Mobile data is charged starting from the IP header, e.g. if you have a TCP/IP connection the packets needed to establish and shut down the connection are also taken into account.
Example: if you send a single byte, the size of the TCP data-gram alone is effectively 41 bytes.
Size of IPv4 Header (without any options) = 20 bytes
Size of TCP Header (without any options) = 20 bytes
This will result in the following packet flow:
3-way TCP handshake
- SYN 40 bytes
- SYN/ACK 44 bytes
- ACK 40 bytes
1 byte is sent in both directions, ACK combined with PSH
- PSH/ACK 41 bytes
- PSH/ACK 41 bytes
TCP connection shutdown
- FIN/ACK 40 bytes
- FIN/ACK 40 bytes
- ACK 40 bytes
- ACK 40 bytes
= 9 packets / 366 bytes total
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