]> Transloadit Ltd

marius@transloadit.com

Apple Inc.

jmehta@apple.com

Apple Inc.

guoye_zhang@apple.com

Cloudflare

lucaspardue.24.7@gmail.com

JellyHive

s.matsson@gmail.com

ART HTTP Internet-Draft 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. Discussion Venues Discussion of this document takes place on the tus-v2 GitHub repository at .

Introduction HTTP clients often encounter interrupted data transfers as a result of canceled requests or dropped connections. Prior to interruption, part of a representation (see ) 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 ) 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.

Conventions and Definitions 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 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 . The terms client and server are imported from . Upload: A sequence of one or more procedures, uniquely identified by a token chosen by a client. Procedure: An HTTP message exchange for that can be used for resumable uploads.

Uploading Overview The Resumable Uploads Protocol consists of several procedures that rely on HTTP message exchanges. The following procedures are defined:

• Upload Transfer Procedure ()
• Offset Retrieving Procedure ()
• Upload Cancellation Procedure ()

A single upload is a sequence of one or more procedures. Each upload is uniquely identified by a token chosen by a client. The token is carried in the Upload-Token header field; see . 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 ), which means that procedures can apply to many forms of content.

Example 1: Complete upload of file with known size In this example, the client first attempts to upload a file with a known size in a single HTTP request. An interruption occurs and the client then attempts to resume the upload using subsequent HTTP requests. 1) The Upload Transfer Procedure () can be used to notify the server that the client wants to begin an upload. The server should then reserve the required resources to accept the upload from the client. The client also begins transferring the entire file in the request body. The request includes the Upload-Token header, which is used for identifying future requests related to this upload. An informational response can be sent to the client to signal the support of resumable upload on the server.

Upload Transfer Procedure Initiation | | | | | Reserve resources | | for Upload-Token | |------------------ | | | | |<----------------- | | | 104 Upload Resumption Supported | |<-------------------------------------------| | | | Flow Interrupted | |------------------------------------------->| | | ]]>

2) If the connection to the server gets interrupted during the Upload Transfer Procedure, the client may want to resume the upload. Before this is possible, the client must know the amount of data that the server was able to receive before the connection got interrupted. To achieve this, the client uses the Offset Retrieving Procedure () to obtain the upload's offset.

Offset Retrieving Procedure | | | | 204 No Content with Upload-Offset | |<-----------------------------------------------| | | ]]>

3) After the Offset Retrieving Procedure () completes, the client can resume the upload by sending the remaining file content to the server, appending to the already stored data in the upload. The Upload-Offset value is included to ensure that the client and server agree on the offset that the upload resumes from.

Resuming Upload | | | | 201 Created on completion | |<-----------------------------------------------| | | ]]>

4) If the client is not interesting in completing the upload anymore, it can instruct the server to delete the upload and free all related resources using the Upload Cancellation Procedure ().

Upload Cancellation Procedure | | | | 204 No Content on completion | |<-----------------------------------------------| | | ]]>

Example 2: Upload as a series of parts In some cases clients might prefer to upload a file as a series of parts sent across multiple HTTP messages. One use case is to overcome server limits on HTTP message content size. Another use case is where the client does not know the final size, such as when file data originates from a streaming source. This example shows how the client, with prior knowledge about the server's resumable upload support, can upload parts of a file over a sequence of procedures. 1) If the client is aware that the server supports resumable upload, it can use the Upload Transfer Procedure with the Upload-Incomplete header to start an upload.

Upload Transfer Procedure Usage | | | | 201 Created with Upload-Incomplete | | on completion | |<-----------------------------------------------| | | ]]>

2) The last part of the upload does not have the Upload-Incomplete header.

Upload Transfer Procedure Last Chunk | | | | 201 Created on completion | |<-----------------------------------------------| | | ]]>

Upload Transfer Procedure The Upload Transfer Procedure is intended for transferring the data chunk. As such, it can be used for either resuming an existing upload, or starting a new upload. A limited form of this procedure MAY be used by the client to start a new upload without the knowledge of server support. 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 client MUST use the same method throughout an entire upload. The server SHOULD reject the attempt to resume an upload with a different method with 400 (Bad Request) response. The request MUST include the Upload-Token header field () which uniquely identifies an upload. The client MUST NOT reuse the token for a different upload. When resuming an upload, the Upload-Offset header field () MUST be set to the resumption offset. The resumption offset 0 indicates a new upload. The absence of the Upload-Offset header field implies the resumption offset of 0. If the end of the request body is not the end of the upload, the Upload-Incomplete header field () MUST be set to true. If the server does not consider the upload associated with the token in the Upload-Token header field active, but the resumption offset is non-zero, it MUST respond with 404 (Not Found) status code. The client MUST NOT perform multiple Upload Transfer Procedures () for the same token in parallel to avoid race conditions and data loss or corruption. The server is RECOMMENDED to take measures to avoid parallel Upload Transfer Procedures: The server MAY terminate any ongoing Upload Transfer Procedure () for the same token. 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 value 0, the offset provided by the immediate previous Offset Retrieving Procedure (), or the end offset of the immediate previous incomplete 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. File metadata can affect how servers might act on the uploaded file. Clients can send Representation Metadata (see ) in the Upload Transfer Procedure request that starts an upload. Servers MAY interpret this metadata or MAY ignore it. Clients SHOULD NOT send representation metadata in subsequent Upload Transfer Procedure requests and servers SHOULD ignore representation received in subsequent requests. 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.

Feature Detection If the client has no knowledge of whether the server supports resumable upload, the Upload Transfer Procedure MAY be used with some additional constraints. In particular, the Upload-Offset header field () and the Upload-Incomplete header field () MUST NOT be sent in the request if the server support is unclear. This allows the upload to function as if it is a regular upload. If the server detects the Upload Transfer Procedure with neither the Upload-Offset header nor the Upload-Incomplete header, and it supports resumable upload, an informational response with 104 (Upload Resumption Supported) status MAY be sent to the client while the request body is being uploaded. The client MUST NOT attempt to resume an upload if it did not receive the 104 (Upload Resumption Supported) informational response, and it does not have other signals of whether the server supporting resumable upload. If the client is aware of the server support, it SHOULD start an upload with the Upload-Offset header set to 0 in order to prevent the unnecessary informational response.

Draft Version Identification

• RFC Editor's Note: Please remove this section and Upload-Draft-Interop-Version from all examples prior to publication of a final version of this document.

The current interop version is 1. Client implementations of draft versions of the protocol MUST send a header field Upload-Draft-Interop-Version with the interop version as its value to its requests. Its ABNF is Server implementations of draft versions of the protocol MUST NOT send a 104 (Upload Resumption Supported) informational response when the interop version indicated by the Upload-Draft-Interop-Version header field in the request is missing or mismatching. Server implementations of draft versions of the protocol MUST also send a header field Upload-Draft-Interop-Version with the interop version as its value to the 104 (Upload Resumption Supported) informational response. Client implementations of draft versions of the protocol MUST ignore a 104 (Upload Resumption Supported) informational response with missing or mismatching interop version indicated by the Upload-Draft-Interop-Version header field. The reason both the client and the server are sending and checking the draft version is to ensure that implementations of the final RFC will not accidentally interop with draft implementations, as they will not check the existence of the Upload-Draft-Interop-Version header field.

Offset Retrieving Procedure If an upload is interrupted, the client MAY attempt to fetch the offset of the incomplete upload by sending a HEAD request to the server with the same Upload-Token header field (). The client MUST NOT initiate this procedure without the knowledge of server support. The request MUST use the HEAD method and include the Upload-Token header. 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 token 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 Transfer Procedure () request with the Upload-Incomplete header being omitted or set to false. The client MUST NOT perform the Offset Retrieving Procedure () while the Upload Transfer Procedures () is in progress. The offset MUST be accepted by a subsequent Upload Transfer Procedure (). Due to network delay and reordering, the server might still be receiving data from an ongoing transfer for the same token, which in the client perspective has failed. The server MAY terminate any transfers for the same token 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 Transfer Procedures () based on the resumption offset from a single Offset Retrieving Procedure (). The response SHOULD include Cache-Control: no-store header to prevent HTTP caching. If the server does not consider the upload associated with this token active, it MUST respond with 404 (Not Found) status code. The client MAY automatically start uploading from the beginning using Upload Transfer Procedure () if 404 (Not Found) status code is received. The client SHOULD NOT automatically retry if a status code other than 204 and 404 is received.

Upload Cancellation Procedure If the client wants to terminate the transfer without the ability to resume, it MAY send a DELETE request to the server along with the Upload-Token which is an indication that the client is no longer interested in uploading this body and the server can release resources associated with this token. The client MUST NOT initiate this procedure without the knowledge of server support. The request MUST use the DELETE method and include the Upload-Token header. 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 token, it MUST send back a 204 (No Content) response. The server MAY terminate any ongoing Upload Transfer Procedure () for the same token before sending the response by abruptly terminating the HTTP connection or stream. If the server does not consider the upload associated with this token 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.

Header Fields

Upload-Token The Upload-Token request header field is an Item Structured Header (see ) carrying the token used for identification of a specific upload. Its value MUST be a byte sequence. Its ABNF is If not otherwise specified by the server, the client is RECOMMENDED to use 256-bit (32 bytes) cryptographically-secure random binary data as the value of the Upload-Token, in order to ensure that it is globally unique and non-guessable. A conforming implementation MUST be able to handle a Upload-Token field value of at least 128 octets.

Upload-Offset The Upload-Offset request and response header field is an Item Structured Header indicating the resumption offset of corresponding upload, counted in bytes. Its value MUST be an integer. Its ABNF is

Upload-Incomplete The Upload-Incomplete request and response header field is an Item Structured Header indicating whether the corresponding upload is considered complete. Its value MUST be a boolean. Its ABNF is

Redirection The 301 (Moved Permanently) status code and the 302 (Found) status code MUST NOT be used in Offset Retrieving Procedure () and Upload Cancellation Procedure () 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 (), it MAY apply the redirection directly in the immediate subsequent Upload Transfer Procedure ().

Security Considerations The tokens inside the Upload-Token header field can be selected by the client which has no knowledge of tokens picked by other client, so uniqueness cannot be guaranteed. If the token is guessable, an attacker can append malicious data to ongoing uploads. To mitigate these issues, 256-bit cryptographically-secure random binary data is recommended for the token. It is OPTIONAL for the server to partition upload tokens based on client identity established through other channels, such as Cookie or TLS client authentication. The client MAY relax the token strength if it is aware of server-side partitioning.

IANA Considerations This specification registers the following entry in the Permanent Message Header Field Names registry established by : Header field name: Upload-Token, Upload-Offset, Upload-Incomplete Applicable protocol: http Status: standard Author/change controller: IETF Specification: This document Related information: n/a This specification registers the following entry in the "HTTP Status Codes" registry: Code: 104 Description: Upload Resumption Supported Specification: This document

Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values. This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.

Acknowledgments TODO acknowledge.

Appendix

Informational Response The server is allowed to respond to Upload Transfer Procedure () 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 Transfer 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 Transfer Procedure (i.e. resumable are supported, the provided upload token 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).

Feature Detection 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. Ideally, the technique used for feature detection meets following criteria (there might not be one approach which fits all requirements, so we have to prioritize them):

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)

Following approaches have already been considered in the past. All except the last approaches have not been deemed acceptable and are therefore not included in the specification. This follow list is a reference for the advantages and disadvantages of some approaches: 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.

Upload Metadata The Upload Transfer Procedure () 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.

FAQ

• 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.