[ovs-discuss] Flow miss/Packet order question

[Ben Pfaff]

I've known about this problem for years now, but I've always ignored it
because it didn't seem to cause trouble in practice.

I have a favorite straw-man approach.  It won't work as-is, but perhaps
it will spur someone to an idea that would work.  Suppose that we
changed the semantics of the kernel "flow_put" so that, when the kernel
puts the flow, it locks the socket buffer of the Netlink socket used to
send packets in that flow to userspace, scans it for other packets in
the flow, removes them from the socket buffer, and executes their
actions.

Problems with this idea include, at least:

        1. Nothing does anything like this with socket buffers (that is,
           there is no precedent for deleting packets queued to
           userspace from a socket buffer) and it seems wrong.

        2. Identifying the socket buffer or buffers to scan is
           problematic (maybe the client would have to identify them).

        3. The flow_put would have to arrange for any other packets
           arriving after it locks the socket buffer to block until it
           finishes processing the buffered packets, so that those
           arriving packets would be processed through the new flow in
           the normal way, but guaranteed to be ordered after the ones
           in the socket buffer.  (This probably isn't hard.)