IPv6中間的路由器不會對過通的IPv6報文進行分片，分片只會在源節點進行。
中間的防火牆可能會阻止ICMP too big的報文，導致源端無法獲知MTU失效。
Since modern networks use MTUs of 1500 bytes or larger, they raised the IPv4 MTU minimum of 576 bytes to 1500 bytes for IPv6.
IPv6 hosts will accept a Path MTU that is as low as 1280 bytes, but not necessarily smaller.
Path MTU historically depends on an ICMP/ICMPv6 Packet Too Big message from the system that discards a datagram because it is too large for the next link.
There is a newer proposal in RFC 4821 that tries various packet sizes to see what works; that helps, but isn’t yet widely implemented.
Do not filter out ICMPv6 Packet Too Big or Destination Unreachable messages, and while you might rate-limit them, do not configure routers to not send them, nor your hosts not to accept them. They enable the network to inform the transport of issues, and as a result, to work around them. More generally, RFC 4890 contains “Recommendations for Filtering ICMPv6 Messages in Firewalls,” which includes filters in the network and those in hosts.
From <https://blogs.cisco.com/enterprise/ipv6-mtu-gotchas-and-other-icmp-issues>
RFC8200
IPv6 requires that every link in the Internet have an MTU of 1280
octets or greater. This is known as the IPv6 minimum link MTU. On
any link that cannot convey a 1280-octet packet in one piece, link-
specific fragmentation and reassembly must be provided at a layer
below IPv6.
MTU：鏈路上最小的報文大小，如果報文小於這個值就無法發送。（直接丟棄還是自動補足？）路由器收到大於MTU的報文直接丟棄，並返回ICMPv6 Packet Too Big消息。源節點必須對報文進行分片或者減小報文的長度。
IPv4和IPv6區別：
1. IPv4最小MTU=576bytes，IPv6最小MTU=1280bytes。
2. IPv4的源節點和中間路由器都能對報文進行分片。而IPv6只有源節點才能對報文分片。（是否以及如何獲知整條鏈路的最小MTU？）和IPv4一樣的是重組只在目的節點進行。
3. 使用ICMPv6消息報告報文超長。
4. 重新定義了Path MTU Discovery。（具體細節要明確）（RFC1981）（RFC8201）Path MTU Discovery is a good feature, but it is static. It requires that hosts keep track of MTUs for different routes, and update them regularly.
5. IPv4報頭里面有fragment字段，IPv6去除了這個字段，添加了fragment擴展報頭。