SOME/IP报文格式-Message Type
Message Type [8 Bit]
The Message Type field is used to differentiate different types of messages and shall contain the following values：
Message Type用来区别不同类型的message，且应当包含如下的值：
| Number | Value | Description |
| 0x00 | REQUEST | A request expecting a response (even void) |
| 0x01 | REQUEST_NO_RETURN | A fire&forget request |
| 0x02 | NOTIFICATION | A request of a notification/event callback expecting no response |
| 0x40 | REQUEST_ACK | Acknowledgment for REQUEST (optional) |
| 0x41 | REQUEST_NO_RETURN_ACK | Acknowledgment for REQUEST_NO_RETURN (informational) |
| 0x42 | NOTIFICATION_ACK | Acknowledgment for NOTIFICATION (informational) |
| 0x80 | RESPONSE | The response message |
| 0x81 | ERROR | The response containing an error |
| 0xC0 | RESPONSE_ACK | The Acknowledgment for RESPONSE (informational) |
| 0xC1 | ERROR_ACK | Acknowledgment for ERROR (informational) |
•Regular request (message type 0x00) will be answered by a response (message type 0x80), when no error occurred. If errors occur an error message (message type 0x81) will be sent. It is also possible to send a request that does not have a response message(message type 0x01). For updating values through notification a callback interface exists (message type 0x02). 常规request （消息类型0x00）将在未发生错误时由response （消息类型0x80）应答。 如果发生错误，则会发送error消息（消息类型0x81）。 也可以发送不需要响应消息的请求（消息类型0x01）。 为了通过notification更新值，存在一个callback接口（消息类型0x02）。 •For all messages an optional acknowledgment (ACK) exists. These care is defined for transport protocols (i.e. UDP) that do not acknowledge a received message. ACKs are only transported when the interface specification requires it. Only the usage of the REQUEST_ACK is currently specified in this document. All other ACKs are currently informational and do not need to be implemented. 对于所有消息，存在可选的acknowledgment 报文（ACK）。 这是为那些并不告知收到消息的某些传输层协议（如UDP）而定义的。 ACK只在interface specification要求时才被传输。 本文档中仅规定了REQUEST_ACK的用法。 所有其他ACK现在都是信息性的，不需要实施。