Structured Headers for HTTP

Abstract

This document describes a set of data types and parsing algorithms associated with them that are intended to make it easier and safer to define and handle HTTP header fields. It is intended for use by new specifications of HTTP header fields as well as revisions of existing header field specifications when doing so does not cause interoperability issues.¶

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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.¶

This Internet-Draft will expire on December 1, 2018.¶

Copyright Notice

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 extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

1. Introduction

Specifying the syntax of new HTTP header fields is an onerous task; even with the guidance in [RFC7231], Section 8.3.1, there are many decisions – and pitfalls – for a prospective HTTP header field author.¶

Once a header field is defined, bespoke parsers for it often need to be written, because each header has slightly different handling of what looks like common syntax.¶

This document introduces a set of common data structures for use in HTTP header field values to address these problems. In particular, it defines a generic, abstract model for header field values, along with a concrete serialisation for expressing that model in textual HTTP headers, as used by HTTP/1 [RFC7230] and HTTP/2 [RFC7540].¶

HTTP headers that are defined as “Structured Headers” use the types defined in this specification to define their syntax and basic handling rules, thereby simplifying both their definition and parsing.¶

Additionally, future versions of HTTP can define alternative serialisations of the abstract model of these structures, allowing headers that use it to be transmitted more efficiently without being redefined.¶

Note that it is not a goal of this document to redefine the syntax of existing HTTP headers; the mechanisms described herein are only intended to be used with headers that explicitly opt into them.¶

To specify a header field that is a Structured Header, see Section 2.¶

Section 4 defines a number of abstract data types that can be used in Structured Headers. Dictionaries and lists are only usable at the “top” level, while the remaining types can be specified appear at the top level or inside those structures.¶

Those abstract types can be serialised into textual headers – such as those used in HTTP/1 and HTTP/2 – using the algorithms described in Section 3.¶

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.¶

This document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], including the DIGIT, ALPHA and DQUOTE rules from that document. It also includes the OWS rule from [RFC7230].¶

This document uses algorithms to specify normative parsing behaviours, and ABNF to illustrate the on-wire format expected. Implementations MUST follow the normative algorithms, but MAY vary in implementation so as the behaviours are indistinguishable from specified behaviour. If there is disagreement between the algorithms and ABNF, the specified algorithms take precedence.¶

2. Defining New Structured Headers

A HTTP header that uses the structures in this specification need to be defined to do so explicitly; recipients and generators need to know that the requirements of this document are in effect. The simplest way to do that is by referencing this document in its definition.¶

The field’s definition will also need to specify the field-value’s allowed syntax, in terms of the types described in Section 4, along with their associated semantics.¶

A header field definition cannot relax or otherwise modify the requirements of this specification, or change the nature of its data structures; doing so would preclude handling by generic software.¶

However, header field authors are encouraged to clearly state additional constraints upon the syntax, as well as the consequences when those constraints are violated. When Structured Headers parsing fails, the header is discarded (see Section 3); in most situations, header-specific constraints should do likewise.¶

Such constraints could include additional structure inside those defined here (e.g., a list of URLs [RFC3986] inside a string).¶

For example:¶

# Foo-Example Header

The Foo-Example HTTP header field conveys information about how
much Foo the message has.

Foo-Example is a Structured Header [RFCxxxx]. Its value MUST be a
dictionary ([RFCxxxx], Section Y.Y).

The dictionary MUST contain:

* Exactly one member whose key is "foo", and whose value is an
  integer ([RFCxxxx], Section Y.Y), indicating the number of foos in
  the message.
* Exactly one member whose key is "barUrls", and whose value is a
  string ([RFCxxxx], Section Y.Y), conveying the Bar URLs for the
  message. See below for processing requirements.

If the parsed header field does not contain both, it MUST be ignored.

"foo" MUST be between 0 and 10, inclusive; other values MUST cause
the header to be ignored.

"barUrls" contains a space-separated list of URI-references
([RFC3986], Section 4.1):

   barURLs = URI-reference *( 1*SP URI-reference )

If a member of barURLs is not a valid URI-reference, it MUST cause
that value to be ignored.

If a member of barURLs is a relative reference ([RFC3986],
Section 4.2), it MUST be resolved ([RFC3986], Section 5) before being
used.

This specification defines minimums for the length or number of various structures supported by Structured Headers implementations. It does not specify maximum sizes in most cases, but header authors should be aware that HTTP implementations do impose various limits on the size of individual header fields, the total number of fields, and/or the size of the entire header block.¶

Note that specifications using Structured Headers do not re-specify its ABNF or parsing algorithms; instead, they should be specified in terms of its abstract data structures.¶

Also, empty header field values are not allowed, and therefore parsing for them will fail.¶

3. Parsing Textual Header Fields

When a receiving implementation parses textual HTTP header fields (e.g., in HTTP/1 or HTTP/2) that are known to be Structured Headers, it is important that care be taken, as there are a number of edge cases that can cause interoperability or even security problems. This section specifies the algorithm for doing so.¶

Given an ASCII string input_string that represents the chosen header’s field-value, and header_type, one of “dictionary”, “list”, “param-list”, or “item”, return the parsed header value.¶

Discard any leading OWS from input_string.
If header_type is “dictionary”, let output be the result of Parsing a Dictionary from Text (Section 4.1.1).
If header_type is “list”, let output be the result of Parsing a List from Text (Section 4.2.1).
If header_type is “param-list”, let output be the result of Parsing a Parameterised List from Text (Section 4.3.1).
Otherwise, let output be the result of Parsing an Item from Text (Section 4.4.1).
Discard any leading OWS from input_string.
If input_string is not empty, fail parsing.
Otherwise, return output.

When generating input_string, parsers MUST combine all instances of the target header field into one comma-separated field-value, as per [RFC7230], Section 3.2.2; this assures that the header is processed correctly.¶

For Lists, Parameterised Lists and Dictionaries, this has the effect of correctly concatenating all instances of the header field.¶

Strings can but SHOULD NOT be split across multiple header instances, because comma(s) inserted upon combination will become part of the string output by the parser.¶

Integers, Floats and Binary Content cannot be split across multiple headers because the inserted commas will cause parsing to fail.¶

If parsing fails – including when calling another algorithm – the entire header field’s value MUST be discarded. This is intentionally strict, to improve interoperability and safety, and specifications referencing this document cannot loosen this requirement.¶

Note that this has the effect of discarding any header field with non-ASCII characters in input_string.¶

4. Structured Header Data Types

This section defines the abstract value types that can be composed into Structured Headers, along with the textual HTTP serialisations of them.¶

4.1. Dictionaries

Dictionaries are unordered maps of key-value pairs, where the keys are identifiers (Section 4.8) and the values are items (Section 4.4). There can be one or more members, and keys are required to be unique.¶

In the textual HTTP serialisation, keys and values are separated by “=” (without whitespace), and key/value pairs are separated by a comma with optional whitespace. Duplicate keys MUST cause parsing to fail.¶

dictionary  = dict-member *( OWS "," OWS dict-member )
dict-member = identifier "=" item

For example, a header field whose value is defined as a dictionary could look like:¶

Example-DictHeader: foo=1.23, en="Applepie", da=*w4ZibGV0w6ZydGUK=*

Typically, a header field specification will define the semantics of individual keys, as well as whether their presence is required or optional. Recipients MUST ignore keys that are undefined or unknown, unless the header field’s specification specifically disallows them.¶

Parsers MUST support dictionaries containing at least 1024 key/value pairs.¶

4.1.1. Parsing a Dictionary from Text

Given an ASCII string input_string, return a mapping of (identifier, item). input_string is modified to remove the parsed value.¶

Let dictionary be an empty, unordered mapping.
While input_string is not empty:
1. Let this_key be the result of running Parse Identifier from Text (Section 4.8.1) with input_string.
2. If dictionary already contains this_key, fail parsing.
3. Consume a “=” from input_string; if none is present, fail parsing.
4. Let this_value be the result of running Parse Item from Text (Section 4.4.1) with input_string.
5. Add key this_key with value this_value to dictionary.
6. Discard any leading OWS from input_string.
7. If input_string is empty, return dictionary.
8. Consume a COMMA from input_string; if no comma is present, fail parsing.
9. Discard any leading OWS from input_string.
10. If input_string is empty, fail parsing.
No structured data has been found; fail parsing.

4.2. Lists

Lists are arrays of items (Section 4.4) with one or more members.¶

In the textual HTTP serialisation, each member is separated by a comma and optional whitespace.¶

list = list-member *( OWS "," OWS list-member )
list-member = item

For example, a header field whose value is defined as a list of strings could look like:¶

Example-StrListHeader: "foo", "bar", "It was the best of times."

Parsers MUST support lists containing at least 1024 members.¶

4.2.1. Parsing a List from Text

Given an ASCII string input_string, return a list of items. input_string is modified to remove the parsed value.¶

Let items be an empty array.
While input_string is not empty:
1. Let item be the result of running Parse Item from Text (Section 4.4.1) with input_string.
2. Append item to items.
3. Discard any leading OWS from input_string.
4. If input_string is empty, return items.
5. Consume a COMMA from input_string; if no comma is present, fail parsing.
6. Discard any leading OWS from input_string.
7. If input_string is empty, fail parsing.
No structured data has been found; fail parsing.

4.3. Parameterised Lists

Parameterised Lists are arrays of a parameterised identifiers.¶

A parameterised identifier is an identifier (Section 4.8) with an optional set of parameters, each parameter having a identifier and an optional value that is an item (Section 4.4). Ordering between parameters is not significant, and duplicate parameters MUST cause parsing to fail.¶

In the textual HTTP serialisation, each parameterised identifier is separated by a comma and optional whitespace. Parameters are delimited from each other using semicolons (“;”), and equals (“=”) delimits the parameter name from its value.¶

param-list = param-id *( OWS "," OWS param-id )
param-id   = identifier *( OWS ";" OWS identifier [ "=" item ] )

For example,¶

Example-ParamListHeader: abc_123;a=1;b=2; c, def_456, ghi;q="19";r=foo

Parsers MUST support parameterised lists containing at least 1024 members, and support members with at least 256 parameters.¶

4.3.1. Parsing a Parameterised List from Text

Given an ASCII string input_string, return a list of parameterised identifiers. input_string is modified to remove the parsed value.¶

Let items be an empty array.
While input_string is not empty:
1. Let item be the result of running Parse Parameterised Identifier from Text (Section 4.3.2) with input_string.
2. Append item to items.
3. Discard any leading OWS from input_string.
4. If input_string is empty, return items.
5. Consume a COMMA from input_string; if no comma is present, fail parsing.
6. Discard any leading OWS from input_string.
7. If input_string is empty, fail parsing.
No structured data has been found; fail parsing.

4.3.2. Parsing a Parameterised Identifier from Text

Given an ASCII string input_string, return a identifier with an mapping of parameters. input_string is modified to remove the parsed value.¶

Let primary_identifier be the result of Parsing a Identifier from Text (Section 4.8.1) from input_string.
Let parameters be an empty, unordered mapping.
In a loop:
1. Discard any leading OWS from input_string.
2. If the first character of input_string is not “;”, exit the loop.
3. Consume a “;” character from the beginning of input_string.
4. Discard any leading OWS from input_string.
5. let param_name be the result of Parsing a Identifier from Text (Section 4.8.1) from input_string.
6. If param_name is already present in parameters, fail parsing.
7. Let param_value be a null value.
8. If the first character of input_string is “=”:
  1. Consume the “=” character at the beginning of input_string.
  2. Let param_value be the result of Parsing an Item from Text (Section 4.4.1) from input_string.
9. Insert (param_name, param_value) into parameters.
Return the tuple (primary_identifier, parameters).

4.4. Items

An item is can be a integer (Section 4.5), float (Section 4.6), string (Section 4.7), or binary content (Section 4.9).¶

item = integer / float / string / binary

4.4.1. Parsing an Item from Text

Given an ASCII string input_string, return an item. input_string is modified to remove the parsed value.¶

Discard any leading OWS from input_string.
If the first character of input_string is a “-“ or a DIGIT, process input_string as a number (Section 4.5.1) and return the result.
If the first character of input_string is a DQUOTE, process input_string as a string (Section 4.7.1) and return the result.
If the first character of input_string is “*”, process input_string as binary content (Section 4.9.1) and return the result.
Otherwise, fail parsing.

4.5. Integers

Abstractly, integers have a range of −9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 inclusive (i.e., a 64-bit signed integer).¶

integer   = ["-"] 1*19DIGIT

Parsers that encounter an integer outside the range defined above MUST fail parsing. Therefore, the value “9223372036854775808” would be invalid. Likewise, values that do not conform to the ABNF above are invalid, and MUST fail parsing.¶

For example, a header whose value is defined as a integer could look like:¶

Example-IntegerHeader: 42

4.5.1. Parsing a Number from Text

NOTE: This algorithm parses both Integers and Floats Section 4.6, and returns the corresponding structure.¶

Let type be “integer”.
Let sign be 1.
Let input_number be an empty string.
If the first character of input_string is “-“, remove it from input_string and set sign to -1.
If input_string is empty, fail parsing.
If the first character of input_string is not a DIGIT, fail parsing.
While input_string is not empty:
1. Let char be the result of removing the first character of input_string.
2. If char is a DIGIT, append it to input_number.
3. Else, if type is “integer” and char is “.”, append char to input_number and set type to “float”.
4. Otherwise, fail parsing.
5. If type is “integer” and input_number contains more than 19 characters, fail parsing.
6. If type is “float” and input_number contains more than 16 characters, fail parsing.
If type is “integer”, parse input_number as an integer and let output_number be the result.
Otherwise:
1. If the final character of input_number is “.”, fail parsing.
2. Parse input_number as a float and let output_number be the result.
Return the product of output_number and sign.

4.6. Floats

Abstractly, floats are integers with a fractional part, that can be stored as IEEE 754 double precision numbers (binary64) ([IEEE754]).¶

The textual HTTP serialisation of floats allows a maximum of fifteen digits between the integer and fractional part, with at least one required on each side, along with an optional “-“ indicating negative numbers.¶

float    = ["-"] (
             DIGIT "." 1*14DIGIT /
            2DIGIT "." 1*13DIGIT /
            3DIGIT "." 1*12DIGIT /
            4DIGIT "." 1*11DIGIT /
            5DIGIT "." 1*10DIGIT /
            6DIGIT "." 1*9DIGIT /
            7DIGIT "." 1*8DIGIT /
            8DIGIT "." 1*7DIGIT /
            9DIGIT "." 1*6DIGIT /
           10DIGIT "." 1*5DIGIT /
           11DIGIT "." 1*4DIGIT /
           12DIGIT "." 1*3DIGIT /
           13DIGIT "." 1*2DIGIT /
           14DIGIT "." 1DIGIT )

Values that do not conform to the ABNF above are invalid, and MUST fail parsing.¶

For example, a header whose value is defined as a float could look like:¶

Example-FloatHeader: 4.5

See Section 4.5.1 for the parsing algorithm for floats.¶

4.7. Strings

Abstractly, strings are zero or more printable ASCII [RFC0020] characters (i.e., the range 0x20 to 0x7E). Note that this excludes tabs, newlines, carriage returns, etc.¶

The textual HTTP serialisation of strings uses a backslash (“\”) to escape double quotes and backslashes in strings.¶

string    = DQUOTE *(chr) DQUOTE
chr       = unescaped / escaped
unescaped = %x20-21 / %x23-5B / %x5D-7E
escaped   = "\" ( DQUOTE / "\" )

For example, a header whose value is defined as a string could look like:¶

Example-StringHeader: "hello world"

Note that strings only use DQUOTE as a delimiter; single quotes do not delimit strings. Furthermore, only DQUOTE and “\” can be escaped; other sequences MUST cause parsing to fail.¶

Unicode is not directly supported in this document, because it causes a number of interoperability issues, and – with few exceptions – header values do not require it.¶

When it is necessary for a field value to convey non-ASCII string content, binary content (Section 4.9) SHOULD be specified, along with a character encoding (preferably, UTF-8).¶

Parsers MUST support strings with at least 1024 characters.¶

4.7.1. Parsing a String from Text

Given an ASCII string input_string, return an unquoted string. input_string is modified to remove the parsed value.¶

Let output_string be an empty string.
If the first character of input_string is not DQUOTE, fail parsing.
Discard the first character of input_string.
While input_string is not empty:
1. Let char be the result of removing the first character of input_string.
2. If char is a backslash (“\”):
  1. If input_string is now empty, fail parsing.
  2. Else:
    1. Let next_char be the result of removing the first character of input_string.
    2. If next_char is not DQUOTE or “\”, fail parsing.
    3. Append next_char to output_string.
3. Else, if char is DQUOTE, return output_string.
4. Else, append char to output_string.
Otherwise, fail parsing.

4.8. Identifiers

Identifiers are short textual identifiers; their abstract model is identical to their expression in the textual HTTP serialisation. Parsers MUST support identifiers with at least 64 characters.¶

identifier = lcalpha *( lcalpha / DIGIT / "_" / "-"/ "*" / "/" )
lcalpha    = %x61-7A ; a-z

Note that identifiers can only contain lowercase letters.¶

4.8.1. Parsing a Identifier from Text

Given an ASCII string input_string, return a identifier. input_string is modified to remove the parsed value.¶

If the first character of input_string is not lcalpha, fail parsing.
Let output_string be an empty string.
While input_string is not empty:
1. Let char be the result of removing the first character of input_string.
2. If char is not one of lcalpha, DIGIT, “_”, “-“, “*” or “/”:
  1. Prepend char to input_string.
  2. Return output_string.
3. Append char to output_string.
Return output_string.

4.9. Binary Content

Arbitrary binary content can be conveyed in Structured Headers.¶

The textual HTTP serialisation encodes the data using Base 64 Encoding [RFC4648], Section 4, and surrounds it with a pair of asterisks (“*”) to delimit from other content.¶

The encoded data is required to be padded with “=”, as per [RFC4648], Section 3.2. It is RECOMMENDED that parsers reject encoded data that is not properly padded, although this might not be possible with some base64 implementations.¶

Likewise, encoded data is required to have pad bits set to zero, as per [RFC4648], Section 3.5. It is RECOMMENDED that parsers fail on encoded data that has non-zero pad bits, although this might not be possible with some base64 implementations.¶

This specification does not relax the requirements in [RFC4648], Section 3.1 and 3.3; therefore, parsers MUST fail on characters outside the base64 alphabet, and on line feeds in encoded data.¶

binary = "*" *(base64) "*"
base64 = ALPHA / DIGIT / "+" / "/" / "="

For example, a header whose value is defined as binary content could look like:¶

Example-BinaryHeader: *cHJldGVuZCB0aGlzIGlzIGJpbmFyeSBjb250ZW50Lg==*

Parsers MUST support binary content with at least 16384 octets after decoding.¶

4.9.1. Parsing Binary Content from Text

Given an ASCII string input_string, return binary content. input_string is modified to remove the parsed value.¶

If the first character of input_string is not “*”, fail parsing.
Discard the first character of input_string.
Let b64_content be the result of removing content of input_string up to but not including the first instance of the character “*”. If there is not a “*” character before the end of input_string, fail parsing.
Consume the “*” character at the beginning of input_string.
Let binary_content be the result of Base 64 Decoding [RFC4648] b64_content, synthesising padding if necessary (note the requirements about recipient behaviour in Section 4.9).
Return binary_content.

5. IANA Considerations

This draft has no actions for IANA.¶

6. Security Considerations

The size of most types defined by Structured Headers is not limited; as a result, extremely large header fields could be an attack vector (e.g., for resource consumption). Most HTTP implementations limit the sizes of size of individual header fields as well as the overall header block size to mitigate such attacks.¶

It is possible for parties with the ability to inject new HTTP header fields to change the meaning of a Structured Headers. In some circumstances, this will cause parsing to fail, but it is not possible to reliably fail in all such circumstances.¶

Appendix A. Changes

A.1. Since draft-ietf-httpbis-header-structure-04 📄 🔍

Remove identifiers from item.
Remove most limits on sizes.
Refine number parsing.

A.2. Since draft-ietf-httpbis-header-structure-03 📄 🔍

Strengthen language around failure handling.

A.3. Since draft-ietf-httpbis-header-structure-02 📄 🔍

Split Numbers into Integers and Floats.
Define number parsing.
Tighten up binary parsing and give it an explicit end delimiter.
Clarify that mappings are unordered.
Allow zero-length strings.
Improve string parsing algorithm.
Improve limits in algorithms.
Require parsers to combine header fields before processing.
Throw an error on trailing garbage.

A.4. Since draft-ietf-httpbis-header-structure-01 📄 🔍

Replaced with draft-nottingham-structured-headers.

A.5. Since draft-ietf-httpbis-header-structure-00 📄 🔍

Added signed 64bit integer type.
Drop UTF8, and settle on BCP137 ::EmbeddedUnicodeChar for h1-unicode-string.
Change h1_blob delimiter to “:” since “’” is valid t_char