Internet Key Exchange version 2 (IKEv2) is a cornerstone protocol for modern IPsec VPNs, offering robust authentication, negotiated cryptographic parameters, and support for mobility and multihoming. One critical but often overlooked aspect of IKEv2 deployments is the rekey interval — the frequency with which IKE_SA (IKE Security Associations) and Child SAs are refreshed. Carefully selecting rekey intervals impacts both the security posture and the performance/availability of a VPN service. This article explores the technical mechanics behind rekeying, trade-offs between security and performance, practical configuration guidance, and monitoring strategies tailored to site operators, enterprise IT teams, and developers maintaining IKEv2-based VPNs.

Why Rekeying Matters

Rekeying is the periodic renewal of cryptographic material to limit the amount of data encrypted under a single key and to ensure forward secrecy. In IKEv2, two types of SAs are relevant:

  • IKE_SA (the control plane SA) — maintains the negotiation context and protects IKE messages.
  • Child SAs (the data plane SAs) — carry the actual IPsec-protected traffic using ESP or AH.

Both SA types have lifetimes expressed in seconds and/or byte limits. When a lifetime expires, peers either rekey (create a new SA) or establish a new set of SAs. Proper lifetimes mitigate cryptographic risks (key compromise, ciphertext analysis) while limiting operational overhead (CPU load, latency spikes, reconnects).

Security Considerations

From a security standpoint, shorter rekey intervals reduce the window of exposure if a key is compromised and lower the volume of data encrypted under one key. Key security concerns include:

  • Key compromise window: If an attacker obtains the current key, shorter lifetimes limit the usable timeframe.
  • Cipher-specific limits: Some ciphers (e.g., AES-GCM) and modes have recommended limits on how many packets or bytes can be securely encrypted with a single key. These limits should influence rekeying based on byte counters as well as time.
  • Perfect Forward Secrecy (PFS): Enforcing PFS (e.g., using a fresh Diffie-Hellman exchange during rekey) prevents past session keys from being derived from future ones if long-term keys are compromised.

For high-security environments, enforcing both short time-based lifetimes and byte-based thresholds — plus mandatory PFS — is common practice. However, that comes at a performance cost.

Performance and Availability Implications

Rekey operations are not free. They introduce CPU and memory overhead on VPN gateways and may momentarily impact packet flow during re-authentication or SA replacement. Key performance considerations include:

  • CPU and crypto acceleration: Frequent rekeys increase the number of ephemeral Diffie-Hellman operations and symmetric key setups. On high-throughput gateways, lack of hardware crypto offload can become a bottleneck.
  • Latency spikes: Rekey negotiation requires message exchanges. Although often sub-second, they can coincide with peak traffic, producing jitter or transient packet loss if not handled gracefully.
  • Session stability for roaming clients: Mobile clients switching networks may rely on long-lived IKE_SAs for seamless handover. Excessively short lifetimes can increase the chance of session interruptions.
  • Scalability: For large numbers of concurrent VPN clients, synchronized rekey events can create CPU spikes. Staggering or randomizing rekey intervals avoids synchronized load.

IKEv2 Rekey Mechanics: What Options Do You Have?

IKEv2 defines several parameters that control rekeying:

  • SA lifetime in seconds (KeyLifetimeSeconds)
  • SA lifetime in kilobytes/bytes (KeyLifetimeBytes)
  • The use of Child SA rekeying versus creating new Child SAs
  • PFS requirement during rekey (which DH group to use)
  • Maximum number of rekey attempts and retransmission timers

Different implementations expose these settings with varying defaults. For example, many IPsec stacks default to 8 hours (28800 seconds) for IKE SA and 1 hour (3600 seconds) for Child SAs, but enterprise policies often mandate shorter durations.

Child SA vs IKE SA Rekeying

Child SAs should generally be rekeyed more frequently than the IKE SA. The IKE SA is primarily a control channel; rekeying it is heavier and less frequent. When a Child SA expires, it can be rekeyed without tearing down the IKE SA. This separation allows balancing security with convenience.

Byte Limits and Cipher Considerations

Byte-based rekeying is essential for ciphers with nonce reuse or bounds on safe data volume (e.g., AES-GCM has practical per-key byte limits). For high-throughput tunnels, a reasonable approach is to combine a moderate time-based interval (e.g., 1–2 hours) with a byte threshold (e.g., 1–10 GB) to trigger a rekey before cipher limits are approached.

Practical Recommendations

There’s no one-size-fits-all interval. Choose lifetimes based on threat model, throughput, client mobility needs, and hardware capabilities. Below are pragmatic guidelines to help you decide.

Conservative (High Security)

  • IKE SA lifetime: 1–2 hours (3600–7200 seconds)
  • Child SA lifetime: 20–60 minutes (1200–3600 seconds)
  • Byte limit: 1–5 GB per Child SA
  • Require PFS on rekey (use at least DH group 14 / 2048-bit or stronger)
  • Use AEAD ciphers (AES-GCM or ChaCha20-Poly1305) but enforce byte counters

This is suitable for environments handling highly sensitive data or regulated traffic.

Balanced (Most Enterprise Deployments)

  • IKE SA lifetime: 4–8 hours (14400–28800 seconds)
  • Child SA lifetime: 1 hour (3600 seconds)
  • Byte limit: 5–20 GB per Child SA
  • PFS recommended on periodic rekeys (e.g., every few Child SA renewals)

This provides a reasonable compromise between security and CPU/latency overhead for typical corporate VPN usage.

Performance-Optimized (Throughput-Critical)

  • IKE SA lifetime: 8–24 hours
  • Child SA lifetime: 2–6 hours
  • Byte limit: high thresholds, but monitor cipher-specific bounds
  • Consider PFS less frequently or schedule PFS rekey windows to off-peak hours
  • Ensure hardware crypto acceleration and staggered rekey schedules

Use this only when operational continuity and throughput are prioritized over the minimal cryptographic exposure window.

Configuration Tips Across Popular Implementations

Implementation details vary, but these common tips apply:

  • Explicitly set both time and byte lifetimes — relying on defaults risks insecure or suboptimal behavior.
  • Use staggered or randomized rekey timers for large user bases to avoid thundering-herd problems.
  • Enable monitoring and alerting on rekey failures, retransmission counts, and SA churn rates.
  • Test rekey behavior under load and during network transitions (e.g., mobile roaming) to ensure acceptable user experience.

For example, on strongSwan you commonly set lifetime and lifetime_data in your conn definitions; on Windows RRAS and many hardware VPNs you will see separate fields for IKE SA and IPsec SA durations. Consult the vendor documentation to apply both time and byte limits.

Monitoring and Troubleshooting Rekey Events

Operational visibility is crucial. Key metrics to collect:

  • SA creation and deletion rates — sudden spikes indicate instability.
  • Rekey failure counts and retransmission rates — high values suggest MTU/path issues, asymmetric routing, or misconfigurations.
  • CPU and hardware crypto utilization correlated with rekey events.
  • Client-side logs for mobile clients (e.g., IKEv2 MOBIKE behaviors).

Common issues and fixes:

  • Frequent rekeys causing CPU overload — increase lifetimes or enable hardware offload.
  • Rekey failures due to MTU fragmentation — ensure IKE fragmentation (RFC 7383) is enabled or adjust MTU and use MSS clamping.
  • Synchronized rekeys causing load spikes — add jitter/randomization to lifetimes or implement phased rekeying.

Advanced Considerations

Two advanced topics deserve attention for high-scale or high-security deployments:

Mobility and Multihoming (MOBIKE)

For mobile clients using MOBIKE, frequent IKE SA rekeys can interfere with seamless handovers. When designing rekey policies for mobile workforces, prefer longer IKE SA lifetimes with more aggressive Child SA rotation, and ensure MOBIKE support is robust in your client/server stack.

Interoperability Between Vendors

Different vendors have different defaults and sometimes incompatible rekey behavior (e.g., enforcing PFS on every rekey). When managing heterogeneous endpoints, explicitly configure SA lifetimes on both ends and test cross-vendor scenarios to avoid unexpected SA teardowns.

Summary

Optimizing IKEv2 rekey intervals requires balancing competing priorities: reducing cryptographic exposure and byte limits versus keeping CPU load, latency, and connection stability within acceptable bounds. Use a risk-based approach:

  • Assess your threat model and compliance requirements to determine acceptable lifetimes.
  • Combine time-based and byte-based triggers, and enforce PFS as appropriate.
  • Instrument and monitor your VPN infrastructure to observe the operational impact of your choices.
  • Stagger rekey schedules and leverage hardware crypto where possible to scale smoothly.

With careful tuning — for example, 1 hour Child SA lifetimes with byte limits for security-sensitive tunnels or 2–6 hour lifetimes for throughput-critical ones — most deployments can strike an effective balance between security and performance.

For more technical guides and configuration examples on secure and optimized VPN deployments, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.