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... AVPs make up the body of control messages, which are used in
the establishment, maintenance, and teardown of control
connections. This basic construct is sometimes referred to as a
Type-Length-Value (TLV ...
... Type-Length-Value (TLV) in some specifications. (See also:
Control Connection, Control Message.)
...
... A general term identifying any one of a wide range of L2
connections. A circuit may be virtual in nature (e.g., an ATM
PVC, an IEEE 802 ...
...
An L2TP control connection is a reliable control channel that is
used to establish, maintain, and release individual L2TP ...
... L2TP sessions
as well as the control connection itself. (See also: Control
Message, Data Channel.)
...
... L2TP node that exists at either end of an L2TP control
connection. May also be referred to as an LAC or LNS, depending
...
... L2TP, Peer refers to the far end of an
L2TP control connection (i.e., the remote LCCE). An LAC's peer
...
... LCCEs in
order to exchange parameters for and maintain an emulated L2
connection. Multiple sessions may be associated with a single
Control Connection ...
... connection. Multiple sessions may be associated with a single
Control Connection.
Zero-Length Body ...
... ZLB messages are used
only to acknowledge messages on the L2TP reliable control
connection. (See also: Control Message.)
...
... LAC-LAC, and LNS-LNS. These models are diagrammed below.
(Dotted lines designate network connections. Solid lines designate
circuit connections ...
... L2TP session. This model typically involves symmetric establishment;
that is, either side of the connection may initiate a session at any
time (or simultaneously, in which a tie breaking mechanism is
...
... data
packets", respectively). Control messages are used in the
establishment, maintenance, and clearing of control connections and
sessions. These messages utilize a reliable control channel ...
... session with L2TP consists of two
steps: (1) Establishing the control connection, and (2) establishing
a session as triggered by an incoming call ...
... session
frames. Multiple sessions may be bound to a single control
connection, and multiple control connections may exist between the
same two LCCEs ...
... frames. Multiple sessions may be bound to a single control
connection, and multiple control connections may exist between the
same two LCCEs.
...
... each message):
Control Connection Management
...
... |T|L|x|x|S|x|x|x|x|x|x|x| Ver | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ns | Nr |
...
... Control Connection ID field contains the identifier for the
control connection. L2TP control connections are named by
...
... control connection. L2TP control connections are named by
identifiers that have local significance only. That is, the same
...
... identifiers that have local significance only. That is, the same
control connection will be given unique Control Connection IDs by
each LCCE ...
... identifiers that have local significance only. That is, the same
control connection will be given unique Control Connection IDs by
each LCCE from within each endpoint ...
... each LCCE from within each endpoint's own Control Connection ID
number space. As such, the Control Connection ...
... Control Connection ID
number space. As such, the Control Connection ID in each message is
that of the intended recipient, not the sender. Non-zero ...
... that of the intended recipient, not the sender. Non-zero Control
Connection IDs are selected and exchanged as Assigned Control
Connection ID AVPs during the creation of a control connection ...
... sender. Non-zero Control
Connection IDs are selected and exchanged as Assigned Control
Connection ID AVPs during the creation of a control connection.
...
... Control
Connection IDs are selected and exchanged as Assigned Control
Connection ID AVPs during the creation of a control connection.
Ns indicates the sequence number ...
... Control Connection Management ...
...
The L2TP control connection handles dynamic establishment, teardown,
and maintenance of the L2TP sessions ...
... and maintenance of the L2TP sessions and of the control connection
itself. The reliable delivery of control messages ...
... Section 4.2.
This section describes typical control connection establishment and
teardown exchanges. It is important to note that, in the diagrams
that follow, the reliable control message ...
... selecting proper configuration parameters (i.e., Hello interval,
window size, etc.) for a control connection, as well as for
determining how to set up associated sessions within the control
connection ...
... control connection, as well as for
determining how to set up associated sessions within the control
connection, password lookup for control connection ...
... control
connection, password lookup for control connection authentication,
control connection ...
... Control Connection Establishment ...
...
Establishment of the control connection involves an exchange of AVPs
that identifies the peer and its capabilities.
...
...
A three-message exchange is used to establish the control connection.
The following is a typical message exchange:
...
... Control Connection Teardown ...
...
Control connection teardown may be initiated by either LCCE and is
accomplished by sending a single StopCCN ...
... StopCCN MUST send an ACK message to acknowledge receipt of the
message and maintain enough control connection state to properly
accept StopCCN ...
... (Clean up)
An implementation may shut down an entire control connection and all
sessions associated with the control connection ...
... control connection and all
sessions associated with the control connection by sending the
StopCCN. Thus, it is not necessary to clear each session ...
... StopCCN. Thus, it is not necessary to clear each session
individually when tearing down the whole control connection.
...
... control message. After the last
session is cleared, the control connection MAY be torn down as well
(and typically is). The following is an example of a typical control
message exchange:
...
... session will be
given different Session IDs by each end of the control connection
for the life of the session. When the L2TP ...
... for the life of the session. When the L2TP control connection is
used for session establishment, Session ...
...
When the L2TP control connection is used for session
establishment, random Cookie values are selected and exchanged as
...
... |T|L|x|x|S|x|x|x|x|x|x|x| Ver | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ns | Nr |
...
... RFC1700]. The initiator of an L2TP control connection picks an
available source UDP port (which may or may not be 1701) and sends to
...
...
Any subsequent traffic associated with this control connection
(either control traffic or data traffic ...
... data traffic from a session established
through this control connection) must use these same UDP ports.
...
... port (as opposed to using the destination port in the packet
initiating the control connection, i.e., 1701) may make it more
difficult for L2TP to traverse some NAT ...
... packets, Nr MAY be updated before the packet is discarded.
Each control connection maintains a queue of control messages to be
...
... response is detected after several retransmissions (a recommended
default is 10, but MUST be configurable), the control connection and
all associated sessions MUST be cleared. As it is the first message
...
... all associated sessions MUST be cleared. As it is the first message
to establish a control connection, the SCCRQ MAY employ a different
retransmission ...
... LCCEs in a timely fashion.
When a control connection is being shut down for reasons other than
loss of connectivity, the state and reliable delivery ...
... L2TP session or control connection, respectively. As with any other
control message, if the Hello message ...
... Hello message is not reliably delivered, the
sending LCCE declares that the control connection is down and resets
its state for the control connection ...
... control connection is down and resets
its state for the control connection. This behavior ensures that a
connectivity failure between the LCCEs is detected independently by
...
... connectivity failure between the LCCEs is detected independently by
each end of a control connection.
Since the control channel ...
...
Periodic keepalive for the control connection MUST be implemented by
sending a Hello if a period of time (a recommended default is 60
seconds, but MUST be configurable) has passed without receiving ...
... message (data or control) from the peer. An LCCE sending Hello
messages across multiple control connections between the same LCCE
endpoints ...
... session frames are multiplexed and demultiplexed between
a given pair of LCCEs. Multiple control connections may exist
between a given pair of LCCEs, and multiple sessions ...
... LCCEs, and multiple sessions may be
associated with a given control connection.
The peer LCCE ...
... AVP that is unrecognized by its recipient,
the session or control connection associated with the control message
containing the AVP ...
... unrecognized AVP is associated with establishment or maintenance of a
Control Connection (e.g., SCCRQ, SCCRP, SCCCN ...
... SCCRP, SCCCN, Hello), then the
associated Control Connection MUST be issued a StopCCN with Result
Code of 2 and Error Code ...
... M bit set is catastrophic
to the session or control connection with which it is associated.
Thus, the M bit should only be set for AVPs ...
... AVPs that are deemed crucial
to proper operation of the session or control connection by the
sender. AVPs ...
... AVP should be ignored or should
result in termination of a session or control connection (see Section
7.1 for more details).
...
... AVP and the Message Type is
unknown to the implementation, the control connection MUST be
cleared. If the M bit is not set, then the implementation may
...
... section.)
shared_key: Derived shared key for this control connection
control_message: The entire contents of the L2TP ...
... The SCCRQ has special treatment as it is the initial message
commencing a new control connection. As such, there is only one
nonce available. Since the nonce ...
... Result Code AVP, Attribute Type 1, indicates the reason for
terminating the control connection or session.
...
...
0 - Reserved.
1 - General request to clear control connection.
2 - General error, Error Code indicates the problem.
...
... 2 - General error, Error Code indicates the problem.
3 - Control connection already exists.
4 - Requester is not authorized to establish a control
connection.
...
... 3 - Control connection already exists.
4 - Requester is not authorized to establish a control
connection.
5 - The protocol version of the requester is not supported,
...
...
0 - No General Error.
1 - No control connection exists yet for this pair of LCCEs.
2 - Length is wrong.
...
... LNS based on policy.
8 - The session or control connection was shut down due to receipt
of an unknown AVP with the M bit ...
... destinations, it should inform the initiator of the
control connection or session. The Error Message MUST contain
...
...
Control Connection Tie Breaker (SCCRQ)
...
... AVP, Attribute Type 5,
indicates that the sender desires a single control connection to
exist between a given pair of LCCEs.
...
... 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits ...
... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Control Connection Tie Breaker Value is an 8-octet random
value that is used to choose a single control connection ...
... Control Connection Tie Breaker Value is an 8-octet random
value that is used to choose a single control connection when two
LCCEs request a control connection ...
... control connection when two
LCCEs request a control connection concurrently. The recipient of
a SCCRQ must check to see if a SCCRQ ...
... so, a tie has been detected. In this case, the LCCE must compare
its Control Connection Tie Breaker value with the one received in
the SCCRQ. The lower value "wins", and the "loser" MUST discard
...
... the SCCRQ. The lower value "wins", and the "loser" MUST discard
its control connection. A StopCCN SHOULD be sent by the winner as
an explicit rejection for the losing SCCRQ ...
... SCCRQ. In the case in which
a tie breaker is present on both sides and the value is equal,
both sides MUST discard their control connections and restart
control connection ...
... control connections and restart
control connection negotiation with a new, random tie breaker
value.
...
... SCCRQ has no tie
breaker value, the initiator that included the Control Connection
Tie Breaker AVP "wins". If neither side issues a tie breaker,
...
... Tie Breaker AVP "wins". If neither side issues a tie breaker,
then two separate control connections are opened.
Applications that employ a distinct and well-known ...
... AVP is referred to as the "Tie
Breaker AVP" and is applicable only to a control connection. In
L2TPv3, the AVP ...
... L2TPv3, the AVP serves the same purpose of tie breaking, but is
applicable to a control connection or a session. The Control
Connection Tie Breaker AVP ...
... applicable to a control connection or a session. The Control
Connection Tie Breaker AVP (present only in Control Connection
...
... session. The Control
Connection Tie Breaker AVP (present only in Control Connection
messages) and Session Tie Breaker AVP ...
... identifier used to
identify an LCCE for control connection setup, tie breaking,
and/or tunnel authentication ...
... StopCCN)
The Assigned Control Connection ID AVP, Attribute Type 61,
contains the ID being assigned to this control connection ...
... Control Connection ID AVP, Attribute Type 61,
contains the ID being assigned to this control connection by the
sender.
...
... 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Control Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Control Connection ID is a 4-octet non-zero unsigned
integer.
...
... AVP establishes the identifier
used to multiplex and demultiplex multiple control connections
between a pair of LCCEs. Once the Assigned Control Connection ...
... control connections
between a pair of LCCEs. Once the Assigned Control Connection ID
AVP has been received by an LCCE ...
... AVP has been received by an LCCE, the Control Connection ID
specified in the AVP MUST be included in the Control Connection ...
... Control Connection ID
specified in the AVP MUST be included in the Control Connection ID
field of all control packets sent to the peer for the lifetime of
...
... field of all control packets sent to the peer for the lifetime of
the control connection. Before the Assigned Control Connection ID
AVP ...
... lifetime of
the control connection. Before the Assigned Control Connection ID
AVP is received from a peer, all control messages ...
... AVP is received from a peer, all control messages MUST be sent to
that peer with a Control Connection ID value of 0 in the header.
Because a Control Connection ...
... Control Connection ID value of 0 in the header.
Because a Control Connection ID value of 0 is used in this special
manner, the zero value MUST NOT be sent as an Assigned Control
Connection ID value.
...
... Because a Control Connection ID value of 0 is used in this special
manner, the zero value MUST NOT be sent as an Assigned Control
Connection ID value.
Under certain circumstances, an LCCE ...
... LCCE may need to send a StopCCN to
a peer without having yet received an Assigned Control Connection
ID AVP from the peer (i.e., SCCRQ ...
... SCCRQ sent, no SCCRP received yet).
In this case, the Assigned Control Connection ID AVP that had been
sent to the peer earlier (i.e., in the SCCRQ ...
... sent to the peer earlier (i.e., in the SCCRQ) MUST be sent as the
Assigned Control Connection ID AVP in the StopCCN. This policy
...
... AVP in the StopCCN. This policy
allows the peer to try to identify the appropriate control
connection via a reverse lookup.
...
... Serial Number is intended to be an easy reference for
administrators on both ends of a control connection to use when
investigating session failure problems. Serial Numbers ...
... PW Capabilities
List AVP it received during control connection establishment.
Attempts to do so MUST result in the call being rejected via a CDN
...
... BPS AVP, Attribute Type 74, contains the
speed of the facility chosen for the connection attempt.
The Attribute Value field ...
... Connect Speed AVP, Attribute Type 75, represents the speed
of the connection from the perspective of the LAC (i.e., data
flowing from the remote system ...
...
Presence of this AVP implies that the connection speed may be
asymmetric with respect to the transmit connect speed given in the
...
... control messages are used to establish, maintain, and
tear down L2TP control connections. All data packets are sent in
network ...
... SCCRQ) is a control message used to
initiate a control connection between two LCCEs. It is sent by
either the LAC ...
... SCCRP is used to indicate
that the SCCRQ was accepted and that establishment of the control
connection should continue.
The following AVPs ...
... sent in reply to an SCCRP. The SCCCN completes the control
connection establishment process.
The following AVP ...
... control message
sent by either LCCE to inform its peer that the control connection is
being shut down and that the control connection should be closed. In
...
... LCCE to inform its peer that the control connection is
being shut down and that the control connection should be closed. In
addition, all active sessions ...
... Control Connection ID
Note that the Assigned Control Connection ID MUST be present if the
StopCCN is sent after an SCCRQ ...
... L2TP control message sent by either
peer of a control connection. This control message is used as a
"keepalive ...
... control message is used as a
"keepalive" for the control connection. See Section 4.2 for a
description of the keepalive mechanism.
...
... connection to the PSTN, an SVC connection, the IP
address of another LCCE, or any other destination ...
... The Explicit Acknowledgement (ACK) message is used only to
acknowledge receipt of a message or messages on the control
connection (e.g., for purposes of updating Ns and Nr values).
Receipt of this message does not trigger an event for the L2TP
...
... Control Connection State Machines ...
... incoming call placement and outgoing call placement, as well as for
initiation of the control connection itself. The state tables do not
encode timeout and retransmission ...
... Receipt of an invalid or unrecoverable malformed control message
SHOULD be logged appropriately and the control connection cleared to
ensure recovery to a known state. The control connection ...
... control connection cleared to
ensure recovery to a known state. The control connection may then be
restarted by the initiator.
...
... AVP, the
associated session or control connection MUST be shut down. If the M
bit is inspected and is found to be 0, the AVP MUST be ignored
...
... and then a "clean up" occurs. Note that, regardless of the initiator
of the control connection destruction, the reliable delivery
mechanism must be allowed to run (see Section 4.2) before destroying
...
... delivery
mechanism must be allowed to run (see Section 4.2) before destroying
the control connection. This permits the control connection
management ...
... mechanism must be allowed to run (see Section 4.2) before destroying
the control connection. This permits the control connection
management messages to be reliably delivered to the peer.
...
... management messages to be reliably delivered to the peer.
Appendix B.1 contains an example of lock-step control connection
establishment.
...
... Control Connection States ...
...
The L2TP control connection protocol is not distinguishable between
the two LCCEs but is distinguishable between the originator and
...
... receiver. The originating peer is the one that first initiates
establishment of the control connection. (In a tie breaker
situation, this is the winner of the tie.) Since either the LAC or
...
... the LNS can be the originator, a collision can occur. See the
Control Connection Tie Breaker AVP in Section 5.4.3 for a description
of this and its resolution.
...
... lose tie breaker, Clean up losing
SCCRQ unacceptable connection
wait-ctl-reply Receive SCCRQ ...
... SCCRQ, Send StopCCN for wait-ctl-reply
win tie breaker losing connection
wait-ctl-reply Receive SCCCN ...
...
The states associated with an LCCE for control connection
establishment are as follows:
...
...
wait-ctl-reply
The originator checks to see if another connection has been
requested from the same peer, and if so, handles the collision
situation described in Section 5.4.3.
...
... SCCCN. If the SCCCN is valid, the control connection
is established; otherwise, it is torn down (sending a StopCCN with
...
...
established
An established connection may be terminated by either a local
condition or the receipt of a StopCCN. In the event of a local
...
... termination, the originator MUST send a StopCCN and clean up the
control connection. If the originator receives a StopCCN, it MUST
also clean up the control connection ...
... control connection. If the originator receives a StopCCN, it MUST
also clean up the control connection.
...
... ATM PVC is provisioned), or a
local event occurs. The LCCE initiates its control connection
establishment state machine and moves to a state ...
... state machine and moves to a state waiting for
confirmation of the existence of a control connection.
wait-control-conn
...
... In this state, the session is waiting for either the control
connection to be opened or for verification that the control
connection is already open. Once an indication that the control
connection ...
... session is waiting for either the control
connection to be opened or for verification that the control
connection is already open. Once an indication that the control
connection has been opened is received, session control messages ...
... control
connection to be opened or for verification that the control
connection is already open. Once an indication that the control
connection has been opened is received, session control messages
...
... idle, wait-control-conn
When an outgoing call request is initiated, a control connection
is created as described above, if not already present. Once the
...
... is created as described above, if not already present. Once the
control connection is established, an OCRQ is sent to the LAC, and
...
... idle state. If a circuit-switched
connection is established, send an OCCN indicating success, and go
to established state ...
... Termination of a Control Connection ...
...
The termination of a control connection consists of either peer
issuing a StopCCN. The sender ...
... retransmission cycle (e.g., 1 + 2 + 4 + 8 ...
seconds) for the acknowledgment of this message before releasing the
control information associated with the control connection. The
recipient of this message should send an acknowledgment of the
message to the peer, then release the associated control information.
...
... message to the peer, then release the associated control information.
When to release a control connection is an implementation issue and
is not specified in this document. A particular implementation may
use whatever policy is appropriate for determining when to release a
...
... is not specified in this document. A particular implementation may
use whatever policy is appropriate for determining when to release a
control connection. Some implementations may leave a control
connection open for a period of time or perhaps indefinitely after
...
... use whatever policy is appropriate for determining when to release a
control connection. Some implementations may leave a control
connection open for a period of time or perhaps indefinitely after
the last session ...
...
the last session for that control connection is cleared. Others may
choose to disconnect the control connection immediately after the
...
... session for that control connection is cleared. Others may
choose to disconnect the control connection immediately after the
last call on the control connection disconnects.
...
... choose to disconnect the control connection immediately after the
last call on the control connection disconnects.
...
... authentication ensures
that an L2TPv3 control connection is only established between two
endpoints that are configured with the proper password ...
...
The shared secret that is used for all control connection, control
message, and AVP security features ...
... ------------------------------------------
Control Connection Management
...
... B.1 : Lock-Step Control Connection Establishment ...
...
In this example, an LCCE establishes a control connection, with the
exchange involving each side alternating in sending messages. This
example shows the final acknowledgment explicitly sent within an ACK
message ...
... ICRQ or OCRQ that will likely
follow from the side that initiated the control connection.
LCCE ...
... sequence number
state on one side of the connection (perhaps as part of a forwarding
plane reset or failover to a standby node ...