proto: Add option to pad application data UDP datagrams to MTU

Author: FlorianUekermann

quinn-proto/src/config/transport.rs

@@ -29,6 +29,7 @@ pub struct TransportConfig {
     pub(crate) initial_mtu: u16,
     pub(crate) min_mtu: u16,
     pub(crate) mtu_discovery_config: Option<MtuDiscoveryConfig>,
+    pub(crate) pad_to_mtu: bool,
     pub(crate) ack_frequency_config: Option<AckFrequencyConfig>,
 
     pub(crate) persistent_congestion_threshold: u32,
@@ -203,6 +204,20 @@ impl TransportConfig {
         self
     }
 
+    /// Pad UDP datagrams carrying application data to current maximum UDP payload size
+    ///
+    /// Disabled by default. UDP datagrams containing loss probes are exempt from padding.
+    ///
+    /// Enabling this helps mitigate traffic analysis by network observers, but it increases
+    /// bandwidth usage. Without this mitigation precise plain text size of application datagrams as
+    /// well as the total size of stream write bursts can be inferred by observers under certain
+    /// conditions. This analysis requires either an uncongested connection or application datagrams
+    /// too large to be coalesced.
+    pub fn pad_to_mtu(&mut self, value: bool) -> &mut Self {
+        self.pad_to_mtu = value;
+        self
+    }
+
     /// Specifies the ACK frequency config (see [`AckFrequencyConfig`] for details)
     ///
     /// The provided configuration will be ignored if the peer does not support the acknowledgement
@@ -340,6 +355,7 @@ impl Default for TransportConfig {
             initial_mtu: INITIAL_MTU,
             min_mtu: INITIAL_MTU,
             mtu_discovery_config: Some(MtuDiscoveryConfig::default()),
+            pad_to_mtu: false,
             ack_frequency_config: None,
 
             persistent_congestion_threshold: 3,
@@ -374,6 +390,7 @@ impl fmt::Debug for TransportConfig {
             initial_mtu,
             min_mtu,
             mtu_discovery_config,
+            pad_to_mtu,
             ack_frequency_config,
             persistent_congestion_threshold,
             keep_alive_interval,
@@ -400,6 +417,7 @@ impl fmt::Debug for TransportConfig {
             .field("initial_mtu", initial_mtu)
             .field("min_mtu", min_mtu)
             .field("mtu_discovery_config", mtu_discovery_config)
+            .field("pad_to_mtu", pad_to_mtu)
             .field("ack_frequency_config", ack_frequency_config)
             .field(
                 "persistent_congestion_threshold",

quinn-proto/src/connection/mod.rs

@@ -509,6 +509,7 @@ impl Connection {
         let mut builder_storage: Option<PacketBuilder> = None;
         let mut sent_frames = None;
         let mut pad_datagram = false;
+        let mut pad_datagram_to_mtu = false;
         let mut congestion_blocked = false;
 
         // Iterate over all spaces and find data to send
@@ -542,6 +543,8 @@ impl Connection {
                 ack_eliciting |= self.can_send_1rtt(frame_space_1rtt);
             }
 
+            pad_datagram_to_mtu |= space_id == SpaceId::Data && self.config.pad_to_mtu;
+
             // Can we append more data into the current buffer?
             // It is not safe to assume that `buf.len()` is the end of the data,
             // since the last packet might not have been finished.
@@ -628,7 +631,7 @@ impl Connection {
                         builder.pad_to(MIN_INITIAL_SIZE);
                     }
 
-                    if num_datagrams > 1 {
+                    if num_datagrams > 1 || pad_datagram_to_mtu {
                         // If too many padding bytes would be required to continue the GSO batch
                         // after this packet, end the GSO batch here. Ensures that fixed-size frames
                         // with heterogeneous sizes (e.g. application datagrams) won't inadvertently
@@ -645,7 +648,8 @@ impl Connection {
                         let packet_len_unpadded = cmp::max(builder.min_size, buf.len())
                             - datagram_start
                             + builder.tag_len;
-                        if packet_len_unpadded + MAX_PADDING < segment_size
+                        if (packet_len_unpadded + MAX_PADDING < segment_size
+                            && !pad_datagram_to_mtu)
                             || datagram_start + segment_size > buf_capacity
                         {
                             trace!(
@@ -904,6 +908,16 @@ impl Connection {
             if pad_datagram {
                 builder.pad_to(MIN_INITIAL_SIZE);
             }
+
+            // If this datagram is a loss probe and `segment_size` is larger than `INITIAL_MTU`,
+            // then padding it to `segment_size` would risk failure to recover from a reduction in
+            // path MTU.
+            // Loss probes are the only packets for which we might grow `buf_capacity`
+            // by less than `segment_size`.
+            if pad_datagram_to_mtu && buf_capacity >= datagram_start + segment_size {
+                builder.pad_to(segment_size as u16);
+            }
+
             let last_packet_number = builder.exact_number;
             builder.finish_and_track(now, self, sent_frames, buf);
             self.path

quinn-proto/src/tests/mod.rs

@@ -3217,6 +3217,72 @@ fn gso_truncation() {
     }
 }
 
+/// Verify that UDP datagrams are padded to MTU if specified in the transport config.
+#[test]
+fn pad_to_mtu() {
+    let _guard = subscribe();
+    const MTU: u16 = 1333;
+    let client_config = {
+        let mut c_config = client_config();
+        let t_config = TransportConfig {
+            initial_mtu: MTU,
+            mtu_discovery_config: None,
+            pad_to_mtu: true,
+            ..TransportConfig::default()
+        };
+        c_config.transport_config(t_config.into());
+        c_config
+    };
+    let mut pair = Pair::default();
+    let (client_ch, server_ch) = pair.connect_with(client_config);
+
+    let initial_ios = pair.client_conn_mut(client_ch).stats().udp_tx.ios;
+    pair.server.capture_inbound_packets = true;
+
+    info!("sending");
+    // Send two datagrams significantly smaller than MTU, but large enough to require two UDP datagrams.
+    const LEN_1: usize = 800;
+    const LEN_2: usize = 600;
+    pair.client_datagrams(client_ch)
+        .send(vec![0; LEN_1].into(), false)
+        .unwrap();
+    pair.client_datagrams(client_ch)
+        .send(vec![0; LEN_2].into(), false)
+        .unwrap();
+    pair.client.drive(pair.time, pair.server.addr);
+
+    // Check padding
+    assert_eq!(pair.client.outbound.len(), 2);
+    assert_eq!(pair.client.outbound[0].0.size, usize::from(MTU));
+    assert_eq!(pair.client.outbound[0].1.len(), usize::from(MTU));
+    assert_eq!(pair.client.outbound[1].0.size, usize::from(MTU));
+    assert_eq!(pair.client.outbound[1].1.len(), usize::from(MTU));
+    pair.drive_client();
+    assert_eq!(pair.server.inbound.len(), 2);
+    assert_eq!(pair.server.inbound[0].2.len(), usize::from(MTU));
+    assert_eq!(pair.server.inbound[1].2.len(), usize::from(MTU));
+    pair.drive();
+
+    // Check that both datagrams ended up in the same GSO batch
+    let final_ios = pair.client_conn_mut(client_ch).stats().udp_tx.ios;
+    assert_eq!(final_ios - initial_ios, 1);
+
+    assert_eq!(
+        pair.server_datagrams(server_ch)
+            .recv()
+            .expect("datagram lost")
+            .len(),
+        LEN_1
+    );
+    assert_eq!(
+        pair.server_datagrams(server_ch)
+            .recv()
+            .expect("datagram lost")
+            .len(),
+        LEN_2
+    );
+}
+
 /// Verify that a large application datagram is sent successfully when an ACK frame too large to fit
 /// alongside it is also queued, in exactly 2 UDP datagrams.
 #[test]