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Although IPv6 can send 64 KBytes long packets, few datalink layer technologies that are used today are able to send a 64 KBytes packet inside a frame. Furthermore, as illustrated in the figure below, another problem is that a host may send a packet that would be too large for one of the datalink layers used by the intermediate routers.
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The need for fragmentation and reassembly
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To solve these problems, IPv6 includes a packet fragmentation and reassembly mechanism. In IPv4, fragmentation was performed by both the hosts and the intermediate routers. However, experience with IPv4 has shown that fragmenting packets in routers was costly [KM1995]_. For this reason, the developers of IPv6 have decided that routers would not fragment packets anymore. In IPv6, fragmentation is only performed by the source host. If a source has to send a packet which is larger than the MTU of the outgoing interface, the packet needs to be fragmented before being transmitted. In IPv6, each packet fragment is an IPv6 packet that includes the `Fragmentation` header. This header is included by the source in each packet fragment. The receiver uses them to reassemble the received fragments.
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IPv6 fragmentation header
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If a router receives a packet that is too long to be forwarded, the packet is dropped and the router returns an ICMPv6 message to inform the sender of the problem. The sender can then either fragment the packet or perform Path MTU discovery. In IPv6, packet fragmentation is performed only by the source by using IPv6 options.
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In IPv6, fragmentation is performed exclusively by the source host and relies on the fragmentation header. This 64 bits header is composed of six fields :
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a `Next Header` field that indicates the type of the header that follows the fragmentation header
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two `Reserved` fields set to `0`.
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the `Fragment Offset` is a 13-bit unsigned integer that contains the offset, in 8 bytes units, of the data following this header, relative to the start of the original packet.
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the `More` flag, which is set to `0` in the last fragment of a packet and to `1` in all other fragments.
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the 32-bit `Identification` field indicates to which original packet a fragment belongs. When a host sends fragmented packets, it should ensure that it does not reuse the same `identification` field for packets sent to the same destination during a period of `MSL` seconds. This is easier with the 32 bits `identification` used in the IPv6 fragmentation header, than with the 16 bits `identification` field of the IPv4 header.
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Some IPv6 implementations send the fragments of a packet in increasing fragment offset order, starting from the first fragment. Others send the fragments in reverse order, starting from the last fragment. The latter solution can be advantageous for the host that needs to reassemble the fragments, as it can easily allocate the buffer required to reassemble all fragments of the packet upon reception of the last fragment. When a host receives the first fragment of an IPv6 packet, it cannot know a priori the length of the entire IPv6 packet.
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