Messages

Messages (Requests, Responses, Notifications, and Files)

This section describes the general format of messages exchanged between hosts and devices that are using the common MMS message framework.

Documentation about the specific messages supported by a given device is provided in section 6 Commands.

About Messages

The host and the device communicate with each other by exchanging blocks of data called Messages, which are standardized wrappers containing a payload that is either a command Request, a command Response, an unsolicited Notification, or a File. For example, the host may send a command request message to the device to change a configuration setting, and the device may send a command response message to indicate the command was successful; when a cardholder inserts a card, the device may send a notification message to the host that a cardholder has initiated a transaction; the host may send the device a file message to load firmware.

Messages can be nested. For example, a top-level secure wrapper request from the host to the device may contain an encrypted or signed command request for the device to unpack, validate, and execute.

Requests and Responses

• Requests and responses are two of the message payload types the host and device exchange inside messages. The combination of a message that contains a request payload and a message that contains the corresponding response payload is referred to generally in this document as a Command.

• The device can only service one command request at a time, and sends each command response within a pre-determined finite amount of time after receiving the request.

• After sending a command request, the host must wait until the device returns a response before sending another request, or until the request is unanswered after a reasonable host-defined timeout period passes.

Notifications

• Notifications are a message payload type the host and device exchange inside messages. The device sends notification messages to the host if the device’s state changes or if an external event occurs, such as a cardholder inserting a card.

• The device can send a notification at any time, and does not expect a response or any specific action from the host.

• By default, the device sends all notifications to the USB interface. To configure the device to send notifications on additional connections, use Command 0x1F02 - Set Notification Subscriptions.

Data Files

• Data Files are a message payload type the host and device exchange inside messages. The device handles them as a stream: it begins storing the payload of the message before it has received the final packet of the message, allowing for much larger payloads than standard requests.

• This streaming behavior is possible because the message is restricted to transferring a file and thus the message payload is primitive data only; it cannot contain composed TLV data objects.

Regardless of connection type, all MMS devices use the same schema for sending and receiving messages, which is documented inMessage Format. For information about transmitting and receiving messages using specific connection types (which involves following connection-specific rules for breaking messages down into transmittable Message Streams), see section 2 Connection Types.