Keep reliable stream packets below path MTU

src/transport.rs

@@ -39,7 +39,10 @@ pub const DEFAULT_KEEPALIVE_AFTER: Duration = Duration::from_secs(2);
 pub const DEFAULT_RETIRED_STREAM_TOMBSTONES: usize = 16 * 1024;
 pub const STREAM_CONTROL_RETRANSMIT_MAX_ATTEMPTS: u32 = 8;
 pub const SERVICE_TARGET_PREFIX: &str = "@dosh-";
-pub const MAX_STREAM_DATA_BYTES: usize = 60 * 1024;
+/// Maximum application bytes in one encrypted UDP stream packet. Keeping this
+/// aligned with terminal output framing avoids IP fragmentation and stays
+/// below macOS route MTUs after Dosh, AEAD, UDP, and IP overhead.
+pub const MAX_STREAM_DATA_BYTES: usize = 1024;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct TransportConfig {
@@ -1675,6 +1678,31 @@ mod tests {
         assert_eq!(third.bytes.len(), 13);
     }
 
+    #[test]
+    fn maximum_stream_chunk_fits_a_safe_udp_datagram() {
+        let body = protocol::to_body(&StreamData {
+            stream_id: u64::MAX,
+            offset: u64::MAX,
+            bytes: vec![0xff; MAX_STREAM_DATA_BYTES],
+        })
+        .unwrap();
+        let packet = protocol::encode_encrypted(
+            PacketKind::StreamData,
+            [0xff; 16],
+            u64::MAX,
+            u64::MAX,
+            &[0xff; 32],
+            CLIENT_TO_SERVER,
+            &body,
+        )
+        .unwrap();
+        assert!(
+            packet.len() <= 1200,
+            "encrypted stream datagram is {} bytes",
+            packet.len()
+        );
+    }
+
     #[test]
     fn queued_large_write_flushes_in_window_sized_chunks_after_open() {
         let mut mux = StreamMux::new(TransportConfig {