Fragmentation in LFI is based on the optimal fragment size. LNS implementation calculates the two optimal fragment sizes, based on two different criteria:
optimal fragment size based on the payload efficiency of the fragment given the fragmentation/transportation header overhead associated with the fragment encapsulation based fragment size
optimal fragment size based on the maximum transmission delay of the fragment set by configuration delay-based fragment size
At the end, only one optimal fragment size is selected. The actual fragments length are of the optimal fragment size.
The parameters required to calculate the optimal fragment sizes are known to the LNS either via configuration or via signaling. These, in-advance known parameters are:
last mile maximum transmission delay (max-fragment-delay obtained via CLI)
last mile ATM Encapsulation (in our example the last mile is ATM but in general it can be Ethernet for MLPPPoE)
MLPPP encapsulation length (depending on the fragment sequence number format)
the last mile on-the-wire rate for the MLPPPoX bundle
Examine closer each of the two optimal fragment sizes.