TCP WindowUpdate - This indicates that the segment was a pure WindowUpdate segment. TCP ZeroWindowViolation - The sender has ignored the zero window condition of the receiver and sent additional bytes of data. If the window is still zero, the sender will double his persist timer before probing again. TCP ZerowindowProbe - The sender is testing to see if the receiver's zero window condition still exists by sending the next byte of data to elicit an ACK from the receiver. Indicates a resource issue on the receiver, as the application is not retrieving data from the TCP buffer in a timely manner. This effectively tells the sender to stop sending because the receiver's buffer is full. TCP ZeroWindow - Occurs when a receiver advertises a receive window size of zero. A clear indication of dropped/missing packets. If the receiver detects a gap in the sequence numbers, it will generate a duplicate ACK for each subsequent packet it receives on that connection, until the missing packet is successfully received (retransmitted). TCP DupACK - Occurs when the same ACK number is seen AND it is lower than the last byte of data sent by the sender. ACK packet sent in response to a "keep-alive" packet. TCP Keep-Alive - Occurs when the sequence number is equal to the last byte of data in the previous packet. This event is a good indicator of packet loss and will likely be accompanied by "TCP Retransmission" events. TCP Previous segment lost - Occurs when a packet arrives with a sequence number greater than the "next expected sequence number" on that connection, indicating that one or more packets prior to the flagged packet did not arrive. TCP_Out-of-order - Occurs when a packet is seen with a sequence number lower than the previously received packet on that connection. Senders should Fast Retransmit upon receipt of 3 duplicate ACKs. Senders receive some packets which sequence number are bigger than the acknowledged packets. TCP Fast Retransmission - Occurs when the sender retransmits a packet before the expiration of the acknowledgement timer. TCP Retransmission - Occurs when the sender retransmits a packet after the expiration of the acknowledgement. When this feature is enabled the sliding window monitoring inside Wireshark will detect and trigger display of interesting events for TCP such as : This feature should not impact too much on the run-time memory requirements of Wireshark but can be disabled if required. This allows much better and more accurate measurements of packet-loss and retransmissions than is available in any other protocol analyzer. This requires some extra state information and memory to be kept by the dissector but allows much better detection of interesting TCP events such as retransmissions. See the TCP Analysis section of the User's Guide for more up to date documentation.īy default Wireshark and TShark will keep track of all TCP sessions and implement its own crude version of Sliding_Windows. ![]() But if there's any chance they're invalid then they can cause this sort of pain.This page might not be accurate. Resets are better when they're provably the correct thing to send. ![]() It's better to drop a packet then to generate a potentially protocol disrupting tcp reset. m state -state RELATED,ESTABLISHED -j ACCEPT A FORWARD -m state -state INVALID -j DROPīasically anytime you have. This should instead be: -A FORWARD -m state -state RELATED,ESTABLISHED -j ACCEPT Reordering is particularly likely with a wireless network. Then packet reordering can result in the firewall considering the packets invalid and thus generating resets which will then break otherwise healthy connections. A FORWARD -p tcp -j REJECT -reject-with tcp-reset If you have something like: -A FORWARD -m state -state RELATED,ESTABLISHED -j ACCEPT One thing to be aware of is that many Linux netfilter firewalls are misconfigured.
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