Keyframe Interval

Understand keyframe interval measurement in video streams — how CallMeter tracks the time between consecutive keyframes, thresholds, and what the interval means for video recovery and bandwidth.

Keyframe Interval measures the time between consecutive keyframes (I-frames) in a video stream, expressed in milliseconds. A shorter interval means the decoder gets fresh reference frames more frequently, enabling faster recovery from corruption. A longer interval saves bandwidth but leaves the decoder dependent on fragile delta frame chains for extended periods.

Think of it as the gap between safety nets under a tightrope. Short gaps (frequent keyframes) mean a fall is caught quickly. Long gaps mean a longer fall before recovery.

How It Works

The video encoder produces keyframes based on its configuration and external events:

Periodic keyframes — Generated at a fixed interval (e.g., every 2 seconds)
Requested keyframes — Produced in response to PLI/FIR requests from the remote side
Scene-change keyframes — Triggered by the encoder detecting a major change in video content

CallMeter measures the elapsed time between each pair of consecutive keyframes on the send side. This gives you a real-time view of how frequently the decoder's reference chain is reset.

Why It Matters

The keyframe interval is a fundamental trade-off in video streaming:

Short interval (under 2000ms) — Fast error recovery, but higher bandwidth usage due to frequent large keyframes
Long interval (over 5000ms) — Efficient bandwidth usage, but slow recovery from packet loss or corruption
Very long interval (over 10000ms) — If a packet loss event occurs at the start of the interval, the viewer may see corrupted or frozen video for 10+ seconds

Thresholds

Level	Interval	Interpretation
Good	2000ms or less	Fast recovery, suitable for interactive video calls
Warning	5000ms	Acceptable for stable networks, slow recovery under loss
Critical	Above 10000ms	Excessively long, video will be severely impacted by any packet loss

Recovery time impact

With a 10-second keyframe interval and no NACK retransmission, a single lost packet can cause up to 10 seconds of video corruption or freezing. For interactive calls, keep the interval at 2 seconds or less.

Common Causes of Abnormal Keyframe Intervals

Cause	Explanation
Very short interval	Excessive keyframe requests from the remote side flooding the encoder with PLI/FIR messages
Very long interval	Encoder configured with a large periodic interval, or periodic keyframes disabled
Irregular intervals	Mix of periodic and requested keyframes creating inconsistent timing
No keyframes at all	Encoder not producing keyframes, or no video data flowing
Decreasing over time	Worsening network conditions causing more frequent keyframe requests

How to Fix It

Configure periodic keyframes — Set the encoder to produce a keyframe at least every 2 seconds for video calls. This ensures recovery even if keyframe requests are lost.
Enable NACK retransmission — NACK-based recovery can fix isolated packet loss without waiting for the next keyframe, reducing the impact of longer intervals.
Monitor request-driven keyframes — If the interval is shorter than configured, check Keyframe Requests Received to see if the remote side is driving extra keyframes due to persistent loss.
Balance bandwidth vs recovery — For bandwidth-constrained links, a 4-second interval with NACK retransmission may be more efficient than a 2-second interval without NACK.
Check encoder configuration — Verify the GOP (Group of Pictures) size or keyframe interval setting in the encoder matches your intended configuration.

Keyframes Sent — Total keyframe count; high count with short interval confirms frequent keyframe production
Keyframe Requests Received — Inbound requests that shorten the effective interval
Video Freeze Duration — Freeze length is bounded by the keyframe interval in the worst case
Packet Loss Rate — Loss between keyframes causes decoder corruption until the next keyframe arrives
Video Bitrate — Frequent keyframes increase average bitrate due to their larger size

RFC Reference

Keyframe (IDR) generation behavior in H.264 is governed by the codec specification (RFC 6184, RTP Payload Format for H.264). VP8 keyframe behavior is defined in RFC 7741 (RTP Payload Format for VP8). The RTCP feedback mechanisms (PLI, FIR) that trigger on-demand keyframes are specified in RFC 4585 and RFC 5104.