《Apache Dubbo微服务开发从入门到精通》——通信协议—— 三、 Dubbo2协议（中） https://developer.aliyun.com/article/1224597
5. 协议SPEC
• Magic-Magic High & Magic Low（16 bits）
Identifies dubbo protocol with value：0xdabb
• Req/Res（1 bit）
Identifies this is a request or response. Request-1; Response-0.
• 2 Way（1 bit）
Only useful when Req/Res is 1（Request），expect for a return value from server or not. Set to 1 if need a return value from server.
• Event（1 bit）
Identifies an event message or not，for example, heartbeat event. Set to 1 if this is an event.
• Serialization ID（5 bit）
Identifies serialization type：the value for fastjson is 6.
• Status（8 bits）
Only useful when Req/Res is 0（Response），identifies the status of response.
｡ 20 - OK
｡ 30 - CLIENT_TIMEOUT
｡ 31 - SERVER_TIMEOUT
｡ 40 - BAD_REQUEST
｡ 50 - BAD_RESPONSE
｡ 60 - SERVICE_NOT_FOUND
｡ 70 - SERVICE_ERROR
｡ 80 - SERVER_ERROR
｡ 90 - CLIENT_ERROR
｡ 100 - SERVER_THREADPOOL_EXHAUSTED_ERROR
• Request ID（64 bits）
Identifies an unique request. Numeric（long）.
• Data Length（32）
Length of the content（the variable part）after serialization, counted by bytes. Numeric（integer）.
• Variable Part
Each part is a byte[] after serialization with specific serialization type, identifies by Serialization ID.
Every part is a byte[] after serialization with specific serialization type, identifies by Serialization ID
• If the content is a Request (Req/Res = 1), each part consists of the content, in turn is：
｡ Dubbo version
｡ Service name
｡ Service version
｡ Method name
｡ Method parameter types
｡ Method arguments
｡ Attachments
• If the content is a Response (Req/Res = 0), each part consists of the content, in turn is:
｡ Return value type, identifies what kind of value returns from server side: RESPONSE_NULL_VALUE - 2, RESPONSE_VALUE - 1, RESPONSE_WITH_EXCEPTION - 0.
｡ Return value, the real value returns from server.
注：对于（Variable Part）变长部分，当前版本的dubbo框架使用json序列化时，在每部分内容间额外增加了换行符作为分隔，请选手在Variable Part的每个part后额外增加换行符，如：
