Using the above method, all multicast data offered by a specific CE is always delivered to all other CEs that are part of the same multicast. It is possible that a number of CEs do not require the delivery of a particular multicast stream because they have no downstream receivers for a specific multicast group. At low traffic volumes, the impact of this is limited. However, at high data rates this could be optimized by devising a mechanism to prune PEs from the distribution tree that although forming part of the customer multicast have no need to deliver a specific multicast stream to the CE attached to them. To facilitate this optimization, the Rosen draft specifies the use of data MDTs. These data MDTs are signaled after the bandwidth for a specific SG exceeds the configurable threshold.
When a PE detects it is transmitting data for the SG in excess of this threshold, it sends an MDT join TLV (at 60 second intervals) over the default MDT to all PEs. All PEs that require the SG specified in the MDT join TLV join the data MDT that is used by the transmitting PE to send the specific SG. PEs that do not require the SG do not join the data MDT, therefore pruning the multicast distribution tree to just the PEs requiring the SG. After providing sufficient time for all PEs to join the data MDT, the transmitting PE switches the specific multicast stream to the data MDT.
PEs that do not require the SG to be delivered, keep state to allow them to join the data MDT as required.
When the bandwidth requirement no longer exceeds the threshold, the PE stops announcing the MDT join TLV. At this point the PEs using the data MDT leave this group and transmission resumes over the default MDT.
Sampling to check if an s,g has exceeded the threshold occurs every ten seconds. If the rate has exceeded the configured rate in that sample period then the data MDT is created. If during that period the transmission rate has not exceeded the configured threshold then the data MDT is not created. If the data MDT is active and the transmission rate in the last sample period has not exceeded the configured rate then the data MDT is torn down and the multicast stream resumes transmission over the default MDT.