diff --git a/crates/shared/src/lib.rs b/crates/shared/src/lib.rs
index f12ea88..33bb1fd 100644
--- a/crates/shared/src/lib.rs
+++ b/crates/shared/src/lib.rs
@@ -7,4 +7,5 @@
 pub mod generator;
 pub mod protocol;
 pub mod save;
+pub mod session;
 pub mod world;
diff --git a/crates/shared/src/protocol.rs b/crates/shared/src/protocol.rs
index e12ff53..539a7aa 100644
--- a/crates/shared/src/protocol.rs
+++ b/crates/shared/src/protocol.rs
@@ -2,8 +2,9 @@
 
 //! Network protocol types and constants.
 //!
-//! The module is split by stream purpose: [`control`]-stream handshake and disconnect messages, and the [`chunk`]-sync request/delivery messages. Control-stream types are re-exported here so callers continue to refer to `shared::protocol::<Type>` regardless of the internal layout, while the chunk types stay namespaced under `shared::protocol::chunk` to keep the two protocols visually distinct.
+//! The module is split by stream purpose: [`control`]-stream handshake and disconnect messages, and the [`chunk`]-sync request/delivery messages, and the periodic [`authority`]-stream state. Control-stream types are re-exported here so callers continue to refer to `shared::protocol::<Type>` regardless of the internal layout, while the chunk types stay namespaced under `shared::protocol::chunk` to keep the two protocols visually distinct.
 
+pub mod authority;
 pub mod chunk;
 mod control;
 
diff --git a/crates/shared/src/protocol/authority.rs b/crates/shared/src/protocol/authority.rs
new file mode 100644
index 0000000..acbabea
--- /dev/null
+++ b/crates/shared/src/protocol/authority.rs
@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+//! Authority-stream messages: periodic state pushed from server to client.
+//!
+//! The authority stream ([`StreamLayout::authority`](super::StreamLayout::authority), id 2) carries state the server is the sole authority over and pushes without being asked. Diagnostics are the first such payload; simulation snapshots will join them on the same stream.
+
+use serde::{Deserialize, Serialize};
+
+/// Messages carried on the authority stream (stream 2): periodic server-authoritative state.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub enum AuthorityMessage {
+    /// Periodic simulation diagnostics, emitted on a fixed wall-clock cadence rather than per tick.
+    ServerStats(ServerStats),
+}
+
+/// A snapshot of the server's simulation health, sent roughly once per second.
+///
+/// The measured figures exist because the nominal tick rate advertised in [`HandshakeAck::tick_rate_hint`](super::HandshakeAck::tick_rate_hint) is a constant: it states what the server intends to run at and can never reveal that it is falling behind. Everything here is observed.
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Default)]
+pub struct ServerStats {
+    /// Ticks actually completed in the reporting window, expressed per second. Below the nominal rate when the server is overrunning its budget.
+    pub measured_tps: f32,
+    /// Mean wall-clock duration of a tick body in the reporting window, in milliseconds, excluding the sleep that pads the tick out to its period.
+    pub mean_tick_ms: f32,
+    /// Longest tick body observed in the reporting window, in milliseconds. A mean within budget alongside a spiking maximum indicates intermittent stalls rather than sustained overload.
+    pub max_tick_ms: f32,
+    /// Share of the nominal tick period consumed by the mean tick body, in percent. Values at or above 100 mean the server no longer has headroom.
+    pub tick_budget_percent: f32,
+    /// Chunks resident in the server's world cache.
+    pub loaded_chunks: u32,
+    /// Chunk generation jobs outstanding in the server's worker pool.
+    pub chunks_in_flight: u32,
+    /// Clients with an established session.
+    pub connected_clients: u32,
+    /// Entities in the server's ECS world, players included.
+    pub entities: u32,
+    /// Players currently in the world.
+    pub players: u32,
+    /// Wall-clock time since the simulation loop started, in seconds.
+    pub uptime_secs: u64,
+}
+
+#[cfg(test)]
+#[path = "../tests/protocol_authority.rs"]
+mod tests;
diff --git a/crates/shared/src/protocol/control.rs b/crates/shared/src/protocol/control.rs
index 54a9338..2965863 100644
--- a/crates/shared/src/protocol/control.rs
+++ b/crates/shared/src/protocol/control.rs
@@ -5,7 +5,7 @@
 use serde::{Deserialize, Serialize};
 
 /// Wire-protocol version. Incremented on any breaking change to the message layout below.
-pub const PROTOCOL_VERSION: u32 = 1;
+pub const PROTOCOL_VERSION: u32 = 2;
 
 /// Messages carried on the control stream (stream 0): handshake and disconnect.
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
diff --git a/crates/shared/src/session.rs b/crates/shared/src/session.rs
new file mode 100644
index 0000000..4430303
--- /dev/null
+++ b/crates/shared/src/session.rs
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+//! Session-level concepts describing the relationship between a client and the server it is playing against.
+
+/// Which kind of server a client session is running against.
+///
+/// Deliberately not part of the wire protocol. The client already knows the answer without asking: it either spawned a server in-process or dialled a socket. A server-declared field would be redundant at best and spoofable at worst, so the value is constructed client-side from facts the client already holds.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ServerKind {
+    /// The server runs in this process, backing single-player.
+    Integrated,
+    /// The server is a separate process reached over the network.
+    Dedicated {
+        /// Whether the server's address is off this machine. Decided from the `SocketAddr` the client dialled, not from anything the server says.
+        remote: bool,
+    },
+}
+
+impl ServerKind {
+    /// Classifies a dedicated server from the address the client dialled.
+    ///
+    /// A loopback address means the process is on this machine (a locally hosted server), which is distinct from an integrated one: it is still a separate process reached over a socket.
+    #[must_use]
+    pub const fn dedicated(addr: std::net::SocketAddr) -> Self {
+        Self::Dedicated {
+            remote: !addr.ip().is_loopback(),
+        }
+    }
+
+    /// Returns a short human-readable label for the session's server kind.
+    #[must_use]
+    pub const fn label(self) -> &'static str {
+        match self {
+            Self::Integrated => "integrated",
+            Self::Dedicated { remote: false } => "dedicated (local)",
+            Self::Dedicated { remote: true } => "dedicated (remote)",
+        }
+    }
+}
+
+#[cfg(test)]
+#[path = "tests/session.rs"]
+mod tests;
diff --git a/crates/shared/src/tests/protocol_authority.rs b/crates/shared/src/tests/protocol_authority.rs
new file mode 100644
index 0000000..beb405a
--- /dev/null
+++ b/crates/shared/src/tests/protocol_authority.rs
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+//! Wire round-trip tests for the authority-stream messages.
+
+use super::*;
+
+#[test]
+fn roundtrip_server_stats() -> Result<(), postcard::Error> {
+    let msg = AuthorityMessage::ServerStats(ServerStats {
+        measured_tps: 19.4,
+        mean_tick_ms: 12.5,
+        max_tick_ms: 48.0,
+        tick_budget_percent: 25.0,
+        loaded_chunks: 4096,
+        chunks_in_flight: 12,
+        connected_clients: 1,
+        entities: 37,
+        players: 1,
+        uptime_secs: 3600,
+    });
+
+    let bytes = postcard::to_stdvec(&msg)?;
+    let decoded: AuthorityMessage = postcard::from_bytes(&bytes)?;
+    assert_eq!(decoded, msg);
+    Ok(())
+}
+
+#[test]
+fn default_stats_survive_a_round_trip() {
+    // The default is what a server reports before its first window has closed; it must decode as cleanly as a populated one.
+    let msg = AuthorityMessage::ServerStats(ServerStats::default());
+    let bytes = postcard::to_stdvec(&msg).unwrap_or_default();
+    assert_eq!(
+        postcard::from_bytes::<AuthorityMessage>(&bytes).ok(),
+        Some(msg)
+    );
+}
diff --git a/crates/shared/src/tests/session.rs b/crates/shared/src/tests/session.rs
new file mode 100644
index 0000000..0536ff5
--- /dev/null
+++ b/crates/shared/src/tests/session.rs
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+//! Unit tests for server-kind classification.
+
+use super::*;
+use std::net::{Ipv4Addr, Ipv6Addr, SocketAddr};
+
+#[test]
+fn loopback_addresses_classify_as_local() {
+    let v4 = SocketAddr::from((Ipv4Addr::LOCALHOST, 25565));
+    let v6 = SocketAddr::from((Ipv6Addr::LOCALHOST, 25565));
+
+    assert_eq!(
+        ServerKind::dedicated(v4),
+        ServerKind::Dedicated { remote: false }
+    );
+    assert_eq!(
+        ServerKind::dedicated(v6),
+        ServerKind::Dedicated { remote: false }
+    );
+}
+
+#[test]
+fn non_loopback_addresses_classify_as_remote() {
+    let addr = SocketAddr::from((Ipv4Addr::new(10, 0, 0, 4), 25565));
+    assert_eq!(
+        ServerKind::dedicated(addr),
+        ServerKind::Dedicated { remote: true }
+    );
+}
+
+#[test]
+fn each_kind_has_a_distinct_label() {
+    assert_eq!(ServerKind::Integrated.label(), "integrated");
+    assert_eq!(
+        ServerKind::Dedicated { remote: false }.label(),
+        "dedicated (local)"
+    );
+    assert_eq!(
+        ServerKind::Dedicated { remote: true }.label(),
+        "dedicated (remote)"
+    );
+}