feat(rust-core): add adaptive keepalive telemetry, MTU handling, and per-client rate limiting APIs

rust/src/tunnel.rs

@@ -64,6 +64,22 @@ pub async fn add_route(subnet: &str, device_name: &str) -> Result<()> {
     Ok(())
 }
 
+/// Action to take after checking a packet against the MTU.
+pub enum TunMtuAction {
+    /// Packet is within MTU limits, forward it.
+    Forward,
+    /// Packet is oversized; the Vec contains the ICMP too-big message to write back into TUN.
+    IcmpTooBig(Vec<u8>),
+}
+
+/// Check a TUN packet against the MTU and return the appropriate action.
+pub fn check_tun_mtu(packet: &[u8], mtu_config: &crate::mtu::MtuConfig) -> TunMtuAction {
+    match crate::mtu::check_mtu(packet, mtu_config) {
+        crate::mtu::MtuAction::Forward => TunMtuAction::Forward,
+        crate::mtu::MtuAction::SendIcmpTooBig(icmp) => TunMtuAction::IcmpTooBig(icmp),
+    }
+}
+
 /// Remove a route.
 pub async fn remove_route(subnet: &str, device_name: &str) -> Result<()> {
     let output = tokio::process::Command::new("ip")