Network Working Group                                      M. Nottingham
Internet-Draft                                             23 April                                               11 May 2022
Intended status: Standards Track
Expires: 25 October 12 November 2022

                  Retrofit Structured Fields for HTTP
                     draft-ietf-httpbis-retrofit-01
                     draft-ietf-httpbis-retrofit-02

Abstract

   This specification defines how 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.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-httpbis-retrofit/.

   Discussion of this document takes place on the HTTP Working Group
   mailing list (mailto:ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/.  Working Group
   information can be found at https://httpwg.org/.

   Source for this draft and an issue tracker can be found at
   https://github.com/httpwg/http-extensions/labels/retrofit.

Status of This Memo

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   This Internet-Draft will expire on 25 October 12 November 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Compatible Fields . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Mapped Fields . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  URLs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Dates . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  ETags . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Links . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.5.  Cookies . . . . . . . . . . . . . . . . . . . . . . . . .  10
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12  13
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .  12  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13  14

1.  Introduction

   Structured Field Values for HTTP [STRUCTURED-FIELDS] introduced a
   data model with associated parsing and serialization algorithms for
   use by new HTTP field values.  Header fields  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 fields.

   *  Canonical form: because a deterministic serialisation algorithm is
      defined for each type, Structure Fields have a canonical
      representation
      representation.

   *  Enhanced API support: a regular data model makes it easier to
      expose field values as a native data structure in implementations 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.

   While

   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.

1.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Compatible Fields

   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.

          +==================================+=================+
          | Field Name                       | Structured Type |
          +==================================+=================+
          | Accept                           | List            |
          +----------------------------------+-----------------+
          | Accept-Encoding                  | List            |
          +----------------------------------+-----------------+
          | Accept-Language                  | List            |
          +----------------------------------+-----------------+
          | Accept-Patch                     | List            |
          +----------------------------------+-----------------+
          | Accept-Post                      | 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      |
          +----------------------------------+-----------------+
          | CDN-Loop                         | List            |
          +----------------------------------+-----------------+
          | Clear-Site-Data                  | List            |
          +----------------------------------+-----------------+
          | Connection                       | List            |
          +----------------------------------+-----------------+
          | Content-Encoding                 | List            |
          +----------------------------------+-----------------+
          | Content-Language                 | List            |
          +----------------------------------+-----------------+
          | Content-Length                   | List            |
          +----------------------------------+-----------------+
          | Content-Type                     | Item            |
          +----------------------------------+-----------------+
          | Cross-Origin-Resource-Policy     | Item            |
          +----------------------------------+-----------------+
          | Expect                           | Dictionary      |
          +----------------------------------+-----------------+
          | Expect-CT                        | Dictionary      |
          +----------------------------------+-----------------+
          | Forwarded                        | Dictionary      |
          +----------------------------------+-----------------+
          | Host                             | Item            |
          +----------------------------------+-----------------+
          | Keep-Alive                       | Dictionary      |
          +----------------------------------+-----------------+
          | Max-Forwards                     | Item            |
          +----------------------------------+-----------------+
          | Origin                           | Item            |
          +----------------------------------+-----------------+
          | Pragma                           | Dictionary      |
          +----------------------------------+-----------------+
          | Prefer                           | Dictionary      |
          +----------------------------------+-----------------+
          | Preference-Applied               | Dictionary      |
          +----------------------------------+-----------------+
          | Retry-After                      | Item            |
          +----------------------------------+-----------------+
          | Sec-WebSocket-Extensions         | List            |
          +----------------------------------+-----------------+
          | Sec-WebSocket-Protocol           | List            |
          +----------------------------------+-----------------+
          | Sec-WebSocket-Version            | Item            |
          +----------------------------------+-----------------+
          | Server-Timing                    | List            |
          +----------------------------------+-----------------+
          | 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            |
          +----------------------------------+-----------------+

                        Table 1 1: Compatible Fields

   Note the following caveats regarding compatibility:

   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 escapes "" 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.

3.  Mapped Fields

   Some HTTP fields can have their field values represented in have syntax that cannot be successfully parsed
   as Structured
   Fields by mapping Fields.  Instead, it is necessary to map them into its data types and then serialising the
   result using a
   separate Structured Field with an alternative field name.

   For example, the Date HTTP header field carries a string representing
   a date:

   Date: Sun, 06 Nov 1994 08:49:37 GMT

   Its value is more efficiently represented as an integer 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: to:

   SF-Date: 784072177

   As in Section 2, these fields are unable to represent carry values that are not parseable,
   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.

3.1.  URLs

   The field names in Table 2 (paired with their mapped field names)
   have values that can be represented as mapped into Structured Fields by
   considering treating the
   original field's value as a string. String.

                +==================+=====================+
                | Field Name       | Mapped Field Name   |
                +==================+=====================+
                | Content-Location | SF-Content-Location |
                +------------------+---------------------+
                | Location         | SF-Location         |
                +------------------+---------------------+
                | Referer          | SF-Referer          |
                +------------------+---------------------+

                           Table 2 2: URL Fields

   For example, a Location field could be represented mapped as:

   SF-Location: "https://example.com/foo"

3.2.  Dates

   The field names in Table 3 (paired with their mapped field names)
   have values that can be represented as mapped into Structured Fields by parsing
   their payload according to Section 5.6.7 of [HTTP] and representing
   the result as an integer Integer number of seconds delta from the Unix Epoch
   (00:00:00 UTC on 1 January 1970, minus leap seconds).

                +=====================+===================+
                | Field Name          | Mapped Field Name |
                +=====================+===================+
                | Date                | SF-Date           |
                +---------------------+-------------------+
                | Expires             | SF-Expires        |
                +---------------------+-------------------+
                | If-Modified-Since   | SF-IMS            |
                +---------------------+-------------------+
                | If-Unmodified-Since | SF-IUS            |
                +---------------------+-------------------+
                | Last-Modified       | SF-LM             |
                +---------------------+-------------------+

                            Table 3 3: Date Fields

   For example, an Expires field could be represented mapped as:

   SF-Expires: 1571965240

3.3.  ETags

   The field value of the ETag header field can be represented as a
   String mapped into the SF-
   ETag Structured Field by representing the entity-tag as a string, String, and
   the weakness flag as a boolean 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:

   SF-ETag: "abcdef"; w=?1

   If-None-Match's field value can be represented as SF-INM, mapped into the SF-INM Structured
   Field, which is a List of the structure described above.

   For example:

   SF-INM: "abcdef"; w=?1, "ghijkl"

3.4.  Links

   The field value of the Link header field [RFC8288] can be represented
   in mapped into
   the SF-Link List Structured Field by representing considering the URI-
   Reference URI-Reference as
   a string, String, and link-param as parameters. Parameters.

   For example:

   SF-Link: "/terms"; rel="copyright"; anchor="#foo"

3.5.  Cookies

   The field values of the Cookie and Set-Cookie fields [RFC6265] [COOKIES] can be
   represented in
   mapped into the SF-Cookie Structured Field (a List) and SF-Set-
   Cookie SF-Set-Cookie
   Structured Field (a Dictionary), respectively.

   In each case, cookie names are serialized as tokens, whereas their Tokens.  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 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.
   defined for them.  This specification defines the parameter types in
   Table 4.

                   +================+=================+
                   | Parameter Name | Structured Type |
                   +================+=================+
                   | Max-Age HttpOnly       | Boolean         |
                   +----------------+-----------------+
                   | Expires        | Integer         |
                   +----------------+-----------------+
                   | Secure Max-Age        | Boolean Integer         |
                   +----------------+-----------------+
                   | HttpOnly Secure         | Boolean         |
                   +----------------+-----------------+
                   | SameSite       | Token           |
                   +----------------+-----------------+

                   Table 4 4: Set-Cookie Parameter Types

   Expires is mapped to an Integer representation of parsed-cookie-date
   (see Part x.x of [COOKIES]) expressed as a number of seconds delta
   from the Unix Epoch (00:00:00 UTC on 1 January 1970, minus leap
   seconds).

   Note that although this mapping is very similar to the syntax of
   Cookie and Set-Cookie headers, cookies in both fields are separated
   by commas, not semicolons, and multiple cookies can appear in each
   field.

   For example:

   SF-Set-Cookie: lang=en-US; lang="en-US"; expires="Wed, 09 Jun 2021 10:18:14 GMT";
                  samesite=Strict
                  samesite=Strict; secure=?1
   SF-Cookie: SID=31d4d96e407aad42, lang=en-US SID="31d4d96e407aad42", lang="en-US"

4.  IANA Considerations

   Please add the following note to the "Hypertext Transfer Protocol
   (HTTP) Field Name Registry":

      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.

                 +=====================+=================+
                 | Field Name          | Structured Type |
                 +=====================+=================+
                 | SF-Content-Location | String          |
                 +---------------------+-----------------+
                 | SF-Location SF-Cookie           | String List            |
                 +---------------------+-----------------+
                 | SF-Referer SF-Date             | String Item            |
                 +---------------------+-----------------+
                 | SF-Date SF-ETag             | Item            |
                 +---------------------+-----------------+
                 | SF-Expires          | Item            |
                 +---------------------+-----------------+
                 | SF-IMS              | Item            |
                 +---------------------+-----------------+
                 | SF-IUS SF-INM              | Item List            |
                 +---------------------+-----------------+
                 | SF-LM SF-IUS              | Item            |
                 +---------------------+-----------------+
                 | SF-ETag SF-Link             | Item List            |
                 +---------------------+-----------------+
                 | SF-INM SF-LM               | List Item            |
                 +---------------------+-----------------+
                 | SF-Link SF-Location         | List String          |
                 +---------------------+-----------------+
                 | SF-Set-Cookie SF-Referer          | Dictionary String          |
                 +---------------------+-----------------+
                 | SF-Cookie SF-Set-Cookie       | List Dictionary      |
                 +---------------------+-----------------+

                            Table 5 5: New Fields

   Finally, add the indicated structured type Structured Type for each existing registry
   entry below: listed in Table 6.

      +==========================================+=================+
      | Field Name                               | Structured Type |
      +==========================================+=================+
      | Accept-CH                                | List            |
      +------------------------------------------+-----------------+
      | Cache-Status                             | List            |
      +------------------------------------------+-----------------+
      | CDN-Cache-Control                        | Dictionary      |
      +------------------------------------------+-----------------+
      | Cross-Origin-Opener-Policy Cross-Origin-Embedder-Policy             | Item            |
      +------------------------------------------+-----------------+
      | Cross-Origin-Opener-Policy-Report-Only Cross-Origin-Embedder-Policy-Report-Only | Item            |
      +------------------------------------------+-----------------+
      | Cross-Origin-Embedder-Policy Cross-Origin-Opener-Policy               | Item            |
      +------------------------------------------+-----------------+
      | Cross-Origin-Embedder-Policy-Report-Only Cross-Origin-Opener-Policy-Report-Only   | Item            |
      +------------------------------------------+-----------------+
      | Origin-Agent-Cluster                     | Item            |
      +------------------------------------------+-----------------+
      | Priority                                 | Dictionary      |
      +------------------------------------------+-----------------+
      | Proxy-Status                             | List            |
      +------------------------------------------+-----------------+

                         Table 6 6: Existing Fields

5.  Security Considerations

   Section 2 identifies existing HTTP fields that can be parsed and
   serialised with the algorithms defined in [STRUCTURED-FIELDS].
   Variances from other implementations 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. Wilander,
              "Cookies: HTTP State Management Mechanism", Work in
              Progress, Internet-Draft, draft-ietf-httpbis-rfc6265bis-
              10, 24 April 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-httpbis-rfc6265bis-10>.

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [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/rfc/rfc2119>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6265>.

   [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/rfc/rfc8174>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/rfc/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/rfc/rfc8941>.

Author's Address

   Mark Nottingham
   Prahran
   Australia
   Email: mnot@mnot.net
   URI:   https://www.mnot.net/