📄 rfc3036.txt
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the LSR playing the active role.
Andersson, et al. Standards Track [Page 16]
RFC 3036 LDP Specification January 2001
The throttled sequence of Initialization NAKs is unlikely to cease
until operator intervention reconfigures one of the LSRs. After
such a configuration action there is no further need to throttle
subsequent session establishment attempts (until their
initialization messages are NAK'd).
Due to the asymmetric nature of session establishment,
reconfiguration of the passive LSR will go unnoticed by the active
LSR without some further action. Section "Hello Message"
describes an optional mechanism an LSR can use to signal potential
LDP peers that it has been reconfigured.
2.5.4. Initialization State Machine
It is convenient to describe LDP session negotiation behavior in
terms of a state machine. We define the LDP state machine to have
five possible states and present the behavior as a state transition
table and as a state transition diagram.
Andersson, et al. Standards Track [Page 17]
RFC 3036 LDP Specification January 2001
Session Initialization State Transition Table
STATE EVENT NEW STATE
NON EXISTENT Session TCP connection established INITIALIZED
established
INITIALIZED Transmit Initialization msg OPENSENT
(Active Role)
Receive acceptable OPENREC
Initialization msg
(Passive Role )
Action: Transmit Initialization
msg and KeepAlive msg
Receive Any other LDP msg NON EXISTENT
Action: Transmit Error Notification msg
(NAK) and close transport connection
OPENREC Receive KeepAlive msg OPERATIONAL
Receive Any other LDP msg NON EXISTENT
Action: Transmit Error Notification msg
(NAK) and close transport connection
OPENSENT Receive acceptable OPENREC
Initialization msg
Action: Transmit KeepAlive msg
Receive Any other LDP msg NON EXISTENT
Action: Transmit Error Notification msg
(NAK) and close transport connection
OPERATIONAL Receive Shutdown msg NON EXISTENT
Action: Transmit Shutdown msg and
close transport connection
Receive other LDP msgs OPERATIONAL
Timeout NON EXISTENT
Action: Transmit Shutdown msg and
close transport connection
Andersson, et al. Standards Track [Page 18]
RFC 3036 LDP Specification January 2001
Session Initialization State Transition Diagram
+------------+
| |
+------------>|NON EXISTENT|<--------------------+
| | | |
| +------------+ |
| Session | ^ |
| connection | | |
| established | | Rx any LDP msg except |
| V | Init msg or Timeout |
| +-----------+ |
Rx Any other | | | |
msg or | |INITIALIZED| |
Timeout / | +---| |-+ |
Tx NAK msg | | +-----------+ | |
| | (Passive Role) | (Active Role) |
| | Rx Acceptable | Tx Init msg |
| | Init msg / | |
| | Tx Init msg | |
| | Tx KeepAlive | |
| V msg V |
| +-------+ +--------+ |
| | | | | |
+---|OPENREC| |OPENSENT|----------------->|
+---| | | | Rx Any other msg |
| +-------+ +--------+ or Timeout |
Rx KeepAlive | ^ | Tx NAK msg |
msg | | | |
| | | Rx Acceptable |
| | | Init msg / |
| +----------------+ Tx KeepAlive msg |
| |
| +-----------+ |
+----->| | |
|OPERATIONAL| |
| |---------------------------->+
+-----------+ Rx Shutdown msg
All other | ^ or Timeout /
LDP msgs | | Tx Shutdown msg
| |
+---+
Andersson, et al. Standards Track [Page 19]
RFC 3036 LDP Specification January 2001
2.5.5. Maintaining Hello Adjacencies
An LDP session with a peer has one or more Hello adjacencies.
An LDP session has multiple Hello adjacencies when a pair of LSRs is
connected by multiple links that share the same label space; for
example, multiple PPP links between a pair of routers. In this
situation the Hellos an LSR sends on each such link carry the same
LDP Identifier.
LDP includes mechanisms to monitor the necessity of an LDP session
and its Hello adjacencies.
LDP uses the regular receipt of LDP Discovery Hellos to indicate a
peer's intent to use the label space identified by the Hello. An LSR
maintains a hold timer with each Hello adjacency which it restarts
when it receives a Hello that matches the adjacency. If the timer
expires without receipt of a matching Hello from the peer, LDP
concludes that the peer no longer wishes to label switch using that
label space for that link (or target, in the case of Targeted Hellos)
or that the peer has failed. The LSR then deletes the Hello
adjacency. When the last Hello adjacency for a LDP session is
deleted, the LSR terminates the LDP session by sending a Notification
message and closing the transport connection.
2.5.6. Maintaining LDP Sessions
LDP includes mechanisms to monitor the integrity of the LDP session.
LDP uses the regular receipt of LDP PDUs on the session transport
connection to monitor the integrity of the session. An LSR maintains
a KeepAlive timer for each peer session which it resets whenever it
receives an LDP PDU from the session peer. If the KeepAlive timer
expires without receipt of an LDP PDU from the peer the LSR concludes
that the transport connection is bad or that the peer has failed, and
it terminates the LDP session by closing the transport connection.
After an LDP session has been established, an LSR must arrange that
its peer receive an LDP PDU from it at least every KeepAlive time
period to ensure the peer restarts the session KeepAlive timer. The
LSR may send any protocol message to meet this requirement. In
circumstances where an LSR has no other information to communicate to
its peer, it sends a KeepAlive message.
An LSR may choose to terminate an LDP session with a peer at any
time. Should it choose to do so, it informs the peer with a Shutdown
message.
Andersson, et al. Standards Track [Page 20]
RFC 3036 LDP Specification January 2001
2.6. Label Distribution and Management
The MPLS architecture [RF3031] allows an LSR to distribute a FEC
label binding in response to an explicit request from another LSR.
This is known as Downstream On Demand label distribution. It also
allows an LSR to distribute label bindings to LSRs that have not
explicitly requested them. [RFC3031] calls this method of label
distribution Unsolicited Downstream; this document uses the term
Downstream Unsolicited.
Both of these label distribution techniques may be used in the same
network at the same time. However, for any given LDP session, each
LSR must be aware of the label distribution method used by its peer
in order to avoid situations where one peer using Downstream
Unsolicited label distribution assumes its peer is also. See Section
"Downstream on Demand label Advertisement".
2.6.1. Label Distribution Control Mode
The behavior of the initial setup of LSPs is determined by whether
the LSR is operating with independent or ordered LSP control. An LSR
may support both types of control as a configurable option.
2.6.1.1. Independent Label Distribution Control
When using independent LSP control, each LSR may advertise label
mappings to its neighbors at any time it desires. For example, when
operating in independent Downstream on Demand mode, an LSR may answer
requests for label mappings immediately, without waiting for a label
mapping from the next hop. When operating in independent Downstream
Unsolicited mode, an LSR may advertise a label mapping for a FEC to
its neighbors whenever it is prepared to label-switch that FEC.
A consequence of using independent mode is that an upstream label can
be advertised before a downstream label is received.
2.6.1.2. Ordered Label Distribution Control
When using LSP ordered control, an LSR may initiate the transmission
of a label mapping only for a FEC for which it has a label mapping
for the FEC next hop, or for which the LSR is the egress. For each
FEC for which the LSR is not the egress and no mapping exists, the
LSR MUST wait until a label from a downstream LSR is received before
mapping the FEC and passing corresponding labels to upstream LSRs.
An LSR may be an egress for some FECs and a non-egress for others.
An LSR may act as an egress LSR, with respect to a particular FEC,
under any of the following conditions:
Andersson, et al. Standards Track [Page 21]
RFC 3036 LDP Specification January 2001
1. The FEC refers to the LSR itself (including one of its directly
attached interfaces).
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