diff --git a/src/config/transport.rs b/src/config/transport.rs
index 2281ee4..cb7b3a8 100644
--- a/src/config/transport.rs
+++ b/src/config/transport.rs
@@ -267,8 +267,10 @@ pub struct EthernetConfig {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub ethertype: Option<u16>,
 
-    /// MTU override. Defaults to the interface's MTU minus 1 (for frame type prefix).
-    /// Cannot exceed the interface's actual MTU.
+    /// MTU override. Defaults to the interface's MTU minus 3 bytes of frame
+    /// header (`[type:1][length:2 LE][payload]`). The 2-byte length field is
+    /// required to trim NIC minimum-frame padding before AEAD verification.
+    /// Cannot exceed the interface's actual MTU minus 3.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub mtu: Option<u16>,
 
diff --git a/src/transport/ethernet/mod.rs b/src/transport/ethernet/mod.rs
index 759b774..60b0dbf 100644
--- a/src/transport/ethernet/mod.rs
+++ b/src/transport/ethernet/mod.rs
@@ -49,7 +49,9 @@ pub struct EthernetTransport {
     local_mac: Option<[u8; 6]>,
     /// Interface name (from config).
     interface: String,
-    /// Effective MTU (interface MTU - 1 for frame type prefix).
+    /// Effective payload MTU: interface MTU minus 3 bytes of frame header
+    /// (`[type:1][length:2 LE][payload]`). The 2-byte length field is required
+    /// to trim NIC minimum-frame padding before AEAD verification.
     effective_mtu: u16,
     /// Discovery buffer for discovered peers.
     discovery_buffer: Arc<DiscoveryBuffer>,