Rare occurence of non-zero ACK packets. The source system is a Linux box,
but it was most likely running some evil tool. Note the source port of
21.

Same in ack_set2.

80.117.4.92:21 - UNKNOWN [56922:113:0:40:.:A:?:?]
  -> 217.8.32.51:21 (link: unspecified)
  -- EXTRA TCP VALUES: ACK=0x41e8a62a, UNUSED=0, URG=0x0
80.117.4.92:21 - UNKNOWN [56922:113:0:40:.:A:?:?]
  -> 217.8.32.52:21 (link: unspecified)
  -- EXTRA TCP VALUES: ACK=0x41e8a62a, UNUSED=0, URG=0x0
80.117.4.92:21 - UNKNOWN [56922:113:0:40:.:A:?:?]
  -> 217.8.32.53:21 (link: unspecified)
  -- EXTRA TCP VALUES: ACK=0x41e8a62a, UNUSED=0, URG=0x0
