Allow initial maximum UDP payload size to be configured

quinn-proto/src/config.rs

@@ -7,7 +7,7 @@ use crate::{
     cid_generator::{ConnectionIdGenerator, RandomConnectionIdGenerator},
     congestion,
     crypto::{self, HandshakeTokenKey, HmacKey},
-    VarInt, VarIntBoundsExceeded, DEFAULT_SUPPORTED_VERSIONS,
+    VarInt, VarIntBoundsExceeded, DEFAULT_SUPPORTED_VERSIONS, INITIAL_MAX_UDP_PAYLOAD_SIZE,
 };
 
 /// Parameters governing the core QUIC state machine
@@ -34,6 +34,7 @@ pub struct TransportConfig {
     pub(crate) packet_threshold: u32,
     pub(crate) time_threshold: f32,
     pub(crate) initial_rtt: Duration,
+    pub(crate) initial_max_udp_payload_size: u16,
 
     pub(crate) persistent_congestion_threshold: u32,
     pub(crate) keep_alive_interval: Option<Duration>,
@@ -151,6 +152,23 @@ impl TransportConfig {
         self
     }
 
+    /// UDP payload size that the network must be capable of carrying
+    ///
+    /// Effective max UDP payload size may change over the life of the connection in the future due
+    /// to path MTU detection, but will never fall below this value.
+    ///
+    /// Must be at least 1200, which is the default, and known to be safe for typical internet
+    /// applications. Larger values are more efficient, but increase the risk of unpredictable
+    /// catastrophic packet loss due to exceeding the network path's IP MTU.
+    ///
+    /// Real-world MTUs can vary according to ISP, VPN, and properties of intermediate network links
+    /// outside of either endpoint's control. Extreme caution should be used when raising this value
+    /// for connections outside of private networks where these factors are fully controlled.
+    pub fn initial_max_udp_payload_size(&mut self, value: u16) -> &mut Self {
+        self.initial_max_udp_payload_size = value.max(INITIAL_MAX_UDP_PAYLOAD_SIZE);
+        self
+    }
+
     /// Number of consecutive PTOs after which network is considered to be experiencing persistent congestion.
     pub fn persistent_congestion_threshold(&mut self, value: u32) -> &mut Self {
         self.persistent_congestion_threshold = value;
@@ -246,6 +264,7 @@ impl Default for TransportConfig {
             packet_threshold: 3,
             time_threshold: 9.0 / 8.0,
             initial_rtt: Duration::from_millis(333), // per spec, intentionally distinct from EXPECTED_RTT
+            initial_max_udp_payload_size: INITIAL_MAX_UDP_PAYLOAD_SIZE,
 
             persistent_congestion_threshold: 3,
             keep_alive_interval: None,

quinn-proto/src/connection/mod.rs

@@ -262,6 +262,7 @@ impl Connection {
                 remote,
                 config.initial_rtt,
                 config.congestion_controller_factory.build(now),
+                config.initial_max_udp_payload_size,
                 now,
                 path_validated,
             ),
@@ -2626,6 +2627,7 @@ impl Connection {
                 remote,
                 self.config.initial_rtt,
                 self.config.congestion_controller_factory.build(now),
+                self.config.initial_max_udp_payload_size,
                 now,
                 false,
             )

quinn-proto/src/connection/paths.rs

@@ -1,7 +1,7 @@
 use std::{cmp, net::SocketAddr, time::Duration, time::Instant};
 
 use super::pacing::Pacer;
-use crate::{congestion, packet::SpaceId, INITIAL_MAX_UDP_PAYLOAD_SIZE, TIMER_GRANULARITY};
+use crate::{congestion, packet::SpaceId, TIMER_GRANULARITY};
 
 /// Description of a particular network path
 pub struct PathData {
@@ -36,6 +36,7 @@ impl PathData {
         remote: SocketAddr,
         initial_rtt: Duration,
         congestion: Box<dyn congestion::Controller>,
+        initial_max_udp_payload_size: u16,
         now: Instant,
         validated: bool,
     ) -> Self {
@@ -46,7 +47,7 @@ impl PathData {
             pacing: Pacer::new(
                 initial_rtt,
                 congestion.initial_window(),
-                INITIAL_MAX_UDP_PAYLOAD_SIZE,
+                initial_max_udp_payload_size,
                 now,
             ),
             congestion,
@@ -55,7 +56,7 @@ impl PathData {
             validated,
             total_sent: 0,
             total_recvd: 0,
-            max_udp_payload_size: INITIAL_MAX_UDP_PAYLOAD_SIZE,
+            max_udp_payload_size: initial_max_udp_payload_size,
             first_packet_after_rtt_sample: None,
         }
     }

quinn-proto/src/lib.rs

@@ -292,6 +292,7 @@ const LOC_CID_COUNT: u64 = 8;
 const RESET_TOKEN_SIZE: usize = 16;
 const MAX_CID_SIZE: usize = 20;
 const MIN_INITIAL_SIZE: u16 = 1200;
+/// <https://www.rfc-editor.org/rfc/rfc9000.html#name-datagram-size>
 const INITIAL_MAX_UDP_PAYLOAD_SIZE: u16 = 1200;
 const TIMER_GRANULARITY: Duration = Duration::from_millis(1);
 /// Maximum number of streams that can be uniquely identified by a stream ID