I Lost 30% of My UDP Packets — and the Network Was Innocent

A receiver pulling a UDP feed was missing roughly 30% of its messages. No errors, no exceptions, no stack traces — just gaps in the sequence numbers. The first suspect is always the network: a flaky switch, a saturated link, a tired NIC. The network was innocent. The packets were being dropped on the receiving host , after they'd already arrived. Here's how to tell the difference, and why it matters. Why UDP makes this sneaky UDP has no retransmission and no backpressure. When a datagram is lost, nobody is notified — not the sender, not the receiver. The packet simply isn't there. That means two completely different failures look identical from the application's point of view: The network dropped the packet before it reached your machine. Your own host accepted the packet and then threw it away after it arrived. The application sees the same thing in both cases: a missing sequence number. But the fix is in a different building depending on which one it is. Where the packets actually go The receive path is: NIC → kernel socket receive buffer → your recv() call. The kernel parks incoming datagrams in a per-socket buffer until your code reads them. If your code doesn't drain that buffer fast enough, it fills, and the kernel drops the overflow. Crucially, the kernel counts those drops. On Linux: # Per-protocol summary — look for "receive buffer errors" netstat -su # Or straight from the kernel counters cat /proc/net/snmp | grep -A1 Udp # InDatagrams ... InErrors RcvbufErrors ... If RcvbufErrors is climbing, the network did its job and your host discarded the datagrams. That single counter collapses a week of "is it the switch?" into about ten seconds of certainty. The actual cause In this case the socket receive buffer was sitting at the default (~208 KB). The sender burst faster than a single receive thread could call recv() . Average throughput looked fine on every dashboard — but the bursts filled the buffer in milliseconds, and everything past the brim was dropped.