Counting Lost ACKS on an IEEE802.11 Wireless Link

#!/usr/bin/env python2

import sys
import dpkt
import struct
import socket

# to collect data run the following commands:
#   iw phy0 interface add mon0 type monitor
#   ip link set dev mon0 up
#   tcpdump -i mon0 -w dump.pcap -s 128 wlan type data and ether dst $(cat /sys/class/net/wlan0/address)

# check argument
if len(sys.argv) < 2:
    print("please specify the dump file to read")
    sys.exit(1)

# open dump file
try:
    reader = dpkt.pcap.Reader(open(sys.argv[1]))
except IOError as e:
    print(e)
    sys.exit(1)
except dpkt.Error as e:
    print(e)
    sys.exit(1)

# check for ieee802.11 dump
if not reader.datalink() == dpkt.pcap.DLT_IEEE802_11_RADIO:
    print("pcap file is not from an 802.11 device")
    sys.exit(1)

# initialize values
subtypes = (
        dpkt.ieee80211.D_QOS_DATA,
        dpkt.ieee80211.D_DATA
        )
sequence_index = 22

# initialize counters
retries = 0
frames = 0
sequence = 0
fragment = 0
duplicates = 0
triples = 0
duplicate = 0
subtype = 0
errors = 0
other_frames = 0

# loop packets
for ts,data in reader:
    # read tap header length
    try:
        tap = dpkt.radiotap.Radiotap(data)
    except dpkt.Error as e:
        errors += 1
        continue

    tap_len = socket.ntohs(tap.length)

    # parse ieee80211 header
    try:
        wlan = dpkt.ieee80211.IEEE80211(data[tap_len:])
    except dpkt.Error as e:
        errors += 1
        continue

    # check frame type
    if wlan.subtype not in subtypes:
        other_frames += 1
        continue

    # read sequence control / fragment
    index = tap_len + sequence_index
    i, = struct.unpack("<H", data[index:index+2])
    seq = i >> 4
    frag = i & 0x000F

    # count retries
    if wlan.retry:
        retries += 1

    # check and count duplicates
    if wlan.retry and seq == sequence:
        if duplicate:
            triples += 1
        duplicates += 1
        duplicate = True
    else:
        duplicate = False

    # check for out-of-sequence by more than one
    if not wlan.retry \
            and seq > sequence + 2\
            and sequence\
            and wlan.subtype == subtype:
        print("missed frames {}:{}".format(sequence + 1, seq - 1))

    # check for out-of-sequence by one
    elif not wlan.retry\
            and seq > sequence + 1\
            and sequence \
            and wlan.subtype == subtype:
        print("missed frame {}".format(sequence + 1))

    # update counters
    frames += 1
    sequence = seq
    fragment = frag
    subtype = wlan.subtype

# print results
print("data frames:  {:6}".format(frames))
print("retries:      {:6}".format(retries))
print("duplicates:   {:6}".format(duplicates))
if triples:
    print("triplicates:  {:6}".format(triples))
if other_frames:
    print("other frames: {:6}".format(other_frames))
if errors:
    print("errors:   {:6}".format(errors))