RFC 8187
Indicating Character Encoding and Language for HTTP Header Field Parameters
RFC 8187 🎀 Indicating Character Encoding and Language for HTTP Header Field Parameters
Okay bestie, let's start with the basics 🎀 This is RFC 8187, called Indicating Character Encoding and Language for HTTP Header Field Parameters. J. Reschke from greenbytes wrote it, it's dated September 2017, and it's a Standards Track doc under the IETF 🦋 It carries ISSN 2070-1721 and it obsoletes RFC 5987 💅
Abstract ✨
So by default, header field values in HTTP messages can't easily hold characters outside the US-ASCII coded character set 🥺 RFC 2231 has an encoding trick for parameters inside MIME header field values, and this doc specifies an encoding for HTTP header fields that's compatible with a simplified profile of RFC 2231's version 💖
This document obsoletes RFC 5987.
Status of This Memo 📜
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community, and it got public review and was approved for publication by the Internet Engineering Steering Group (IESG) 🫶 If you want more on Internet Standards, check Section 2 of RFC 7841.
Info on the current status, any errata, and how to give feedback lives at https://www.rfc-editor.org/info/rfc8187 🌸
Copyright Notice 🌈
Copyright (c) 2017 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) as they stood on the publication date. Read these carefully, bestie, because they spell out your rights and restrictions for this doc 📖 Code Components pulled out of this document must include Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as the Simplified BSD License describes ⭐
Table of Contents 🗂️
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Comparison to RFC 2231 and Definition of the Encoding . . . . 3
3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 4
3.2. Parameter Value Character Encoding and Language
Information . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.1. Definition . . . . . . . . . . . . . . . . . . . . . 4
3.2.2. Historical Notes . . . . . . . . . . . . . . . . . . 6
3.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Language Specification in Encoded Words . . . . . . . . . 7
4. Guidelines for Usage in HTTP Header Field Definitions . . . . 7
4.1. When to Use the Extension . . . . . . . . . . . . . . . . 8
4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Changes from RFC 5987 . . . . . . . . . . . . . . . 12
Appendix B. Implementation Report . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 131. Introduction 🦋
Using characters outside the US-ASCII coded character set ([RFC0020]) in HTTP header fields ([RFC7230]) is kinda tricky, ngl 😭 Here's why:
- The HTTP spec discourages non-US-ASCII characters in field values. It puts them into the "obs-text" Augmented Backus-Naur Form (ABNF) production ([RFC7230], Section 3.2) 🙈
- It also stays silent on a default character encoding scheme for field values. So any non-US-ASCII characters would have to be specific to the field definition, or they'd need some other kind of out-of-band information 💀
- Some APIs assume a default encoding so they can map the octet sequences from the HTTP message into character sequences. For example, the XMLHttpRequest API ([XMLHttpRequest]) uses the Interface Definition Language type "ByteString", which effectively means the ISO-8859-1 character encoding scheme ([ISO-8859-1]) gets used 🍓
On the other hand, RFC 2231 defines an encoding mechanism for parameters inside MIME header fields ([RFC2231]). Unlike HTTP messages, those fields do need to be sent over non-binary transports 🧚 This document specifies an encoding suitable for use in HTTP header fields, and it's compatible with a simplified profile of the encoding defined in RFC 2231. It can be applied to any HTTP header field that uses the common "parameter" ("name=value") syntax ✨
This document obsoletes [RFC5987] and moves it to "Historic" status. The changes are summarized in Appendix A 💜
Note: For the rest of this document, RFC 2231 is only referenced to explain why certain features were adopted, so those references are purely informative.
Note: This encoding does not apply to message payloads sent over HTTP, such as when using the media type "multipart/form-data" ([RFC7578]). Don't get it twisted, bestie 🙅♀️
2. Notational Conventions 📐
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 [RFC2119] 🔥
This specification uses the ABNF notation defined in [RFC5234]. The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and LWSP (linear whitespace) 🎀
This specification uses terminology defined in [RFC6365], namely: "character encoding scheme" (abbreviated to "character encoding" below), "charset", and "coded character set" 🌷
Note that this differs from RFC 2231, which uses the term "character set" for "character encoding scheme" 👀
3. Comparison to RFC 2231 and Definition of the Encoding 💅
RFC 2231 defines several extensions to MIME. The sections below talk about whether and how they apply to HTTP header fields 🧠
In short:
- Parameter Continuations aren't needed (Section 3.1),
- Character Encoding and Language Information are useful, so a simple subset is specified (Section 3.2), and
- Language Specifications in Encoded Words aren't needed (Section 3.3).
3.1. Parameter Continuations 🧵
Section 3 of [RFC2231] defines a mechanism that deals with the length limitations that apply to MIME headers. Those limitations don't apply to HTTP ([RFC7231], Appendix A.6) 🙌
So parameter continuations aren't part of the encoding defined by this specification 🚫
3.2. Parameter Value Character Encoding and Language Information 🌍
Section 4 of [RFC2231] explains how to embed language information into parameter values, and how to encode non-ASCII characters. It deals with restrictions in both MIME and HTTP header field parameters 💫
But RFC 2231 doesn't specify a mandatory-to-implement character encoding, which makes it hard for senders to pick one 😵 So recipients implementing this specification MUST support the "UTF-8" character encoding [RFC3629] 🫶
Furthermore, RFC 2231 allows the character encoding information to be left out. The encoding defined here does not allow that 🙅♀️
3.2.1. Definition 🩷
The presence of extended parameter values is usually indicated by a parameter name ending in an asterisk character. But note that this is just a convention. The extended parameter values need to be explicitly specified in the definition of the header field using this extension (see Section 4) 🔍
The ABNF for extended parameter values is below, so screenshot it bestie 📸
ext-value = charset "'" [ language ] "'" value-chars
; like RFC 2231's <extended-initial-value>
; (see [RFC2231], Section 7)
charset = "UTF-8" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in Section 2.3 of [RFC2978]
; except that the single quote is not included
; SHOULD be registered in the IANA charset registry
language = <Language-Tag, see [RFC5646], Section 2.1>
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see [RFC3986], Section 2.1
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )The value part of an extended parameter (ext-value) is a token made of three parts, separated by single quote characters 🎀
- the REQUIRED character encoding name (charset),
- the OPTIONAL language information (language), and
- a character sequence representing the actual value (value-chars).
Both character encoding names and language tags are restricted to the US-ASCII coded character set, and they're matched case-insensitively (see Section 2.3 of [RFC2978] and Section 2.1.1 of [RFC5646]) 🔤
Inside the value part, characters that aren't in attr-char get encoded into an octet sequence using the specified character encoding. That octet sequence is then percent-encoded as specified in Section 2.1 of [RFC3986] 🌸
Producers MUST use the "UTF-8" ([RFC3629]) character encoding. Extension character encodings (mime-charset) are reserved for future use 🔮
Note: Recipients should be ready to handle encoding errors, like malformed or incomplete percent escape sequences, or non-decodable octet sequences, in a robust way. This specification doesn't mandate any specific behavior. For instance, all of these strategies are acceptable: - ignoring the parameter, - stripping a non-decodable octet sequence, and - substituting a non-decodable octet sequence with a replacement character, such as the Unicode character U+FFFD (Replacement Character) 🦋
Okay bestie, picking right back up! 💖
3.2.2. Historical Notes ✨
The RFC 7230 token production ([RFC7230], Section 3.2.6) is a little different from the one RFC 2231 borrowed (imported from Section 5.1 of [RFC2045]), because curly braces (i.e., "{" and "}") are excluded from it. So these two characters are excluded from the attr-char production too. 🎀
The <mime-charset> ABNF defined here is different from the one in Section 2.3 of [RFC2978], because it does not allow the single quote character (see also RFC Errata ID 1912 [Err1912]). 🙈 Honestly no biggie in practice, since no character encoding names using that character had been registered at the time of this writing. 💅
For backwards compatibility with RFC 2231, the encoding defined by this specification deviates from common parameter syntax by not allowing the quoted-string notation. 🦋 Implementations using generic parser components might not be able to detect the use of quoted-string notation, and so they might accept that format as well, even though it's invalid. 😭
[RFC5987] did require support for ISO-8859-1 ([ISO-8859-1]) too. For compatibility with legacy code, recipients are encouraged to support this encoding as well. 🌈
3.2.3. Examples 🧚
Non-extended notation, using "token":
foo: bar; title=EconomyNon-extended notation, using "quoted-string":
foo: bar; title="US-$ rates"Extended notation, using the Unicode character U+00A3 ("£", POUND SIGN):
foo: bar; title*=utf-8'en'%C2%A3%20ratesNote: The Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, and then percent-encoded. Also, note that the space character was encoded as %20, as it is not contained in attr-char. 🍓
Extended notation, using the Unicode characters U+00A3 ("£", POUND SIGN) and U+20AC ("€", EURO SIGN):
foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20ratesNote: The Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, and then percent-encoded. Likewise, the Unicode euro sign character U+20AC was encoded into the octet sequence E2 82 AC, and then percent-encoded. Also note that HEXDIG allows both lowercase and uppercase characters, so recipients must understand both, and that the language information is optional, while the character encoding is not. 🫶
3.3. Language Specification in Encoded Words 🌸
Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to also support language specification in encoded words. RFC 2616, the now-obsolete HTTP/1.1 specification, did refer to RFC 2047 ([RFC2616], Section 2.2). However, it wasn't clear to which header field it applied. Consequently, the current revision of the HTTP/1.1 specification has deprecated use of the encoding forms defined in RFC 2047 (see Section 3.2.4 of [RFC7230]). 💀
Thus, this specification does not include this feature. 🔥
4. Guidelines for Usage in HTTP Header Field Definitions 👑
Specifications of HTTP header fields that use the extensions defined in Section 3.2 ought to clearly state that. A simple way to achieve this is to normatively reference this specification and to include the ext-value production into the ABNF for specific header field parameters. 💜
For instance:
foo = token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see RFC 8187, Section 3.2>Note: The Parameter Value Continuation feature defined in Section 3 of [RFC2231] makes it impossible to have multiple instances of extended parameters with identical names, as the processing of continuations would become ambiguous. Thus, specifications using this extension are advised to disallow this case for compatibility with RFC 2231. 🙈
Note: This specification does not automatically assign a new interpretation to parameter names ending in an asterisk. As pointed out above, it's up to the specification for the non-extended parameter to "opt in" to the syntax defined here. That being said, some existing implementations are known to automatically switch to using this notation when a parameter name ends with an asterisk; thus, using parameter names ending in an asterisk for something else is likely to cause interoperability problems. 😭
4.1. When to Use the Extension 💅
Section 4.2 of [RFC2277] requires that protocol elements containing human-readable text be able to carry language information. Thus, the ext-value production ought to always be used when the parameter value is of a textual nature and its language is known. 🦋
Furthermore, the extension ought to also be used whenever the parameter value needs to carry characters not present in the US-ASCII coded character set ([RFC0020]). Note that it would be unacceptable to define a new parameter that would be restricted to a subset of the Unicode character set. ✨
4.2. Error Handling 🎀
Header field specifications need to define whether multiple instances of parameters with identical names are allowed and how they should be processed. This specification suggests that a parameter using the extended syntax takes precedence. This would allow producers to use both formats without breaking recipients that do not understand the extended syntax yet. 🌈
Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''%e2%82%ac%20exchange%20ratesIn this case, the sender provides an ASCII version of the title for legacy recipients, but also includes an internationalized version for recipients understanding this specification. The latter obviously ought to prefer the new syntax over the old one. 👑
5. Security Considerations 🔥
⚠️ Serious business alert, bestie! Please read this part carefully. 🚨
The format described in this document makes it possible to transport non-ASCII characters, and thus enables character "spoofing" scenarios in which a displayed value appears to be something other than it is. 😱
Furthermore, there are known attack scenarios related to decoding UTF-8. 💀
See Section 10 of [RFC3629] for more information on both topics. 🧚
In addition, the extension specified in this document makes it possible to transport multiple language variants for a single parameter. Such use might allow spoofing attacks where different language versions of the same parameter are not equivalent. Whether this attack is effective as an attack depends on the parameter specified. 🙈🔥
6. IANA Considerations 🌸
This document does not require any IANA actions. 💖
7. References 💜
7.1. Normative References 🦋
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
October 2000, <https://www.rfc-editor.org/info/rfc2978>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[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,
<https://www.rfc-editor.org/info/rfc7231>.
---
<lavender>More to come, so stay tuned, bestie!</lavender> 🦋💜
# 7.2. Informative References ✨
okay so these are the "fyi, go read this if you're curious" sources bestie 📚🫶 nothing in here is required reading, but they give the full backstory 💅
- **[Err1912]** RFC Errata, "Erratum ID 1912, RFC 2978", `https://www.rfc-editor.org/errata/eid1912`.
- **[ISO-8859-1]** International Organization for Standardization, "Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1", ISO/IEC 8859-1:1998, 1998. 🌈
- **[RFC2045]** Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, `https://www.rfc-editor.org/info/rfc2045`. 💌
- **[RFC2047]** Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text", RFC 2047, DOI 10.17487/RFC2047, November 1996, `https://www.rfc-editor.org/info/rfc2047`. 🎀
- **[RFC2231]** Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, DOI 10.17487/RFC2231, November 1997, `https://www.rfc-editor.org/info/rfc2231`. this is the big ancestor of the whole encoding, so iconic 👑
- **[RFC2277]** Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277, January 1998, `https://www.rfc-editor.org/info/rfc2277`. 🦋
- **[RFC2616]** Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/RFC2616, June 1999, `https://www.rfc-editor.org/info/rfc2616`. the OG HTTP/1.1 doc, a real classic 🍓
- **[RFC5987]** Reschke, J., "Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010, `https://www.rfc-editor.org/info/rfc5987`. the one this doc is updating, so it's giving main character 🧚
- **[RFC5988]** Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010, `https://www.rfc-editor.org/info/rfc5988`. 🔗
- **[RFC6266]** Reschke, J., "Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)", RFC 6266, DOI 10.17487/RFC6266, June 2011, `https://www.rfc-editor.org/info/rfc6266`. 📎
- **[RFC6365]** Hoffman, P. and J. Klensin, "Terminology Used in Internationalization in the IETF", BCP 166, RFC 6365, DOI 10.17487/RFC6365, September 2011, `https://www.rfc-editor.org/info/rfc6365`. the glossary bestie, very helpful ✨
- **[RFC7578]** Masinter, L., "Returning Values from Forms: multipart/form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015, `https://www.rfc-editor.org/info/rfc7578`. 📝
- **[RFC7616]** Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI 10.17487/RFC7616, September 2015, `https://www.rfc-editor.org/info/rfc7616`. 🔐
- **[RFC8053]** Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi, T., and Y. Ioku, "HTTP Authentication Extensions for Interactive Clients", RFC 8053, DOI 10.17487/RFC8053, January 2017, `https://www.rfc-editor.org/info/rfc8053`. 🫶
- **[XMLHttpRequest]** WhatWG, "XMLHttpRequest", `https://xhr.spec.whatwg.org/`. 💜
---
# Appendix A. Changes from RFC 5987 💅
okay so here's the glow-up list, what changed since RFC 5987 🦋 the vibe is "new and improved" 🌸
This section summarizes the changes compared to [RFC5987]:
- The document title was changed to "Indicating Character Encoding and Language for HTTP Header Field Parameters". new name, same slay ✨
- The introduction was rewritten to better explain the issues around non-ASCII characters in field values. 🎀
- The requirement to support the "ISO-8859-1" encoding was removed. bye bye, ISO-8859-1 🙈
- This document no longer attempts to redefine a generic "parameter" ABNF (it turned out that there really isn't a generic definition of parameters in HTTP; for instance, there are subtle differences with respect to whitespace handling). ngl this one is so relatable, there's no one-size-fits-all "parameter" in HTTP lol 💀
- A note about defects in error handling in current implementations was removed, as it was no longer accurate. 🍓
---
# Appendix B. Implementation Report 🌈
the receipts! here's who's actually using this encoding out in the wild 🫶
The encoding defined in this document is currently used in four different HTTP header fields:
- "Authentication-Control", defined in [RFC8053], 🔐
- "Authorization" (as used in HTTP Digest Authentication, defined in [RFC7616]), 🔑
- "Content-Disposition", defined in [RFC6266], and 📎
- "Link", defined in [RFC5988]. 🔗
As the encoding is a profile/clarification of the one defined in [RFC2231] in 1997, many user agents already supported it for use in "Content-Disposition" when [RFC5987] was published. so it had a head start, very main character 👑
Since the publication of [RFC5987], three more popular desktop user agents have added support for this encoding; see `http://purl.org/NET/http/content-disposition-tests#encoding-2231-char` for details. At this time, the current versions of all major desktop user agents support it. the whole squad is in ✨
Note that the implementation in Internet Explorer 9 does not support the ISO-8859-1 character encoding; this document revision acknowledges that UTF-8 is sufficient for expressing all code points and removes the requirement to support ISO-8859-1. 💖 UTF-8 really said "I can hold all of it" 💅
The "Link" header field, on the other hand, was more recently specified in [RFC5988]. At the time of this writing, no user agent except Firefox supported the "title*" parameter (starting with release 15). 🦊 Firefox was out here doing the most, bestie 🙌
Section 3.4 of [RFC7616] defines the "username*" parameter for use in HTTP Digest Authentication. At the time of writing, no user agent implemented this extension. 🥺 a lowkey underrated parameter, we see you 🫶
---
# Acknowledgements 🎀
big thank-you energy to everyone who helped make this happen 🫶💖
Thanks to Martin Dürst and Frank Ellermann for help figuring out ABNF details, to Graham Klyne and Alexey Melnikov for general review, to Chris Newman for pointing out an RFC 2231 incompatibility, and to Benjamin Carlyle, Roar Lauritzsen, Eric Lawrence, and James Manger for implementers feedback. 🧚
Furthermore, thanks to the members of the IETF HTTP Working Group for the feedback specific to this update of RFC 5987. 🌸
---
# Author's Address 🦋
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Münster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: `http://greenbytes.de/tech/webdav/`
📦 where this page comes from
- ietf: rfc8187 2017-09
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms