source.txt

INTERNET DRAFT                                      Michiel B. de Jong
Document: draft-dejong-remotestorage-${VERSION}                  (independent)
                                                             F. Kooman
                                                         (independent)
                                                              S. Kippe
Intended Status: Proposed Standard                       (independent)
Expires: ${EXPIRES}                                  ${DATE}


                            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 ${EXPIRES}.

Copyright Notice

   Copyright (c) ${YEAR} 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...................................................2
   2. Terminology....................................................3
   3. Storage model..................................................3
   4. Requests.......................................................4
   5. Response codes.................................................7
   6. Versioning.....................................................8
   7. CORS headers...................................................8
   8. Session description............................................9
   9. Bearer tokens and access control...............................9
  10. Application-first bearer token issuance.......................10
  11. Storage-first bearer token issuance...........................12
  12. Example wire transcripts......................................12
     12.1. WebFinger................................................12
     12.2. OAuth dialog form........................................13
     12.3. OAuth dialog form submission.............................14
     12.4. OPTIONS preflight........................................14
     12.5. Initial PUT..............................................15
     12.6. Subsequent PUT...........................................15
     12.7. GET......................................................16
     12.8. DELETE...................................................17
  13. Distributed versioning........................................18
  14. Security Considerations.......................................19
  15. IANA Considerations...........................................20
  16. Acknowledgments...............................................20
  17. References....................................................20
     17.1. Normative References.....................................20
     17.2. Informative References...................................21
  18. Authors' addresses............................................22


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
       *  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 five 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 (ETag)

    The list also contains, for each document in the list:

       * content type (media type, a.k.a. MIME type)
       * content length
       * last-modified date

    Apart from this folder and document metadata, the server should also
    be able to produce the current content of each document.

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 to HTTP/2 [HTTP/2].

    A request is considered successful if the HTTP response code is in
    the 2xx range (e.g. 200 OK, 201 Created), and unsuccessful if an
    error occurred or a condition was not met, e.g. response code 404
    Not Found, 304 Not Modified.

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

        URI_ENCODE( <storage_root> '/' )

    Subsequently, let <parent_folder> be the URL of a folder, i.e. ends
    with a '/', 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.

    If a document with document_name <x> exists, then no folder with
    folder_name <x> can exist in the same parent folder, and vice versa.

    Item names MAY contain all characters, before URI_ENCODE, except '/'
    and the null character '\0' and MUST NOT have zero length. Item
    names MUST NOT be equal to '.' or to '..', as those have a special
    semantic in URIs (Section 5.2.4 of [URI]).

    A document description is a map containing one string-valued 'ETag'
    field, one string-valued 'Content-Type', one integer-valued
    'Content-Length' field, and one string-valued 'Last-Modified' field.
    They represent the document's current version, its content type, its
    content length, and last-modfied date respectively. The
    last-modified date MUST be formatted as HTTP-date (Section 7.1.1.1
    of [HTTP]). 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 MUST be responded to with a
    JSON-LD [JSON-LD] document (content type 'application/ld+json'),
    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 MUST 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,
             "Last-Modified": "Sat, 2 Jun 2018 15:58:23 GMT"
           },
           "def/": {
             "ETag": "1337ABCD1337ABCD1337ABCD"
           }
         }
       }


    GET requests to empty 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.

    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.

    In contexts outside of this document, non-empty folders may be
    called 'existent', while empty folders may be called 'non-existent'.

    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
    compressed to other clients when requested, since the two bodies
    would not be identical byte-for-byte.

    Servers MAY support Content-Range headers [RANGE] on GET requests,
    but whether or not they do SHOULD be announced both through the
    "http://tools.ietf.org/html/rfc7233" option mentioned below in
    section 10 and through the HTTP 'Accept-Ranges' response header.

    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 7.2].

    If no valid Content-Type header was received as part of a PUT
    request, the server MAY refuse to process the request, and instead
    respond with a descriptive error message in the body, as well as a
    http response code from the 4xx range.

    RemoteStorage does not place any restrictions on the value of
    Content-Type other than what is defined in [HTTP, section 3.1.1.5].

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

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

5. Response codes

    Response codes SHOULD be given as defined by [HTTP, section 6] and
    [BEARER, section 3.1]. The following is a non-normative list of
    status codes that are likely to occur in practice:

       * 2xx for all successful requests.
       * 304 for a conditional GET request whose precondition
             fails (see "Versioning" below),
       * 401 for all requests that require a valid bearer token and
             where no valid one was sent (see also [BEARER, section
             3.1]),
       * 403 for all requests that have insufficient scope, e.g.
             accessing a <module> for which no scope was obtained, or
             accessing data outside the user's <storage_root>,
       * 404 for all DELETE, GET and HEAD requests to documents that do
             not exist on the storage,
       * 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 precondition
             fails (see "Versioning" below),
       * 413 if the payload is too large, e.g. when the server has a
             maximum upload size for documents
       * 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,
       * 429 if the client makes too frequent requests or is suspected
             of malicious activity,
       * 4xx for all malformed requests, e.g. reserved characters in the
             path [URI, section 2.2], as well as for all PUT and DELETE
             requests to folders,
       * 500 if an internal server error occurred,
       * 507 in case the account is over its storage quota,

    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 GET, HEAD, PUT and DELETE requests MUST return an
    'ETag' header [HTTP] with, in the case of GET and HEAD the current
    version, in the case of PUT, the new version, and in case of DELETE,
    the version that was deleted. All successful GET requests MUST
    return a 'Cache-Control: no-cache' header. PUT and DELETE requests
    MAY have an 'If-Match' request header [COND], and MUST fail with a
    412 response code if that does not match the document's current
    version.

    GET requests MAY have a comma-separated list of revisions in an
    'If-None-Match' header [COND], and SHOULD be responded to with a 304
    response if that list includes the document or folder's current
    version. A PUT request MAY have an 'If-None-Match: *' header [COND],
    in which case it MUST fail with a 412 response code if the document
    already exists.

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

7. CORS headers

    All responses MUST carry CORS headers [CORS]. The server MUST also
    reply to preflight OPTIONS requests as per CORS.



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-${VERSION}' 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
    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 relative to <storage_root>
                    that start with '/' <module> '/' or
                    '/public/' <module> '/',

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

    As a special exceptions, GET and HEAD 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]. If supported, this SHOULD be announce through the
    "http://tools.ietf.org/html/rfc6750#section-2.3" WebFinger property
    as per section 10 below.

10. Application-first bearer token issuance

    To make a remoteStorage server available as 'the remoteStorage of
    the person identified by <uri>', exactly one link of the following
    format SHOULD be added to the WebFinger record [WEBFINGER] for
    <uri>:

    {
      "href": <storage_root>,
      "rel": "http://tools.ietf.org/id/draft-dejong-remotestorage",
      "properties": {
        "http://remotestorage.io/spec/version": <storage_api>,
        "http://tools.ietf.org/html/rfc6749#section-4.2": <auth-dialog>,
        "...": "...",
      }
    }

    A common way of identifying persons as <user> at <host> is through a
    URI of the format "acct:<user>@<host>". Persons who use a personal
    domain name, not shared with any other users, can be identified by
    a URI of the format "http://<host>/" (see [WEBFINGER, section 4.1]).

    Here <storage_root> and <storage_api> are as per "Session
    description" above, and <auth-dialog> SHOULD be either null 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
    for accessing the account (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. Note that an account
    will often belong to just one human user, but may also belong to a
    group of multiple users (the remoteStorage of <group> at <host>).

    If <auth-dialog> is null, 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 '...' ellipses indicate that more properties may be present.
    Non-breaking examples that have been proposed so far, include a
    "http://tools.ietf.org/html/rfc6750#section-2.3" property, set to
    the string value "true" if the server supports passing the bearer
    token in the URI query parameter as per section 2.3 of [BEARER],
    instead of in the request header.

    Another example is "http://tools.ietf.org/html/rfc7233" with a
    string value of "GET" if Content-Range headers are supported for
    GET requests as per [RANGE].

    Both these proposals are non-breaking extensions, since the client
    will have a way to work around it if these features are not present
    (e.g. retrieve the protected resource asynchronously in the first
    case, or request the entire resource in the second case).

    A "http://remotestorage.io/spec/web-authoring" property has been
    proposed with a string value of the fully qualified domain name to
    which web authoring content is published if the server supports web
    authoring as per [AUTHORING]. Note that this extension is a breaking
    extension in the sense that it divides users into "haves", whose
    remoteStorage accounts allow them to author web content, and
    "have-nots", whose remoteStorage account does not support this
    functionality.

    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, the server MUST ignore the value of
    the client_id parameter in favor of relying on the origin of the
    redirect_uri parameter for unique client identification. See section
    4 of [ORIGIN] for computing the origin.

11. Storage-first bearer token issuance

    To request that the application connects to the user account
    <account> ' ' <host>, providers MAY redirect to applications with a
    'remotestorage' field in the URL fragment, with the user account as
    value.

    The appplication MUST make sure this request is intended by the
    user. It SHOULD ask for confirmation from the user whether they want
    to connect to the given provider account. After confirmation, it
    SHOULD connect to the given provider account, as defined in Section
    10.

    If the 'remotestorage' field exists in the URL fragment, the
    application SHOULD ignore any other parameters such as
    'access_token' or 'state', to ensure compatibility with servers
    that implement older versions of this specification.

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: *
         Content-Type: application/jrd+json

         {
           "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": "http://tools.ietf.org/id/draft-dejong-remotestorag\
e",
             "properties": {
               "http://remotestorage.io/spec/version": "draft-dejong-re\
motestorage-${VERSION}",
               "http://tools.ietf.org/html/rfc6749#section-4.2": "https\
://3pp.io:4439/oauth/michiel",
               "http://tools.ietf.org/html/rfc6750#section-2.3": null,
               "http://tools.ietf.org/html/rfc7233": null,
               "http://remotestorage.io/spec/web-authoring": null
             }
           }]
         }

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 account'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\
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&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=j2YnG\
            tXjzzzHNjkd1CJxoQubA1o%3D&token_type=bearer

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
        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"

    In case the document already exists on the server, it would respond
    with something like:

        HTTP/1.1 412 Precondition Failed
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "2182694048000"

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 Precondition Failed or a
    200 OK status:

        HTTP/1.1 200 OK
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "2182694048000"

12.7. GET

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

        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"

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

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

    Or a 200 OK status, plus a response body:

        HTTP/1.1 200 OK
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        Content-Type: application/json; charset=UTF-8
        Content-Length: 106
        ETag: "1382694048000"
        Cache-Control: no-cache

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

    If the GET URL would have been "/storage/michiel/myfavoritedrinks/",
    a 200 OK response would have a folder description as the response
    body:

        HTTP/1.1 200 OK
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        Content-Type: application/ld+json
        Content-Length: 171
        ETag: "1382694048000"
        Cache-Control: no-cache

        {"@context":"http://remotestorage.io/spec/folder-version","ite\
ms":{"test":{"ETag":"1382694048000","Content-Type":"application/json; \
charset=UTF-8","Content-Length":106,"Last-Modified":"Sat, 2 Jun 2018 1\
5:58:23 GMT"}}}

    If the GET URL would have been a non-existing document like
    "/storage/michiel/myfavoritedrinks/x", the response would have a 404
    Not Found status, and no ETag header:

        HTTP/1.1 404 Not Found
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com

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 Precondition Failed or a 200
    OK status:

        HTTP/1.1 412 Precondition Failed
        Access-Control-Allow-Origin: https://drinks-unhosted.5apps.com
        ETag: "2182694048000"



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,
    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 Precondition Failed 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
    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 account 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 revocation 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 [CAPABILITIES] 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.

15. IANA Considerations

    This document registers 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"
      * "http://tools.ietf.org/html/rfc7233"
      * "http://remotestorage.io/spec/web-authoring"

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, Markus Lanthaler,
    and Markus Unterwaditzer, 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.

    [URI]
        Fielding, R., "Uniform Resource Identifier (URI): Generic
        Syntax", RFC 3986, 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.

    [ORIGIN]
        "Section 4: Origin of a URI", in: Barth, A., "The Web Origin
        Concept", RFC6454, December 2011.

17.2. Informative References

    [HTTPS]
        Rescorla, E., "HTTP Over TLS", RFC2818, May 2000.

    [HTTP]
        Fielding et al., "Hypertext Transfer Protocol (HTTP/1.1):
        Semantics and Content", RFC7231, June 2014.

    [COND]
        Fielding et al., "Hypertext Transfer Protocol (HTTP/1.1):
        Conditional Requests", RFC7232, June 2014.

    [RANGE]
        Fielding et al., "Hypertext Transfer Protocol (HTTP/1.1):
        Conditional Requests", RFC7233, June 2014.

    [HTTP/2]
        M. Belshe, R. Peon, M. Thomson, Ed. "Hypertext Transfer Protocol
        Version 2 (HTTP/2)", RFC7540, May 2015.

    [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.

    [KERBEROS]
        C. Neuman et al., "The Kerberos Network Authentication Service
        (V5)", RFC4120, July 2005.

    [BEARER]
        M. Jones, D. Hardt, "The OAuth 2.0 Authorization Framework:
        Bearer Token Usage", RFC6750, October 2012.

    [AUTHORING]
        "Using remoteStorage for web authoring", reSite wiki, retrieved
        September 2014. https://github.com/michielbdejong/resite/wiki
        /Using-remoteStorage-for-web-authoring

    [CAPABILITIES]
        J. Tennison (ed.), "Good Practices for Capability URLs",
        http://www.w3.org/TR/capability-urls/, February 2014.

18. Authors' addresses

    Michiel B. de Jong
    (independent)

    Email: michiel@unhosted.org


    F. Kooman
    (independent)

    Email: fkooman@tuxed.net


    S. Kippe
    (independent)

    Email: sebastian@kip.pe