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+# MSC3870: Async media upload extension: upload to URL
+
+This MSC proposes an extension to [MSC2246](https://github.com/matrix-org/matrix-spec-proposals/pull/2246)
+(async media uploads) that allows a media server to give clients a URL to use for uploading media
+data after the MXID is created.
+
+The rationale behind this is to enable clients to upload direct to the backing datastore, for example
+by using [S3 presigned URLs](https://docs.aws.amazon.com/AmazonS3/latest/userguide/PresignedUrlUploadObject.html)
+and/or [S3 transfer acceleration](https://docs.aws.amazon.com/AmazonS3/latest/userguide/transfer-acceleration-getting-started.html)
+(this is not limited to S3, just well documented by AWS). By enabling clients to upload to a different
+URL server owners could improve upload performance significantly by using a CDN with closer Points
+of Presence to their users than the homeserver itself.
+
+When combined with [MSC3860](https://github.com/matrix-org/matrix-spec-proposals/pull/3860) (media
+download redirect URLs) this makes it possible to run a "lean" media server that orchestrates where
+media is uploaded to and downloaded from but does not directly handle much media itself. Note this 
+doesn't include thumbnails or URL previews which would still go through the media server.
+
+
+## Proposal
+
+The proposal modifies the MSC2246 create endpoint to optionally include a URL that a compatible
+client may use to upload the content for the created MXID. This is considered optional and the
+client may continue to upload via the new upload endpoint defined in MSC2246.
+
+`POST /_matrix/media/v1/create`
+
+**Example response**
+
+```json
+{
+  "content_uri": "mxc://example.com/AQwafuaFswefuhsfAFAgsw",
+  "unused_expires_at": 1647257217083,
+  "upload_method": "PUT",
+  "upload_url": "https://cdn.example.com/media-repo/upload/XAPw4CtrzArk?signed=h4tGOHvCu"
+}
+```
+
+If the client chooses to upload media via the `upload_url` field it must use the method specified
+by the `upload_method` field. Clients should expect the upload URL to work until `unused_expires_at`
+time, afterwhich a new media would need to be created. Upload request based on the above response:
+
+`PUT https://cdn.example.com/media-repo/upload/XAPw4CtrzArk?signed=h4tGOHvCu`
+
+The response to the upload is implementation specific depending on the target server, the client
+should interpret standard HTTP responses, a 200 or 201 indicating successful upload. Once the upload
+is complete, the client must notify the media server via a new endpoint:
+
+`POST /_matrix/media/v1/upload/{serverName}/{mediaId}/complete`
+
+**Example response**
+
+```json
+{
+  "content_uri": "mxc://example.com/AQwafuaFswefuhsfAFAgsw"
+}
+```
+
+This allows the media server to perform any post-upload actions before either allowing the media
+to be downloaded (and returning the URI to the client as above) or rejecting the upload.
+
+This means that there is potential for a client to send an event referencing an MXC that is then
+uploaded and rejected. Note that this is also a problem MSC2246 faces, whatever solution applies
+there can be applied here.
+
+
+## Alternatives
+
+### Run the whole homeserver behind CDN
+
+Server owners could run their own Points of Presence to receive and handle media content and pass
+through any non-media requests to a central homeserver (or run a distributed homeserver if/when
+that exists). There is a significant cost (management time & infrastructure) associated with this.
+
+
+## Security Considerations
+
+Security of presigned URL uploads is well documented but important to be aware of when implementing
+support for this.
+
+
+## Unstable Prefix
+
+While this MSC is not in a released version of the spec, implementations should use `com.beeper.msc3880`
+as a prefix and as an unstable_features flag in the /versions endpoint.
+
+```
+ POST /_matrix/media/unstable/com.beeper.msc3870/upload/{serverName}/{mediaId}/complete
+```