receivers/sessions referenced in a replication context is reached. This threshold value SHOULD be valued at 2 by default, as it is worth opening a dedicated L2TP multicast session for two group members sharing the same desired reception state (which means that two L2TP unicast sessions are concerned). In this case, the OSL will reference two distinct L2TP sessions.
The actual receipt by the LNS of multicast traffic requested by end- users can also be taken into account to decide whether the associated L2TP multicast session has to be opened.
Whenever an OSL gets empty, the LNS MUST stop sending multicast traffic over the corresponding L2TP multicast session. Then the L2TP multicast session MUST be torn down as described in Section 7.
Filter-mode changes for a group can also trigger the opening or the termination of L2TP multicast sessions in the following ways:
a) From INCLUDE Mode to EXCLUDE Mode
When a group state filter-mode switches from INCLUDE to EXCLUDE, only one replication context (and its associated L2TP multicast session) issued from this group state can exist (see Section 4.1). The LNS SHOULD keep one replication context previously created for this group state and it has to update it with:
- a new source-list that has to be excluded from forwarding - a new OSL
The LNS MUST send an OSL update to the LAC to reflect L2TP session list changes (section 6.2), whenever appropriate. The unused L2TP multicast sessions that correspond to previously created replication contexts for the group SHOULD be terminated, either actively or passively by emptying their corresponding OSLs.
The remaining L2TP multicast session MAY also be terminated if the number of receivers is below a predefined threshold (see Section 7). To limit the duration of temporary packet loss or duplicates to receivers, the LNS has to minimize delay between OSL updates messages sent to the LAC. Therefore, one can assume that terminating a multicast session passively gives the smoothest transition.
b) From EXCLUDE Mode to INCLUDE Mode
When a group state filter-mode switches from EXCLUDE to INCLUDE, multiple replication contexts issued by this group state may be created (see Section 4.1). The LNS SHOULD keep the replication context previously created for this group state and it has to update it accordingly with the following information:
- a new list of sources that has to be forwarded. This list has
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