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 is performed at the expiry of the user-configurable retry-timer. By default, the retry time is constant. The exponential back-off timer procedures doubles 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 because of a Path message timeout and no Resv was received, the next attempt is 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. After 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 continues 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 decreases the probability of overwhelming a congested LSR with new state while the previous states installed by the same LSP are lingering and is only 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 further decreases 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.