deciphering and verification of the checksum of enciphered and encapsulated udp datagrams , particularly those which enclose a tunneling protocol such as l2tp , are achieved by the provision of a checksum verifier in parallel with a decipher block . checksum logic creates a pseudo udp header needed for checksum verification using fields that would occur at the start of the packet that encapsulates the udp datagram . the first part of the packet to be deciphered is the udp header ; checksum logic can latch the checksum field into a local register . as the rest of the packet is deciphered the checksum verifier processes the data at the same time . eventually the checksum logic will acquire a complete checksum which can be compared with the checksum that had been previously latched , so as to verify the checksum .