Shows how headers wrap payload data inside the physical MTU frame size limit.
Calculate effective TCP Maximum Segment Size (MSS) and optimal interface MTU after accounting for WireGuard, OpenVPN, IPsec ESP, GRE, and L2TP encapsulation overheads. Eliminate packet fragmentation and black-hole dropouts.
Calculate effective Maximum Segment Size (MSS) and interface MTU after accounting for VPN, transport, and tunnel layer encapsulation overheads.
Misconfigured MTU is a primary cause of connection dropouts, slow downloads, or partial page loads over VPNs.
Run the following command to check if a specific payload size can pass without fragmentation. If it fails, reduce the size until it passes.
ping -f -l 1472 8.8.8.8
Run the following command. The -M do flag sets the Don't Fragment (DF) bit. If you get "frag needed", reduce the packet size.
ping -M do -s 1472 8.8.8.8
MSS Clamping is a router-level technique that intercepts the TCP 3-way handshake SYN packets and alters the declared Maximum Segment Size (MSS) option to match the path's bottleneck MTU. Since it operates during the handshake phase, it automatically prevents clients from sending oversized packets, completely resolving "black hole" router dropouts without needing manual adjustments on client endpoints.
Ethernet interfaces default to a 1500-byte MTU. When packets are encapsulated within VPN tunnels (WireGuard, IPsec, OpenVPN), header overhead reduces the available payload capacity.
1420 (1500 - 80B over IPv6).