Several network protocols use checksums to ensure data integrity. Applying checksums as described here is also known as redundancy checking.
Wireshark will validate the checksums of many protocols, e.g. IP, TCP, UDP, etc.
It will do the same calculation as a “normal receiver” would do, and shows the checksum fields in the packet details with a comment, e.g. [correct] or [invalid, must be 0x12345678].
Checksum validation can be switched off for various protocols in the Wireshark protocol preferences, e.g. to (very slightly) increase performance.
If the checksum validation is enabled and it detected an invalid checksum, features like packet reassembly won’t be processed. This is avoided as incorrect connection data could “confuse” the internal database.
The checksum calculation might be done by the network driver, protocol driver or even in hardware.
For example: The Ethernet transmitting hardware calculates the Ethernet CRC32 checksum and the receiving hardware validates this checksum. If the received checksum is wrong Wireshark won’t even see the packet, as the Ethernet hardware internally throws away the packet.
Higher level checksums are “traditionally” calculated by the protocol implementation and the completed packet is then handed over to the hardware.
Recent network hardware can perform advanced features such as IP checksum calculation, also known as checksum offloading. The network driver won’t calculate the checksum itself but will simply hand over an empty (zero or garbage filled) checksum field to the hardware.
Checksum offloading often causes confusion as the network packets to be transmitted are handed over to Wireshark before the checksums are actually calculated. Wireshark gets these “empty” checksums and displays them as invalid, even though the packets will contain valid checksums when they leave the network hardware later.
Checksum offloading can be confusing and having a lot of [invalid] messages on the screen can be quite annoying. As mentioned above, invalid checksums may lead to unreassembled packets, making the analysis of the packet data much harder.
You can do two things to avoid this checksum offloading problem: