Retrofit Structured Fields for HTTP

This specification nominates a selection of existing HTTP fields as having syntax that is compatible with Structured Fields, so that they can be handled as such (subject to certain caveats).¶

To accommodate some additional fields whose syntax is not compatible, it also defines mappings of their semantics into new Structured Fields. It does not specify how to negotiate their use.¶

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• Improved interoperability and security: precisely defined parsing and serialisation algorithms are typically not available for fields defined with just ABNF and/or prose.
• Reuse of common implementations: many parsers for other fields are specific to a single field or a small family of fields.
• Canonical form: because a deterministic serialisation algorithm is defined for each type, Structure Fields have a canonical representation.
• Enhanced API support: a regular data model makes it easier to expose field values as a native data structure in implementations.
• Alternative serialisations: While [STRUCTURED-FIELDS] defines a textual serialisation of that data model, other, more efficient serialisations of the underlying data model are also possible.

However, a field needs to be defined as a Structured Field for these benefits to be realised. Many existing fields are not, making up the bulk of header and trailer fields seen in HTTP traffic on the internet.¶

This specification defines how a selection of existing HTTP fields can be handled as Structured Fields, so that these benefits can be realised -- thereby making them Retrofit Structured Fields.¶

It does so using two techniques. Section 2 lists compatible fields -- those that can be handled as if they were Structured Fields due to the similarity of their defined syntax to that in Structured Fields. Section 3 lists mapped fields -- those whose syntax needs to be transformed into an underlying data model which is then mapped into that defined by Structured Fields.¶

Note that while implementations can parse and serialise compatible fields as Structured Fields subject to the caveats in Section 2, a sender cannot generate mapped fields from Section 3 and expect them to be understood and acted upon by the recipient without prior negotiation. This specification does not define such a mechanism.¶

The HTTP fields listed in Table 1 can usually have their values handled as Structured Fields according to the listed parsing and serialisation algorithms in [STRUCTURED-FIELDS], subject to the listed caveats.¶

The listed types are chosen for compatibility with the defined syntax of the field as well as with actual internet traffic. However, not all instances of these fields will successfully parse. This might be because the field value is clearly invalid, or it might be because it is valid but not parseable as a Structured Field.¶

An application using this specification will need to consider how to handle such field values. Depending on its requirements, it might be advisable to reject such values, treat them as opaque strings, or attempt to recover a structured value from them in an ad hoc fashion.¶

Parameter and Dictionary keys:

HTTP parameter names are case-insensitive (per Section 5.6.6 of [HTTP]), but Structured Fields require them to be all-lowercase. Although the vast majority of parameters seen in typical traffic are all-lowercase, compatibility can be improved by force-lowercasing parameters when encountered. Likewise, many Dictionary-based fields (e.g., Cache-Control, Expect-CT, Pragma, Prefer, Preference-Applied, Surrogate-Control) have case-insensitive keys, and compatibility can be improved by force-lowercasing them.

Parameter delimitation:

The parameters rule in HTTP (see Section 5.6.6 of [HTTP]) allows whitespace before the ";" delimiter, but Structured Fields does not. Compatibility can be improved by allowing such whitespace.

String quoting:

Section 5.6.4 of [HTTP] allows backslash-escaping most characters in quoted strings, whereas Structured Field Strings only escape "\" and DQUOTE. Compatibility can be improved by unescaping other characters before processing as Strings.

Token limitations:

In Structured Fields, tokens are required to begin with an alphabetic character or "*", whereas HTTP tokens allow a wider range of characters. This prevents use of mapped values that begin with one of these characters. For example, media types, field names, methods, range-units, character and transfer codings that begin with a number or special character other than "*" might be valid HTTP protocol elements, but will not be able to be parsed as Structured Field Tokens.

Integer limitations:

Structured Fields Integers can have at most 15 digits; larger values will not be able to be represented in them.

IPv6 Literals:

Fields whose values can contain IPv6 literal addresses (such as CDN-Loop, Host, and Origin) are not compatible when those values are parsed as Structured Fields Tokens, because the brackets used to delimit them are not allowed in Tokens.

Empty Field Values:

Empty and whitespace-only field values are considered errors in Structured Fields. For compatible fields, an empty field indicates that the field should be silently ignored.

Alt-Svc:

Some ALPN tokens (e.g., h3-Q43) do not conform to key's syntax. Since the final version of HTTP/3 uses the h3 token, this shouldn't be a long-term issue, although future tokens may again violate this assumption.

Content-Length:

Content-Length is defined as a List because it is not uncommon for implementations to mistakenly send multiple values. See Section 8.6 of [HTTP] for handling requirements.

Retry-After:

Only the delta-seconds form of Retry-After is supported; a Retry-After value containing a http-date will need to be either converted into delta-seconds or represented as a raw value.

Some HTTP field values have syntax that cannot be successfully parsed as Structured Fields. Instead, it is necessary to map them into a separate Structured Field with an alternative name.¶

As in Section 2, these fields are unable to carry values that are not valid Structured Fields, and so an application using this specification will need to how to support such values. Typically, handling them using the original field name is sufficient.¶

Each field name listed below indicates a replacement field name and a means of mapping its original value into a Structured Field.¶

• The "Structured Type" column indicates the type of the field (per RFC8941), if any, and may be "Dictionary", "List" or "Item". A prefix of "*" indicates that it is a retrofit type (i.e., not natively Structured); see [this specification].

  Note that field names beginning with characters other than ALPHA or "*" will not be able to be represented as a Structured Fields Token, and therefore may be incompatible with being mapped into fields that refer to it; see [this specification].

Then, add a new column, "Structured Type", with the values from Section 2 assigned to the nominated registrations, prefixing each with "*" to indicate that it is a retrofit type.¶

Then, add the field names in Table 5, with the corresponding Structured Type as indicated, a status of "permanent" and referring to this document.¶

Finally, add the indicated Structured Type for each existing registry entry listed in Table 6.¶

Section 2 identifies existing HTTP fields that can be parsed and serialised with the algorithms defined in [STRUCTURED-FIELDS]. Variances from existing parser behavior might be exploitable, particularly if they allow an attacker to target one implementation in a chain (e.g., an intermediary). However, given the considerable variance in parsers already deployed, convergence towards a single parsing algorithm is likely to have a net security benefit in the longer term.¶

Section 3 defines alternative representations of existing fields. Because downstream consumers might interpret the message differently based upon whether they recognise the alternative representation, implementations are prohibited from generating such fields unless they have negotiated support for them with their peer. This specification does not define such a mechanism, but any such definition needs to consider the implications of doing so carefully.¶

6. Normative References

[COOKIES]

Chen, L., Englehardt, S., West, M., and J. , “Cookies: HTTP State Management Mechanism”, Internet-Draft draft-ietf-httpbis-rfc6265bis-10 (work in progress), April 2022.

[HTTP]

Fielding, R., Nottingham, M., and J. , “HTTP Semantics”, Internet-Draft draft-ietf-httpbis-semantics-19 (work in progress), September 2021.

[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>.

[RFC8174]

Leiba, B., “Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words”, BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8288]

Nottingham, M., “Web Linking”, RFC 8288, DOI 10.17487/RFC8288, October 2017, <https://www.rfc-editor.org/info/rfc8288>.

[STRUCTURED-FIELDS]

Nottingham, M. and P-H. Kamp, “Structured Field Values for HTTP”, RFC 8941, DOI 10.17487/RFC8941, February 2021, <https://www.rfc-editor.org/info/rfc8941>.