Problem - How to diagnose when users see Google Re-Captcha accessing the website?
Solution - Run and capture a session using any packet analyzer. In case, the packet capture shows too many packets for any of the below-
- TCP Spurious Retransmission
- TCP Dup ACK
- TCP Retransmission
- TCP Out-of-Order
Different TCP Packets Explained
- When TCP timer expires, it is assumed that the packet is lost, and TCP retransmits the same packet.
- Duplicate acknowledgements are generated by TCP to retransmit a packet if lost and are a part of retransmission and packet recovery.
- TCP starts a retransmission timer when each outbound segment is handed down to IP. If no acknowledgment has been received for the data in a given segment before the timer expires, the segment is retransmitted
- Spurious packet retransmission can be caused by either spurious timeout (STO) or spurious fast retransmit (SFR). The “lost” packets are unnecessarily retransmitted and the evoked congestion control process causes network underutilization.
- TCP out-of-order can mean particular frame was received in a different order from which it was sent (after a later packet in the sequence). It may indicate that there are multiple paths between source and destination - and one travels a through a longer path.
All the above is noticed by Google and categorized as unusual traffic which causes to display the Google Re-Captcha to ensure that the user accessing the website is Genuine and not a robot.
To read more about this, follow the link below to Google Support Documentation
"Unusual traffic from your computer network" - Google Search Help