The Cache-Status HTTP Response Header Field
draft-ietf-httpbis-cache-header-03

HTTP                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                           March 1, 2020
Expires: September 2, 2020

              The Cache-Status HTTP Response Header Field
                   draft-ietf-httpbis-cache-header-03

Abstract

   To aid debugging, HTTP caches often append headers to a response
   detailing how they handled the request.  This specification codifies
   that practice and updates it for HTTP's current caching model.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   Discussion of this draft takes place on the HTTP working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].

   Working Group information can be found at https://httpwg.org/ [2];
   source code and issues list for this draft can be found at
   https://github.com/httpwg/http-extensions/labels/cache-header [3].

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 2, 2020.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Cache-Status HTTP Response Header Field . . . . . . . . .   3
     2.1.  The hit parameter . . . . . . . . . . . . . . . . . . . .   4
     2.2.  The fwd parameter . . . . . . . . . . . . . . . . . . . .   4
     2.3.  The fwd-status parameter  . . . . . . . . . . . . . . . .   5
     2.4.  The ttl parameter . . . . . . . . . . . . . . . . . . . .   5
     2.5.  The stored parameter  . . . . . . . . . . . . . . . . . .   5
     2.6.  The collapsed parameter . . . . . . . . . . . . . . . . .   5
     2.7.  The key parameter . . . . . . . . . . . . . . . . . . . .   5
   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   To aid debugging, HTTP caches often append headers to a response
   detailing how they handled the request.

   Unfortunately, the semantics of these headers are often unclear, and
   both the semantics and syntax used vary greatly between
   implementations.

   This specification defines a single, new HTTP response header field,
   "Cache-Status" for this purpose.

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1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses ABNF as defined in [RFC5234], along with the "%s"
   extension for case sensitivity defined in [RFC7405].

2.  The Cache-Status HTTP Response Header Field

   The Cache-Status HTTP response header indicates caches' handling of
   the request corresponding to the response it occurs within.

   Its value is a List [I-D.ietf-httpbis-header-structure]:

   Cache-Status   = sh-list

   Each member of the list represents a cache that has handled the
   request.  The first member of the list represents the cache closest
   to the origin server, and the last member of the list represents the
   cache closest to the client (possibly including the user agent's
   cache itself, if it chooses to append a value).

   Caches determine when it is appropriate to add the Cache-Status
   header field to a response.  Some might decide to add it to all
   responses, whereas others might only do so when specifically
   configured to, or when the request contains a header that activates a
   debugging mode.

   When adding a value to the Cache-Status header field, caches SHOULD
   preserve the existing contents of the header, to allow debugging of
   the entire chain of caches handling the request.

   Each list member identifies the cache that inserted that value, and
   MUST have a type of either sh-string or sh-token.  Depending on the
   deployment, this might be a product or service name (e.g.,
   ExampleCache or "Example CDN"), a hostname ("cache-3.example.com"),
   and IP address, or a generated string.

   Each member of the list can also have parameters that describe that
   cache's handling of the request.  While all of these parameters are
   OPTIONAL, caches are encouraged to provide as much information as
   possible.

   This specification defines these parameters:

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   hit          = sh-boolean
   fwd          = sh-token
   fwd-status   = sh-integer
   ttl          = sh-integer
   stored       = sh-boolean
   collapsed    = sh-boolean
   key          = sh-string

2.1.  The hit parameter

   "hit", when true, indicates that the request was satisfied by the
   cache; i.e., it did not go forward, and the response was obtained
   from the cache (possibly with modifications; e.g., if the request was
   conditional, a 304 Not Modified could be generated from cache).

   "hit" and "fwd" are exclusive; only one of them should appear on each
   list member.

2.2.  The fwd parameter

   "fwd" indicates why the request went forward.

   It can have one of the following values:

   o  uri-miss - The cache did not contain any responses that matched
      the request URI

   o  vary-miss - The cache contained a response that matched the
      request URI, but could not select a response based upon this
      request's headers and stored Vary headers.

   o  miss - The cache did not contain any responses that could be used
      to satisfy this request (to be used when an implementation cannot
      distinguish between uri-miss and vary-miss)

   o  stale - The cache was able to select a response for the request,
      but it was stale

   o  request - The cache was able to select a fresh response for the
      request, but client request headers (e.g., Cache-Control request
      directives) did not allow its use

   o  bypass - The cache was configured to not handle this request

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2.3.  The fwd-status parameter

   "fwd-status" indicates what status code the next hop server returned
   in response to the request.  Only meaningful when "fwd" is present;
   if "fwd-status" is not present but "fwd" is, it defaults to the
   status code sent in the response.

   This parameter is useful to distinguish cases when the next hop
   server sends a 304 Not Modified response to a conditional request, or
   a 206 Partial Response due to a range request.

2.4.  The ttl parameter

   "ttl" indicates the response's remaining freshness lifetime as
   calculated by the cache, as an integer number of seconds, measured
   when the response is sent by the cache.  This includes freshness
   assigned by the cache; e.g., through heuristics, local configuration,
   or other factors.  May be negative, to indicate staleness.

2.5.  The stored parameter

   "stored" indicates whether the cache stored the forward response; a
   true value indicates that it did.  Only meaningful when fwd is
   present.

2.6.  The collapsed parameter

   "collapsed" indicates whether this request was collapsed together
   with one or more other forward requests; if true, the response was
   successfully reused; if not, a new request had to be made.  If not
   present, the request was not collapsed with others.  Only meaningful
   when fwd is present.

2.7.  The key parameter

   "key" conveys a representation of the cache key used for the
   response.  Note that this may be implementation-specific.

3.  Examples

   The most minimal cache hit:

   Cache-Status: ExampleCache; hit

   ... but a polite cache will give some more information, e.g.:

   Cache-Status: ExampleCache; hit; ttl=376

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   A stale hit just has negative freshness:

   Cache-Status: ExampleCache; hit; ttl=-412

   Whereas a complete miss is:

   Cache-Status: ExampleCache; fwd=uri-miss

   A miss that successfully validated on the back-end server:

   Cache-Status: ExampleCache; fwd=stale; fwd-status=304

   A miss that was collapsed with another request:

   Cache-Status: ExampleCache; fwd=uri-miss; collapsed

   A miss that the cache attempted to collapse, but couldn't:

   Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0

   Going through two layers of caching, both of which were hits, and the
   second collapsed with other requests:

   Cache-Status: OriginCache; hit; ttl=1100; collapsed,
                 "CDN Company Here"; hit; ttl=545

4.  Security Considerations

   Information about a cache's content can be used to infer the activity
   of those using it.  Generally, access to sensitive information in a
   cache is limited to those who are authorised to access that
   information (using a variety of techniques), so this does not
   represent an attack vector in the general sense.

   However, if the Cache-Status header is exposed to parties who are not
   authorised to obtain the response it occurs within, it could expose
   information about that data.

   For example, if an attacker were able to obtain the Cache-Status
   header from a response containing sensitive information and access
   were limited to one person (or limited set of people), they could
   determine whether that information had been accessed before.  This is
   similar to the information exposed by various timing attacks, but is
   arguably more reliable, since the cache is directly reporting its
   state.

   Mitigations include use of encryption (e.g., TLS [RFC8446])) to
   protect the response, and careful controls over access to response

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   headers (as are present in the Web platform).  When in doubt, the
   Cache-Status header field can be omitted.

5.  References

5.1.  Normative References

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-13 (work in progress),
              August 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  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/info/rfc5234>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

5.2.  Informative References

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

5.3.  URIs

   [1] https://lists.w3.org/Archives/Public/ietf-http-wg/

   [2] https://httpwg.org/

   [3] https://github.com/httpwg/http-extensions/labels/cache-header

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Author's Address

   Mark Nottingham
   Fastly

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

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