The IPv6 specification grew out of the efforts of the IPng (IP next generation) Working Group to define a successor protocol to IPv4. Specified originally in IETF RFC 1883 (1995), since replaced by RFC 2460 (1998), IPv6 overcomes many of the limitations of IPv4, most notably with respect to address space, integration of application level protocols, quality of service (QoS), and security.The total size of the IPv6 datagram, including the header, is increased beyond the IPv4 limit of 65,535 octets to support jumbo payloads. At a minimum, all network links must support a maximum transmission unit (MTU) of at least 1,280 octets. As indicated in Figure I-2, the IPv6 header is 40 octets, compared to the IPv4 header of 20 octets, and can be extended as necessary through optional headers associated with higher layer protocols such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). In a departure from IPv4, the IP header and any extension headers associated with the datagram are in addition to the payload length.The IPv6 datagram contains the following fields: