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87 lines
2.9 KiB
Markdown
87 lines
2.9 KiB
Markdown
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## 802.11 packet injection
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The Linux wireless networking stack (i.e. driver, mac80211, cfg80211, net_dev, user app) is a robust implementation supporting a plethora of wireless devices. As robust as it is, it also has a drawback when it comes to single-layer testing.
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Ping and Iperf are well established performance measurement tools. However, using such tools to measure 802.11 PHY performance can be misleading, simply because they touch multiple layers in the network stack.
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Luckily, the mac80211 Linux subsystem provides packet injection functionality and it allows us to have finer control over physical layer testing.
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To this end, we have adapted a [packetspammer](https://github.com/gnychis/packetspammer) application originally written by Andy Green <andy@warmcat.com> and maintained by George Nychis <gnychis@gmail.com>.
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### inject_80211
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Userspace program to inject 802.11 packets through mac80211 supported (softmac) wireless devices.
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### Options
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```
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-m/--hw_mode <hardware operation mode> (a,g,n)
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-r/--rate_index <rate/MCS index> (0,1,2,3,4,5,6,7)
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-i/--sgi_flag (0,1)
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-n/--num_packets <number of packets>
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-s/--payload_size <payload size in bytes>
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-d/--delay <delay between packets in usec>
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-h this menu
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```
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### Example:
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```
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iw dev wlan0 interface add mon0 type monitor && ifconfig mon0 up
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inject_80211 -m n -r 0 -n 64 -s 100 mon0 # Inject 10 802.11n packets at 6.5Mbps bitrate and 64bytes size
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```
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### Link performance test
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To make a profound experimental analysis on the physical layer performance, we can rely on automation scripts.
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The following script will inject 100 802.11n packets at different bitrates and payload sizes.
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```
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#!/bin/bash
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HW_MODE='n'
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COUNT=100
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DELAY=1000
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RATE=( 0 1 2 3 4 5 6 7 )
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SIZE=( $(seq -s' ' 50 100 1450) ) # paload size in bytes
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IF="mon0"
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for (( i = 0 ; i < ${#PAYLOAD[@]} ; i++ )) do
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for (( j = 0 ; j < ${#RATE[@]} ; j++ )) do
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inject_80211 -m $HW_MODE -n $COUNT -d $DELAY -r ${RATE[$j]} -s ${SIZE[$i]} $IF
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sleep 1
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done
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done
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```
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On the receiver side, we can use tcpdump to collect the pcap traces.
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```
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iw dev wlan0 interface add mon0 type monitor && ifconfig mon0 up
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tcpdump -i mon0 -w trace.pcap 'wlan addr1 ff:ff:ff:ff:ff:ff and wlan addr2 66:55:44:33:22:11'
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```
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Wlan addresses *ff:ff:ff:ff:ff:ff* and *66:55:44:33:22:11* are specific to our injector application.
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Next, we analyze the collected pcap traces using the analysis tool provided.
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```
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analyze_80211 trace.pcap
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```
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An excerpt from a sample analysis looks the following
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```
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HW MODE RATE(Mbps) SGI SIZE(bytes) COUNT Duration(sec)
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======= ========== === =========== ===== =============
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802.11n 6.5 OFF 54 100 0.11159
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802.11n 13.0 OFF 54 100 0.11264
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802.11n 19.5 OFF 54 100 0.11156
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802.11n 26.0 OFF 54 100 0.11268
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802.11n 39.0 OFF 54 100 0.11333
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802.11n 52.0 OFF 54 100 0.11149
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802.11n 58.5 OFF 54 100 0.11469
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802.11n 65.0 OFF 54 100 0.11408
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```
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