How much network overhead does TLS add compared to a non-encrypted connection?

Question

(Approximately) how many more bits of data must be transferred over the network during an encrypted connection compared to an unencrypted connection?

IIUC, once the TLS

Answer 1

In 2020, TLS 1.2 and 1.3 are more typical with AES-GCM being a streaming cipher mode with lower overhead.

See https://tools.ietf.org/id/draft-mattsson-uta-tls-overhead-01.xml#rfc.section.3.

Per packet, the overhead for AES-GCM is 29 bytes. The TCP MSS may be as large as 1460 (https://blog.apnic.net/2014/12/15/ip-mtu-and-tcp-mss-missmatch-an-evil-for-network-performance/). So for a large download (where the maximum MSS is used), the overhead would be 29:1431 which is 2.03%.

(Handshake overhead is separate being once-off)