HTTP Working Group M. Nottingham
Internet-Draft August 22, 2024
Intended status: Standards Track
Expires: February 23, 2025
Retrofit Structured Fields for HTTP
draft-ietf-httpbis-retrofit-latest
Abstract
This specification nominates a selection of existing HTTP fields
whose values are compatible with Structured Fields syntax, 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 Structured Fields.
It does not specify how to convey them in HTTP messages.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Using Retrofit Structured Fields . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Compatible Fields . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Caveats . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Mapped Fields . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. URLs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. ETags . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Cookies . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Normative References . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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 bring advantages that include:
o Improved interoperability and security: precisely defined parsing
and serialisation algorithms are typically not available for
fields defined with just ABNF and/or prose.
o Reuse of common implementations: many parsers for other fields are
specific to a single field or a small family of fields.
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o Canonical form: because a deterministic serialisation algorithm is
defined for each type, Structure Fields have a canonical
representation.
o Enhanced API support: a regular data model makes it easier to
expose field values as a native data structure in implementations.
o 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.
1.1. Using Retrofit Structured Fields
Retrofitting data structures onto existing and widely-deployed HTTP
fields requires careful handling to assure interoperability and
security. This section highlights considerations for applications
that use Retrofit Structured Fields.
While the majority of field values seen in HTTP traffic should be
able to be parsed or mapped successfully, some will not. An
application using Retrofit Structured Fields will need to define how
unsuccessful values will be handled.
For example, an API that exposes field values using Structured Fields
data types might make the field value available as a string in cases
where the field did not successfully parse or map.
The mapped field values described in Section 3 are not compatible
with the original syntax of their fields, and so cannot be used
unless parties processing them have explicitly indicated their
support for that form of the field value. An application using
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Retrofit Structured Fields will need to define how to negotiate
support for them.
For example, an alternative serialization of fields that takes
advantage of Structured Fields would need to establish an explicit
negotiation mechanism to assure that both peers would handle that
serialization appropriately before using it.
See also the security considerations in Section 5.
1.2. 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
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 have values that can be handled as
Structured Field Values according to the parsing and serialisation
algorithms in [STRUCTURED-FIELDS] corresponding to the listed top-
level type, subject to the caveats in Section 2.1.
The top-level 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 as
a Structured Field Value. 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 Field 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 |
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| 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 |
| 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 |
| 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 |
+----------------------------------+-----------------+
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Table 1: Compatible Fields
2.1. Caveats
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
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.
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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 Field values. Instead, it is necessary to map them
into a Structured Field value.
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:
@784111777
Unlike those listed in Section 2, these representations are not
compatible with the original fields' syntax, and MUST NOT be used
unless they are explicitly and unambiguously supported. For example,
this means that sending them to a next-hop recipient in HTTP requires
prior negotiation. This specification does not define how to do so.
3.1. URLs
The field names in Table 2 have values that can be mapped into
Structured Field values by treating the original field's value as a
String.
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+------------------+
| Field Name |
+------------------+
| Content-Location |
| Location |
| Referer |
+------------------+
Table 2: URL Fields
For example, this Location field:
Location: https://example.com/foo
would have a mapped value of:
"https://example.com/foo"
3.2. Dates
The field names in Table 3 have values that can be mapped into
Structured Field values by parsing their payload according to
Section 5.6.7 of [HTTP] and representing the result as a Date.
+---------------------+
| Field Name |
+---------------------+
| Date |
| Expires |
| If-Modified-Since |
| If-Unmodified-Since |
| Last-Modified |
+---------------------+
Table 3: Date Fields
For example, an Expires field's value could be mapped as:
@1659578233
3.3. ETags
The field value of the ETag header field can be mapped into a
Structured Field value 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.
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For example, this ETag header field:
ETag: W/"abcdef"
would have a mapped value of:
"abcdef"; w
If-None-Match's field value can be mapped into a Structured Field
value 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 a Structured
Field value in the same manner.
For example, this If-None-Match field:
If-None-Match: W/"abcdef", "ghijkl", *
would have a mapped value of:
"abcdef"; w, "ghijkl", *
3.4. Cookies
The field values of the Cookie and Set-Cookie fields [COOKIES] can be
mapped into Structured Fields Lists.
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.
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+----------------+-----------------+
| 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, this Set-Cookie field:
Set-Cookie: Lang=en-US; Expires=Wed, 09 Jun 2021 10:18:14 GMT;
samesite=Strict; secure
would have a mapped value of:
("Lang" "en-US"); expires=@1623233894;
samesite=Strict; secure
And this Cookie field:
Cookie: SID=31d4d96e407aad42; lang=en-US
would have a mapped value of:
("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.
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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.
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 values 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", draft-ietf-httpbis-
rfc6265bis-15 (work in progress), July 2024.
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-sfbis-06 (work in progress),
April 2024.
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Author's Address
Mark Nottingham
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
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