HTTP Working Group M. Kleidl, Ed. Internet-Draft Transloadit Intended status: Standards Track G. Zhang, Ed. Expires: December 6, 2025 Apple Inc. L. Pardue, Ed. Cloudflare June 04, 2025 Resumable Uploads for HTTP draft-ietf-httpbis-resumable-upload-09 Abstract Data transfer using the Hypertext Transfer Protocol (HTTP) is often interrupted by canceled requests or dropped connections. If the intended recipient can indicate how much of the data was received prior to interruption, a sender can resume data transfer at that point instead of attempting to transfer all of the data again. 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. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at . Discussion of this document takes place on the HTTP Working Group mailing list (), which is archived at . Working Group information can be found at . Source for this draft and an issue tracker can be found at . 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 https://datatracker.ietf.org/drafts/current/. Kleidl, et al. Expires December 6, 2025 [Page 1] Internet-Draft Resumable Uploads June 2025 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 December 6, 2025. Copyright Notice Copyright (c) 2025 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Example 1: Complete upload of representation data with known size . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Example 2: Upload as a series of parts . . . . . . . . . 7 4. Upload Resource . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. State . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.1.1. Offset . . . . . . . . . . . . . . . . . . . . . . . 10 4.1.2. Completeness . . . . . . . . . . . . . . . . . . . . 11 4.1.3. Length . . . . . . . . . . . . . . . . . . . . . . . 11 4.1.4. Limits . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Upload Creation . . . . . . . . . . . . . . . . . . . . . 14 4.2.1. Client Behavior . . . . . . . . . . . . . . . . . . . 14 4.2.2. Server Behavior . . . . . . . . . . . . . . . . . . . 15 4.2.3. Examples . . . . . . . . . . . . . . . . . . . . . . 16 4.3. Offset Retrieval . . . . . . . . . . . . . . . . . . . . 18 4.3.1. Client Behavior . . . . . . . . . . . . . . . . . . . 18 4.3.2. Server Behavior . . . . . . . . . . . . . . . . . . . 19 4.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 19 4.4. Upload Append . . . . . . . . . . . . . . . . . . . . . . 20 4.4.1. Client Behavior . . . . . . . . . . . . . . . . . . . 20 4.4.2. Server Behavior . . . . . . . . . . . . . . . . . . . 21 4.4.3. Example . . . . . . . . . . . . . . . . . . . . . . . 22 Kleidl, et al. Expires December 6, 2025 [Page 2] Internet-Draft Resumable Uploads June 2025 4.5. Upload Cancellation . . . . . . . . . . . . . . . . . . . 23 4.5.1. Client Behavior . . . . . . . . . . . . . . . . . . . 23 4.5.2. Server Behavior . . . . . . . . . . . . . . . . . . . 23 4.5.3. Example . . . . . . . . . . . . . . . . . . . . . . . 24 4.6. Concurrency . . . . . . . . . . . . . . . . . . . . . . . 24 5. Status Code 104 (Upload Resumption Supported) . . . . . . . . 25 6. Media Type application/partial-upload . . . . . . . . . . . . 25 7. Problem Types . . . . . . . . . . . . . . . . . . . . . . . . 26 7.1. Mismatching Offset . . . . . . . . . . . . . . . . . . . 26 7.2. Completed Upload . . . . . . . . . . . . . . . . . . . . 26 7.3. Inconsistent Length . . . . . . . . . . . . . . . . . . . 27 8. Content Codings . . . . . . . . . . . . . . . . . . . . . . . 27 9. Transfer Codings . . . . . . . . . . . . . . . . . . . . . . 28 10. Integrity Digests . . . . . . . . . . . . . . . . . . . . . . 28 10.1. Representation Digests . . . . . . . . . . . . . . . . . 28 10.2. Content Digests . . . . . . . . . . . . . . . . . . . . 29 11. Responses to Uploads . . . . . . . . . . . . . . . . . . . . 29 12. Upload Strategies . . . . . . . . . . . . . . . . . . . . . . 30 12.1. Optimistic Upload Creation . . . . . . . . . . . . . . . 30 12.1.1. Upgrading To Resumable Uploads . . . . . . . . . . . 31 12.2. Careful Upload Creation . . . . . . . . . . . . . . . . 31 13. Security Considerations . . . . . . . . . . . . . . . . . . . 32 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 14.1. HTTP Fields . . . . . . . . . . . . . . . . . . . . . . 33 14.2. HTTP Status Code . . . . . . . . . . . . . . . . . . . . 33 14.3. Media Type . . . . . . . . . . . . . . . . . . . . . . . 33 14.4. HTTP Problem Types . . . . . . . . . . . . . . . . . . . 34 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 15.1. Normative References . . . . . . . . . . . . . . . . . . 35 15.2. Informative References . . . . . . . . . . . . . . . . . 36 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 37 A.1. Since draft-ietf-httpbis-resumable-upload-08 . . . . . . 37 A.2. Since draft-ietf-httpbis-resumable-upload-07 . . . . . . 37 A.3. Since draft-ietf-httpbis-resumable-upload-06 . . . . . . 37 A.4. Since draft-ietf-httpbis-resumable-upload-05 . . . . . . 38 A.5. Since draft-ietf-httpbis-resumable-upload-04 . . . . . . 38 A.6. Since draft-ietf-httpbis-resumable-upload-03 . . . . . . 38 A.7. Since draft-ietf-httpbis-resumable-upload-02 . . . . . . 39 A.8. Since draft-ietf-httpbis-resumable-upload-01 . . . . . . 39 A.9. Since draft-ietf-httpbis-resumable-upload-00 . . . . . . 39 A.10. Since draft-tus-httpbis-resumable-uploads-protocol-02 . . 39 A.11. Since draft-tus-httpbis-resumable-uploads-protocol-01 . . 39 A.12. Since draft-tus-httpbis-resumable-uploads-protocol-00 . . 40 Appendix B. Draft Version Identification . . . . . . . . . . . . 40 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 40 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 Kleidl, et al. Expires December 6, 2025 [Page 3] Internet-Draft Resumable Uploads June 2025 1. Introduction Data transfer using the Hypertext Transfer Protocol ([HTTP]) is often interrupted by canceled requests or dropped connections. If the intended recipient can indicate how much of the data was received prior to interruption, a sender can resume data transfer at that point instead of attempting to transfer all of the data again. HTTP range requests (see Section 14 of [HTTP]) support this concept of resumable data transfers for downloads from server to client. This specification defines a mechanism for resumable uploads from client to server in a way 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 send the remaining representation data. Alternatively, they can cancel the upload entirely. Unlike ranged downloads, this protocol does not support transferring an upload as multiple requests in parallel. Utilizing resumable uploads, applications can recover from unintended interruptions, but also interrupt an upload on purpose to later resume it, for example, when a user wants to pause an upload, the device's network connectivity changes, or bandwidth should be saved for higher priority tasks. The document introduces the concept of an upload resource to facilitate resumable uploads (Section 4) and defines new header fields to communicate the state of the upload (Section 4.1), the status code "104 (Upload Resumption Supported)" to indicate the server's support for resumable uploads (Section 5), and the "application/partial-upload" media type to label partial representation data when resuming an upload (Section 6). 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Some examples in this document contain long lines that may be folded, as described in [RFC8792]. The terms Structured Header, Item, Dictionary, String, Integer, and Boolean are imported from [STRUCTURED-FIELDS]. Kleidl, et al. Expires December 6, 2025 [Page 4] Internet-Draft Resumable Uploads June 2025 The terms "representation", "representation data", "representation metadata", "content", "client" and "server" are from Section 3 of [HTTP]. The term "URI" is used as defined in Section 4 of [HTTP]. The term "patch document" is taken from [PATCH]. An _upload resource_ is a temporary resource on the server that facilitates the resumable upload of one representation (Section 4). 3. Overview Resumable uploads are supported in HTTP through use of a temporary resource, an _upload resource_ (Section 4), that is separate from the resource being uploaded to and specific to that upload. By interacting with the upload resource, a client can retrieve the current offset of the upload (Section 4.3), append to the upload (Section 4.4), and cancel the upload (Section 4.5). The remainder of this section uses examples to illustrate different interactions with the upload resource. HTTP message exchanges, and thereby resumable uploads, use representation data (see Section 8.1 of [HTTP]). This means that resumable uploads can be used with many forms of content, such as static files, in-memory buffers, data from streaming sources, or on-demand generated data. 3.1. Example 1: Complete upload of representation data with known size In this example, the client first attempts to upload representation data with a known size in a single HTTP request to the resource at "/project/123/files". An interruption occurs and the client then attempts to resume the upload using subsequent HTTP requests to the upload resource at "/uploads/abc". 1) The client notifies the server that it wants to begin an upload (Section 4.2). The server reserves the required resources to accept the upload from the client, and the client begins transferring the entire representation data in the request content. An interim response can be sent to the client, which signals the server's support of resumable upload as well as the upload resource's URI via the Location header field (Section 10.2.2 of [HTTP]). Kleidl, et al. Expires December 6, 2025 [Page 5] Internet-Draft Resumable Uploads June 2025 Client Server | | | POST /project/123/files | | Upload-Complete: ?1 | |------------------------------------------->| | | | | Reserve resources | | for upload | |-----------------. | | | | |<----------------' | | | 104 Upload Resumption Supported | | Location: /uploads/abc | |<-------------------------------------------| | | X--------------Flow Interrupted--------------X Figure 1: Upload Creation 2) If the connection to the server is interrupted, the client might want to resume the upload. However, before this is possible the client needs to know the amount of representation data that the server received before the interruption. It does so by retrieving the offset (Section 4.3) from the upload resource. Client Server | | | HEAD /uploads/abc | |------------------------------------------------>| | | | 204 No Content | | Upload-Offset: X | |<------------------------------------------------| | | Figure 2: Offset Retrieval 3) The client can resume the upload by sending the remaining representation data to the upload resource (Section 4.4), appending to the already stored representation data in the upload using the "application/partial-upload" media type. The "Upload-Offset" value is included to ensure that the client and server agree on the offset that the upload resumes from. Once the remaining representation data is transferred, the server processes the entire representation and responds with whatever the initial request to "/project/123/files" would have produced if it had not been interrupted, e.g. a "200 (OK)" response. Kleidl, et al. Expires December 6, 2025 [Page 6] Internet-Draft Resumable Uploads June 2025 Client Server | | | PATCH /uploads/abc | | Upload-Complete: ?1 | | Upload-Offset: X | | Content-Type: application/partial-upload | |------------------------------------------------>| | | | 200 OK | |<------------------------------------------------| | | Figure 3: Upload Append 4) If the client is not interested in completing the upload, it can instruct the upload resource to delete the upload and free all related resources (Section 4.5). Client Server | | | DELETE /uploads/abc | |------------------------------------------------>| | | | 204 No Content | |<------------------------------------------------| | | Figure 4: Upload Cancellation 3.2. Example 2: Upload as a series of parts In some cases, clients might prefer to upload a representation as a series of parts sent serially 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 of the representation data, such as when the 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 representation incrementally. 1) If the client is aware that the server supports resumable upload, it can start an upload with the "Upload-Complete" field value set to false and the first part of the representation. Kleidl, et al. Expires December 6, 2025 [Page 7] Internet-Draft Resumable Uploads June 2025 Client Server | | | POST /project/123/files | | Upload-Complete: ?0 | |------------------------------------------------>| | | | 201 Created | | Location: /uploads/abc | |<------------------------------------------------| | | Figure 5: Upload creation with partial representation data 2) Subsequent, intermediate parts are appended (Section 4.4) with the "Upload-Complete" field value set to false, indicating that they are not the last part of the representation data. The offset value in the "Upload-Offset" header field is taken from the previous response when creating the upload or appending to it. Client Server | | | PATCH /uploads/abc | | Upload-Complete: ?0 | | Upload-Offset: X | | Content-Type: application/partial-upload | |------------------------------------------------>| | | | 204 No Content | |<------------------------------------------------| | | Figure 6: Appending partial representation data to upload 3) If the connection was interrupted, the client might want to resume the upload, similar to the previous example (Section 3.1). The client retrieves the offset (Section 4.3) to learn the amount of representation data received by the server and then continues appending the remaining parts to the upload as in the previous step. Kleidl, et al. Expires December 6, 2025 [Page 8] Internet-Draft Resumable Uploads June 2025 Client Server | | | HEAD /uploads/abc | |------------------------------------------------>| | | | 204 No Content | | Upload-Offset: Y | |<------------------------------------------------| | | | PATCH /uploads/abc | | Upload-Complete: ?0 | | Upload-Offset: Y | | Content-Type: application/partial-upload | |------------------------------------------------>| | | | 204 No Content | |<------------------------------------------------| | | Figure 7: Resuming an interrupted upload 4) The request to append the last part of the representation data has a "Upload-Complete" field value set to true to indicate the complete transfer. Once the remaining representation data is transferred, the server processes the entire representation and responds with whatever the initial request to "/project/123/files" would have produced if it had received the entire representation, e.g. a "200 (OK)" response. Client Server | | | PATCH /uploads/abc | | Upload-Offset: Z | | Upload-Complete: ?1 | | Content-Type: application/partial-upload | |------------------------------------------------>| | | | 200 OK | |<------------------------------------------------| | | Figure 8: Appending remaining representation data 4. Upload Resource A resumable upload is enabled through interaction with an upload resource. When a resumable upload begins, the server is asked to create an upload resource through a request to another resource (Section 4.2). This upload resource is responsible for handling the Kleidl, et al. Expires December 6, 2025 [Page 9] Internet-Draft Resumable Uploads June 2025 upload of a representation. Using the upload resource, the client can query the upload progress (Section 4.3), append representation data (Section 4.4), or cancel the upload (Section 4.5). An upload resource is specific to the upload of one representation. For uploading multiple representations, multiple upload resources have to be used. The server can clean up an upload resource and make it inaccessible immediately after the upload is complete. However, if a client didn't receive the last response acknowledging the upload's completion and the upload resource is not available anymore, the client cannot verify the upload's state with the server. Therefore, the server SHOULD keep the upload resource available for a reasonable amount of time after the upload is complete. An upload resource SHOULD NOT reuse the URI from a previous upload resource, unless reasonable time has passed to ensure that no client will attempt to access the previous upload resource. Otherwise, a client might access the upload resource corresponding to a different representation than it intends to transfer. 4.1. State The state of an upload consists of the following properties that are tracked by the upload resource. 4.1.1. Offset The offset is the number of bytes from the representation data that have been received, either during the creation of the upload resource (Section 4.2) and by appending to it (Section 4.4). The offset can be retrieved from the upload resource (Section 4.3) and is required when appending representation data (Section 4.4) to synchronize the client and resource regarding the amount of transferred representation data. Representation data received by the upload resource cannot be removed again and, therefore, the offset MUST NOT decrease. If the upload resource loses representation data, the server MUST consider the upload resource invalid and reject further interaction with it. The "Upload-Offset" request and response header field conveys the offset. "Upload-Offset" is an Item Structured Header Field ([STRUCTURED-FIELDS]). Its value is a non-negative Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) and indicates the current offset as viewed by the message sender. Other values MUST cause the entire header field to be ignored. Kleidl, et al. Expires December 6, 2025 [Page 10] Internet-Draft Resumable Uploads June 2025 The "Upload-Offset" header field in responses serves as an acknowledgement of the received representation data and as a guarantee that no retransmission of it will be necessary. Clients can use this guarantee to free resources associated to transferred representation data. 4.1.2. Completeness An upload is incomplete until it is explicitly marked as completed by the client. After this point, no representation data can be appended anymore. The "Upload-Complete" request and response header field conveys the completeness state. "Upload-Complete" is an Item Structured Header Field ([STRUCTURED-FIELDS]). Its value is a Boolean (Section 3.3.6 of [STRUCTURED-FIELDS]) and indicates whether the upload is complete or not. Other values MUST cause the entire header field to be ignored. An upload is marked as completed if a request for creating the upload resource (Section 4.2) or appending to it (Section 4.4) included the "Upload-Complete" header field with a true value and the request content was fully received. 4.1.3. Length The length of an upload is the number of bytes of representation data that the client intends to upload. Even the client might not know the total length of the representation data when starting the transfer, for example, because the representation is taken from a streaming source. However, a client SHOULD communicate the length to the upload resource as soon as it becomes known. There are two different ways for the client to indicate and the upload resource to discover the length from requests for creating the upload resource (Section 4.2) or appending to it (Section 4.4): o If the request includes the "Upload-Complete" field value set to true and a valid "Content-Length" header field, the request content is the remaining representation data. The length is then the sum of the current offset (Section 4.1.1) and the "Content- Length" header field value. o The request can include the "Upload-Length" request and response header field. "Upload-Length" is an Item Structured Header Field ([STRUCTURED-FIELDS]). Its value is a non-negative Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) and indicates the number of Kleidl, et al. Expires December 6, 2025 [Page 11] Internet-Draft Resumable Uploads June 2025 bytes of the entire representation data. Other values MUST cause the entire header field to be ignored. If both indicators are present in the same request, their indicated lengths MUST match. If multiple requests include indicators, their indicated values MUST match. A server can use the problem type [PROBLEM] of "https://iana.org/assignments/http-problem- types#inconsistent-upload-length" (Section 7.3) in responses to indicate inconsistent length values. The upload resource might not know the length until the upload is complete. Note that the length and offset values do not determine whether an upload is complete. Instead, the client uses the "Upload-Complete" (Section 4.1.2) header field to indicate that a request completes the upload. The offset could match the length, but the upload can still be incomplete. 4.1.4. Limits An upload resource MAY enforce one or multiple limits, which are communicated to the client via the "Upload-Limit" response header field. "Upload-Limit" is a Dictionary Structured Header Field ([STRUCTURED-FIELDS]). Its value is a Dictionary (Section 3.2 of [STRUCTURED-FIELDS]). Other values MUST cause the entire header field to be ignored. The following key-value pairs are defined: o The value under the "max-size" key specifies a maximum size for the representation data, counted in bytes. The server might not create an upload resource if the length (Section 4.1.3) deduced from the upload creation request is larger than the maximum size. The upload resource can stop the upload if the offset (Section 4.1.1) exceeds the maximum size. The value is an Integer. o The value under the "min-size" key specifies a minimum size for the representation data, counted in bytes. The server might not create an upload resource if the length (Section 4.1.3) deduced from the upload creation request is smaller than the minimum size or no length can be deduced at all. The value is an Integer. o The value under the "max-append-size" key specifies a maximum size counted in bytes for the request content in a single upload append request (Section 4.4). The server might reject requests exceeding Kleidl, et al. Expires December 6, 2025 [Page 12] Internet-Draft Resumable Uploads June 2025 this limit. A client that is aware of this limit MUST NOT send larger upload append requests. The value is an Integer. o The value under the "min-append-size" key specifies a minimum size counted in bytes for the request content in a single upload append request (Section 4.4). The server might reject requests below this limit. A client that is aware of this limit MUST NOT send smaller upload append requests. The value is an Integer. Requests completing the upload by including the "Upload-Complete: ?1" header field are exempt from this limit. o The value under the "max-age" key specifies the remaining lifetime of the upload resource in seconds counted from the generation of the response. After the resource's lifetime is reached, the server might make the upload resource inaccessible and a client SHOULD NOT attempt to access the upload resource as these requests will likely fail. The lifetime MAY be extended but SHOULD NOT be reduced unless the server makes the upload resource immediately inaccessible. The value is an Integer. Except for the "max-age" limit, the existence of a limit or its value MUST NOT change throughout the lifetime of the upload resource. When parsing the "Upload-Limit" header field, unrecognized keys MUST be ignored and MUST NOT fail the parsing to facilitate the addition of new limits in the future. Keys with values other than defined here MUST be ignored. A server that supports the creation of a resumable upload resource (Section 4.2) for a target URI MUST include the "Upload-Limit" header field with the corresponding limits in a response to an "OPTIONS" request sent to this target URI. If a server supports the creation of upload resources for any target URI, it SHOULD include the "Upload-Limit" header field with the corresponding limits in a response to an "OPTIONS" request with the "*" target unless the server is not capable of handling "OPTIONS *" requests. If the server does not apply any limits, it MUST use "min-size=0" instead of an empty header value. A client can use an "OPTIONS" request to discover support for resumable uploads and potential limits before creating an upload resource. To reduce the liklihood of failing requests, the limits announced in an "OPTIONS" response SHOULD NOT be less restrictive than the limits applied to an upload once the upload resource has been created, unless the request to create an upload resource included additional information that warrants different limits. For example, a server might announce a general maximum size limit of 1GB, but reduce it to 100MB when the media type indicates an image. Kleidl, et al. Expires December 6, 2025 [Page 13] Internet-Draft Resumable Uploads June 2025 4.2. Upload Creation 4.2.1. Client Behavior A client can start a resumable upload from any request that can carry content by including the "Upload-Complete" header field (Section 4.1.2). As a consequence, all request methods that allow content are possible, such as "POST", "PUT", and "PATCH". The "Upload-Complete" header field is set to true if the request content includes the entire representation data that the client intends to upload. This is also a requirement for transparently upgrading to resumable uploads from traditional uploads (Section 12.1.1). If the client knows the representation data's length, it SHOULD include the "Upload-Length" header field (Section 4.1.3) in the request to help the server allocate necessary resources for the upload and provide early feedback if the representation violates a limit (Section 4.1.4). The client SHOULD respect any limits (Section 4.1.4) announced in the "Upload-Limit" header field in interim or final responses. In particular, if the allowed maximum size is less than the amount of representation data the client intends to upload, the client SHOULD stop the current request immediately and cancel the upload (Section 4.5). The request content can be empty. If the "Upload-Complete" header field is then set to true, the client intends to upload an empty representation. An "Upload-Complete" header field is set to false is also valid. This can be used to retrieve the upload resource's URI before transferring any representation data. Since interim responses are optional, this technique provides another mechanism to learn the URI, at the cost of an additional round-trip before data upload can commence. Representation metadata included in the initial request (see Section 8.3 of [HTTP]) can affect how servers act on the uploaded representation data. The "Content-Type" header field (Section 8.3 of [HTTP]) indicates the media type of the representation. The "Content-Disposition" header field ([CONTENT-DISPOSITION]) can be used to transmit a filename. The "Content-Encoding" header field (Section 8.4 of [HTTP]) names the content codings applied to the representation. If the client received a final response with a Kleidl, et al. Expires December 6, 2025 [Page 14] Internet-Draft Resumable Uploads June 2025 o "2xx (Successful)" status code and the entire representation data was transferred in the request content, the upload is complete and the response belongs to the targeted resource processing the representation. o "2xx (Successful)" status code and not the entire representation data was transferred in the request content, the "Location" response header field points the client to the created upload resource. The client can continue appending representation data to it (Section 4.4). o "4xx (Client Error)" status code, the client SHOULD NOT attempt to retry or resume the upload, unless the semantics of the response allow or recommend the client to retry the request. o "5xx (Server Error)" status code or no final response at all due to connectivity issues, the client MAY automatically attempt upload resumption by retrieving the current offset (Section 4.3) if it received the URI of the upload resource in a "104 (Upload Resumption Supported)" interim response. 4.2.2. Server Behavior Upon receiving a request with the "Upload-Complete" header field, the server can choose to offer resumption support by creating an upload resource. If so, it SHOULD announce the upload resource by sending an interim response with the "104 (Upload Resumption Supported)" status code and the "Location" header field pointing to the upload resource unless the server is not capable of sending interim responses. The interim response MUST include the "Upload-Limit" header field with the corresponding limits (Section 4.1.4) if existing. The interim response allows the client to resume the upload even if the message exchange gets later interrupted. The resource targeted by this initial request is responsible for processing the representation data transferred in the resumable upload according to the method and header fields in the initial request, while the upload resource enables resuming the transfer. If the "Upload-Complete" request header field is set to true, the client intends to transfer the entire representation data in one request. If the request content was fully received, no resumable upload is needed and the resource proceeds to process the request and generate a response. If the "Upload-Complete" header field is set to false, the client intends to transfer the representation over multiple requests. If the request content was fully received, the server MUST announce the Kleidl, et al. Expires December 6, 2025 [Page 15] Internet-Draft Resumable Uploads June 2025 upload resource by referencing it in the "Location" response header field. Servers are RECOMMENDED to use the "201 (Created)" status code. The response MUST include the "Upload-Limit" header field with the corresponding limits if existing. The server MUST record the length according to Section 4.1.3 if the necessary header fields are included in the request. While the request content is being received, the server MAY send additional interim responses with a "104 (Upload Resumption Supported)" status code and the "Upload-Offset" header field set to the current offset to inform the client about the upload progress. These interim responses MUST NOT include the "Location" header field. If the server does not receive the entire request content, for example because of canceled requests or dropped connections, it SHOULD append as much of the request content as possible to the upload resource. The upload resource MUST NOT be considered complete then. 4.2.3. Examples A) The following example shows an upload creation, where the entire 100 bytes are transferred in the initial request. The server sends multiple interim responses and one final response from processing the uploaded representation. POST /project/123/files HTTP/1.1 Host: example.com Upload-Complete: ?1 Content-Length: 100 Upload-Length: 100 [content (100 bytes)] HTTP/1.1 104 Upload Resumption Supported Location: https://example.com/upload/b530ce8ff Upload-Limit: max-size=1000000000 HTTP/1.1 104 Upload Resumption Supported Upload-Offset: 50 HTTP/1.1 200 OK Location: https://example.com/upload/b530ce8ff Upload-Limit: max-size=1000000000 Content-Type: application/json {"attachmentId": "b530ce8ff"} Kleidl, et al. Expires December 6, 2025 [Page 16] Internet-Draft Resumable Uploads June 2025 B) The following example shows an upload creation, where only the first 25 bytes of a 100 bytes upload are transferred. The server acknowledges the received representation data and that the upload is not complete yet. The client can continue appending data. POST /upload HTTP/1.1 Host: example.com Upload-Complete: ?0 Content-Length: 25 Upload-Length: 100 [partial content (25 bytes)] HTTP/1.1 104 Upload Resumption Supported Location: https://example.com/upload/b530ce8ff HTTP/1.1 201 Created Location: https://example.com/upload/b530ce8ff Upload-Limit: max-size=1000000000 C) The following example shows an upload creation, where the server responds with a 5xx status code. Thanks to the interim response containing the upload resource URI, the client can resume the upload. POST /upload HTTP/1.1 Host: example.com Upload-Complete: ?1 Content-Length: 100 Upload-Length: 100 [content (100 bytes)] HTTP/1.1 104 Upload Resumption Supported Location: https://example.com/upload/b530ce8ff HTTP/1.1 500 Internal Server Error D) The following example shows an upload creation being rejected by the server. The client cannot continue the upload. POST /upload HTTP/1.1 Host: example.com Upload-Complete: ?1 Content-Length: 100 Upload-Length: 100 [content (100 bytes)] Kleidl, et al. Expires December 6, 2025 [Page 17] Internet-Draft Resumable Uploads June 2025 HTTP/1.1 400 Bad Request 4.3. Offset Retrieval 4.3.1. Client Behavior If the client wants to resume the upload after an interruption, it has to know the amount of representation data received by the upload resource so far. It can fetch the offset by sending a "HEAD" request to the upload resource. Upon a successful response, the client can continue the upload by appending representation data (Section 4.4) starting at the offset indicated by the "Upload-Offset" response header field. The offset can be less than or equal to the number of bytes of representation data that 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 representation data it has already sent, the upload MUST be considered a failure. The client SHOULD cancel the upload (Section 4.5) after rejecting the offset. The client MUST NOT perform offset retrieval while creation (Section 4.2) or appending (Section 4.4) is in progress as this can cause the previous request to be terminated by the server as described in Section 4.6. If the client received a response with a o "2xx (Successful)" status code, the client can continue appending representation data to it (Section 4.4) if the upload is not complete yet. o "307 (Temporary Redirect)" or "308 (Permanent Redirect)" status code, the client MAY retry retrieving the offset from the new URI. o "4xx (Client Error)" status code, the client SHOULD NOT attempt to retry or resume the upload, unless the semantics of the response allow or recommend the client to retry the request. o "5xx (Server Error)" status code or no final response at all due to connectivity issues, the client MAY retry retrieving the offset. Kleidl, et al. Expires December 6, 2025 [Page 18] Internet-Draft Resumable Uploads June 2025 4.3.2. Server Behavior A successful response to a "HEAD" request against an upload resource o MUST include the offset in the "Upload-Offset" header field (Section 4.1.1), o MUST include the completeless state in the "Upload-Complete" header field (Section 4.1.2), o MUST include the length in the "Upload-Length" header field if known (Section 4.1.3), o MUST indicate the limits in the "Upload-Limit" header field (Section 4.1.4), and o SHOULD include the "Cache-Control" header field with the value "no-store" to prevent HTTP caching ([CACHING]). The resource SHOULD NOT generate a response with the "301 (Moved Permanently)" and "302 (Found)" status codes because clients might follow the redirect without preserving the "HEAD" method. 4.3.3. Example A) The following example shows an offset retrieval request. The server indicates the current offset and that the upload is not complete yet. The client can continue to append representation data. HEAD /upload/b530ce8ff HTTP/1.1 Host: example.com HTTP/1.1 204 No Content Upload-Offset: 100 Upload-Complete: ?0 Upload-Length: 500 Upload-Limit: max-age=3600 Cache-Control: no-store B) The following example shows on offset retrieval request for a completed upload. The client does not need to continue the upload. HEAD /upload/b530ce8ff HTTP/1.1 Host: example.com Kleidl, et al. Expires December 6, 2025 [Page 19] Internet-Draft Resumable Uploads June 2025 HTTP/1.1 204 No Content Upload-Offset: 500 Upload-Complete: ?1 Upload-Length: 500 Cache-Control: no-store 4.4. Upload Append 4.4.1. Client Behavior A client can continue the upload and append representation data by sending a "PATCH" request with the "application/partial-upload" media type (Section 6) to the upload resource. The request content is the representation data to append. The client MUST indicate the offset of the request content inside the representation data by including the "Upload-Offset" request header field. To ensure that the upload resource will accept request, the offset SHOULD be taken from an immediate previous response for retrieving the offset (Section 4.3) or appending representation data (Section 4.4). The request MUST include the "Upload-Complete" header field. Its value is true if the end of the request content is the end of the representation data. If the content is then fully received by the upload resource, the upload will be complete. The request content can be empty. If the "Upload-Complete" field is then set to true, the client wants to complete the upload without appending additional representation data. If the client received a final response with a o "2xx (Successful)" status code and the remaining representation data was transferred in the request content, the upload is complete and the corresponding response belongs to the resource processing the representation according to the initial request (see Section 4.2). o "2xx (Successful)" status code and not the entire remaining representation data was transferred in the request content, the client can continue appending representation data. o "307 (Temporary Redirect)" or "308 (Permanent Redirect)" status code, the client MAY retry appending to the new URI. Kleidl, et al. Expires December 6, 2025 [Page 20] Internet-Draft Resumable Uploads June 2025 o "4xx (Client Error)" status code, the client SHOULD NOT attempt to retry or resume the upload, unless the semantics of the response allow or recommend the client to retry the request. o "5xx (Server Error)" status code or no final response at all due to connectivity issues, the client MAY automatically attempt upload resumption by retrieving the current offset (Section 4.3). 4.4.2. Server Behavior An upload resource applies a "PATCH" request with the "application/ partial-upload" media type (Section 6) by appending the patch document in the request content to the upload resource. If the upload resource does not receive the entire patch document, for example because of canceled requests or dropped connections, it SHOULD append as much of the patch document as possible, starting at its beginning and without discontinuities. Appending a continuous section starting at the patch document's beginning constitutes a successful PATCH as defined in Section 2 of [PATCH]. If the "Upload-Offset" request header field value does not match the current offset (Section 4.1.1), the upload resource MUST reject the request with a "409 (Conflict)" status code. The response MUST include the correct offset in the "Upload-Offset" header field. The response can use the problem type [PROBLEM] of "https://iana.org/assignments/http-problem-types#mismatching-upload- offset" (Section 7.1). If the upload is already complete (Section 4.1.2), the server MUST NOT modify the upload resource and MUST reject the request. The server can use the problem type [PROBLEM] of "https://iana.org/assignments/http-problem-types#completed-upload" in the response (Section 7.2). If the "Upload-Complete" request header field is set to true, the client intends to transfer the remaining representation data in one request. If the request content was fully received, the upload is marked as complete and the upload resource SHOULD generate the response that matches what the resource, that was targeted by the initial upload creation (Section 4.2), would have generated if it had received the entire representation in the initial request. However, the response MUST include the "Upload-Complete" header field with a true value, allowing clients to identify whether a response, in particular error responses, is related to the resumable upload itself or the processing of the upload representation. Kleidl, et al. Expires December 6, 2025 [Page 21] Internet-Draft Resumable Uploads June 2025 If the "Upload-Complete" request header field is set to false, the client intends to transfer the remaining representation over multiple requests. If the request content was fully received, the upload resource acknowledges the appended data by sending a "2xx (Successful)" response. If the request didn't complete the upload, any response, successful or not, MUST include the "Upload-Complete" header field with a false value, indicating that this response does not belong to the processing of the uploaded representation. The upload resource MUST record the length according to Section 4.1.3 if the necessary header fields are included in the request. If the length is known, the upload resource MUST prevent the offset from exceeding the upload length by stopping to append bytes once the offset reaches the length, rejecting the request, marking the upload resource invalid and rejecting any further interaction with it. It is not sufficient to rely on the "Content-Length" header field for enforcement because the header field might not be present. While the request content is being received, the server SHOULD send interim responses with a "104 (Upload Resumption Supported)" status code and the "Upload-Offset" header field set to the current offset to inform the client about the upload progress. These interim responses MUST NOT include the "Location" header field. 4.4.3. Example A) The following example shows an upload append request. The client transfers the next 100 bytes at an offset of 100 and does not indicate that the upload is then completed. The server generates one interim response and finally acknowledges the new offset: PATCH /upload/b530ce8ff HTTP/1.1 Host: example.com Upload-Complete: ?0 Upload-Offset: 100 Content-Length: 100 Content-Type: application/partial-upload [content (100 bytes)] HTTP/1.1 104 Upload Resumption Supported Upload-Offset: 150 HTTP/1.1 204 No Content Upload-Complete: ?0 Kleidl, et al. Expires December 6, 2025 [Page 22] Internet-Draft Resumable Uploads June 2025 B) The next example shows an upload append, where the client transfers the remaining 200 bytes and completes the upload. The server processes the uploaded representation and generates the responding response, in this example containing extracted meta data: PATCH /upload/b530ce8ff HTTP/1.1 Host: example.com Upload-Complete: ?1 Upload-Offset: 200 Content-Length: 100 Content-Type: application/partial-upload [content (100 bytes)] HTTP/1.1 200 OK Upload-Complete: ?1 Content-Type: application/json { "metadata": { [...] } } 4.5. Upload Cancellation 4.5.1. Client Behavior If the client wants to terminate the transfer without the ability to resume, it can send a "DELETE" request to the upload resource. Doing so is an indication that the client is no longer interested in continuing the upload, and that the server can release any resources associated with it. 4.5.2. Server Behavior Upon receiving a "DELETE" request, the server SHOULD deactivate the upload resource. The server SHOULD terminate any in-flight requests to the upload resource before sending the response by abruptly terminating their HTTP connection(s) or stream(s) as described in Section 4.6. The resource SHOULD NOT generate a response with the "301 (Moved Permanently)" and "302 (Found)" status codes because clients might follow the redirect without preserving the "DELETE" method. Kleidl, et al. Expires December 6, 2025 [Page 23] Internet-Draft Resumable Uploads June 2025 4.5.3. Example The following example shows an upload cancellation: DELETE /upload/b530ce8ff HTTP/1.1 Host: example.com HTTP/1.1 204 No Content 4.6. Concurrency Resumable uploads, as defined in this document, do not permit uploading representation data in parallel to the same upload resource. The client MUST NOT perform multiple representation data transfers for the same upload resource in parallel. Even if the client is well behaving and doesn't send concurrent requests, network interruptions can occur in such a way that the client considers a request as failed while the server is unaware of the problem and considers the request still ongoing. The client might then try to resume the upload with the best intentions, resulting in concurrent requests from the server's perspective. Therefore, the server MUST take measures to prevent race conditions, data loss and corruption from concurrent requests to append representation data (Section 4.4) and/or cancellation (Section 4.5) to the same upload resource. In addition, the server MUST NOT send outdated information in responses when retrieving the offset (Section 4.3). This means that the offset sent by the server MUST be accepted in a subsequent request to append representation data if no other request to append representation data or cancel was received in the meantime. In other words, clients have to be able to use received offsets. The RECOMMENDED approach is as follows: If an upload resource receives a new request to retrieve the offset (Section 4.3), append representation data (Section 4.4), or cancel the upload (Section 4.5) while a previous request for creating the upload (Section 4.2) or appending representation data (Section 4.4) is still ongoing, the resource SHOULD prevent race conditions, data loss, and corruption by terminating the previous request before processing the new request. Due to network delay and reordering, the resource might still be receiving representation data from an ongoing transfer for the same upload resource, which in the client's perspective has failed. Since the client is not allowed to perform multiple transfers in parallel, the upload resource can assume that the previous attempt has already failed. Therefore, the server MAY abruptly terminate the previous HTTP connection or stream. Kleidl, et al. Expires December 6, 2025 [Page 24] Internet-Draft Resumable Uploads June 2025 Since implementing this approach is not always technically possible or feasible, other measures can be considered as well. A simpler approach is that the server only processes a new request to retrieve the offset (Section 4.3), append representation data (Section 4.4), or cancellation (Section 4.5) once all previous requests have been processed. This effectively implements exclusive access to the upload resource through an access lock. However, since network interruptions can occur in ways that cause the request to hang from the server's perspective, it might take the server significant time to realize the interruption and time out the request. During this period, the client will be unable to access the resource and resume the upload, causing friction for the end users. Therefore, the recommended approach is to terminate previous requests to enable quick resumption of uploads. 5. Status Code 104 (Upload Resumption Supported) The "104 (Upload Resumption Supported)" status code is can be used for two purposes: o When responding to requests to create uploads, an interim response with the "104 (Upload Resumption Supported)" status code can be sent to indicate the server's support for resumable uploads, as well as the URI and limits of the corresponding upload resource in the "Location" and "Upload-Limit" header fields, respectively (see Section 4.2). This notifies the client early about the ability to resume the upload in case of network interruptions. o While processing the content of a request to append representation data or create an upload, the server can regularly send interim responses with the "104 (Upload Resumption Supported)" status code to indicate the current upload progress in the "Upload-Offset" header field (see Section 4.2 and Section 4.4). This allows the client to show more accurate progress information about the amount of data receive by the server. In addition, clients can use this information to release representation data that was buffered, knowing that it doesn't have to be re-transmitted. 6. Media Type application/partial-upload The "application/partial-upload" media type describes a contiguous block from the representation data that should be uploaded to a resource. There is no minimum block size and the block might be empty. The block can be a subset of the representation data, where the start and/or end of the block don't line up with the start and/or end of the representation data respectively. Kleidl, et al. Expires December 6, 2025 [Page 25] Internet-Draft Resumable Uploads June 2025 7. Problem Types 7.1. Mismatching Offset This section defines the "https://iana.org/assignments/http-problem- types#mismatching-upload-offset" problem type [PROBLEM]. A server can use this problem type when responding to an upload append request (Section 4.4) to indicate that the "Upload-Offset" header field in the request does not match the upload resource's offset. Two problem type extension members are defined: the "expected-offset" and "provided-offset" members. A response using this problem type SHOULD populate both members, with the value of "expected-offset" taken from the upload resource and the value of "provided-offset" taken from the upload append request. The following example shows an example response, where the resource's offset was 100, but the client attempted to append at offset 200: # NOTE: '\' line wrapping per RFC 8792 HTTP/1.1 409 Conflict Content-Type: application/problem+json { "type":"https://iana.org/assignments/http-problem-types#\ mismatching-upload-offset", "title": "offset from request does not match offset of resource", "expected-offset": 100, "provided-offset": 200 } 7.2. Completed Upload This section defines the "https://iana.org/assignments/http-problem- types#completed-upload" problem type [PROBLEM]. A server can use this problem type when responding to an upload append request (Section 4.4) to indicate that the upload has already been completed and cannot be modified. The following example shows an example response: Kleidl, et al. Expires December 6, 2025 [Page 26] Internet-Draft Resumable Uploads June 2025 # NOTE: '\' line wrapping per RFC 8792 HTTP/1.1 400 Bad Request Content-Type: application/problem+json { "type":"https://iana.org/assignments/http-problem-types#\ completed-upload", "title": "upload is already completed" } 7.3. Inconsistent Length This section defines the "https://iana.org/assignments/http-problem- types#inconsistent-upload-length" problem type [PROBLEM]. A server can use this problem type when responding to an upload creation (Section 4.2) or upload append request (Section 4.4) to indicate that the request includes inconsistent upload length values, as described in Section 4.1.3. The following example shows an example response: # NOTE: '\' line wrapping per RFC 8792 HTTP/1.1 400 Bad Request Content-Type: application/problem+json { "type":"https://iana.org/assignments/http-problem-types#\ inconsistent-upload-length", "title": "inconsistent length values for upload" } 8. Content Codings Since the codings listed in "Content-Encoding" are a characteristic of the representation (see Section 8.4 of [HTTP]), both the client and the server always compute the values for "Upload-Offset" and optionally "Upload-Length" on the content coded data (that is, the representation data). Moreover, the content codings are retained throughout the entire upload, meaning that the server is not required to decode the representation data to support resumable uploads. See Appendix A of [DIGEST-FIELDS] for more information. Kleidl, et al. Expires December 6, 2025 [Page 27] Internet-Draft Resumable Uploads June 2025 9. Transfer Codings Unlike "Content-Encoding" (see Section 8.4.1 of [HTTP]), "Transfer- Encoding" (see Section 6.1 of [RFC9112]) is a property of the message, not of the representation. Moreover, transfer codings can be applied in transit (e.g., by proxies). This means that a client does not have to consider the transfer codings to compute the upload offset, while a server is responsible for transfer decoding the message before computing the upload offset. The same applies to the value of "Upload-Length". Please note that the "Content-Length" header field cannot be used in conjunction with the "Transfer- Encoding" header field. 10. Integrity Digests The integrity of an entire upload or individual upload requests can be verifying using digests from [DIGEST-FIELDS]. 10.1. Representation Digests Representation digests help verify the integrity of the entire representation data that has been uploaded so far, which might strech across multiple requests. If the client knows the integrity digest of the entire representation data before creating an upload resource, it can include the "Repr- Digest" header field when creating an upload (Section 4.2). Once the upload is completed, the server can compute the integrity digest of the received representation data and compare it to the provided digest. If the digests don't match, the server SHOULD consider the upload failed, not process the representation further, and signal the failure to the client. This way, the integrity of the entire representation data can be protected. Alternatively, when creating an upload (Section 4.2), the client can ask the server to compute and return the integrity digests using a "Want-Repr-Digest" field conveying the preferred algorithms. The response SHOULD include at least one of the requested digests, but might not include it. The server SHOULD compute the representation digests using the preferred algorithms once the upload is complete and include the corresponding "Repr-Digest" header field in the response. Alternatively, the server can compute the digest continuously during the upload and include the "Repr-Digest" header field in responses to upload creation (Section 4.2) and upload appending requests (Section 4.4) even when the upload is not completed yet. This allows the client to simultaneously compute the digest of the transmitted representation data, compare its digest to the server's digest, and spot data integrity issues. If an upload is Kleidl, et al. Expires December 6, 2025 [Page 28] Internet-Draft Resumable Uploads June 2025 spread across multiple requests, data integrity issues can be found even before the upload is fully completed. 10.2. Content Digests Content digests help verify the integrity of the content in an individual request. If the client knows the integrity digest of the content from an upload creation (Section 4.2) or upload appending (Section 4.4) request, it can include the "Content-Digest" header field in the request. Once the content has been received, the server can compute the integrity digest of the received content and compare it to the provided digest. If the digests don't match the server SHOULD consider the transfer failed, not append the content to the upload resource, and signal the failure to the client. This way, the integrity of an individual request can be protected. 11. Responses to Uploads HTTP uploads often not only transfer a representation to the server but also send back information to the client. For resumable uploads, this works similar to conventional HTTP uploads. The server can include information in the response to the request, which transferred the remaining representation data and included the "Upload-Complete: ?1" header field. Clients can regard this response as the final response for the entire upload, as detailed in Section 4.2 and Section 4.4. However, due to network interruptions, the upload resource might receive the remaining representation data, complete the upload, and send a response to the client, which then does not receive the response. The client can learn that the upload is complete by retrieving its state (Section 4.3), but resumable uploads as defined in this document do not offer functionality to recover the missed response. To address this issue, the server can send the desired information in header fields, which are included in both the final response and responses when the client fetches the upload state via a HEAD request (Section 4.3). This way, the client can retrieve the information from the header even if it failed to receive the final response. If the piece of information is too large for header fields, the server could make it available as a separate resource for retrieval and point the client to its URI in an appropriate header field. Kleidl, et al. Expires December 6, 2025 [Page 29] Internet-Draft Resumable Uploads June 2025 12. Upload Strategies The definition of the upload creation request (Section 4.2) provides the client with flexibility to choose whether the representation data is fully or partially transferred in the first request, or if no representation data is included at all. Which behavior is best largely depends on the client's capabilities, its intention to avoid data re-transmission, and its knowledge about the server's support for resumable uploads. The following subsections describe two typical upload strategies that are suited for common environments. Note that these modes are never explicitly communicated to the server and clients are not required to stick to one strategy, but can mix and adapt them to their needs. 12.1. Optimistic Upload Creation An "optimistic upload creation" can be used independent of the client's knowledge about the server's support for resumable uploads. However, the client must be capable of handling and processing interim responses. An upload creation request then includes the full representation data because the client anticipates that it will be transferred without interruptions or resumed if an interruption occurs. The benefit of this method is that if the upload creation request succeeded, the representation data was transferred in a single request without additional round trips. A possible drawback is that the client might be unable to resume an upload. If an upload is interrupted before the client received a "104 (Upload Resumption Supported)" interim response with the upload resource's URI, the client cannot resume that upload due to the missing URI. The interim response might not be received if the interruption happens too early in the message exchange, the server does not support resumable uploads at all, the server does not support sending the "104 (Upload Resumption Supported)" interim response, or an intermediary dropped the interim response. Without a 104 response, the client needs to either treat the upload as failed or retry the entire upload creation request if this is allowed by the application. A client might wait for a limited duration to receive a 104 (Upload Resumption Supported) interim response before starting to transmit the request content. This way, the client can learn about the resource's support for resumable uploads and/or the upload resource's URI. This is conceptually similar to how a client might wait for a Kleidl, et al. Expires December 6, 2025 [Page 30] Internet-Draft Resumable Uploads June 2025 100 (Continue) interim response (see Section 10.1.1 of [HTTP]) before committing to work. 12.1.1. Upgrading To Resumable Uploads Optimistic upload creation allows clients and servers to automatically upgrade non-resumable uploads to resumable ones. In a non-resumable upload, the representation is transferred in a single request, usually "POST" or "PUT", without any ability to resume from interruptions. The client can offer the server to upgrade such a request to a resumable upload by adding the "Upload-Complete: ?1" header field to the original request. The "Upload-Length" header field SHOULD be added if the representation data's length is known upfront. The request is not changed otherwise. A server that supports resumable uploads at the target URI can create an upload resource and send its URI in a "104 (Upload Resumption Supported)" interim response for the client to resume the upload after interruptions. A server that does not support resumable uploads or does not want to upgrade to a resumable upload for this request ignores the "Upload-Complete: ?1" header. The transfer then falls back to a non-resumable upload without additional cost. This upgrade can also be performed transparently by a library or program that acts as a HTTP client by sending requests on behalf of a user. When the user instructs the client to send a non-resumable request, the client can perform the upgrade transparently and handle potential interruptions and resumptions under the hood without involving the user. The last response received by the client is considered the response for the entire upload and should be provided to the user. 12.2. Careful Upload Creation For a "careful upload creation" the client knows that the server supports resumable uploads and sends an empty upload creation request without including any representation data. Upon successful response reception, the client can use the included upload resource URI to transmit the representation data (Section 4.4) and resume the upload at any stage if an interruption occurs. The client should inspect the response for the "Upload-Limit" header field, which would indicate limits applying to the remaining upload procedure. The retransmission of representation data or the ultimate upload failure that can happen with an "optimistic upload creation" is therefore avoided at the expense of an additional request that does not carry representation data. Kleidl, et al. Expires December 6, 2025 [Page 31] Internet-Draft Resumable Uploads June 2025 This approach is best suited if the client cannot receive interim responses, e.g. due to a limitation in the provided HTTP interface, or if large representations are transferred where the cost of the additional request is minuscule compared to the effort of transferring the representation itself. 13. Security Considerations The upload resource URI is the identifier used for modifying the upload. Without further protection of this URI, an attacker may obtain information about an upload, append data to it, or cancel it. To prevent this, the server SHOULD ensure that only authorized clients can access the upload resource. To reduce the risk of unauthorized access, it is RECOMMENDED to generate upload resource URIs in such a way that makes it hard to be guessed by unauthorized clients. In addition, servers may embed information about the storage or processing location of the uploaded representation in the upload resource URI to make routing requests more efficient. If so, they MUST ensure that no internal information is leaked in the URI that is not intended to be exposed. Uploaded representation data and its metadata are untrusted input. Server operators have to be careful of where uploaded data is written and subsequently accessed, especially if the operations cause the representation to be processed or executed by the server. In addition, metadata MUST be validated and/or sanitized if the server takes its values into consideration for processing or storing the representation. Some servers or intermediaries provide scanning of content uploaded by clients. Any scanning mechanism that relies on receiving a complete representation in a single request message can be defeated by resumable uploads because content can be split across multiple messages. Servers or intermediaries wishing to perform content scanning SHOULD consider how resumable uploads can circumvent scanning and take appropriate measures. Possible strategies include waiting for the upload to complete before scanning the entire representation, or disabling resumable uploads. Resumable uploads are vulnerable to Slowloris-style attacks [SLOWLORIS]. A malicious client may create upload resources and keep them alive by regularly sending "PATCH" requests with no or small content to the upload resources. This could be abused to exhaust server resources by creating and holding open uploads indefinitely with minimal work. Servers SHOULD provide mitigations for Slowloris attacks, such as increasing the maximum number of clients the server will allow, limiting the number of uploads a single client is allowed to make, imposing restrictions on the minimum transfer speed an Kleidl, et al. Expires December 6, 2025 [Page 32] Internet-Draft Resumable Uploads June 2025 upload is allowed to have, and restricting the length of time an upload resource can exist. 14. IANA Considerations 14.1. HTTP Fields IANA is asked to register the following entries in the "Hypertext Transfer Protocol (HTTP) Field Name Registry": +-----------------+-----------+-------------+-----------------------+ | Field Name | Status | Structured | Reference | | | | Type | | +-----------------+-----------+-------------+-----------------------+ | Upload-Offset | permanent | Item | Section 4.1.1 of this | | | | | document | | | | | | | Upload-Complete | permanent | Item | Section 4.1.2 of this | | | | | document | | | | | | | Upload-Length | permanent | Item | Section 4.1.3 of this | | | | | document | | | | | | | Upload-Limit | permanent | Dictionary | Section 4.1.4 of this | | | | | document | +-----------------+-----------+-------------+-----------------------+ 14.2. HTTP Status Code IANA is asked to register the following entry in the "HTTP Status Codes" registry: Value: 104 (suggested value) Description: Upload Resumption Supported Specification: Section 5 of this document 14.3. Media Type IANA is asked to register the following entry in the "Media Types" registry: Type name: application Subtype name: partial-upload Required parameters: N/A Kleidl, et al. Expires December 6, 2025 [Page 33] Internet-Draft Resumable Uploads June 2025 Optional parameters: N/A Encoding considerations: binary Security considerations: see Section 13 of this document Interoperability considerations: N/A Published specification: Section 6 of this document Applications that use this media type: Applications that transfer files over unreliable networks or want pause- and resumable uploads. Fragment identifier considerations: N/A Additional information: o Deprecated alias names for this type: N/A o Magic number(s): N/A o File extension(s): N/A o Macintosh file type code(s): N/A o Windows Clipboard Name: N/A Person and email address to contact for further information: See the Authors' Addresses section of this document. Intended usage: COMMON Restrictions on usage: N/A Author: See the Authors' Addresses section of this document. Change controller: IETF 14.4. HTTP Problem Types IANA is asked to register the following entry in the "HTTP Problem Types" registry: Type URI: https://iana.org/assignments/http-problem- types#mismatching-upload-offset Title: Mismatching Upload Offset Kleidl, et al. Expires December 6, 2025 [Page 34] Internet-Draft Resumable Uploads June 2025 Recommended HTTP status code: 409 Reference: Section 7.1 of this document IANA is asked to register the following entry in the "HTTP Problem Types" registry: Type URI: https://iana.org/assignments/http-problem-types#completed- upload Title: Upload Is Completed Recommended HTTP status code: 400 Reference: Section 7.2 of this document IANA is asked to register the following entry in the "HTTP Problem Types" registry: Type URI: https://iana.org/assignments/http-problem- types#inconsistent-upload-length Title: Inconsistent Upload Length Values Recommended HTTP status code: 400 Reference: Section 7.3 of this document 15. References 15.1. Normative References [CACHING] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", STD 98, RFC 9111, DOI 10.17487/RFC9111, June 2022, . [CONTENT-DISPOSITION] Reschke, J., "Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)", RFC 6266, DOI 10.17487/RFC6266, June 2011, . [DIGEST-FIELDS] Polli, R. and L. Pardue, "Digest Fields", RFC 9530, DOI 10.17487/RFC9530, February 2024, . Kleidl, et al. Expires December 6, 2025 [Page 35] Internet-Draft Resumable Uploads June 2025 [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, . [PATCH] Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC 5789, DOI 10.17487/RFC5789, March 2010, . [PROBLEM] Nottingham, M., Wilde, E., and S. Dalal, "Problem Details for HTTP APIs", RFC 9457, DOI 10.17487/RFC9457, July 2023, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112, June 2022, . [STRUCTURED-FIELDS] Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024, . 15.2. Informative References [RFC8792] Watsen, K., Auerswald, E., Farrel, A., and Q. Wu, "Handling Long Lines in Content of Internet-Drafts and RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020, . [SLOWLORIS] "RSnake" Hansen, R., "Welcome to Slowloris - the low bandwidth, yet greedy and poisonous HTTP client!", June 2009, . Kleidl, et al. Expires December 6, 2025 [Page 36] Internet-Draft Resumable Uploads June 2025 15.3. URIs [1] https://tus.io/ Appendix A. Changes This section is to be removed before publishing as an RFC. A.1. Since draft-ietf-httpbis-resumable-upload-08 o Clarify definitions of new header fields. o Make handling of OPTIONS * optional. o Require server to announce limits using Upload-Limit. o Require clients to adhere to known limits. o Rephrase requirements for concurrency handling, focusing on the outcome. o Remove requirement for 204 status code for DELETE responses. o Increase the draft interop version. o Add section about 104 status code. o Rephrase recommendation for sending information back to client. A.2. Since draft-ietf-httpbis-resumable-upload-07 o Clarify server handling when upload length is exceeded. o Extend security considerations about upload resource URIs, representation metadata, and untrusted inputs. o Allow clients to retry for appropriate 4xx responses. A.3. Since draft-ietf-httpbis-resumable-upload-06 o Minor editorial improvements to introduction and examples. o Define structured types for new header fields. Kleidl, et al. Expires December 6, 2025 [Page 37] Internet-Draft Resumable Uploads June 2025 A.4. Since draft-ietf-httpbis-resumable-upload-05 o Increase the draft interop version. o Numerous editorial changes. o Rename "expires" limit to "max-age". o Require "Upload-Complete", but not "Upload-Offset" or "Upload- Limit", for append responses. o Add problem type for inconsistent length values. o Reduce use of "file" in favor of "representation". A.5. Since draft-ietf-httpbis-resumable-upload-04 o Clarify implications of "Upload-Limit" header. o Allow client to fetch upload limits upfront via "OPTIONS". o Add guidance on upload creation strategy. o Add "Upload-Length" header to indicate length during creation. o Describe possible usage of "Want-Repr-Digest". A.6. Since draft-ietf-httpbis-resumable-upload-03 o Add note about "Content-Location" for referring to subsequent resources. o Require "application/partial-upload" for appending to uploads. o Explain handling of content and transfer codings. o Add problem types for mismatching offsets and completed uploads. o Clarify that completed uploads must not be appended to. o Describe interaction with Digest Fields from RFC9530. o Require that upload offset does not decrease over time. o Add Upload-Limit header field. o Increase the draft interop version. Kleidl, et al. Expires December 6, 2025 [Page 38] Internet-Draft Resumable Uploads June 2025 A.7. Since draft-ietf-httpbis-resumable-upload-02 o Add upload progress notifications via informational responses. o Add security consideration regarding request filtering. o Explain the use of empty requests for creation uploads and appending. o Extend security consideration to include resource exhaustion attacks. o Allow 200 status codes for offset retrieval. o Increase the draft interop version. A.8. Since draft-ietf-httpbis-resumable-upload-01 o Replace Upload-Incomplete header with Upload-Complete. o Replace terminology about procedures with HTTP resources. o Increase the draft interop version. A.9. Since draft-ietf-httpbis-resumable-upload-00 o Remove Upload-Token and instead use Server-generated upload URL for upload identification. o Require the Upload-Incomplete header field in Upload Creation Procedure. o Increase the draft interop version. A.10. Since draft-tus-httpbis-resumable-uploads-protocol-02 None A.11. Since draft-tus-httpbis-resumable-uploads-protocol-01 o Clarifying backtracking and preventing skipping ahead during the Offset Receiving Procedure. o Clients auto-retry 404 is no longer allowed. Kleidl, et al. Expires December 6, 2025 [Page 39] Internet-Draft Resumable Uploads June 2025 A.12. Since draft-tus-httpbis-resumable-uploads-protocol-00 o Split the Upload Transfer Procedure into the Upload Creation Procedure and the Upload Appending Procedure. Appendix B. Draft Version Identification This section is to be removed before publishing as an RFC. To assist the development of implementations and interoperability testing while this document is still a draft, an interop version is defined. Implementations of this draft use the interop version to identify the iteration of the draft that they implement. The interop version is bumped for breaking changes. The current interop version is 8. 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. The "Upload-Draft-Interop-Version" field value is an Integer. 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. Acknowledgments This document is based on an Internet-Draft specification written by Jiten Mehta, Stefan Matsson, and the authors of this document. Kleidl, et al. Expires December 6, 2025 [Page 40] Internet-Draft Resumable Uploads June 2025 The tus v1 protocol [1] 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. The authors would like to thank Mark Nottingham for substantive contributions to the text, and Roy T. Fielding and Julian Reschke for their thorough reviews of the document. Authors' Addresses Marius Kleidl (editor) Transloadit Email: marius@transloadit.com Guoye Zhang (editor) Apple Inc. Email: guoye_zhang@apple.com Lucas Pardue (editor) Cloudflare Email: lucas@lucaspardue.com Kleidl, et al. Expires December 6, 2025 [Page 41]