A Packet's Journey Through the OpenBSD Network Stack

When debugging network issues, it is important to understand when
certain things happen.  Tcpdump provides valuable insight, pf
transforms packets, pseudo devices add features, and netstat counters
show action.  The call graph of the functions within the kernel is
the base to comprehend the relation between these sources of
information.

The layering of kernel code in hardware drivers, pseudo devices,
IP processing, forwarding and protocol layer is explained.  The
kernel provides the socket interface to userland processes.  Packet
forwarding happens within the kernel.  Bridge code uses certain
shortcuts.  pf is a swiss knife that can manipulate traffic in
multiple layers.  IPsec has an independent interface that overrides
routing.  Routing itself and neighbor discovery is a necessary step
that has its tentacles everywhere.  Checksum calculation can be
performed by hardware offloading.

By using examples with a single packets, their way through the
kernel is shown.  The possible branches, configuration options, and
measurement output are put in correlation.