\subsection{Raft}%
\label{sub:Raft}
Raft is a consensus algorithm designed to replicate a state machine across distributed peers, and sees broad usage in distributed databases, key-value stores, distributed file systems, distributed load-balancers, and container orchestration. Historically, verification efforts of Raft using both constructive, mechanized proving techniques \cite{Woos_Wilcox_Anton_Tatlock_Ernst_Anderson_2016, Wilcox_Woos_Panchekha_Tatlock_Wang_Ernst_Anderson, Ongaro} and automated verification \cite{Ongaro} have only reasoned about the protocol under certain assumptions about the stability of the communication channels. However, no previous approach to Raft verification has reasoned about an on-channel attacker \textit{external} to the protocol itself. Uniquely, \korg enables us to study Raft under insecure communication channels.

\subsection{TCP}%
\label{sub:TCP}

TCP (Transmission Control Protocol) is a transport-layer protocol designed to establish reliable, ordered communications between two peers. TCP is ubiquitous in today's internet, and therefore has seen ample formal verification efforts \cite{Cluzel_Georgiou_Moy_Zeller_2021, Smith_1997, Pacheco2022}, including using \promela and \spin \cite{Pacheco2022}. A previous version of \korg has been applied TCP in \cite{Pacheco2022, Hippel2022}; 
in particular, we study our \korg extensions using the \promela models from Pacheco et al., which includes a "gold" model whose underlying state machine is derived via an NLP-based algorithm applied to the SCTP RFC \cite{rfc9260} and a "canonical" model hand-written by domain experts \cite{Pacheco2022}. Additionally, we borrow the four LTL properties used in \cite{Pacheco2022}, as detailed below:
\[
\begin{aligned}
\phi_1 &= \text{\parbox[t]{20em}{No half-open connections.}} \\
\phi_2 &= \text{\parbox[t]{20em}{Passive/active establishment eventually succeeds.}} \\
\phi_3 &= \text{\parbox[t]{20em}{Peers don't get stuck.}} \\
\phi_4 &= \text{\parbox[t]{20em}{\texttt{SYN\_RECEIVED} is eventually followed by \texttt{ESTABLISHED}, \texttt{FIN\_WAIT\_1}, or \texttt{CLOSED}.}}
\end{aligned}
\]

Evaluating the canonical TCP model using \korg led us to identify edge-cases in the connection establishment routine that weren't accounted for, leading us to construct a "revised" TCP model accounting for these missing edge cases. The resulting breakdown of attacks discovered is shown in Figure \ref{res:tcp-table}.

\begin{figure}[h!]
\centering
\begin{scriptsize}
\begin{tabular}{|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|@{}c@{}|}
\hline
& \multicolumn{3}{c|}{\footnotesize \raisebox{-0.15ex}{Drop Attacker} } & \multicolumn{3}{c|}{\footnotesize \raisebox{-0.15ex}{Replay Attacker}} & \multicolumn{3}{c|}{\footnotesize \raisebox{-0.15ex}{Reorder Attacker}} \\
\hline
& \: Gold \: & \: Expert \: & \: Revised \: & \: Gold \: & \: Expert \: & \: Revised \: & \: Gold \: & \: Expert \: & \: Revised \: \\
\hline
$\phi_1$ & \rule{0pt}{8pt} & & & & & & & & \\
$\phi_2$ & \rule{0pt}{8pt} & x & x & & x & x & & x & \\
$\phi_3$ & \rule{0pt}{8pt} & & & & & & & & \\
$\phi_4$ & \rule{0pt}{8pt} x & & & & & & x & & \\
\hline
\end{tabular}
\end{scriptsize}

\label{res:tcp-table}
\caption{Automatically discovered attacks against the gold, canonical (labeled "expert"), and revised TCP models for $\phi_1$ through $\phi_4$. "x" indicates an attack was discovered, and no "x" indicates \korg proved the absence of an attack via an exhaustive search. Full attack traces are available in the artifact.}
\end{figure}
