The mobility feature is included in the design of the IPv6 protocol, by which a mobile node does not need special features on the visiting network to be able to use it. In this way the mobile node could connect to different networks maintaining active their connections and continuing at a transparent way with the applications that were using.
The field ‘Total Lengh’which is part of the IPv4 header, is not found in the IPv6 header. Its function was to count the size of the packet payload plus the size of a variable lenght header. As the IPv6 header has a fixed size, the presence of this field is unnecessary.