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INTERNET DRAFT                                      Michiel B. de Jong
Document: draft-dejong-remotestorage-03                  (independent)
                                                             F. Kooman
Intended Status: Proposed Standard                       (independent)
Expires: 15 December 2014                                 13 June 2014


                            remoteStorage

Abstract

    This draft describes a protocol by which client-side applications,
    running inside a web browser, can communicate with a data storage
    server that is hosted on a different domain name. This way, the
    provider of a web application need not also play the role of data
    storage provider. The protocol supports storing, retrieving, and
    removing individual documents, as well as listing the contents of an
    individual folder, and access control is based on bearer tokens.

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."

   This Internet-Draft will expire on 15 December 2014.

Copyright Notice

   Copyright (c) 2013 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.


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Table of Contents

   1. Introduction...................................................2
   2. Terminology....................................................3
   3. Storage model..................................................3
   4. Requests.......................................................4
   5. Response codes.................................................6
   6. Versioning.....................................................7
   7. CORS headers...................................................8
   8. Session description............................................8
   9. Bearer tokens and access control...............................9
  10. Application-first bearer token issuance........................9
  11. Storage-first bearer token issuance...........................11
  12. Example wire transcripts......................................12
     12.1. WebFinger................................................12
     12.2. OAuth dialog form........................................13
     12.3. OAuth dialog form submission.............................13
     12.4. OPTIONS preflight........................................14
     12.5. Initial PUT..............................................14
     12.6. Subsequent PUT...........................................15
     12.7. GET......................................................15
     12.8. DELETE...................................................16
  13. Distributed versioning........................................16
  14. Security Considerations.......................................18
  15. IANA Considerations...........................................19
  16. Acknowledgments...............................................19
  17. References....................................................19
     17.1. Normative References.....................................19
     17.2. Informative References...................................19
  18. Authors' addresses............................................20


1.  Introduction

    Many services for data storage are available over the internet. This
    specification describes a vendor-independent interface for such
    services. It is based on https, CORS and bearer tokens. The
    metaphor for addressing data on the storage is that of folders
    containing documents and subfolders. The actions the interface
    exposes are:

       *  GET a folder: retrieve the names and current versions of the
          documents and subfolders currently contained by the folder


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       *  GET a document: retrieve its content type, current version,
          and contents

       *  PUT a document: store a new version, its content type, and
          contents, conditional on the current version

       *  DELETE a document: remove it from the storage, conditional on
          the current version

       *  HEAD a folder or document: like GET, but omitting the response
          body

    The exact details of these four actions are described in this
    specification.

2. Terminology

    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 RFC 2119 [WORDS].

    "SHOULD" and "SHOULD NOT" are appropriate when valid exceptions to a
    general requirement are known to exist or appear to exist, and it is
    infeasible or impractical to enumerate all of them.  However, they
    should not be interpreted as permitting implementors to fail to
    implement the general requirement when such failure would result in
    interoperability failure.

3. Storage model

    The server stores data in nodes that form a tree structure.
    Internal nodes are called 'folders' and leaf nodes are called
    'documents'. For a folder, the server stores references to nodes
    contained in the folder, and it should be able to produce a list of
    them, with for each contained item:

       * item name
       * item type (folder or document)
       * current version
       * content type
       * content length




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    For a document, the server stores, and should be able to produce:

       * current version
       * content type
       * content length
       * content

4. Requests

    Client-to-server requests SHOULD be made over https [HTTPS], and
    servers MUST comply with HTTP/1.1 [HTTP]. Specifically, they
    MUST support chunked transfer coding on PUT requests. Servers MAY
    also offer an optional switch from https to SPDY [SPDY].

    The root folder of the storage tree is represented by the following
    URL:

        URI_ENCODE( <storage_root> '/' )

    Subsequently, if <parent_folder> is the URL of a folder, then the
    URL of an item contained in it is:

        URI_ENCODE( <parent_folder> <document_name> )

    for a document, or:

        URI_ENCODE( <parent_folder> <folder_name> '/' )

    for a folder. Item names MAY contain all characters except '/' and
    the null character, and MUST NOT have zero length.

    A document description is a map containing one string-valued 'ETag'
    field, one string-valued 'Content-Type' and one integer-valued
    'Content-Length' field. They represent the document's current
    version, its content type, and its content length respectively. Note
    that content length is measured in octets (bytes), not in
    characters.

    A folder description is a map containing a string-valued 'ETag'
    field, representing the folder's current version.

    A successful GET request to a folder SHOULD be responded to with a
    JSON-LD [JSON-LD] document (content type 'application/ld+json'),


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    containing as its 'items' field a map in which contained documents
    appear as entries <item_name> to a document description, and
    contained non-empty folders appear as entries <item_name> '/' to a
    folder description. It SHOULD also contain an '@context' field with
    the value 'http://remotestorage.io/spec/folder-description'. For
    instance:

       {
         "@context": "http://remotestorage.io/spec/folder-description",
         "items": {
           "abc": {
             "ETag": "DEADBEEFDEADBEEFDEADBEEF",
             "Content-Type": "image/jpeg",
             "Content-Length": 82352
           },
           "def/": {
             "ETag": "1337ABCD1337ABCD1337ABCD"
           }
         }
       }

    All folders are treated as existing, and therefore GET requests to
    untouched folders SHOULD be responded to with a folder description
    with no items (the items field set to '{}'). However, an empty
    folder MUST NOT be listed as an item in its parent folder.

    Also, since folders exist automatically, PUT and DELETE requests
    only need to be made to documents, and never to folders. A document
    PUT will make all ancestor folders along its path become non-empty;
    deleting the last document from a subtree will make that whole
    subtree become empty. Folders will therefore show up in their parent
    folder descriptions if and only if their subtree contains at least
    one document.

    A successful GET request to a document SHOULD be responded to with
    the full document contents in the body, the document's content type
    in a 'Content-Type' header, its content length in octets (not in
    characters) in a 'Content-Length' header, and the document's current
    version as a strong ETag in an 'ETag' header.

    Note that the use of strong ETags prohibits changing the response
    body based on request headers; in particular, the server will not be
    able to serve the same document uncompressed to some clients and


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    gzipped when requested by the client, since the two bodies would not
    be identical byte-for-byte.

    Servers MAY support Content-Range headers [HTTP] on GET requests,
    but whether or not they do SHOULD be announced through the <ranges>
    variable mentioned below in section 10.

    A successful PUT request to a document MUST result in:

       * the request body being stored as the document's new content,
       * parent and further ancestor folders being silently created as
         necessary, with the document (name and version) being added to
         its parent folder, and each folder added to its subsequent
         parent,
       * the value of its Content-Type header being stored as the
         document's new content type,
       * its version being updated, as well as that of its parent folder
         and further ancestor folders, using a strong validator [HTTP,
         section 13.3.3].

    The response MUST contain a strong ETag header, with the document's
    new version (for instance a hash of its contents) as its value.

    A successful DELETE request to a document MUST result in:

       * the deletion of that document from the storage, and from its
         parent folder,
       * silent deletion of the parent folder if it is left empty by
         this, and so on for further ancestor folders,
       * the version of its parent folder being updated, as well as that
         of further ancestor folders.

    A successful OPTIONS request SHOULD be responded to as described in
    the CORS section below.

    A successful HEAD request SHOULD be responded to like to the
    equivalent GET request, but omitting the response body.

5. Response codes

    Response codes SHOULD be given as defined by [HTTP, section 10] and
    [BEARER, section 3.1]. The following is a non-normative checklist
    of status codes that are likely to occur in practice:


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       * 500 if an internal server error occurs,
       * 429 if the client makes too frequent requests or is suspected
             of malicious activity,
       * 414 if the request URI is too long,
       * 416 if Range requests are supported by the server and the Range
             request can not be satisfied,
       * 401 for all requests that don't have a bearer token with
             sufficient permissions,
       * 404 for all DELETE and GET requests to nodes that do not exist
             on the storage,
       * 304 for a conditional GET request whose pre-condition
             fails (see "Versioning" below),
       * 409 for a PUT request where any folder name in the path
             clashes with an existing document's name at the same
             level, or where the document name coincides with an
             existing folder's name at the same level.
       * 412 for a conditional PUT or DELETE request whose pre-condition
             fails (see "Versioning" below),
       * 507 in case the user's account is over its storage quota,
       * 4xx for all malformed requests (e.g. foreign characters in the
             path), as well as for all PUT and DELETE requests to
             folders,
       * 2xx for all successful requests.

    Clients SHOULD also handle the case where a response takes too long
    to arrive, or where no response is received at all.

6. Versioning

    All successful requests MUST return an 'Expires: 0' header, and an
    'ETag' header [HTTP] with, in the case of GET, the current version,
    in the case of PUT, the new version, and in case of DELETE, the
    version that was deleted. PUT and DELETE requests MAY have an
    'If-Match' request header [HTTP], and MUST fail with a 412 response
    code if that doesn't match the document's current version.

    GET requests MAY have a comma-separated list of revisions in an
    'If-None-Match' header [HTTP], and SHOULD be responded to with a 412
    response if that list includes the document or folder's current
    version. A PUT request MAY have an 'If-None-Match: *' header [HTTP],
    in which case it MUST fail with a 412 response code if the document
    already exists.


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    In all 'ETag', 'If-Match' and 'If-None-Match' headers, revision
    strings should appear inside double quotes (").

    A provider MAY offer version rollback functionality to its users,
    but this specification does not define the user interface for that.

7. CORS headers

    All responses MUST carry CORS headers [CORS]. The server MUST also
    reply to OPTIONS requests as per CORS. For GET requests, a wildcard
    origin MAY be returned, but for PUT and DELETE requests, the
    response MUST echo back the Origin header sent by the client.

8. Session description

    The information that a client needs to receive in order to be able
    to connect to a server SHOULD reach the client as described in the
    'bearer token issuance' sections below. It consists of:

       * <storage_root>, consisting of 'https://' followed by a server
         host, and optionally a server port and a path prefix as per
         [IRI]. Examples:
         * 'https://example.com' (host only)
         * 'https://example.com:8080' (host and port)
         * 'https://example.com/path/to/storage' (host, port and
           path prefix; note there is no trailing slash)
       * <access_token> as per [OAUTH]. The token SHOULD be hard to
         guess and SHOULD NOT be reused from one client to another. It
         can however be reused in subsequent interactions with the same
         client, as long as that client is still trusted. Example:
         * 'ofb24f1ac3973e70j6vts19qr9v2eei'
       * <storage_api>, always 'draft-dejong-remotestorage-03' for this
         alternative version of the specification.

    The client can make its requests using https with CORS and bearer
    tokens, to the URL that is the concatenation of <storage_root> with
    '/' plus one or more <folder> '/' strings indicating a path in the
    folder tree, followed by zero or one <document> strings, indicating
    a document. For example, if <storage_root> is
    "https://storage.example.com/bob", then to retrieve the folder
    contents of the /public/documents/ folder, or to retrieve a
    'draft.txt' document from that folder, the client would make


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    requests to, respectively:

    * https://storage.example.com/bob/public/documents/
    * https://storage.example.com/bob/public/documents/draft.txt

9. Bearer tokens and access control

    A bearer token represents one or more access scopes. These access
    scopes are represented as strings of the form <module> <level>,
    where the <module> string SHOULD be lower-case alphanumerical, other
    than the reserved word 'public', and <level> can be ':r' or ':rw'.
    The access the bearer token gives is the sum of its access scopes,
    with each access scope representing the following permissions:

    '*:rw') any request,

    '*:r') any GET or HEAD request,

    <module> ':rw') any requests to paths that start with
                    '/' <module> '/' or '/public/' <module> '/',

    <module> ':r') any GET or HEAD requests to paths that start with
                   '/' <module> '/' or '/public/' <module> '/',

    As a special exceptions, GET requests to a document (but not a
    folder) whose path starts with '/public/' are always allowed. They,
    as well as OPTIONS requests, can be made without a bearer token.
    Unless [KERBEROS] is used (see section 10 below), all other requests
    SHOULD present a bearer token with sufficient access scope, using a
    header of the following form (no double quotes here):

       Authorization: Bearer <access_token>

    In addition, providing the access token via a HTTP query parameter
    for GET requests MAY be supported by the server, although its use
    is not recommended, due to its security deficiencies; see [BEARER,
    section 2.3].

10. Application-first bearer token issuance

    To make a remoteStorage server available as 'the remoteStorage of
    <user> at <host>', exactly one link of the following format SHOULD
    be added to the webfinger record [WEBFINGER] of <user> at <host>:


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    {
      "href": <storage_root>,
      "rel": "remotestorage",
      "properties": {
        "http://remotestorage.io/spec/version": <storage_api>,
        "http://tools.ietf.org/html/rfc6749#section-4.2": <auth-dialog>,
        "http://tools.ietf.org/html/rfc6750#section-2.3": <query-param>,
        "https://tools.ietf.org/html/rfc2616#section-14.16": <ranges>
      }
    }

    Here <storage_root> and <storage_api> are as per "Session
    description" above, and <auth-dialog> SHOULD be eihter the boolean
    value false or a URL where an OAuth 2.0 implicit-grant flow dialog
    [OAUTH] is presented.

    If <auth-dialog> is a URL, the user can supply their credentials
    there (how, is out of scope), and allow or reject a request by the
    connecting application to obtain a bearer token for a certain list
    of access scopes.

    If <auth-dialog> is false, the client will not have a way to obtain
    an access token, and SHOULD send all requests without Authorization
    header, and rely on Kerberos [KERBEROS] instead for requests that
    would normally be sent with a bearer token, but servers SHOULD NOT
    impose any such access barriers for resources that would normally
    not require an access token.

    The <query-param> variable SHOULD have the boolean value true if
    the server supports passing the bearer token in the URI query
    parameter as per section 2.3 of [BEARER], and false otherwise.

    The <ranges> variable SHOULD have a string value of "GET" if
    Content-Range headers are supported for GET requests as per
    [HTTP, section 14.16], and the boolean value false if not.

    The server MAY expire bearer tokens, and MAY require the user to
    register applications as OAuth clients before first use; if no
    client registration is required, then the server MAY ignore the
    client_id parameter in favor of relying on the redirect_uri
    parameter for client identification.



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11. Storage-first bearer token issuance

    The provider MAY also present a dashboard to the user, where they
    have some way to add open web app manifests [MANIFEST]. Adding a
    manifest to the dashboard is considered equivalent to clicking
    'accept' in the dialog of the application-first flow. Removing one
    is considered equivalent to revoking its access token.

    As an equivalent to OAuth's 'scope' parameter, a 'datastores-access'
    field SHOULD be present in the root of such an application manifest
    document, with entries <module> -> '{"access": "readonly"}' for
    <level> 'r' or '{"access": "readwrite"}' for <level> 'rw', as
    prescribed in [DATASTORE].

    When the user gestures they want to use a certain application whose
    manifest is present on the dashboard, the dashboard SHOULD redirect
    to the application or open it in a new window. To mimic coming back
    from the OAuth dialog, it MAY add 'access_token' and 'scope'
    fields to the URL fragment.

    Regardless of whether 'access_token' and 'scope' are specified, it
    SHOULD add a 'remotestorage' field to the URL fragment, with a
    value of the form <user> '@' <host>. When the application detects
    this parameter, it SHOULD resolve the webfinger record for <user> at
    <host> and extract the <storage_root> and <storage_api> information.

    If no access_token was given, then the application SHOULD also
    extract the <auth_endpoint> information from webfinger, and continue
    as per application-first bearer token issuance.

    Note that whereas a remoteStorage server SHOULD offer support of the
    application-first flow with webfinger and OAuth, it MAY choose not
    to support the storage-first flow, provided that users will easily
    remember their <user> '@' <host> webfinger address at that provider.
    Applications SHOULD, however, support both flows, which means
    checking the URL for a 'remotestorage' parameter, but giving the
    user a way to specify their webfinger address if there is none.

    If a server provides an application manifest dashboard, then it
    SHOULD merge the list of applications there with the list of
    issued access tokens as specified by OAuth into one list. Also,
    the interface for revoking an access token as specified by OAuth


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    SHOULD coincide with removing an application from the dashboard.

    Servers MAY also provide a way to create access tokens directly from
    their user interface. Such functionality would be aimed mainly at
    developers, to manually copy and paste a token into a script or
    debug tool, thus bypassing the need for an OAuth dance. Clients
    SHOULD NOT rely on this in production.

12. Example wire transcripts

    The following examples are not normative ("\" indicates a line was
    wrapped).

12.1. WebFinger

    In application-first, an in-browser application might issue the
    following request, using XMLHttpRequest and CORS:

         GET /.well-known/webfinger?resource=acct:michiel@michielbdejon\
g.com HTTP/1.1
         Host: michielbdejong.com

    and the server's response might look like this:

         HTTP/1.1 200 OK
         Access-Control-Allow-Origin: *
         Access-Control-Allow-Methods: GET
         Access-Control-Allow-Headers: If-Match, If-None-Match
         Access-Control-Expose-Headers: ETag, Content-Type, Content-Len\
gth

         {
           "links":[{
             "href": "https://michielbdejong.com:7678/inbox",
             "rel": "post-me-anything"
           }, {
             "href": "https://michielbdejong.com/me.jpg",
             "rel": "avatar"
           }, {
             "href": "https://3pp.io:4439/storage/michiel",
             "rel": "remotestorage",
             "properties": {
               "http://remotestorage.io/spec/version": "draft-dejong-re\


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motestorage-03",
               "http://tools.ietf.org/html/rfc6750#section-4.2": "https\
://3pp.io:4439/oauth/michiel",
               "http://tools.ietf.org/html/rfc6750#section-2.3": false,
               "http://tools.ietf.org/html/rfc2616#section-14.16": false
             }
           }]
         }

12.2. OAuth dialog form

    Once the in-browser application has discovered the server's OAuth
    end-point, it will typically redirect the user to this URL, in
    order to obtain a bearer token. Say the application is hosted on
    https://drinks-unhosted.5apps.com/ and wants read-write access to
    the user's "myfavoritedrinks" scope:

        GET /oauth/michiel?redirect_uri=https%3A%2F%2Fdrinks-unhosted.5\
apps.com%2F&scope=myfavoritedrinks%3Arw&client_id=https%3A%2F%2Fdrinks-\
unhosted.5apps.com&response_type=token HTTP/1.1
        Host: 3pp.io

    The server's response might look like this (truncated for brevity):

        HTTP/1.1 200 OK

        <!DOCTYPE html>
        <html lang="en">
          <head>
            <title>Allow access?</title>
        ...


12.3. OAuth dialog form submission

    When the user submits the form, the request would look something
    like this:

        POST /oauth HTTP/1.1
        Host: 3pp.io:4439
        Origin: https://3pp.io:4439
        Content-Type: application/x-www-form-urlencoded
        Referer: https://3pp.io:4439/oauth/michiel?redirect_uri=https%3\


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A%2F%2Fdrinks-unhosted.5apps.com%2F&scope=myfavoritedrinks%3Arw&client_\
id=https%3A%2F%2Fdrinks-unhosted.5apps.com&response_type=token

        client_id=https%3A%2F%2Fdrinks-unhosted.5apps.com&redirect_uri=\
https%3A%2F%2Fdrinks-unhosted.5apps.com%2F&response_type=token&scope=my\
favoritedrinks%3Arw&state=&username=michiel&password=something&allow=Al\
low

    To which the server could respond with a 302 redirect, back to the
    origin of the requesting application:

        HTTP/1.1 302 Found
        Location:https://drinks-unhosted.5apps.com/#access_token=j2YnGt\
XjzzzHNjkd1CJxoQubA1o%3D&token_type=bearer&state=

12.4. OPTIONS preflight

    When an in-browser application makes a cross-origin request which
    may affect the server-state, the browser will make a preflight
    request first, with the OPTIONS verb, for instance:

        OPTIONS /storage/michiel/myfavoritedrinks/ HTTP/1.1
        Host: 3pp.io:4439
        Access-Control-Request-Method: GET
        Origin: https://drinks-unhosted.5apps.com
        Access-Control-Request-Headers: Authorization
        Referer: https://drinks-unhosted.5apps.com/

    To which the server can for instance respond:

        HTTP/1.1 200 OK
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        Access-Control-Allow-Methods: GET, PUT, DELETE
        Access-Control-Allow-Headers: Authorization, Content-Length, Co\
ntent-Type, Origin, X-Requested-With, If-Match, If-None-Match

12.5. Initial PUT

    An initial PUT may contain an 'If-None-Match: *' header, like this:

        PUT /storage/michiel/myfavoritedrinks/test HTTP/1.1
        Host: 3pp.io:4439
        Content-Length: 91


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        Origin: https://drinks-unhosted.5apps.com
        Authorization: Bearer j2YnGtXjzzzHNjkd1CJxoQubA1o=
        Content-Type: application/json; charset=UTF-8
        Referer: https://drinks-unhosted.5apps.com/?
        If-None-Match: *

        {"name":"test","@context":"http://remotestorage.io/spec/modules\
/myfavoritedrinks/drink"}

    And the server may respond with either a 201 Created or a 200 OK
    status:

        HTTP/1.1 201 Created
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "1382694045000"

12.6. Subsequent PUT

    A subsequent PUT may contain an 'If-Match' header referring to the
    ETag previously returned, like this:

        PUT /storage/michiel/myfavoritedrinks/test HTTP/1.1
        Host: 3pp.io:4439
        Content-Length: 91
        Origin: https://drinks-unhosted.5apps.com
        Authorization: Bearer j2YnGtXjzzzHNjkd1CJxoQubA1o=
        Content-Type: application/json; charset=UTF-8
        Referer: https://drinks-unhosted.5apps.com/?
        If-Match: "1382694045000"

        {"name":"test", "updated":true, "@context":"http://remotestorag\
e.io/spec/modules/myfavoritedrinks/drink"}

    And the server may respond with a 412 Conflict or a 200 OK status:

        HTTP/1.1 200 OK
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "1382694048000"

12.7. GET

    A GET request would also include the bearer token, and optionally
    an If-None-Match header:


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        GET /storage/michiel/myfavoritedrinks/test HTTP/1.1
        Host: 3pp.io:4439
        Origin: https://drinks-unhosted.5apps.com
        Authorization: Bearer j2YnGtXjzzzHNjkd1CJxoQubA1o=
        Referer: https://drinks-unhosted.5apps.com/?
        If-None-Match: "1382694045000", "1382694048000"

        {"name":"test", "updated":true, "@context":"http://remotestora\
ge.io/spec/modules/myfavoritedrinks/drink"}

    And the server may respond with a 304 Not Modified or a 200 OK
    status:

        HTTP/1.1 304 Not Modified
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "1382694048000"

12.8. DELETE

    A DELETE request may look like this:

        DELETE /storage/michiel/myfavoritedrinks/test HTTP/1.1
        Host: 3pp.io:4439
        Origin: https://drinks-unhosted.5apps.com
        Authorization: Bearer j2YnGtXjzzzHNjkd1CJxoQubA1o=
        Content-Type: application/json; charset=UTF-8
        Referer: https://drinks-unhosted.5apps.com/?
        If-Match: "1382694045000"

    And the server may respond with a 412 Conflict or a 200 OK status:

        HTTP/1.1 412 Conflict
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "1382694048000"

13. Distributed versioning

    This section is non-normative, and is intended to explain some of
    the design choices concerning ETags and folder listings. At the
    same time it will hopefully help readers who intend to develop an
    application that uses remoteStorage as its per-user data storage.
    When multiple clients have read/write access to the same document,


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    versioning conflicts may occur. For instance, client A may make
    a PUT request that changes the document from version 1 to version
    2, after which client B may make a PUT request attempting to change
    the same document from version 1 to version 3.

    In this case, client B can add an 'If-Match: "1"' header, which
    would trigger a 412 Conflict response code, since the current
    version ("2") does not match the version required as a condition by
    the header If-Match header ("1").

    Client B is now aware of the conflict, and may consult the user,
    saying the update to version 3 failed. The user may then choose,
    through the user interface of client B, whether version 2 or
    version 3 should be kept, or maybe the document should be reverted
    on the server to version 1, or a merged version 4 is needed. Client
    B may then make a request that puts the document to the version the
    user wishes; this time setting an 'If-Match: "2"' header instead.

    Both client A and client B would periodically poll the root
    folder of each scope they have access to, to see if the version
    of the root folder changed. If it did, then one of the versions
    listed in there will necessarily have changed, and the client can
    make a GET request to that child folder or document, to obtain
    its latest version.

    Because an update in a document will result in a version change of
    its containing folder, and that change will propagate all the way
    to the root folder, it is not necessary to poll each document for
    changes individually.

    As an example, the root folder may contain 10 directories,
    each of which contain 10 directories, which each contain 10
    documents, so their paths would be for instance '/0/0/1', '/0/0/2',
    etcetera. Then one GET request to the root folder '/' will be
    enough to know if any of these 1000 documents has changed.

    Say document '/7/9/2' has changed; then the GET request to '/' will
    come back with a different ETag, and entry '7/' will have a
    different value in its JSON content. The client could then request
    '/7/', '/7/9/', and '/7/9/2' to narrow down the one document that
    caused the root folder's ETag to change.

    Note that the remoteStorage server does not get involved in the


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    conflict resolution. It keeps the canonical current version at all
    times, and allows clients to make conditional GET and PUT requests,
    but it is up to whichever client discovers a given version
    conflict, to resolve it.

14. Security Considerations

    To prevent man-in-the-middle attacks, the use of https instead of
    http is important for both the interface itself and all end-points
    involved in webfinger, OAuth, and (if present) the storage-first
    application launch dashboard.

    A malicious party could link to an application, but specifying a
    remoteStorage user address that it controls, thus tricking the user
    into using a trusted application to send sensitive data to the wrong
    remoteStorage server. To mitigate this, applications SHOULD clearly
    display to which remoteStorage server they are sending the user's
    data.

    Applications could request scopes that the user did not intend to
    give access to. The user SHOULD always be prompted to carefully
    review which scopes an application is requesting.

    An application may upload malicious html pages and then trick the
    user into visiting them, or upload malicious client-side scripts,
    that take advantage of being hosted on the user's domain name. The
    origin on which the remoteStorage server has its interface SHOULD
    therefore NOT be used for anything else, and the user SHOULD be
    warned not to visit any web pages on that origin. In particular, the
    OAuth dialog and launch dashboard or token revokation interface
    SHOULD be on a different origin than the remoteStorage interface.

    Where the use of bearer tokens is impractical, a user may choose to
    store documents on hard-to-guess URLs whose path after
    <storage_root> starts with '/public/', while sharing this URL only
    with the intended audience. That way, only parties who know the
    document's hard-to-guess URL, can access it. The server SHOULD
    therefore make an effort to detect and stop brute-force attacks that
    attempt to guess the location of such documents.

    The server SHOULD also detect and stop denial-of-service attacks
    that aim to overwhelm its interface with too much traffic.



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15. IANA Considerations

    This document registers the 'remotestorage' link relation, as well
    as the following WebFinger properties:
      * "http://remotestorage.io/spec/version"
      * "http://tools.ietf.org/html/rfc6749#section-4.2"
      * "http://tools.ietf.org/html/rfc6750#section-2.3"
      * "https://tools.ietf.org/html/rfc2616#section-14.16

16. Acknowledgements

    The authors would like to thank everybody who contributed to the
    development of this protocol, including Kenny Bentley, Javier Diaz,
    Daniel Groeber, Bjarni Runar, Jan Wildeboer, Charles Schultz, Peter
    Svensson, Valer Mischenko, Michiel Leenaars, Jan-Christoph
    Borchardt, Garret Alfert, Sebastian Kippe, Max Wiehle, Melvin
    Carvalho, Martin Stadler, Geoffroy Couprie, Niklas Cathor, Marco
    Stahl, James Coglan, Ken Eucker, Daniel Brolund, elf Pavlik, Nick
    Jennings, Markus Sabadello, Steven te Brinke, Matthias Treydte,
    Rick van Rein, Mark Nottingham, Julian Reschke, and Markus
    Lanthaler, among many others.

17. References

17.1. Normative References

    [WORDS]
        Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

    [IRI]
        Duerst, M., "Internationalized Resource Identifiers (IRIs)",
        RFC 3987, January 2005.
    [WEBFINGER]
        Jones, P., Salguerio, G., Jones, M, and Smarr, J.,
        "WebFinger", RFC7033, September 2013.

    [OAUTH]
        "Section 4.2: Implicit Grant", in: Hardt, D. (ed), "The OAuth
        2.0 Authorization Framework", RFC6749, October 2012.

17.2. Informative References



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    [HTTPS]
        Rescorla, E., "HTTP Over TLS", RFC2818, May 2000.
    [HTTP]
        Fielding et al., "Hypertext Transfer Protocol -- HTTP/1.1",
        RFC2616, June 1999.

    [SPDY]
        Mark Belshe, Roberto Peon, "SPDY Protocol - Draft 3.1", http://
        www.chromium.org/spdy/spdy-protocol/spdy-protocol-draft3-1,
        September 2013.

    [JSON-LD]
        M. Sporny, G. Kellogg, M. Lanthaler, "JSON-LD 1.0", W3C
        Proposed Recommendation,
        http://www.w3.org/TR/2014/REC-json-ld-20140116/, January 2014.

    [CORS]
        van Kesteren, Anne (ed), "Cross-Origin Resource Sharing --
        W3C Candidate Recommendation 29 January 2013",
        http://www.w3.org/TR/cors/, January 2013.

    [MANIFEST]
        Mozilla Developer Network (ed), "App manifest -- Revision
        330541", https://developer.mozilla.org/en-
        US/Apps/Build/Manifest$revision/566677, April 2014.

    [DATASTORE]
        "WebAPI/DataStore", MozillaWiki, retrieved May 2014.
        https://wiki.mozilla.org/WebAPI/DataStore#Manifest

    [KERBEROS]
        C. Neuman et al., "The Kerberos Network Authentication Service
        (V5)", RFC4120,
        https://tools.ietf.org/html/rfc4120

    [BEARER]
        M. Jones, D. Hardt, "The OAuth 2.0 Authorization Framework:
        Bearer Token Usage", RFC6750,
        http://tools.ietf.org/html/rfc6750#section-2.3, October 2012.






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18. Authors' addresses

    Michiel B. de Jong
    (independent)

    Email: michiel@michielbdejong.com


    F. Kooman
    (independent)

    Email: fkooman@tuxed.net


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