HA Session Synchronization Queue Drop under Extreme TCP Session Creation Burst
FortiOS High Availability (FGCP) cluster primary nodes experience kernel session-sync ring buffer overflow when sudden TCP connection creation bursts exceed 60,000 cps, silently dropping state packets to secondary units and causing mass session resets during failover.
Technical Root Cause Analysis
In a FortiGate FGCP cluster (Active-Passive or Active-Active), the primary appliance synchronizes connection state information to subordinate cluster members across dedicated heartbeat interfaces (typically ha1 and ha2). Each state change—including TCP SYN-ACK progression, sequence validation, NAT port mapping, and helper session state—is queued in the kernel hasync ring buffer.
In FortiOS builds 7.2.3 through 7.2.6, the kernel ring buffer is configured with a static depth of 4,096 entries per queue. When sudden connection bursts occur (such as microservices scaling, database failovers, or bulk API processing generating >60,000 connections/sec), the producer thread in the network stack exhausts the ring buffer faster than the transmission thread can serialize frames onto the heartbeat Ethernet medium. As a result, the kernel discards session synchronization frames without backpressure signaling to the forwarding engine. The secondary node's session table drifts out of alignment, creating silent state holes.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Workaround & Remediation |
|---|---|---|---|
| FortiOS 7.2 | 7.2.3 – 7.2.6 | 7.2.7+ | Apply session-sync-filter and expand HA queue depth |
| FortiOS 7.4 | 7.4.0 – 7.4.1 | 7.4.2+ | Upgrade recommended for dynamic ring sizing |
| FortiOS 7.0 | 7.0.10 – 7.0.14 | 7.0.15+ | Tune session sync filter for non-TCP protocols |
Platform Impact: Predominantly impacts enterprise HA clusters (FG-100F, FG-200F, FG-400F, FG-600F, and FG-1000F) deployed in high-throughput datacenter perimeters and financial application gateways.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands to check HA cluster synchronization state, inspect heartbeat interface packet discards, and verify session sync queues:
Diagnostic Commands
# 1. Check overall HA cluster status and node synchronization state
diagnose sys ha status
# 2. View global session table counters and active sessions
diagnose sys session stat
# 3. Check HA session synchronization drop and transmit statistics
diagnose sys session sync-stat
# 4. Dump heartbeat interface packet drop counters (replace 'ha1' with your HB port)
diagnose sys ha dump-by dev ha1Remediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the Ferrite interactive platform:
Frequently Asked Questions
During sudden traffic surges (>60,000 cps), session state generation exceeds heartbeat transmission throughput, overflowing the kernel ring buffer and causing silent packet drops.
Standard SNMP traps may not trigger immediately because heartbeat links remain UP. The issue only manifests when drop counters increment in diagnose sys session sync-stat or when failover resets active connections.
Excluding high-frequency, stateless traffic (like DNS queries, ICMP pings, and short UDP streams) reduces heartbeat sync volume by up to 60%, allowing critical enterprise TCP sessions to be synchronized without drops.