Hello Sarah,
in addition to skipping the header checksum that is present in IPv4 header IPv6 is simplified in terms of options.
IPv4 header size is actually variable depending on the possible presence of options like route alert and padding.
IPv6 header uses a different approach: the IPv6 header has a fixed size and the options are moved to additional headers called extension headers that are placed between the OSI Layer 3 header (the IPv6 itself) and the possible upper layer protocol like TCP or UDP ( OSI layer 4).
|IPv6 header | EH1|EH2|...| upper layer TCP or UDP
There are a number of these extension headers, each of them performs a different function. If they appear they appear in a deterministic order.
To be noted among these extension headers are included AH and ESP as these protocols are provided to IPv6 as a common facility (this why OSPFv3 relies on AH or ESP extension headers instead of modifying OSPFv3 PDUs)
This makes the processing of IPv6 packets more efficient as the IPv6 header has a fixed size and there is a protocol type pointer to the next extension header tha explains what follows up. Only devices that need to process the extension headers process the packet deeper.
We could also say that it is a more structured approach in IPv6 in comparison to IPv4.
Hope to help
Giuseppe
