Enhancements To Rsvp Control Plane Congestion Control - Alcatel-Lucent 7450 Manual

Enhancements to RSVP control plane congestion control

Enhancements to RSVP control plane congestion control

The RSVP control plane makes use of a global flow control mechanism to adjust the rate of Path
messages for unmapped LSP paths sent to the network under congestion conditions. When a Path
message for establishing a new LSP path or retrying an LSP path that failed is sent out, the control
plane keeps track of the rate of successful establishment of these paths and adjusts the number of
Path messages it sends per second to reflect the success ratio.
In addition, an option to enable an exponential back-off retry-timer is available. When an LSP path
establishment attempt fails, the path is put into retry procedures and a new attempt will be
performed at the expiry of the user-configurable retry-timer. By default, the retry time is constant.
The exponential back-off timer procedures will double the value of the user configurable retry-
timer value at every failure of the attempt to adjust to the potential network congestion that caused
the failure. An LSP establishment fails if no Resv message was received and the Path message
retry-timer expired, or a PathErr message was received before the timer expired.
Three enhancements to this flow-control mechanism to improve congestion handling in the rest of
the network are supported.
The first enhancement is the change to the LSP path retry procedure. If the establishment attempt
failed due to a Path message timeout and no Resv was received, the next attempt will be performed
at the expiry of a new LSP path initial retry-timer instead of the existing retry-timer. While the
LSP path initial retry-timer is still running, a refresh of the Path message using the same path and
the same LSP-id is performed according to the configuration of the refresh-timer. Once the LSP
path initial retry-timer expires, the ingress LER then puts this path on the regular retry-timer to
schedule the next path signaling using a new computed path by CSPF and a new LSP-id.
The benefits of this enhancement is that the user can now control how many refreshes of the
pending PATH state can be performed before starting a new retry-cycle with a new LSP-id. This is
all done without affecting the ability to react faster to failures of the LSP path, which will continue
to be governed by the existing retry-timer. By configuring the LSP path initial retry-timer to values
that are larger than the retry-timer, the ingress LER will decrease the probability of overwhelming
a congested LSR with new state while the previous states installed by the same LSP are lingering
and will only be removed after the refresh timeout period expires.
The second enhancement consists of applying a jitter +/- 25% to the value of the retry-timer
similar to how it is currently done for the refresh timer. This will further decrease the probability
that ingress LER nodes synchronize their sending of Path messages during the retry-procedure in
response to a congestion event in the network.
The third enhances the RSVP flow control mechanism by taking into account new parameters:
outstanding CSPF requests, Resv timeouts and Path timeouts.
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