what does wireshark check when it reports the error " new fragment overlaps old data"

Question

what does wireshark check when it reports the error " new fragment overlaps old data"? i mean what kind of analysis does wireshark does when it reports this error in TCP ?

Answer 1

Lower layer packets (IP) should contain each fragment of the higher layer packet (TCP) only once. If a lower layer packet contains fragment data which is also found in another lower layer packet, this gets reported.

              +-----------+
              |bytes 1..37|
              +-----------+
          Higher layer packet
+———–+ +————+ +————+
|bytes 1..20| |bytes 21..30| |bytes 27..37|
+———–+ +————+ +————+
packet 1       packet 2       packet 3

This is an example of three fragments which overlap bytes 27, 28, 29 and 30, which are found in lower layer packet 2 and 3.

              +———–+
|bytes 1..37|
+———–+
Higher layer packet
+———–+ +————+ +————+
|bytes 1..20| |bytes 21..37| |bytes 21..37|
+———–+ +————+ +————+
packet 1       packet 2       packet 3

This is an example of three fragments where packet 3 is a retransmission of the fragment already seen in packet 2. This is a common cause of overlapping fragment data.