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Versions: 00 01 draft-ietf-httpbis-cache-digest

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   The most recent (often, unpublished) draft is at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

   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 http://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 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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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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.

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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