Properly count records for IPFIX and NetFlow v9

IPFIX and NetFlow v9 have different semantics for the sequence number
carried in the header.

src/apps/ipfix/ipfix.lua

@@ -89,10 +89,10 @@ local function padded_length(len)
  return bit.band(len + max_padding, bit.bnot(max_padding))
 end
 
--- Sadly, for NetFlow v9, the header needs to know the number of
--- records in a message. So before flushing out a message, a FlowSet
--- will append the record count, and then the exporter needs to slurp
--- this data off before adding the NetFlow/IPFIX header.
+-- The header needs to know the number of records in a message. So
+-- before flushing out a message, a FlowSet will append the record
+-- count, and then the exporter needs to slurp this data off before
+-- adding the NetFlow/IPFIX header.
 local uint16_ptr_t = ffi.typeof('uint16_t*')
 local function add_record_count(pkt, count)
  pkt.length = pkt.length + 2
@@ -358,7 +358,13 @@ function IPFIX:send_template_records(out)
  for _, flow_set in ipairs(self.flow_sets) do
  pkt = flow_set:append_template_record(pkt)
  end
- add_record_count(pkt, #self.flow_sets)
+ if self.version == 9 then
+ add_record_count(pkt, #self.flow_sets)
+ else
+ -- For IPFIX, template records are not accounted for in the
+ -- sequence number of the header
+ add_record_count(pkt, 0)
+ end
  link.transmit(out, pkt)
 end
 
@@ -367,18 +373,23 @@ function IPFIX:add_ipfix_header(pkt, count)
  local header = ffi.cast(self.header_ptr_t, pkt.data)
 
  header.version = htons(self.version)
+ header.sequence_number = htonl(self.sequence_number)
  if self.version == 9 then
+ -- record_count counts the number of all records in this packet
+ -- (template and data)
  header.record_count = htons(count)
+ -- sequence_number counts the number of exported packets
+ self.sequence_number = self.sequence_number + 1
  header.uptime = htonl(to_milliseconds(engine.now() - self.boot_time))
  elseif self.version == 10 then
+ -- sequence_number counts the cumulative number of data records
+ -- (i.e. excluding template and option records)
+ self.sequence_number = self.sequence_number + count
  header.byte_length = htons(pkt.length)
  end
  header.timestamp = htonl(math.floor(C.get_unix_time()))
- header.sequence_number = htonl(self.sequence_number)
  header.observation_domain = htonl(self.observation_domain)
 
- self.sequence_number = self.sequence_number + 1
-
  return pkt
 end