public MsRegAck(byte[] buf, int start, int end)
: base(buf, start, end) {
int ptr=buf[start+0]==1?start+4:start+2;
this.TopicId=(ushort)((buf[ptr++]<<8) | buf[ptr++]);
this.MessageId=(ushort)((buf[ptr++]<<8) | buf[ptr++]);
this.RetCode=(MsReturnCode)buf[ptr];
this.ReqTyp=MsMessageType.REGISTER;
}