Mar 3, 2020 · A 16-bit checksum only has 65,536 unique values, so it will be fairly common to repeat checksum values on multiple segments. What you have is not going to be unusual with different segment payloads. The source TCP calculates the checksum as it creates a segment, and the destination TCP calculates the checksum when it receives a segment.

May 12, 2016 · Because, the checksum is being calculated by the NIC, and not by the operating system. The wiki page you linked to did explain this: If you capture on a recent Ethernet NIC, you may see many such "checksum errors". This is due to TCP Checksum offloading often being implemented on those NICs and thus, for packets being transmitted by the machine.

You see the "incorrect" checksums due to a feature called TCP checksum offloading. The checksum fields for outgoing TCP packets are not pre-calculated by the operating system but instead set to 0 and left for calculation by the NIC processor. The Wireshark FAQ has a more detailed explanation.

Jan 13, 2019 · Checksum offload is generally quite reliable. The last NIC I knew of that had a bug around this was a 100 Mbps Sun hme interface, which had a problem with UDP checksum offloads for IPv6 only.

Oct 8, 2017 · If the TCP checksum is corrupted, then it will not match the TCP pseudo header and payload. There should only be one checksum that matches the pseudo header and payload, but there are multiple TCP pseudo header and payload combinations that will resolve to the same checksum. It is a one-way function.

May 28, 2020 · C rejects the packet because of an invalid checksum. The source of my confusion comes from the Wikipedia pages for UDP, TCP, and ICMPv6 which state that the checksum is computed with a pseudo-header where the next header field is set to the protocol: 17 for UDP, 6 for TCP, 58 for ICMPv6.

The checksum field is the 16 bit one's complement of the one's complement sum of all 16-bit words in the header and text. This seems a little crazy to me that the TCP checksum, which is placed in the header (as opposed to a footer), depends upon bytes sent after the header. This is a hindrance to hardware optimised TCP.

Something that should be noted here, and that most people overlook completely, is the fact, that the TCP checksum is actually a very poor checksum. so the checksum of TCP is no good at all?! so how can TCP be a reliable method when there is a chance that it transfers a corrupted packet and the end system won't notice it? how is this even possible?

May 16, 2013 · The complement is taken to make checksum validation simpler — instead of calculating the checksum again and then comparing the calculated value with the checksum field in the header (which is in the middle of the summed data), it is possible to sum (using one's complement arithmetic) all 16-bit words in the header (including the checksum word) and compare the result with zero.

Mar 3, 2017 · TCP checksum offloading. As already mentioned - is a technique where CPU does not calculate the TCP checksum but leaves this to NIC. The NIC may calculate it incorrectly. faulty layer 3 device. This needs to be at layer 3 as TCP checksum errors may occur after a successfull Ethernet CRC check - which is more robust than TCP checksum.