The desc explicitly states this is the count of IPsec bytes compression overhead — processing overhead attributable to the compression step, the send-direction mirror of cpvIpsecDecomprOverhead. This carries the same CPU-cost tradeoff consideration as the decompression overhead counter, but measured on the sending side, relevant if this gateway rather than the peer is the one showing elevated CPU on tunnels with compression enabled. e.g. a modest but nonzero cpvIpsecComprOverhead consistent with the CPU cost of compressing outbound traffic on that one IPComp-enabled tunnel.