smolmix + sdk: throughput and latency tuning for the Goblin read tunnel

256KB TCP buffers (was 8KB) + burst 64 (was 1) in smolmix; an
IpMixStream::from_client constructor + best_ipr helper and reply-SURBs
10->4 in the ipr wrapper so a caller can back the tunnel with a
high-traffic-profile MixnetClient. No behavior change to the default
connect_new path used by the scoped exit.

sdk/rust/nym-sdk/src/ipr_wrapper/ip_mix_stream.rs

@@ -53,22 +53,52 @@ impl IpMixStream {
     ///
     /// Returns a ready-to-use tunnel with allocated IP addresses.
     pub async fn new() -> Result<Self, Error> {
+        let ipr_address = Self::best_ipr().await?;
+        Self::new_with_ipr(ipr_address).await
+    }
+
+    /// Auto-discover the best available IPR exit for the mainnet.
+    ///
+    /// Exposed so a caller that wants to back the tunnel with its own tuned
+    /// [`MixnetClient`] (via [`IpMixStream::from_client`]) can still get the
+    /// auto-selected exit that [`IpMixStream::new`] would have used.
+    pub async fn best_ipr() -> Result<Recipient, Error> {
         let network_defaults = NymNetworkDetails::new_mainnet();
         let api_client =
             create_nym_api_client(network_defaults.nym_api_urls.ok_or(Error::NoNymAPIUrl)?)?;
-        let ipr_address = get_best_ipr(api_client).await?;
-        Self::new_with_ipr(ipr_address).await
+        get_best_ipr(api_client).await
     }
 
     /// Connect to a specific IPR address.
     ///
     /// Use this when you already know the IPR `Recipient` address (e.g. for
     /// testing against a specific exit node). For automatic discovery, use
-    /// [`IpMixStream::new`] instead.
+    /// [`IpMixStream::new`] instead. Backs the tunnel with a default
+    /// [`MixnetClient::connect_new`] client (full cover traffic, poisson per-hop
+    /// delays); for a tuned client use [`IpMixStream::from_client`].
     pub async fn new_with_ipr(ipr_address: Recipient) -> Result<Self, Error> {
         nym_network_defaults::setup_env(None::<&str>);
-        let mut client = MixnetClient::connect_new().await?;
-        let mut stream = client.open_stream(ipr_address, Some(10)).await?;
+        let client = MixnetClient::connect_new().await?;
+        Self::from_client(client, ipr_address).await
+    }
+
+    /// Establish the IP tunnel over a caller-provided, already-connected
+    /// [`MixnetClient`].
+    ///
+    /// This is the low-anonymity-tuning seam: the caller builds the client with
+    /// whatever traffic/anonymity [`DebugConfig`](crate::mixnet::config) it wants
+    /// (e.g. a higher-throughput / lower-cover-traffic preset for a public read
+    /// tunnel) and passes it in. Everything after the client — the stream open,
+    /// the IPR connect handshake and the allocated-IP bookkeeping — is identical
+    /// to [`new_with_ipr`](Self::new_with_ipr).
+    pub async fn from_client(
+        mut client: MixnetClient,
+        ipr_address: Recipient,
+    ) -> Result<Self, Error> {
+        // 4 reply-SURBs per stream (was 10): the IPR replies steadily, so a
+        // smaller SURB budget still keeps the return path fed while cutting the
+        // per-write SURB overhead on this public read tunnel.
+        let mut stream = client.open_stream(ipr_address, Some(4)).await?;
 
         info!("Connecting to IP packet router at {ipr_address}");
         let allocated_ips = Self::connect_tunnel(&mut stream).await?;

sdk/rust/nym-sdk/src/lib.rs

@@ -141,6 +141,11 @@ pub use nym_network_defaults::NymContracts;
 /// println!("API: {:?}", network.endpoints);
 /// ```
 pub use nym_network_defaults::NymNetworkDetails;
+/// Export the network (mainnet by default) environment configuration. Callers
+/// that build a [`MixnetClient`](mixnet::MixnetClient) themselves (rather than
+/// via [`MixnetClient::connect_new`](mixnet::MixnetClient::connect_new)) should
+/// call this first, mirroring what the higher-level constructors do internally.
+pub use nym_network_defaults::setup_env;
 /// Validator/API endpoint configuration.
 pub use nym_network_defaults::ValidatorDetails;
 

smolmix/core/src/device.rs

@@ -33,7 +33,11 @@ impl NymAsyncDevice {
         let mut capabilities = DeviceCapabilities::default();
         capabilities.medium = Medium::Ip;
         capabilities.max_transmission_unit = 1500;
-        capabilities.max_burst_size = Some(1);
+        // Let the reactor process up to 64 packets per poll loop instead of one.
+        // max_burst_size = Some(1) serialized all I/O to a single packet per
+        // reactor wake, which throttles throughput badly over the mixnet's long
+        // RTT; 64 lets a burst drain in one pass.
+        capabilities.max_burst_size = Some(64);
 
         Self {
             rx,

smolmix/core/src/tunnel.rs

@@ -217,11 +217,19 @@ impl Tunnel {
         // Configure smoltcp: raw IP mode (no Ethernet), /32 for the allocated IP,
         // default route via unspecified (the IPR does the actual routing).
         let iface_config = Config::new(HardwareAddress::Ip);
-        let net_config = NetConfig::new(
+        let mut net_config = NetConfig::new(
             iface_config,
             IpCidr::new(IpAddress::from(allocated_ips.ipv4), 32),
             vec![IpAddress::from(Ipv4Address::UNSPECIFIED)],
         );
+        // Lift the per-socket TCP window from tokio-smoltcp's 8 KB default to
+        // 256 KB. The mixnet RTT is large (5 hops + deliberate per-hop delay),
+        // so an 8 KB window caps bulk throughput at ~8 KB per round trip; 256 KB
+        // lets many segments stay in flight, which is what makes relay/NIP-11/
+        // price reads fast. Only the two TCP buffers change — every other
+        // `BufferSize` field keeps its default.
+        net_config.buffer_size.tcp_rx_size = 262144;
+        net_config.buffer_size.tcp_tx_size = 262144;
 
         // Net::new spawns the smoltcp reactor as a background task. After this,
         // tcp_connect/udp_bind create sockets managed by that reactor.