MSC3859: Add well known media domain proposal

proposals/3859-well-known-media-domain.md

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+# MSC3859: .well-known Media Domain
+
+Currently Matrix clients and servers discover homeservers and identify servers via the `.well-known/matrix/client` and 
+`.well-known/matrix/server` endpoints. This is currently limited to the homeserver and for clients the identity server.
+This MSC proposes adding an additional field that can be used to specify a media server that can optionally be used 
+for `/_matrix/media/*` endpoints.
+
+The reasoning behind this change is to support hosting media behind a CDN designed and optimised specifically for 
+serving media. Since media transfers and endpoints are very different to the other JSON based endpoints a separate 
+domain offers maximum flexibility for server owners. This is particularly important for path optimisation between
+client and server over long distances where home internet providers generally offer inferior routing between countries
+than CDN providers do. By allowing clients to upload/download from the closest CDN Point of Presence they will benefit from improved
+upload and download performance.
+
+This cannot be achieved currently unless the entire homeserver is served behind a CDN, which may have other cost or
+management implications (or simply be impossible, depending on CDN). A separate domain allows server admins to
+decide the best setup for their users.
+
+
+## Proposals
+
+### Extend the client .well-known
+
+Add a new optional field, `m.media_server` that can specify a separate URL to be used for media requests. Clients can 
+then optionally use this field for all media endpoints, including both download and upload.
+
+```json
+{
+  "m.homeserver": {
+    "base_url": "https://matrix.example.com"
+  },
+  "m.media_server": {
+    "base_url": "https://matrix-media.example.com"
+  }
+}
+```
+
+To ensure backwards compatibility the homeserver base URL must also be able to serve the same media requests (via a 
+proxy or some other backend system).
+
+### Extend the server .well-known
+
+This is the same as above but for the server endpoint:
+
+```json
+{
+  "m.server": "matrix.example.com:443",
+  "m.media_server": "matrix-media.example.com:443"
+}
+```
+
+As above, the non-media endpoint must also serve media requests.
+
+
+## Alternatives
+
+### Redirects
+
+For the download path, the homeserver could send a redirect response to a CDN backed domain. This is proposed in 
+[MSC3860](https://github.com/matrix-org/matrix-spec-proposals/pull/3860).
+
+For the upload path this is not possible under HTTP.
+
+
+## Security Considerations
+
+Server admins may have to manage two distinct domains/installs increasing management & attack vector.
+
+
+## Unstable Prefix
+
+While not released in the Matrix spec implementations should use field `com.beeper.msc3859.m.media_server` in place of 
+`m.media_server` in the well known responses:
+
+```json5
+// Client
+{
+  "com.beeper.msc3859.m.media_server": {
+    "base_url": "https://matrix-media.example.com"
+  }
+}
+// Server
+{
+  "com.beeper.msc3859.m.media_server": "matrix-media.example.com:443"
+}