Checksum, IP-UDP-TCP checksum all are On or Off by default ??
IPv4 header checksum is a cheksum of header and it is enabled by default. Ipv4 leaves to upper layer the duty to detect possible errors in the payload.
To be noted checksum of IPv6 header should be disabled by default.
TCP checksum is enabled by default and covers all the paylod and the TCP header.
UDP checksum is enabled by default and covers payload and UDP pseudo header.
the reason for this is that before passing data to the application the TCP/IP stack, that is running on the operating system on the host, can decide if data is correct or not.
Hope to help