* The next expected sequence number is one less than the current sequence number. Of the following conditions are true for that segment: If the single data byte from a Zero Window Probe is dropped by the receiver (notĪCKed), then a subsequent segment should not be flagged as retransmission if all Number, the segment size is one, and last-seen window size in the Set when the sequence number is equal to the next expected sequence It might take a long time (sometimes several minutes) to resume a paused connection, even if the underlying condition that caused the zero window clears up quickly. However, in most cases this indicates a performance or capacity problem on the receiving end. In some specific cases this is normal - for example, a printer might use a zero window to pause the transmission of a print job while it loads or reverses a sheet of paper. If the receiver can’t accept any more data it will set the window value to zero, which tells the sender to pause its transmission. The window field in each TCP header advertises the amount of data a receiver can accept. Set when the receive window size is zero and none of SYN, FIN, or RST are set. The acknowledgment number is equal to the last-seen acknowledgment number. The sequence number is equal to the next expected sequence number. The window size is non-zero and not equal to the last-seen window size. TCP Spurious RetransmissionĬhecks for a retransmission based on analysis data in the reverseĭirection. Set when the current sequence number is greater than the next expected sequence number. Set when the SYN flag is set (not SYN+ACK), we have an existing conversation using the same addresses and ports, and the sequence number is different than the existing conversation’s initial sequence number. The threshold is either the value shown in the “iRTT” (_rtt) field under “SEQ/ACK analysis” if it is present, or the default value of 3ms if it is not. The last segment arrived within the Out-Of-Order RTT threshold. The next expected sequence number and the next sequence number differ. The next expected sequence number is greater than the current sequence number. In the forward direction, the segment length is greater than zero or the SYN or FIN is set.
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