👤 Owner: Customer (with IM guidance)
Phase 4: Critical - MSS Clamping
Why MSS Clamping is Required
- GRE adds 24 bytes of overhead (20 bytes IP + 4 bytes GRE)
- Default Ethernet MTU: 1500 bytes
- Without clamping: Packet fragmentation or drops
Required Configuration:
| Setting | Value |
|---|---|
| TCP MSS Clamp | 1436 bytes or lower |
| Tunnel MTU | 1476 bytes |
Verification Command:
curl https://mss.magictransit.tools/probe -H "Target:<customer_ip>:<port>"
Important: Apply MSS clamp on egress WAN interface, NOT the GRE tunnel interface.