HTTP
Internet Engineering Task Force (IETF) P. Meenan, Ed.
Internet-Draft
Request for Comments: 9842 Google LLC
Intended status:
Category: Standards Track Y. Weiss, Ed.
Expires: 1 March 2025
ISSN: 2070-1721 Shopify Inc
28
August 2024 2025
Compression Dictionary Transport
draft-ietf-httpbis-compression-dictionary-19
Abstract
This document specifies a mechanism for dictionary-based compression
in the Hypertext Transfer Protocol (HTTP). By utilizing this
technique, clients and servers can reduce the size of transmitted
data, leading to improved performance and reduced bandwidth
consumption. This document extends existing HTTP compression methods
and provides guidelines for the delivery and use of compression
dictionaries within the HTTP protocol.
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-compression-
dictionary/.
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/compression-
dictionary.
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This Internet-Draft will expire on 1 March 2025.
https://www.rfc-editor.org/info/rfc9842.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Version Upgrade . . . . . . . . . . . . . . . . . . . 3
1.1.2. Common Content . . . . . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Dictionary Negotiation . . . . . . . . . . . . . . . . . . . 6
2.1. Use-As-Dictionary . . . . . . . . . . . . . . . . . . . . 6
2.1.1. match . . . . . . . . . . . . . . . . . . . . . . . . 6 "match"
2.1.2. match-dest . . . . . . . . . . . . . . . . . . . . . 7 "match-dest"
2.1.3. id . . . . . . . . . . . . . . . . . . . . . . . . . 7 "id"
2.1.4. type . . . . . . . . . . . . . . . . . . . . . . . . 8 "type"
2.1.5. Examples . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Available-Dictionary . . . . . . . . . . . . . . . . . . 8
2.2.1. Dictionary freshness requirement . . . . . . . . . . 9 Freshness Requirement
2.2.2. Dictionary URL matching . . . . . . . . . . . . . . . 9 Matching
2.2.3. Multiple matching dictionaries . . . . . . . . . . . 10 Matching Dictionaries
2.3. Dictionary-ID . . . . . . . . . . . . . . . . . . . . . . 10
3. The 'compression-dictionary' "compression-dictionary" Link Relation Type . . . . . . . 11
4. Dictionary-Compressed Brotli . . . . . . . . . . . . . . . . 11
5. Dictionary-Compressed Zstandard . . . . . . . . . . . . . . . 12
6. Negotiating the content encoding . . . . . . . . . . . . . . 13 Content Encoding
6.1. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 13
6.2. Content-Encoding . . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.1. Content Encoding . . . . . . . . . . . . . . . . . . . . 14 Registration
7.2. Header Field Registration . . . . . . . . . . . . . . . . 14
7.3. Link Relation Registration . . . . . . . . . . . . . . . 15
8. Compatibility Considerations . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9.1. Changing content . . . . . . . . . . . . . . . . . . . . 15 Content
9.2. Reading content . . . . . . . . . . . . . . . . . . . . . 16 Content
9.3. Security Mitigations . . . . . . . . . . . . . . . . . . 16
9.3.1. Cross-origin protection . . . . . . . . . . . . . . . 16 Cross-Origin Protection
9.3.2. Response readability . . . . . . . . . . . . . . . . 16 Readability
9.3.3. Server Responsibility . . . . . . . . . . . . . . . . 17
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
This specification defines a mechanism for using designated HTTP
[HTTP] responses as an external dictionary for future HTTP responses
for compression schemes that support using external dictionaries
(e.g., Brotli [RFC7932] and Zstandard [RFC8878]).
This document describes the HTTP headers used for negotiating
dictionary usage and registers content encoding content-encoding values for
compressing with Brotli and Zstandard using a negotiated dictionary.
The negotiation of dictionary usage leverages HTTP's content
negotiation (see Section 12 of [HTTP]) and is usable with all
versions of HTTP.
1.1. Use Cases
Any HTTP response can be specified to be used for use as a compression
dictionary for future HTTP requests requests, which provides a lot of
flexibility. There are two Two common use cases that are seen
frequently: frequently are
described below.
1.1.1. Version Upgrade
Using a previous version of a resource as a dictionary for a newer
version enables delivery of a delta-compressed version of the
changes, usually resulting in significantly smaller responses than
what can be achieved by compression alone.
For example:
Client Server
| |
| GET /app.v1.js |
|------------------------------------------------->|
| |
| 200 OK |
| Use-As-Dictionary: match="/app*js" |
| <full app.v1.js resource - 100KB compressed> |
|<-------------------------------------------------|
| |
Some time later ...
Client Server
| |
| GET /app.v2.js |
| Available-Dictionary: :pZGm1A...2a2fFG4=: |
| Accept-Encoding: gzip, br, zstd, dcb, dcz |
|------------------------------------------------->|
| |
| 200 OK |
| Content-Encoding: dcb |
| <delta-compressed app.v2.js resource - 1KB> |
|<-------------------------------------------------|
| |
Figure 1: Version Upgrade Example
1.1.2. Common Content
If several resources share common patterns in their responses responses, then
it can be useful to reference an external dictionary that contains
those common patterns, effectively compressing them out of the
responses. Some examples of this are common template HTML for
similar pages across a site and common keys and values in API calls.
For example:
Client Server
| |
| GET /index.html |
|--------------------------------------------------->|
| |
| 200 OK |
| Link: <.../dict>; rel="compression-dictionary" |
| <full index.html resource - 100KB compressed> |
|<---------------------------------------------------|
| |
| GET /dict |
|--------------------------------------------------->|
| |
| 200 OK |
| Use-As-Dictionary: match="/*html" |
|<---------------------------------------------------|
| |
Some time later ...
Client Server
| |
| GET /page2.html |
| Available-Dictionary: :pZGm1A...2a2fFG4=: |
| Accept-Encoding: gzip, br, zstd, dcb, dcz |
|--------------------------------------------------->|
| |
| 200 OK |
| Content-Encoding: dcb |
| <delta-compressed page2.html resource - 10KB> |
|<---------------------------------------------------|
| |
Figure 2: Common Content Example
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.
This document uses the following terminology from Section 3 of
[STRUCTURED-FIELDS] to specify syntax and parsing: Dictionary,
String, Inner List, Token, and Byte Sequence.
This document uses the line folding strategies described in
[FOLDING].
This document also uses terminology from [HTTP] and [HTTP-CACHING].
2. Dictionary Negotiation
2.1. Use-As-Dictionary
When responding to a an 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 "Use-As-Dictionary" response
header.
The Use-As-Dictionary "Use-As-Dictionary" response header is a Structured Field
[STRUCTURED-FIELDS] Dictionary with values for "match", "match-dest",
"id", and "type".
2.1.1. match "match"
The "match" value of the Use-As-Dictionary "Use-As-Dictionary" response header is a
String value that provides the URL Pattern to use for request
matching (see [URLPATTERN]).
The URL Pattern used for matching does not support using regular
expressions.
The following algorithm is used to validate that a given String value
is a valid URL Pattern that does not use regular expressions and is
for the same Origin (Section 4.3.1 of [HTTP]) as the dictionary. It
will return TRUE for a valid match pattern and FALSE for an invalid
pattern that MUST NOT be used: 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 URL pattern created by running the steps to
create a URL pattern by setting input=MATCH, input=MATCH and baseURL=URL (see
Part create of [URLPATTERN]).
4. If the result of running the "has regexp groups" steps for
PATTERN returns TRUE TRUE, then return FALSE (see Part has regexp
groups of [URLPATTERN]).
5. Return TRUE.
The "match" value is required and MUST be included in the Use-As-
Dictionary "Use-As-
Dictionary" response header for the dictionary to be considered
valid.
Operating at the HTTP-level, HTTP level, the specified match patterns will
operate on the percent-encoded version of the URL path (see Section 2
of [URL]).
For example example, the URL "http://www.example.com/düsseldorf" would be
encoded as "http://www.example.com/d%C3%BCsseldorf" and a relevant
match pattern would be:
Use-As-Dictionary: match="/d%C3%BCsseldorf"
2.1.2. match-dest "match-dest"
The "match-dest" value of the Use-As-Dictionary header is an Inner
List of String values that provides a list of Fetch request
destinations for the dictionary to match (see Part RequestDestination
of [FETCH]).
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.
2.1.3. id "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 zero, then it MUST be
sent to the server in a "Dictionary-ID" request header when the
client sends an "Available-Dictionary" request header for the same
dictionary (see Section 2.2).
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.
2.1.4. type "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 defined as a dictionary format which that 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".
2.1.5. Examples
2.1.5.1. Path Prefix
A response that contained a response header:
NOTE: '\' line wrapping per RFC 8792
Use-As-Dictionary: \
match="/product/*", match-dest=("document")
Would specify matching any document request for a URL with a path
prefix of /product/ on the same Origin (Section 4.3.1 of [HTTP]) as
the original request.
2.1.5.2. Versioned Directories
A response that contained a response header:
Use-As-Dictionary: match="/app/*/main.js"
Would match any path that starts with "/app/" and ends with
"/main.js".
2.2. Available-Dictionary
When a an HTTP client makes a request for a resource for which it has
an appropriate dictionary, it can add a an "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
[STRUCTURED-FIELDS] Byte Sequence containing the SHA-256 [SHA-256]
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:
Available-Dictionary: :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:
2.2.1. Dictionary freshness requirement Freshness Requirement
To be considered as a match, the dictionary resource MUST be either
fresh [HTTP-CACHING] or allowed to be served stale (see eg [RFC5861]).
2.2.2. Dictionary URL matching Matching
When a dictionary is stored as a result of a "Use-As-Dictionary"
directive, it includes a "match" string and an optional "match-dest"
string that are used to match an outgoing request from a client to
the available dictionaries.
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 and a "match-
dest" string was provided with the dictionary:
* 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 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 (see Section 4.3.1 of [HTTP]).
5. Let MATCH be the value of "match" for the given dictionary.
6. Let PATTERN be a URL pattern created by running the steps to
create a URL pattern by setting input=MATCH, input=MATCH and baseURL=URL (see
Part create of [URLPATTERN]).
7. Return the result of running the "match" steps on PATTERN with
input=URL
input=URL, which will check for a match between the request URL
and the supplied "match" string (see Part match of [URLPATTERN]).
2.2.3. Multiple matching dictionaries 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.
2. Given equivalent destination precedence, the dictionary with the
longest "match" takes precedence.
3. Given equivalent destination and match length precedence, the
most recently fetched dictionary takes precedence.
2.3. Dictionary-ID
When a an HTTP client makes a request for a resource for which it has
an appropriate dictionary and the dictionary was stored with a server-
provided
server-provided "id" in the Use-As-Dictionary response then response, the client
MUST echo the stored "id" in a "Dictionary-ID" request header.
The "Dictionary-ID" request header is a Structured Field
[STRUCTURED-FIELDS] String of up to 1024 characters (after any
decoding) and MUST be identical to the server-provided "id".
For example, given a an HTTP response that set a dictionary ID:
Use-As-Dictionary: match="/app/*/main.js", id="dictionary-12345"
A future request that matches the given dictionary will include both
the hash and the ID:
Available-Dictionary: :pZGm1Av0IEBKARczz7exkNYsZb8LzaMrV7J32a2fFG4=:
Dictionary-ID: "dictionary-12345"
3. The 'compression-dictionary' "compression-dictionary" Link Relation Type
This specification defines the 'compression-dictionary' "compression-dictionary" link relation
type [WEB-LINKING] that provides a mechanism for a an HTTP response to
provide a URL for a compression dictionary that is related to, to but not
directly used by the current HTTP response.
The 'compression-dictionary' "compression-dictionary" link relation type indicates that
fetching and caching the specified resource is likely to be
beneficial for future requests. The response to some of those future
requests are likely to be able have the ability to use the indicated resource as a
compression dictionary.
Clients can fetch the provided resource at a time that they determine
would be appropriate.
The response to the fetch for the compression dictionary needs to
include a "Use-As-Dictionary" and caching response headers for it to
be usable as a compression dictionary. The link relation only
provides the mechanism for triggering the fetch of the dictionary.
The following example shows a link to a resource at
https://example.org/dict.dat that is expected to produce a
compression dictionary:
Link: <https://example.org/dict.dat>; rel="compression-dictionary"
4. Dictionary-Compressed Brotli
The "dcb" content encoding identifies a resource that is a
"Dictionary-Compressed Brotli" stream.
A "Dictionary-Compressed Brotli" stream has a fixed header that is
followed by a Shared Brotli [SHARED-BROTLI] stream. The header
consists of a fixed 4-byte sequence and a 32-byte hash of the
external dictionary that was used. The Shared Brotli stream is
created using the referenced external dictionary and a compression
window that is at most 16 MB in size.
The dictionary used for the "dcb" content encoding is a "raw"
dictionary type as defined in Section 2.1.4 and is treated as a
prefix dictionary as defined in section Section 9.2 of the Shared Brotli
Compressed Data Format draft. [SHARED-BROTLI] [SHARED-BROTLI].
The 36-byte fixed header is as follows:
Magic_Number: 4 fixed bytes: bytes -- 0xff, 0x44, 0x43, 0x42.
SHA_256_Hash: 32 bytes. SHA-256 hash digest of the dictionary
[SHA-256].
Clients that announce support for dcb content encoding MUST be able
to decompress resources that were compressed with a window size of up
to 16 MB.
With Brotli compression, the full dictionary is available during
compression and decompression independent of the compression window,
allowing for delta-compression of resources larger than the
compression window.
5. Dictionary-Compressed Zstandard
The "dcz" content encoding identifies a resource that is a
"Dictionary-Compressed Zstandard" stream.
A "Dictionary-Compressed Zstandard" stream is a binary stream that
starts with a 40-byte fixed header and is followed by a Zstandard
[RFC8878] stream of the response that has been compressed with an
external dictionary.
The dictionary used for the "dcz" content encoding is a "raw"
dictionary type as defined in Section 2.1.4 and is treated as a raw
dictionary as per section Section 5 of RFC 8878. [RFC8878].
The 40-byte header consists of a fixed 8-byte sequence followed by
the 32-byte SHA-256 hash of the external dictionary that was used to
compress the resource:
Magic_Number: 8 fixed bytes: bytes -- 0x5e, 0x2a, 0x4d, 0x18, 0x20, 0x00,
0x00, 0x00.
SHA_256_Hash: 32 bytes. SHA-256 hash digest of the dictionary
[SHA-256].
The 40-byte header is a Zstandard skippable frame (little-endian
0x184D2A5E) with a 32-byte length (little-endian 0x00000020) that is
compatible with existing Zstandard decoders.
Clients that announce support for dcz content encoding MUST be able
to decompress resources that were compressed with a window size of at
least 8 MB or 1.25 times the size of the dictionary, which ever whichever is
greater, up to a maximum of 128 MB.
The window size used will be encoded in the content (currently, this
can be expressed in powers of two only) and it MUST be lower than
this limit. An implementation MAY treat a window size that exceeds
the limit as a decoding error.
With Zstandard compression, the full dictionary is available during
compression and decompression until the size of the input exceeds the
compression window. Beyond that point point, the dictionary becomes
unavailable. Using a compression window that is 1.25 times the size
of the dictionary allows for full delta compression of resources that
have grown by 25% between releases while still giving the client
control over the memory it will need to allocate for a given
response.
6. Negotiating the content encoding Content Encoding
When a compression dictionary is available for use compressing the
response to a given request, the encoding to be used is negotiated
through the regular mechanism for negotiating content encoding in
HTTP through the "Accept-Encoding" request header and "Content-
Encoding" response header.
The dictionary to use is negotiated separately and advertised in the
"Available-Dictionary" request header.
6.1. Accept-Encoding
When a dictionary is available for use on a given request, request and the
client chooses to make dictionary-based content-encoding content encoding available,
the client adds the dictionary-aware content encodings that it
supports to the "Accept-Encoding" request header. e.g.: For example:
Accept-Encoding: gzip, deflate, br, zstd, dcb, dcz
When a client does not have a stored dictionary that matches the
request,
request or chooses not to use one for the request, the client MUST
NOT send its dictionary-aware content-encodings content encodings in the "Accept-
Encoding" request header.
6.2. Content-Encoding
If a server supports one of the dictionary encodings advertised by
the client and chooses to compress the content of the response using
the dictionary that the client has advertised advertised, then it sets the
"Content-Encoding" response header to the appropriate value for the
algorithm selected. e.g.: For example:
Content-Encoding: dcb
If the response is cacheable, it MUST include a "Vary" header to
prevent caches from serving dictionary-compressed resources to
clients that don't support them or serving the response compressed
with the wrong dictionary: dictionary. For example:
Vary: accept-encoding, available-dictionary
7. IANA Considerations
7.1. Content Encoding Registration
IANA is asked to enter has added the following into entries to the "HTTP Content Coding
Registry" registry maintained at <https://www.iana.org/assignments/
http-parameters/http-parameters.xhtml>:
* <https://www.iana.org/assignments/http-
parameters/>:
Name: dcb
*
Description: "Dictionary-Compressed Brotli" data format.
*
Reference: This document
* Notes: RFC 9842, Section 4
IANA is asked to enter the following into the "HTTP Content Coding
Registry" registry maintained at <https://www.iana.org/assignments/
http-parameters/http-parameters.xhtml>:
*
Name: dcz
*
Description: "Dictionary-Compressed Zstandard" data format.
*
Reference: This document
* Notes: RFC 9842, Section 5
7.2. Header Field Registration
IANA is asked has added the following entries to update the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" registry maintained at
<https://www.iana.org/assignments/http-fields/http-fields.xhtml>
according to the table below:
+======================+===========+==============================+
<https://www.iana.org/assignments/http-fields/>:
+======================+===========+=======================+
| Field Name | Status | Reference |
+======================+===========+==============================+
+======================+===========+=======================+
| Use-As-Dictionary | permanent | RFC 9842, Section 2.1 of this document |
+----------------------+-----------+------------------------------+
+----------------------+-----------+-----------------------+
| Available-Dictionary | permanent | RFC 9842, Section 2.2 of this document |
+----------------------+-----------+------------------------------+
+----------------------+-----------+-----------------------+
| Dictionary-ID | permanent | RFC 9842, Section 2.3 of this document |
+----------------------+-----------+------------------------------+
+----------------------+-----------+-----------------------+
Table 1
7.3. Link Relation Registration
IANA is asked has added the following entry to update the "Link Relation Types"
registry maintained at <https://www.iana.org/assignments/link-relations/link-
relations.xhtml>:
* <https://www.iana.org/assignments/link-
relations/>:
Relation Name: compression-dictionary
*
Description: Refers to a compression dictionary used for content
encoding.
*
Reference: This document, RFC 9842, Section 3
8. Compatibility Considerations
It is not unusual for there devices to be devices on the network path that
intercept, inspect inspect, and process HTTP requests (web proxies,
firewalls, intrusion detection systems, etc). etc.). To minimize the risk
of these devices incorrectly processing dictionary-compressed
responses, compression dictionary transport MUST only be used in
secure contexts (HTTPS).
9. Security Considerations
The security considerations for Brotli [RFC7932], Shared Brotli
[SHARED-BROTLI]
[SHARED-BROTLI], and Zstandard [RFC8878] apply to the dictionary-based dictionary-
based versions of the respective algorithms.
9.1. Changing content Content
The dictionary must be treated with the same security precautions as
the content, content because a change to the dictionary can result in a change
to the decompressed content.
The dictionary is validated using a an 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.
9.2. Reading content Content
The compression attacks in Section 2.6 of [RFC7457] show that it's a
bad idea to compress data from mixed (e.g. (e.g., public and private)
sources -- the
sources. The data sources include not only the compressed data but
also the dictionaries. For example, if you compress secret cookies are compressed
using a public-data-only dictionary, you still leak information about the cookies.
Not only can the cookies is
still leaked.
The dictionary can reveal information about the compressed
data, but data and
vice versa, versa. That is, data compressed with the dictionary can reveal
the
contents of the dictionary when an adversary can control parts of the
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.
9.3. Security Mitigations
If any of the mitigations do not pass, the client MUST drop the
response and return an error.
9.3.1. Cross-origin protection Cross-Origin Protection
To make sure that a dictionary can only impact content from the same
origin where the dictionary was served, the URL Pattern used for
matching a dictionary to requests (Section 2.1.1) is guaranteed to be
for the same origin that the dictionary is served from.
9.3.2. Response readability 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 Cross-Origin Resource Sharing (CORS) check (see
Part CORS check of [FETCH]).
9.3.3. Server Responsibility
As with any usage of compressed content in a secure context, a
potential timing attack exists if the attacker can control any part
of the dictionary, dictionary or if it can read the dictionary and control any
part of the content being compressed, compressed while performing multiple
requests that vary the dictionary or injected content. Under such an
attack, the changing size or processing time of the response reveals
information about the content, which might be sufficient to read the
supposedly secure response.
In general, a server can mitigate such attacks by preventing
variations per request, as in preventing active use of multiple
dictionaries for the same content, disabling compression when any
portion of the content comes from uncontrolled sources, and securing
access and control over the dictionary content in the same way as the
response content. In addition, the following requirements on a
server are intended to disable dictionary-aware compression when the
client provides CORS request header fields that indicate a cross-
origin request context.
The following algorithm will return FALSE for cross-origin requests
where precautions such as not using dictionary-based compression
should be considered:
1. If there is no "Sec-Fetch-Site" request header then header, return TRUE.
2. if If the value of the "Sec-Fetch-Site" request header is "same-
origin" then
origin", return TRUE.
3. If there is no "Sec-Fetch-Mode" request header then header, return TRUE.
4. If the value of the "Sec-Fetch-Mode" request header is "navigate"
or "same-origin" then "same-origin", return TRUE.
5. If the value of the "Sec-Fetch-Mode" request header is "cors":
* If the response does not include an "Access-Control-Allow-
Origin" response header then header, return FALSE.
* If the request does not include an "Origin" request header
then header,
return FALSE.
* If the value of the "Access-Control-Allow-Origin" response
header is "*" then "*", return TRUE.
* If the value of the "Access-Control-Allow-Origin" response
header matches the value of the "Origin" request header then header,
return TRUE.
6. return Return FALSE.
10. 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 [RFC6265]. This
includes partitioning the storage as cookies are partitioned as well
as clearing the dictionaries whenever cookies are cleared.
11. References
11.1. Normative References
[FETCH] WHATWG, "Fetch - Living Standard", WHATWG Living Standard,
<https://fetch.spec.whatwg.org/>. Commit snapshot:
<https://fetch.spec.whatwg.org/commit-
snapshots/5a9680638ebfc2b3b7f4efb2bef0b579a2663951/>
[FOLDING] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
"Handling Long Lines in Content of Internet-Drafts and
RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
<https://www.rfc-editor.org/rfc/rfc8792>.
<https://www.rfc-editor.org/info/rfc8792>.
[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>.
<https://www.rfc-editor.org/info/rfc9110>.
[HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/rfc/rfc9111>.
<https://www.rfc-editor.org/info/rfc9111>.
[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>.
<https://www.rfc-editor.org/info/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>. <https://www.rfc-editor.org/info/rfc8174>.
[RFC8878] Collet, Y. and M. Kucherawy, Ed., "Zstandard Compression
and the 'application/zstd' Media Type", RFC 8878,
DOI 10.17487/RFC8878, February 2021,
<https://www.rfc-editor.org/rfc/rfc8878>.
<https://www.rfc-editor.org/info/rfc8878>.
[SHA-256] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/rfc/rfc6234>.
<https://www.rfc-editor.org/info/rfc6234>.
[SHARED-BROTLI]
Alakuijala, J., Duong, T., Kliuchnikov, E., Szabadka, Z.,
and L. Vandevenne, "Shared Brotli Compressed Data Format", September 2022,
<https://datatracker.ietf.org/doc/draft-vandevenne-shared-
brotli-format/>.
RFC 9841, DOI 10.17487/RFC9841, August 2025,
<https://www.rfc-editor.org/info/rfc9841>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", May RFC 9651, DOI 10.17487/RFC9651, September 2024,
<https://datatracker.ietf.org/doc/draft-ietf-httpbis-
sfbis/>.
<https://www.rfc-editor.org/info/rfc9651>.
[URL] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
<https://www.rfc-editor.org/info/rfc3986>.
[URLPATTERN]
WHATWG, "URL Pattern - Living Standard", WHATWG Living Standard,
<https://urlpattern.spec.whatwg.org/>. Commit snapshot:
<https://urlpattern.spec.whatwg.org/commit-
snapshots/696b4029d52e5854044bac6b72cdb198cb962ed0/>
[WEB-LINKING]
Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/rfc8288>.
<https://www.rfc-editor.org/info/rfc8288>.
11.2. Informative References
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, DOI 10.17487/RFC5861, May 2010,
<https://www.rfc-editor.org/rfc/rfc5861>.
<https://www.rfc-editor.org/info/rfc5861>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/rfc/rfc6265>.
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/rfc/rfc7457>. <https://www.rfc-editor.org/info/rfc7457>.
[RFC7932] Alakuijala, J. and Z. Szabadka, "Brotli Compressed Data
Format", RFC 7932, DOI 10.17487/RFC7932, July 2016,
<https://www.rfc-editor.org/rfc/rfc7932>.
<https://www.rfc-editor.org/info/rfc7932>.
Authors' Addresses
Patrick Meenan (editor)
Google LLC
Email: pmeenan@google.com
Yoav Weiss (editor)
Shopify Inc
Email: yoav.weiss@shopify.com