diff --git a/rups/src/lib.rs b/rups/src/lib.rs
index 0952659..a8b5467 100644
--- a/rups/src/lib.rs
+++ b/rups/src/lib.rs
@@ -19,6 +19,8 @@ pub mod tokio;
 mod cmd;
 mod config;
 mod error;
+/// Bi-directional networking protocol implementation.
+mod proto;
 #[cfg(feature = "ssl")]
 mod ssl;
 mod util;
diff --git a/rups/src/proto/mod.rs b/rups/src/proto/mod.rs
new file mode 100644
index 0000000..887bbb4
--- /dev/null
+++ b/rups/src/proto/mod.rs
@@ -0,0 +1,112 @@
+///! NUT protocol implementation (v1.2).
+///! Documentation: https://networkupstools.org/docs/developer-guide.chunked/ar01s09.html
+
+macro_rules! impl_words {
+    (
+        $(
+            $(#[$attr:meta])+
+            $name:ident($word:tt),
+        )*
+    ) => {
+        pub(crate) enum Word {
+            /// A string argument.
+            Arg,
+            /// End-of-line.
+            EOL,
+            $(
+                /// Protocol word.
+                $(#[$attr])*
+                #[allow(dead_code)]
+                $name,
+            )*
+        }
+
+        impl Word {
+            /// Matches a raw string into the corresponding word.
+            /// Passing `None` will always return `EOL`. Passing an unrecognized
+            /// string returns `None`.
+            pub(crate) fn decode(raw: Option<&str>) -> Option<Self> {
+                if let Some(raw) = raw {
+                    match raw {
+                        $($word => Some(Self::$name),)*
+                        _ => None
+                    }
+                } else {
+                    Some(Self::EOL)
+                }
+            }
+
+            /// Encodes a `Word` into a string.
+            /// This function cannot encode `Arg` or `EOL` (either returns `None`).
+            pub(crate) fn encode(&self) -> Option<&str> {
+                match self {
+                    Self::Arg | Self::EOL => None,
+                    $(Self::$name => Some($word),)*
+                }
+            }
+        }
+    };
+}
+
+impl_words! {
+    /// Begins a `LIST`.
+    Begin("BEGIN"),
+    /// Describes a client connected to a UPS.
+    Client("CLIENT"),
+    /// Represents an executable command.
+    Cmd("CMD"),
+    /// Describes a command (`CMD`).
+    CmdDesc("CMDDESC"),
+    /// Describes a variable (`VAR` or `RW`).
+    Desc("DESC"),
+    /// Ends a block of sentences.
+    End("END"),
+    /// An enumerable type.
+    Enum("ENUM"),
+    /// An error response.
+    Err("ERR"),
+    /// Executes a forced shut down (FSD).
+    Fsd("FSD"),
+    /// Serverbound query.
+    Get("GET"),
+    /// Client requesting a list of commands supported by the server.
+    Help("HELP"),
+    /// Executes an instant command.
+    InstCmd("INSTCMD"),
+    /// Queries or describes a list.
+    List("LIST"),
+    /// Client requests login to a UPS device.
+    Login("LOGIN"),
+    /// Client logs out.
+    Logout("LOGOUT"),
+    /// Client verifying it has master-level access to the UPS device.
+    Master("MASTER"),
+    /// Client requests the network version.
+    NetworkVersion("NETVER"),
+    /// Represents the amount of logins to a UPS device.
+    NumLogins("NUMLOGINS"),
+    /// Clientbound response for a good outcome.
+    Ok("OK"),
+    /// Client setting password.
+    Password("PASSWORD"),
+    /// Represents a range of numerical values.
+    Range("RANGE"),
+    /// Represents a mutable variable.
+    Rw("RW"),
+    /// Client requests to set the value of a mutable variable.
+    Set("SET"),
+    /// Client requests the connection be upgraded to TLS.
+    StartTLS("STARTTLS"),
+    /// Represents the type of a variable.
+    Type("TYPE"),
+    /// Represents a UPS device.
+    Ups("UPS"),
+    /// Represents the description of a UPS device.
+    UpsDesc("UPSDESC"),
+    /// Client setting username.
+    Username("USERNAME"),
+    /// Represents a variable.
+    Var("VAR"),
+    /// Client requests the server version.
+    Version("VERSION"),
+}