.  Row-Based Selection . . . . . . . . . . . . . . . . . . .   5
     2.2.  Column-Based Selection  . . . . . . . . . . . . . . . . .   6
     2.3.  Cell-Based Selection  . . . . . . . . . . . . . . . . . .   6
     2.4.  Multi-Selections  . . . . . . . . . . . . . . . . . . . .   7
   3.  Fragment Identification Syntax  . . . . . . . . . . . . . . .   7
   4.  Fragment Identifier Processing  . . . . . . . . . . . . . . .   8
     4.1.  Syntax Errors in Fragment Identifiers . . . . . . . . . .   8
     4.2.  Semantics of Fragment Identifiers . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  The text/csv media type . . . . . . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12

1. Introduction

This memo updates the text/csv media type defined in RFC 4180 [RFC4180] by defining URI fragment identifiers for text/csv MIME entities.

The change to the text/csv media type registration required IESG approval, as the IESG is the change controller for that registration. The IESG has, after consultation with the IETF community, approved the change, which is specified in Section 5 of this document.

This section gives an introduction to the general concepts of text/csv MIME entities and URI fragment identifiers and discusses the need for fragment identifiers for text/csv and deployment issues. Section 2 discusses the principles and methods on which this memo is based. Section 3 defines the syntax, and Section 4 discusses processing of text/csv fragment identifiers.

1.1. What is text/csv?

Internet Media Types (often referred to as "MIME types") as defined in RFC 2045 [RFC2045] and RFC 2046 [RFC2046] are used to identify different types and subtypes of media. The text/csv media type is defined in RFC 4180 [RFC4180], using US-ASCII [ASCII] as the default character encoding (other character encodings can be used as well). Apart from a media type parameter for specifying the character encoding ("charset"), there is a second media type parameter ("header") that indicates whether there is a header row in the CSV document or not.

1.2. Why text/csv Fragment Identifiers?

URIs are the identification mechanism for resources on the Web. The URI syntax specified in RFC 3986 [RFC3986] optionally includes a so- called "fragment identifier", separated by a number sign ("#"). The fragment identifier consists of additional reference information to be interpreted by the client after the retrieval action has been successfully completed. The semantics of a fragment identifier is a property of the media type resulting from a retrieval action, regardless of the URI scheme used in the URI reference. Therefore, the format and interpretation of fragment identifiers is dependent on the media type of the retrieval result.

1.2.1. Motivation

Similar to the motivation in RFC 5147 [RFC5147], which defines fragment identifiers for plain text files, referring to specific parts of a resource can be very useful because it enables users and applications to create more specific references. Users can create references to a particular point of interest within a resource, rather than referring to the complete resource. Even though it is suggested that fragment identification methods are specified in a media type's registration (see [RFC6838]), many media types do not have fragment identification methods associated with them.

Fragment identifiers are only useful if supported by the client, because they are only interpreted by the client. Therefore, a new fragment identification method will require some time to be adopted by clients, and older clients will not support it. However, because the URI still works even if the fragment identifier is not supported (the resource is retrieved, but the fragment identifier is not interpreted), rapid adoption is not highly critical to ensure the success of a new fragment identification method.

1.2.2. Use Cases

Fragment identifiers for text/csv as defined in this memo make it possible to refer to specific parts of a text/csv MIME entity. Use cases include, but are not limited to, selecting a part for visual rendering, stream processing, making assertions about a certain value (provenance, confidence, comments, etc.), or data integration.

1.3. Incremental Deployment

As long as text/csv fragment identifiers are not supported universally, it is important to consider the implications of incremental deployment. Clients (for example, Web browsers) not supporting the text/csv fragment identifier described in this memo will work with URI references to text/csv MIME entities, but they will fail to understand the identification of the sub-resource specified by the fragment identifier, and thus will behave as if the complete resource was referenced. This is a reasonable fallback behavior and, in general users, should take into account the possibility that a program interpreting a given URI will fail to interpret the fragment identifier part. Since fragment identifier evaluation is local to the client (and happens after retrieving the MIME entity), there is no reliable way for a server to determine whether a requesting client is using a URI containing a fragment identifier.

1.4. Notation Used in this Memo

The capitalized 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 RFC 2119 [RFC2119].

2. Fragment Identification Methods

This memo specifies fragment identification using the following methods: "row" for row selections, "col" for columns selections, and "cell" for cell selections.

Throughout the sections below, the following example table in CSV (having 7 rows, including one header row, and 3 columns) is used:

   date,temperature,place
   2011-01-01,1,Galway
   2011-01-02,-1,Galway
   2011-01-03,0,Galway
   2011-01-01,6,Berkeley
   2011-01-02,8,Berkeley
   2011-01-03,5,Berkeley

2.1. Row-Based Selection

To select a specific record, the "row" scheme followed by a single number is used (the first row is at position 1).

   http://example.com/data.csv#row=4

The above CSV fragment identifies the fourth row:

   2011-01-03,0,Galway

Fragments can also select ranges of rows:

   http://example.com/data.csv#row=5-7

The above CSV fragment identifies three consecutive rows:

   2011-01-01,6,Berkeley
   2011-01-02,8,Berkeley
   2011-01-03,5,Berkeley

The value "*" can be used to indicate the last row, so the previous URI is equivalent to:

   http://example.com/data.csv#row=5-*

2.2. Column-Based Selection

To select values from a certain column, the "col" scheme is used, followed by a position (the first column is at position 1):

   http://example.com/data.csv#col=2

The above CSV fragment addresses the second column, identifying the column:

   temperature
   1
   -1
   0
   6
   8
   5

The "col" scheme can also be used to identify ranges of columns:

   http://example.com/data.csv#col=1-2

The above CSV fragment addresses the first and second column:

   date,temperature
   2011-01-01,1
   2011-01-02,-1
   2011-01-03,0
   2011-01-01,6
   2011-01-02,8
   2011-01-03,5

As for rows, the value "*" can be used to indicate the last column.

2.3. Cell-Based Selection

To select particular fields, the "cell" scheme is used, followed by a row number, a comma, and a column number.

   http://example.com/data.csv#cell=4,1

The above CSV fragment addresses the field in the first column within the fourth row, yielding:

   2011-01-03

It is also possible to select cell-based fragments that have more than just one cell, in which case the cell selection uses the same range syntax as for row and column range selections. For these selections, the syntax uses the upper left-hand cell as the starting point of the selection, followed by a minus sign, and then the lower right-hand cell as the end point of the selection.

   http://example.com/data.csv#cell=4,1-6,2

The above CSV fragment selects a region that starts at the fourth row and the first column and ends at the sixth row and the second column:

   2011-01-03,0
   2011-01-01,6
   2011-01-02,8

2.4. Multi-Selections

Row, column, and cell selections can make more than one selection, in which case the individual selections are separated by semicolons. In these cases, the resulting fragment may be a disjoint fragment, such as the selection "#row=3;6" for the example CSV, which would select the third and the sixth row. It is up to the user agent to decide how to handle disjoint fragments, but since they are allowed, user agents should be prepared to handle disjoint fragments.

3. Fragment Identification Syntax

The syntax for the text/csv fragment identifiers is as follows.

The following syntax definition uses ABNF as defined in RFC 5234 [RFC5234], including the rule DIGIT.

   NOTE:  In the descriptions that follow, specified text values MUST be
   
      used exactly as given, using exactly the indicated lowercase
      letters.  In this respect, the ABNF usage differs from [RFC5234].
   
   csv-fragment =  rowsel / colsel / cellsel
   rowsel       =  "row=" singlespec 0*( ";" singlespec)
   colsel       =  "col=" singlespec 0*( ";" singlespec)
   cellsel      =  "cell=" cellspec 0*( ";" cellspec)
   singlespec   =  position [ "-" position ]
   cellspec     =  cellrow "," cellcol [ "-" cellrow "," cellcol ]
   cellrow      =  position
   cellcol      =  position
   position     =  number / "*"
   number       =  1*( DIGIT )

4. Fragment Identifier Processing

Applications implementing support for the mechanism described in this memo MUST behave as described in the following sections.

4.1. Syntax Errors in Fragment Identifiers

If a fragment identifier contains a syntax error (i.e., does not conform to the syntax specified in Section 3), then it MUST be ignored by clients. Clients MUST NOT make any attempt to correct or guess fragment identifiers. Syntax errors MAY be reported by clients.

4.2. Semantics of Fragment Identifiers

Rows and columns in CSV are counted from one. Positions thus refer to the rows and columns starting from position 1, which identifies the first row or column of a CSV. The special character "*" can be used to refer to the last row or column of a CSV, thus allowing fragment identifiers to easily identify ranges that extend to the last row or column.

If single selections refer to non-existing rows or columns (i.e., beyond the size of the CSV), they MUST be ignored.

If ranges extend beyond the size of the CSV (by extending to rows or columns beyond the size of the CSV), they MUST be interpreted to only extend to the actual size of the CSV.

If selections of ranges of rows, ranges of columns, or ranges of cells are specified in a way so that they select "inversely" (i.e., "#row=10-5" or "#cell=10,10-5,5"), they MUST be ignored.

Each specification of an identified region is processed independently, and ignored specifications (because of reasons listed in the previous paragraphs) do not cause the whole fragment identifier to fail, they just mean that this single specification is ignored. For the example file, the fragment identifier "#row=1-2;5-4;13-16" does identify the first two rows: the second specification is an "inverse" specification and thus is ignored, and the third specification targets rows beyond the actual size of the CSV and thus is also ignored.

The complete fragment identifier identifies all the successfully evaluated identified parts as a single identified fragment. This fragment can be disjoint because of multiple selections. Multiple selections also can result in overlapping individual parts, and it is up to the user agent how to process such a fragment and whether the individual parts are still made accessible (i.e., visualized in visual user agents) or are presented as one unit. For example, the fragment identifier "#row=3-6;4-5" contains a second identified part that is completely contained in the first identified part. Whether a user agent maintains this selection as two parts, or simply signals that the identified fragment spans from the third to the sixth row, is up for the user agent to decide.

5. IANA Considerations

IANA has added a reference to this specification in the text/csv media type registration.

5.1. The text/csv media type

The Internet media type [RFC6838] for a CSV document is text/csv. The following registration has been copied from the original registration of text/csv [RFC4180], with the exception of the added fragment identification considerations and added security considerations for fragment identifiers.

   Type name:  text
   
   Subtype name:  csv
   
   Required parameters:  none
   
   Optional parameters:  charset, header

The "charset" parameter specifies the charset employed by the CSV content. In accordance with RFC 6657 [RFC6657], the charset parameter SHOULD be used, and if it is not present, UTF-8 SHOULD be assumed as the default (this implies that US-ASCII CSV will work, even when not specifying the "charset" parameter). Any charset defined by IANA for the "text" tree may be used in conjunction with the "charset" parameter.

The "header" parameter indicates the presence or absence of the header line. Valid values are "present" or "absent". Implementors choosing not to use this parameter must make their own decisions as to whether the header line is present or absent.

   Encoding considerations:  CSV MIME entities consist of binary data
      [RFC6838].  As per Section 4.1.1. of RFC 2046 [RFC2046], this
      media type uses CRLF to denote line breaks.  However, implementers
      should be aware that some implementations may use other values.

Security considerations:

Text/csv consists of nothing but passive text data that should not pose any direct risks. However, it is possible that malicious data may be included in order to exploit buffer overruns or other bugs in the program processing the text/csv data.

The text/csv format provides no confidentiality or integrity protection, so if such protections are needed, they must be supplied externally.

The fact that software implementing fragment identifiers for CSV and software not implementing them differs in behavior, and the fact that different software may show documents or fragments to users in different ways, can lead to misunderstandings on the part of users. Such misunderstandings might be exploited in a way similar to spoofing or phishing.

Implementers and users of fragment identifiers for CSV text should also be aware of the security considerations in RFC 3986 [RFC3986] and RFC 3987 [RFC3987].

   Interoperability considerations:  Due to lack of a single
      specification, there are considerable differences among
      implementations.  Implementers should "be conservative in what you
      do, be liberal in what you accept from others" (RFC 793 [RFC0793])
      when processing CSV files.  An attempt at a common definition can
      be found in Section 2.  Implementations deciding not to use the
      optional "header" parameter must make their own decision as to
      whether the header is absent or present.
   
   Published specification:  While numerous private specifications exist
      for various programs and systems, there is no single "master"
      specification for this format.  An attempt at a common definition
      can be found in Section 2 of RFC 4180 [RFC4180].
   
   Applications that use this media type:  Spreadsheet programs and
      various data conversion utilities.
   
   Fragment identifier considerations:  Fragment identification for
      text/csv is supported by using fragment identifiers as specified
      by RFC 7111.

Additional information:

      Magic number(s):  none
      
      File extension(s):  CSV
      
      Macintosh file type code(s):  TEXT

Person & email address to contact for further information:

      Yakov Shafranovich <ietf@shaftek.org> and
      Erik Wilde <dret@berkeley.edu>
   
   Intended usage:  COMMON
   
   Restrictions on usage:  none

Author:

      Yakov Shafranovich <ietf@shaftek.org> and
      Erik Wilde <dret@berkeley.edu>
   
   Change controller:  IESG

6. Security Considerations

The security considerations for text/csv fragment identifiers are listed in the respective section of the media type registration in Section 5.1.

7. Acknowledgements

Thanks for comments and suggestions provided by Nevil Brownlee, Richard Cyganiak, Ian Davis, Gannon Dick, Leigh Dodds, and Barry Leiba.

8. References

8.1. Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.
   
   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.
   
   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.
   
   [RFC4180]  Shafranovich, Y., "Common Format and MIME Type for Comma-
              Separated Values (CSV) Files", RFC 4180, October 2005.
   
   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.
   
   [RFC6657]  Melnikov, A. and J. Reschke, "Update to MIME regarding
              "charset" Parameter Handling in Textual Media Types", RFC
              6657, July 2012.

8.2. Informative References

   [ASCII]    ANSI X3.4-1986, "Coded Character Set - 7-Bit American
              National Standard Code for Information Interchange", STD
              63, RFC 3629, 1992.
   
   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.
   
   [RFC5147]  Wilde, E. and M. Duerst, "URI Fragment Identifiers for the
              text/plain Media Type", RFC 5147, April 2008.
   
   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

Authors' Addresses

Michael Hausenblas
MapR Technologies
32 Bushypark Lawn
Galway
Ireland

   Phone: +353-86-0215164
   EMail: mhausenblas@maprtech.com
   URI:   http://mhausenblas.info

Erik Wilde
UC Berkeley

   EMail: dret@berkeley.edu
   URI:   http://dret.net/netdret/

Jeni Tennison
Open Data Institute
65 Clifton Street
London EC2A 4JE
U.K.

   Phone: +44-797-4420482
   EMail: jeni@jenitennison.com
   URI:   http://www.jenitennison.com/blog/