Network & Infrastructure / Segmentation, TLS & Secure Connectivity
IPsec overhead and the effective inner-packet size budget
An encrypted site link carries small requests while large database transfers stall. Encapsulation, padding and the actual outer path can reduce the usable inner packet size.
Environment and inputs: Stateful network boundaries, TLS endpoints and IPsec gateways; use the deployed security policy and implementation manual. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Record selected IPsec mode and algorithms, outer transport, NAT traversal and path MTUs. Compare packet-size failure thresholds with counters and feedback from the tunnel endpoints.
Explain the result
IPsec overhead is not one universal constant. The effective budget depends on negotiated protection and encapsulation, so a value copied from another tunnel may be wrong.
Plan the fix
Calculate and validate the deployed path budget, permit required feedback and apply scoped transport adjustment only where justified. Include lower-MTU backup links in the design.
Check the result and rollback
Test TCP, UDP and relevant IPv6 traffic at representative sizes across both paths. Restore prior settings if performance degrades or a new proposal changes overhead beyond the tested assumption.