This is a running count of reverse-direction packets that have entered this transit tunnel at this node, and per its own description it's only populated on certain platforms. Since this is an Unsigned32 counter, it wraps back to zero after roughly 4.3 billion packets, so on a busy core tunnel a monitoring system needs to poll it often enough to catch each wrap and compute a proper rate rather than reading it as a lifetime total. A busy transit tunnel might show roughly 566,858,439 reverse-direction packets counted since the counter last wrapped or was reset.