add graph comments for size of request and response.

@@ -150,13 +150,29 @@ void SrsKafkaRequestHeader::set_api_key(SrsKafkaApiKey key)
     api_key = (int16_t)key;
 }
 
-SrsKafkaResponse::SrsKafkaResponse()
+SrsKafkaResponseHeader::SrsKafkaResponseHeader()
 {
+    size = 0;
     correlation_id = 0;
 }
 
-SrsKafkaResponse::~SrsKafkaResponse()
+SrsKafkaResponseHeader::~SrsKafkaResponseHeader()
+{
+}
+
+int SrsKafkaResponseHeader::header_size()
+{
+    return 4;
+}
+
+int SrsKafkaResponseHeader::message_size()
+{
+    return size - header_size();
+}
+
+int SrsKafkaResponseHeader::total_size()
 {
+    return 4 + size;
 }
 
 SrsKafkaMessage::SrsKafkaMessage()

@@ -179,18 +179,24 @@ public:
     virtual ~SrsKafkaRequestHeader();
 public:
     /**
-     * the size of header, exclude the 4bytes size.
-     * @remark total_size = 4 + header_size + message_size.
+     * the layout of request:
+     *      +-----------+----------------------------------+
+     *      |   4B size |       [size] bytes               |
+     *      +-----------+------------+---------------------+
+     *      |   4B size |   header   |    message          |
+     *      +-----------+------------+---------------------+
+     *      |  total size = 4 + header + message           |
+     *      +----------------------------------------------+
+     * where the header is specifies this request header without the start 4B size.
+     * @remark size = 4 + header + message.
      */
     virtual int header_size();
     /**
      * the size of message, the bytes left after the header.
-     * @remark total_size = 4 + header_size + message_size.
      */
     virtual int message_size();
     /**
-     * the total size of the request, 4bytes + size of header and message.
-     * @remark total_size = 4 + header_size + message_size.
+     * the total size of the request, includes the 4B size.
      */
     virtual int total_size();
 public:
@@ -210,21 +216,53 @@ public:
 };
 
 /**
- * the common kafka response.
- * The response will always match the paired request (e.g. we will 
+ * the header of response, include the size of response.
+ * The response will always match the paired request (e.g. we will
  * send a MetadataResponse in return to a MetadataRequest).
  * @see https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol#AGuideToTheKafkaProtocol-Responses
  */
-class SrsKafkaResponse
+class SrsKafkaResponseHeader
 {
-protected:
+private:
     /**
-     * The server passes back whatever integer the client supplied as the correlation in the request.
+     * The MessageSize field gives the size of the subsequent request or response
+     * message in bytes. The client can read requests by first reading this 4 byte
+     * size as an integer N, and then reading and parsing the subsequent N bytes
+     * of the request.
+     */
+    int32_t size;
+private:
+    /**
+     * This is a user-supplied integer. It will be passed back in
+     * the response by the server, unmodified. It is useful for matching
+     * request and response between the client and server.
      */
     int32_t correlation_id;
 public:
-    SrsKafkaResponse();
-    virtual ~SrsKafkaResponse();
+    SrsKafkaResponseHeader();
+    virtual ~SrsKafkaResponseHeader();
+public:
+    /**
+     * the layout of response:
+     *      +-----------+----------------------------------+
+     *      |   4B size |       [size] bytes               |
+     *      +-----------+------------+---------------------+
+     *      |   4B size |  4B header |    message          |
+     *      +-----------+------------+---------------------+
+     *      |  total size = 4 + 4 + message                |
+     *      +----------------------------------------------+
+     * where the header is specifies this request header without the start 4B size.
+     * @remark size = 4 + 4 + message.
+     */
+    virtual int header_size();
+    /**
+     * the size of message, the bytes left after the header.
+     */
+    virtual int message_size();
+    /**
+     * the total size of the request, includes the 4B size.
+     */
+    virtual int total_size();
 };
 
 /**
@@ -316,6 +354,15 @@ public:
     virtual ~SrsKafkaTopicMetadataRequest();
 };
 
+class SrsKafkaTopicMetadataResponse
+{
+private:
+    SrsKafkaRequestHeader header;
+public:
+    SrsKafkaTopicMetadataResponse();
+    virtual ~SrsKafkaTopicMetadataResponse();
+};
+
 /**
  * the kafka protocol stack, use to send and recv kakfa messages.
  */