---
# SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: Apache-2.0
title: Request Plane
---

## Overview

Dynamo supports multiple transport mechanisms for its request plane (the communication layer between services). You can choose from three different request plane modes based on your deployment requirements:

- **TCP** (default): Direct TCP connection for optimal performance
- **NATS**: Message broker-based request plane
- **HTTP**: HTTP/2-based request plane

This guide explains how to configure and use request plane in your Dynamo deployment.

## What is a Request Plane?

The request plane is the transport layer that handles communication between Dynamo services (e.g., frontend to backend, worker to worker). Different request planes offer different trade-offs:

| Request Plane | Suitable For | Characteristics |
|--------------|----------|-----------------|
| **NATS** | Production deployments with KV routing | Requires NATS infrastructure, provides pub/sub patterns, highest flexibility |
| **TCP** | Low-latency direct communication | Direct connections, minimal overhead |
| **HTTP** | Standard deployments, debugging | HTTP/2 protocol, easier observability with standard tools, widely compatible |

## Request Plane vs KV Event Plane

Dynamo has **two independent communication planes**:

- **Request plane** (**`DYN_REQUEST_PLANE`**): how **RPC requests** flow between components (frontend → router → worker), via `tcp`, `http`, or `nats`.
- **KV event plane** (currently only **NATS** is supported): how **KV cache events** (and optional router replica sync) are distributed/persisted for KV-aware routing.

**Note:** If you are using `tcp` or `http` request plane with KV events enabled on the router (the default router-side setting), NATS is automatically initialized. SGLang requires explicit `--kv-events-config` and TRT-LLM requires `--publish-events-and-metrics` to publish events. For vLLM, KV events are currently auto-configured when prefix caching is active (deprecated — use `--kv-events-config` explicitly to prepare for a future release where all backends will default to off). You can optionally configure `NATS_SERVER` environment variable (e.g., `NATS_SERVER=nats://nats-hostname:port`) to specify a custom NATS server; otherwise, it defaults to `localhost:4222`. To disable the router's KV event listener, use `--no-router-kv-events` on the frontend.

Because they are independent, you can mix them.

For example, a deployment with TCP request plane can use different KV event planes:
- **JetStream KV events**: requests use TCP, KV routing still uses NATS JetStream + object store for persistence.
- **NATS Core KV events (local indexer)**: requests use TCP, KV events use NATS Core pub/sub and persistence lives on workers.
- **no KV events**: requests use TCP and KV routing runs without events (no NATS required, but no event-backed persistence).

## Configuration

### Environment Variable

Set the request plane mode using the `DYN_REQUEST_PLANE` environment variable:

```bash
export DYN_REQUEST_PLANE=<mode>
```

Where `<mode>` is one of:
- `tcp` (default)
- `nats`
- `http`

The value is case-insensitive.

### Default Behavior

If `DYN_REQUEST_PLANE` is not set or contains an invalid value, Dynamo defaults to `tcp`.

## Usage Examples

### Using TCP (Default)

TCP is the default request plane and provides direct, low-latency communication between services.

**Configuration:**

```bash
# TCP is the default, so no need to set DYN_REQUEST_PLANE explicitly
# But you can explicitly set it if desired:
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   # 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 (NATS is initialized when the router listens for KV events; disable with `--no-router-kv-events`)
- Low-latency requirements

**TCP Configuration Options:**

Additional TCP-specific environment variables:
- `DYN_TCP_RPC_HOST`: Server host address (default: auto-detected)
- `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_SHRINK_MESSAGE_SIZE`: Threshold for shrinking the zero-copy decoder buffer back to initial size after processing large messages (default: 8MB, max: DYN_TCP_MAX_MESSAGE_SIZE)
- `DYN_TCP_REQUEST_TIMEOUT`: Request timeout for TCP client (default: 10 seconds)
- `DYN_TCP_POOL_SIZE`: Connection pool size for TCP client (default: 50)
- `DYN_TCP_CONNECT_TIMEOUT`: Connect timeout for TCP client (default: 3 seconds)
- `DYN_TCP_CHANNEL_BUFFER`: Request channel buffer size for TCP client (default: 100)

### Using HTTP

HTTP/2 provides a standards-based request plane that's easy to debug and widely compatible.

**Configuration:**

```bash
# Optional: Configure HTTP server host and port
export DYN_HTTP_RPC_HOST=0.0.0.0      # Default host
export DYN_HTTP_RPC_PORT=8888         # Default port
export DYN_HTTP_RPC_ROOT_PATH=/v1/rpc # Default path

# Run your Dynamo service
DYN_REQUEST_PLANE=http python -m dynamo.frontend --http-port=8000 &
DYN_REQUEST_PLANE=http python -m dynamo.vllm --model Qwen/Qwen3-0.6B
```

**When to use HTTP:**
- Standard deployments requiring HTTP compatibility
- Debugging scenarios (use curl, browser tools, etc.)
- Integration with HTTP-based infrastructure
- Load balancers and proxies that work with HTTP

**HTTP Configuration Options:**

Additional HTTP-specific environment variables:
- `DYN_HTTP_RPC_HOST`: Server host address (default: auto-detected)
- `DYN_HTTP_RPC_PORT`: Server port (default: 8888)
- `DYN_HTTP_RPC_ROOT_PATH`: Root path for RPC endpoints (default: /v1/rpc)

`DYN_HTTP2_*`: Various HTTP/2 client configuration options
- `DYN_HTTP2_MAX_FRAME_SIZE`: Maximum frame size for HTTP client (default: 1MB)
- `DYN_HTTP2_MAX_CONCURRENT_STREAMS`: Maximum concurrent streams for HTTP client (default: 1000)
- `DYN_HTTP2_POOL_MAX_IDLE_PER_HOST`: Maximum idle connections per host for HTTP client (default: 100)
- `DYN_HTTP2_POOL_IDLE_TIMEOUT_SECS`: Idle timeout for HTTP client (default: 90 seconds)
- `DYN_HTTP2_KEEP_ALIVE_INTERVAL_SECS`: Keep-alive interval for HTTP client (default: 30 seconds)
- `DYN_HTTP2_KEEP_ALIVE_TIMEOUT_SECS`: Keep-alive timeout for HTTP client (default: 10 seconds)
- `DYN_HTTP2_ADAPTIVE_WINDOW`: Enable adaptive flow control (default: true)

### Using NATS

NATS provides durable jetstream messaging for request plane and can be used for KV events (and router replica sync).

**Prerequisites:**
- NATS server must be running and accessible
- Configure NATS connection via standard Dynamo NATS environment variables

```bash
# Explicitly set to NATS
export DYN_REQUEST_PLANE=nats

# Run your Dynamo service
DYN_REQUEST_PLANE=nats python -m dynamo.frontend --http-port=8000 &
DYN_REQUEST_PLANE=nats python -m dynamo.vllm --model Qwen/Qwen3-0.6B
```

**When to use NATS:**
- Production deployments with service discovery
- KV-aware routing with accurate cache state tracking (requires NATS for event transport). Note: approximate mode (`--no-router-kv-events`) provides KV routing without NATS but with reduced accuracy.
- Need for message replay and persistence features

Limitations:
- NATS does not support payloads beyond 16MB (use TCP for larger payloads)

## Complete Example

Here's a complete example showing how to launch a Dynamo deployment with different request planes:

See [`examples/backends/vllm/launch/agg_request_planes.sh`](https://github.com/ai-dynamo/dynamo/tree/main/examples/backends/vllm/launch/agg_request_planes.sh) for a complete working example that demonstrates launching Dynamo with TCP, HTTP, or NATS request planes.


## Real-World Example

The Dynamo repository includes a complete example demonstrating all three request planes:

**Location:** `examples/backends/vllm/launch/agg_request_planes.sh`

```bash
cd examples/backends/vllm/launch

# Run with TCP
./agg_request_planes.sh --tcp

# Run with HTTP
./agg_request_planes.sh --http

# Run with NATS
./agg_request_planes.sh --nats
```

## Architecture Details

### Network Manager

The request plane implementation is centralized in the Network Manager (`lib/runtime/src/pipeline/network/manager.rs`), which:

1. Reads the `DYN_REQUEST_PLANE` environment variable at startup
2. Creates the appropriate server and client implementations
3. Provides a transport-agnostic interface to the rest of the codebase
4. Manages all network configuration and lifecycle

### Transport Abstraction

All request plane implementations conform to common trait interfaces:
- `RequestPlaneServer`: Server-side interface for receiving requests
- `RequestPlaneClient`: Client-side interface for sending requests

This abstraction means your application code doesn't need to change when switching request planes.

### Configuration Loading

Request plane configuration is loaded from environment variables at startup and cached globally. The configuration hierarchy is:

1. **Mode Selection**: `DYN_REQUEST_PLANE` (defaults to `tcp`)
2. **Transport-Specific Config**: Mode-specific environment variables (e.g., `DYN_TCP_*`, `DYN_HTTP2_*`)

## Migration Guide

### From NATS to TCP

1. Stop your Dynamo services
2. Set environment variable `DYN_REQUEST_PLANE=tcp`
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


### From NATS to HTTP

1. Stop your Dynamo services
2. Set environment variable `DYN_REQUEST_PLANE=http`
3. Optionally configure HTTP-specific settings (`DYN_HTTP_RPC_PORT`, etc.)
4. Restart your services

### Testing the Migration

After switching request planes, verify your deployment:

```bash
# Test with a simple request
curl http://localhost:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "Qwen/Qwen3-0.6B",
    "messages": [{"role": "user", "content": "Hello!"}]
  }'
```

## Troubleshooting

### Issue: Services Can't Communicate

**Symptoms:** Requests timeout or fail to reach the backend

**Solutions:**
- Verify all services use the same `DYN_REQUEST_PLANE` setting
- Check that server ports are not blocked by k8s network policies or firewalls
- For TCP/HTTP: Ensure host/port configurations are correct and accessible
- For NATS: Verify NATS server is running and accessible

### Issue: "Invalid request plane mode" Error

**Symptoms:** Service fails to start with configuration error

**Solutions:**
- Check `DYN_REQUEST_PLANE` spelling (valid values: `nats`, `tcp`, `http`)
- Value is case-insensitive but must be one of the three options
- If not set, defaults to `tcp`

### Issue: Port Conflicts

**Symptoms:** Server fails to start due to "address already in use"

**Solutions:**
- 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

### Latency

- **TCP**: Lowest latency due to direct connections and binary serialization
- **HTTP**: Moderate latency with HTTP/2 overhead
- **NATS**: Moderate latency due to nats jet stream persistence


### Resource Usage

- **TCP**: Minimal infrastructure (NATS required only if using KV events, disable router-side with `--no-router-kv-events`)
- **HTTP**: Minimal infrastructure (NATS required only if using KV events, disable router-side with `--no-router-kv-events`)
- **NATS**: Requires running NATS server (additional memory/CPU)
