(I understand that nox-classic is no longer maintained. However, if pox
is using some of the same methodology, maybe this issue still exists.)
Packets received via a Packet_In can either be dealt with by a Flow Mod
message (ofp_flow_mod) or by a Packet Out (ofp_packet_out)
message, or by not responding.
The method of dropping packets by having an empty action list in a Flow
Mod rule is ok by OpenFlow 1.0 spec:
3.3 Actions
Each flow entry is associated with zero or more actions that dictate
how the
switch handles matching packets. If no forward actions are present,
the packet
is dropped.
Suppose I get a Packet In with a packet that I want to just drop on the
floor. If I was given the full packet and got no buffer_id, then it is
ok to just ignore the packet as the switch did not keep a copy.
But, if I got a buffer id for the packet, it is NOT ok to just not
respond. The action of the switch in such a case seems to be undefined
(in OF 1.0 at least)
A switch must gracefully handle the case where a bu ffered
packet_in message yields no response from the controller.
It makes sense that the switch would actually deliver that packet rather
than drop it (thinking the controller was too slow to respond).
In any case, you don't want the switch hanging onto that packet buffer
when it could be freed.
To drop the packet's buffer, without using a flow rule, I should do
something like a send the packet for this buffer_id, but with an empty
action list.
Unfortunately, it appears that the only function like that that takes a
buffer_id is send_openflow_buffer, but that function throws an error if
the action list is empty. Why it does so isn't documented in the
function. I don't see a reason for this in the OF 1.0 spec.
As a workaround, I put in a single action of rewriting the dst MAC
address with 0.
