What does "broadcast/multicast datagrams undelivered" in netstat indicate?

Question

I'm diagnosing an UDP packet loss issue in FreeBSD. netstat -s -p udp has the following output:

udp:
        116974545 datagrams received
        0 with incomplete header
        0 with bad data length field
        0 with bad checksum
        198 with no checksum
        6313 dropped due to no socket
        119696 broadcast/multicast datagrams undelivered
        41534 dropped due to full socket buffers
        0 not for hashed pcb
        116807002 delivered
        955 datagrams output
        0 times multicast source filter matched

Although most of the output is straight forward, I'm really confused about what does broadcast/multicast datagrams undelivered indicate. Is that the total number of undelivered multicast packets? Or does that refer to the number caused by a specific reason, like drooped due to full socket buffers does?

Answer 1

"broadcast/multicast datagrams undelivered" indicates total number of received multicast/broadcast UDP packets with no socket willing to accept them. The counter is bumped in two cases:

Case #1 -- accounts for all/most of undelivered UDP multicast/broadcast packets in your case:

531            /*
532             * No matching pcb found; discard datagram.  (No need
533             * to send an ICMP Port Unreachable for a broadcast
534             * or multicast datgram.)
535             */
536            UDPSTAT_INC(udps_noportbcast);

Case #2 would also bump "dropped due to no socket" counter which is probably not your situation.

596        UDPSTAT_INC(udps_noport);
597        if (m->m_flags & (M_BCAST | M_MCAST)) {
598            UDPSTAT_INC(udps_noportbcast);
599            goto badunlocked;
600        }