HTTP Working Group                                                       M. Nottingham
Internet-Draft                                                    Fastly
Intended status: Standards Track                        January 27,                        November 2, 2019
Expires: July 31, 2019 May 5, 2020

                 The Cache Cache-Status HTTP Response Header
                   draft-ietf-httpbis-cache-header-00
                   draft-ietf-httpbis-cache-header-01

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 July 31, 2019. May 5, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Cache Cache-Status HTTP Response Header . . . . . . . . . . . .   3
     2.1.  The fwd parameter . . . . . .   3 . . . . . . . . . . . . . .   4
     2.2.  The fwd-res parameter . . . . . . . . . . . . . . . . . .   4
     2.3.  The fwd-stored parameter  . . . . . . . . . . . . . . . .   4
     2.4.  The res-fresh parameter . . . . . . . . . . . . . . . . .   5
     2.5.  The cache-fresh parameter . . . . . . . . . . . . . . . .   5
     2.6.  The collapse-hit parameter  . . . . . . . . . . . . . . .   5
     2.7.  The collapse-wait parameter . . . . . . . . . . . . . . .   5
     2.8.  The key parameter . . . . . . . . . . . . . . . . . . . .   5
   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     4.2.   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
     4.3.
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   7   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7   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"
   "Cache-Status" for this purpose.

   For example:

  Cache: HIT_FRESH; node="reverse-proxy.example.com:80";
                    key="https://example.com/foo|Accept-Encoding:gzip",
         HIT_STALE; node="FooCDN parent"; fresh=-45; age=200; latency=3,
         MISS; node="FooCDN edge"; fresh=-45; age=200; latency=98

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 Cache-Status HTTP Response Header

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

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

   Cache

   Cache-Status   = sh-param-list sh-list

   Each member of the parameterised 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 user agent (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 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 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.

   Identifiers in the parameterised

   The list members are expected to be
   cache-actions:

   cache-action   = %s"HIT_FRESH"
                  / %s"HIT_STALE"
                  / %s"HIT_REFRESH_MODIFIED"
                  / %s"HIT_REFRESH_NOT_MODIFIED"
                  / %s"HIT_REFRESH_STALE"
                  / %s"MISS"
                  / %s"MISS_CLIENT"
                  / %s"BYPASS"
                  / %s"ERROR"

   The semantics of cache-actions are:

   o  HIT_FRESH - The cache used a fresh stored response to satisfy identify the
      request without going forward

   o  HIT_STALE - The cache used that inserted the value, and MUST
   have a stale stored response to satisfy type of either sh-string or sh-token.  Depending on the
      request without going forward

   o  HIT_REFRESH_MODIFIED - The cache had
   deployment, this might be a stale stored response, went
      forward to validate it, product or service name (e.g.,
   ExampleCache or "Example CDN"), a hostname ("cache-3.example.com"),
   and used IP address, or a generated string.

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

   fwd           = sh-token
   fwd-res       = sh-token
   fwd-stored    = sh-boolean
   res-fresh     = sh-integer
   cache-fresh   = sh-integer
   collapse-hit  = sh-boolean
   collapse-wait = sh-integer
   key           = sh-string

2.1.  The fwd parameter

   "fwd" indicates why the request

   o  HIT_REFRESH_NOT_MODIFIED - The cache had a stale stored response, went forward to validate it, and used forward.  If it is not present,
   the stored response value defaults to
      satisfy "none".

   It can have one of the request

   o  HIT_REFRESH_STALE following values: * none - The cache had request did not
   go forward; i.e., it was a stale stored response, went
      forward to validate it, hit, and encountered a problem, so was served from the stored
      response cache.  *
   bypass - The cache was used configured to satisfy the not handle this request

   o  MISS * uri-
   miss - The cache did not have a stored response, so contain any responses that matched the
   request
      was forwarded

   o  MISS_CLIENT URI * vary-miss - The client included request directives (e.g.,
      Pragma, Cache-Control) that prevented the cache from returning contained a
      response, so response that matched
   the request was forwarded

   o  BYPASS URI, but could not select a response based upon this
   request's headers.  * miss - The cache was configured did not contain any responses
   that could be used to forward the satisfy this request without
      attempting (to be used when an
   implementation cannot distinguish between uri-miss and vary-miss) *
   res-stale - The cache was able to use select a stored response

   o  ERROR for the request,
   but it was stale * req-stale - The cache was unable able to use select a stored fresh
   response or obtain
      one by going forward

   Caches SHOULD use for the most specific cache-action to a given response, request, but are client request headers (e.g., Cache-
   Control request directives) did not required to allow its use all cache-actions.  Future updates to
   this specification can add additional cache-actions.

   Each member

2.2.  The fwd-res parameter

   "fwd-res" indicates what the result of the Cache header forward request was.  It
   is only valid when fwd is "res-stale" or "req-stale", and defaults to
   "full" if not present when fwd is one of those values.

   It can also have any (or all, or none) one of the following parameters:

   node           = sh-string
   fresh          = sh-integer
   age            = sh-integer
   cacheable      = sh-boolean
   key            = sh-string
   latency        = sh-integer
   cl_nm          = sh-boolean

   Their semantics are:

   o  "node" values: * full - indicates that the
   response was a string identifying for complete response (any status code except 304 Not
   Modified and 206 Partial Response) * partial - indicates that the cache node.  MAY be
   response was a
      hostname, IP address, or alias.

   o  "fresh" 206 Partial Response * notmod - an integer indicating indicates that the cache's estimation of
   response was a 304 Not Modified

2.3.  The fwd-stored parameter

   "fwd-stored" indicates whether the cache stored the response; a true
   value indicates that it did.  Only valid when fwd is not "none".

2.4.  The res-fresh parameter

   "res-fresh" indicates the response's remaining freshness lifetime ([RFC7234], (as
   per [I-D.ietf-httpbis-cache], Section 4.2.1) of this response in
      seconds, including any locally applied configuration.  MAY be
      negative.

   o  "age" - 4.2.1), as an integer indicating the cache's estimation number of
   seconds.  This does not include freshness assigned by the age
      ([RFC7234], Section 4.2.3) cache (see
   "cache-fresh").  May be negative, to indicate staleness.

2.5.  The cache-fresh parameter

   "cache-fresh" indicates the response's remaining freshness lifetime
   as calculated by the cache, as an integer number of this response in seconds.  MUST be 0
      or greater.

   o  "cacheable" - a boolean indicating whether  This
   includes freshness assigned by the cache can store
      this response, according cache; e.g., through heuristics,
   local configuration, or other factors.  May be negative, to [RFC7234], Section 3 indicate
   staleness.

   If both cache-fresh and any locally
      applied configuration.

   o  "key" - a string representing res-fresh appear as parameters on the key same
   value, it implies that the cache has
      associated with freshness overrode the response
   freshness.

2.6.  The collapse-hit parameter

   "collapse-hit" indicates whether this response.  This might include request was collapsed together
   with one or more other forward requests; if true, the response was
   successfully reused; if not, a new request
      URL, had to be made.  If not
   present, the request headers, and other values.

   o  "latency" - an integer indicating was not collapsed with others.

2.7.  The collapse-wait parameter

   "collapse-wait" indicates the amount of time in
      milliseconds between that the receipt of a complete set of request
      headers and sending cache held the complete set
   request while waiting to see if it could be successfully collapsed,
   as an integer number of response headers milliseconds.

2.8.  The key parameter

   "key" conveys a representation of this
      response, from the viewpoint of cache key used for the cache.
   response.  Note that this may not
      include buffering time in transport protocols and similar delays.

   o  "cl_nm" - be implementation-specific.

3.  Examples

   The most minimal cache hit:

   Cache-Status: ExampleCache

   ... but a boolean indicating whether the response to polite cache will give some more information, e.g.:

   Cache-Status: ExampleCache; res-fresh=376
   A "negative" hit (i.e., the client
      had cache imposed its own freshness
   lifetime):

   Cache-Status: ExampleCache; cache-fresh=415

   A stale hit just has negative freshness:

   Cache-Status: ExampleCache; res-fresh=-412

   Whereas a 304 Not Modified status code.

   While all of these parameters are OPTIONAL, caches are encouraged to
   use complete miss is:

   Cache-Status: ExampleCache; fwd=uri-miss

   A miss that validated on the 'node' parameter back-end server:

   Cache-Status: ExampleCache; fwd=res-stale; fwd-res=notmod

   A miss that was collapsed with another request:

   Cache-Status: ExampleCache; fwd=uri-miss; collapse-hit=?1

   A miss that the cache attempted to identify themselves.

3. collapse, but couldn't:

   Cache-Status: ExampleCache; fwd=uri-miss;
                 collapse-hit=?0; collapse-wait=240

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

   Cache-Status: "CDN Company Here"; res-fresh=545,
                 OriginCache; cache-fresh=1100; collapse-hit=?1

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

4.

5.  References

4.1.

5.1.  Normative References

   [I-D.ietf-httpbis-cache]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Caching", draft-ietf-httpbis-cache-05 (work in progress),
              July 2019.

   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-09
              draft-ietf-httpbis-header-structure-13 (work in progress),
              December 2018.
              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>.

   [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,
              <https://www.rfc-editor.org/info/rfc7234>.

   [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>.

4.2.

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>.

4.3.

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

Author's Address

   Mark Nottingham
   Fastly

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