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> Chapter 15 - IPv6 > Introduction > IPv6 packet structure

IPv6 packet structure

Each IPv6 packet consists of a mandatory fixed header and optional extension headers, and carries a payload, which is typically either a datagram and/or Transport Layer information. The payload could also contain data for the Internet Layer or Link Layer. Unlike IPv4, IPv6 packets aren't fragmented by routers, requiring hosts to implement Maximum Transmission Unit (MTU) Path Discovery for MTUs larger than the smallest MTU (which is 1280 octets).

Jumbograms and jumbo payloads

In IPv6, packets which exceed the MTU of the underlying network are labelled jumbograms, which consist of a jumbo payload. A jumbogram typically exceeds the IP MTU size limit of 65,535 octets, and provides the jumbo payload option, which can allow up to nearly 4GiB of payload data, as defined in RFC 2675. When the MTU is determined to be too large, the receiving host sends a 'Packet too Big' ICMPv6 type 2 message to the sender.

Fragmentation and reassembly

As noted, packets that are too large for the MTU require hosts to perform MTU Path Discovery to determine the maximum size of packets to send. Packets that are too large require a 'Fragment' extension header, to divide the payload into segments that are 8 octets in length (except for the last fragment, which is smaller). Packets are reassembled according to the extension header and the fragment offset.