SAMSUNG Electronics This document presents a new method for the PMTU discovery for IPv6. To discover the PMTU of a path, a source node measures its actual PMTU using the newly defined Hop-by-Hop Option header, whereas a source node initially assumes that the PMTU of a path is the known MTU of the first hop in the path in the previous one [1981]. In order to measure the actual PMTU, the source node sends the IP packet with the newly defined Hop-by-Hop Option header to the destination node with the first data packet when node is beginning. This can eliminate the chance of occurrence of several iterations of the somewhat complex discovery cycle (sending a packet, receiving a Packet Too Big message, reducing a packet size). Most of all, since existing PMTU has a weak point for security and denial-of-service attacks, this document suggests a security function when PMTU is going on. All IPv6 nodes, including both hosts and routers, must implement newly Options as defined in this specification.