The maximum size of an IPv6 datagram depends on the Maximum Transmission Unit (MTU) and on the Payload Length field size. The Payload Length field is 16 bits wide, which means that a normal payload cannot be larger than 64K octets, unless a specific feature named Jumbogram, discussed in Section 3.6.4, is used.
The MTU is the largest size a given link layer technology can support for datagrams. For any link, IPv4 mandates a minimum MTU of 68 octets, while the recommended MTU is 576 (see Table 3.7). This enables any IPv4 stack to send a 68 octet datagram with the assurance that it is guaranteed to be forwarded up to the destination. 68 octets is very small, since most current link layer technologies have a minimum MTU of 1500.
In IPv6, the minimum MTU is 1280 octets. However, the mandatory minimum fragment reassembly buffer size is 1500 octets, which tends towards a 1500 octet MTU as the most efficient MTU.
Important to remember:
The minimum MTU for IPv6 is 1280 octets and the most efficient MTU is 1500. The maximum datagram size is 64K octets.
Since routers do not fragment IPv6 datagrams, if a datagram is larger than the size of one link in the path to the destination, the router connected to that link and receiving the datagram sends an ICMP error message to the source and the datagram is ...