Packet Loss

RTP runs over UDP, which does not retransmit. A packet that is dropped by a congested router, corrupted on a wireless link, or discarded by a firewall is simply gone. The receiver notices because sequence numbers skip, and it reports the loss fraction back to the sender in RTCP receiver reports, as RFC 3550 specifies.

Each lost packet removes about 20 milliseconds of speech. Scattered losses below one percent are rarely audible. Losses of a few percent produce clicks and dropped syllables. Above five to ten percent, conversation becomes tiring, and bursts of consecutive loss are worse than the same number of packets lost at random.

Codecs mitigate loss in different ways. G.711 has no built-in concealment, though receivers often repeat or interpolate the previous frame. Opus, defined in RFC 6716, includes packet loss concealment and optional forward error correction, where each packet carries a low-quality copy of the previous one so a single loss can be repaired.

Complete loss in one direction is a different problem. If no packets arrive at all, the cause is almost always NAT or firewall rules rather than network quality, and the one-way audio troubleshooting article covers it. Partial loss points to congestion, Wi-Fi interference or an overloaded link, and is addressed by prioritising voice traffic, reducing competing bandwidth use or moving to a wired connection.