For fast datapath, the router core will only be invoked upon datapath
misses. And it needs to yield routing decisions in a form that can be
used to configure the in-kernel datapath. So here we make the router
core yield FlowOps, which represent operations which actually do
something with a packet. The router core obtains these in turn from
the Bridge or the Overlay, depending on what it wants to do with
a packet.
For fast datapath, it's critical that the router makes decisions based
only on the required packet header information (as the same header
information will be used when configuring the ODP flow rules). So we
pass only that information into the router core, in the form of
PacketKeys.
The overall effect is to curry the router core: Rather than taking a
packet and doing something with it, it takes packet header information
and returns a function that does something with a packet.
If gopacket is compiled against an older pcap.h that doesn't have
pcap_set_immediate_mode, it supplies a dummy definition that
always returns PCAP_ERROR. That becomes "Generic error", which
is not very helpful. The real pcap_set_set_immediate never
returns PCAP_ERROR, so this turns it into a more informative
message.
Bryan Boreham