This attribute indicates the latest learned Path MTU of the tunnel.
An MTU is the maximum transmission unit of an arbitrary internet
path. When a large amount of IP datagrams are to be transmitted, it
is usually preferable that the datagrams be of the largest size that
does not require fragmentation anywhere along the path from the
source to the destination. This datagram size is referred to as the
Path MTU (PMTU). It is equal to the minimum of the MTUs of
each hop in the path.