feat(proto): validate negotiated datagram budgets

PLAN.md keeps MVP traffic to one Ethernet frame per QUIC datagram with no
fragmentation. The relay already negotiates a datagram budget, but the client
and gateway send paths still relied on Quinn to reject oversized encoded
Ethernet datagrams.

Add a shared protocol validation helper for encoded datagram length versus the
negotiated QUIC budget. Thread the negotiated budget into client and gateway
send boundaries, reject oversized datagrams before send, and count those valid
but unsent frames as local drops.

Document the budget check in the workspace decomposition and gateway behavior.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-proto -p lanparty-client-core -p lanparty-gateway
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Refs: PLAN.md

README.md

@@ -20,6 +20,7 @@ Monorepo for a Layer 2 over QUIC LAN party bridge.
 Transport-agnostic tunnel contract shared by all binaries:
 
 - overlay datagram header encoding and decoding
+- negotiated QUIC datagram budget validation before send
 - Ethernet frame header parsing
 - MAC address parsing and identity validation
 - QUIC datagram to TAP MTU budget helpers
@@ -59,7 +60,7 @@ Platform-neutral remote client relay session:
 - welcome/reject handling with assigned peer id and effective TAP MTU
 - QUIC DATAGRAM support and negotiated datagram budget diagnostics
 - reliable relay control-event reads for peer lifecycle messages
-- Ethernet frame send/receive helpers over QUIC DATAGRAM
+- Ethernet frame send/receive helpers over QUIC DATAGRAM with budget checks
 - client tunnel statistics for frame/datagram rx/tx and drops
 - reliable client stats snapshot sends for relay diagnostics
 - best-effort graceful disconnect messages before QUIC close
@@ -158,10 +159,12 @@ cargo run -p lanparty-gateway -- \
 The gateway connects to the relay as `role = gateway`, completes the
 control-stream hello/welcome handshake, opens an AF_PACKET socket on the LAN
 interface with promiscuous packet membership, and bridges Ethernet frames
-between the relay and wired LAN until shutdown. `--relay` accepts a DNS name or
-socket address; bare hosts default to UDP/443. It tracks remote-client source
-MACs seen from relay traffic and periodically emits small CAM refresh frames so
-the physical switch keeps those MACs associated with the gateway port. Gateway
+between the relay and wired LAN until shutdown. Sends fail before fragmentation
+when an encoded Ethernet datagram exceeds the negotiated QUIC datagram budget.
+`--relay` accepts a DNS name or socket address; bare hosts default to UDP/443.
+It tracks remote-client source MACs seen from relay traffic and periodically
+emits small CAM refresh frames so the physical switch keeps those MACs
+associated with the gateway port. Gateway
 frame logs include direction, peer id when present, MACs, ethertype/length,
 frame length, action, and drop reason. The gateway also tracks frame/datagram
 counters and periodically sends stats snapshots to the relay. Relay lifecycle

crates/lanparty-client-core/src/lib.rs

@@ -26,7 +26,9 @@ use lanparty_ctrl::{
     encode_control_message,
 };
 use lanparty_obs::{QuicDiagnostics, TunnelStats};
-use lanparty_proto::{EthernetFrame, FrameType, MacAddr, decode_datagram, encode_datagram};
+use lanparty_proto::{
+    EthernetFrame, FrameType, MacAddr, decode_datagram, encode_datagram, validate_datagram_budget,
+};
 use quinn::{ClientConfig, Endpoint, crypto::rustls::QuicClientConfig};
 use rustls::pki_types::CertificateDer;
 
@@ -266,6 +268,7 @@ impl ClientSession {
         ClientRelayIo::new(
             self.connection.clone(),
             self.welcome.clone(),
+            self.quic_max_datagram_size,
             Arc::clone(&self.stats),
         )
     }
@@ -305,6 +308,7 @@ impl ClientSession {
 pub struct ClientRelayIo {
     connection: quinn::Connection,
     welcome: ServerWelcome,
+    quic_max_datagram_size: u16,
     stats: Arc<ClientTunnelStats>,
 }
 
@@ -313,11 +317,13 @@ impl ClientRelayIo {
     fn new(
         connection: quinn::Connection,
         welcome: ServerWelcome,
+        quic_max_datagram_size: u16,
         stats: Arc<ClientTunnelStats>,
     ) -> Self {
         Self {
             connection,
             welcome,
+            quic_max_datagram_size,
             stats,
         }
     }
@@ -327,6 +333,11 @@ impl ClientRelayIo {
         &self.welcome
     }
 
+    #[must_use]
+    pub const fn quic_max_datagram_size(&self) -> u16 {
+        self.quic_max_datagram_size
+    }
+
     pub fn send_ethernet(&self, frame: &[u8]) -> Result<()> {
         let ethernet_frame = match EthernetFrame::parse(frame) {
             Ok(frame) => frame,
@@ -343,6 +354,12 @@ impl ClientRelayIo {
             frame,
         )
         .context("failed to encode client Ethernet datagram")?;
+        if let Err(error) =
+            validate_datagram_budget(datagram.len(), usize::from(self.quic_max_datagram_size))
+        {
+            self.stats.record_dropped_frame();
+            return Err(error).context("client Ethernet datagram exceeds negotiated QUIC budget");
+        }
 
         self.connection
             .send_datagram(Bytes::from(datagram))
@@ -799,6 +816,10 @@ mod tests {
         );
         let relay_io = client.relay_io();
         assert_eq!(relay_io.welcome().peer_id(), 2);
+        assert_eq!(
+            relay_io.quic_max_datagram_size(),
+            client.quic_max_datagram_size()
+        );
 
         relay_io
             .send_ethernet(&ethernet_frame(b"to relay"))

crates/lanparty-gateway/src/lib.rs

@@ -33,7 +33,7 @@ use lanparty_obs::TunnelStats;
 use lanparty_obs::{FrameAction, FrameDirection, FrameLog};
 use lanparty_proto::{
     EthernetFrame, FrameType, MAX_STANDARD_ETHERNET_FRAME_LEN, MacAddr, decode_datagram,
-    encode_datagram,
+    encode_datagram, validate_datagram_budget,
 };
 use quinn::{ClientConfig, Endpoint, crypto::rustls::QuicClientConfig};
 use rustls::pki_types::CertificateDer;
@@ -196,6 +196,7 @@ pub struct GatewayConnection {
     connection: quinn::Connection,
     config: GatewayConfig,
     welcome: ServerWelcome,
+    quic_max_datagram_size: u16,
     stats: Arc<GatewayTunnelStats>,
 }
 
@@ -228,8 +229,19 @@ impl GatewayConnection {
         &self.welcome
     }
 
+    #[must_use]
+    pub const fn quic_max_datagram_size(&self) -> u16 {
+        self.quic_max_datagram_size
+    }
+
     pub fn send_ethernet(&self, frame: &[u8]) -> Result<()> {
-        send_gateway_ethernet(&self.connection, &self.welcome, &self.stats, frame)
+        send_gateway_ethernet(
+            &self.connection,
+            &self.welcome,
+            self.quic_max_datagram_size,
+            &self.stats,
+            frame,
+        )
     }
 
     pub async fn recv_ethernet(&self) -> Result<ReceivedEthernetFrame> {
@@ -263,6 +275,7 @@ impl GatewayConnection {
             endpoint,
             connection,
             welcome,
+            quic_max_datagram_size,
             stats,
             ..
         } = self;
@@ -292,7 +305,13 @@ impl GatewayConnection {
                 }
                 lan_frame = read_lan_ethernet(&packet_socket) => {
                     let lan_frame = lan_frame?;
-                    send_gateway_ethernet(&connection, &welcome, &stats, &lan_frame)?;
+                    send_gateway_ethernet(
+                        &connection,
+                        &welcome,
+                        quic_max_datagram_size,
+                        &stats,
+                        &lan_frame,
+                    )?;
                     println!(
                         "{}",
                         gateway_frame_log_line(
@@ -363,6 +382,7 @@ impl GatewayConnection {
 fn send_gateway_ethernet(
     connection: &quinn::Connection,
     welcome: &ServerWelcome,
+    quic_max_datagram_size: u16,
     stats: &GatewayTunnelStats,
     frame: &[u8],
 ) -> Result<()> {
@@ -381,6 +401,12 @@ fn send_gateway_ethernet(
         frame,
     )
     .context("failed to encode gateway Ethernet datagram")?;
+    if let Err(error) =
+        validate_datagram_budget(datagram.len(), usize::from(quic_max_datagram_size))
+    {
+        stats.record_dropped_frame();
+        return Err(error).context("gateway Ethernet datagram exceeds negotiated QUIC budget");
+    }
 
     connection
         .send_datagram(Bytes::from(datagram))
@@ -747,6 +773,7 @@ pub async fn connect_gateway(config: GatewayConfig) -> Result<GatewayConnection>
             connection,
             config,
             welcome,
+            quic_max_datagram_size: hello_datagram_size,
             stats: Arc::default(),
         }),
         ControlMessage::Reject(reject) => bail!(
@@ -968,6 +995,7 @@ mod tests {
         assert_eq!(gateway.config().interface(), "eth0");
         assert_eq!(gateway.welcome().room_id(), 7);
         assert_eq!(gateway.welcome().peer_id(), 1);
+        assert!(gateway.quic_max_datagram_size() <= 1400);
 
         let event = tokio::time::timeout(Duration::from_secs(5), gateway.recv_control_event())
             .await

crates/lanparty-proto/src/lib.rs

@@ -20,5 +20,5 @@ pub use mtu::{
 };
 pub use overlay::{
     FrameType, OVERLAY_HEADER_LEN, OVERLAY_MAGIC, OVERLAY_VERSION, OverlayHeader, OverlayPacket,
-    ProtoError, decode_datagram, encode_datagram,
+    ProtoError, decode_datagram, encode_datagram, validate_datagram_budget,
 };

crates/lanparty-proto/src/overlay.rs

@@ -170,6 +170,8 @@ pub enum ProtoError {
     UnknownFrameType(u8),
     #[error("payload length {len} exceeds wire maximum {max}")]
     PayloadTooLarge { len: usize, max: usize },
+    #[error("encoded datagram length {len} exceeds negotiated QUIC datagram budget {max}")]
+    DatagramExceedsBudget { len: usize, max: usize },
     #[error("declared payload length {declared} does not match actual length {actual}")]
     PayloadLengthMismatch { declared: usize, actual: usize },
     #[error("Ethernet frame is too short: got {actual} bytes, need at least {minimum}")]
@@ -190,6 +192,20 @@ pub fn encode_datagram(
     Ok(datagram)
 }
 
+pub fn validate_datagram_budget(
+    datagram_len: usize,
+    max_datagram_size: usize,
+) -> Result<(), ProtoError> {
+    if datagram_len > max_datagram_size {
+        return Err(ProtoError::DatagramExceedsBudget {
+            len: datagram_len,
+            max: max_datagram_size,
+        });
+    }
+
+    Ok(())
+}
+
 pub fn decode_datagram(bytes: &[u8]) -> Result<OverlayPacket<'_>, ProtoError> {
     let header = OverlayHeader::decode(bytes)?;
     let payload = &bytes[OVERLAY_HEADER_LEN..];
@@ -281,4 +297,18 @@ mod tests {
             ProtoError::PayloadTooLarge { .. }
         ));
     }
+
+    #[test]
+    fn rejects_datagrams_over_negotiated_budget() {
+        let datagram = encode_datagram(FrameType::Ethernet, 1, 2, 0, &[1, 2, 3]).unwrap();
+
+        assert!(validate_datagram_budget(datagram.len(), datagram.len()).is_ok());
+        assert_eq!(
+            validate_datagram_budget(datagram.len(), datagram.len() - 1).unwrap_err(),
+            ProtoError::DatagramExceedsBudget {
+                len: datagram.len(),
+                max: datagram.len() - 1
+            }
+        );
+    }
 }