Min Packet Spacing

What It Measures

Min Packet Spacing records the smallest time gap between two consecutive RTP packets within a 2-second measurement window. For a G.711 stream with 20ms packetization, packets should arrive approximately 20ms apart. If the minimum spacing is 1ms, it means two packets arrived almost simultaneously — they were bunched together.

Think of it as the opposite of max spacing. Where max spacing shows the longest pause, min spacing shows the shortest gap. Packets that arrive in rapid succession reveal that something in the network path is buffering and then releasing them in bursts.

Why It Matters

Packet bunching is a telltale sign of network buffering behavior:

• Buffer drain pattern — When a network device buffers several packets and then forwards them all at once, the receiver sees very low spacing between those packets followed by a gap. This is the classic "clumping" pattern.
• Traffic shaping evidence — Token bucket shapers accumulate packets when the bucket is empty, then release a burst when tokens are replenished. Min spacing during the burst approaches zero.
• Jitter buffer impact — Bunched packets arrive faster than the playout rate. The jitter buffer fills rapidly during the burst and slowly drains during the gap. If it overflows during the burst, packets are discarded.
• Network device behavior — Very low min spacing combined with high Max Packet Spacing is the fingerprint of a buffering device in the path.

How CallMeter Measures It

CallMeter measures the arrival time of every RTP packet and calculates the inter-packet spacing. Within each 2-second measurement window, the minimum spacing value is recorded. This captures the tightest packet bunching in that period.

Thresholds

This metric does not have fixed thresholds because some degree of spacing variation is normal. Use it as a diagnostic alongside other spacing metrics:

What Causes Low Min Spacing

• Network buffering — Intermediate routers or switches holding packets in queues before forwarding them in batches.
• Traffic shaping — QoS shapers that regulate traffic flow by buffering and releasing in controlled bursts.
• WiFi aggregation — WiFi frame aggregation (A-MPDU) bunches multiple packets into a single wireless frame, which are then delivered nearly simultaneously.
• VPN tunnel batching — VPN gateways that batch multiple encapsulated packets for transmission efficiency.
• Sender-side batching — The remote endpoint's network stack may batch packets before transmission, especially under CPU contention.

How to Fix It

1. Pair with max spacing. Very low min spacing almost always co-occurs with high max spacing. Together, they confirm a buffering device in the path.
2. Identify the buffering device. Use traceroute and test segments of the path to isolate where the bunching originates.
3. Reduce buffer sizes. If the buffering is at a device you control, reduce its output queue size. Smaller queues mean less bunching but may increase drops under congestion.
4. Disable WiFi aggregation. If the media path crosses a WiFi link, test with A-MPDU aggregation disabled (if the access point allows it).
5. Check VPN configuration. Some VPN implementations have configurable batching intervals. Reduce the interval for lower-latency delivery.

Related Metrics

• Max Packet Spacing — The other extreme; high max with low min confirms buffering
• Avg Packet Spacing — Should be close to the codec's packetization interval
• Jitter — Bunching contributes to jitter measurements
• Out-of-Order Packets — Bunching may cause minor reordering