Clarify default MTU

Author: Serock3

talpid-core/src/tunnel/mod.rs

@@ -154,14 +154,30 @@ impl TunnelMonitor {
             + Clone
             + 'static,
     {
-        #[cfg(target_os = "linux")]
-        let detect_mtu = params.options.mtu.is_none();
+        let default_mtu = if cfg!(target_os = "android") {
+            // Set the MTU to the lowest possible whilst still allowing for IPv6 to help
+            // with wireless carriers that do a lot of encapsulation.
+            1280
+        } else {
+            1380
+        };
 
         #[cfg(any(target_os = "linux", target_os = "windows"))]
-        args.runtime
-            .block_on(Self::assign_mtu(&args.route_manager, params));
-        let config = talpid_wireguard::config::Config::from_parameters(params)?;
+        // Detects the MTU of the device, calculates what the virtual device MTU should be use that
+        // as the default mtu
+        let default_mtu = args
+            .runtime
+            .block_on(
+                args.route_manager
+                    .get_mtu_for_route(params.connection.peer.endpoint.ip()),
+            )
+            .map(|mtu| Self::clamp_mtu(params, mtu))
+            .unwrap_or(default_mtu);
 
+        #[cfg(target_os = "linux")]
+        let detect_mtu = params.options.mtu.is_none();
+
+        let config = talpid_wireguard::config::Config::from_parameters(params, default_mtu)?;
         let monitor = talpid_wireguard::WireguardMonitor::start(
             config,
             params.options.quantum_resistant,
@@ -174,11 +190,10 @@ impl TunnelMonitor {
             monitor: InternalTunnelMonitor::Wireguard(monitor),
         })
     }
-
-    /// Set the MTU in the tunnel parameters based on the inputted device MTU and some
+    /// Calculates the MTU in the tunnel parameters based on the inputted device MTU and some
     /// calculations. `peer_mtu` is the detected device MTU.
     #[cfg(any(target_os = "linux", target_os = "windows"))]
-    fn set_mtu(params: &mut wireguard_types::TunnelParameters, peer_mtu: u16) {
+    fn clamp_mtu(params: &wireguard_types::TunnelParameters, peer_mtu: u16) -> u16 {
         // Some users experience fragmentation issues even when we take the interface MTU and
         // subtract the header sizes. This is likely due to some program that they use which does
         // not change the interface MTU but adds its own header onto the outgoing packets. For this
@@ -202,31 +217,10 @@ impl TunnelMonitor {
             false => MIN_IPV4_MTU,
             true => MIN_IPV6_MTU,
         };
-        let tunnel_mtu = peer_mtu
-            .saturating_sub(total_header_size)
-            .clamp(min_mtu, max_peer_mtu);
-        params.options.mtu = Some(tunnel_mtu);
-    }
 
-    /// Detects the MTU of the device, calculates what the virtual device MTU should be and sets
-    /// that in the tunnel parameters.
-    #[cfg(any(target_os = "linux", target_os = "windows"))]
-    async fn assign_mtu(
-        route_manager: &RouteManagerHandle,
-        params: &mut wireguard_types::TunnelParameters,
-    ) {
-        // Only calculate the mtu automatically if the user has not set any
-        if params.options.mtu.is_none() {
-            match route_manager
-                .get_mtu_for_route(params.connection.peer.endpoint.ip())
-                .await
-            {
-                Ok(mtu) => Self::set_mtu(params, mtu),
-                Err(e) => {
-                    log::error!("Could not get the MTU for route {}", e);
-                }
-            }
-        }
+        peer_mtu
+            .saturating_sub(total_header_size)
+            .clamp(min_mtu, max_peer_mtu)
     }
 
     #[cfg(not(target_os = "android"))]

talpid-wireguard/src/config.rs

@@ -30,14 +30,6 @@ pub struct Config {
     pub obfuscator_config: Option<ObfuscatorConfig>,
 }
 
-/// Set the MTU to the lowest possible whilst still allowing for IPv6 to help with wireless
-/// carriers that do a lot of encapsulation.
-const DEFAULT_MTU: u16 = if cfg!(target_os = "android") {
-    1280
-} else {
-    1380
-};
-
 /// Configuration errors
 #[derive(err_derive::Error, Debug)]
 pub enum Error {
@@ -52,12 +44,16 @@ pub enum Error {
 
 impl Config {
     /// Constructs a Config from parameters
-    pub fn from_parameters(params: &wireguard::TunnelParameters) -> Result<Config, Error> {
+    pub fn from_parameters(
+        params: &wireguard::TunnelParameters,
+        default_mtu: u16,
+    ) -> Result<Config, Error> {
         Self::new(
             &params.connection,
             &params.options,
             &params.generic_options,
             &params.obfuscation,
+            default_mtu,
         )
     }
 
@@ -67,9 +63,11 @@ impl Config {
         wg_options: &wireguard::TunnelOptions,
         generic_options: &GenericTunnelOptions,
         obfuscator_config: &Option<ObfuscatorConfig>,
+        default_mtu: u16,
     ) -> Result<Config, Error> {
         let mut tunnel = connection.tunnel.clone();
-        let mtu = wg_options.mtu.unwrap_or(DEFAULT_MTU);
+
+        let mtu = wg_options.mtu.unwrap_or(default_mtu);
 
         if tunnel.addresses.is_empty() {
             return Err(Error::InvalidTunnelIpError);