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continued functioning of the LSR. Failure to receive an expected keepalive message
causes an LSR to terminate the LDP session.
Label mapping and distribution use downstream-unsolicited, independent control.
With downstream-unsolicited, independent control, an LSR creates a label binding
whenever it learns a new IGP route; the LSR sends a label mapping message
immediately to all of its peer LSRs—upstream and downstream—without having
received a label request message from any peer. The LSR sends the label mapping
message regardless of whether it has received a label mapping message from a
downstream LSR. This is the label distribution method employed in a topology-driven
MPLS network.
A downstream LSR can send a label withdrawal message to recall a label that it
previously mapped. If an LSR that has received a label mapping subsequently
determines that it no longer needs that label, it can send a label release message
that frees the label for use.
RSVP-TE Messages and Sessions
RSVP is described in RFC 2205. Multiple RFCs enable extensions to RSVP for traffic
engineering. The router supports the extended version of RSVP, referred to as
RSVP-TE.
RSVP-TE is “ unreliable” because it does not use TCP to exchange messages. In
contrast to LDP—a hard-state protocol—RSVP-TE is a
soft-state protocol, meaning
that much of the session information is embedded in a state machine on each LSR.
The state machine must be refreshed periodically to avoid session termination. LSRs
send path messages to downstream peers to create and refresh local path states.
LSRs send resv messages to upstream peers in response to path messages to create
and refresh local resv states. A session is ended if the state machine is not refreshed
within the RSVP tunnel timeout period, which is determined as follows:
For example, for the default values,
RSVP-TE messages carry objects consisting of type-length-values (TLVs). The label
request object instructs the endpoint LSR to return an resv message to establish the
LSP. The resv message contains the label object , the label used for the FEC. Both the
path and resv messages carry the record route object , which records the route traversed
by the message.
An upstream LSR sends a pathtear message when its path state times out as a result
of not being refreshed. The pathtear message removes the path and resv states in
each LSR as it proceeds downstream. Downstream LSRs similarly send the resvtear
message when their resv state times out to remove the resv states in upstream LSRs.
MPLS Label Distribution Protocols
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Chapter 2: MPLS Overview