IPv6 path MTU discovery
The links that a packet passes from a source to a destination may have different MTUs. In IPv6, when the packet size exceeds the path MTU of a link, the packet is fragmented at the source end of the link to reduce the processing pressure on intermediate devices and to use network resources effectively.
The path MTU discovery mechanism is designed to find the minimum MTU of all links in the path between a source and a destination. Figure 56 shows how a source host discovers the path MTU to a destination host.
Figure 56: Path MTU discovery process
The source host compares its MTU with the packet to be sent, performs necessary fragmentation, and sends the resulting packet to the destination host.
If the MTU supported by a forwarding interface is smaller than the packet, the device discards the packet and returns an ICMPv6 error packet containing the interface MTU to the source host.
After receiving the ICMPv6 error packet, the source host uses the returned MTU to limit the packet size, performs fragmentation, and sends the resulting packet to the destination host.
Step 2 and step 3 are repeated until the destination host receives the packet. In this way, the source host decides the minimum MTU of all links in the path to the destination host.