feat(quic): make send window configurable

transports/quic/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 0.13.0
 
+- Add `Config::max_connection_send_data` to configure the connection-level
+  send window independently from `max_connection_data`.
+  This enables tuning for high BDP while preserving Quinn's default send window when unset.
+
 - Remove `async-std` support.
   See [PR 5954](https://github.com/libp2p/rust-libp2p/pull/5954)
 

transports/quic/src/config.rs

@@ -26,6 +26,16 @@ use quinn::{
 };
 
 /// Config for the transport.
+///
+/// Performance or resource requirements can be improved in some cases by tuning these values for
+/// the particular application and/or network configuration.
+///
+/// Flow-control window sizes should reflect expected round-trip time, link capacity, and available
+/// memory. Larger windows improve headroom for high-bandwidth, high-latency links, but increase
+/// worst-case memory usage.
+///
+/// Defaults follow Quinn and are tuned for roughly 100 Mbit/s at 100 ms RTT. For details see:
+/// https://github.com/quinn-rs/quinn/blob/f853e5e0826a00f4b80551986eba0ca251ae1b45/quinn-proto/src/config/transport.rs
 #[derive(Clone)]
 pub struct Config {
     /// Timeout for the initial handshake when establishing a connection.
@@ -44,13 +54,20 @@ pub struct Config {
     /// concurrently by the remote peer.
     pub max_concurrent_stream_limit: u32,
 
-    /// Max unacknowledged data in bytes that may be sent on a single stream.
+    /// Maximum amount of stream data in bytes that may be received on a single stream
+    /// before the peer is blocked.
     pub max_stream_data: u32,
 
-    /// Max unacknowledged data in bytes that may be sent in total on all streams
-    /// of a connection.
+    /// Maximum amount of stream data in bytes that may be received in total across all
+    /// streams of a connection before the peer is blocked.
     pub max_connection_data: u32,
 
+    /// Optional override for the maximum amount of stream data in bytes for sending on
+    /// a connection.
+    ///
+    /// If unset, Quinn's default [`quinn::TransportConfig::send_window`] is used.
+    pub max_connection_send_data: Option<u32>,
+
     /// Support QUIC version draft-29 for dialing and listening.
     ///
     /// Per default only QUIC Version 1 / [`libp2p_core::multiaddr::Protocol::QuicV1`]
@@ -93,6 +110,7 @@ impl Config {
             max_concurrent_stream_limit: 256,
             keep_alive_interval: Duration::from_secs(5),
             max_connection_data: 15_000_000,
+            max_connection_send_data: None,
 
             // Ensure that one stream is not consuming the whole connection.
             max_stream_data: 10_000_000,
@@ -114,6 +132,14 @@ impl Config {
         self.mtu_discovery_config = None;
         self
     }
+
+    /// Override the connection-level send window.
+    ///
+    /// If not set, Quinn's default send window is used.
+    pub fn max_connection_send_data(mut self, value: u32) -> Self {
+        self.max_connection_send_data = Some(value);
+        self
+    }
 }
 
 /// Represents the inner configuration for [`quinn`].
@@ -134,6 +160,7 @@ impl From<Config> for QuinnConfig {
             max_concurrent_stream_limit,
             keep_alive_interval,
             max_connection_data,
+            max_connection_send_data,
             max_stream_data,
             support_draft_29,
             handshake_timeout: _,
@@ -152,6 +179,10 @@ impl From<Config> for QuinnConfig {
         transport.stream_receive_window(max_stream_data.into());
         transport.receive_window(max_connection_data.into());
         transport.mtu_discovery_config(mtu_discovery_config);
+        if let Some(max_connection_send_data) = max_connection_send_data {
+            transport.send_window(max_connection_send_data.into());
+        }
+
         let transport = Arc::new(transport);
 
         let mut server_config = quinn::ServerConfig::with_crypto(server_tls_config);