This read-only integer holds the ifIndex of the specific physical link within the IMA group that the near end uses as its timing reference for recovering the transmit clock from the ATM layer. Admins use this to know exactly which physical link is "special" as the clock source, since problems on that particular link can have an outsized effect on the whole group's timing stability. For example, if the link identified by this ifIndex goes down, the admin should expect the group to select a new timing reference link and may see a brief timing disruption.