draft-ietf-httpbis-retrofit-02.txt   draft-ietf-httpbis-retrofit-latest.txt 
Network Working Group M. Nottingham Network Working Group M. Nottingham
Internet-Draft May 11, 2021 Internet-Draft May 13, 2022
Intended status: Standards Track Intended status: Standards Track
Expires: November 12, 2021 Expires: November 14, 2022
Retrofit Structured Fields for HTTP Retrofit Structured Fields for HTTP
draft-ietf-httpbis-retrofit-02 draft-ietf-httpbis-retrofit-latest
Abstract Abstract
This specification nominates a selection of existing HTTP fields as This specification nominates a selection of existing HTTP fields as
having syntax that is compatible with Structured Fields, so that they having syntax that is compatible with Structured Fields, so that they
can be handled as such (subject to certain caveats). can be handled as such (subject to certain caveats).
To accommodate some additional fields whose syntax is not compatible, To accommodate some additional fields whose syntax is not compatible,
it also defines mappings of their semantics into new Structured it also defines mappings of their semantics into new Structured
Fields. It does not specify how to negotiate their use. Fields. It does not specify how to negotiate their use.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months 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." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 12, 2021. This Internet-Draft will expire on November 14, 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
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Table 1: Compatible Fields Table 1: Compatible Fields
Note the following caveats regarding compatibility: Note the following caveats regarding compatibility:
Parameter and Dictionary keys: HTTP parameter names are case- Parameter and Dictionary keys: HTTP parameter names are case-
insensitive (per Section 5.6.6 of [HTTP]), but Structured Fields insensitive (per Section 5.6.6 of [HTTP]), but Structured Fields
require them to be all-lowercase. Although the vast majority of require them to be all-lowercase. Although the vast majority of
parameters seen in typical traffic are all-lowercase, parameters seen in typical traffic are all-lowercase,
compatibility can be improved by force-lowercasing parameters when compatibility can be improved by force-lowercasing parameters when
encountered. Likewise, many Dictionary-based fields (e.g., Cache- parsing. Likewise, many Dictionary-based fields (e.g., Cache-
Control, Expect-CT, Pragma, Prefer, Preference-Applied, Surrogate- Control, Expect-CT, Pragma, Prefer, Preference-Applied, Surrogate-
Control) have case-insensitive keys, and compatibility can be Control) have case-insensitive keys, and compatibility can be
improved by force-lowercasing them. improved by force-lowercasing them when parsing.
Parameter delimitation: The parameters rule in HTTP (see Parameter delimitation: The parameters rule in HTTP (see
Section 5.6.6 of [HTTP]) allows whitespace before the ";" Section 5.6.6 of [HTTP]) allows whitespace before the ";"
delimiter, but Structured Fields does not. Compatibility can be delimiter, but Structured Fields does not. Compatibility can be
improved by allowing such whitespace. improved by allowing such whitespace when parsing.
String quoting: Section 5.6.4 of [HTTP] allows backslash-escaping String quoting: Section 5.6.4 of [HTTP] allows backslash-escaping
most characters in quoted strings, whereas Structured Field most characters in quoted strings, whereas Structured Field
Strings only escape "\" and DQUOTE. Compatibility can be improved Strings only escape "\" and DQUOTE. Compatibility can be improved
by unescaping other characters before processing as Strings. by unescaping other characters before parsing.
Token limitations: In Structured Fields, tokens are required to Token limitations: In Structured Fields, tokens are required to
begin with an alphabetic character or "*", whereas HTTP tokens begin with an alphabetic character or "*", whereas HTTP tokens
allow a wider range of characters. This prevents use of mapped allow a wider range of characters. This prevents use of mapped
values that begin with one of these characters. For example, values that begin with one of these characters. For example,
media types, field names, methods, range-units, character and media types, field names, methods, range-units, character and
transfer codings that begin with a number or special character transfer codings that begin with a number or special character
other than "*" might be valid HTTP protocol elements, but will not other than "*" might be valid HTTP protocol elements, but will not
be able to be parsed as Structured Field Tokens. be able to be parsed as Structured Field Tokens.
Integer limitations: Structured Fields Integers can have at most 15 Integer limitations: Structured Fields Integers can have at most 15
digits; larger values will not be able to be represented in them. digits; larger values will not be able to be represented in them.
IPv6 Literals: Fields whose values can contain IPv6 literal IPv6 Literals: Fields whose values contain IPv6 literal addresses
addresses (such as CDN-Loop, Host, and Origin) are not compatible (such as CDN-Loop, Host, and Origin) are not able to be
when those values are parsed as Structured Fields Tokens, because represented as Structured Fields Tokens, because the brackets used
the brackets used to delimit them are not allowed in Tokens. to delimit them are not allowed in Tokens.
Empty Field Values: Empty and whitespace-only field values are Empty Field Values: Empty and whitespace-only field values are
considered errors in Structured Fields. For compatible fields, an considered errors in Structured Fields. For compatible fields, an
empty field indicates that the field should be silently ignored. 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 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, syntax, and therefore cannot be represented as a Token. Since the
this shouldn't be a long-term issue, although future tokens may final version of HTTP/3 uses the "h3" token, this shouldn't be a
again violate this assumption. long-term issue, although future tokens may again violate this
assumption.
Content-Length: Content-Length is defined as a List because it is Content-Length: Note that Content-Length is defined as a List
not uncommon for implementations to mistakenly send multiple because it is not uncommon for implementations to mistakenly send
values. See Section 8.6 of [HTTP] for handling requirements. multiple values. See Section 8.6 of [HTTP] for handling
requirements.
Retry-After: Only the delta-seconds form of Retry-After is Retry-After: Only the delta-seconds form of Retry-After can be
supported; a Retry-After value containing a http-date will need to represented; a Retry-After value containing a http-date will need
be either converted into delta-seconds or represented as a raw to be either converted into delta-seconds to be conveyed as a
value. Structured Field Value.
3. Mapped Fields 3. Mapped Fields
Some HTTP field values have syntax that cannot be successfully parsed Some HTTP field values have syntax that cannot be successfully parsed
as Structured Fields. Instead, it is necessary to map them into a as Structured Fields. Instead, it is necessary to map them into a
separate Structured Field with an alternative name. separate Structured Field with an alternative name.
For example, the Date HTTP header field carries a date: For example, the Date HTTP header field carries a date:
Date: Sun, 06 Nov 1994 08:49:37 GMT Date: Sun, 06 Nov 1994 08:49:37 GMT
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