feat: use os assigned free port for tcp rpc (#4891)

Author: biswapanda

docs/guides/request_plane.md

@@ -108,13 +108,15 @@ export DYN_REQUEST_PLANE=tcp
 
 # Optional: Configure TCP server host and port
 export DYN_TCP_RPC_HOST=0.0.0.0  # Default host
-export DYN_TCP_RPC_PORT=9999     # Default port
+# export DYN_TCP_RPC_PORT=9999   # Optional: specify a fixed port
 
 # Run your Dynamo service
 DYN_REQUEST_PLANE=tcp python -m dynamo.frontend --http-port=8000 &
 DYN_REQUEST_PLANE=tcp python -m dynamo.vllm --model Qwen/Qwen3-0.6B
 ```
 
+**Note:** By default, TCP uses an OS-assigned free port (port 0). This is ideal for environments where multiple services may run on the same machine or when you want to avoid port conflicts. If you need a specific port (e.g., for firewall rules), set `DYN_TCP_RPC_PORT` explicitly.
+
 **When to use TCP:**
 - Simple deployments with direct service-to-service communication (e.g. frontend to backend)
 - Minimal infrastructure requirements (no NATS needed)
@@ -124,7 +126,7 @@ DYN_REQUEST_PLANE=tcp python -m dynamo.vllm --model Qwen/Qwen3-0.6B
 
 Additional TCP-specific environment variables:
 - `DYN_TCP_RPC_HOST`: Server host address (default: auto-detected)
-- `DYN_TCP_RPC_PORT`: Server port (default: 9999)
+- `DYN_TCP_RPC_PORT`: Server port. If not set, the OS assigns a free port automatically (recommended for most deployments). Set explicitly only if you need a specific port for firewall rules.
 - `DYN_TCP_MAX_MESSAGE_SIZE`: Maximum message size for TCP client (default: 32MB)
 - `DYN_TCP_REQUEST_TIMEOUT`: Request timeout for TCP client (default: 10 seconds)
 - `DYN_TCP_POOL_SIZE`: Connection pool size for TCP client (default: 50)
@@ -228,7 +230,7 @@ Request plane configuration is loaded from environment variables at startup and
 
 1. Stop your Dynamo services
 2. Set environment variable `DYN_REQUEST_PLANE=tcp`
-3. Optionally configure TCP-specific settings (`DYN_TCP_RPC_PORT`, etc.)
+3. Optionally configure TCP-specific settings (e.g., `DYN_TCP_RPC_HOST`). Note: `DYN_TCP_RPC_PORT` is optional; if not set, an OS-assigned free port is used automatically.
 4. Restart your services
 
 
@@ -279,7 +281,7 @@ curl http://localhost:8000/v1/chat/completions \
 **Symptoms:** Server fails to start due to "address already in use"
 
 **Solutions:**
-- TCP default port: 9999 (adjust environment variable `DYN_TCP_RPC_PORT`)
+- TCP: By default, TCP uses an OS-assigned free port, so port conflicts should be rare. If you explicitly set `DYN_TCP_RPC_PORT` to a specific port and get conflicts, either change the port or remove the setting to use automatic port assignment.
 - HTTP default port: 8888 (adjust environment variable `DYN_HTTP_RPC_PORT`)
 
 ## Performance Considerations

lib/llm/src/discovery/model_manager.rs

@@ -330,12 +330,11 @@ impl ModelManager {
         // Register router via discovery mechanism
         let discovery = endpoint.component().drt().discovery();
         let instance_id = discovery.instance_id();
-        let request_plane_mode = endpoint.drt().request_plane();
 
         // Build transport for router endpoint based on request plane mode
         // Use KV_ROUTER_COMPONENT as the component name to distinguish from the generate endpoint's component
         let router_endpoint_id = router_endpoint_id(endpoint.id().namespace);
-        let transport = build_transport_type(request_plane_mode, &router_endpoint_id, instance_id);
+        let transport = build_transport_type(endpoint, &router_endpoint_id, instance_id).await?;
 
         let discovery_spec = DiscoverySpec::Endpoint {
             namespace: router_endpoint_id.namespace.clone(),

lib/runtime/src/component/endpoint.rs

@@ -89,30 +89,6 @@ impl EndpointConfigBuilder {
         let request_plane_mode = endpoint.drt().request_plane();
         tracing::info!("Endpoint starting with request plane mode: {request_plane_mode}",);
 
-        // Register health check target in SystemHealth if provided
-        if let Some(health_check_payload) = &health_check_payload {
-            // Build transport based on request plane mode
-            let transport = build_transport_type(request_plane_mode, &endpoint_id, connection_id);
-
-            let instance = Instance {
-                component: endpoint_id.component.clone(),
-                endpoint: endpoint_id.name.clone(),
-                namespace: endpoint_id.namespace.clone(),
-                instance_id: connection_id,
-                transport,
-            };
-            tracing::debug!(endpoint_name = %endpoint.name, "Registering endpoint health check target");
-            let guard = system_health.lock();
-            guard.register_health_check_target(
-                &endpoint.name,
-                instance,
-                health_check_payload.clone(),
-            );
-            if let Some(notifier) = guard.get_endpoint_health_check_notifier(&endpoint.name) {
-                handler.set_endpoint_health_check_notifier(notifier)?;
-            }
-        }
-
         // Register with graceful shutdown tracker if needed
         if graceful_shutdown {
             tracing::debug!(
@@ -137,9 +113,33 @@ impl EndpointConfigBuilder {
         let component_name_for_task = endpoint_id.component.clone();
         let endpoint_name_for_task = endpoint_id.name.clone();
 
-        // Get the unified request plane server (works for all transport types)
+        // Get the unified request plane server
         let server = endpoint.drt().request_plane_server().await?;
 
+        // Register health check target in SystemHealth if provided
+        if let Some(health_check_payload) = &health_check_payload {
+            // Build transport based on request plane mode
+            let transport = build_transport_type(&endpoint, &endpoint_id, connection_id).await?;
+
+            let instance = Instance {
+                component: endpoint_id.component.clone(),
+                endpoint: endpoint_id.name.clone(),
+                namespace: endpoint_id.namespace.clone(),
+                instance_id: connection_id,
+                transport,
+            };
+            tracing::debug!(endpoint_name = %endpoint.name, "Registering endpoint health check target");
+            let guard = system_health.lock();
+            guard.register_health_check_target(
+                &endpoint.name,
+                instance,
+                health_check_payload.clone(),
+            );
+            if let Some(notifier) = guard.get_endpoint_health_check_notifier(&endpoint.name) {
+                handler.set_endpoint_health_check_notifier(notifier)?;
+            }
+        }
+
         tracing::info!(
             endpoint = %endpoint_name_for_task,
             transport = server.transport_name(),
@@ -198,7 +198,7 @@ impl EndpointConfigBuilder {
         let discovery = endpoint.drt().discovery();
 
         // Build transport for discovery service based on request plane mode
-        let transport = build_transport_type(request_plane_mode, &endpoint_id, connection_id);
+        let transport = build_transport_type(&endpoint, &endpoint_id, connection_id).await?;
 
         let discovery_spec = crate::discovery::DiscoverySpec::Endpoint {
             namespace: endpoint_id.namespace.clone(),
@@ -232,11 +232,14 @@ impl EndpointConfigBuilder {
 /// - HTTP: Uses full URL path including endpoint name (e.g., http://host:port/v1/rpc/endpoint_name)
 /// - TCP: Includes endpoint name for routing (e.g., host:port/endpoint_name)
 /// - NATS: Uses subject-based addressing (unique per endpoint)
-pub fn build_transport_type(
+///
+/// # Errors
+/// Returns an error if TCP mode is used but the TCP server hasn't been started yet.
+fn build_transport_type_inner(
     mode: RequestPlaneMode,
     endpoint_id: &EndpointId,
     connection_id: u64,
-) -> TransportType {
+) -> Result<TransportType> {
     match mode {
         RequestPlaneMode::Http => {
             let http_host = crate::utils::get_http_rpc_host_from_env();
@@ -252,23 +255,54 @@ pub fn build_transport_type(
                 endpoint_id.name
             );
 
-            TransportType::Http(http_endpoint)
+            Ok(TransportType::Http(http_endpoint))
         }
         RequestPlaneMode::Tcp => {
             let tcp_host = crate::utils::get_tcp_rpc_host_from_env();
+            // If a fixed port is explicitly configured, use it directly (no init ordering dependency).
+            // Otherwise, use the actual bound port (set by TCP server after binding when port 0 is used).
             let tcp_port = std::env::var("DYN_TCP_RPC_PORT")
                 .ok()
                 .and_then(|p| p.parse::<u16>().ok())
-                .unwrap_or(9999);
+                .unwrap_or(crate::pipeline::network::manager::get_actual_tcp_rpc_port()?);
 
             // Include endpoint name for proper TCP routing
             // TCP client parses this format and adds x-endpoint-path header for server-side routing
             let tcp_endpoint = format!("{}:{}/{}", tcp_host, tcp_port, endpoint_id.name);
 
-            TransportType::Tcp(tcp_endpoint)
+            Ok(TransportType::Tcp(tcp_endpoint))
         }
-        RequestPlaneMode::Nats => {
-            TransportType::Nats(nats::instance_subject(endpoint_id, connection_id))
+        RequestPlaneMode::Nats => Ok(TransportType::Nats(nats::instance_subject(
+            endpoint_id,
+            connection_id,
+        ))),
+    }
+}
+
+/// Build transport type, ensuring TCP server is initialized when needed.
+///
+/// In TCP mode with an OS-assigned port (`DYN_TCP_RPC_PORT` unset or invalid), the server must bind
+/// before we can construct a correct transport address. This helper ensures that initialization
+/// occurs, then delegates to the internal builder.
+pub async fn build_transport_type(
+    endpoint: &Endpoint,
+    endpoint_id: &EndpointId,
+    connection_id: u64,
+) -> Result<TransportType> {
+    let mode = endpoint.drt().request_plane();
+
+    if mode == RequestPlaneMode::Tcp {
+        // Only force server init when we *don't* have a valid explicit port.
+        let has_fixed_port = std::env::var("DYN_TCP_RPC_PORT")
+            .ok()
+            .and_then(|p| p.parse::<u16>().ok())
+            .is_some();
+
+        if !has_fixed_port {
+            // Ensure request plane server is initialized before building transport.
+            let _ = endpoint.drt().request_plane_server().await?;
         }
     }
+
+    build_transport_type_inner(mode, endpoint_id, connection_id)
 }

lib/runtime/src/pipeline/network/ingress/shared_tcp_endpoint.rs

@@ -11,7 +11,7 @@ use crate::pipeline::network::PushWorkHandler;
 use anyhow::Result;
 use bytes::Bytes;
 use dashmap::DashMap;
-use parking_lot::Mutex;
+use parking_lot::{Mutex, RwLock};
 use std::net::SocketAddr;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicU64, Ordering};
@@ -36,7 +36,10 @@ fn get_max_message_size() -> usize {
 /// Shared TCP server that handles multiple endpoints on a single port
 pub struct SharedTcpServer {
     handlers: Arc<DashMap<String, Arc<EndpointHandler>>>,
+    /// The address to bind to (may have port 0 for OS-assigned port)
     bind_addr: SocketAddr,
+    /// The actual bound address (populated after bind_and_start, contains actual port)
+    actual_addr: RwLock<Option<SocketAddr>>,
     cancellation_token: CancellationToken,
 }
 
@@ -55,11 +58,83 @@ impl SharedTcpServer {
     pub fn new(bind_addr: SocketAddr, cancellation_token: CancellationToken) -> Arc<Self> {
         Arc::new(Self {
             handlers: Arc::new(DashMap::new()),
+            // address we requested to bind to.
             bind_addr,
+            // actual address after free port assignment (if DYN_TCP_RPC_PORT is not specified)
+            actual_addr: RwLock::new(None),
             cancellation_token,
         })
     }
 
+    /// Bind the server and start accepting connections.
+    ///
+    /// This method binds to the configured address first, then starts the accept loop.
+    /// If the configured port is 0, the OS will assign a free port.
+    /// The actual bound address is stored and can be retrieved via `actual_address()`.
+    ///
+    /// Returns the actual bound address (useful when port 0 was specified).
+    pub async fn bind_and_start(self: Arc<Self>) -> Result<SocketAddr> {
+        tracing::info!("Binding TCP server to {}", self.bind_addr);
+
+        let listener = TcpListener::bind(&self.bind_addr).await?;
+        let actual_addr = listener.local_addr()?;
+
+        tracing::info!(
+            requested = %self.bind_addr,
+            actual = %actual_addr,
+            "TCP server bound successfully"
+        );
+
+        // Store the actual bound address
+        *self.actual_addr.write() = Some(actual_addr);
+
+        // Start accepting connections in a background task
+        let server = self.clone();
+        tokio::spawn(async move {
+            server.accept_loop(listener).await;
+        });
+
+        Ok(actual_addr)
+    }
+
+    /// Get the actual bound address (after bind_and_start has been called).
+    ///
+    /// Returns None if the server hasn't been started yet.
+    pub fn actual_address(&self) -> Option<SocketAddr> {
+        *self.actual_addr.read()
+    }
+
+    /// Internal accept loop - runs after binding
+    async fn accept_loop(self: Arc<Self>, listener: TcpListener) {
+        let cancellation_token = self.cancellation_token.clone();
+
+        loop {
+            tokio::select! {
+                accept_result = listener.accept() => {
+                    match accept_result {
+                        Ok((stream, peer_addr)) => {
+                            tracing::trace!("Accepted TCP connection from {}", peer_addr);
+
+                            let handlers = self.handlers.clone();
+                            tokio::spawn(async move {
+                                if let Err(e) = Self::handle_connection(stream, handlers).await {
+                                    tracing::error!("TCP connection error: {}", e);
+                                }
+                            });
+                        }
+                        Err(e) => {
+                            tracing::error!("Failed to accept TCP connection: {}", e);
+                        }
+                    }
+                }
+                _ = cancellation_token.cancelled() => {
+                    tracing::info!("SharedTcpServer received cancellation signal, shutting down");
+                    return;
+                }
+            }
+        }
+    }
+
     #[allow(clippy::too_many_arguments)]
     pub async fn register_endpoint(
         &self,
@@ -93,7 +168,7 @@ impl SharedTcpServer {
         tracing::info!(
             "Registered endpoint '{}' with shared TCP server on {}",
             endpoint_name,
-            self.bind_addr
+            self.actual_address().unwrap_or(self.bind_addr)
         );
 
         Ok(())
@@ -129,37 +204,16 @@ impl SharedTcpServer {
         }
     }
 
+    /// Start the server (legacy method - prefer bind_and_start for new code).
+    ///
+    /// This method is kept for backwards compatibility. It binds and starts
+    /// the server but doesn't return the actual bound address.
     pub async fn start(self: Arc<Self>) -> Result<()> {
-        tracing::info!("Starting shared TCP server on {}", self.bind_addr);
-
-        let listener = TcpListener::bind(&self.bind_addr).await?;
-        let cancellation_token = self.cancellation_token.clone();
-
-        loop {
-            tokio::select! {
-                accept_result = listener.accept() => {
-                    match accept_result {
-                        Ok((stream, peer_addr)) => {
-                            tracing::trace!("Accepted TCP connection from {}", peer_addr);
-
-                            let handlers = self.handlers.clone();
-                            tokio::spawn(async move {
-                                if let Err(e) = Self::handle_connection(stream, handlers).await {
-                                    tracing::debug!("TCP connection error: {}", e);
-                                }
-                            });
-                        }
-                        Err(e) => {
-                            tracing::error!("Failed to accept TCP connection: {}", e);
-                        }
-                    }
-                }
-                _ = cancellation_token.cancelled() => {
-                    tracing::info!("SharedTcpServer received cancellation signal, shutting down");
-                    return Ok(());
-                }
-            }
-        }
+        let cancel_token = self.cancellation_token.clone();
+        self.bind_and_start().await?;
+        // Wait for cancellation (the accept loop runs in background)
+        cancel_token.cancelled().await;
+        Ok(())
     }
 
     async fn handle_connection(
@@ -378,7 +432,10 @@ impl super::unified_server::RequestPlaneServer for SharedTcpServer {
     }
 
     fn address(&self) -> String {
-        format!("tcp://{}:{}", self.bind_addr.ip(), self.bind_addr.port())
+        // Return actual bound address if available (after bind_and_start),
+        // otherwise fall back to configured bind address
+        let addr = self.actual_address().unwrap_or(self.bind_addr);
+        format!("tcp://{}:{}", addr.ip(), addr.port())
     }
 
     fn transport_name(&self) -> &'static str {