IP Packet Fragment Reassembly for L2TP Overview
You can configure the service interfaces on the MX Series routers with modular port concentrators (MPCs) to support reassemble fragmented IP packets for an L2TP connection. When packets are transmitted over an L2TP connection, the packets may be fragmented during transmission and need to be reassembled before they are processed further. Efficient reassembly is important for network throughput, scalability, and graceful response to congestion.
Fragmentation of IP packets for transmission and the need to reassemble the IP packets at a destination is a feature of how Layer 2 (the frame layer) and Layer 3 (the packet layer) operate. The maximum size of a frame, set by the Maximum Transmission Unit (MTU) value, and the maximum size of a packet are determined independently. Typically the packet size can far exceed the MTU size defined for the outgoing connection. If the packet size (data plus IP and other headers) exceeds the configured frame size (usually set by the transport medium limits), the packet must be fragmented and split across multiple frames for transmission. Frames are always processed immediately, when they arrive (if error-free), but packet fragments cannot be processed until the whole packet has been reassembled. Each packet fragment inside a frame series, except the last packet fragment, has the more fragments (MF) IP header bit set, indicating that this packet is part of a whole. The last packet fragment inside a frame does not have this MF bit set and therefore ends the fragment sequence. After all of the fragments of a packet have arrived, the entire packet can be reassembled.
In an L2TP connection, packets are transmitted between the L2TP Access Concentrator (LAC) and the L2TP Network Server (LNS). For an IP packet being transmitted over an L2TP connection, the packet is fragmented at the LAC, at an LNS, or at any intermediate router. IP reassembly parameters configured on the service interfaces of the LAC and the LNS determine how the fragments are reassembled at the service interfaces to ensure efficient reassembly over an L2TP connection.
