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Linux Checksum

union {

    __wsum                       csum;
    struct {
        __u16  csum_start;
        __u16  csum_offset;
   };
};

Main Variables/Direction	skb→ip_summed	skb→csum / csum_start and csum_offset
Input	CHECKSUM_NONE 0	csum valid	SW need do L4 payload + pseudo header verification
	CHECKSUM_UNNECESSARY 1	csum valid	HW already do L4 payload + pseudo header verification
	CHECKSUM_COMPLETE 2	csum valid (stored with L4 payload checksum)	SW need to do skb→csum + pseudo header verification
Output	CHECKSUM_NONE 0	csum_start/end valid	SW already do necessary L4 checksum calculation
	CHECKSUM_PARTIAL 3	csum_start/end valid	SW already do pseudo header and HW need to further calculate its payload based on

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