为什么ORC文件的页脚会存储文件头SIZE？

Question

来自：https://orc.apache.org/specification/ORCv2/

ORC文件页脚的代码：

message Footer {
 // the length of the file header in bytes (always 3)
 optional uint64 headerLength = 1;
 // the length of the file header and body in bytes
 optional uint64 contentLength = 2;
 // the information about the stripes
 repeated StripeInformation stripes = 3;
 // the schema information
 repeated Type types = 4;
 // the user metadata that was added
 repeated UserMetadataItem metadata = 5;
 // the total number of rows in the file
 optional uint64 numberOfRows = 6;
 // the statistics of each column across the file
 repeated ColumnStatistics statistics = 7;
 // the maximum number of rows in each index entry
 optional uint32 rowIndexStride = 8;
}

我了解到ORC文件头始终以其文件签名开头，即3字节字符串：“ ORC”，以使任何文件阅读器都知道其ORC文件。 那么，ORC文件的FOOTER中包含文件签名长度（始终为3个字节）的意义是什么？