Internet DRAFT - draft-ietf-httpbis-retrofit
draft-ietf-httpbis-retrofit
HTTP M. Nottingham
Internet-Draft 31 March 2023
Intended status: Standards Track
Expires: 2 October 2023
Retrofit Structured Fields for HTTP
draft-ietf-httpbis-retrofit-06
Abstract
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.
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
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 2 October 2023.
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. Code Components
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provided without warranty as described in the Revised BSD License.
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. Cookies . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. Normative References . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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. 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.
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* 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.
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.
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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 |
+----------------------------------+-----------------+
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| 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 |
+----------------------------------+-----------------+
| DNT | Item |
+----------------------------------+-----------------+
| Expect | Dictionary |
+----------------------------------+-----------------+
| Expect-CT | 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 |
+----------------------------------+-----------------+
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| Sec-WebSocket-Version | Item |
+----------------------------------+-----------------+
| Server-Timing | List |
+----------------------------------+-----------------+
| Surrogate-Control | Dictionary |
+----------------------------------+-----------------+
| TE | List |
+----------------------------------+-----------------+
| Timing-Allow-Origin | List |
+----------------------------------+-----------------+
| Trailer | List |
+----------------------------------+-----------------+
| Transfer-Encoding | List |
+----------------------------------+-----------------+
| Upgrade-Insecure-Requests | Item |
+----------------------------------+-----------------+
| Vary | List |
+----------------------------------+-----------------+
| X-Content-Type-Options | Item |
+----------------------------------+-----------------+
| X-Frame-Options | Item |
+----------------------------------+-----------------+
| X-XSS-Protection | List |
+----------------------------------+-----------------+
Table 1: Compatible Fields
Note the following caveats regarding compatibility:
Parsing differences: Some values may fail to parse as Structured
Fields, even though they are valid according to their originally
specified syntax. For example, HTTP parameter names are case-
insensitive (per Section 5.6.6 of [HTTP]), but Structured Fields
require them to be all-lowercase. Likewise, many Dictionary-based
fields (e.g., Cache-Control, Expect-CT, Pragma, Prefer,
Preference-Applied, Surrogate-Control) have case-insensitive keys.
Similarly, the parameters rule in HTTP (see Section 5.6.6 of
[HTTP]) allows whitespace before the ";" delimiter, but Structured
Fields does not. And, Section 5.6.4 of [HTTP] allows backslash-
escaping most characters in quoted strings, whereas Structured
Field Strings only escape "\" and DQUOTE. The vast majority of
fields seen in typical traffic do not exhibit these behaviors.
Error handling: Parsing algorithms specified (or just widely
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implemented) for current HTTP headers may differ from those in
Structured Fields in details such as error handling. For example,
HTTP specifies that repeated directives in the Cache-Control
header field have a different precedence than that assigned by a
Dictionary structured field (which Cache-Control is mapped to).
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 represented 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 contain IPv6 literal addresses
(such as CDN-Loop, Host, and Origin) are not able to be
represented 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, and therefore cannot be represented as a Token. 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: Note that 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 can be
represented; a Retry-After value containing a http-date will need
to be converted into delta-seconds to be conveyed as a Structured
Field Value.
3. Mapped Fields
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.
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For example, the Date HTTP header field carries a date:
Date: Sun, 06 Nov 1994 08:49:37 GMT
Its value would be mapped to:
SF-Date: @784111777
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.
3.1. URLs
The field names in Table 2 (paired with their mapped field names)
have values that can be mapped into Structured Fields by treating the
original field's value as a String.
+==================+=====================+
| Field Name | Mapped Field Name |
+==================+=====================+
| Content-Location | SF-Content-Location |
+------------------+---------------------+
| Location | SF-Location |
+------------------+---------------------+
| Referer | SF-Referer |
+------------------+---------------------+
Table 2: URL Fields
For example, this Location field
Location: https://example.com/foo
could be 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 mapped into Structured Fields by parsing
their payload according to Section 5.6.7 of [HTTP] and representing
the result as a Date.
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+=====================+========================+
| Field Name | Mapped Field Name |
+=====================+========================+
| Date | SF-Date |
+---------------------+------------------------+
| Expires | SF-Expires |
+---------------------+------------------------+
| If-Modified-Since | SF-If-Modified-Since |
+---------------------+------------------------+
| If-Unmodified-Since | SF-If-Unmodified-Since |
+---------------------+------------------------+
| Last-Modified | SF-Last-Modified |
+---------------------+------------------------+
Table 3: Date Fields
For example, an Expires field could be mapped as:
SF-Expires: @1659578233
3.3. ETags
The field value of the ETag header field can be mapped into the SF-
ETag 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, this:
ETag: W/"abcdef"
SF-ETag: "abcdef"; w
If-None-Match's field value can be mapped into the SF-If-None-Match
Structured Field, which is a List of the structure described above.
When a field value contains "*", it is represented as a Token.
Likewise, If-Match's field value can be mapped into the SF-If-Match
Structured Field in the same manner.
For example:
SF-If-None-Match: "abcdef"; w, "ghijkl", *
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3.4. Cookies
The field values of the Cookie and Set-Cookie fields [COOKIES] can be
mapped into the SF-Cookie Structured Field (a List) and SF-Set-Cookie
Structured Field (a List), respectively.
In each case, a cookie is represented as an Inner List containing two
Items; the cookie name and value. The cookie name is always a
String; the cookie value is a String, unless it can be successfully
parsed as the textual representation of another, bare Item structured
type (e.g., Byte Sequence, Decimal, Integer, Token, or Boolean).
Cookie attributes map to Parameters on the Inner List, with the
parameter name being forced to lowercase. Cookie attribute values
are Strings unless a specific type is defined for them. This
specification defines types for existing cookie attributes in
Table 4.
+================+=================+
| Parameter Name | Structured Type |
+================+=================+
| Domain | String |
+----------------+-----------------+
| HttpOnly | Boolean |
+----------------+-----------------+
| Expires | Date |
+----------------+-----------------+
| Max-Age | Integer |
+----------------+-----------------+
| Path | String |
+----------------+-----------------+
| Secure | Boolean |
+----------------+-----------------+
| SameSite | Token |
+----------------+-----------------+
Table 4: Set-Cookie Parameter Types
The Expires attribute is mapped to a Date representation of parsed-
cookie-date (see Section 5.1.1 of [COOKIES]).
For example, these unstructured fields:
Set-Cookie: lang=en-US; Expires=Wed, 09 Jun 2021 10:18:14 GMT;
samesite=Strict; secure
Cookie: SID=31d4d96e407aad42; lang=en-US
can be mapped into:
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SF-Set-Cookie: ("lang" "en-US"); expires=@1623233894;
samesite=Strict; secure
SF-Cookie: ("SID" "31d4d96e407aad42"), ("lang" "en-US")
4. IANA Considerations
Please add the following note to the "Hypertext Transfer Protocol
(HTTP) Field Name Registry":
A prefix of "*" in the Structured Type column indicates that it is
a retrofit type (i.e., not natively Structured); see RFC nnnn.
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.
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+========================+=================+
| Field Name | Structured Type |
+========================+=================+
| SF-Content-Location | Item |
+------------------------+-----------------+
| SF-Cookie | List |
+------------------------+-----------------+
| SF-Date | Item |
+------------------------+-----------------+
| SF-ETag | Item |
+------------------------+-----------------+
| SF-Expires | Item |
+------------------------+-----------------+
| SF-If-Match | List |
+------------------------+-----------------+
| SF-If-Modified-Since | Item |
+------------------------+-----------------+
| SF-If-None-Match | List |
+------------------------+-----------------+
| SF-If-Unmodified-Since | Item |
+------------------------+-----------------+
| SF-Last-Modified | Item |
+------------------------+-----------------+
| SF-Location | Item |
+------------------------+-----------------+
| SF-Referer | Item |
+------------------------+-----------------+
| SF-Set-Cookie | List |
+------------------------+-----------------+
Table 5: New Fields
Finally, add a new column to the "Cookie Attribute Registry"
established by [COOKIES] with the title "Structured Type", using
information from Table 4.
5. Security Considerations
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.
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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] Bingler, S., West, M., and J. Wilander, "Cookies: HTTP
State Management Mechanism", Work in Progress, Internet-
Draft, draft-ietf-httpbis-rfc6265bis-11, 7 November 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
rfc6265bis-11>.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[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>.
[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>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", Work in Progress, Internet-Draft, draft-ietf-
httpbis-sfbis-01, 22 December 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
sfbis-01>.
Author's Address
Mark Nottingham
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
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