IP-in-IP MTU Calculator
IP-in-IP adds about 20 bytes to every packet, so over a normal 1500-byte link the inner MTU is 1480.
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Encapsulation in the Path
Inner MTU
1500 − 20 bytes of headers
TCP MSS (IPv4)
TCP MSS (IPv6)
Largest Ping (IPv4)
Clamp MSS (nftables)
nft add rule inet filter forward tcp flags syn tcp option maxseg size set 1440
MTU & MSS Calculator
IP-in-IP on a 1500-byte link
Overhead20 bytes
Inner MTU1480
TCP MSS (IPv4)1440
TCP MSS (IPv6)1420
Over PPPoE (1492)1472
Testping -M do -s 1452 <host>
If the inner interface keeps an MTU of 1500, packets larger than 1480 bytes are fragmented or dropped. The usual symptom: ping and small requests work, large downloads and TLS handshakes hang.
Set the IP-in-IP interface's MTU to 1480, or clamp TCP MSS to 1440 on the router.
MTU by Tunnel
Questions
What MTU should I use for IP-in-IP?
1480 over a standard 1500-byte link; 1472 over PPPoE.
How much overhead does IP-in-IP add?
About 20 bytes per packet.
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