Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Standards Track G. Eriksson
Expires: September 22, 2016 C. Holmberg
Ericsson
March 21, 2016
Caching Secure HTTP Content using Blind Caches
draft-thomson-http-bc-00
Abstract
A mechanism is described whereby a server can use client-selected
shared cache.
Status of This Memo
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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
proxy.
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
server.
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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
sent.
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
[I-D.ietf-httpbis-encryption-encoding]
Thomson, M., "Encrypted Content-Encoding for HTTP", draft-
ietf-httpbis-encryption-encoding-01 (work in progress),
March 2016.
[I-D.thomson-http-mice]
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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[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,
<http://www.rfc-editor.org/info/rfc7230>.
[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>.
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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,
<http://www.rfc-editor.org/info/rfc7540>.
[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
[I-D.reschke-http-oob-encoding]
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,
<http://www.rfc-editor.org/info/rfc7231>.
Authors' Addresses
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com
Goeran AP Eriksson
Ericsson
Email: goran.ap.eriksson@ericsson.com
Christer Holmberg
Ericsson
Email: christer.holmberg@ericsson.com
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