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+# MSC2246: Asynchronous media uploads
+Sending media to Matrix currently requires that clients first upload the media
+to the content repository and then send the event. This is a problem for some
+use cases, such as bridges that want to preserve message order, as reuploading
+a large file would block all messages.
+
+## Proposal
+This proposal proposes a way to send the event containing media before actually
+uploading the media, which would make the aformentioned bridge message order
+preservation possible without blocking all other messages behind a long upload.
+
+In the future, this new functionality could be used for streaming file
+transfers, as requested in [matrix-spec#432].
+
+### Content repository behavior
+The proposal adds two new endpoints to the content repository API and modifies
+the download and thumbnail endpoints.
+
+#### `POST /_matrix/media/v1/create`
+Create a new MXC URI without content. Like `/upload`, this endpoint requires
+auth, can be rate limited, and returns the `content_uri` that can be used in
+events.
+
+The request body should be an empty JSON object. In the future, the body could
+be used for metadata about the file, such as the mime type or access control
+settings (related: [MSC701]).
+
+The server may optionally enforce a maximum age for unused media IDs to delete
+media IDs when the client doesn't start the upload in time, or when the upload
+was interrupted and not resumed in time. The server should include the maximum
+POSIX millisecond timestamp to complete the upload in the `unused_expires_at`
+field in the response JSON. The recommended default expiration is 24 hours which
+should be enough time to accommodate users on poor connection who find a better
+connection to complete the upload.
+
+##### Rate Limiting
+
+The server should rate limit requests to create media.
+
+The server should limit the number of concurrent *pending media uploads* a given
+user can have. A pending media upload is a created MXC URI that (a) is not
+expired (the `unused_expires_at` timestamp has not passed) and (b) the media has
+not yet been uploaded for.
+
+In both cases, the server should respond with `M_LIMIT_EXCEEDED` optionally
+providing details in the `error` field, but servers may wish to obscure the
+exact limits that are used and not provide such details.
+
+##### Example response
+```json
+{
+  "content_uri": "mxc://example.com/AQwafuaFswefuhsfAFAgsw",
+  "unused_expires_at": 1647257217083
+}
+```
+
+#### `PUT /_matrix/media/v3/upload/{serverName}/{mediaId}`
+Upload content to a MXC URI that was created earlier. This endpoint requires
+auth. If the upload is successful, an empty JSON object and status code 200 is
+returned. Rate limiting additionally can apply here.
+
+If the endpoint is called with a media ID that already has content, the request
+should be rejected with the error code `M_CANNOT_OVERWRITE_MEDIA` and HTTP
+status code 409.
+
+If the upload request comes from a user other than the one who created the media
+ID, the request should be rejected with an `M_FORBIDDEN` error.
+
+If the serverName/mediaId combination is not known, not local, or expired, an
+`M_NOT_FOUND` error is returned.
+
+If the MXC's `unused_expires_at` is reached before the upload completes, the
+server may either respond immediately with `M_NOT_FOUND` or allow the upload to
+continue.
+
+For other errors, such as file size, file type or user quota errors, the normal
+`/upload` rules apply.
+
+#### Changes to the `/download` and `/thumbnail` endpoints
+A new query parameter, `timeout_ms` is added to the endpoints that can
+download media. It's an integer that specifies the maximum number of
+milliseconds that the client is willing to wait to start receiving data.
+The default value is 20000 (20 seconds). The content repository can and should
+impose a maximum value for this parameter. The content repository can also
+choose to respond before the timeout if it desires.
+
+If the media is available immediately (for example in the case of a
+non-asynchronous upload), the content repository should ignore this parameter.
+
+If the MXC has expired, the content repository should respond with `M_NOT_FOUND`
+and a HTTP 404 status code.
+
+If the data is not available when the server chooses to respond, the content
+repository returns a `M_NOT_YET_UPLOADED` error with a HTTP 504 status code.
+
+For the `/download` endpoint, the server could also stream data directly as it
+is being uploaded. However, streaming creates several implementation and spec
+complications (e.g. how to stream if the media repo has multiple workers, what
+to do if the upload is interrupted), so specifying exactly how streaming works
+is left for another MSC.
+
+## Potential issues
+Other clients may time out the download if the sender takes too long to upload
+media.
+
+## Alternatives
+
+## Security considerations
+
+The primary attack vector that must be prevented is a malicious user creating a
+large number of MXC URIs and sending them to a room without uploading the
+corresponding media. Clients in that room would then attempt to download the
+media, holding open connections to the server and potentially exhausting the
+number of available connections.
+
+This attack vector is stopped in multiple ways:
+
+1. Limits on `/create` prevent users from creating MXC URIs too quickly and also
+   require them to finish uploading files (or let some of their MXCs expire)
+   before creating new MXC URIs.
+
+2. Servers are free to respond to `/download` and `/thumbnail` requests before
+   the `timeout_ms` has been reached and respond with `M_NOT_YET_UPLOADED`. For
+   example, if the server is under connection count pressure, it can choose to
+   respond to waiting download connections with `M_NOT_YET_UPLOADED` to free
+   connections in the pool.
+
+3. Once the media is expired, servers can respond immediately to `/download` and
+   `/thumbnail` requests with `M_NOT_FOUND`.
+
+## Future work
+
+Future MSCs might wish to address large file uploads. One approach would be to
+add metadata to the `/create` call via a query parameter (for example
+`?large_file_upload=true`. Servers would have the ability to impose restrictions
+on how many such "large file" uploads a user can have concurrently. For such a
+situation, the server would likely send a more generous `unused_expires_at`
+timestamp to allow for a long-running upload.
+
+## Unstable prefix
+While this MSC is not in a released version of the spec, implementations should
+use `fi.mau.msc2246` as a prefix and as an `unstable_features` flag in the
+`/versions` endpoint.
+
+* `POST /_matrix/media/unstable/fi.mau.msc2246/create`
+* `PUT /_matrix/media/unstable/fi.mau.msc2246/upload/{serverName}/{mediaId}`
+* `?fi.mau.msc2246.timeout_ms`
+* `FI.MAU.MSC2246_NOT_YET_UPLOADED`
+* `FI.MAU.MSC2246_CANNOT_OVERWRITE_MEDIA`
+
+[matrix-spec#432]: https://github.com/matrix-org/matrix-spec/issues/432
+[MSC701]: https://github.com/matrix-org/matrix-doc/issues/701