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copied!<p><strong>edited from original:</strong> OK, so IPv6 routers cannot fragment packets, they are dropped and a ICMPv6 packet-too-big is returned. This means end nodes perform negotiation on link MTU. However this all describes the link layer, as per OSI the higher layers shouldn't have to concern themselves with all the details of the lower layers.</p> <p>Consider in IPv4 hardware may support jumbograms upto 9,000 bytes but datagram size can reach to 64KB. With TCP the OS can use the underlying link size and optimally carve up the data-stream, all well and good. With UDP however there is limited support for handling such negotiation above the system administrator noticing an issues and re-configuring software.</p> <p>So say you have some software with a fixed datagram size of 8,000 bytes, to migrate from IPv4 to IPv6 there is the choice of fragmenting if the end-to-end MTU is say only 1,500 bytes, or dropping everything. In a nice cleanup you are asking to drop fragmentation completely, but this would break the application requiring a rewrite of more than just the socket init handling.</p> <p>IPv6 is still IP, the idea isn't to break everything, the changes presented though drop all the features that affect performance: recommending a 1,500 minimum (like 576 in IPv4, cf 68/1,280 abs. min), not fragmenting en route.</p> <p>Cisco page detailing various MTU limits due to ASIC hardware:</p> <p><a href="http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml" rel="nofollow">http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml</a></p> <p><strong>Examples</strong></p> <p>On Linux 2.6.38-10-generic with <a href="http://miru.hk/tmp/ipv4.dump" rel="nofollow">IPv4</a> and <a href="http://miru.hk/tmp/ipv6.dump" rel="nofollow">IPv6</a>. IPv4 shows the fragmentation in Wireshark, IPv6 is only showing application layer fragmentation.</p> <p><code>tcpdump</code> shows this:</p> <pre><code>11:13: IP aiko.hk.miru.hk.37505 > 239.192.0.1.7600: UDP, length 1972 11:13: IP aiko.hk.miru.hk.37505 > 239.192.0.1.7600: UDP, length 1117 11:15: IP6 fe80::230:1bff:feb7:a209.51993 > ff08::1.7600: UDP, length 1137 </code></pre> <p>The packets are PGM above UDP above IPv4 or IPv6. 3,000 byte payload with 2,000 byte MTU so there should be IP fragmentation and PGM fragmentation.</p>

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