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L2TP
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... endpoints to
reside on different devices interconnected by a packet-switched
network. With L2TP, a user has an L2 connection to an access
concentrator (e.g., modem ...
... PPP session to a location other than
the point at which it was physically received, L2TP can be used to
make all channels terminate at a single NAS ...
... Remote System and LAC results in a corresponding L2TP Session
within a previously established Tunnel ...
...
A node that acts as one side of an L2TP tunnel endpoint and is a
peer to the L2TP Network Server ...
... L2TP tunnel endpoint and is a
peer to the L2TP Network Server (LNS). The LAC sits between an
...
... LNS requires tunneling with the
L2TP protocol as defined in this document. The connection from
the LAC ...
...
A node that acts as one side of an L2TP tunnel endpoint and is a
peer to the L2TP Access Concentrator ...
... LAC and LNS. There is a one to one relationship
between established L2TP Sessions and their associated Calls. (See
also: Call).
...
... A LAC Client (a Host which runs L2TP natively) may also participate
in tunneling to the Home LAN ...
... tunnel. Control
messages utilize a reliable Control Channel within L2TP to guarantee
delivery (see section 5.1 for details). Data messages ...
... PPP Frames |
+-------------------+ +-----------------------+
| L2TP Data Messages| | L2TP Control Messages ...
... | L2TP Data Messages| | L2TP Control Messages |
+-------------------+ +-----------------------+
...
... Control Messages |
+-------------------+ +-----------------------+
| L2TP Data Channel | | L2TP Control Channel ...
... +------------------------------------------------+
Figure 3.0 L2TP Protocol Structure
Figure 3.0 depicts the relationship of PPP ...
... Figure 3.0 depicts the relationship of PPP frames and Control
Messages over the L2TP Control and Data Channels. PPP Frames are
...
... passed over an unreliable Data Channel encapsulated first by an L2TP
header and then a Packet Transport ...
... ATM,
etc. Control messages are sent over a reliable L2TP Control Channel
which transmits packets in-band ...
... L2TP Header Format ...
... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3.1 L2TP Message Header
...
... L2F [RFC2341] packets should they arrive intermixed with
L2TP packets. Packets received with an unknown Ver field MUST be
discarded.
...
... 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 ...
... session within a tunnel.
L2TP sessions are named by identifiers that have local significance
...
...
The Offset Size field, if present, specifies the number of octets
past the L2TP header at which the payload data is expected to start ...
... 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
offset padding.
...
... encoding message types and bodies is used
throughout L2TP. This encoding will be termed AVP (Attribute-Value
...
... AVPs defined within
this document. Any vendor wishing to implement their own L2TP
extensions can use their own Vendor ID along with private Attribute
...
... The Error Codes defined below pertain to types of errors that are
not specific to any particular L2TP request, but rather to
protocol or message format errors. If an L2TP ...
... L2TP request, but rather to
protocol or message format errors. If an L2TP reply indicates in
its Result Code that a general error occurred, the General Error
value ...
... The Protocol Version AVP, Attribute Type 2, indicates the L2TP
protocol version of the sender ...
... 1. Rev field is a 1 octet unsigned integer containing 0. This
pertains to L2TP protocol version 1, revision 0. Note this is not
the same version number ...
...
For devices which do not have a firmware revision (general purpose
computers running L2TP software modules, for instance), the
revision of the L2TP software module may be reported instead.
...
... computers running L2TP software modules, for instance), the
revision of the L2TP software module may be reported instead.
This AVP ...
... tunnels between the LNS and LAC. The L2TP
peer MUST place this value in the Tunnel ID header field ...
... tunneling a PPP session with L2TP consists of
two steps, (1) establishing the Control Connection for a Tunnel ...
... Control Connection MUST be
established before an incoming or outgoing call is initiated. An L2TP
Session MUST be established before L2TP ...
... Control Connection######## |
[Remote] | | | |
[System]------Call----------*============L2TP Session=============* |
PPP ...
... | | | |
[Remote] | | | |
[System]------Call----------*============L2TP Session=============* |
PPP ...
... control connection includes securing the
identity of the peer, as well as identifying the peer's L2TP version,
framing, and bearer capabilities, etc.
...
... link framing, and
transparency bytes, encapsulated in L2TP, and forwarded over the
appropriate tunnel. The LNS ...
...
A keepalive mechanism is employed by L2TP in order to differentiate
tunnel outages from extended periods of no control or data activity
...
... (see section 3.1) belong to this transport. The upper level
functions of L2TP are not concerned with retransmission or ordering
of control messages ...
... The last received message number, Nr, is used to acknowledge messages
received by an L2TP peer. It contains the sequence number of the
message the peer expects to receive next (e.g. the last Ns of a non-
...
... for flow controlling control messages. An L2TP implementation is
expected to be able to keep up with incoming control messages,
...
... The following control connection messages are used to establish,
clear and maintain L2TP tunnels. All data is sent in network order
(high order octets first). Any "reserved" or "empty" fields MUST be
...
... to answer the call if it has not already done so. It also allows the
LNS to indicate necessary parameters for the L2TP session.
...
... ICCN is used to indicate that the ICRP was accepted, the call has
been answered, and that the L2TP session should move to the
established state ...
... LAC for this call and provides the LAC with parameter
information for both the L2TP session, and the call that is to be
placed
...
...
This section describes the operation of various L2TP control
connection functions and the Control Connection messages which are
...
... L2TP Over Specific Media ...
...
L2TP is self-describing, operating at a level above the media over
which it is carried. However, some details of its connection to media
...
... L2TP uses the registered UDP port 1701 [RFC1700]. The entire L2TP
packet, including payload and L2TP ...
... datagram. The initiator of an L2TP tunnel picks an available source
UDP port (which may or may not be 1701), and sends to the desired
destination address ...
... initiating the tunnel, i.e., 1701) may make it more difficult for
L2TP to traverse some NAT devices. Implementors should consider the
...
... L2TP packet travels over the IP
substrate. L2TP makes no special efforts to optimize this. A LAC
implementation MAY cause its LCP ...
... LAC environments in which the MTU's of the
path over which the L2TP packets are likely to travel have a
consistent value.
...
... consistent value.
The default for any L2TP implementation is that UDP checksums MUST be
enabled for both control and data messages ...
... UDP checksums MUST be
enabled for both control and data messages. An L2TP implementation
MAY provide an option to disable UDP checksums for data messages ...
... two packet types (L2F uses a value of 1, and the L2TP version
described in this document uses a value of 2). An L2TP ...
... L2TP version
described in this document uses a value of 2). An L2TP implementation
running on a system which does not support L2F MUST silently discard ...
... per-packet indication of error, such as TCP
header compression. Sequencing may be handled by using L2TP data
message sequence numbers if any protocol being transported by the PPP ...
... no upper PPP protocol will encounter lost packets. If L2TP sequence
numbers are enabled, L2TP can detect the packet loss ...
... lost packets. If L2TP sequence
numbers are enabled, L2TP can detect the packet loss. In the case of
an LNS ...
... L2TP encounters several security issues in its operation. The
general approach of L2TP to these issues is documented here.
...
... encryption, integrity and authentication services for all L2TP
traffic. This secure transport ...
... traffic. This secure transport operates on the entire L2TP packet
and is functionally independent of PPP and the protocol being carried
...
... PPP and the protocol being carried
by PPP. As such, L2TP is only concerned with confidentiality,
authenticity, and integrity ...
... confidentiality,
authenticity, and integrity of the L2TP packets between its tunnel
endpoints (the LAC and LNS ...
... authentication provided by IPsec tunnel mode and that
provided by L2TP secured with IPsec provide an equivalent level of
security ...
... requirements for access control mechanisms are not a part
of the L2TP specification and as such are outside the scope of this
document.
...
...
Dory Leifer made valuable refinements to the protocol definition of
L2TP and contributed to the editing of this document.
Steve Cobb and Evan Caves redesigned the state machine ...