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Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                               S. Loreto
Expires: October 28, 2016                                       Ericsson
                                                          April 26, 2016


                 'Out-Of-Band' Content Coding for HTTP
                   draft-reschke-http-oob-encoding-06

Abstract

   This document describes an Hypertext Transfer Protocol (HTTP) content
   coding that can be used to describe the location of a secondary
   resource that contains the payload.

Editorial Note (To be removed by RFC Editor before publication)

   Distribution of this document is unlimited.  Although this is not a
   work item of the HTTPbis Working Group, comments should be sent to
   the Hypertext Transfer Protocol (HTTP) mailing list at
   ietf-http-wg@w3.org [1], which may be joined by sending a message
   with subject "subscribe" to ietf-http-wg-request@w3.org [2].

   Discussions of the HTTPbis Working Group are archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   XML versions, latest edits, and issue tracking for this document are
   available from <https://github.com/reschke/oobencoding> and
   <http://greenbytes.de/tech/webdav/#draft-reschke-http-oob-encoding>.

   The changes in this draft are summarized in Appendix C.6.

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."




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   This Internet-Draft will expire on October 28, 2016.

Copyright Notice

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

   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
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   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.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  'Out-Of-Band' Content Coding . . . . . . . . . . . . . . . . .  4
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Processing Steps . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Problem Reporting  . . . . . . . . . . . . . . . . . . . .  7
       3.4.1.  Server Not Reachable . . . . . . . . . . . . . . . . .  7
       3.4.2.  Resource Not Found . . . . . . . . . . . . . . . . . .  7
       3.4.3.  Payload Unusable . . . . . . . . . . . . . . . . . . .  7
     3.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.5.1.  Basic Example  . . . . . . . . . . . . . . . . . . . .  8
       3.5.2.  Example for an attempt to use 'out-of-band'
               cross-origin . . . . . . . . . . . . . . . . . . . . .  9
       3.5.3.  Example involving an encrypted resource  . . . . . . . 10
       3.5.4.  Example For Problem Reporting  . . . . . . . . . . . . 11
   4.  Content Codings and Range Requests . . . . . . . . . . . . . . 11
   5.  Feature Discovery  . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     6.1.  Content Modifications  . . . . . . . . . . . . . . . . . . 12
     6.2.  Content Stealing . . . . . . . . . . . . . . . . . . . . . 12
     6.3.  Use in Requests  . . . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Alternatives, or: why not a new Status Code?  . . . . 15
   Appendix B.  Open Issues . . . . . . . . . . . . . . . . . . . . . 16
     B.1.  Accessing the Secondary Resource Too Early . . . . . . . . 16
     B.2.  Resource maps  . . . . . . . . . . . . . . . . . . . . . . 16
     B.3.  Fragmenting  . . . . . . . . . . . . . . . . . . . . . . . 17
     B.4.  Relation to Content Encryption . . . . . . . . . . . . . . 17
     B.5.  Reporting  . . . . . . . . . . . . . . . . . . . . . . . . 17
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 18
     C.1.  Changes since draft-reschke-http-oob-encoding-00 . . . . . 18
     C.2.  Changes since draft-reschke-http-oob-encoding-01 . . . . . 18
     C.3.  Changes since draft-reschke-http-oob-encoding-02 . . . . . 18
     C.4.  Changes since draft-reschke-http-oob-encoding-03 . . . . . 19
     C.5.  Changes since draft-reschke-http-oob-encoding-04 . . . . . 19
     C.6.  Changes since draft-reschke-http-oob-encoding-05 . . . . . 19
   Appendix D.  Acknowledgements  . . . . . . . . . . . . . . . . . . 19







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1.  Introduction

   This document describes an Hypertext Transfer Protocol (HTTP) content
   coding (Section 3.1.2.1 of [RFC7231]) that can be used to describe
   the location of a secondary resource that contains the payload.

   The primary use case for this content coding is to enable origin
   servers to securely delegate the delivery of content to a secondary
   server that might be "closer" to the client (with respect to network
   topology) and/or able to cache content ([SCD]), leveraging content
   encryption ([ENCRYPTENC]).

2.  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].

   This document reuses terminology used in the base HTTP
   specifications, namely Section 2 of [RFC7230] and Section 3 of
   [RFC7231].

3.  'Out-Of-Band' Content Coding

3.1.  Overview

   The 'Out-Of-Band' content coding is used to direct the recipient to
   retrieve the actual message representation (Section 3 of [RFC7231])
   from a secondary resource, such as a public cache:

   1.  Client performs a request

   2.  Received response specifies the 'out-of-band' content coding; the
       payload of the response contains additional meta data, plus the
       location of the secondary resource

   3.  Client performs GET request on secondary resource (usually again
       via HTTP(s))

   4.  Secondary server provides payload

   5.  Client combines above representation with additional
       representation metadata obtained from the primary resource








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     Client                  Secondary Server           Origin Server

        sends GET request with Accept-Encoding: out-of-band
   (1) |---------------------------------------------------------\
                      status 200 and Content-Coding: out-of-band |
   (2) <---------------------------------------------------------/

        GET to secondary server
   (3) |---------------------------\
                           payload |
   (4) <---------------------------/

   (5)
      Client and combines payload received in (4)
      with metadata received in (2).

3.2.  Definitions

   The name of the content coding is "out-of-band".

   The payload format uses JavaScript Object Notation (JSON, [RFC7159]),
   describing an object describing secondary resources; currently only
   defining one member:

   'sr'  A REQUIRED string array containing at least one URI reference
      (Section 4.1 of [RFC3986]) of a secondary resource (URI references
      that are relative references are resolved against the URI of the
      primary resource).

   The payload format uses an array so that the origin server can
   specify multiple secondary resources.  The ordering within the array
   reflects the origin server's preference (if any), with the most
   preferred secondary resource location being first.  Clients receiving
   a response containing multiple URIs are free to choose which of these
   to use.

   In some cases, the origin server might want to specify a "fallback
   URI"; identifying a secondary resource served by the origin server
   itself, but otherwise equivalent "regular" secondary resources.  Any
   secondary resource hosted by the origin server can be consired to be
   a "fallback"; origin servers will usually list them last in the "sr"
   array so that they only will be used by clients when there is no
   other choice.

   New specifications can define new OPTIONAL header fields, thus
   clients MUST ignore unknown fields.  Extension specifications will
   have to update this specification. [[anchor3: or we define a
   registry]]



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3.3.  Processing Steps

   Upon receipt of an 'out-of-band' encoded response, a client first
   needs to obtain the secondary resource's presentation.  This is done
   using an HTTP GET request (independantly of the original request
   method).

   In order to prevent any leakage of information, the GET request for
   the secondary resource MUST only contain information provided by the
   origin server or the secondary server itself, namely HTTP
   authentication credentials ([RFC7235]) and cookies ([RFC6265]).

   Furthermore, the request MUST include an "Origin" header field
   indicating the origin of the original resource ([RFC6454], Section
   7).  The secondary server MUST verify that the specified origin is
   authorized to retrieve the given payload (or otherwise return an
   appropriate 4xx status code).

   After receipt of the secondary resource's payload, the client then
   reconstructs the original message by:

   1.  Unwrapping the encapsulated HTTP message by removing any transfer
       and content codings.

   2.  Replacing/setting any response header fields from the primary
       response except for framing-related information such as Content-
       Length, Transfer-Encoding and Content-Encoding.

   If the client is unable to retrieve the secondary resource's
   representation (host can't be reached, non 2xx response status code,
   payload failing integrity check, etc.), it can choose an alternate
   secondary resource (if specified), try the fallback URI (if given),
   or simply retry the request to the origin server without including
   'out-of-band' in the Accept-Encoding request header field.  In the
   latter case, it can be useful to inform the origin server about what
   problems were encountered when trying to access the secondary
   resource; see Section 3.4 for details.

   Note that although this mechanism causes the inclusion of external
   content, it will not affect the application-level security properties
   of the reconstructed message, such as its web origin ([RFC6454]).

   The cacheability of the response for the secondary resource does not
   affect the cacheability of the reconstructed response message, which
   is the same as for the origin server's response.

   Use of the 'out-of-band' coding is similar to HTTP redirects
   ([RFC7231], Section 6.4) in that it can lead to cycles.  Unless with



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   HTTP redirects, the client however is in full control: it does not
   need to advertise support for the 'out-of-band' coding in requests
   for secondary resources.  Alternatively, it can protect itself just
   like for HTTP redirects -- by limiting the number of indirections it
   supports.

   Note that because the server's response depends on the request's
   Accept-Encoding header field, the response usually will need to be
   declared to vary on that.  See Section 7.1.4 of [RFC7231] and Section
   2.3 of [RFC7232] for details.

3.4.  Problem Reporting

   When the client fails to obtain the secondary resource, it can be
   useful to inform the origin server about the condition.  This can be
   accomplished by adding a "Link" header field ([RFC5988]) to a
   subsequent request to the origin server, detailing the URI of the
   secondary resource and the failure reason.

   The following link extension relations are defined: [[linkrel:
   Missing: "TLS handshake failure"]]

3.4.1.  Server Not Reachable

   Used in case the server was not reachable.

   Link relation:

   http://purl.org/NET/linkrel/not-reachable

3.4.2.  Resource Not Found

   Used in case the server responded, but the object could not be
   obtained.

   Link relation:

   http://purl.org/NET/linkrel/resource-not-found

3.4.3.  Payload Unusable

   Used in case the the payload could be obtained, but wasn't usable
   (for instance, because integrity checks failed).

   Link relation:

   http://purl.org/NET/linkrel/payload-unusable




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3.5.  Examples

3.5.1.  Basic Example

   Client request of primary resource at https://www.example.com/test:

     GET /test HTTP/1.1
     Host: www.example.com
     Accept-Encoding: gzip, out-of-band


   Response:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:00 GMT
     Content-Type: text/plain
     Cache-Control: max-age=10, public
     Content-Encoding: out-of-band
     Content-Length: 133
     Vary: Accept-Encoding

     {
       "sr": [
         "http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00",
         "/c/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"
       ]
     }

   (note that the Content-Type header field describes the media type of
   the secondary's resource representation, and the origin server
   supplied a fallback URI)

   Client request for secondary resource:

     GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
     Host: example.net
     Origin: https://www.example.com














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   Response:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:10 GMT
     Cache-Control: private
     Content-Length: 15

     Hello, world.

   (Note no Content-Type header field is present here because the
   secondary server truly does not know the media type of the payload)

   Final message after recombining header fields:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:00 GMT
     Content-Length: 15
     Cache-Control: max-age=10, public
     Content-Type: text/plain

     Hello, world.

3.5.2.  Example for an attempt to use 'out-of-band' cross-origin

   Section 3.3 requires the client to include an "Origin" header field
   in the request to a secondary server.  The example below shows how
   the server for the secondary resource would respond to a request
   which contains an "Origin" header field identifying an unauthorized
   origin.

   Continuing with the example from Section 3.5.1, and a secondary
   server that is configured to allow only access for requests initiated
   by "https://www.example.org":

   Client request for secondary resource:

     GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
     Host: example.net
     Origin: https://www.example.com


   Response:

     HTTP/1.1 403 Forbidden
     Date: Thu, 14 May 2015 18:52:10 GMT


   Note that a request missing the "Origin" header field would be



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   treated the same way.

   [[anchor6: Any reason why to *mandate* a specific 4xx code?]]

3.5.3.  Example involving an encrypted resource

   Given the example HTTP message from Section 5.4 of [ENCRYPTENC], a
   primary resource could use the 'out-of-band' coding to specify just
   the location of the secondary resource plus the contents of the
   "Crypto-Key" header field needed to decrypt the payload:

   Response:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:00 GMT
     Content-Encoding: aesgcm, out-of-band
     Content-Type: text/plain
     Encryption: keyid="a1"; salt="vr0o6Uq3w_KDWeatc27mUg"
     Crypto-Key: keyid="a1"; aesgcm="csPJEXBYA5U-Tal9EdJi-w"
     Content-Length: 85
     Vary: Accept-Encoding

     {
       "sr": [
         "http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"
       ]
     }

   (note that the Content-Type header field describes the media type of
   the secondary's resource representation)

   Response for secondary resource:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:10 GMT
     Content-Length: ...

     VDeU0XxaJkOJDAxPl7h9JD5V8N43RorP7PfpPdZZQuwF
   (payload body shown in base64 here)

   Final message undoing all content codings:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:00 GMT
     Content-Length: 15
     Content-Type: text/plain

     I am the walrus



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      Note: in this case, the ability to undo the 'aesgcm' is needed to
      process the response.  If 'aesgcm' wasn't listed as acceptable
      content coding in the request, the origin server wouldn't be able
      to use the 'out-of-band' mechanism.

3.5.4.  Example For Problem Reporting

   Client requests primary resource as in Section 3.5.1, but the attempt
   to access the secondary resource fails.

   Response:

     HTTP/1.1 404 Not Found
     Date: Thu, 08 September 2015 16:49:00 GMT
     Content-Type: text/plain
     Content-Length: 20

     Resource Not Found

   Client retries with the origin server and includes Link header field
   reporting the problem:

     GET /test HTTP/1.1
     Host: www.example.com
     Accept-Encoding: gzip, out-of-band
     Link: <http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00>;
           rel="http://purl.org/NET/linkrel/resource-not-found"


4.  Content Codings and Range Requests

   The combination of content codings ([RFC7231], Section 3.1.2 with
   range requests ([RFC7233]) can lead to surprising results, as
   applying the range request happens after applying content codings.

   Thus, for a request for the bytes starting at position 100000 of a
   video:

     GET /test.mp4 HTTP/1.1
     Host: www.example.com
     Range: bytes=100000-
     Accept-Encoding: identity









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   ...a successful response would use status code 206 (Partial Content)
   and have a payload containing the octets starting at position 100000.

     HTTP/1.1 206 Partial Content
     Date: Thu, 08 September 2015 16:49:00 GMT
     Content-Type: video/mp4
     Content-Length: 134567
     Content-Range: bytes 100000-234566/234567

     (binary data)

   However, if the request would have allowed the use of 'out-of-band'
   coding:

     GET /test.mp4 HTTP/1.1
     Host: www.example.com
     Range: bytes=100000-
     Accept-Encoding: out-of-band


   ...a server might return an empty payload (if the out-of-band coded
   response body would be shorter than 100000 bytes, as would be usually
   the case).

   Thus, in order to avoid unnecessary network traffic, servers SHOULD
   NOT apply range request processing to responses using ouf-of-band
   content coding (or, in other words: ignore "Range" request header
   fields in this case).

5.  Feature Discovery

   New content codings can be deployed easily, as the client can use the
   "Accept-Encoding" header field (Section 5.3.4 of [RFC7231]) to signal
   which content codings are supported.

6.  Security Considerations

6.1.  Content Modifications

   This specification does not define means to verify that the payload
   obtained from the secondary resource really is what the origin server
   expects it to be.  Content signatures can address this concern (see
   [CONTENTSIG] and [MICE]).

6.2.  Content Stealing

   The 'out-of-band' content coding could be used to circumvent the
   same-origin policy ([RFC6454], Section 3) of user agents: an



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   attacking site which knows the URI of a secondary resource would use
   the 'out-of-band' coding to trick the user agent to read the contents
   of the secondary resource, which then, due to the security properties
   of this coding, would be handled as if it originated from the
   origin's resource.

   This scenario is addressed by the client requirement to include the
   "Origin" request header field and the server requirement to verify
   that the request was initiated by an authorized origin.

      Note: similarities with the "Cross-Origin Resource Sharing"
      protocol ([CORS]) are intentional.

   Requiring the secondary resource's payload to be encrypted
   ([ENCRYPTENC]) is an additional mitigation.

6.3.  Use in Requests

   In general, content codings can be used in both requests and
   responses.  This particular content coding has been designed for
   responses.  When supported in requests, it creates a new attack
   vector where the receiving server can be tricked into including
   content that the client might not have access to otherwise (such as
   HTTP resources behind a firewall).

7.  IANA Considerations

   The IANA "HTTP Content Coding Registry", located at
   <http://www.iana.org/assignments/http-parameters>, needs to be
   updated with the registration below:

   Name:  out-of-band

   Description:  Payload needs to be retrieved from a secondary resource

   Reference:  Section 3 of this document

8.  References

8.1.  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",



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                 STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,
                 <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5988]     Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/
                 RFC5988, October 2010,
                 <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6265]     Barth, A., "HTTP State Management Mechanism", RFC 6265,
                 DOI 10.17487/RFC6265, April 2011,
                 <http://www.rfc-editor.org/info/rfc6265>.

   [RFC7159]     Bray, T., "The JavaScript Object Notation (JSON) Data
                 Interchange Format", RFC 7159, DOI 10.17487/RFC7159,
                 March 2014, <http://www.rfc-editor.org/info/rfc7159>.

   [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>.

   [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>.

   [RFC7235]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 RFC 7235, DOI 10.17487/RFC7235, June 2014,
                 <http://www.rfc-editor.org/info/rfc7235>.

8.2.  Informative References

   [CONTENTSIG]  Thomson, M., "Content-Signature Header Field for HTTP",
                 draft-thomson-http-content-signature-00 (work in
                 progress), July 2015.

   [CORS]        van Kesteren, A., "Cross-Origin Resource Sharing", W3C
                 Recommendation REC-cors-20140116, January 2014,
                 <http://www.w3.org/TR/2014/REC-cors-20140116/>.

                 Latest version available at
                 <http://www.w3.org/TR/cors/>.

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

   [MICE]        Thomson, M., "Merkle Integrity Content Encoding",



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                 draft-thomson-http-mice-00 (work in progress),
                 January 2016.

   [RFC2017]     Freed, N. and K. Moore, "Definition of the URL MIME
                 External-Body Access-Type", RFC 2017, DOI 10.17487/
                 RFC2017, October 1996,
                 <http://www.rfc-editor.org/info/rfc2017>.

   [RFC4483]     Burger, E., "A Mechanism for Content Indirection in
                 Session Initiation Protocol (SIP) Messages", RFC 4483,
                 DOI 10.17487/RFC4483, May 2006,
                 <http://www.rfc-editor.org/info/rfc4483>.

   [RFC6454]     Barth, A., "The Web Origin Concept", RFC 6454,
                 DOI 10.17487/RFC6454, December 2011,
                 <http://www.rfc-editor.org/info/rfc6454>.

   [RFC7232]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 RFC 7232, DOI 10.17487/RFC7232, June 2014,
                 <http://www.rfc-editor.org/info/rfc7232>.

   [RFC7233]     Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
                 "Hypertext Transfer Protocol (HTTP/1.1): Range
                 Requests", RFC 7233, DOI 10.17487/RFC7233, June 2014,
                 <http://www.rfc-editor.org/info/rfc7233>.

   [SCD]         Thomson, M., Eriksson, G., and C. Holmberg, "An
                 Architecture for Secure Content Delegation using HTTP",
                 draft-thomson-http-scd-00 (work in progress),
                 March 2016.

URIs

   [1]  <mailto:ietf-http-wg@w3.org>

   [2]  <mailto:ietf-http-wg-request@w3.org?subject=subscribe>

Appendix A.  Alternatives, or: why not a new Status Code?

   A plausible alternative approach would be to implement this
   functionality one level up, using a new redirect status code (Section
   6.4 of [RFC7231]).  However, this would have several drawbacks:

   o  Servers will need to know whether a client understands the new
      status code; thus some additional signal to opt into this protocol
      would always be needed.




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   o  In redirect messages, representation metadata (Section 3.1 of
      [RFC7231]), namely "Content-Type", applies to the response
      message, not the redirected-to resource.

   o  The origin-preserving nature of using a content coding woudld be
      lost.

   Another alternative would be to implement the indirection on the
   level of the media type using something similar to the type "message/
   external-body", defined in [RFC2017] and refined for use in the
   Session Initiation Protocol (SIP) in [RFC4483].  This approach though
   would share most of the drawbacks of the status code approach
   mentioned above.

Appendix B.  Open Issues

B.1.  Accessing the Secondary Resource Too Early

   One use-case for this protocol is to enable a system of "blind
   caches", which would serve the secondary resources.  These caches
   might only be populated on demand, thus it could happen that whatever
   mechanism is used to populate the cache hasn't finished when the
   client hits it (maybe due to race conditions, or because the cache is
   behind a middlebox which doesn't allow the origin server to push
   content to it).

   In this particular case, it can be useful if the client was able to
   "piggyback" the URI of the fallback for the primary resource, giving
   the secondary server a means by which it could obtain the payload
   itself.  This information could be provided in yet another Link
   header field:

     GET /bae27c36-fa6a-11e4-ae5d-00059a3c7a00 HTTP/1.1
     Host: example.net
     Link: <http://example.com/c/bae27c36-fa6a-11e4-ae5d-00059a3c7a00>;
           rel="http://purl.org/NET/linkrel/primary-resource"


   (continuing the example from Section 3.5.1)

B.2.  Resource maps

   When 'out-of-band' coding is used as part of a caching solution, the
   additional round trips to the origin server can be a significant
   performance problem; in particular, when many small resources need to
   be loaded (such as scripts, images, or video fragments).  In cases
   like these, it could be useful for the origin server to provide a
   "resource map", allowing to skip the round trips to the origin server



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   for these mapped resources.  Plausible ways to transmit the resource
   map could be:

   o  as extension in the 'out-of-band' coding JSON payload, or

   o  as separate resource identified by a "Link" response header field.

   This specification does not define a format, nor a mechanism to
   transport the map, but it's a given that some specification using
   'out-of-band' coding will do.

B.3.  Fragmenting

   It might be interesting to divide the original resource's payload
   into fragments, each of which being mapped to a distinct secondary
   resource.  This would allow to not store the full payload of a
   resource in a single cache, thus

   o  distribute load,

   o  caching different parts of the resource with different
      characteristics (such as only distribute the first minutes of a
      long video), or

   o  fetching specific parts of a resource (similar to byte range
      requests), or

   o  hide information from the secondary server.

   Another benefit might be that it would allow the origin server to
   only serve the first part of a resource itself (reducing time to play
   of a media resource), while delegating the remainder to a cache
   (however, this might require further adjustments of the 'out-of-band'
   payload format).

B.4.  Relation to Content Encryption

   Right now this specification is orthogonal to [ENCRYPTENC]/[MICE];
   that is, it could be used for public content such as software
   downloads.  However, the lack of mandatory encryption affects the
   security considerations (which currently try to rule attack vectors
   caused by ambient authority ([RFC6265], Section 8.2).  We need to
   decide whether we need this level of independance.

B.5.  Reporting

   This specification already defines hooks through which a client can
   report failures when accessing secondary resources (see Section 3.4).



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   However, it would be useful if there were also ways to report on
   statistics such as:

   o  Success (Cache Hit) rates, and

   o  Bandwidth to secondary servers.

   This could be implemented using a new service endpoint and a (JSON?)
   payload format.

   Similarly, a reporting facility for use by the secondary servers
   could be useful.

Appendix C.  Change Log (to be removed by RFC Editor before publication)

C.1.  Changes since draft-reschke-http-oob-encoding-00

   Mention media type approach.

   Explain that clients can always fall back not to use oob when the
   secondary resource isn't available.

   Add Vary response header field to examples and mention that it'll
   usually be needed
   (<https://github.com/reschke/oobencoding/issues/6>).

   Experimentally add problem reporting using piggy-backed Link header
   fields (<https://github.com/reschke/oobencoding/issues/7>).

C.2.  Changes since draft-reschke-http-oob-encoding-01

   Updated ENCRYPTENC reference.

C.3.  Changes since draft-reschke-http-oob-encoding-02

   Add MICE reference.

   Remove the ability of the secondary resource to contain anything but
   the payload (<https://github.com/reschke/oobencoding/issues/11>).

   Changed JSON payload to be an object containing an array of URIs plus
   additional members.  Specify "fallback" as one of these additional
   members, and update Appendix B.1 accordingly).

   Discuss extensibility a bit.






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C.4.  Changes since draft-reschke-http-oob-encoding-03

   Mention "Content Stealing" thread.

   Mention padding.

C.5.  Changes since draft-reschke-http-oob-encoding-04

   Reduce information leakage by disallowing ambient authority
   information being sent to the secondary resource.  Require "Origin"
   to be included in request to secondary resource, and require seconday
   server to check it.

   Mention "Origin" + server check on secondary resource as defense to
   content stealing.

   Update ENCRYPTENC reference, add SCD reference.

   Mention fragmentation feature.

   Discuss relation with range requests.

C.6.  Changes since draft-reschke-http-oob-encoding-05

   Remove redundant Cache-Control: private from one example response
   (the response payload is encrypted anyway).

   Mention looping.

   Remove 'metadata' payload element.

   Align with changes in ENCRYPTENC spec.

   Fix incorrect statement about what kind of cookies/credentials can be
   used in the request to the secondary resource.

   Rename "URIs" to "sr" ("secondary resources") and treat the fallback
   URI like a regular secondary resource.

   Mention reporting protocol ideas.

Appendix D.  Acknowledgements

   Thanks to Christer Holmberg, Daniel Lindstrom, Erik Nygren, Goran
   Eriksson, John Mattsson, Kevin Smith, Magnus Westerlund, Mark
   Nottingham, Martin Thomson, and Roland Zink for feedback on this
   document.




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Authors' Addresses

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/


   Salvatore Loreto
   Ericsson
   Torshamnsgatan 21
   Stochholm  16483
   Sweden

   EMail: salvatore.loreto@ericsson.com
































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