Network Working Group Y. Shafranovich Internet-Draft Amazon Web Services (AWS) Intended status: Informational 5 September 2023 Expires: 8 March 2024 Common Format and MIME Type for Comma-Separated Values (CSV) Files draft-shafranovich-rfc4180-bis-05 Abstract This RFC documents the common format used for Comma-Separated Values (CSV) files and updates the associated MIME type "text/csv". Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents 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 "work in progress." This Internet-Draft will expire on 8 March 2024. Copyright Notice Copyright (c) 2023 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 (https://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 1.1. Terminology 1.2. Motivation For and Status of This Document 2. Definition of the CSV Format 2.1. High level description 2.2. Default charset, binary content and line break values 2.3. ABNF Grammar 3. Common implementation concerns 3.1. Null values 3.2. Empty files 3.3. Empty lines 3.4. Fields spanning multiple lines 3.5. Unique header names 3.6. Whitespace outside quoted fields 3.7. Other field separators 3.8. Escaping double quotes 3.9. BOM header 3.10. Bidirectional text 3.11. Comments 3.12. IANA Considerations 4. Update to MIME Type Registration of text/csv 5. Security Considerations 6. Acknowledgments 7. References 7.1. Normative References 7.2. Informative References Appendix A. Major changes since RFC4180 Appendix B. Changes since the -00 draft Appendix C. Changes since the -01 draft Appendix D. Changes since the -02 draft Appendix E. Changes since the -03 draft Appendix F. Changes since the -04 draft Appendix G. Note to Readers Author's Address 1. Introduction The comma separated values format (CSV) has been used as a common way to exchange data between disparate systems and applications for many years. Surprisingly, while this format is very popular, it has never been formally documented and didn't have a media type registered. This was addressed in 2005 via publication of [RFC4180] and the concurrent registration of the "text/csv" media type. Since the publication of [RFC4180], the CSV format has evolved and this specification seeks to reflect these changes as well as update the "text/csv" media type registration. 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 1.2. Motivation For and Status of This Document The original motivation of [RFC4180] was to provide a reference in order to register the media type "text/csv". It tried to document existing practices at the time based on the approaches used by most implementations. This document continues to do the same, and updates the original document to reflect current practices for generating and consuming of CSV files. Both [RFC4180] and this document are published as informational RFC for the benefit of the Internet community and not intended to be used as formal standards. Implementers should consult [RFC1796] and [RFC2026] for crucial differences between IETF standards and informational RFCs. 2. Definition of the CSV Format While there had been various specifications and implementations for the CSV format (for ex. [CREATIVYST], [EDOCEO], [CSVW] and [ART])), prior to publication of [RFC4180] there is no attempt to provide a common specification. This section documents the format that seems to be followed by most implementations (incorporating changes since the publication of [RFC4180] and listing common implementation concerns). 2.1. High level description The CSV format uses line breaks to separate records, and commas to separate fields within a given record. The format is described as follows: 1. Each record is located on a separate line, ended by a line break (CR, LF or CRLF) indicating the end of this record. For example: aaa,bbb,cccCRLF zzz,yyy,xxxCRLF 2. The last record in the file MUST have an ending line break indicating the end of a record. For example: aaa,bbb,cccCRLF zzz,yyy,xxxCRLF 3. The first record in the file MAY be an optional header and MUST follow the same format as normal records. This header contains names corresponding to the fields in the file and SHOULD contain the same number of fields as the records in the rest of the file. For example: field_name_1,field_name_2,field_name_3CRLF aaa,bbb,cccCRLF zzz,yyy,xxxCRLF 4. Within each record, there MAY be zero or more fields, separated by commas. Each record SHOULD contain the same number of fields throughout the file. Spaces are considered part of a field and SHOULD NOT be ignored. The last field in the record MUST NOT be followed by a comma (since this will indicate an empty field following the comma). For example: aaa,bbb,cccCRLF 5. Each field MAY be enclosed in double quotes. If fields are not enclosed with double quotes, then double quotes MUST NOT appear inside the fields. For example: "aaa","bbb","ccc"CRLF zzz,yyy,xxxCRLF 6. Fields containing line breaks (CR, LF or CRLF), double quotes, or commas MUST be enclosed in double quotes. For example: "aaa","b CRLF bb","ccc"CRLF zzz,yyy,xxxCRLF 7. A double quote appearing inside a field MUST be escaped by preceding it with another double quote. For example: "aaa","b""bb","ccc"CRLF 2.2. Default charset, binary content and line break values Since the initial publication of [RFC4180], the default charset for "text/*" media types has been changed to UTF-8 (as per [RFC6657]) and [RFC7111]. This document reflects this change and the default charset for CSV files is now UTF-8. As per section 4.2.1 of [RFC6838], the "text/*" media types are defined as those reasonable to present to the user. While [RFC4180] restricted CSV contents to printable ASCII only, [RFC7111] updated the MIME registration to allow binary content in CSV entities. Therefore, this document has been updated to allow binary content within CSV files. Although section 4.1.1. of [RFC2046] defines CRLF to denote line breaks, implementers MAY recognize a single CR or LF as a line break (similar to section 3.1.1.3 of [RFC7231]). However, some implementations MAY use other values. 2.3. ABNF Grammar The ABNF grammar (as per [RFC5234]) appears as follows: file = [header] *(record) header = [field] *(COMMA field) linebreak record = [field] *(COMMA field) linebreak field = (escaped / non-escaped) escaped = DQUOTE *(textdata / COMMA / CR / LF / 2DQUOTE) DQUOTE non-escaped = *(textdata) textdata = %x00-09 / %x0B-0C / %x0E-21 / %x23-2B / %x2D-7F / UTF8-data ; all characters except LF, CR, DQUOTE and COMMA linebreak = CR / LF / CRLF COMMA = %x2C CR = %x0D ; as per section B.1 of [RFC5234] CRLF = CR LF ; as per section B.1 of [RFC5234] DQUOTE = %x22 ; as per section B.1 of [RFC5234] LF = %x0A ; as per section B.1 of [RFC5234] UTF8-data = UTF8-2 / UTF8-3 / UTF8-4 ; as per section 4 of [RFC3629] Note that the authoritative definition of UTF-8 is in section 2.5 of [UNICODE]. 3. Common implementation concerns This section describes some common concerns that may arise when producing or parsing CSV files. These are not part of the formal definition of CSV and are included for awareness only. Implementers may also use other means to handle these use cases including approaches like [CSVW]. 3.1. Null values Some implementations (such as databases) treat empty fields and null values differently. For these implementations, there is a need to define a special value representing a null. However, this specification does not attempt to define a default value for nulls. Example of a CSV file with nulls (if "NULL" is used to mark nulls): field_name_1,field_name_2,field_name_3CRLF aaa,bbb,cccCRLF zzz,NULL,xxxCRLF 3.2. Empty files Implementers should be aware that in accordance to this specification a file does not need to contain any comments or records. Therefore, an empty file with zero bytes is considered valid. 3.3. Empty lines This specification recommends but doesn't require having the same number of fields in every line. This allows CSV files to have empty lines without any fields at all. Implementors may choose to skip empty lines instead of parsing them but this specification does not dictate such behavior. Example of a CSV file with empty lines: field_name_1,field_name_2,field_name_3CRLF aaa,bbb,cccCRLF CRLF zzz,yyy,xxxCRLF However, if the records are only made up of one field it is not possible to differentiate between an empty line, and an empty and unquoted field. This differentiation might play an important role in some implementations such as database exports/imports. Example of a CSV file with empty lines and only one field per record: aaaCRLF CRLF bbbCRLF Note that some implementations may interpret the presence of a line break after the last record in the file as a start of a new but empty record. 3.4. Fields spanning multiple lines When quoted fields are used, it is possible for a field to span multiple lines, even when line breaks appear within such field. 3.5. Unique header names Implementers should be aware that some applications may treat header values as unique (either case-sensitive or case-insensitive). 3.6. Whitespace outside quoted fields When quoted fields are used, this document does not allow whitespace between double quotes and commas. Implementers should be aware that some applications may be more lenient and allow whitespace outside the double quotes. 3.7. Other field separators This document defines a comma as a field separator but implementers should be aware that some applications may use different values, especially with non-English languages. Those are outside the scope of this document and implementers should consult other efforts such as [CSVW]. 3.8. Escaping double quotes This document prescribes that a double quote appearing inside a field must be escaped by preceding it with another double quote. Implementers should be aware that some applications may choose to use a different escaping mechanism. 3.9. BOM header Applications that create text files with unicode character encoding might write a BOM (byte order mark) header in order to support multiple unicode encodings (like UTF-16 and UTF-32). Some applications might be able to read and properly interpret such a header, others could break. Implementors should review section 6 of [RFC3629] and section 23.8 of [UNICODE]. 3.10. Bidirectional text While most of the world's written languages are displayed left-to- right, many languages such as ones based on Hebrew or Arabic scripts are displayed primarily right-to-left. Implementers should consult the "bidirectional display" part in section 5 of [RFC6365] for further guidance. 3.11. Comments Some implementations may use the hash sign ("#") to mark lines that are meant to be commented lines. Such lines may contain any character until terminated by a line break (CR, LF or CRLF) and might appear in any line of the file (before or after the header). Comments should not be confused with a subsequent line of a multi- line field. If a first field of a record starts with a hash, it should be surrounded with double quotes to avoid being mistaken for a comment as per Section 2.1. Example of a CSV file containing comments: #commentCRLF
aaa,bbb,cccCRLF
#comment 2CRLF
"aaa","this is CRLF
# not a comment","ccc"CRLF
"#aaa",bbb,cccCRLF 3.12. IANA Considerations As per [RFC6838], IANA is directed to update the MIME type registration for "text/csv" with the content in Section 4 and add a reference to this document within the registration. The update to the media type registration is copied from the current one which consists of the original registration from [RFC4180] as updated by [RFC7111] and updated based on this document. 4. Update to MIME Type Registration of text/csv Type name: text Subtype name: csv Required parameters: none Optional parameters: charset 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 defined in [RFC4180] is deprecated and SHOULD NOT be used. Encoding considerations: CSV files and CSV MIME entities can consist of binary data as per section 4.8 of [RFC6838]. Although section 4.1.1. of [RFC2046] defines CRLF to denote line breaks, implementers MAY also recognize a single CR or LF as a line break (similar to section 3.1.1.3 of [RFC7231]). However, 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. Implementers and users should also be aware that some software applications may interpret certain characters in the beginning of CSV fields as referring to code or formulas, thus resulting in malicious code execution. This is known as "CSV injection" and users consuming CSV files should filter out such characters. 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" ([RFC0793]) when processing CSV files. An attempt at a common definition can be found in section 2 of (to be replaced with the RFC number). There are numerous differences between different CSV implementations, many of which are addressed in section 4 of (to be replaced with the RFC number of this document). 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 (to be replaced with the RFC number). 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 [RFC7111]. 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 5. Security Considerations All security considerations discussed in Section 4 still apply. 6. Acknowledgments In addition to everyone thanked previously in [RFC4180], the author would like to thank acknowledge the contributions of the following people to this document: Alperen Belgic, Abed BenBrahim, Damon Koach, Barry Leiba, Oliver Siegmar, Marco Diniz Sousa and Greg Skinner. A special thank you to L.T.S. 7. References 7.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, November 1996, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC4180] Shafranovich, Y., "Common Format and MIME Type for Comma- Separated Values (CSV) Files", RFC 4180, DOI 10.17487/RFC4180, October 2005, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC6657] Melnikov, A. and J. Reschke, "Update to MIME regarding "charset" Parameter Handling in Textual Media Types", RFC 6657, DOI 10.17487/RFC6657, July 2012, . [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013, . [RFC7111] Hausenblas, M., Wilde, E., and J. Tennison, "URI Fragment Identifiers for the text/csv Media Type", RFC 7111, DOI 10.17487/RFC7111, January 2014, . [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 7.2. Informative References [ART] Raymond, E., "The Art of Unix Programming, Chapter 5", September 2003, . [CREATIVYST] Repici, J., "HOW-TO: The Comma Separated Value (CSV) File Format", 2010, . [CSVW] W3C, "Model for Tabular Data and Metadata on the Web", December 2015, . [EDOCEO] Edoceo, Inc., "Comma Separated Values (CSV) Standard File Format", 2020, . [RFC1796] Huitema, C., Postel, J., and S. Crocker, "Not All RFCs are Standards", RFC 1796, DOI 10.17487/RFC1796, April 1995, . [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, . [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, . [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, . [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987, January 2005, . [RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, . [UNICODE] The Unicode Consortium, "The Unicode Standard, Version 15.0.0", September 2022, . Appendix A. Major changes since [RFC4180] * Added a section clarifying motivation for this document and standards status * Changing default encoding to UTF-8 and adding Unicode to the ABNF grammar * Allowing CR, LF and CRLF for line breaks * Allowing binary content including HTAB in CSV files * Mandating a line break at the end of the last line in the file * Making records and headers optional, thus allowing for an empty file * Adding a section on common implementation concerns * Removed "header" parameter for the MIME type since it is not used Appendix B. Changes since the -00 draft * Added CSV injection to security considerations (#30) * Added a reference to RFC 7111 (#27) Appendix C. Changes since the -01 draft * No changes yet, refreshed to keep draft alive Appendix D. Changes since the -02 draft * Refreshed to keep draft alive * Contact information and GitHub link changes * Minor updates on language * Added a section on bidi handling Appendix E. Changes since the -03 draft * Moved comments to the common practices section and removed from the ABNF grammar (#32) * Added more clarifications to the format section * Made ABNF grammar match the document * Added a note about text content to the format section Appendix F. Changes since the -04 draft * No changes yet, refreshed to keep draft alive Appendix G. Note to Readers *Note to the RFC Editor:* Please remove this section prior to publication. Development of this draft takes place on Github at: https://github.com/yakovsh/rfc4180-bis Comments can also be sent to the ART mailing list at: https://www.ietf.org/mailman/listinfo/art Full list of changes can be viewed via the IETF document tracker: https://tools.ietf.org/html/draft-shafranovich-rfc4180-bis Author's Address Yakov Shafranovich Amazon Web Services (AWS) Email: yakovsh@amazon.com or ietf@shaftek.org