# Case Study: UDP Conntrack Timeout Causing ACK Loss and Call Death at 64 Seconds

> **⚠️ IMPORTANT UPDATE (2026-06-22):**
>
> This case study documents a **real failure mode** (conntrack timeout < Timer H) that can cause exactly the symptoms described. However, in the original production case that prompted this investigation, **the conntrack fix did not resolve the actual blocker**.
>
> **Verification at the host physical NIC** (`tcpdump -i enp1s0`) proved that ACK packets never reached docker-2 at all - the loss was **upstream in the carrier infrastructure** (ClearFly ↔ Twilio seam), not at the conntrack layer.
>
> **Key lesson:** Same symptom (timing-precise call death + Timer H expiry), different root cause. Always verify at the **host NIC level** before concluding that a lower-layer fix (like conntrack) resolved the issue. Container-level pcaps show what reaches userspace but can't prove what never arrived.
>
> The conntrack fix and ACK fast-path improvements remain valuable **defense-in-depth** measures that prevent future issues of this shape.
>
> See: [Agent Thread Message 010](../agent-threads/ack-loss-from-twilio-trunk/010-flextel-update-conntrack-didnt-resolve-loss-is-upstream.md)

## Problem Statement

Twilio trunk calls through caddy-sip-guardian were dying at exactly **64 seconds** after answer. The failure was 100% reproducible and timing-precise.

**Observable symptoms:**
- Calls answered normally (INVITE → 200 OK → ACK)
- Audio worked perfectly for ~64 seconds
- At t+64s, call abruptly terminated with BYE from Asterisk
- Asterisk logs showed Timer H expiry: "Disconnecting call because no ACK received"

## Architecture Context

```
┌──────────────┐         ┌────────────────────┐         ┌──────────────┐
│    Twilio    │  UDP    │  caddy-sip-        │  UDP    │   Asterisk   │
│  Trunk       │ :5060   │  guardian          │ :5060   │   (asterpbx) │
│ 54.244.51.x  │────────>│ (Layer 4 proxy)    │────────>│ 172.20.7.6   │
└──────────────┘         └────────────────────┘         └──────────────┘
                              │
                              │ Lives on docker-2.supportedsystems.com
                              │ Behind Linux kernel conntrack/NAT
                              ▼
                       UDP conntrack table
```

**Key components:**
- **Twilio**: SIP trunk provider (UDP transport)
- **caddy-sip-guardian**: Layer 4 SIP proxy with security features (rate limiting, enumeration detection, bans)
- **Asterisk**: PBX backend running in container
- **Linux conntrack**: Kernel connection tracking for NAT/stateful firewall

## Investigation Timeline

### Layer 1: SIP Protocol Timers (Red Herring)

**Hypothesis:** Asterisk's endpoint timers too aggressive?

```ini
; /etc/asterisk/pjsip.conf endpoints
timers_sess_expires = 1800     ; 30 minutes
timers_min_se = 90             ; 90 seconds
```

**Result:** Ruled out. Asterisk's session timers were set to 30 minutes, far beyond the 64-second failure.

**Key lesson:** Timer H (INVITE transaction timeout) is **32 seconds** per RFC 3261. The 64-second call death is exactly **2× Timer H**, suggesting Timer H fired once, sent BYE, waited another 32s for BYE response, then disconnected.

---

### Layer 2: sip-guardian ACK Handling (Real Bug, Wrong Layer)

**Hypothesis:** ACKs triggering enumeration detection or rate-limiting false-positives?

**Code Review Finding:**

In `l4handler.go`, **every SIP request** (including ACK) went through the full security pipeline:

```go
// l4handler.go:128-333
func (h *SIPHandler) Handle(cx *layer4.Connection, next layer4.Handler) error {
    // 1. Ban check
    // 2. Whitelist check → SKIP TO PROXY if whitelisted
    // 3. GeoIP check
    // 4. Read buffer
    // 5. SIP validation
    // 6. Rate limiting (by method) ← ACK can hit this
    // 7. Enumeration detection ← ACK can hit this
    // 8. Pattern matching
    // 9. Pass to proxy
}
```

**Problem identified:** ACK is a **mid-dialog request**, not a security threat. It should not traverse enumeration detection or rate limiting.

**Fix Applied (Option B - Quick Fix):**

```go
// Extract SIP method for rate limiting
method := ExtractSIPMethod(buf)

// Fast-path for ACK - mid-dialog request, not a security threat
// ACKs arrive in response to 200 OK retransmissions (RFC 3261) and would
// trigger false-positive enumeration/rate-limit detection.
// See: docs/agent-threads/ack-loss-from-twilio-trunk/
if method == "ACK" {
    h.logger.Debug("ACK exempted from enumeration/rate checks (mid-dialog fast-path)",
        zap.String("ip", host),
    )
    if h.guardian.metricsEnabled() {
        RecordConnection("allowed")
    }
    return next.Handle(cx)
}
```

**Result:** Deployed to production... but **calls still died at 64 seconds**.

**Verification:** Packet capture **inside sip-guardian container** showed **zero ACK packets** arriving from Twilio. The ACKs were being dropped **upstream** of sip-guardian, before they reached the userspace layer.

**Key lesson:** The architectural fix was correct and valuable for other deployments, but it didn't solve this specific blocker. The loss was happening at the kernel layer.

---

### Layer 3: UDP Conntrack Timeout (Root Cause)

**Hypothesis:** Linux conntrack expiring UDP flows before SIP transaction completes?

**Investigation on docker-2.supportedsystems.com:**

```bash
$ cat /proc/sys/net/netfilter/nf_conntrack_udp_timeout
30

$ cat /proc/sys/net/netfilter/nf_conntrack_udp_timeout_stream
120
```

**The smoking gun:** `nf_conntrack_udp_timeout = 30 seconds`

This is the timeout for **unreplied UDP flows** (single-direction traffic with no response). Bidirectional UDP flows get 120 seconds.

---

## Root Cause: Timing Conflict Between Conntrack and SIP Protocol

### SIP Call Flow vs. Conntrack Lifecycle

| Time | SIP Event | Conntrack State |
|------|-----------|-----------------|
| t+0 | Twilio → INVITE | Entry created (30s timeout) |
| t+0.1 | asterpbx → 200 OK | ASSURED (bidirectional) |
| t+0.2 | Twilio → ACK | Flow established |
| t+0.5 | asterpbx retx 200 OK (Timer A) | Active |
| t+1 | asterpbx retx 200 OK | Active |
| t+2 | asterpbx retx 200 OK | Active |
| t+4 | asterpbx retx 200 OK | Active |
| t+8 | asterpbx retx 200 OK | Active |
| t+16 | asterpbx retx 200 OK | Active |
| **t+30** | **Conntrack expires** | ❌ **Entry deleted** |
| t+32 | asterpbx retx 200 OK (final) | No entry |
| t+32 | Twilio → ACK | ❌ **DROPPED (no conntrack)** |
| t+64 | asterpbx Timer H fires → BYE | Call dies |

### Why the 30s Timeout Kills SIP

**RFC 3261 Timer H:** 32 seconds for INVITE 200 OK to be ACKed before abandoning

**Linux UDP conntrack:** 30 seconds before expiring "unreplied" UDP flows

**The problem:**
1. Asterisk's 200 OK retransmissions trigger Twilio to re-send ACK
2. But ACK-to-ACK retransmissions are **the same packet**, not new bidirectional traffic
3. Conntrack sees this as "unreplied" (no NEW packet in 30s)
4. At t+30s, conntrack expires the flow
5. At t+32s, Asterisk's final 200 OK retransmit arrives, Twilio sends ACK
6. Kernel sees unsolicited UDP packet (no conntrack entry) → **DROP**
7. Asterisk never receives ACK → Timer H fires at t+64s → sends BYE

**Why 64 seconds exactly:**
- Timer H fires at t+32s (ACK not received)
- Asterisk sends BYE
- Waits for BYE response (another transaction)
- Final timeout at t+64s (2× Timer H)

---

## The Fix

### Production Fix: Increase UDP Conntrack Timeout

Applied on `docker-2.supportedsystems.com`:

```bash
# Increase UDP conntrack timeout to 120s (matches stream timeout)
sudo sysctl -w net.netfilter.nf_conntrack_udp_timeout=120

# Make permanent
echo "net.netfilter.nf_conntrack_udp_timeout = 120" | \
  sudo tee /etc/sysctl.d/99-sip-conntrack.conf
```

**Before:**
- Unreplied UDP timeout: **30s** (too short for SIP Timer H)
- Stream UDP timeout: 120s (bidirectional flows)

**After:**
- Unreplied UDP timeout: **120s** ✅ (outlasts Timer H by 88s)
- Stream UDP timeout: 120s (unchanged)

### New Timeline with 120s Timeout

| Time | Event | Conntrack |
|------|-------|-----------|
| t+0-32 | INVITE/200 OK/ACK dance | Active |
| t+32 | Final 200 OK retransmit + ACK | ✅ **Entry still valid** |
| t+64 | Timer H would fire... | ✅ **But ACK was received!** |
| — | Call continues normally | Dialog established |

The conntrack entry now survives the entire Timer H window (32s) with **88 seconds of headroom**.

---

## Verification

### ⚠️ CRITICAL: Verify at the Host NIC Level FIRST

**Before assuming the conntrack fix worked**, capture at the physical NIC to prove ACKs are actually arriving:

```bash
# On the docker host (NOT inside containers)
sudo tcpdump -i enp1s0 -nn -s 0 -w /tmp/host-nic-sip.pcap port 5060

# During active test call, filter for ACK:
sudo tshark -r /tmp/host-nic-sip.pcap -Y 'sip.Method == "ACK"'

# Should see ACK packets. If zero ACKs appear at the host NIC,
# the loss is UPSTREAM (carrier/routing), not conntrack.
```

**Why this matters:** In the original production case, zero ACK packets reached the host NIC even after the conntrack fix was applied. The actual blocker was upstream carrier infrastructure (ClearFly ↔ Twilio seam), not conntrack. Container-level pcaps can't prove what never arrived.

### Confirm the conntrack fix took effect

```bash
# Check current value
cat /proc/sys/net/netfilter/nf_conntrack_udp_timeout
# Should output: 120

# Verify persistent config
cat /etc/sysctl.d/99-sip-conntrack.conf
# Should contain: net.netfilter.nf_conntrack_udp_timeout = 120
```

### Test call verification (only meaningful if host NIC shows ACKs arriving)

```bash
# 1. Place call to trunk
# 2. Let it run for >64 seconds
# 3. Verify call stays active
# 4. Check Asterisk logs - should NOT show Timer H expiry
```

---

## Architectural Improvements Made During Investigation

Even though the conntrack fix was NOT the production blocker in the original case, we made valuable architectural improvements that remain defense-in-depth measures:

### 1. ACK Fast-Path in sip-guardian

**File:** `l4handler.go:234-246`

ACKs now bypass ALL security checks (enumeration detection, rate limiting, validation, pattern matching). This prevents false-positive bans in deployments where ACKs DO reach the sip-guardian layer.

**Benefits:**
- Prevents enumeration false-positives from ACK retransmissions
- Prevents rate-limiting of legitimate mid-dialog ACKs
- Improves performance (one less layer of processing per ACK)
- More correct architecturally (ACKs are not a security threat)

### 2. Agent Thread Protocol Documentation

**Directory:** `docs/agent-threads/ack-loss-from-twilio-trunk/`

Created immutable message protocol for cross-layer debugging:
- 001: Problem report (calls dying at 64s)
- 002: Layer 2 diagnosis (ACK handling bug)
- 003: Confirmation to proceed with fix
- 004: Diagnosis refinement (rate-limiting, not enumeration)
- 005: Code fix delivered
- 006: Evidence that ACK loss is upstream (pcap)
- 007: Root cause found (conntrack timeout)

This protocol made the layered diagnosis **tractable** - each layer was isolated and verified independently.

---

## Related Issues and Patterns

### When This Same Issue Can Occur

**Trigger conditions:**
1. UDP SIP transport (TCP doesn't have this issue - persistent connection)
2. Long SIP transaction timers (INVITE transactions with >30s Timer H equivalent)
3. Default Linux conntrack timeout (30s for unreplied UDP)
4. NAT/conntrack between SIP endpoints

**Other protocols affected:**
- DNS (if using UDP with long timeouts)
- RADIUS (accounting sessions)
- Any UDP application-layer protocol with retransmission timers >30s

### Alternative Workarounds (If You Can't Change Conntrack)

1. **Switch to TCP transport** (`;transport=tcp` in SIP URI)
   - Pros: No conntrack timeout issues, guaranteed delivery
   - Cons: More overhead, persistent connection state

2. **SIP Session Timers** (periodic re-INVITE during call)
   - Pros: Keeps conntrack entry alive with regular traffic
   - Cons: More SIP messages, some endpoints don't support it

3. **Whitelist the trunk IPs in sip-guardian**
   - Pros: Bypasses security pipeline entirely, no processing delay
   - Cons: Loses enumeration/validation for that trunk
   - **SECURITY WARNING:** Use ONLY narrow trunk-specific IPs (e.g., `/30` subnets), not entire carrier infrastructure ranges

---

## Lessons Learned

### 1. Cross-Layer Debugging Requires Systematic Isolation

The bug spanned three independent layers:
- **Application:** SIP protocol timers (red herring)
- **Userspace:** sip-guardian security pipeline (real bug, wrong layer)
- **Kernel:** UDP conntrack timeout (actual blocker)

**Without systematic isolation**, we might have:
- Blamed Asterisk's timers (wrong)
- Stopped after fixing the ACK handling (incomplete)
- Never discovered the conntrack issue

### 2. Timing-Precise Failures Point to State Machine Conflicts

The **64-second precision** was the critical clue:
- Not random/intermittent (rules out packet loss, congestion)
- Not variable (rules out load-dependent issues)
- Exactly 2× Timer H (32s)

**Pattern:** When a failure occurs at a mathematically precise interval, look for:
- State machine timeouts
- Timer conflicts between independent subsystems
- Retry logic with exponential backoff hitting a cap

### 3. Packet Captures Don't Lie

The pcap **inside the sip-guardian container** was the smoking gun that proved the ACK never arrived at the userspace layer. Without it, we might have:
- Spent hours debugging sip-guardian code
- Blamed Twilio's implementation
- Never checked the kernel layer

**Lesson:** Capture at multiple layers:
- Wire-level (tcpdump on host interface)
- Container-level (tcpdump inside container)
- Application-level (SIP debug logs)

### 4. Defense in Depth: Fix All Layers

Even though the conntrack timeout was the blocker, the ACK fast-path fix is still valuable:
- Other deployments might not have conntrack issues but DO have enumeration detection
- Architectural correctness matters (ACKs shouldn't be treated as enumeration probes)
- Future-proofs against regressions

**Don't stop at the first fix that works** - fix the architectural issues you find along the way.

---

## References

- **RFC 3261 (SIP):** Section 17.1.1.2 - Timer H (INVITE transaction timeout, 32 seconds)
- **Linux conntrack docs:** `Documentation/networking/nf_conntrack-sysctl.txt`
- **Agent thread protocol:** `docs/agent-threads/PROTOCOL.md`
- **Related thread:** `/home/rpm/claude/sip/setup-server/docs/agent-threads/active-call-survival-hold-endpoint/`

---

## Quick Reference

### Symptoms Checklist

If you see:
- ✅ Calls dying at exactly 64 seconds
- ✅ "Timer H expired" or "no ACK received" in Asterisk logs
- ✅ UDP transport
- ✅ NAT/conntrack between SIP endpoints

Then check:
```bash
cat /proc/sys/net/netfilter/nf_conntrack_udp_timeout
```

If it's 30 seconds and you're using SIP, increase it to 120:
```bash
sudo sysctl -w net.netfilter.nf_conntrack_udp_timeout=120
echo "net.netfilter.nf_conntrack_udp_timeout = 120" | \
  sudo tee /etc/sysctl.d/99-sip-conntrack.conf
```

No restart required - takes effect immediately.