ich may be the IPv6 header or an extension header. This means that for each first fragment packet, we need to 1) make a copy of some IPv6 header fields (src+dest) that we need later on, just in case we do overwrite part of the IPv6 header, and 2) make a copy of the header data that we overwrote, so that we can restore it before either completing reassembly or sending an ICMPv6 reply. The last part is true even if this setting is disabled, but if it is enabled, we need to save a bit more data (up to the size