The mLDP and RSVP-TE S-PMSIs support two types of data thresholds: bandwidth-driven and receiver-PE-driven. The threshold evaluation and bandwidth driven threshold functionality are described in Use of data MDTs.
In addition to the bandwidth threshold functionality, an operator can enable receiver-PE-driven threshold behavior. Receiver PE thresholds ensure that S-PMSI is only created when BW savings in P-instance justify extra signaling required to establish a new S-PMSI. For example, the number of receiver PEs interested in a specific C-multicast flow is meaningfully smaller than the number of receiver PEs for default PMSI (I-PMSI or wildcard S-PMSI). To ensure that S-PMSI is not constantly created/deleted, two thresholds need to be specified: receiver PE add threshold and receiver PE delete threshold (expected to be significantly higher).
When a (C-S, C-G) crosses a data threshold to create S-PMSI, instead of regular S-PMSI signaling, sender PE originates S-PMSI explicit tracking procedures to detect how many receiver PEs are interested in a specific (C-S, C-G). When receiver PEs receive an explicit tracking request, each receiver PE responds, indicating whether there are multicast receivers present for that (C-S, C-G) on the specific PE (PE is interested in a specific (C-S, C-G)). If the geo-redundancy feature is enabled, receiver PEs do not respond to explicit tracking requests for suppressed sources and therefore only Receiver PEs with an active join are counted against the configured thresholds on Source PEs.
Upon regular sampling and check interval, if the previous check interval had a non-zero receiver PE count (one interval delay to not trigger S-PMSI prematurely) and current count of receiver PEs interested in the specific (C-S, C-G) is non-zero and is less than the configured receiver PE add threshold, Source PE sets up S-PMSI for this (C-S, C-G) following standard ng-MVPN procedures augmented with explicit tracking for S-PMSI being established.
Data threshold timer should be set to ensure enough time is given for explicit tracking to complete (for example, setting the timer to value that is too low may create S-PMSI prematurely).
Upon regular data-delay-interval expiry processing, when BW threshold validity is being checked, a current receiver PE count is also checked (for example, explicit tracking continues on the established S-PMSI). If BW threshold no longer applies or the receiver PEs exceed receiver PE delete threshold, the S-PMSI is torn down and (C-S, C-G) joins back the default PMSI.
Changing of thresholds (including enabling disabling the thresholds) is allowed in service. The configuration change is evaluated at the next periodic threshold evaluation.
The explicit tracking procedures follow RFC 6513/6514 with clarification and wildcard S-PMSI explicit tracking extensions as described in IETF Draft: draft-dolganow-l3vpn-expl-track-00.