smartproxy/readme.hints.md

# SmartProxy Development Hints

## Byte Tracking and Metrics

### Throughput Drift Issue (Fixed)

**Problem**: Throughput numbers were gradually increasing over time for long-lived connections.

**Root Cause**: The `byRoute()` and `byIP()` methods were dividing cumulative total bytes (since connection start) by the window duration, causing rates to appear higher as connections aged:
- Hour 1: 1GB total / 60s = 17 MB/s ✓
- Hour 2: 2GB total / 60s = 34 MB/s ✗ (appears doubled!)
- Hour 3: 3GB total / 60s = 50 MB/s ✗ (keeps rising!)

**Solution**: Implemented dedicated ThroughputTracker instances for each route and IP address:
- Each route and IP gets its own throughput tracker with per-second sampling
- Samples are taken every second and stored in a circular buffer
- Rate calculations use actual samples within the requested window
- Default window is now 1 second for real-time accuracy

### What Gets Counted (Network Interface Throughput)

The byte tracking is designed to match network interface throughput (what Unifi/network monitoring tools show):

**Counted bytes include:**
- All application data
- TLS handshakes and protocol overhead
- TLS record headers and encryption padding
- HTTP headers and protocol data
- WebSocket frames and protocol overhead
- TLS alerts sent to clients

**NOT counted:**
- PROXY protocol headers (sent to backend, not client)
- TCP/IP headers (handled by OS, not visible at application layer)

**Byte direction:**
- `bytesReceived`: All bytes received FROM the client on the incoming connection
- `bytesSent`: All bytes sent TO the client on the incoming connection
- Backend connections are separate and not mixed with client metrics

### Double Counting Issue (Fixed)

**Problem**: Initial data chunks were being counted twice in the byte tracking:
1. Once when stored in `pendingData` in `setupDirectConnection()`
2. Again when the data flowed through bidirectional forwarding

**Solution**: Removed the byte counting when storing initial chunks. Bytes are now only counted when they actually flow through the `setupBidirectionalForwarding()` callbacks.

### HttpProxy Metrics (Fixed)

**Problem**: HttpProxy forwarding was updating connection record byte counts but not calling `metricsCollector.recordBytes()`, resulting in missing throughput data.

**Solution**: Added `metricsCollector.recordBytes()` calls to the HttpProxy bidirectional forwarding callbacks.

### Metrics Architecture

The metrics system has multiple layers:
1. **Connection Records** (`record.bytesReceived/bytesSent`): Track total bytes per connection
2. **Global ThroughputTracker**: Accumulates bytes between samples for overall rate calculations
3. **Per-Route ThroughputTrackers**: Dedicated tracker for each route with per-second sampling
4. **Per-IP ThroughputTrackers**: Dedicated tracker for each IP with per-second sampling
5. **connectionByteTrackers**: Track cumulative bytes and metadata for active connections

Key features:
- All throughput trackers sample every second (1Hz)
- Each tracker maintains a circular buffer of samples (default: 1 hour retention)
- Rate calculations are accurate for any requested window (default: 1 second)
- All byte counting happens exactly once at the data flow point
- Unused route/IP trackers are automatically cleaned up when connections close

### Understanding "High" Byte Counts

If byte counts seem high compared to actual application data, remember:
- TLS handshakes can be 1-5KB depending on cipher suites and certificates
- Each TLS record has 5 bytes of header overhead
- TLS encryption adds 16-48 bytes of padding/MAC per record
- HTTP/2 has additional framing overhead
- WebSocket has frame headers (2-14 bytes per message)

This overhead is real network traffic and should be counted for accurate throughput metrics.

### Byte Counting Paths

There are two mutually exclusive paths for connections:

1. **Direct forwarding** (route-connection-handler.ts):
   - Used for TCP passthrough, TLS passthrough, and direct connections
   - Bytes counted in `setupBidirectionalForwarding` callbacks
   - Initial chunk NOT counted separately (flows through bidirectional forwarding)

2. **HttpProxy forwarding** (http-proxy-bridge.ts):
   - Used for TLS termination (terminate, terminate-and-reencrypt)
   - Initial chunk counted when written to proxy
   - All subsequent bytes counted in `setupBidirectionalForwarding` callbacks
   - This is the ONLY counting point for these connections

### Byte Counting Audit (2025-01-06)

A comprehensive audit was performed to verify byte counting accuracy:

**Audit Results:**
- ✅ No double counting detected in any connection flow
- ✅ Each byte counted exactly once in each direction
- ✅ Connection records and metrics updated consistently
- ✅ PROXY protocol headers correctly excluded from client metrics
- ✅ NFTables forwarded connections correctly not counted (kernel handles)

**Key Implementation Points:**
- All byte counting happens in only 2 files: `route-connection-handler.ts` and `http-proxy-bridge.ts`
- Both use the same pattern: increment `record.bytesReceived/Sent` AND call `metricsCollector.recordBytes()`
- Initial chunks handled correctly: stored but not counted until forwarded
- TLS alerts counted as sent bytes (correct - they are sent to client)

For full audit details, see `readme.byte-counting-audit.md`

## Connection Cleanup

### Zombie Connection Detection

The connection manager performs comprehensive zombie detection every 10 seconds:
- **Full zombies**: Both incoming and outgoing sockets destroyed but connection not cleaned up
- **Half zombies**: One socket destroyed, grace period expired (5 minutes for TLS, 30 seconds for non-TLS)
- **Stuck connections**: Data received but none sent back after threshold (5 minutes for TLS, 60 seconds for non-TLS)

### Cleanup Queue

Connections are cleaned up through a batched queue system:
- Batch size: 100 connections
- Processing triggered immediately when batch size reached
- Otherwise processed after 100ms delay
- Prevents overwhelming the system during mass disconnections

## Keep-Alive Handling

Keep-alive connections receive special treatment based on `keepAliveTreatment` setting:
- **standard**: Normal timeout applies
- **extended**: Timeout multiplied by `keepAliveInactivityMultiplier` (default 6x)
- **immortal**: No timeout, connections persist indefinitely

## PROXY Protocol

The system supports both receiving and sending PROXY protocol:
- **Receiving**: Automatically detected from trusted proxy IPs (configured in `proxyIPs`)
- **Sending**: Enabled per-route or globally via `sendProxyProtocol` setting
- Real client IP is preserved and used for all connection tracking and security checks

## Metrics and Throughput Calculation

The metrics system tracks throughput using per-second sampling:

1. **Byte Recording**: Bytes are recorded as data flows through connections
2. **Sampling**: Every second, accumulated bytes are stored as a sample
3. **Rate Calculation**: Throughput is calculated by summing bytes over a time window
4. **Per-Route/IP Tracking**: Separate ThroughputTracker instances for each route and IP

Key implementation details:
- Bytes are recorded in the bidirectional forwarding callbacks
- The instant() method returns throughput over the last 1 second
- The recent() method returns throughput over the last 10 seconds
- Custom windows can be specified for different averaging periods

### Throughput Spikes Issue

There's a fundamental difference between application-layer and network-layer throughput:

**Application Layer (what we measure)**:
- Bytes are recorded when delivered to/from the application
- Large chunks can arrive "instantly" due to kernel/Node.js buffering
- Shows spikes when buffers are flushed (e.g., 20MB in 1 second = 160 Mbit/s)

**Network Layer (what Unifi shows)**:
- Actual packet flow through the network interface
- Limited by physical network speed (e.g., 20 Mbit/s)
- Data transfers over time, not in bursts

The spikes occur because:
1. Data flows over network at 20 Mbit/s (takes 8 seconds for 20MB)
2. Kernel/Node.js buffers this incoming data
3. When buffer is flushed, application receives large chunk at once
4. We record entire chunk in current second, creating artificial spike

**Potential Solutions**:
1. Use longer window for "instant" measurements (e.g., 5 seconds instead of 1)
2. Track socket write backpressure to estimate actual network flow
3. Implement bandwidth estimation based on connection duration
4. Accept that application-layer != network-layer throughput