
The retain header is at te end of the blobs.

   char           magic;         // 0x00: 'R'
   uint8_t        versionMajor;  // 0x01: 5
   uint8_t        versionMinor;  // 0x02: 0
   c_counter_t    counter;       // 0x03: 1
   uint32_t       len;           // 0x04: 0x34 (0x40-0xC)
   uint32_t       dataCrc32;     // 0x08: 0xaabbccdd

