Internet DRAFT - draft-meenan-httpbis-compression-dictionary
draft-meenan-httpbis-compression-dictionary
HTTP P. Meenan
Internet-Draft Y. Weiss
Intended status: Informational Google LLC
Expires: 10 February 2024 9 August 2023
Compression Dictionary Transport
draft-meenan-httpbis-compression-dictionary-05
Abstract
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 [RFC7932] and Zstandard [RFC8878]).
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://pmeenan.github.io/i-d-compression-dictionary/draft-meenan-
httpbis-compression-dictionary.html. Status information for this
document may be found at https://datatracker.ietf.org/doc/draft-
meenan-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/.
Source for this draft and an issue tracker can be found at
https://github.com/pmeenan/i-d-compression-dictionary.
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
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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."
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Dictionary Negotiation . . . . . . . . . . . . . . . . . . . 3
2.1. Use-As-Dictionary . . . . . . . . . . . . . . . . . . . . 3
2.1.1. match . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.2. ttl . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.3. type . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.4. hashes . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.5. Examples . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Sec-Available-Dictionary . . . . . . . . . . . . . . . . 5
2.2.1. Dictionary freshness requirement . . . . . . . . . . 6
2.2.2. Dictionary URL matching . . . . . . . . . . . . . . . 6
2.2.3. Multiple matching dictionaries . . . . . . . . . . . 7
3. Negotiating the compression algorithm . . . . . . . . . . . . 7
3.1. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 8
3.2. Content-Encoding . . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
4.1. Content Encoding . . . . . . . . . . . . . . . . . . . . 8
4.2. Header Field Registration . . . . . . . . . . . . . . . . 9
5. Compatibility Considerations . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Changing content . . . . . . . . . . . . . . . . . . . . 9
6.2. Reading content . . . . . . . . . . . . . . . . . . . . . 10
6.3. Security Mitigations . . . . . . . . . . . . . . . . . . 10
6.3.1. Cross-origin protection . . . . . . . . . . . . . . . 10
6.3.2. Response readability . . . . . . . . . . . . . . . . 10
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
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 [RFC7932] and Zstandard [RFC8878]).
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.
This document uses the line folding strategies described in
[FOLDING].
2. Dictionary Negotiation
2.1. 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 pattern specified in the Use-As-Dictionary response
header.
The Use-As-Dictionary response header is a Structured Field [RFC8941]
sf-dictionary with values for "match", "ttl", "type" and "hashes".
2.1.1. match
The "match" value of the Use-As-Dictionary header is a sf-string
value that provides an URL-matching pattern for requests where the
dictionary can be used.
The sf-string is parsed as a URL [RFC3986], and supports absolute
URLs as well as relative URLs. When stored, any relative URLs MUST
be expanded so that only absolute URL patterns are used for matching
against requests.
The match URL supports using * as a wildcard within the match string
for pattern-matching multiple URLs. URLs with a natural * in them
are not directly supported unless they can rely on the behavior of *
matching an arbitrary string.
The [Origin] of the URL in the "match" pattern MUST be the same as
the origin of the request that specifies the "Use-As-Dictionary"
response and MUST not include a * wildcard.
The "match" value is required and MUST be included in the Use-As-
Dictionary sf-dictionary for the dictionary to be considered valid.
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2.1.2. ttl
The "ttl" value of the Use-As-Dictionary header is a sf-integer value
that provides the time in seconds that the dictionary is valid for
(time to live).
The "ttl" is independent of the cache lifetime of the resource being
used for the dictionary. If the underlying resource is evicted from
cache then it is also removed but this allows for setting an explicit
time to live for use as a dictionary independent of the underlying
resource in cache. Expired resources can still be useful as
dictionaries while they are in cache and can be used for fetching
updates of the expired resource. It can also be useful to
artificially limit the life of a dictionary in cases where the
dictionary is updated frequently which can help limit the number of
possible incoming dictionary variations.
The "ttl" value is optional and defaults to 31536000 (1 year).
2.1.3. type
The "type" value of the Use-As-Dictionary header is a sf-string 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".
2.1.4. hashes
The "hashes" value of the Use-As-Dictionary header is a inner-list
value that provides a list of supported hash algorithms in order of
server preference.
The dictionaries are identified by the hash of their contents and
this value allows for negotiation of the algorithm to use.
The "hashes" value is optional and defaults to (sha-256).
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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/*", ttl=604800, hashes=(sha-256 sha-512)
Would specify matching any URL with a path prefix of /product/ on the
same [Origin] as the original request, expiring as a dictionary in 7
days independent of the cache lifetime of the resource, and advertise
support for both sha-256 and sha-512 hash algorithms.
2.1.5.2. Versioned Directories
A response that contained a response header:
Use-As-Dictionary: match="/app/*/main.js"
Would match main.js in any directory under /app/, expiring as a
dictionary in one year and support using the sha-256 hash algorithm.
2.2. Sec-Available-Dictionary
When a HTTP client makes a request for a resource for which it has an
appropriate dictionary, it can add a "Sec-Available-Dictionary"
request header to the request to indicate to the server that it has a
dictionary available to use for compression.
The "Sec-Available-Dictionary" request header is a lowercase
Base16-encoded [RFC4648] hash of the contents of a single available
dictionary calculated using one of the algorithms advertised as being
supported by the server.
Its syntax is defined by the following [ABNF]:
Sec-Available-Dictionary = hvalue
hvalue = 1*hchar
hchar = DIGIT / "a" / "b" / "c" / "d" / "e" / "f"
The client MUST only send a single "Sec-Available-Dictionary" request
header with a single hash value for the best available match that it
has available.
For example:
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NOTE: '\' line wrapping per RFC 8792
Sec-Available-Dictionary: \
a591a6d40bf420404a011733cfb7b190d62c65bf0bcda32b57b277d9ad9f146e
2.2.1. Dictionary freshness requirement
To be considered as a match, the dictionary must not yet be expired
as a dictionary. When iterating through dictionaries looking for a
match, the expiration time of the dictionary is calculated by taking
the last time the dictionary was written and adding the "ttl" seconds
from the "Use-As-Dictionary" response. If the current time is beyond
the expiration time of the dictionary, it MUST be ignored.
2.2.2. Dictionary URL matching
When a dictionary is stored as a result of a "Use-As-Dictionary"
directive, it includes a "match" string with the URL pattern of
request URLs that the dictionary can be used for.
When comparing request URLs to the available dictionary match
patterns, the comparison should account for the * wildcard when
matching against request URLs. This can be accomplished with the
following algorithm which returns TRUE for a successful match and
FALSE for no-match:
1. Let MATCH represent the absolute URL pattern from the "match"
value for the given dictionary.
2. LET URL represent the request URL being checked.
3. If there are no * characters in MATCH:
* If the MATCH and URL strings are identical, return TRUE.
* Else, return FALSE.
4. If there is a single * character in MATCH and it is at the end of
the string:
* If the MATCH string is identical to the start of the URL
string, return TRUE.
* Else, return FALSE.
5. Split the MATCH string by the * character into an array of
MATCHES (excluding the * deliminator from the individual
entries).
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6. If there is not a * character at the end of MATCH:
* Pop the last entry in MATCHES from the end of the array into
PATTERN.
* If PATTERN is identical to the end of the URL string, remove
the end of the URL string to the beginning of the match to
PATTERN.
* Else, return FALSE.
7. Pop the first entry in MATCHES from the front of the array into
PATTERN.
* If PATTERN is not identical to the start of the URL string,
return FALSE.
8. Pop each entry off of the front of the MATCHES array into
PATTERN. For each PATTERN, in order:
* Search for the first match of PATTERN in URL, starting from
the position of the end of the previous match.
* If no match is found, return FALSE.
9. Return TRUE.
2.2.3. Multiple matching dictionaries
When there are multiple dictionaries that match a given request URL,
the client MUST pick the dictionary with the longest match pattern
string length.
3. 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:
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+==================+=============================+
| Content-Encoding | Description |
+==================+=============================+
| br-d | Brotli using an external |
| | compression dictionary |
+------------------+-----------------------------+
| zstd-d | Zstandard using an external |
| | compression dictionary |
+------------------+-----------------------------+
Table 1
The dictionary to use is negotiated separately and advertised in the
"Sec-Available-Dictionary" request header.
3.1. Accept-Encoding
The client adds the algorithms that it supports to the "Accept-
Encoding" request header. e.g.:
Accept-Encoding: gzip, deflate, br, zstd, br-d, zstd-d
3.2. 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.:
Content-Encoding: br-d
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:
Vary: accept-encoding, sec-available-dictionary
4. IANA Considerations
4.1. Content Encoding
IANA is asked to update the "HTTP Content Coding Registry" registry
([RFC9110]) according to the table below:
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+========+=============================================+===========+
| Name | Description | Reference |
+========+=============================================+===========+
| br-d | A stream of bytes compressed using the | [RFC7932] |
| | Brotli protocol with an external dictionary | |
+--------+---------------------------------------------+-----------+
| zstd-d | A stream of bytes compressed using the | [RFC8878] |
| | Zstandard protocol with an external | |
| | dictionary | |
+--------+---------------------------------------------+-----------+
Table 2
4.2. Header Field Registration
IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field
Name Registry" registry ([RFC9110]) according to the table below:
+==========================+===========+================+
| Field Name | Status | Reference |
+==========================+===========+================+
| Use-As-Dictionary | permanent | Section 2.1 of |
| | | this document |
+--------------------------+-----------+----------------+
| Sec-Available-Dictionary | permanent | Section 2.2 of |
| | | this document |
+--------------------------+-----------+----------------+
Table 3
5. 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).
6. Security Considerations
The security considerations for Brotli [RFC7932] and Zstandard
[RFC8878] apply to the dictionary-based versions of the respective
algorithms.
6.1. 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.
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6.2. 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.
6.3. Security Mitigations
If any of the mitigations do not pass, the client MUST drop the
response and return an error.
6.3.1. Cross-origin protection
To make sure that a dictionary can only impact content from the same
origin where the dictionary was served, the "match" pattern used for
matching a dictionary to requests MUST be for the same origin that
the dictionary is served from.
6.3.2. 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).
6.3.2.1. Same-Origin
On the client-side, same-origin determination is defined in the fetch
spec (https://html.spec.whatwg.org/multipage/browsers.html#origin).
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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.
6.3.2.2. Cross-Origin
For requests that are not same-origin (Section 6.3.2.1), 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:
o Response MAY be used as a dictionary.
o Response MAY be compressed by an available dictionary.
- Else:
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o Response MUST NOT be used as a dictionary.
o Response MUST NOT be compressed by an available
dictionary.
* For servers:
- If the response does not include an "Access-Control-Allow-
Origin" response header:
o Response MUST NOT be used as a dictionary.
o Response MUST NOT be compressed by an available
dictionary.
- If the request does not include an "Origin" request header:
o Response MUST NOT be used as a dictionary.
o Response MUST NOT be compressed by an available
dictionary.
- If the value of the "Access-Control-Allow-Origin" response
header is "*":
o Response MAY be used as a dictionary.
o 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:
o Response MAY be used as a dictionary.
o 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.
7. 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.
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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.
8. References
8.1. Normative References
[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>.
[RFC9110] 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>.
8.2. Informative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[HTTP] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/rfc/rfc7230>.
[Origin] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/rfc/rfc6454>.
[RFC3986] 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>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/rfc/rfc4648>.
[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>.
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[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>.
[RFC8941] Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
Authors' Addresses
Patrick Meenan
Google LLC
Email: pmeenan@google.com
Yoav Weiss
Google LLC
Email: yoavweiss@google.com
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