mtu parameter, fixed. This mtu of 4568 allows a type 3 Source Route
Frame (info=4472) to be IP encapsulated over Token Ring.
The components are:
AC_FC (2) +
SNAP (12+8) +
SNAP_IP (12+8+20) +
SR (12+18) +
TYPE3_INFO (4472) +
FCS (4)
---------------------
total 4568 bytes