TCP tuning for high-latency links

February 2026

A single TCP flow can only have one window of data in flight per round trip. On a 250 ms path that limit shows up quickly: a 4 MB window caps you at about 128 Mbit/s no matter how fast the link is.

Bandwidth-delay product

Multiply bandwidth by RTT to get the window you need. 100 Mbit/s × 0.25 s is roughly 3 MB. Make sure the maximum socket buffers are comfortably above that.

net.core.default_qdisc = fq
net.ipv4.tcp_congestion_control = bbr
net.ipv4.tcp_rmem = 4096 87380 67108864
net.ipv4.tcp_wmem = 4096 65536 67108864

Congestion control

BBR models the path instead of reacting only to loss, which helps on long paths with occasional random loss. Always measure before and after; the right answer depends on the path.