At the very least, this doxygen is wrong since the implementation just passes through to device->GetMtu().  But the XXX suggests to deprecate or delete it, since it is available on the device API.

class Ipv4Interface
  /**
   * This function a pass-through to NetDevice GetMtu, modulo
   * the  LLC/SNAP header i.e., ipv4MTU = NetDeviceMtu - LLCSNAPSIZE
   * \returns the Maximum Transmission Unit associated to this interface.
   *
   * XXX deprecated?  This is duplicate API to GetDevice ()->GetMtu ()
   */
  uint16_t GetMtu (void) const;