Unlike in IPv4, an IPv6 router does not fragment a packet unless it is the source of the packet. Intermediate nodes (routers) do not perform fragmentation. The fields used in the IPv4 header for fragmentation do not exist in the IPv6 header.
When an IPv6 router receives a packet larger than the MTU (maximum transmission unit) of the egress interface, the router drops the packet and sends an ICMPv6 Packet Too Big message back to the source. The Packet Too Big message includes the MTU size of the link in bytes so that the source can change the size of the packet for retransmission.
Data is usually transmitted as a series of packets, sometimes referred to as a packet train. The larger the packets, the fewer packets that may be needed to be transmitted. So it is preferred to use the largest-size packet possible that can be supported by all the links from the source to the destination. This is known as the Path MTU (PMTU).
A device can use Path MTU discovery to determine the minimum link MTU along the path. RFC 1981, Path MTU Discovery for IP Version 6, suggests that IPv6 devices should perform ICMPv6 PTMU discovery to avoid source fragmentation.
About the Author
Rick Graziani has been an instructor of computer networking and computer science courses at Cabrillo College in Aptos, California since 1994. Rick also teaches networking courses in the Computer Engineering department at the University of California, Santa Cruz and is on the Curriculum Engineering team for Cisco Networking Academy.
Organizations are increasingly transitioning to IPv6, the next generation protocol for defining how devices of all kinds communicate over networks. Now fully updated, IPv6 Fundamentals offers a thorough, friendly, and easy-to-understand introduction to the knowledge and skills you need to deploy and operate IPv6 networks.
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