Checksum

Covers a pseudoheader and entire TCP segment

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.
If a segment contains an odd number of header and text octets to be checksummed,
the last octet is padded on the right with zeros to form a 16 bit word for checksum purposes.
The pad is not transmitted as part of the segment.
While computing the checksum, the checksum field itself is replaced with zeros.
The checksum also covers a 96 bit pseudo header conceptually prefixed to the TCP header.
This pseudo header contains the Source Address, the Destination Address, the Protocol, and TCP length.
This gives the TCP protection against misrouted segments.
This information is carried in the Internet Protocol and is transferred across the TCP/Network interface in the arguments or results of calls by the TCP on the IP.

The TCP Length is the TCP header length plus the data length in octets
(this is not an explicitly transmitted quantity, but is computed),
and it does not count the 12 octets of the pseudo header.