Now that we've discussed how fragmentation works in IPv6, let's
briefly dig into the ICMPv6 packet too big message.
First, a note on the Wireshark column titled Payload length.
In IPv4, the length field of the packet measures the length of
the whole IPv4 packet, including the IP header,
which is typically 20 bytes. In IPv6, the length field only includes
the payload size, so we need to add the typical 40‑byte IPv6 header
size to see the actual IPv6 packet size.
The first set of ICMPv6 ping packets have an IPv6 packet
size of 1400, which is 1360 plus 40.
The next ICMPv6 echo‑request has a total IPv6 packet size of 1401,
which is slightly too large to fit through the network.
The Globomantics router R1 responds to the client that the packet is too big.
This message uses ICMP type 2 and code 0,
whereas the IPv4 equivalent used a destination
unreachable code rather than its own type.
There isn't a big operational impact to that, but it is interesting
to note. The most important bit of information in this packet is
the MTU, which the Globomantics client can use to adjust its own
MTU for future transmissions.
Finally, the offending packet is encapsulated,
helping the client to identify the oversized packet it tried to send. Overall, the process is very similar to IPv4.