SLI Messages
Understand SLI (Slice Loss Indication) messages — how partial frame corruption is reported and what it means for video quality.
SLI stands for Slice Loss Indication. It is a targeted feedback message from a video receiver telling the sender that specific parts of a video frame were lost or corrupted. Unlike PLI, which signals total picture loss, SLI pinpoints exactly which portions of the frame are affected.
Think of it as circling the damaged section of a photograph instead of throwing the whole photo away. The sender can repair just the damaged area rather than resending the entire picture.
How It Works
Modern video codecs divide each frame into smaller units called macroblocks (or coding tree units in newer codecs). When the receiver detects that certain macroblocks within a frame are lost or corrupted — but the rest of the frame is intact — it sends an SLI message identifying the affected region.
The SLI message includes three pieces of information:
- First — The macroblock address where the loss begins
- Number — How many consecutive macroblocks are affected
- PictureID — Which frame the loss occurred in
This precision allows the sender to refresh only the damaged region in subsequent frames, which is more bandwidth-efficient than sending a full keyframe.
Less common in practice
SLI is less widely implemented than PLI or NACK. Many video systems skip SLI and go directly to PLI when any frame corruption is detected. You may see zero SLI messages even when other feedback metrics are active. This is normal and does not indicate a problem.
Why It Matters
SLI messages represent a middle ground in the video error recovery spectrum. They indicate partial corruption — not as minor as a single lost packet (NACK) but not as severe as total picture loss (PLI). When your video system supports SLI, it enables more efficient recovery because only the affected region needs to be repaired.
For SIP testing, the presence of SLI messages means the video codec and feedback path are working at a granular level. The absence of SLI messages when PLI counts are high may suggest that the system does not support SLI and instead escalates directly to full keyframe requests, which is less bandwidth-efficient.
Common Causes
| Cause | Explanation |
|---|---|
| Localized packet loss | Loss affecting only part of a frame's data |
| Partial frame corruption | Network errors damaging specific packets within a frame |
| MTU fragmentation | Large frames split across packets, with some fragments lost |
How to Fix It
- Address packet loss — SLI is a symptom of packet loss affecting parts of video frames. Follow the network troubleshooting guidance for NACK and Packet Loss.
- Check MTU configuration — If SLI correlates with larger frame sizes, path MTU issues may be causing fragmentation and partial loss. Verify that MTU settings are consistent across the network path.
- Monitor alongside PLI — Compare SLI and PLI counts. If SLI is working effectively, you should see SLI messages handling partial corruption while PLI only triggers for complete picture loss.
Related Metrics
- PLI Messages — Full picture loss, more severe than SLI
- FIR Messages — Complete decoder state loss, the most severe feedback
- NACK Messages — Packet-level retransmission, the first line of defense
- Packet Loss — The underlying metric that drives all feedback messages
RFC Reference
SLI is defined in RFC 4585 (Extended RTP Profile for RTCP-Based Feedback), Section 6.3.2. It uses the RTCP Payload-Specific Feedback Message type (PT=206) with FMT=2. The SLI feedback message carries macroblock-level loss information to enable targeted error recovery.
FIR Messages
Understand FIR (Full Intra Request) messages — the strongest keyframe request mechanism, when it triggers, and what it means for video quality.
SIP INFO PFU
Understand SIP INFO Picture Fast Update — the legacy keyframe request mechanism that operates through SIP signaling instead of RTCP feedback.