Resumable Uploads for HTTP

HTTP clients often encounter interrupted data transfers as a result of canceled requests or dropped connections. Prior to interruption, part of a representation may have been exchanged. To complete the data transfer of the entire representation, it is often desirable to issue subsequent requests that transfer only the remainder of the representation. HTTP range requests support this concept of resumable downloads from server to client. This document describes a mechanism that supports resumable uploads from client to server using HTTP.¶

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HTTP clients often encounter interrupted data transfers as a result of canceled requests or dropped connections. Prior to interruption, part of a representation (see Section 3.2 of [HTTP]) might have been exchanged. To complete the data transfer of the entire representation, it is often desirable to issue subsequent requests that transfer only the remainder of the representation. HTTP range requests (see Section 14 of [HTTP]) support this concept of resumable downloads from server to client.¶

HTTP methods such as POST or PUT can be used by clients to request processing of representation data enclosed in the request message. The transfer of representation data from client to server is often referred to as an upload. Uploads are just as likely as downloads to suffer from the effects of data transfer interruption. Humans can play a role in upload interruptions through manual actions such as pausing an upload. Regardless of the cause of an interruption, servers may have received part of the representation before its occurrence and it is desirable if clients can complete the data transfer by sending only the remainder of the representation. The process of sending additional parts of a representation using subsequent HTTP requests from client to server is herein referred to as a resumable upload.¶

Connection interruptions are common and the absence of a standard mechanism for resumable uploads has lead to a proliferation of custom solutions. Some of those use HTTP, while others rely on other transfer mechanisms entirely. An HTTP-based standard solution is desirable for such a common class of problem.¶

This document defines the Resumable Uploads Protocol, an optional mechanism for resumable uploads using HTTP that is backwards-compatible with conventional HTTP uploads. When an upload is interrupted, clients can send subsequent requests to query the server state and use this information to the send remaining data. Alternatively, they can cancel the upload entirely. Different from ranged downloads, this protocol does not support transferring different parts of the same representation in parallel.¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

The terms byte sequence, Item, string, sf-binary, sf-boolean, sf-integer, sf-string, and sf-token are imported from [STRUCTURED-FIELDS].¶

The terms client and server are imported from [HTTP].¶

Upload: A sequence of one or more procedures, uniquely identified by an upload URL chosen by a server.¶

Procedure: An HTTP message exchange for that can be used for resumable uploads.¶

• Upload Creation Procedure (Section 4)
• Offset Retrieving Procedure (Section 5)
• Upload Appending Procedure (Section 6)
• Upload Cancellation Procedure (Section 7)

A single upload is a sequence of one or more procedures. Each upload is uniquely identified by an upload URL chosen by a server.¶

The remainder of this section uses examples of a file upload to illustrate permutations of procedure sequence. Note, however, that HTTP message exchanges use representation data (see Section 8.1 of [HTTP]), which means that procedures can apply to many forms of content.¶

The Upload Creation Procedure is intended for starting a new upload. A limited form of this procedure MAY be used by the client without the knowledge of server support of the Resumable Uploads Protocol.¶

This procedure is designed to be compatible with a regular upload. Therefore all methods are allowed with the exception of GET, HEAD, DELETE, and OPTIONS. All response status codes are allowed. The client is RECOMMENDED to use the POST method if not otherwise intended. The server MAY only support a limited number of methods.¶

The request MUST include the Upload-Incomplete header field (Section 9.2). It MUST be set to true if the end of the request body is not the end of the upload. Otherwise, it MUST be set to false. This header field can be used for request identification by a server. The request MUST NOT include the Upload-Offset header.¶

If the request is valid, the server SHOULD create an upload resource. If so, the server MUST include the Location header in the response and set to the upload URL, which points to the created upload resource. The client MAY use this upload URL to execute the Offset Retrieving Procedure (Section 5), Upload Appending Procedure (Section 6), or Upload Cancellation Procedure (Section 7).¶

As soon as the upload resource is available, the server MAY send an informational response with 104 (Upload Resumption Supported) status to the client while the request body is being uploaded. In this informational response, the Location header field MUST be set to the upload URL.¶

The server MUST send the Upload-Offset header in the response if it considers the upload active, either when the response is a success (e.g. 201 (Created)), or when the response is a failure (e.g. 409 (Conflict)). The value MUST be equal to the end offset of the entire upload, or the begin offset of the next chunk if the upload is still incomplete. The client SHOULD consider the upload failed if the response status code indicates a success but the offset in the Upload-Offset header field in the response does not equal to the begin offset plus the number of bytes uploaded in the request.¶

If the request completes successfully and the entire upload is complete, the server MUST acknowledge it by responding with a successful status code between 200 and 299 (inclusive). Server is RECOMMENDED to use 201 (Created) response if not otherwise specified. The response MUST NOT include the Upload-Incomplete header with the value of true.¶

If the request completes successfully but the entire upload is not yet complete indicated by the Upload-Incomplete header, the server MUST acknowledge it by responding with the 201 (Created) status code, the Upload-Incomplete header set to true.¶

If the client received an informational repsonse with the upload URL, it MAY automatically attempt upload resumption when the connection is terminated unexpectedly, or if a server error status code between 500 and 599 (inclusive) is received. The client SHOULD NOT automatically retry if a client error status code between 400 and 499 (inclusive) is received.¶

File metadata can affect how servers might act on the uploaded file. Clients can send Representation Metadata (see Section 8.3 of [HTTP]) in the Upload Creation Procedure request that starts an upload. Servers MAY interpret this metadata or MAY ignore it. The Content-Type header can be used to indicate the MIME type of the file. The Content-Disposition header can be used to transmit a filename. If included, the parameters SHOULD be either filename, filename* or boundary.¶

If an upload is interrupted, the client MAY attempt to fetch the offset of the incomplete upload by sending a HEAD request to the upload URL, as obtained from the Upload Creation Procedure (Section 4). The client MUST NOT initiate this procedure without the knowledge of server support.¶

The request MUST NOT include the Upload-Offset header or the Upload-Incomplete header. The server MUST reject the request with the Upload-Offset header or the Upload-Incomplete header by sending a 400 (Bad Request) response.¶

If the server considers the upload associated with this upload URL active, it MUST send back a 204 (No Content) response. The response MUST include the Upload-Offset header set to the current resumption offset for the client. The response MUST include the Upload-Incomplete header which is set to true if and only if the upload is incomplete. An upload is considered complete if and only if the server completely and successfully received a corresponding Upload Creation Procedure (Section 4) or Upload Appending Procedure (Section 6) request with the Upload-Incomplete header being omitted or set to false.¶

The client MUST NOT perform the Offset Retrieving Procedure (Section 5) while the Upload Creation Procedure (Section 4) or the Upload Appending Procedure (Section 6) is in progress.¶

The offset MUST be accepted by a subsequent Upload Appending Procedure (Section 6). Due to network delay and reordering, the server might still be receiving data from an ongoing transfer for the same upload URL, which in the client perspective has failed. The server MAY terminate any transfers for the same upload URL before sending the response by abruptly terminating the HTTP connection or stream. Alternatively, the server MAY keep the ongoing transfer alive but ignore further bytes received past the offset.¶

The client MUST NOT start more than one Upload Appending Procedures (Section 6) based on the resumption offset from a single Offset Retrieving Procedure (Section 5).¶

The response SHOULD include Cache-Control: no-store header to prevent HTTP caching.¶

If the server does not consider the upload associated with this upload URL active, it MUST respond with 404 (Not Found) status code.¶

The resumption offset can be less than or equal to the number of bytes the client has already sent. The client MAY reject an offset which is greater than the number of bytes it has already sent during this upload. The client is expected to handle backtracking of a reasonable length. If the offset is invalid for this upload, or if the client cannot backtrack to the offset and reproduce the same content it has already sent, the upload MUST be considered a failure. The client MAY use the Upload Cancellation Procedure (Section 7) to cancel the upload after rejecting the offset.¶

The client SHOULD NOT automatically retry if a client error status code between 400 and 499 (inclusive) is received.¶

The Upload Appending Procedure is used for resuming an existing upload.¶

The request MUST use the PATCH method and be sent to the upload URL, as obtained from the Upload Creation Procedure (Section 4). The Upload-Offset header field (Section 9.1) MUST be set to the resumption offset.¶

If the end of the request body is not the end of the upload, the Upload-Incomplete header field (Section 9.2) MUST be set to true.¶

The server SHOULD respect representation metadata received in the Upload Creation Procedure (Section 4) and ignore any representation metadata received in the Upload Appending Procedure (Section 6).¶

If the server does not consider the upload associated with the upload URL active, it MUST respond with 404 (Not Found) status code.¶

The client MUST NOT perform multiple upload transfers for the same upload URL using Upload Creation Procedures (Section 4) or Upload Appending Procedures (Section 6) in parallel to avoid race conditions and data loss or corruption. The server is RECOMMENDED to take measures to avoid parallel upload transfers: The server MAY terminate any ongoing Upload Creation Procedure (Section 4) or Upload Appending Procedure (Section 6) for the same upload URL. Since the client is not allowed to perform multiple transfers in parallel, the server can assume that the previous attempt has already failed. Therefore, the server MAY abruptly terminate the previous HTTP connection or stream.¶

If the offset in the Upload-Offset header field does not match the offset provided by the immediate previous Offset Retrieving Procedure (Section 5), or the end offset of the immediate previous incomplete successful transfer, the server MUST respond with 409 (Conflict) status code.¶

The server MUST send the Upload-Offset header in the response if it considers the upload active, either when the response is a success (e.g. 201 (Created)), or when the response is a failure (e.g. 409 (Conflict)). The value MUST be equal to the end offset of the entire upload, or the begin offset of the next chunk if the upload is still incomplete. The client SHOULD consider the upload failed if the response status code indicates a success but the offset in the Upload-Offset header field in the response does not equal to the begin offset plus the number of bytes uploaded in the request.¶

If the request completes successfully and the entire upload is complete, the server MUST acknowledge it by responding with a successful status code between 200 and 299 (inclusive). Server is RECOMMENDED to use 201 (Created) response if not otherwise specified. The response MUST NOT include the Upload-Incomplete header with the value of true.¶

If the request completes successfully but the entire upload is not yet complete indicated by the Upload-Incomplete header, the server MUST acknowledge it by responding with the 201 (Created) status code, the Upload-Incomplete header set to true.¶

The client MAY automatically attempt upload resumption when the connection is terminated unexpectedly, or if a server error status code between 500 and 599 (inclusive) is received. The client SHOULD NOT automatically retry if a client error status code between 400 and 499 (inclusive) is received.¶

If the client wants to terminate the transfer without the ability to resume, it MAY send a DELETE request to the upload URL, as obtained from the Upload Creation Procedure (Section 4). It is an indication that the client is no longer interested in uploading this body and the server can release resources associated with this upload URL. The client MUST NOT initiate this procedure without the knowledge of server support.¶

The request MUST use the DELETE method. The request MUST NOT include the Upload-Offset header or the Upload-Incomplete header. The server MUST reject the request with the Upload-Offset header or the Upload-Incomplete header by sending a 400 (Bad Request) response.¶

If the server has successfully deactivated this upload URL, it MUST send back a 204 (No Content) response.¶

The server MAY terminate any ongoing Upload Creation Procedure (Section 4) or Upload Appending Procedure (Section 6) for the same upload URL before sending the response by abruptly terminating the HTTP connection or stream.¶

If the server does not consider the upload associated with this upload URL active, it MUST respond with 404 (Not Found) status code.¶

If the server does not support cancellation, it MUST respond with 405 (Method Not Allowed) status code.¶

1. The request URL is not an upload URL (i.e. does not point to an upload resource): Upload Creation Procedure (Section 4)
2. The request URL is an upload URL and the method is HEAD: Offset Retrieving Procedure (Section 5).
3. The request URL is an upload URL and the method is DELETE: Upload Cancellation Procedure (Section 7).
4. The request URL is an upload URL and the Upload-Offset header is present: Upload Appending Procedure (Section 6).
5. Otherwise: Not a resumable upload.

The 301 (Moved Permanently) status code and the 302 (Found) status code MUST NOT be used in Offset Retrieving Procedure (Section 5) and Upload Cancellation Procedure (Section 7) responses. A 308 (Permanent Redirect) response MAY be persisted for all subsequent procedures. If client receives a 307 (Temporary Redirect) response in the Offset Retrieving Procedure (Section 5), it MAY apply the redirection directly in the immediate subsequent Upload Appending Procedure (Section 6).¶

The upload URL obtained through the Upload Creation Procedure (Section 4) is the identifier used for modifying the upload. Without further protection of this upload URL, an attacker may use the upload URL to obtain information about an upload, append data to it, or cancel it. To prevent this, the server SHOULD ensure that only authorized clients can perform the Offset Retrieving Procedure (Section 5), Upload Appending Procedure (Section 6), or Upload Cancellation Procedure (Section 7) for a given upload URL and otherwise reject the procedure. In addition, the upload URL SHOULD be generated in such a way that makes it hard to be guessed by non-authorized clients.¶

Header field name: Upload-Offset, Upload-Incomplete¶

Applicable protocol: http¶

Status: standard¶

Author/change controller: IETF¶

Specification: This document¶

Related information: n/a¶

Code: 104 (suggested value)¶

Description: Upload Resumption Supported¶

Specification: This document¶

• Remove Upload-Token and instead use Server-generated upload URL for upload identification.
• Require the Upload-Incomplete header field in Upload Creation Procedure.
• Increase the draft interop version.

The server is allowed to respond to Upload Creation Procedure (Section 4) requests with a 104 (Upload Resumption Supported) intermediate response as soon as the server has validated the request. This way, the client knows that the server supports resumable uploads before the complete response for the Upload Creation Procedure is received. The benefit is the clients can defer starting the actual data transfer until the server indicates full support of the incoming Upload Creation Procedure (i.e. resumable are supported, the provided upload URL is active etc).¶

On the contrary, support for intermediate responses (the 1XX range) in existing software is limited or not at all present. Such software includes proxies, firewalls, browsers, and HTTP libraries for clients and server. Therefore, the 104 (Upload Resumption Supported) status code is optional and not mandatory for the successful completion of an upload. Otherwise, it might be impossible in some cases to implement resumable upload servers using existing software packages. Furthermore, as parts of the current internet infrastructure currently have limited support for intermediate responses, a successful delivery of a 104 (Upload Resumption Supported) from the server to the client should be assumed.¶

We hope that support for intermediate responses increases in the near future, to allow a wider usage of 104 (Upload Resumption Supported).¶

This specification includes a section about feature detection (it was called service discovery in earlier discussions, but this name is probably ill-suited). The idea is to allow resumable uploads to be transparently implemented by HTTP clients. This means that application developers just keep using the same API of their HTTP library as they have done in the past with traditional, non-resumable uploads. Once the HTTP library gets updated (e.g. because mobile OS or browsers start implementing resumable uploads), the HTTP library can transparently decide to use resumable uploads without explicit configuration by the application developer. Of course, in order to use resumable uploads, the HTTP library needs to know whether the server supports resumable uploads. If no support is detected, the HTTP library should use the traditional, non-resumable upload technique. We call this process feature detection.¶

1. Avoid additional roundtrips by the client, if possible (i.e. an additional HTTP request by the client should be avoided).
2. Be backwards compatible to HTTP/1.1 and existing network infrastructure: This means to avoid using new features in HTTP/2, or features which might require changes to existing network infrastructure (e.g. nginx or HTTP libraries)
3. Conserve the user's privacy (i.e. the feature detection should not leak information to other third-parties about which URLs have been connected to)

Include a support statement in the SETTINGS frame. The SETTINGS frame is a HTTP/2 feature and is sent by the server to the client to exchange information about the current connection. The idea was to include an additional statement in this frame, so the client can detect support for resumable uploads without an additional roundtrip. The problem is that this is not compatible with HTTP/1.1. Furthermore, the SETTINGS frame is intended for information about the current connection (not bound to a request/response) and might not be persisted when transmitted through a proxy.¶

Include a support statement in the DNS record. The client can detect support when resolving a domain name. Of course, DNS is not semantically the correct layer. Also, DNS might not be involved if the record is cached or retrieved from a hosts files.¶

Send a HTTP request to ask for support. This is the easiest approach where the client sends an OPTIONS request and uses the response to determine if the server indicates support for resumable uploads. An alternative is that the client sends the request to a well-known URL to obtain this response, e.g. /.well-known/resumable-uploads. Of course, while being fully backwards-compatible, it requires an additional roundtrip.¶

Include a support statement in previous responses. In many cases, the file upload is not the first time that the client connects to the server. Often additional requests are sent beforehand for authentication, data retrieval etc. The responses for those requests can also include a header which indicates support for resumable uploads. There are two options: - Use the standardized Alt-Svc response header. However, it has been indicated to us that this header might be reworked in the future and could also be semantically different from our intended usage. - Use a new response header Resumable-Uploads: https://example.org/files/* to indicate under which endpoints support for resumable uploads is available.¶

Send a 104 intermediate response to indicate support. The clients normally starts a traditional upload and includes a header indicate that it supports resumable uploads (e.g. Upload-Offset: 0). If the server also supports resumable uploads, it will immediately respond with a 104 intermediate response to indicate its support, before further processing the request. This way the client is informed during the upload whether it can resume from possible connection errors or not. While an additional roundtrip is avoided, the problem with that solution is that many HTTP server libraries do not support sending custom 1XX responses and that some proxies may not be able to handle new 1XX status codes correctly.¶

Send a 103 Early Hint response to indicate support. This approach is the similar to the above one, with one exception: Instead of a new 104 (Upload Resumption Supported) status code, the existing 103 (Early Hint) status code is used in the intermediate response. The 103 code would then be accompanied by a header indicating support for resumable uploads (e.g. Resumable-Uploads: 1). It is unclear whether the Early Hints code is appropriate for that, as it is currently only used to indicate resources for prefetching them.¶

The Upload Creation Procedure (Section 4) allows the Content-Type and Content-Disposition header to be included. They are intended to be a standardized way of communicating the file name and file type, if available. However, this is not without controversy. Some argue that since these headers are already defined in other specifications, it is not necessary to include them here again. Furthermore, the Content-Disposition header field's format is not clearly enough defined. For example, it is left open which disposition value should be used in the header. There needs to be more discussion whether this approach is suited or not.¶

However, from experience with the tus project, users are often asking for a way to communicate the file name and file type. Therefore, we believe it is help to explicitly include an approach for doing so.¶

• Are multipart requests supported? Yes, requests whose body is encoded using the multipart/form-data are implicitly supported. The entire encoded body can be considered as a single file, which is then uploaded using the resumable protocol. The server, of course, must store the delimiter ("boundary") separating each part and must be able to parse the multipart format once the upload is completed.

This document is based on an Internet-Draft specification written by Jiten Mehta, Stefan Matsson, and the authors of this document.¶

The tus v1 protocol is a specification for a resumable file upload protocol over HTTP. It inspired the early design of this protocol. Members of the tus community helped significantly in the process of bringing this work to the IETF.¶

TODO: more acknowledgements.¶