  The Transmission Control Protocol (TCP) is one of the most important
protocols in today's Internet. Its specification and implementations have been
refined for almost forty years. The Linux TCP stack is one of the most widely
used TCP stacks given its utilisation on servers and Android smartphones and
tablets. However, TCP and its implementations evolve very slowly. In this
paper, we demonstrate how to leverage the eBPF virtual machine that is part of
the recent versions of the Linux kernel to make the TCP stack easier to extend.
  We demonstrate a variety of use cases where the eBPF code is injected inside
a running kernel to update or tune the TCP implementation. We first implement
the TCP User Timeout Option. Then we propose a new option that enables a client
to request a server to use a specific congestion control scheme. Our third
extension is a TCP option that sets the initial congestion window. We then
demonstrate how eBPF code can be used to tune the acknowledgment strategy.
