Høiland-Jørgensen: The inner workings of TCP zero-copy

Since the memory is being copied directly from userspace to the
network device, the userspace application has to keep it around
unmodified, until it has finished sending. The sendmsg()
syscall itself is asynchronous, and will return without waiting for
this. Instead, once the memory buffers are no longer needed by the
stack, the kernel will return a notification to userspace that the
buffers can be reused.