This header is formatted:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|T|L|x|x|S|x|O|P|x|x|x|x| Ver | Length (opt) |
| Tunnel ID | Session ID |
| Ns (opt) | Nr (opt) |
| Offset Size (opt) | Offset pad... (opt) |
Figure 3.1 L2TP Message Header
The Type (T) bit indicates the type of message. It is set to 0 for a
data message and 1 for a control message.
If the Length (L) bit is 1, the Length field is present. This bit
MUST be set to 1 for control messages.
The x bits are reserved for future extensions. All reserved bits MUST
be set to 0 on outgoing messages and ignored on incoming messages.
If the Sequence (S) bit is set to 1 the Ns and Nr fields are present.
The S bit MUST be set to 1 for control messages.
If the Offset (O) bit is 1, the Offset Size field is present. The O
bit MUST be set to 0 (zero) for control messages.
If the Priority (P) bit is 1, this data message should receive
preferential treatment in its local queuing and transmission. LCP
echo requests used as a keepalive for the link, for instance, should
generally be sent with this bit set to 1. Without it, a temporary
interval of local congestion could result in interference with
keepalive messages and unnecessary loss of the link. This feature is
only for use with data messages. The P bit MUST be set to 0 for all
Ver MUST be 2, indicating the version of the L2TP data message header
described in this document. The value 1 is reserved to permit
detection of L2F [RFC2341] packets should they arrive intermixed with
L2TP packets. Packets received with an unknown Ver field MUST be
The Length field indicates the total length of the message in octets.
Tunnel ID indicates the identifier for the control connection. L2TP
tunnels are named by identifiers that have local significance only.
That is, the same tunnel will be given different Tunnel IDs by each
end of the tunnel. Tunnel ID in each message is that of the intended
recipient, not the sender. Tunnel IDs are selected and exchanged as
Assigned Tunnel ID AVPs during the creation of a tunnel.
Session ID indicates the identifier for a session within a tunnel.
L2TP sessions are named by identifiers that have local significance
only. That is, the same session will be given different Session IDs
by each end of the session. Session ID in each message is that of the
intended recipient, not the sender. Session IDs are selected and
exchanged as Assigned Session ID AVPs during the creation of a
Ns indicates the sequence number for this data or control message,
beginning at zero and incrementing by one (modulo 2**16) for each
message sent. See Section 5.8 and 5.4 for more information on using
Nr indicates the sequence number expected in the next control message
to be received. Thus, Nr is set to the Ns of the last in-order
message received plus one (modulo 2**16). In data messages, Nr is
reserved and, if present (as indicated by the S-bit), MUST be ignored
upon receipt. See section 5.8 for more information on using this
field in control messages.
The Offset Size field, if present, specifies the number of octets
past the L2TP header at which the payload data is expected to start.
Actual data within the offset padding is undefined. If the offset
field is present, the L2TP header ends after the last octet of the