Rsvp-Te Messages And Sessions - Juniper JUNOSE 11.2.X BGP AND MPLS Configuration Manual

JunosE 11.2.x BGP and MPLS Configuration Guide

RSVP-TE Messages and Sessions

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established, LDP peers exchange keepalive messages that verify 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 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.
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