operation linebacker, the designation for the new interdiction campaign, would have four objectives: to isolate north vietnam from its outside sources of supply by destroying railroad bridges and rolling stock in and around hanoi and northeastward toward the chinese frontier; the targeting of primary storage areas and marshalling yards; to destroy storage and transshipment points; and finally, to eliminate (or at least damage) the north's air defense system. with nearly 85 percent of north vietnam's imports (which arrived by sea) blocked by "pocket money", the administration and the pentagon believed that this would cut its final lines of communication with its socialist allies. the people's republic of china alone shipped an average of 22,000 tons of supplies a month over two rail lines and eight major roads that linked it with north vietnam.