An apparatus and system are disclosed for securing a heat sink against a central processing unit. The subrack stores, from a front side, the plurality of plug-in units at a pitch including a clearance for preventing interference to an adjacent unit. The respective maximum refractive indices of the central core portion, the depressed portion and the cladding portion satisfy the relationship \u201cN1>N3>N2\u201d. The cooling section is provided at the main body. Each of said inlet and outlet ducts have a one-way valve. The method further entails submitting the search key to a routing function where the routing function identifies the outbound packet’s next hop address.