[RFC Note] 2661-5.0

5.0 there are two necessary steps for protocol operations to transmit a PPP session through an L2TP tunnel: 1. Establish a tunnel control connection; 2. Establish a session triggered by an inbound or outbound call request. The tunnel and corresponding control connections must be created before the incoming or outgoing call initialization. An L2TP session must be created before the L2TP transmits the PPP frame. Multiple sessions can coexist in the same tunnel, and multiple tunnels can coexist between the same lac and LNS.

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5.3 once the PPP Frame Forwarding is completed, the LAC can receive the PPP frame of the remote system, remove CRC, establish a link, and the transparent bytes are encapsulated in L2TP for forwarding in the appropriate tunnel. (I don't know what it means here. I understand that PPP frames have undergone CRC verification, removing the extra content, and the rest of the byte streams are relatively transparent for L2TP encapsulation ). When lNS receives an L2TP group, it processes the encapsulated PPP frame, just as it receives a PPP frame from the local PPP interface. The session ID and tunnel ID (specified by the peer) are placed in the headers of all outgoing messages, which are associated with the message sender. In such a way, the PPP frame in the LNS-LAC pair is multivariate and demultiplexed in a single tunnel. Multi-tunnel can coexist in a given LNS-LAC pair, and multi-sessions can coexist in a single tunnel. If the session ID and tunnel ID are O, no ID is assigned. In this case, the session id avp must specify the ID, and the tunnel ID is the same. 5.4 The sequence number is defined in the Control Message Header of L2TP in the data channel, which is optional for data messages. It is used to provide a reliable control transmission and optional data message sequence. Both parties maintain their respective sequence numbers for data sessions in control connections and tunnels. Unlike the L2TP control channel, the L2TP data channel does not use sequence numbers to resend lost data messages. Data Messages can use sequence numbers to detect group loss and restore the order of the original group (Retransmission group is used ). The LAC can request the sequence number to appear in the data message through sequence AVP. If AVP appears during session creation, the sequence number is controlled by LNS. LNS can or cannot control the sequence number by sending a data message (carrying or not carrying a sequence number) (this can be done during a session ). In this case, if the LAC receives a data message without a sequence number, it must stop sending the sequence number. If the LAC receives a data message with a sequence number, it must send the sequence number in a later outgoing data message. LNS can be unordered initially. When groups are rearranged or lost, we recommend that you use the sequence number during initial negotiation. Sequence numbers are not used only when risks are acceptable.