% Chapter Template

\chapter{Methodology} % Main chapter title

\label{Methodology}

This chapter describes the methodology used to benchmark peer-to-peer
mesh VPN implementations. The experimental design prioritizes
reproducibility at every layer; from dependency management to network
conditions; enabling independent verification of results and
facilitating future comparative studies.

\section{Experimental Setup}

\subsection{Hardware Configuration}

All experiments were conducted on three bare-metal servers with
identical specifications:

\begin{itemize}
  \item \textbf{CPU:} Intel Model 94, 4 cores / 8 threads
  \item \textbf{Memory:} 64 GB RAM
  \item \textbf{Network:} 1 Gbps Ethernet (e1000e driver; one machine
    uses r8169)
  \item \textbf{Cryptographic acceleration:} AES-NI, AVX, AVX2, PCLMULQDQ,
    RDRAND, SSE4.2
\end{itemize}

The presence of hardware cryptographic acceleration is relevant because
many VPN implementations leverage AES-NI for encryption, and the results
may differ on systems without these features.

\subsection{Network Topology}

The three machines are connected via a direct 1 Gbps LAN on the same
network segment. This baseline topology provides a controlled environment
with minimal latency and no packet loss, allowing the overhead introduced
by each VPN implementation to be measured in isolation.

To simulate real-world network conditions, Linux traffic control
(\texttt{tc netem}) is used to inject latency, jitter, packet loss,
and reordering. These impairments are applied symmetrically on all
machines, meaning effective round-trip impairment is approximately
double the per-machine values.

\section{VPNs Under Test}

Ten VPN implementations were selected for evaluation, spanning a range
of architectures from centralized coordination to fully decentralized
mesh topologies. Table~\ref{tab:vpn_selection} summarizes the selection.

\begin{table}[H]
  \centering
  \caption{VPN implementations included in the benchmark}
  \label{tab:vpn_selection}
  \begin{tabular}{lll}
    \hline
    \textbf{VPN} & \textbf{Architecture} & \textbf{Notes} \\
    \hline
    Tailscale (Headscale) & Coordinated mesh & Open-source
    coordination server \\
    ZeroTier & Coordinated mesh & Global virtual Ethernet \\
    Nebula & Coordinated mesh & Slack's overlay network \\
    Tinc & Fully decentralized & Established since 1998 \\
    Yggdrasil & Fully decentralized & Spanning-tree routing \\
    Mycelium & Fully decentralized & End-to-end encrypted IPv6 overlay \\
    Hyprspace & Fully decentralized & libp2p-based, IPFS-compatible \\
    EasyTier & Fully decentralized & Rust-based, multi-protocol \\
    VpnCloud & Fully decentralized & Lightweight, kernel bypass option \\
    WireGuard & Point-to-point & Reference baseline (not a mesh VPN) \\
    \hline
    Internal (no VPN) & N/A & Baseline for raw network performance \\
    \hline
  \end{tabular}
\end{table}

WireGuard is included as a reference point despite not being a mesh VPN.
Its minimal overhead and widespread adoption make it a useful comparison
for understanding the cost of mesh coordination and NAT traversal logic.

\subsection{Selection Criteria}

VPNs were selected based on:
\begin{itemize}
  \item \textbf{NAT traversal capability:} All selected VPNs can establish
    connections between peers behind NAT without manual port forwarding.
  \item \textbf{Decentralization:} Preference for solutions without mandatory
    central servers, though coordinated-mesh VPNs were included for comparison.
  \item \textbf{Active development:} Only VPNs with recent commits and
    maintained releases were considered.
  \item \textbf{Linux support:} All VPNs must run on Linux.
\end{itemize}

\subsection{Configuration Methodology}

Each VPN is built from source within the Nix flake, ensuring that all
dependencies are pinned to exact versions. VPNs not packaged in nixpkgs
(Hyprspace, EasyTier, VpnCloud, qperf) have dedicated build expressions
under \texttt{pkgs/} in the flake.

Cryptographic material (WireGuard keys, Nebula certificates, ZeroTier
identities) is generated deterministically via Clan's vars generator
system. For example, WireGuard keys are generated as:

\begin{verbatim}
wg genkey > "$out/private-key"
wg pubkey < "$out/private-key" > "$out/public-key"
\end{verbatim}

Generated keys are stored in version control under
\texttt{vars/per-machine/\{name\}/} and read at NixOS evaluation time,
making key material part of the reproducible configuration.

\section{Benchmark Suite}

The benchmark suite includes both synthetic throughput tests and
real-world workloads. This combination addresses a limitation of prior
work that relied exclusively on iperf3.

\subsection{Ping}

Measures round-trip latency and packet delivery reliability.

\begin{itemize}
  \item \textbf{Method:} 100 ICMP echo requests at 200 ms intervals,
    1-second per-packet timeout, repeated for 3 runs.
  \item \textbf{Metrics:} RTT (min, avg, max, mdev), packet loss percentage,
    per-packet RTTs.
\end{itemize}

\subsection{iPerf3}

Measures bulk data transfer throughput.

\textbf{TCP variant:} 30-second bidirectional test with RSA authentication
and zero-copy mode (\texttt{-Z}) to minimize CPU overhead.

\textbf{UDP variant:} Same configuration with unlimited target bandwidth
(\texttt{-b 0}) and 64-bit counters.

\textbf{Parallel TCP variant:} Tests concurrent mesh traffic by running
TCP streams on all machines simultaneously in a circular pattern
(A$\rightarrow$B, B$\rightarrow$C, C$\rightarrow$A) for 60 seconds.
This simulates contention across the mesh.

\begin{itemize}
  \item \textbf{Metrics:} Throughput (bits/s), retransmits, congestion window,
    jitter (UDP), packet loss (UDP).
\end{itemize}

\subsection{qPerf}

Measures connection-level performance rather than bulk throughput.

\begin{itemize}
  \item \textbf{Method:} One qperf instance per CPU core in parallel, each
    running for 30 seconds. Bandwidth from all cores is summed per second.
  \item \textbf{Metrics:} Total bandwidth (Mbps), CPU usage, time to first
    byte (TTFB), connection establishment time.
\end{itemize}

\subsection{RIST Video Streaming}

Measures real-time multimedia streaming performance.

\begin{itemize}
  \item \textbf{Method:} The sender generates a 4K (3840$\times$2160) test
    pattern at 30 fps using ffmpeg with H.264 encoding (ultrafast preset,
    zerolatency tuning) at 25 Mbps target bitrate. The stream is transmitted
    over the RIST protocol to a receiver on the target machine for 30 seconds.
  \item \textbf{Encoding metrics:} Actual bitrate, frame rate, dropped frames.
  \item \textbf{Network metrics:} Packets dropped, packets recovered via
    RIST retransmission, RTT, quality score (0--100), received bitrate.
\end{itemize}

RIST (Reliable Internet Stream Transport) is a protocol designed for
low-latency video contribution over unreliable networks, making it a
realistic test of VPN behavior under multimedia workloads.

\subsection{Nix Cache Download}

Measures sustained download performance using a real-world workload.

\begin{itemize}
  \item \textbf{Method:} A Harmonia Nix binary cache server on the target
    machine serves the Firefox package. The client downloads it via
    \texttt{nix copy} through the VPN. Benchmarked with hyperfine:
    1 warmup run followed by 2 timed runs. The local cache and Nix's
    SQLite metadata are cleared between runs.
  \item \textbf{Metrics:} Mean duration (seconds), standard deviation,
    min/max duration.
\end{itemize}

This benchmark tests realistic HTTP traffic patterns and sustained
sequential download performance, complementing the synthetic throughput
tests.

\section{Network Impairment Profiles}

Four impairment profiles simulate a range of network conditions, from
ideal to severely degraded. Impairments are applied via Linux traffic
control (\texttt{tc netem}) on every machine's primary interface.
Table~\ref{tab:impairment_profiles} shows the per-machine values;
effective round-trip impairment is approximately doubled.

\begin{table}[H]
  \centering
  \caption{Network impairment profiles (per-machine egress values)}
  \label{tab:impairment_profiles}
  \begin{tabular}{lccccc}
    \hline
    \textbf{Profile} & \textbf{Latency} & \textbf{Jitter} &
    \textbf{Loss} & \textbf{Reorder} & \textbf{Correlation} \\
    \hline
    Baseline & ;  & ;  & ;  & ;  & ;  \\
    Low & 2 ms & 2 ms & 0.25\% & 0.5\% & 25\% \\
    Medium & 4 ms & 7 ms & 1.0\% & 2.5\% & 50\% \\
    High & 12 ms & 30 ms & 5.0\% & 10\% & 50\% \\
    \hline
  \end{tabular}
\end{table}

The ``Low'' profile approximates a well-provisioned continental
connection, ``Medium'' represents intercontinental links or congested
networks, and ``High'' simulates severely degraded conditions such as
satellite links or highly congested mobile networks.

A 30-second stabilization period follows TC application before
measurements begin, allowing queuing disciplines to settle.

\section{Experimental Procedure}

\subsection{Automation}

The benchmark suite is fully automated via a Python orchestrator
(\texttt{vpn\_bench/}). For each VPN under test, the orchestrator:

\begin{enumerate}
  \item Cleans all state directories from previous VPN runs
  \item Deploys the VPN configuration to all machines via Clan
  \item Restarts the VPN service on every machine (with retry:
    up to 3 attempts, 2-second backoff)
  \item Verifies VPN connectivity via a connection-check service
    (120-second timeout)
  \item For each impairment profile:
    \begin{enumerate}
      \item Applies TC rules via context manager (guarantees cleanup)
      \item Waits 30 seconds for stabilization
      \item Executes all benchmarks
      \item Clears TC rules
    \end{enumerate}
  \item Collects results and metadata
\end{enumerate}

\subsection{Retry Logic}

Tests use a retry wrapper with up to 2 retries (3 total attempts),
5-second initial delay, and 700-second maximum total time. The number
of attempts is recorded in test metadata so that retried results can
be identified during analysis.

\subsection{Statistical Analysis}

Each metric is summarized as a statistics dictionary containing:

\begin{itemize}
  \item \textbf{min / max:} Extreme values observed
  \item \textbf{average:} Arithmetic mean across samples
  \item \textbf{p25 / p50 / p75:} Quartiles via \texttt{statistics.quantiles()}
\end{itemize}

Multi-run tests (ping, nix-cache) aggregate across runs. Per-second
tests (qperf, RIST) aggregate across all per-second samples.

The approach uses empirical percentiles rather than parametric
confidence intervals, which is appropriate for benchmark data that
may not follow a normal distribution. The nix-cache test (via hyperfine)
additionally reports standard deviation.

\section{Reproducibility}

Reproducibility is ensured at every layer of the experimental stack.

\subsection{Dependency Pinning}

Every external dependency is pinned via \texttt{flake.lock}, which records
cryptographic hashes (\texttt{narHash}) and commit SHAs for each input.
Key pinned inputs include:

\begin{itemize}
  \item \textbf{nixpkgs:} Follows \texttt{clan-core/nixpkgs}, ensuring a
    single version across the dependency graph
  \item \textbf{clan-core:} The Clan framework, pinned to a specific commit
  \item \textbf{VPN sources:} Hyprspace, EasyTier, Nebula locked to
    exact commits
  \item \textbf{Build infrastructure:} flake-parts, treefmt-nix, disko,
    nixos-facter-modules
\end{itemize}

Custom packages not in nixpkgs (qperf, VpnCloud, iperf with auth patches,
phantun, EasyTier, Hyprspace) are built from source within the flake.

\subsection{Declarative System Configuration}

Each benchmark machine runs NixOS, where the entire operating system is
defined declaratively. There is no imperative package installation or
configuration drift. Given the same NixOS configuration, two machines
will have identical software, services, and kernel parameters.

Machine deployment is atomic: the system either switches to the new
configuration entirely or rolls back.

\subsection{Inventory-Driven Topology}

Clan's inventory system maps machines to service roles declaratively.
For each VPN, the orchestrator writes an inventory entry assigning
machines to roles (e.g., Nebula lighthouse vs.\ peer). The Clan module
system translates this into NixOS configuration; systemd services,
firewall rules, peer lists, and key references. The same inventory
entry always produces the same NixOS configuration.

\subsection{State Isolation}

Before installing a new VPN, the orchestrator deletes all state
directories from previous runs, including VPN-specific directories
(\texttt{/var/lib/zerotier-one}, \texttt{/var/lib/nebula}, etc.) and
benchmark directories. This prevents cross-contamination between tests.

\subsection{Data Provenance}

Every test result includes metadata recording:

\begin{itemize}
  \item Wall-clock duration
  \item Number of attempts (1 = first try succeeded)
  \item VPN restart attempts and duration
  \item Connectivity wait duration
  \item Source and target machine names
  \item Service logs (on failure)
\end{itemize}

Results are organized hierarchically by VPN, TC profile, and machine
pair. Each profile directory contains a \texttt{tc\_settings.json}
snapshot of the exact impairment parameters applied.
