Internet DRAFT - draft-ietf-httpbis-resumable-upload
draft-ietf-httpbis-resumable-upload
HTTP M. Kleidl, Ed.
Internet-Draft Transloadit
Intended status: Standards Track G. Zhang, Ed.
Expires: 5 September 2024 Apple Inc.
L. Pardue, Ed.
Cloudflare
4 March 2024
Resumable Uploads for HTTP
draft-ietf-httpbis-resumable-upload-03
Abstract
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.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-httpbis-resumable-
upload/.
Discussion of this document takes place on the HTTP Working Group
mailing list (mailto:ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group
information can be found at https://httpwg.org/.
Source for this draft and an issue tracker can be found at
https://github.com/httpwg/http-extensions/labels/resumable-upload.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 September 2024.
Copyright Notice
Copyright (c) 2024 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/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Example 1: Complete upload of file with known size . . . 4
3.2. Example 2: Upload as a series of parts . . . . . . . . . 6
4. Upload Creation . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Feature Detection . . . . . . . . . . . . . . . . . . . . 10
4.2. Draft Version Identification . . . . . . . . . . . . . . 11
5. Offset Retrieval . . . . . . . . . . . . . . . . . . . . . . 11
6. Upload Append . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Upload Cancellation . . . . . . . . . . . . . . . . . . . . . 15
8. Header Fields . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Upload-Offset . . . . . . . . . . . . . . . . . . . . . . 16
8.2. Upload-Complete . . . . . . . . . . . . . . . . . . . . . 16
9. Redirection . . . . . . . . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . 18
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Appendix A. Informational Response . . . . . . . . . . . . . . . 18
Appendix B. Feature Detection . . . . . . . . . . . . . . . . . 19
Appendix C. Upload Metadata . . . . . . . . . . . . . . . . . . 21
Appendix D. FAQ . . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Since draft-ietf-httpbis-resumable-upload-02 . . . . . . . . . 22
Since draft-ietf-httpbis-resumable-upload-01 . . . . . . . . . 22
Since draft-ietf-httpbis-resumable-upload-00 . . . . . . . . . 22
Since draft-tus-httpbis-resumable-uploads-protocol-02 . . . . . 23
Since draft-tus-httpbis-resumable-uploads-protocol-01 . . . . . 23
Since draft-tus-httpbis-resumable-uploads-protocol-00 . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. 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 Section 3.2 of [HTTP]) might have been
exchanged. To complete the data transfer of the entire
representation, it is often desirable to issue subsequent requests
that transfer only the remainder of the representation. HTTP range
requests (see Section 14 of [HTTP]) support this concept of resumable
downloads from server to client.
HTTP methods such as POST or PUT can be used by clients to request
processing of representation data enclosed in the request message.
The transfer of representation data from client to server is often
referred to as an upload. Uploads are just as likely as downloads to
suffer from the effects of data transfer interruption. Humans can
play a role in upload interruptions through manual actions such as
pausing an upload. Regardless of the cause of an interruption,
servers may have received part of the representation before its
occurrence and it is desirable if clients can complete the data
transfer by sending only the remainder of the representation. The
process of sending additional parts of a representation using
subsequent HTTP requests from client to server is herein referred to
as a resumable upload.
Connection interruptions are common and the absence of a standard
mechanism for resumable uploads has lead to a proliferation of custom
solutions. Some of those use HTTP, while others rely on other
transfer mechanisms entirely. An HTTP-based standard solution is
desirable for such a common class of problem.
This document defines an optional mechanism for HTTP that enables
resumable uploads in a way that is backwards-compatible with
conventional HTTP uploads. When an upload is interrupted, clients
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can send subsequent requests to query the server state and use this
information to send the remaining data. Alternatively, they can
cancel the upload entirely. Different from ranged downloads, this
protocol does not support transferring different parts of the same
representation in parallel.
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.
The terms Byte Sequence, Item, String, Token, Integer, and Boolean
are imported from [STRUCTURED-FIELDS].
The terms client and server are from [HTTP].
3. Overview
Resumable uploads are supported in HTTP through use of a temporary
resource, an _upload resource_, that is separate from the resource
being uploaded to (hereafter, the _target resource_) and specific to
that upload. By interacting with the upload resource, a client can
retrieve the current offset of the upload (Section 5), append to the
upload (Section 6), and cancel the upload (Section 7).
The remainder of this section uses an example of a file upload to
illustrate different interactions with the upload resource. Note,
however, that HTTP message exchanges use representation data (see
Section 8.1 of [HTTP]), which means that resumable uploads can be
used with many forms of content -- not just static files.
3.1. 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 to the target resource. An
interruption occurs and the client then attempts to resume the upload
using subsequent HTTP requests to the upload resource.
1) The client notifies the server that it wants to begin an upload
(Section 4). The server reserves the required resources to accept
the upload from the client, and the client begins transferring the
entire file in the request content.
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An informational response can be sent to the client, which signals
the server's support of resumable upload as well as the upload
resource URL via the Location header field (Section 10.2.2 of
[HTTP]).
Client Server
| |
| POST |
|------------------------------------------->|
| |
| | Reserve resources
| | for upload
| |-----------------.
| | |
| |<----------------'
| |
| 104 Upload Resumption Supported |
| with upload resouce URL |
|<-------------------------------------------|
| |
| Flow Interrupted |
|------------------------------------------->|
| |
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 data that the server received
before the interruption. It does so by retrieving the offset
(Section 5) from the upload resource.
Client Server
| |
| HEAD to upload resource URL |
|------------------------------------------------>|
| |
| 204 No Content with Upload-Offset |
|<------------------------------------------------|
| |
Figure 2: Offset Retrieval
3) The client can resume the upload by sending the remaining file
content to the upload resource (Section 6), 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.
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Client Server
| |
| PATCH to upload resource URL with Upload-Offset |
|------------------------------------------------>|
| |
| 201 Created on completion |
|<------------------------------------------------|
| |
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 7).
Client Server
| |
| DELETE to upload resource URL |
|------------------------------------------------>|
| |
| 204 No Content on completion |
|<------------------------------------------------|
| |
Figure 4: Upload Cancellation
3.2. Example 2: Upload as a series of parts
In some cases, clients might prefer to upload a file 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, 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
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 file.
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Client Server
| |
| POST with Upload-Complete: ?0 |
|------------------------------------------------>|
| |
| 201 Created with Upload-Complete: ?0 |
| and Location on completion |
|<------------------------------------------------|
| |
Figure 5: Incomplete Upload Creation
2) Subsequently, parts are appended (Section 6). The last part of
the upload has a Upload-Complete field value set to true to indicate
the complete transfer.
Client Server
| |
| PATCH to upload resource URL with |
| Upload-Offset and Upload-Complete: ?1 |
|------------------------------------------------>|
| |
| 201 Created on completion |
|<------------------------------------------------|
| |
Figure 6: Upload Append Last Chunk
4. Upload Creation
When a resource supports resumable uploads, the first step is
creating the upload resource. To be compatible with the widest range
of resources, this is accomplished by including the Upload-Complete
header field in the request that initiates the upload.
As a consequence, resumable uploads support all HTTP request methods
that can carry content, such as POST, PUT, and PATCH. Similarly, the
response to the upload request can have any status code. Both the
method(s) and status code(s) supported are determined by the
resource.
Upload-Complete MUST be set to false if the end of the request
content is not the end of the upload. Otherwise, it MUST be set to
true. This header field can be used for request identification by a
server. The request MUST NOT include the Upload-Offset header field.
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If the request is valid, the server SHOULD create an upload resource.
Then, the server MUST include the Location header field in the
response and set its value to the URL of the upload resource. The
client MAY use this URL for offset retrieval (Section 5), upload
append (Section 6), and upload cancellation (Section 7).
Once the upload resource is available and while the request content
is being uploaded, the target resource MAY send one or more
informational responses with a 104 (Upload Resumption Supported)
status code to the client. In the first informational response, the
Location header field MUST be set to the URL pointing to the upload
resource. In subsequent informational responses, the Location header
field MUST NOT be set. An informational response MAY contain the
Upload-Offset header field with the current upload offset as the
value to inform the client about the upload progress. In subsequent
informational responses, the upload offset MUST NOT be smaller than
in previous informational responses. In addition, later offset
retrievals (Section 5) MUST NOT receive an upload offset that is less
than the offset reported in the latest informational response,
allowing the client to free associated resources.
The server MUST send the Upload-Offset header field 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 Upload-Offset field 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 has a status code that indicates a
success but the offset indicated in the Upload-Offset field value
does not equal the total of 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). Servers are
RECOMMENDED to use 201 (Created) unless otherwise specified. The
response MUST NOT include the Upload-Complete header field with the
value of false.
If the request completes successfully but the entire upload is not
yet complete, as indicated by an Upload-Complete field value of false
in the request, the server MUST acknowledge it by responding with the
201 (Created) status code and an Upload-Complete header value set to
false.
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If the request includes an Upload-Complete field value set to true
and a valid Content-Length header field, the client attempts to
upload a fixed-length resource in one request. In this case, the
upload's final size is the Content-Length field value and the server
MUST record it to ensure its consistency.
The request content MAY be empty. If the Upload-Complete header
field is then set to true, the client intends to upload an empty
entity. An Upload-Complete header field is set to false is also
valid. This can be used to create an upload resource URL before
transferring data, which can save client or server resources. Since
informational responses are optional, this technique provides another
mechanism to learn the URL, at the cost of an additional round-trip
before data upload can commence.
The following example shows an upload creation. The client transfers
the entire 100 bytes in the first request. The server generates two
informational responses to transmit the upload resource's URL and
progress information, and one final response to acknowledge the
completed upload:
POST /upload HTTP/1.1
Host: example.com
Upload-Draft-Interop-Version: 5
Upload-Complete: ?1
Content-Length: 100
[content (100 bytes)]
HTTP/1.1 104 Upload Resumption Supported
Upload-Draft-Interop-Version: 5
Location: https://example.com/upload/b530ce8ff
HTTP/1.1 104 Upload Resumption Supported
Upload-Draft-Interop-Version: 5
Upload-Offset: 50
HTTP/1.1 201 Created
Location: https://example.com/upload/b530ce8ff
Upload-Offset: 100
The next example shows an upload creation, where only the first 25
bytes are transferred. The server acknowledges the received data and
that the upload is not complete yet:
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POST /upload HTTP/1.1
Host: example.com
Upload-Draft-Interop-Version: 5
Upload-Complete: ?0
Content-Length: 25
[partial content (25 bytes)]
HTTP/1.1 201 Created
Location: https://example.com/upload/b530ce8ff
Upload-Complete: ?0
Upload-Offset: 25
If the client received an informational response with the upload URL
in the Location field value, it MAY automatically attempt upload
resumption when the connection is terminated unexpectedly, or if a
5xx status is received. The client SHOULD NOT automatically retry if
it receives a 4xx status code.
File metadata can affect how servers might act on the uploaded file.
Clients can send representation metadata (see Section 8.3 of [HTTP])
in the request that starts an upload. Servers MAY interpret this
metadata or MAY ignore it. The Content-Type header field
(Section 8.3 of [HTTP]) can be used to indicate the MIME type of the
file. The Content-Disposition header field ([RFC6266]) can be used
to transmit a filename; if included, the parameters SHOULD be either
filename, filename* or boundary.
4.1. Feature Detection
If the client has no knowledge of whether the resource supports
resumable uploads, a resumable request can be used with some
additional constraints. In particular, the Upload-Complete field
value (Section 8.2) MUST NOT be false if the server support is
unclear. This allows the upload to function as if it is a regular
upload.
Servers SHOULD use the 104 (Upload Resumption Supported)
informational response to indicate their support for a resumable
upload request.
Clients MUST NOT attempt to resume an upload unless they receive 104
(Upload Resumption Supported) informational response, or have other
out-of-band methods to determine server support for resumable
uploads.
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4.2. 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 5.
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.
5. Offset Retrieval
If an upload is interrupted, the client MAY attempt to fetch the
offset of the incomplete upload by sending a HEAD request to the
upload resource.
The request MUST NOT include an Upload-Offset or Upload-Complete
header field. The server MUST reject requests with either of these
fields by responding with a 400 (Bad Request) status code.
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If the server considers the upload resource to be active, it MUST
respond with a 204 (No Content) or 200 (OK) status code. The
response MUST include the Upload-Offset header field, with the value
set to the current resumption offset for the target resource. The
response MUST include the Upload-Complete header field; the value is
set to true only if the upload is complete.
An upload is considered complete only if the server completely and
successfully received a corresponding creation request (Section 4) or
append request (Section 6) with the Upload-Complete header value set
to true.
The client MUST NOT perform offset retrieval while creation
(Section 4) or append (Section 6) is in progress.
The offset MUST be accepted by a subsequent append (Section 6). Due
to network delay and reordering, the server might still be receiving
data from an ongoing transfer for the same upload resource, which in
the client perspective has failed. The server MAY terminate any
transfers for the same upload resource 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 append (Section 6) based on
the resumption offset from a single offset retrieving (Section 5)
request.
In order to prevent HTTP caching, the response SHOULD include a
Cache-Control header field with the value no-store.
If the server does not consider the upload resource to be active, it
MUST respond with a 404 (Not Found) status code.
The resumption offset can be less than or equal to the number of
bytes the client has already sent. The client MAY reject an offset
which is greater than the number of bytes it has already sent during
this upload. The client is expected to handle backtracking of a
reasonable length. If the offset is invalid for this upload, or if
the client cannot backtrack to the offset and reproduce the same
content it has already sent, the upload MUST be considered a failure.
The client MAY cancel the upload (Section 7) after rejecting the
offset.
The following example shows an offset retrieval request. The server
indicates the new offset and that the upload is not complete yet:
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HEAD /upload/b530ce8ff HTTP/1.1
Host: example.com
Upload-Draft-Interop-Version: 5
HTTP/1.1 204 No Content
Upload-Offset: 100
Upload-Complete: ?0
Cache-Control: no-store
The client SHOULD NOT automatically retry if a client error status
code between 400 and 499 (inclusive) is received.
6. Upload Append
Upload appending is used for resuming an existing upload.
The request MUST use the PATCH method and be sent to the upload
resource. The Upload-Offset field value (Section 8.1) MUST be set to
the resumption offset.
If the end of the request content is not the end of the upload, the
Upload-Complete field value (Section 8.2) MUST be set to false.
The server SHOULD respect representation metadata received during
creation (Section 4) and ignore any representation metadata received
from appending (Section 6).
If the server does not consider the upload associated with the upload
resource active, it MUST respond with a 404 (Not Found) status code.
The client MUST NOT perform multiple upload transfers for the same
upload resource in parallel. This helps avoid race conditions, and
data loss or corruption. The server is RECOMMENDED to take measures
to avoid parallel upload transfers: The server MAY terminate any
creation (Section 4) or append (Section 6) for the same upload URL.
Since the client is not allowed to perform multiple transfers in
parallel, the server can assume that the previous attempt has already
failed. Therefore, the server MAY abruptly terminate the previous
HTTP connection or stream.
If the offset indicated by the Upload-Offset field value does not
match the offset provided by the immediate previous offset retrieval
(Section 5), or the end offset of the immediate previous incomplete
successful transfer, the server MUST respond with a 409 (Conflict)
status code.
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While the request content is being uploaded, the target resource MAY
send one or more informational responses with a 104 (Upload
Resumption Supported) status code to the client. These informational
responses MUST NOT contain the Location header field. They MAY
include the Upload-Offset header field with the current upload offset
as the value to inform the client about the upload progress. The
same restrictions on the Upload-Offset header field in informational
responses from the upload creation (Section 4) apply.
The server MUST send the Upload-Offset header field 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 status code indicates a success but the offset indicated by the
Upload-Offset field value does not equal the total of 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). Servers are
RECOMMENDED to use a 201 (Created) response if not otherwise
specified. The response MUST NOT include the Upload-Complete header
field with the value set to false.
If the request completes successfully but the entire upload is not
yet complete indicated by the Upload-Complete field value set to
false, the server MUST acknowledge it by responding with a 201
(Created) status code and the Upload-Complete field value set to
true.
If the request includes the Upload-Complete field value set to true
and a valid Content-Length header field, the client attempts to
upload the remaining resource in one request. In this case, the
upload's final size is the sum of the upload's offset and the
Content-Length header field. If the server does not have a record of
the upload's final size from creation or the previous append, the
server MUST record the upload's final size to ensure its consistency.
If the server does have a previous record, that value MUST match the
upload's final size. If they do not match, the server MUST reject
the request with a 400 (Bad Request) status code.
The request content MAY be empty. If the Upload-Complete field is
then set to true, the client wants to complete the upload without
appending additional data.
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The following example shows an upload append. The client transfers
the next 100 bytes at an offset of 100 and does not indicate that the
upload is then completed. The server acknowledges the new offset:
PATCH /upload/b530ce8ff HTTP/1.1
Host: example.com
Upload-Offset: 100
Upload-Draft-Interop-Version: 5
Content-Length: 100
[content (100 bytes)]
HTTP/1.1 201 Created
Upload-Offset: 200
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.
7. Upload Cancellation
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.
The client MUST NOT initiate cancellation without the knowledge of
server support.
The request MUST use the DELETE method. The request MUST NOT include
an Upload-Offset or Upload-Complete header field. The server MUST
reject the request with a Upload-Offset or Upload-Complete header
field with a 400 (Bad Request) status code.
If the server successfully deactivates the upload resource, it MUST
respond with a 204 (No Content) status code.
The server MAY terminate any in-flight requests to the upload
resource before sending the response by abruptly terminating their
HTTP connection(s) or stream(s).
If the server does not consider the upload resource to be active, it
MUST respond with a 404 (Not Found) status code.
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If the server does not support cancellation, it MUST respond with a
405 (Method Not Allowed) status code.
The following example shows an upload cancellation:
DELETE /upload/b530ce8ff HTTP/1.1
Host: example.com
Upload-Draft-Interop-Version: 5
HTTP/1.1 204 No Content
8. Header Fields
8.1. Upload-Offset
The Upload-Offset request and response header field indicates the
resumption offset of corresponding upload, counted in bytes. The
Upload-Offset field value is an Integer.
8.2. Upload-Complete
The Upload-Complete request and response header field indicates
whether the corresponding upload is considered complete. The Upload-
Complete field value is a Boolean.
The Upload-Complete header field MUST only be used if support by the
resource is known to the client (Section 4.1).
9. Redirection
The 301 (Moved Permanently) and 302 (Found) status codes MUST NOT be
used in offset retrieval (Section 5) and upload cancellation
(Section 7) responses. For other responses, the upload resource MAY
return a 308 (Permanent Redirect) status code and clients SHOULD use
new permanent URI for subsequent requests. If the client receives a
307 (Temporary Redirect) response to an offset retrieval (Section 5)
request, it MAY apply the redirection directly in an immediate
subsequent upload append (Section 6).
10. Security Considerations
The upload resource URL is the identifier used for modifying the
upload. Without further protection of this URL, 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. In addition, the upload
resource URL SHOULD be generated in such a way that makes it hard to
be guessed by unauthorized clients.
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Some servers or intermediaries provide scanning of content uploaded
by clients. Any scanning mechanism that relies on receiving a
complete file 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 a full file, 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 upload is
allowed to have, and restricting the length of time an upload
resource can exist.
11. IANA Considerations
IANA is asked to register the following entries in the "Hypertext
Transfer Protocol (HTTP) Field Name Registry":
+=================+===========+==============================+
| Field Name | Status | Reference |
+=================+===========+==============================+
| Upload-Complete | permanent | Section 8.2 of this document |
+-----------------+-----------+------------------------------+
| Upload-Offset | permanent | Section 8.1 of this document |
+-----------------+-----------+------------------------------+
Table 1
IANA is asked to register the following entry in the "HTTP Status
Codes" registry:
Value: 104 (suggested value)
Description: Upload Resumption Supported
Specification: This document
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12. References
12.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6266] Reschke, J., "Use of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
DOI 10.17487/RFC6266, June 2011,
<https://www.rfc-editor.org/rfc/rfc6266>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.
12.2. Informative References
[SLOWLORIS]
"RSnake" Hansen, R., "Welcome to Slowloris - the low
bandwidth, yet greedy and poisonous HTTP client!", June
2009, <https://web.archive.org/web/20150315054838/
http://ha.ckers.org/slowloris/>.
Appendix A. Informational Response
The server is allowed to respond to upload creation (Section 4)
requests with a 104 (Upload Resumption Supported) intermediate
response as soon as the server has validated the request. This way,
the client knows that the server supports resumable uploads before
the complete response is received. The benefit is the clients can
defer starting the actual data transfer until the server indicates
full support (i.e. resumable are supported, the provided upload URL
is active etc).
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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).
Appendix B. 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)
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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 field which indicates support for resumable
uploads. There are two options: - Use the standardized Alt-Svc
response header field. However, it has been indicated to us that
this header field might be reworked in the future and could also be
semantically different from our intended usage. - Use a new response
header field 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 field
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
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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 field 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.
Appendix C. Upload Metadata
When an upload is created (Section 4), the Content-Type and Content-
Disposition header fields are allowed 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 header fields 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 field. 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.
Appendix D. FAQ
* *Are multipart requests supported?* Yes, requests whose content is
encoded using the multipart/form-data are implicitly supported.
The entire encoded content 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.
Acknowledgments
This document is based on an Internet-Draft specification written by
Jiten Mehta, Stefan Matsson, and the authors of this document.
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The tus v1 protocol (https://tus.io/) 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.
Changes
This section is to be removed before publishing as an RFC.
Since draft-ietf-httpbis-resumable-upload-02
* Add upload progress notifications via informational responses.
* Add security consideration regarding request filtering.
* Explain the use of empty requests for creation uploads and
appending.
* Extend security consideration to include resource exhaustion
attacks.
* Allow 200 status codes for offset retrieval.
* Increase the draft interop version.
Since draft-ietf-httpbis-resumable-upload-01
* Replace Upload-Incomplete header with Upload-Complete.
* Replace terminology about procedures with HTTP resources.
* Increase the draft interop version.
Since draft-ietf-httpbis-resumable-upload-00
* Remove Upload-Token and instead use Server-generated upload URL
for upload identification.
* Require the Upload-Incomplete header field in Upload Creation
Procedure.
* Increase the draft interop version.
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Since draft-tus-httpbis-resumable-uploads-protocol-02
None
Since draft-tus-httpbis-resumable-uploads-protocol-01
* Clarifying backtracking and preventing skipping ahead during the
Offset Receiving Procedure.
* Clients auto-retry 404 is no longer allowed.
Since draft-tus-httpbis-resumable-uploads-protocol-00
* Split the Upload Transfer Procedure into the Upload Creation
Procedure and the Upload Appending Procedure.
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
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