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Versions: 00 01                                                         
Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             G. Eriksson
Expires: September 22, 2016                                  C. Holmberg
                                                          March 21, 2016

             Caching Secure HTTP Content using Blind Caches


   A mechanism is described whereby a server can use client-selected
   shared cache.

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
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   This Internet-Draft will expire on September 22, 2016.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Shared Caching for HTTPS  . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Same-Host Secure Content Delegation . . . . . . . . . . . . .   3
     2.1.  Signaling Presence of a Proxy . . . . . . . . . . . . . .   3
     2.2.  Enabling Proxy Use  . . . . . . . . . . . . . . . . . . .   4
     2.3.  Proxy Identification and Authentication . . . . . . . . .   5
   3.  Performance Optimizations . . . . . . . . . . . . . . . . . .   5
     3.1.  Proxy Cache Priming . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Shared Caching for HTTPS

   Shared caches allow an HTTP server to offload the responsibility for
   delivering certain content.  Content in the shared cache can be
   accessed efficiently by multiple clients, saving the origin server
   from having to serve those requests and ensuring that clients receive
   responses to cached requests more quickly.

   Proxy caching is the most common configuration for shared caching.  A
   proxy cache is either explicitly configured by a client, discovered
   as a result of being automatically configured, or interposed
   automatically by an on-path network entity (this latter case being
   called a transparent proxy).

   HTTPS [RFC2818] prevents the use of proxies by creating an
   authenticated end-to-end connection to the origin server or its
   gateway that is authenticated.  This provides a critical protection
   against man-in-the-middle attacks, but it also prevents the proxy
   from acting as a shared cache.

   Thus, clients use the CONNECT pseudo-method (Section 4.3.6 of
   [RFC7231]) with any explicitly configured proxies to create an end-
   to-end tunnel and will refuse to send a query for an "https" URI to a

   This document describes a method for conditionally delegating the
   hosting of secure content to the same server.  This delegation allows
   a client to send a request for an "https" resource via a proxy rather
   than insisting on an end-to-end TLS connection.  This enables shared
   caching for a limited set of "https" resources, as selected by the

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1.1.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

   This document uses the term "proxy cache" to refer to a proxy
   [RFC7230] that operates an HTTP cache [RFC7234].

2.  Same-Host Secure Content Delegation

   The secure content delegation mechanism defined in [SCD] is used to
   create a separate resource that contains encrypted and integrity
   protected content.

   A client that signals a willingness to support this feature can be
   provided an response with an out-of-band encoding
   [I-D.reschke-http-oob-encoding] that identifies this resource.  The
   client can then make a request for that content to a proxy cache
   rather than directly to the origin server.

   In this document, the origin server is able to act in the role of the
   CDN in [SCD].  However, all of the considerations that apply to
   having a third party host content apply to the proxy cache.  Thus,
   integrity and confidentiality protections against the proxy cache are
   the primary consideration.

2.1.  Signaling Presence of a Proxy

   Without a clear signal from the client that a caching proxy is
   present, an origin is unable to send a response with out-of-band
   encoding.  A value of "out-of-band" in the Accept-Encoding header
   field might only indicate willingness to use the secure content
   delegation mechanism.

   The BC header field indicates that a client is connected to a proxy
   cache that it is willing to use for out-of-band requests.  The value
   of the BC header field is a simple boolean, represented as a "0" or
   "1".  A value that is present and set to "1" indicates that a proxy
   cache is present and available for use.  This header field can be
   used even if the current request was not routed via a proxy.

      BC = "0" / "1"

   Issue:  What signal do we need from the proxy cache that it supports
      this mode of operation?  Can we expect that a proxy cache will
      happily accept a request for an HTTPS URL?

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   Issue:  Do we want to identify the proxy so that the origin can make
      some sort of judgment about the proxy?  Probably not.  We
      shouldn't be relying on the origin server making judgments about
      the character of proxies.

2.2.  Enabling Proxy Use

   It is not sufficient to couple the acceptance and use of out-of-band
   content encoding with the use of a proxy.  Without an additional
   signal, a resource using secure content delegation to a CDN [SCD]
   could trigger a request via a proxy.

   The security properties of delegation via a CDN and via a caching
   proxy are similar only to the extent that a third party is involved.
   However, it might be the case that the CDN has a stronger
   relationship with the origin server and additional constraints on its
   actions, such as contractual limitations.  Such constraints might
   make delegation to the CDN acceptable to the origin server.  A
   caching proxy might not be considered acceptable.

   Therefore, a clear signal from the origin server is needed to allow
   the client to identify which resources are safe to retrieve from a
   proxy-cache.  A "proxy" extension to the JSON format defined in
   [I-D.reschke-http-oob-encoding] is added that signals to the client
   that the out-of-band content MAY be retrieved by making a request to
   a proxy.

   The "proxy" attribute is a boolean value.  In its absence, the value
   is assumed to be false.  If present and set to true, a client can
   send the request for the out-of-band content to a proxy instead of
   the identified server.

   Clients MUST NOT send a request via a proxy if the message containing
   the out-of-band content encoding does not include header fields for
   message integrity and encryption, such as the M-I header field
   [I-D.thomson-http-mice] or the Crypto-Key header field
   [I-D.ietf-httpbis-encryption-encoding].  Absence of these header
   fields indicate an error on the part of the origin server, since
   integrity and confidentiality protection are mandatory.

   Alternative:  The "proxy" attribute might be replaced by a rule that
      stated that same-origin out-of-band encoding implied permission to
      route via a proxy.  However, the gain here is minimal, it saves
      only on the explicit indication.

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2.3.  Proxy Identification and Authentication

   This mechanism does not work with a transparent caching proxy.  Since
   the request is made over end-to-end HTTPS in the absence of a proxy,
   the feature will not be used unless the proxy is known to the client.

   A proxy cache MUST therefore be expressly configured or discovered.
   This produces a name and possibly a port number for the proxy.  The
   proxy MUST be contacted using HTTPS [RFC2818] and authenticated using
   the configured or discovered domain name.

3.  Performance Optimizations

   As noted in [SCD], the secondary request required by out-of-band
   content encoding imposes a performance penalty.  This can be
   mitigated by priming clients with information about the location and
   disposition of resources prior to the client making a request.  A
   resource map described in [SCD] might be provided to clients to
   eliminate the latency involved in making requests of the origin
   server for resources that might be cached.

3.1.  Proxy Cache Priming

   A client that makes a request of an origin server via an unprimed
   proxy cache will suffer additional latency as a consequence of the
   cache having to make a request to the origin server.

   The following options are possible:

   o  Clients can speculatively make requests to a proxy cache based on
      information it learns from a resource map.  To avoid a potential
      waste of resources as a result of receiving complete responses,
      these might either be limited to HEAD requests; HTTP/2 [RFC7540]
      flow control might be used to allow only limited information to be

   o  The origin server might provide the proxy cache with "prefetch"
      link relations in responses to requests for secondary resources.
      These link relations might identify other resources that the proxy
      might retrieve speculatively.  This does not improve the latency
      of the initial request, but could improve subsequent requests.

4.  Security Considerations

   All the considerations of [SCD] apply.  In particular, content that
   is distributed with the assistance of a proxy cache MUST include
   integrity and confidentiality protection.  That means that the M-I
   header field [I-D.thomson-http-mice] and the Crypto-Key header field

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   [I-D.ietf-httpbis-encryption-encoding] or equivalent information MUST
   be present in responses that include an out-of-band content encoding.

   Clients that receive a response without the information necessary to
   ensure integrity and confidentiality protection against a proxy cache
   MUST NOT make a request to a proxy to retrieve that response.
   Clients could treat such a response as failed, make the request
   directly to the origin server, or retry a request without the out-of-
   band token in the Accept-Encoding header field (for idempotent
   methods only).

5.  IANA Considerations

   This document has no IANA actions.  It should.

6.  References

6.1.  Normative References

              Thomson, M., "Encrypted Content-Encoding for HTTP", draft-
              ietf-httpbis-encryption-encoding-01 (work in progress),
              March 2016.

              Thomson, M., "Merkle Integrity Content Encoding", draft-
              thomson-http-mice-00 (work in progress), January 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

   [RFC7230]  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,

   [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,

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   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [SCD]      Ericsson, G., Holmberg, C., and M. Thomson, "An
              Architecture for Secure Content Delegation using HTTP",
              February 2016, <draft-eriksson-http-scd.html>.

6.2.  Informative References

              Reschke, J. and S. Loreto, "'Out-Of-Band' Content Coding
              for HTTP", draft-reschke-http-oob-encoding-04 (work in
              progress), March 2016.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

Authors' Addresses

   Martin Thomson

   Email: martin.thomson@gmail.com

   Goeran AP Eriksson

   Email: goran.ap.eriksson@ericsson.com

   Christer Holmberg

   Email: christer.holmberg@ericsson.com

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