The total cost of establishing a TLS/DTLS session includes message size overhead, computational time, and energy consumption.
Handshake Message Overhead: \[\text{Total Bytes} = \text{ClientHello} + \text{ServerHello} + \text{Certificate} + \text{CertificateVerify} + \text{Finished}\]
Session Resumption Savings: \[\text{Savings Ratio} = \frac{\text{Full Handshake Cost} - \text{PSK Resumption Cost}}{\text{Full Handshake Cost}}\]
Working through an example:
Given: nRF52840 BLE sensor reporting every 15 minutes for 2 years
Full TLS 1.3 Handshake (ECDHE-P256):
- Time: \(T_{\text{full}} = 1,200 \text{ ms}\)
- Current draw: \(I = 8 \text{ mA}\)
- Energy per handshake: \(E_{\text{full}} = I \times \frac{T_{\text{full}}}{3600 \text{ s/h}} = 8 \times \frac{1.2}{3600} = 0.00267 \text{ mAh}\)
Annual handshakes: \(96 \text{ per day} \times 365 \text{ days} = 35{,}040 \text{ handshakes/year}\)
Annual energy cost: \(E_{\text{annual}} = 35{,}040 \times 0.00267 = 93.5 \text{ mAh/year}\)
Battery fraction: \(\frac{93.5}{2400} = 3.9\%\) of battery per year for handshakes alone
PSK Resumption:
- Time: \(T_{\text{PSK}} = 80 \text{ ms}\)
- Energy per resumption: \(E_{\text{PSK}} = 8 \times \frac{0.08}{3600} = 0.000178 \text{ mAh}\)
- Annual energy: \(35{,}040 \times 0.000178 = 6.2 \text{ mAh/year}\)
- Battery fraction: \(\frac{6.2}{2400} = 0.26\%\) per year
Energy savings: \(93.5 - 6.2 = 87.3 \text{ mAh/year}\) (93.4% reduction)
Result: PSK session resumption reduces TLS handshake energy consumption by 93%, freeing up 87.3 mAh/year for other device functions. For a device where TLS handshakes are the dominant energy cost (e.g., a sensor that sleeps between reports), this savings can extend battery life from approximately 2 years to well over 5 years.
In practice: Without session resumption, devices performing 96 daily connections consume 3.9% of a 2400 mAh battery per year on handshakes alone. With other energy costs (radio, sensing, MCU), this can be the difference between meeting a 2-year battery target or falling short. PSK session resumption makes the handshake energy nearly negligible.
