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IPv6 MTU Gotchas and other ICMP issues

From my home network, I can successfully ping or traceroute to some IPv6 hosts, but I cannot subsequently open a web page or use other applications with it.  How can this be?  Maximum Transmission Unit (MTU) gotchas…

HISTORY

There is a subtle difference between IPv4 and IPv6 fragmentation strategies. IPv4 routers fragment traffic in the network when needed and then the receiving host reassembles those fragments.  This generally works well, but there are a number of potential issues.  Because of these issues, the IETF developed means for higher layer protocols such as TCP to determine the smallest MTU on a path and send appropriately sized datagrams in order to avoid fragmentation. The IPv6 designers presumed the presence of this Path MTU Discovery so that in IPv6, fragmentation no longer happens in the network but only at the hosts -- and then only in special cases in that absolutely require it.

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