Retrofit Structured Fields for HTTP

Prahran VIC Australia mnot@mnot.net https://www.mnot.net/

Internet-Draft This specification defines how a selection of existing HTTP fields can be handled as Structured Fields. About This Document Status information for this document may be found at . information can be found at . Source for this draft and an issue tracker can be found at .

Introduction Structured Field Values for HTTP introduced a data model with associated parsing and serialisation algorithms for use by new HTTP field values. Header fields that are defined as Structured Fields can realise a number of benefits, including:

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

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 BCP 14 when, and only when, they appear in all capitals, as shown here.

Compatible Fields HTTP fields with the following names can usually have their values handled as Structured Fields according to the listed parsing and serialisation algorithms in , 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 (see ). 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.

Accept - List
Accept-Encoding - List
Accept-Language - List
Accept-Patch - List
Accept-Ranges - List
Access-Control-Allow-Credentials - Item
Access-Control-Allow-Headers - List
Access-Control-Allow-Methods - List
Access-Control-Allow-Origin - Item
Access-Control-Expose-Headers - List
Access-Control-Max-Age - Item
Access-Control-Request-Headers - List
Access-Control-Request-Method - Item
Age - Item
Allow - List
ALPN - List
Alt-Svc - Dictionary
Alt-Used - Item
Cache-Control - Dictionary
Connection - List
Content-Encoding - List
Content-Language - List
Content-Length - List
Content-Type - Item
Cross-Origin-Resource-Policy - Item
Expect - Item
Expect-CT - Dictionary
Host - Item
Keep-Alive - Dictionary
Origin - Item
Pragma - Dictionary
Prefer - Dictionary
Preference-Applied - Dictionary
Retry-After - Item
Surrogate-Control - Dictionary
TE - List
Timing-Allow-Origin: List
Trailer - List
Transfer-Encoding - List
Vary - List
X-Content-Type-Options - Item
X-Frame-Options - Item
X-XSS-Protection - List

Note the following caveats:

Parameter names:: HTTP parameter names are case-insensitive (as per ), 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.
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.
Cache-Control, Expect-CT, Pragma, Prefer, Preference-Applied, Surrogate-Control:: These Dictionary-based fields consider the key to be case-insensitive, but Structured Fields requires keys to be all-lowercase. Although the vast majority of values seen in typical traffic are all-lowercase, compatibility can be improved by force-lowercasing these Dictionary keys when encountered.
Content-Length:: Content-Length is defined as a List because it is not uncommon for implementations to mistakenly send multiple values. See 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.

Mapped Fields Some HTTP fields can have their values represented in Structured Fields by mapping them into its data types and then serialising the result using an alternative field name. For example, the Date HTTP header field carries a string representing a date: Its value is more efficiently represented as an integer number of delta seconds from the Unix epoch (00:00:00 UTC on 1 January 1970, minus leap seconds). Thus, the example above would be mapped as: As in , these fields are unable to represent values that are not parseable, 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.

URLs The following field names (paired with their replacement field names) have values that can be represented as Structured Fields by considering the original field's value as a string.

Content-Location - SF-Content-Location
Location - SF-Location
Referer - SF-Referer

For example, a Location field could be represented as:

Dates The following field names (paired with their replacement field names) have values that can be represented as Structured Fields by parsing their payload according to and representing the result as an integer number of seconds delta from the Unix Epoch (00:00:00 UTC on 1 January 1970, minus leap seconds).

Date - SF-Date
Expires - SF-Expires
If-Modified-Since - SF-IMS
If-Unmodified-Since - SF-IUS
Last-Modified - SF-LM

For example, an Expires field could be represented as:

ETags The field value of the ETag header field can be represented as a String Structured Field by representing the entity-tag as a string, and the weakness flag as a boolean "w" parameter on it, where true indicates that the entity-tag is weak; if 0 or unset, the entity-tag is strong. For example: If-None-Match's field value can be represented as SF-INM, which is a List of the structure described above. For example:

Links The field value of the Link header field can be represented in the SF-Link List Structured Field by representing the URI-Reference as a string, and link-param as parameters. For example:

Cookies The field values of the Cookie and Set-Cookie fields can be represented in the SF-Cookie Structured Field (a List) and SF-Set-Cookie Structured Field (a Dictionary), respectively. In each case, cookie names are serialized as tokens, whereas their values are serialised as Strings, unless they can be represented accurately and unambiguously using the textual representation of another structured types (e.g., an Integer or Decimal). Set-Cookie parameters map to parameters on the appropriate SF-Set-Cookie member, with the parameter name being forced to lowercase. Set-Cookie parameter values are Strings unless a specific type is defined. This specification defines the following parameter types:

Max-Age: Integer
Secure: Boolean
HttpOnly: Boolean
SameSite: Token

Note that cookies in both fields are separated by commas, not semicolons, and multiple cookies can appear in each field. For example:

IANA Considerations Please add the following note to the HTTP Field Name Registry:

The "Structured Type" column indicates the type of the field as 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].

Then, add a new column, "Structured Type", with the values from assigned to the nominated registrations, prefixing each with "*" to indicate that it is a retrofit type. Then, add the following field names into the HTTP Field Name Registry, with the corresponding Structured Type as indicated, a status of "permanent" and referring to this document:

SF-Content-Location - String
SF-Location - String
SF-Referer - String
SF-Date - Integer
SF-Expires - Integer
SF-IMS - Integer
SF-IUS - Integer
SF-LM - Integer
SF-ETag - Item
SF-INM - List
SF-Link - List
SF-Set-Cookie - Dictionary
SF-Cookie - List

Security Considerations identifies existing HTTP fields that can be parsed and serialised with the algorithms defined in . Variances from other implementations 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. 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.

Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. HTTP Semantics Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230. Structured Field Values for HTTP This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Web Linking This specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types"). It also defines the serialisation of such links in HTTP headers with the Link header field. HTTP State Management Mechanism This document defines the HTTP Cookie and Set-Cookie header fields. These header fields can be used by HTTP servers to store state (called cookies) at HTTP user agents, letting the servers maintain a stateful session over the mostly stateless HTTP protocol. Although cookies have many historical infelicities that degrade their security and privacy, the Cookie and Set-Cookie header fields are widely used on the Internet. This document obsoletes RFC 2965. [STANDARDS-TRACK]

Data Supporting Field Compatibility To help guide decisions about compatible fields, the HTTP response headers captured by the HTTP Archive https://httparchive.org in September 2021 (representing more than 528,000,000 HTTP exchanges) were parsed as Structured Fields using the types listed in , with the indicated number of successful header instances, failures, and the resulting failure rate: Note that this data set only includes response headers, although some request headers are present, indicated with an asterisk (because, the Web). Also, Dictionary and Parameter keys have not been force-lowercased, with the result that any values containing uppercase keys are considered to fail. The top thirty header fields in that data set that were not considered compatible are (* indicates that the field is mapped in ):

*date: 524,810,577
server: 470,777,294
*last-modified: 383,437,099
*expires: 292,109,781
*etag: 255,788,799
strict-transport-security: 111,993,787
x-cache: 70,713,258
via: 55,983,914
cf-ray: 54,556,881
p3p: 54,479,183
report-to: 54,056,804
cf-cache-status: 53,536,789
nel: 44,815,769
x-powered-by: 37,281,354
content-security-policy-report-only: 33,104,387
*location: 30,533,957
x-amz-cf-pop: 28,549,182
x-amz-cf-id: 28,444,359
content-security-policy: 25,404,401
x-served-by: 23,277,252
x-cache-hits: 21,842,899
*link: 20,761,372
x-timer: 18,780,130
content-disposition: 18,516,671
x-request-id: 16,048,668
referrer-policy: 15,596,734
x-cdn: 10,153,756
x-amz-version-id: 9,786,024
x-amz-request-id: 9,680,689
x-dc: 9,557,728