code: func (b TCP) CalculateChecksum(partialChecksum uint16) uint16 {
  	// Calculate the rest of the checksum.
  	return Checksum(b[:b.DataOffset()], partialChecksum)
  }
docs: // CalculateChecksum calculates the checksum of the tcp segment.
  // partialChecksum is the checksum of the network-layer pseudo-header
  // and the checksum of the segment data.