Network Working Group K. Oku
Internet-Draft DeNA Co, Ltd.
Intended status: Informational M. Nottingham
Expires: July 10, 2016 January 7, 2016
Cache Digests for HTTP/2
draft-kazuho-h2-cache-digest-00
Abstract
This specification defines a HTTP/2 frame type to allow clients to
inform the server of their cache's contents. Servers can then use
this to inform their choices of what to push to clients.
Note to Readers
The issues list for this draft can be found at
https://github.com/mnot/I-D/labels/h2-cache-digest .
The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/h2-cache-digest/ .
Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
pages/h2-cache-digest .
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 10, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The CACHE_DIGEST Frame . . . . . . . . . . . . . . . . . . . 3
2.1. Computing the Digest-Value . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Normative References . . . . . . . . . . . . . . . . . . 5
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
HTTP/2 [RFC7540] allows a server to "push" synthetic request/response
pairs into a client's cache optimistically. While there is strong
interest in using this facility to improve perceived Web browsing
performance, it is sometimes counterproductive because the client
might already have cached the "pushed" response.
When this is the case, the bandwidth used to "push" the response is
effectively wasted, and represents opportunity cost, because it could
be used by other, more relevant responses. HTTP/2 allows a stream to
be cancelled by a client using a RST_STREAM frame in this situation,
but there is still at least one round trip of potentially wasted
capacity even then.
This specification defines a HTTP/2 frame type to allow clients to
inform the server of their cache's contents using a Golumb-Rice Coded
Set. Servers can then use this to inform their choices of what to
push to clients.
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1.1. Notational Conventions
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 [RFC2119].
2. The CACHE_DIGEST Frame
The CACHE_DIGEST frame type is 0xf1. NOTE: This is an experimental
value; if standardised, a permanent value will be assigned.
A CACHE_DIGEST frame can be sent from a client to a server on any
stream in the "open" state, and conveys a digest of the contents of
the cache associated with that stream, as explained in Section 2.1.
In typical use, a client will send CACHE_DIGEST immediately after the
first request on a connection for a given origin, on the same stream,
because there is usually a short period of inactivity then, and
servers can benefit most when they understand the state of the cache
before they begin pushing associated assets (e.g., CSS, JavaScript
and images).
Clients MAY send CACHE_DIGEST at other times, but servers ought not
expect frequent updates; instead, if they wish to continue to utilise
the digest, they will need update it with responses sent to that
client on the connection.
Servers MUST NOT use any but the most recent CACHE_DIGEST for a given
origin as current, and MUST treat an empty Digest-Value as
effectively clearing all stored digests for that origin.
CACHE_DIGEST has no defined meaning when sent from servers to
clients, and MAY be ignored.
+---------------------------------------------------------------+
| Digest-Value? (*) ...
+---------------------------------------------------------------+
The CACHE_DIGEST frame payload has the following fields:
o Digest-Value: An optional sequence of octets containing the digest
as computed in Section 2.1.
2.1. Computing the Digest-Value
The set of URLs that is used to compute Digest-Value MUST only
include URLs that share origins [RFC6454] with the stream that
CACHE_DIGEST is sent on, and they MUST be fresh [RFC7234].
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A client MAY choose a subset of the available stored responses to
include in the set. Additionally, it MUST choose a parameter, "P",
that indicates the probability of a false positive it is willing to
tolerate, expressed as "1/P".
"P" MUST be a power of 2.
To compute a digest-value for the set "URLs" and "P":
1. Let N be the count of "URLs"' members, rounded up to power of 2.
2. Let "hash-values" be an empty array of integers.
3. Append 0 to "hash-values".
4. For each "URL" in URLs, follow these steps:
1. Convert "URL" to an ASCII string by percent-encoding as
appropriate [RFC3986].
2. Let "key" be the SHA-256 message digest [RFC6234] of URL,
expressed as an integer.
3. Append "key" modulo ( "N" * "P" ) to "hash-values".
5. Sort "hash-values" in ascending order.
6. Let "digest" be an empty array of bits.
7. Write log base 2 of "N" and "P" to "digest" as octets.
8. For each "V" in "hash-values":
1. Let "W" be the value following "V" in "hash-values".
2. If "W" and "V" are equal, continue to the next "V".
3. Let "D" be the result of "W - V - 1".
4. Let "Q" be the integer result of "D / P".
5. Let "R" be the result of "D modulo P".
6. Write "Q" '1' bits to "digest".
7. Write 1 '0' bit to "digest".
8. Write "R" to "digest" as binary, using log2(P) bits.
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9. If "V" is the second-to-last member of "hash-values", stop
iterating through "hash-values" and continue to the next
step.
9. If the length of "digest" is not a multiple of 8, pad it with 1s
until it is.
3. IANA Considerations
This draft currently has no requirements for IANA. If the
CACHE_DIGEST frame is standardised, it will need to be assigned a
frame type.
4. Security Considerations
The contents of a User Agent's cache can be used to re-identify or
"fingerprint" the user over time, even when other identifiers (e.g.,
Cookies [RFC6265]) are cleared.
CACHE_DIGEST allows such cache-based fingerprinting to become
passive, since it allows the server to discover the state of the
client's cache without any visible change in server behaviour.
As a result, clients MUST mitigate for this threat when the user
attempts to remove identifiers (e.g., "clearing cookies"). This
could be achieved in a number of ways; for example: by clearing the
cache, by changing one or both of N and P, or by adding new,
synthetic entries to the digest to change its contents.
TODO: discuss how effective the suggested mitigations actually would
be.
Additionally, User Agents SHOULD NOT send CACHE_DIGEST when in
"privacy mode."
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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,
<http://www.rfc-editor.org/info/rfc3986>.
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[RFC6234] 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,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
5.2. Informative References
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<http://www.rfc-editor.org/info/rfc6265>.
Appendix A. Acknowledgements
Thanks to Adam Langley and Giovanni Bajo for their explorations of
Golumb-coded sets. In particular, see
http://giovanni.bajo.it/post/47119962313/golomb-coded-sets-smaller-
than-bloom-filters , which refers to sample code.
Authors' Addresses
Kazuho Oku
DeNA Co, Ltd.
Email: kazuhooku@gmail.com
Mark Nottingham
Email: mnot@mnot.net
URI: https://www.mnot.net/
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