what should NIC do in order to support TSO?

HI All: As the title. I saw someone mentioned that NIC should support S/G DMA, TCP checksum offload and auto insert IP header,TCP header. The S/G DMA is easy to understand since large mount of data can’t be allocated into one single page, but why it need hardware to auto insert TCP header? As I know, the TCP header should only exist in the first fragments, and it ‘s already in skb->data, for other fragments, it’s in skb->frags, hardware only need to insert IP header for them, so I don’t see such need to auto insert TCP header by Hardware. And another question about checksum offload is that, how can hardware support rx checksum verification if received packets are fragmented? Thanks for your answer in advance. Rick