IPv6 Fragmentation Header

Prerequisite – Internet Protocol version 6 (IPv6) Header
In IPv4 fragmentation is done whenever required, at destination or at routers whereas in IPv6 only source is supposed to do fragmentation but not routers. This can only be done when source knows path Maximum Transmission Unit (MTU). In Ipv6 “do not fragment” bit is always 1 where case is not same in IPv4 and ‘More fragment’ bit is only flag in fragmentation header which is of one bit. Two bits are reserved for future use as you can see in picture below.

Internet Protocol Version 6 Fragmentation Header –

The IPv6 sender may perform fragmentation at source because an IPv6 router cannot perform a fragmentation, so if packet is too large for next hop, router will generate an ICMP packet to let the source know that packet is too large in size.

The fragmentation header tries to minimize use of fragmentation as much as possible by supporting minimum packet size of 1280 Bytes. As shown in above picture how fragmentation is taken place according to MTU that sender knows.

IPv6 and other Extension headers are unfragmentable part because every fragment has to go through nodes or routers and at every router, information stored in these extensions headers are required. That is why IPv6 packet is divided into two parts. One is unfragmentable part and other is fragmentable part. The unfragmentable part does not encounter any modification in between and another part being fragmentable is divided into many small fragments as fragment 1, fragment 2, and so on.

After small fragments being created fragmentation header and particular fragment ( as fragment 1 ) is connected to unfragmentable part and is send to destination. Payload length may change after fragmentation and after fragmentation header is added corresponding fields like next header, identification number, fragment offset, and more fragment bits are filled appropriately.

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