INTERNET DRAFT Michiel B. de Jong Document: draft-dejong-remotestorage-07 (independent) F. Kooman Intended Status: Proposed Standard (independent) Expires: 29 November 2016 2 June 2016 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 29 November 2016. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect 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. de Jong [Page 1] Internet-Draft remoteStorage June 2016 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 de Jong [Page 2] Internet-Draft remoteStorage June 2016 * 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 * content type * content length For a document, the server stores, and should be able to produce: de Jong [Page 3] Internet-Draft remoteStorage June 2016 * 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 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( '/' ) Subsequently, let be the URL of a folder, i.e. ends with a '/', then the URL of an item contained in it is: URI_ENCODE( ) for a document, or: URI_ENCODE( '/' ) for a folder. 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' and one integer-valued 'Content-Length' field. They represent the document's current version, its content type, and its content length respectively. Note de Jong [Page 4] Internet-Draft remoteStorage June 2016 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 to a document description, and contained non-empty folders appear as entries '/' 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 }, "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'. de Jong [Page 5] Internet-Draft remoteStorage June 2016 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 not exactly one Content-Type header was received as part of a PUT request, the Content-Type header value contains non-ASCII characters, or it is unreasonably long, the server MAY refuse to process the request, and instead respond with a descriptive error message in the body, and a http response code from the 4xx range. 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 de Jong [Page 6] Internet-Draft remoteStorage June 2016 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: * 500 if an internal server error occurred, * 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 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 for which no scope was obtained, or accessing data outside the user's , * 404 for all DELETE, GET and HEAD requests to documents that do not exist on the storage, * 304 for a conditional GET request whose precondition 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 precondition fails (see "Versioning" below), * 507 in case the account is over its storage quota, * 4xx for all malformed requests, e.g. reserved characters in the path [URI, section 2.2], as well as for all PUT and DELETE de Jong [Page 7] Internet-Draft remoteStorage June 2016 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 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. All successful requests MUST return 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. All successful GET requests MUST return an '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. de Jong [Page 8] Internet-Draft remoteStorage June 2016 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: * , 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) * 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' * , always 'draft-dejong-remotestorage-06' 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 with '/' plus one or more '/' strings indicating a path in the folder tree, followed by zero or one strings, indicating a document. For example, if 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 , where the string SHOULD be lower-case alphanumerical, other than the reserved word 'public', and 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: de Jong [Page 9] Internet-Draft remoteStorage June 2016 '*:rw') any request, '*:r') any GET or HEAD request, ':rw') any requests to paths relative to that start with '/' '/' or '/public/' '/', ':r') any GET or HEAD requests to paths relative to that start with '/' '/' or '/public/' '/', 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 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 ', exactly one link of the following format SHOULD be added to the WebFinger record [WEBFINGER] for : { "href": , "rel": "http://tools.ietf.org/id/draft-dejong-remotestorage", "properties": { "http://remotestorage.io/spec/version": , "http://tools.ietf.org/html/rfc6749#section-4.2": , "...": "...", } de Jong [Page 10] Internet-Draft remoteStorage June 2016 } A common way of identifying persons as at is through a URI of the format "acct:@". Persons who use a personal domain name, not shared with any other users, can be identified by a URI of the format "http:///" (see [WEBFINGER, section 4.1]). Here and are as per "Session description" above, and SHOULD be either null or a URL where an OAuth 2.0 implicit-grant flow dialog [OAUTH] is presented. If 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 at ). If 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 de Jong [Page 11] Internet-Draft remoteStorage June 2016 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 ' ' , 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: de Jong [Page 12] Internet-Draft remoteStorage June 2016 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-06", "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-\ de Jong [Page 13] Internet-Draft remoteStorage June 2016 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 Allow access? ... 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: de Jong [Page 14] Internet-Draft remoteStorage June 2016 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" 12.6. Subsequent PUT A subsequent PUT may contain an 'If-Match' header referring to the ETag previously returned, like this: de Jong [Page 15] Internet-Draft remoteStorage June 2016 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: 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 de Jong [Page 16] Internet-Draft remoteStorage June 2016 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}}} 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 Conflict or a 200 OK status: HTTP/1.1 412 Conflict de Jong [Page 17] Internet-Draft remoteStorage June 2016 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, 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, de Jong [Page 18] Internet-Draft remoteStorage June 2016 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. de Jong [Page 19] Internet-Draft remoteStorage June 2016 Where the use of bearer tokens is impractical, a user may choose to store documents on hard-to-guess URLs [CAPABILITIES] whose path after 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] de Jong [Page 20] Internet-Draft remoteStorage June 2016 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, de Jong [Page 21] Internet-Draft remoteStorage June 2016 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 de Jong [Page 22]