tcpdump and wifi retransmissions

Question

With tcpdump, I am dumping traffic going out of and coming in from a wifi interface.

Are wifi retransmissions captured by tcpdump?

I am measuring the round-trip time of packets, so knowing this would greatly affect my measurements.

Answer 1

It is indeed possible to use Wireshark to monitor for retransmissions as mentioned here. But it probably depends on which machine you do the sniffing - a sending host may not make the retransmitted frames available to Wireshark running on that machine as they may be handled directly by the Wi-Fi card. However if the packets were monitored on another machine then you should see them. For more info on retransmissions see this question/answer.

Generally to see link layer interactions you will need to utilise Monitor mode capture and may need to play with the WLAN Interface Settings, such as Promiscuous mode (some cards need Promiscuous mode on, whilst others need it off to capture traffic).

It is possible to filter for link-layer retransmissions in Wireshark using the following filter:

wlan.fc.retry == 1

Answer 2

According to this link TCP Timeout and Retransmission retransmissions are also captures.

Take a look at Figure 21.1: tcpdump output

Answer 3

As far as I know, there is no wifi re-transmission at the physical link layer. It does however check for interference signal before initiating transmission. When there is a strong interference signal on the same channel detected, transmission is deferred. However, if an interference signal comes in during transmission and cause error, the error is detected as packet lost or corrupted packet at the data link layer. So I think you should be able to capture all re-transmission using wireshark and I suppose tcpdump as well.