UTP_HEADER_SIZE

const val UTP_HEADER_SIZE: Int = 20

The fixed uTP header size: 20 bytes. Matches sizeof(utp_header) in libtorrent (utp_stream.cpp uses acquire_packet(sizeof(utp_header)) everywhere).

Layout (BEP-29, all multi-byte fields big-endian / network order):

0       4       8               16              24              32
+-------+-------+---------------+---------------+---------------+
| type | ver | extension | connection_id |
+-------+-------+---------------+---------------+---------------+
| timestamp_microseconds |
+---------------+---------------+---------------+---------------+
| timestamp_difference_microseconds |
+---------------+---------------+---------------+---------------+
| wnd_size |
+---------------+---------------+---------------+---------------+
| seq_nr | ack_nr |
+---------------+---------------+---------------+---------------+