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Dusseault 3 Internet-Draft OSAF 4 Expires: February 28, 2005 August 30, 2004 6 Partial Document Changes (PATCH Method) for HTTP 7 draft-dusseault-http-patch-05 9 Status of this Memo 11 This document is an Internet-Draft and is subject to all provisions 12 of section 3 of RFC 3667. By submitting this Internet-Draft, each 13 author represents that any applicable patent or other IPR claims of 14 which he or she is aware have been or will be disclosed, and any of 15 which he or she become aware will be disclosed, in accordance with 16 RFC 3668. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as 21 Internet-Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on February 28, 2005. 36 Copyright Notice 38 Copyright (C) The Internet Society (2004). 40 Abstract 42 Several applications extending HTTP require a feature to do partial 43 resource modification. Existing HTTP functionality only allows a 44 complete replacement of a document. This proposal adds a new HTTP 45 method, PATCH, to modify an existing HTTP resource. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 50 2. Patch Formats . . . . . . . . . . . . . . . . . . . . . . . . 5 51 3. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 6 52 3.1 PATCH Method . . . . . . . . . . . . . . . . . . . . . . . 6 53 3.2 PATCH Response . . . . . . . . . . . . . . . . . . . . . . 7 54 3.2.1 Success Response . . . . . . . . . . . . . . . . . . . 7 55 3.2.2 Error handling . . . . . . . . . . . . . . . . . . . . 7 56 3.3 Advertising Support in OPTIONS . . . . . . . . . . . . . . 9 57 4. Interdependencies with other Standards . . . . . . . . . . . . 11 58 4.1 PATCH and Access Control (RFC3744) . . . . . . . . . . . . 11 59 4.2 PATCH and Instance Manipulations (RFC3230) . . . . . . . . 11 60 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 61 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 62 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 63 7.1 Normative References . . . . . . . . . . . . . . . . . . . . 14 64 7.2 Non-Normative References . . . . . . . . . . . . . . . . . . 14 65 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14 66 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 67 B. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 68 B.1 Changes from -00 . . . . . . . . . . . . . . . . . . . . . 16 69 B.2 Changes from -01 . . . . . . . . . . . . . . . . . . . . . 16 70 B.3 Changes from -02 . . . . . . . . . . . . . . . . . . . . . 16 71 B.4 Changes from -03 . . . . . . . . . . . . . . . . . . . . . 16 72 B.5 Changes from -04 . . . . . . . . . . . . . . . . . . . . . 17 73 Intellectual Property and Copyright Statements . . . . . . . . 18 75 1. Introduction 77 Three use cases initially motivated this proposal 79 1. WebDAV [4] is used by authoring applications to store and share 80 files on the internet. For example, Adobe Photoshop has a 81 Workgroup feature allowing the user to browse a repository and 82 save the file. Currently, Photoshop only publishes the file to 83 the repository rarely, because Photoshop files are typically 84 large and upload is slow. Worse, large uploads are more likely 85 to be interrupted. Although HTTP [2] provides byte range 86 downloads, it does not provide a mechanism for partial uploads. 87 2. DeltaV [7] extends WebDAV to do versioning. In versioning 88 environments, a large number of files may be updated with very 89 small changes. For example, a programmer may change the name of 90 a function used in a hundred source files. Versioning 91 applications typically send deltas or patches to the server to 92 modify these files, however DetaV does not yet have this 93 functionality. 94 3. The SIMPLE WG is devising a way to store and modify configuration 95 information. The biggest feature missing from HTTP is the 96 ability to modify information in a very lightweight manner, so 97 that the client that decides to change its presence state from 98 "free" to "busy" doesn't have to upload a large document. This 99 can be accomplished through changes to a HTTP resource as well. 101 Other working groups (like netconf) are also considering manipulating 102 large files using HTTP GET and PUT. Sometimes the files aren't that 103 large but the device is small or bandwidth is limited, as when phones 104 need to add a new contact to an address book file. This feature 105 would allow much more efficient changes to files. 107 This specification defines a new HTTP 1.1 method for patches. A new 108 method is necessary to improve interoperability and prevent errors. 109 The PUT method is already defined to overwrite a resource with a 110 complete new body, and MUST NOT be reused to do partial changes. 111 Otherwise, proxies and caches and even clients and servers may get 112 confused as to the result of the operation. 114 Note that byte ranges are already used in HTTP to do partial 115 downloads (GET method). However, they are not defined for uploads, 116 and there are some missing pieces for uploads. For example, the HTTP 117 specification does not define a particularly informative error to 118 send if the byte range in a PUT is invalid. Byte ranges (or some 119 other kind of range) could be made to work in this specification but 120 a more flexible mechanism (one that could also encompass XML patch 121 formats) was desired, as well as a method that would not confuse 122 caching proxies. Reliable and tested patch algorithms already exist, 123 and this specification takes advantage of that existing work. 125 Other patch formats ("delta encodings") are defined for HTTP in RFC 126 3229 [5]. That specification defines delta encodings for cache 127 updates, not for user write operations. It does mean that servers 128 can reuse delta encoding algorithms to support both that 129 specification and this proposal. 131 This specification defines the new method PATCH to alter a single 132 existing resource, in place, by applying a patch. The operation is 133 atomic. Note that WebDAV MOVE and COPY requests, if supported by the 134 HTTP server, can be useful to independently rename or copy a whole 135 resource before applying PATCH to either the source or destination 136 URL to modify the contents. 138 2. Patch Formats 140 A set of changes for a resource is itself a document, called a patch 141 document. The patch format is uniquely identified through a MIME 142 type. Servers advertise supported patch formats by advertising these 143 MIME types, and clients specify which one they're using by including 144 the MIME type in the request. MIME types were specifically chosen so 145 that there would be a well-defined way for other PATCH extensions to 146 define their own patch formats and how to use them. 148 This specification only defines usage of the platform-portable gdiff 149 [3] format identified as 'application/gdiff'. Servers SHOULD support 150 gdiff for all authorable resources, that is all resources that 151 support PUT. Some requirements apply only to specific patch formats, 152 and in this specification those requirements are spelled out only for 153 gdiff. 155 3. Mechanisms 157 3.1 PATCH Method 159 The PATCH method requests that the request body (a patch document) be 160 applied to the resource identified by the Request-URI. The server 161 MUST NOT create a new resource with the contents of the request body, 162 although it MAY (depending on the patch document format) apply the 163 request body to an empty entity to result in the content for the new 164 resource. The target resource's content type MUST be one to which 165 the patch format applies. The server MUST apply the entire patch 166 atomically and never provide (e.g. in response to a GET during this 167 operation) a partially-patched body. If the entire patch file cannot 168 be successfully applied then the server MUST fail the entire request, 169 applying none of the changes. See error handling section for details 170 on status codes and possible error conditions. 172 PATCH request bodies MUST NOT be cached. A cache MAY mark the 173 resource identified in the Request-URI as stale if it sees a 174 successful response to the PATCH request. 176 The PATCH request MUST have a body. It MUST include the Content-Type 177 header with a MIME [1] type value identifying the patch format used 178 in the request body. The request body MUST be in some format which 179 has the semantics of defining a change to an existing document. 181 If the gdiff format is used: 183 o The client MUST verify that it is applying the patch document to a 184 known entity. There are two reliable ways to do this. The first 185 way is to find out the resource ETag at the time the body is 186 downloaded, and use that Etag in the If-Match header on the PATCH 187 request to make sure the resource is still unchanged. The second 188 way to use WebDAV LOCK/UNLOCK to reserve the file (first LOCK, 189 then GET, then PATCH, then UNLOCK). Gdiff collisions from 190 multiple users are more dangerous than PUT collisions, because a 191 gdiff that is not operating from a known base point may corrupt 192 the resource. Therefore, if neither strong ETags nor LOCKS are 193 available from the server, the client MUST use If-Unmodified-Since 194 as a less-reliable safeguard. 195 o If the Request-URI does not identify an existing resource, the 196 server SHOULD (subject of course to access control and other 197 restrictions) create a resource with an empty body and apply the 198 gdiff changes to that empty entity. A client SHOULD verify that 199 the URL is unmapped, as expected, with use of the "If-None-Match: 200 *" header. 202 Simple PATCH example 204 PATCH /file.txt HTTP/1.1 205 Host: www.example.com 206 Content-type: application/gdiff 207 If-Match: "e0023aa4e" 208 Content-Length: 100 210 [gdiff-binary-body] 212 Figure 1 214 This example illustrates use of the gdiff algorithm on an existing 215 text file. 217 3.2 PATCH Response 219 3.2.1 Success Response 221 A successful response with the 204 No Content status code implies 222 that no new resource was created. A successful response with the 201 223 Created status code informs the client that a new resource was 224 created. 226 The server SHOULD provide a MD5 hash of the resource entity after the 227 patch was applied. This allows the client to verify the success of 228 the operation. 230 As with PUT, the PATCH method MUST change the resource's ETag if the 231 resulting entity is not identical to the original. If the server 232 supports strong ETags, the server MUST return a strong ETag for use 233 in future client operations. The server MUST return the 234 Last-Modified header if it does not support strong ETags. 236 Successful PATCH response to existing text file 238 HTTP/1.1 204 No Content 239 Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== 240 ETag: "e0023aa4e" 242 3.2.2 Error handling 244 This proposal uses the same mechanism as DeltaV (defined in section 245 1.6 of RFC3253) to add machine-parsable info to provide more detail 246 than HTTP status codes can. Existing HTTP status codes are not 247 infinitely extensible but XML elements and namespaces are more so, 248 and it's simple to treat the HTTP error code as a rough category and 249 put detailed error codes in the body. Clients that do not use the 250 extra information ignore the bodies of error responses. These error 251 codes are not meant to be displayed directly to end-users, so there 252 is no language code or other display information. Clients MUST 253 ignore any unrecognized elements within the XML response body because 254 extensions allow implementors to add custom debug information to the 255 response. 257 The PATCH method can return the following errors. All these errors 258 are represented as XML elements in an XML document, where the 259 specific error element appears inside a root element called "error" 260 in the "DAV:" namespace. The new elements defined in this 261 specification are all in the "urn:ietf:params:xml:ns:patch" 262 namespace. 264 delta-format-unsupported: Used with 403 Forbidden status code. 265 Returned by the server when it doesn't support the patch format 266 chosen by the client. 268 delta-format-forbidden-on-resource: Used with 403 Forbidden when the 269 patch format chosen by the client is supported by the server but 270 not allowed on this kind of resource. 272 delta-format-badly-formatted: Used with 400 Bad Request when the 273 server finds that the patch document provided by the client was 274 badly formatted or non-compliant. The definition of badly 275 formatted or non-compliant depends on the patch format chosen, but 276 generally if the server finds it can't handle the current patch 277 even though it supports the format used, this error ought to be 278 appropriate. 280 patch-empty-resource: Used with 409 Conflict when the resource 281 addressed in the Request-URI exists but is empty, and the patch 282 format cannot be applied to an empty document. Note that some 283 patch formats may be applied to an empty document, in which case 284 this error wouldn't be used. 286 patch-result-invalid: Used with 409 Conflict when the resource could 287 be patched but the result of the patch would be a resource which 288 is invalid. This could mean, for example, that a XML resource 289 would become an invalid XML file if the patch specified that a 290 close element text line should be deleted. 292 "404 Not Found" can be used (with no body/error element) when the URL 293 in by the Request-URI does not map to a resource and the server 294 cannot apply the patch document to a new empty resource (thus this 295 error wouldn't be used with gdiff patch documents). 297 Other status codes defined in RFC2616 may also be used under the 298 appropriate circumstances, with no response body. For example, an 299 unauthenticated user may be prompted to authenticate, in order to use 300 PATCH, with "401 Unauthorized". An authenticated user who does not 301 have sufficient privilege to use PATCH may receive a "403 Forbidden" 302 response. 304 3.2.2.1 Example error response with body detail 306 HTTP/1.1 409 Conflict 307 Content-Type: text/xml; charset="utf-8" 308 Content-Length: xxx 310 311 312 314 316 3.3 Advertising Support in OPTIONS 318 The server advertises its support for the features described here 319 with OPTIONS response headers. The "Allow" OPTIONS header is already 320 defined in HTTP 1.1 to contain all the allowed methods on the 321 addressed resource, so the server MUST add PATCH if it is allowed. 323 Clients also need to know whether the server supports special patch 324 formats, so this document introduces a new OPTIONS response header 325 "Accept-Patch". "Accept-Patch" MUST appear in the OPTIONS response 326 for any resource where the PATCH method is shown as an allowed 327 method. 329 OPTIONS * is not used to advertise support for PATCH because the 330 patch formats supported are likely to change from one resource to 331 another. A server MAY include the Accept-Patch header in response to 332 OPTIONS *, and its value MAY be the union of known supported patch 333 formats. 335 Accept-Patch = "Accept-Patch" ":" #media-type 337 Example: OPTIONS request and response for specific resource 339 [request] 341 OPTIONS /example/buddies.xml HTTP/1.1 342 Host: www.example.com 344 [response] 346 HTTP/1.1 200 OK 347 Allow: GET, PUT, POST, OPTIONS, HEAD, TRACE, DELETE, PATCH 348 Accept-Patch: example/xcap+xml, application/gdiff 350 The examples show a server that supports PATCH generally, with two 351 formats supported (one of them is fictional). On some resources, for 352 example on XML files, different kinds of patch formats more 353 appropriate to the resource may be supported. 355 4. Interdependencies with other Standards 357 4.1 PATCH and Access Control (RFC3744) 359 If the server supports WebDAV Access Control [8], then the PATCH 360 request SHOULD be subject to the same access control permissions as 361 the PUT request. 363 4.2 PATCH and Instance Manipulations (RFC3230) 365 A patch document is modelled as an instance being sent to the server, 366 following the model of RFC3230 [6]. Thus, if the server supports 367 instance manipulations, the client MAY apply a supported manipulation 368 to the patch document after it is generated (for example, a 369 compression algorithm could be applied to the patch document). On 370 the receiving end, the server MUST undo the instance manipulation 371 then apply the resulting document as a patch. 373 5. IANA Considerations 375 This document uses URNs to describe XML namespaces and XML schemas 376 conforming to a registry mechanism described in [RFC3688]. 378 Registration request for the patch namespace: 380 URI: urn:ietf:params:xml:ns:patch 382 Registrant Contact: See the "Author's Address" section of this 383 document. 385 XML: None. Namespace URIs do not represent an XML specification. 387 6. Security Considerations 389 The security considerations for PATCH are nearly identical to the 390 security considerations for PUT. In addition, one might be concerned 391 that a document that is patched might be more likely to be corrupted, 392 but that concern is addressed through use of MD5 digests. 394 7. References 396 7.1 Normative References 398 [1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 399 Extensions (MIME) Part Two: Media Types", RFC 2046, November 400 1996. 402 [2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., 403 Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- 404 HTTP/1.1", RFC 2616, June 1999. 406 [3] van Hoff, A. and J. Payne, "Generic Diff Format Specification", 407 W3C NOTE-gdiff-19970901, August 1997, 408 . 410 7.2 Non-Normative References 412 [4] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D. Jensen, 413 "HTTP Extensions for Distributed Authoring -- WEBDAV", RFC 2518, 414 February 1999. 416 [5] Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland, 417 Y., van Hoff, A. and D. Hellerstein, "Delta encoding in HTTP", 418 RFC 3229, January 2002. 420 [6] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP", RFC 3230, 421 January 2002. 423 [7] Clemm, G., Amsden, J., Ellison, T., Kaler, C. and J. Whitehead, 424 "Versioning Extensions to WebDAV (Web Distributed Authoring and 425 Versioning)", RFC 3253, March 2002. 427 [8] Clemm, G., Reschke, J., Sedlar, E. and J. Whitehead, "Web 428 Distributed Authoring and Versioning (WebDAV) Access Control 429 Protocol", RFC 3744, May 2004. 431 Author's Address 433 Lisa Dusseault 434 Open Source Application Foundation 435 2064 Edgewood Dr. 436 Palo Alto, CA 94303 437 US 439 EMail: lisa@osafoundation.org 441 Appendix A. Acknowledgements 443 PATCH is not a new concept, it first appeared in HTTP in drafts of 444 version 1.1 written by Roy Fielding and Henrik Frystyk. 446 Thanks to Adam Roach, Chris Sharp, Julian Reschke, Geoff Clemm, Scott 447 Lawrence, Jeffrey Mogul, Roy Fielding, Greg Stein, Jim Luther, Alex 448 Rousskov, Jamie Lokier and Joe Hildebrand for review and advice on 449 this document. 451 Appendix B. Changes 453 B.1 Changes from -00 455 OPTIONS support: removed "Patch" header definition and used Allow and 456 new "Accept-Patch" headers instead. 458 Supported patch formats: removed vcdiff and diffe as these do not 459 have defined MIME types and did not seem to be strongly desired. 461 PATCH method definition: Clarified cache behavior. 463 B.2 Changes from -01 465 Removed references to XCAP - not yet a RFC. 467 Fixed use of MIME types (this "fix" now obsolete) 469 Explained how to use MOVE or COPY in conjunction with PATCH, to 470 create a new resource based on an existing resource in a different 471 location. 473 B.3 Changes from -02 475 Clarified that MOVE and COPY are really independent of PATCH. 477 Clarified when an ETag must change, and when Last-Modified must be 478 used. 480 Clarified what server should do if both Content-Type and IM headers 481 appear in PATCH request. 483 Filled in missing reference to DeltaV and ACL RFCs. 485 Stopped using 501 Unsupported for unsupported patch formats. 487 Clarified what a static resource is. 489 Refixed use of MIME types for patch formats. 491 Limited the scope of some restrictions to apply only to 'gdiff' 492 usage. 494 B.4 Changes from -03 496 Various typographical, terminology consistency, and other minor 497 clarifications or fixes. 499 B.5 Changes from -04 501 Moved paragraphs on ACL and RFC3230 interoperability to new section. 503 Added security considerations. 505 Added IANA considerations, registration of new namespace, and 506 discontinued use of "DAV:" namespace for new elements. 508 Added example of error response. 510 Intellectual Property Statement 512 The IETF takes no position regarding the validity or scope of any 513 Intellectual Property Rights or other rights that might be claimed to 514 pertain to the implementation or use of the technology described in 515 this document or the extent to which any license under such rights 516 might or might not be available; nor does it represent that it has 517 made any independent effort to identify any such rights. 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