1.1.d Explain IP operations

1.1.d [iii] IPv4 and IPv6 fragmentation

IP implements datagram fragmentation, breaking it into smaller pieces, so that packets can pass through a link with a smaller maximum transmission unit (MTU) than the original datagram size. The Identification field, and Fragment offset field along with Don’t Fragment (DF) and More Fragment (MF) flags in the IP protocol header are used for fragmentation and reassembly of IP datagrams.

The details of the fragmentation mechanism, as well as the overall architectural approach to packet fragmentation, are different between IPv4 and IPv6. In IPv4, routers perform fragmentation, whereas in IPv6, routers do not fragment, but drop the packets that are larger than the MTU. While the header formats are different for IPv4 and IPv6, analogous fields are used for fragmentation , so the algorithm can be reused for fragmentation and reassembly.

In IPv4, hosts must make a best-effort attempt to reassemble fragmented IP datagrams with a total reassembled size of up to 576 bytes – equal to the minimum MTU for IPv4. They may also attempt to reassemble fragmented IP datagrams larger than 576 bytes. In IPv6, this minimum MTU is increased to 1,280 bytes larger than the minimum MTU for IPv4.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1064-1071).  . Kindle Edition.