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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.

{
  "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:

{
  "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.

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:

// 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"
}