
A Tale of 2 Protocols: IPv4, IPv6, MTUs and Fragmentation - prakash
http://www.potaroo.net/ispcol/2009-01/mtu6.html
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pmjordan
Wow, that was long. Interesting, but excessively verbose.

I'll attempt a summary:

Larger (~1500 bytes gross) IPv6 datagrams are currently not being routed
correctly over certain networks. The author has deduced that a combination of
multiple factors is to blame:

\- IPv6 fragmentation happens at the source, if a packet attempts to cross a
link with an MTU that is too small, an ICMPv6 packet informs the sender.

\- IPv4 fragmentation typically happens on the way, as necessary. TCP supports
an optional IPv6-like source fragmentation mode which isn't widely used as
ICMPv4 packets are often filtered by firewalls, etc.

\- IPv6 coverage isn't perfect yet, so there are IPv6-over-IPv4 tunnels in
place in many cases. Tunneling adds size to a packet.

\- Some tunnels refuse to do IPv4-based fragmentation for performance reasons.
Because of the extra bytes, most packets would have to be fragmented. Instead,
they use the TCP/IPv4 source fragmentation mode, but the relevant ICMP packets
get lost along the way, so the tunnel can't operate correctly. Packets are
eaten.

I hope I've understood & summarised it correctly. If anyone with more patience
or understanding of these things could correct me, that'd be great.

