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ART HTTP compression dictionary shared brotli zstandard dictionary delta compression This specification defines a mechanism for using designated HTTP responses as an external dictionary for future HTTP responses for compression schemes that support using external dictionaries (e.g., Brotli (RFC 7932) and Zstandard (RFC 8878)). About This Document Status information for this document may be found at . Discussion of this document takes place on the HTTP Working Group mailing list (), which is archived at . Working Group information can be found at . Source for this draft and an issue tracker can be found at .

Introduction This specification defines a mechanism for using designated responses as an external dictionary for future HTTP responses for compression schemes that support using external dictionaries (e.g., Brotli and Zstandard ). This document describes the HTTP headers used for negotiating dictionary usage and registers media types for content encoding Brotli and Zstandard using a negotiated dictionary.

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 when, and only when, they appear in all capitals, as shown here. This document uses the following terminology from to specify syntax and parsing: Dictionary, String, Inner List, Token, and Byte Sequence. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . This document uses the line folding strategies described in . This document also uses terminology from and .

Dictionary Negotiation

Use-As-Dictionary When responding to a HTTP Request, a server can advertise that the response can be used as a dictionary for future requests for URLs that match the rules specified in the Use-As-Dictionary response header. The Use-As-Dictionary response header is a Structured Field Dictionary with values for "match", "match-dest", "id", and "type".

match The "match" value of the Use-As-Dictionary header is a String value that provides the URLPattern to use for request matching (https://urlpattern.spec.whatwg.org/). The URLPattern used for matching does not support using Regular expressions. The following algorithm will return TRUE for a valid match pattern and FALSE for an invalid pattern that MUST NOT be used:

1. Let MATCH be the value of "match" for the given dictionary.
2. Let URL be the URL of the dictionary request.
3. Let PATTERN be a URLPattern constructed by setting input=MATCH, and baseURL=URL (https://urlpattern.spec.whatwg.org/).
4. If PATTERN has regexp groups then return FALSE (https://urlpattern.spec.whatwg.org/#urlpattern-has-regexp-groups).
5. Return True.

The "match" value is required and MUST be included in the Use-As-Dictionary Dictionary for the dictionary to be considered valid.

match-dest The "match-dest" value of the Use-As-Dictionary header is an Inner List of String values that provides a list of request destinations for the dictionary to match (https://fetch.spec.whatwg.org/#concept-request-destination). An empty list for "match-dest" MUST match all destinations. For clients that do not support request destinations, the client MUST treat it as an empty list and match all destinations. The "match-dest" value is optional and defaults to an empty list.

id The "id" value of the Use-As-Dictionary header is a String value that specifies a server identifier for the dictionary. If an "id" value is present and has a string length longer than zero then it MUST be sent to the server in a "Dictionary-ID" request header when the dictionary is advertised as being available. The server identifier MUST be treated as an opaque string by the client. The server identifier MUST NOT be relied upon by the server to guarantee the contents of the dictionary. The dictionary hash MUST be validated before use. The "id" value string length (after any decoding) supports up to 1024 characters. The "id" value is optional and defaults to the empty string.

type The "type" value of the Use-As-Dictionary header is a Token value that describes the file format of the supplied dictionary. "raw" is the only defined dictionary format which represents an unformatted blob of bytes suitable for any compression scheme to use. If a client receives a dictionary with a type that it does not understand, it MUST NOT use the dictionary. The "type" value is optional and defaults to raw.

Examples

Path Prefix A response that contained a response header: Would specify matching any document request for a URL with a path prefix of /product/ on the same as the original request.

Versioned Directories A response that contained a response header: Would match main.js in any directory under /app/ and expiring as a dictionary in one year.

Available-Dictionary When a HTTP client makes a request for a resource for which it has an appropriate dictionary, it can add a "Available-Dictionary" request header to the request to indicate to the server that it has a dictionary available to use for compression. The "Available-Dictionary" request header is a Structured Field Byte Sequence containing the hash of the contents of a single available dictionary. The client MUST only send a single "Available-Dictionary" request header with a single hash value for the best available match that it has available. For example:

Dictionary freshness requirement To be considered as a match, the dictionary resource MUST be either fresh or allowed to be served stale (see eg ).

Dictionary URL matching When a dictionary is stored as a result of a "Use-As-Dictionary" directive, it includes "match" and "match-dest" strings that are used to match an outgoing request from a client to the available dictionaries. Dictionaries MUST have been served from the same {Origin} as the outgoing request to match. To see if an outbound request matches a given dictionary, the following algorithm will return TRUE for a successful match and FALSE for no-match:

1. If the current client supports request destinations:
   • Let DEST be the value of "match-dest" for the given dictionary.
   • Let REQUEST_DEST be the value of the destination for the current request.
   • If DEST is not an empty list and if REQUEST_DEST is not in the DEST list of destinations, return FALSE
2. Let BASEURL be the URL of the dictionary request.
3. Let URL represent the URL of the outbound request being checked.
4. If the {Origin} of BASEURL and the {Origin} of URL are not the same, return FALSE.
5. Let MATCH be the value of "match" for the given dictionary.
6. Let PATTERN be a URLPattern constructed by setting input=MATCH, and baseURL=BASEURL (https://urlpattern.spec.whatwg.org/).
7. Return the result of running the "test" method of PATTERN with input=URL (https://urlpattern.spec.whatwg.org/#ref-for-dom-urlpattern-test)

Multiple matching dictionaries When there are multiple dictionaries that match a given request URL, the client MUST pick a single dictionary using the following rules: 1. For clients that support request destinations, a dictionary that specifies and matches a "match-dest" takes precedence over a match that does not use a destination. 1. Given equivalent destination precedence, the dictionary with the longest "match" takes precedence. 1. Given equivalent destination and match length precedence, the most recently fetched dictionary takes precedence.

Dictionary-ID When a HTTP client makes a request for a resource for which it has an appropriate dictionary and the dictionary was stored with a server-provided "id" in the Use-As-Dictionary response then the client MUST echo the stored "id" in a "Dictionary-ID" request header. The "Dictionary-ID" request header is a Structured Field String of up to 1024 characters (after any decoding) and MUST be identical to the server-provided "id". For example:

Content-Dictionary When a HTTP server responds with a resource that is encoded with a dictionary the server MUST send the hash of the dictionary that was used in the "Content-Dictionary" response header. The "Content-Dictionary" response header is a Structured Field Byte Sequence containing the hash of the contents of the dictionary that was used to encode the response. If the HTTP response contains a "Content-Dictionary" response header with the hash of a dictionary that the client does not have available then the client cannot decode or use the HTTP response. For example:

Negotiating the compression algorithm When a compression dictionary is available for use for a given request, the algorithm to be used is negotiated through the regular mechanism for negotiating content encoding in HTTP. This document introduces two new content encoding algorithms:

Content-Encoding	Description
br-d	Brotli using an external compression dictionary
zstd-d	Zstandard using an external compression dictionary

The dictionary to use is negotiated separately and advertised in the "Available-Dictionary" request header.

Accept-Encoding The client adds the algorithms that it supports to the "Accept-Encoding" request header. e.g.:

Content-Encoding If a server supports one of the dictionary algorithms advertised by the client and chooses to compress the content of the response using the dictionary that the client has advertised then it sets the "Content-Encoding" response header to the appropriate value for the algorithm selected. e.g.: If the response is cacheable, it MUST include a "Vary" header to prevent caches serving dictionary-compressed resources to clients that don't support them or serving the response compressed with the wrong dictionary:

IANA Considerations

Content Encoding IANA is asked to update the "HTTP Content Coding Registry" registry () according to the table below:

Name	Description	Reference
br-d	A stream of bytes compressed using the Brotli protocol with an external dictionary
zstd-d	A stream of bytes compressed using the Zstandard protocol with an external dictionary

Header Field Registration IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field Name Registry" registry () according to the table below:

Field Name	Status	Reference
Use-As-Dictionary	permanent	of this document
Available-Dictionary	permanent	of this document
Dictionary-ID	permanent	of this document
Content-Dictionary	permanent	of this document

Compatibility Considerations To minimize the risk of middle-boxes incorrectly processing dictionary-compressed responses, compression dictionary transport MUST only be used in secure contexts (HTTPS).

Security Considerations The security considerations for Brotli and Zstandard apply to the dictionary-based versions of the respective algorithms.

Changing content The dictionary must be treated with the same security precautions as the content, because a change to the dictionary can result in a change to the decompressed content. The dictionary is validated using a SHA-256 hash of the content to make sure that the client and server are both using the same dictionary. The strength of the SHA-256 hash algorithm isn't explicitly needed to counter attacks since the dictionary is being served from the same origin as the content. That said, if a weakness is discovered in SHA-256 and it is determined that the dictionary negotiation should use a different hash algorithm, the "Use-As-Dictionary" response header can be extended to specify a different algorithm and the server would just ignore any "Available-Dictionary" requests that do not use the updated hash.

Reading content The CRIME attack shows that it's a bad idea to compress data from mixed (e.g. public and private) sources -- the data sources include not only the compressed data but also the dictionaries. For example, if you compress secret cookies using a public-data-only dictionary, you still leak information about the cookies. Not only can the dictionary reveal information about the compressed data, but vice versa, data compressed with the dictionary can reveal the contents of the dictionary when an adversary can control parts of data to compress and see the compressed size. On the other hand, if the adversary can control the dictionary, the adversary can learn information about the compressed data.

Security Mitigations If any of the mitigations do not pass, the client MUST drop the response and return an error.

Cross-origin protection To make sure that a dictionary can only impact content from the same origin where the dictionary was served, the URLPattern used for matching a dictionary to requests is guaranteed to be for the same origin that the dictionary is served from.

Response readability For clients, like web browsers, that provide additional protection against the readability of the payload of a response and against user tracking, additional protections MUST be taken to make sure that the use of dictionary-based compression does not reveal information that would not otherwise be available. In these cases, dictionary compression MUST only be used when both the dictionary and the compressed response are fully readable by the client. In browser terms, that means that both are either same-origin to the context they are being fetched from or that the response is cross-origin and passes the CORS check (https://fetch.spec.whatwg.org/#cors-check).

Same-Origin On the client-side, same-origin determination is defined in the fetch spec (https://html.spec.whatwg.org/multipage/browsers.html#origin). On the server-side, a request with a "Sec-Fetch-Site:" request header with a value of "same-origin" is to be considered a same-origin request.

• For any request that is same-origin:
  • Response MAY be used as a dictionary.
  • Response MAY be compressed by an available dictionary.

Cross-Origin For requests that are not same-origin (), the "mode" of the request can be used to determine the readability of the response. For clients that conform to the fetch spec, the mode of the request is stored in the RequestMode attribute of the request (https://fetch.spec.whatwg.org/#requestmode). For servers responding to clients that expose the request mode information, the value of the mode is sent in the "Sec-Fetch-Mode" request header. If a "Sec-Fetch-Mode" request header is not present, the server SHOULD allow for the dictionary compression to be used.

1. If the mode is "navigate" or "same-origin":
   • Response MAY be used as a dictionary.
   • Response MAY be compressed by an available dictionary.
2. If the mode is "cors":
   • For clients, apply the CORS check from the fetch spec (https://fetch.spec.whatwg.org/#cors-check) which includes credentials checking restrictions that may not be possible to check on the server.
     • If the CORS check passes:
       • Response MAY be used as a dictionary.
       • Response MAY be compressed by an available dictionary.
     • Else:
       • Response MUST NOT be used as a dictionary.
       • Response MUST NOT be compressed by an available dictionary.
   • For servers:
     • If the response does not include an "Access-Control-Allow-Origin" response header:
       • Response MUST NOT be used as a dictionary.
       • Response MUST NOT be compressed by an available dictionary.
     • If the request does not include an "Origin" request header:
       • Response MUST NOT be used as a dictionary.
       • Response MUST NOT be compressed by an available dictionary.
     • If the value of the "Access-Control-Allow-Origin" response header is "*":
       • Response MAY be used as a dictionary.
       • Response MAY be compressed by an available dictionary.
     • If the value of the "Access-Control-Allow-Origin" response header matches the value of the "Origin" request header:
       • Response MAY be used as a dictionary.
       • Response MAY be compressed by an available dictionary.
3. If the mode is any other value (including "no-cors"):
   • Response MUST NOT be used as a dictionary.
   • Response MUST NOT be compressed by an available dictionary.

Privacy Considerations Since dictionaries are advertised in future requests using the hash of the content of the dictionary, it is possible to abuse the dictionary to turn it into a tracking cookie. To mitigate any additional tracking concerns, clients MUST treat dictionaries in the same way that they treat cookies. This includes partitioning the storage as cookies are partitioned as well as clearing the dictionaries whenever cookies are cleared.

References Normative References This document defines two strategies for handling long lines in width-bounded text content. One strategy, called the "single backslash" strategy, is based on the historical use of a single backslash ('\') character to indicate where line-folding has occurred, with the continuation occurring with the first character that is not a space character (' ') on the next line. The second strategy, called the "double backslash" strategy, extends the first strategy by adding a second backslash character to identify where the continuation begins and is thereby able to handle cases not supported by the first strategy. Both strategies use a self-describing header enabling automated reconstitution of the original content. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages. This document obsoletes RFC 7234. This document defines two independent HTTP Cache-Control extensions that allow control over the use of stale responses by caches. This document is not an Internet Standards Track specification; it is published for informational purposes. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document defines the concept of an "origin", which is often used as the scope of authority or privilege by user agents. Typically, user agents isolate content retrieved from different origins to prevent malicious web site operators from interfering with the operation of benign web sites. In addition to outlining the principles that underlie the concept of origin, this document details how to determine the origin of a URI and how to serialize an origin into a string. It also defines an HTTP header field, named "Origin", that indicates which origins are associated with an HTTP request. [STANDARDS-TRACK] This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. Federal Information Processing Standard, FIPS This specification defines a lossless compressed data format that compresses data using a combination of the LZ77 algorithm and Huffman coding, with efficiency comparable to the best currently available general-purpose compression methods. Zstandard, or "zstd" (pronounced "zee standard"), is a lossless data compression mechanism. This document describes the mechanism and registers a media type, content encoding, and a structured syntax suffix to be used when transporting zstd-compressed content via MIME. Despite use of the word "standard" as part of Zstandard, readers are advised that this document is not an Internet Standards Track specification; it is being published for informational purposes only. This document replaces and obsoletes RFC 8478.