Did you know that FreeBSD has more than one TCP stack and that TCP stacks are pluggable at run time? Since FreeBSD 12, FreeBSD has support pluggable TCP stacks, and today we will look at the RACK TCP Stack. The FreeBSD RACK stack takes this pluggable TCP feature to an extreme: rather than just swapping the congestion control algorithm, FreeBSD now supports dynamically loading and an entirely separate TCP stack. With the RACK stack loaded, TCP flows can be handled either by the default FreeBSD TCP stack or by the RACK stack.

While new protocols are constantly being developed, the venerable Transmission Control Protocol (TCP) still accounts for most global traffic. The FreeBSD kernel TCP stack offers a lot of opportunities to tweak different performance features. The options it includes allow a lot of flexibility in the configuration of machines without having to do custom kernel builds.
Find out how to make use of the Initial Window, what the TCP Segment OffLoad is, and how to use TCP Buffer Tuning to your advantage.