L2TPv3 Data Channel

After a session is established through the control session, the L2TP endpoint is ready to send and receive data traffic, using the header shown in the top half of Figure A-4.

Figure A-4. L2TP Data Message and Session Headers (RFC 3931)

Each frame has a Session ID field in the session header, which is in the bottom half of Figure A-4. This field contains the session identifier value negotiated during setup (with an ICRQ/ICRP exchange, for example). No fixed mapping exists between data and control channels. A single data channel can carry frames belonging to multiple sessions. And, the session ID is used to multiplex different traffic streams onto the data channel. The Cookie field, also shown in Figure A-4, contains the cookie 64-bit value also negotiated during session establishment.

Although the data header has a Sequence Number field, the data channel is not reliable. The protocol can detect missing, duplicate, or out-of-order packets but does not resend. That is left to higher-layer protocols.

The RFC allows for the data channel to be set up either using the native control protocol or statically or using another control mechanism.