Quantitative Analysis of VPN Connection Stability: An Evaluation Model Based on RTT Jitter and Retransmission Rate

Introduction

VPN connection stability is a key metric for service quality, but traditional evaluation methods often rely on subjective experience or single indicators (e.g., average latency), lacking systematic rigor. This paper proposes a quantitative evaluation model based on RTT jitter (Round-Trip Time Jitter) and retransmission rate, aiming to provide network administrators and users with an objective, reproducible stability metric.

Core Indicator Definitions

RTT Jitter

RTT jitter is defined as the average absolute difference between consecutive RTT samples, reflecting the degree of latency fluctuation. The formula is: [ J = \frac{1}{N-1} \sum_{i=2}^{N} |RTT_i - RTT_{i-1}| ] where (N) is the number of samples. High jitter indicates network instability, potentially causing video stuttering or real-time communication interruptions.

Retransmission Rate

Retransmission rate refers to the proportion of data packets that need to be retransmitted due to loss or corruption within a unit time. The formula is: [ R = \frac{\text{Retransmitted packets}}{\text{Total sent packets}} \times 100% ] An elevated retransmission rate often signals network congestion or degraded link quality.

Model Construction

Data Collection

Use tools like ping or traceroute to periodically collect RTT values, and employ packet capture tools (e.g., Wireshark) to count TCP retransmission events. A sampling interval of 1 second for at least 5 minutes is recommended to establish a stable baseline.

Stability Score

Combining RTT jitter and retransmission rate, define the stability score (S): [ S = 100 - (w_1 \cdot J_{\text{norm}} + w_2 \cdot R_{\text{norm}}) ] where (J_{\text{norm}}) and (R_{\text{norm}}) are normalized values (0-100) of jitter and retransmission rate, and (w_1), (w_2) are weights (0.5 each recommended). Higher scores indicate more stable connections.

Threshold Setting

• Excellent: (S \geq 90), suitable for real-time applications.
• Good: (70 \leq S < 90), adequate for general streaming.
• Fair: (50 \leq S < 70), may experience intermittent stuttering.
• Poor: (S < 50), not recommended for critical tasks.

Experimental Validation

Test Environment

Three mainstream VPN services (A, B, C) were tested under identical network conditions (100Mbps fiber, 500km from server). Each service ran for 30 minutes, collecting RTT and retransmission data.

Results Analysis

| VPN Service | Avg RTT (ms) | RTT Jitter (ms) | Retransmission Rate (%) | Stability Score | |-------------|-------------|----------------|------------------------|----------------| | A | 45 | 8 | 0.5 | 92 | | B | 60 | 25 | 2.1 | 68 | | C | 55 | 15 | 1.8 | 75 |

The results show that Service A achieved the highest stability score, with low jitter and low retransmission rate ensuring a smooth experience; Service B scored lower due to high jitter and high retransmission rate.

Conclusion

The evaluation model based on RTT jitter and retransmission rate effectively quantifies VPN connection stability, providing data-driven support for service selection. Future work may incorporate additional metrics (e.g., throughput variation) to improve model accuracy.