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Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                               S. Loreto
Expires: July 3, 2016                                           Ericsson
                                                       December 31, 2015


                 'Out-Of-Band' Content Coding for HTTP
                   draft-reschke-http-oob-encoding-02

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.2.

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 July 3, 2016.

Copyright Notice

   Copyright (c) 2015 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
   to this document.  Code Components extracted from this document must
   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.

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.  Problem Reporting  . . . . . . . . . . . . . . . . . . . .  6
       3.3.1.  Server Not Reachable . . . . . . . . . . . . . . . . .  7
       3.3.2.  Resource Not Found . . . . . . . . . . . . . . . . . .  7
       3.3.3.  Payload Unusable . . . . . . . . . . . . . . . . . . .  7
     3.4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.4.1.  Basic Example  . . . . . . . . . . . . . . . . . . . .  7
       3.4.2.  Example involving an encrypted resource  . . . . . . .  9
       3.4.3.  Example For Problem Reporting  . . . . . . . . . . . . 10
   4.  Feature Discovery  . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     5.1.  Content Modifications  . . . . . . . . . . . . . . . . . . 11
     5.2.  Use in Requests  . . . . . . . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Alternatives, or: why not a new Status Code?  . . . . 13
   Appendix B.  Open Issues . . . . . . . . . . . . . . . . . . . . . 13
     B.1.  Range Requests . . . . . . . . . . . . . . . . . . . . . . 13
     B.2.  Accessing the Secondary Resource Too Early . . . . . . . . 13
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 14
     C.1.  Changes since draft-reschke-http-oob-encoding-00 . . . . . 14
     C.2.  Changes since draft-reschke-http-oob-encoding-01 . . . . . 14



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   Appendix D.  Acknowledgements  . . . . . . . . . . . . . . . . . . 14


















































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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 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, leveraging content
   encryption, as described in [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 GET 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 wrapped HTTP message

   5.  Client unwraps that representation (obtaining a full HTTP
       message)

   6.  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) |---------------------------\
              wrapped HTTP message |
   (4) <---------------------------/

   (5, 6)
      Client and combines HTTP message 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 array of objects describing secondary resources, each
   containing some of the members below:

   'URI'  A REQUIRED string containing the URI reference (Section 4.1 of
      [RFC3986]) of the secondary resource.

   'metadata'  An OPTIONAL object containing additional members,
      representing header field values to be recombined with the
      metadata from the secondary resource and which can not appear as
      header fields in the response message itself (header fields that
      occur multiple times need to be combined into a single field value
      as per Section 3.2.2 of [RFC7230]; header field names are lower-
      cased).

   The payload format uses a JSON array so that the origin server can
   specify multiple secondary resources.  When a client receives a
   response containing multiple entries, it is free to choose which of
   these to use.

   The representation of the secondary resource needs to use a media
   type capable of representing a full HTTP message.  For now the only
   supported type is "application/http" (Section 8.3.2 of [RFC7230]).

   The client then obtains the original message by:






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

       The latter might require additional metadata that could be
       present in the "metadata" object, such as the "Crypto-Key" header
       field described in Section 4 of [ENCRYPTENC].

   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.

   3.  Replacing/setting any header fields with those present as members
       in the "metadata" object. [[anchor3: Do we have a use case for
       this?]]

   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), 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 3.3 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.

   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.

3.3.  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:





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3.3.1.  Server Not Reachable

   Used in case the server was not reachable.

   Link relation:

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

3.3.2.  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

3.3.3.  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

3.4.  Examples

3.4.1.  Basic Example

   Client request of primary resource:

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
















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   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: 76
     Vary: Accept-Encoding

     [{
       "URI": "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)

   Client request for secondary resource:

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


   Response:

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 18:52:10 GMT
     Content-Type: application/http
     Cache-Control: private
     Content-Length: 115

     HTTP/1.1 200 OK
     Date: Thu, 14 May 2015 17:00:00 GMT
     Content-Length: 15
     Content-Language: en

     Hello, world.














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   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
     Content-Language: en

     Hello, world.

   In this example, Cache-Control, Content-Length, and Date have been
   set/overwritten with data from the primary resource's representation.

3.4.2.  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" encoding 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: out-of-band
     Content-Type: text/plain
     Content-Length: 194
     Vary: Accept-Encoding

     [{
       "URI": "http://example.net/bae27c36-fa6a-11e4-ae5d-00059a3c7a00"
       "metadata": {
         "crypto-key": "keyid=\"a1\";
                            aesgcm128=\"csPJEXBYA5U-Tal9EdJi-w\""
       }
     }]

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











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   Response for secondary resource:

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

     HTTP/1.1 200 OK
     Content-Length: 32
     Content-Encoding: aesgcm128
     Encryption: keyid="a1"; salt="vr0o6Uq3w_KDWeatc27mUg"

     fuag8ThIRIazSHKUqJ5OduR75UgEUuM76J8UFwadEvg
   (payload body shown in base64 here)

   Final message after recombining header fields:

     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

3.4.3.  Example For Problem Reporting

   Client requests primary resource as in Section 3.4.1, 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"




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4.  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.

5.  Security Considerations

5.1.  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]).

5.2.  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).

6.  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

7.  References

7.1.  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",
                 STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,



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                 <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>.

   [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>.

7.2.  Informative References

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

   [ENCRYPTENC]  Thomson, M., "Encrypted Content-Encoding for HTTP",
                 draft-ietf-httpbis-encryption-encoding-00 (work in
                 progress), December 2015.

   [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>.



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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.

   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.  Range Requests

   We probably need to handle Range Requests.  How would this work?
   Passing down the Range request header field to the secondary
   resource?

   What about codes other than 200 and 206?

B.2.  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



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   content to it).

   In this particular case, it can be useful if the client was able to
   "piggyback" the URI of 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/test>;
           rel="http://purl.org/NET/linkrel/primary-resource"


   (continuing the example from Section 3.4.1)

   What's unclear is whether it's ok for the client to reveal the URI if
   the primary resource, and under which conditions it's ok for the
   secondary server to access it.  All it needs is the potentially
   encrypted payload, so maybe yet another URI on the origin server is
   needed.

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.

Appendix D.  Acknowledgements

   Thanks to Christer Holmberg, Daniel Lindstrom, Goran Eriksson, John
   Mattsson, Kevin Smith, 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
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: salvatore.loreto@ericsson.com
































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