matrix-spec-proposals/content/client-server-api/modules/send_to_device.md

type	weight
module	80

Send-to-Device messaging

This module provides a means by which clients can exchange signalling messages without them being stored permanently as part of a shared communication history. A message is delivered exactly once to each client device.

The primary motivation for this API is exchanging data that is meaningless or undesirable to persist in the room DAG - for example, one-time authentication tokens or key data. It is not intended for conversational data, which should be sent using the normal /rooms/<room_id>/send API for consistency throughout Matrix.

Client behaviour

To send a message to other devices, a client should call /sendToDevice. Only one message can be sent to each device per transaction, and they must all have the same event type. The device ID in the request body can be set to * to request that the message be sent to all known devices.

If there are send-to-device messages waiting for a client, they will be returned by /sync, as detailed in Extensions to /sync. Clients should inspect the type of each returned event, and ignore any they do not understand.

Server behaviour

Servers should store pending messages for local users until they are successfully delivered to the destination device. When a client calls /sync with an access token which corresponds to a device with pending messages, the server should list the pending messages, in order of arrival, in the response body.

When the client calls /sync again with the next_batch token from the first response, the server should infer that any send-to-device messages in that response have been delivered successfully, and delete them from the store.

If there is a large queue of send-to-device messages, the server should limit the number sent in each /sync response. 100 messages is recommended as a reasonable limit.

If the client sends messages to users on remote domains, those messages should be sent on to the remote servers via federation.

Protocol definitions

{{% http-api spec="client-server" api="to_device" %}}

Extensions to /sync

This module adds the following properties to the /sync response:

Parameter	Type	Description
to_device	ToDevice	Optional. Information on the send-to-device messages for the client device.

ToDevice

Parameter	Type	Description
events	[Event]	List of send-to-device messages.

Event

Parameter	Type	Description
content	EventContent	The content of this event. The fields in this object will vary depending on the type of event.
sender	string	The Matrix user ID of the user who sent this event.
type	string	The type of event.

Example response:

{
  "next_batch": "s72595_4483_1934",
  "rooms": {"leave": {}, "join": {}, "invite": {}},
  "to_device": {
    "events": [
      {
        "sender": "@alice:example.com",
        "type": "m.new_device",
        "content": {
          "device_id": "XYZABCDE",
          "rooms": ["!726s6s6q:example.com"]
        }
      }
    ]
  }
}