feat(relay): rate limit client unknown unicast floods

Client-originated unknown unicast now has its own relay-side token bucket.
When a client sends to an unlearned unicast MAC, the relay consumes from that
bucket before flooding the frame to the room.

Known unicast to a registered client bypasses this limit, and broadcast or
multicast traffic continues to use its separate bucket. Keeping the buckets in
room state matches the existing forwarding-policy ownership and lets unit tests
drive explicit instants.

This is the second rate-limit slice from PLAN.md. A total bandwidth cap remains
future work.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-relay
- cargo clippy -p lanparty-relay --all-targets -- -D warnings
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

README.md

@@ -80,7 +80,7 @@ Public relay binary and relay-owned room state:
 - stable effective room MTU chosen before Ethernet datagrams flow
 - live Ethernet datagram forwarding with no ingress reflection
 - L2 safety filters for jumbo, switch-control, DHCP-server, and IPv6-RA frames
-- client broadcast/multicast burst limiting
+- client broadcast/multicast and unknown-unicast burst limiting
 - malformed peer datagram disconnect threshold
 - peer leave cleanup for room membership and MAC indexes
 

crates/lanparty-relay/src/lib.rs

@@ -22,6 +22,8 @@ pub use server::RelayServer;
 pub const DEFAULT_MAX_CLIENTS_PER_ROOM: usize = 16;
 const CLIENT_MULTICAST_BURST_FRAMES: u32 = 64;
 const CLIENT_MULTICAST_REFILL_FRAMES_PER_SECOND: u32 = 32;
+const CLIENT_UNKNOWN_UNICAST_BURST_FRAMES: u32 = 64;
+const CLIENT_UNKNOWN_UNICAST_REFILL_FRAMES_PER_SECOND: u32 = 32;
 const ETHERTYPE_IPV4: u16 = 0x0800;
 const ETHERTYPE_EAPOL: u16 = 0x888e;
 const ETHERTYPE_SLOW_PROTOCOLS: u16 = 0x8809;
@@ -286,6 +288,7 @@ struct Room {
     clients: HashMap<u32, PeerInfo>,
     clients_by_mac: HashMap<MacAddr, u32>,
     client_multicast_limits: HashMap<u32, FrameRateLimit>,
+    client_unknown_unicast_limits: HashMap<u32, FrameRateLimit>,
 }
 
 impl Room {
@@ -299,6 +302,7 @@ impl Room {
             clients: HashMap::new(),
             clients_by_mac: HashMap::new(),
             client_multicast_limits: HashMap::new(),
+            client_unknown_unicast_limits: HashMap::new(),
         }
     }
 
@@ -335,7 +339,9 @@ impl Room {
                 self.clients_by_mac.insert(mac, peer.peer_id());
                 self.clients.insert(peer.peer_id(), peer.clone());
                 self.client_multicast_limits
-                    .insert(peer.peer_id(), FrameRateLimit::new());
+                    .insert(peer.peer_id(), client_multicast_limit());
+                self.client_unknown_unicast_limits
+                    .insert(peer.peer_id(), client_unknown_unicast_limit());
             }
         }
 
@@ -424,6 +430,7 @@ impl Room {
         let mac = peer.mac().expect("client peer info has MAC");
         self.clients_by_mac.remove(&mac);
         self.client_multicast_limits.remove(&peer.peer_id());
+        self.client_unknown_unicast_limits.remove(&peer.peer_id());
 
         Ok(peer)
     }
@@ -471,15 +478,21 @@ impl Room {
             return Ok(ForwardingDecision::rate_limited());
         }
 
-        let targets = if frame.destination().is_multicast() {
+        let destination = frame.destination();
+        let targets = if destination.is_multicast() {
             self.all_peer_ids_except(ingress_peer_id)
-        } else if let Some(client_peer_id) = self.clients_by_mac.get(&frame.destination()) {
+        } else if let Some(client_peer_id) = self.clients_by_mac.get(&destination) {
             if *client_peer_id == ingress_peer_id {
                 Vec::new()
             } else {
                 vec![*client_peer_id]
             }
         } else {
+            if ingress_role == Role::Client
+                && !self.allow_client_unknown_unicast(ingress_peer_id, now)
+            {
+                return Ok(ForwardingDecision::rate_limited());
+            }
             self.all_peer_ids_except(ingress_peer_id)
         };
 
@@ -493,7 +506,14 @@ impl Room {
     fn allow_client_multicast(&mut self, peer_id: u32, now: Instant) -> bool {
         self.client_multicast_limits
             .entry(peer_id)
-            .or_insert_with(FrameRateLimit::new)
+            .or_insert_with(client_multicast_limit)
+            .allow(now)
+    }
+
+    fn allow_client_unknown_unicast(&mut self, peer_id: u32, now: Instant) -> bool {
+        self.client_unknown_unicast_limits
+            .entry(peer_id)
+            .or_insert_with(client_unknown_unicast_limit)
             .allow(now)
     }
 
@@ -528,13 +548,23 @@ impl Room {
 #[derive(Debug, Clone)]
 struct FrameRateLimit {
     tokens: u32,
+    capacity: u32,
+    refill_per_second: u32,
     last_refill: Instant,
 }
 
 impl FrameRateLimit {
-    fn new() -> Self {
+    fn new(capacity: u32, refill_per_second: u32) -> Self {
+        assert!(capacity > 0, "rate-limit capacity must be nonzero");
+        assert!(
+            refill_per_second > 0,
+            "rate-limit refill rate must be nonzero"
+        );
+
         Self {
-            tokens: CLIENT_MULTICAST_BURST_FRAMES,
+            tokens: capacity,
+            capacity,
+            refill_per_second,
             last_refill: Instant::now(),
         }
     }
@@ -555,17 +585,27 @@ impl FrameRateLimit {
             return;
         }
 
-        let refill =
-            elapsed_secs.saturating_mul(u64::from(CLIENT_MULTICAST_REFILL_FRAMES_PER_SECOND));
+        let refill = elapsed_secs.saturating_mul(u64::from(self.refill_per_second));
         let refill = refill.min(u64::from(u32::MAX)) as u32;
-        self.tokens = self
-            .tokens
-            .saturating_add(refill)
-            .min(CLIENT_MULTICAST_BURST_FRAMES);
+        self.tokens = self.tokens.saturating_add(refill).min(self.capacity);
         self.last_refill = now;
     }
 }
 
+fn client_multicast_limit() -> FrameRateLimit {
+    FrameRateLimit::new(
+        CLIENT_MULTICAST_BURST_FRAMES,
+        CLIENT_MULTICAST_REFILL_FRAMES_PER_SECOND,
+    )
+}
+
+fn client_unknown_unicast_limit() -> FrameRateLimit {
+    FrameRateLimit::new(
+        CLIENT_UNKNOWN_UNICAST_BURST_FRAMES,
+        CLIENT_UNKNOWN_UNICAST_REFILL_FRAMES_PER_SECOND,
+    )
+}
+
 fn safety_drop_reason(ingress_role: Role, frame: EthernetFrame<'_>) -> Option<DropReason> {
     if frame.is_jumbo() {
         return Some(DropReason::JumboFrame);
@@ -950,6 +990,49 @@ mod tests {
         assert_eq!(decision.action(), FrameAction::Forwarded);
     }
 
+    #[test]
+    fn rate_limits_client_unknown_unicast_after_burst() {
+        let mut registry = RoomRegistry::default();
+        let gateway = registry.join(gateway_hello()).unwrap();
+        let client_one = registry.join(client_hello(1)).unwrap();
+        let client_two = registry.join(client_hello(2)).unwrap();
+        let unknown_unicast = ethernet(physical_mac(), mac(1));
+        let known_unicast = ethernet(mac(2), mac(1));
+        let now = Instant::now();
+
+        for _ in 0..CLIENT_UNKNOWN_UNICAST_BURST_FRAMES {
+            let decision = registry
+                .forward_ethernet_at(&room(), client_one.peer().peer_id(), &unknown_unicast, now)
+                .unwrap();
+            assert_eq!(decision.action(), FrameAction::Forwarded);
+            assert_eq!(
+                decision.targets(),
+                &[gateway.peer().peer_id(), client_two.peer().peer_id()]
+            );
+        }
+
+        let decision = registry
+            .forward_ethernet_at(&room(), client_one.peer().peer_id(), &unknown_unicast, now)
+            .unwrap();
+        assert_rate_limited(&decision);
+
+        let decision = registry
+            .forward_ethernet_at(&room(), client_one.peer().peer_id(), &known_unicast, now)
+            .unwrap();
+        assert_eq!(decision.action(), FrameAction::Forwarded);
+        assert_eq!(decision.targets(), &[client_two.peer().peer_id()]);
+
+        let decision = registry
+            .forward_ethernet_at(
+                &room(),
+                client_one.peer().peer_id(),
+                &unknown_unicast,
+                now + Duration::from_secs(1),
+            )
+            .unwrap();
+        assert_eq!(decision.action(), FrameAction::Forwarded);
+    }
+
     #[test]
     fn drops_client_frames_with_forged_source_mac() {
         let mut registry = RoomRegistry::default();