Sequence Number Jumps
Large discontinuities in RTP sequence numbers — indicates network path changes, stream switches, or SSRC transitions.
Sequence Number Jumps
| Property | Value |
|---|---|
| Key | sequence_number_jumps |
| Unit | Jumps (count) |
| Type | Counter |
| Direction | Receive |
| RFC | RFC 3550 |
What It Measures
Sequence Number Jumps counts large forward discontinuities in the RTP sequence number that are too big to be explained by normal packet loss. If the receiver expects packet 1,000 and the next packet it receives is 5,000, that 4,000-packet gap is not a loss event — it is a jump that indicates something structural changed in the media stream.
The distinction between a gap and a jump is magnitude. A gap of 5-10 packets is burst loss. A jump of hundreds or thousands of packets indicates the sequence number space changed — typically because the media source changed, the stream was redirected, or a network device rewrote the sequence numbers.
Why It Matters
Sequence jumps are diagnostic markers for significant stream events:
- Network path change — If a media proxy or SBC switches the upstream source, the new source has its own sequence number space.
- SSRC transition — A new SSRC from the remote endpoint may carry a completely different sequence number range.
- Stream switching — In conferencing or forking scenarios, the active speaker change may present as a sequence jump.
- Middlebox intervention — Some SBCs or media gateways may renumber sequence numbers when taking over a stream.
A call with zero jumps had a stable, consistent media path. Jumps indicate the path or source changed during the call.
How CallMeter Measures It
CallMeter monitors the RTP sequence number progression and distinguishes between normal loss gaps (small) and jumps (large). When the gap between the expected and received sequence number exceeds a codec-specific threshold, it is counted as a jump rather than a loss event.
Thresholds
This metric does not have fixed thresholds. Jumps are structural events:
| Jumps | Interpretation |
|---|---|
| 0 | Stable media path throughout the call |
| 1 | One path change or stream switch (may be expected) |
| 2-3 | Multiple path changes — verify against call flow |
| 4+ | Unstable media path or misconfigured middlebox |
What Causes Sequence Jumps
- Call transfer (REFER) — The new party has a different sequence number space.
- Media proxy failover — A redundant proxy taking over the stream starts with its own sequence numbers.
- SBC re-INVITE — Some SBCs re-establish media on re-INVITE with new sequence number state.
- Conferencing bridge changes — Active speaker switching in a multipoint conference may manifest as sequence jumps.
- SSRC collision — RFC 3550 collision handling produces a new SSRC with a new sequence space.
How to Fix It
- Correlate with signaling. Check SIP logs for REFER, re-INVITE, or UPDATE messages that coincide with each jump.
- Check middlebox behavior. If jumps occur without signaling events, a media proxy or SBC may be re-establishing the stream unnecessarily.
- Verify SSRC handling. Some endpoints do not properly signal SSRC changes, causing the receiver to interpret a new stream as a jump in the existing one.
- Review failover configuration. If media proxy failover is causing jumps, verify that the failover is necessary and that the proxy preserves sequence continuity where possible.
Related Metrics
- Sequence Gaps — Smaller gaps caused by packet loss rather than path changes
- Sequence Resets — Full sequence counter restarts
- Timestamp Jumps — Corresponding timestamp discontinuities