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How FEC repairs lost video packets
Video over IP is carried in packets. If a packet is lost in transit, the receiver cannot use that packet unless it obtains the data another way. FEC adds repair information to the transmission in advance, giving the receiver a chance to rebuild certain missing packets from the packets and repair data that did arrive.
- Group media packets. The sender arranges packets into a protection pattern defined by the chosen FEC format.
- Send repair data. In the generic RTP format specified by IETF RFC 5109, the sender uses XOR parity to create FEC packets associated with the media packets.
- Check for missing packets. The receiver uses the association information and the packets it received to determine whether a missing packet can be recovered.
- Reconstruct what the pattern permits. If the available parity and surviving packets contain enough information for that loss pattern, the receiver can rebuild the missing data in time for playback.
Some protection arrangements can give more protection to selected portions of a payload than to others. Recovery is conditional: which packets were lost and how the sender configured the protection determine whether reconstruction is possible. FEC is not a promise that all loss will be repaired.
When FEC is useful for real-time video
FEC is most relevant when a stream has to play on schedule. A retransmission may eventually deliver a missing packet, but the round trip and retry can take longer than the receiver’s playback deadline. Repair data already travelling with the stream can allow recovery without that wait.
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That protection has a cost. Repair packets use network capacity and require processing at the sender and receiver. RFC 5109 also cautions against allowing total traffic to rise substantially as loss increases: adding more repair traffic to an already congested path can make conditions worse. FEC should therefore be budgeted and monitored alongside the media stream, not treated as free protection.
FEC, retransmission, and packet redundancy are different approaches
| Approach | What is sent | How recovery works | Main constraint |
|---|---|---|---|
| FEC | Media packets plus repair information | The receiver uses received media and repair data to reconstruct some missing packets. | Extra bandwidth is required, and recoverability depends on the loss pattern and protection configuration. |
| Retransmission | The original packet is sent again after a loss is detected and a request can be acted on. | The receiver uses the resent packet if it arrives before the playback deadline. | The request-and-return delay may be too long for real-time playback. |
| Packet redundancy, such as the ST 2022-7 category | Redundant packet delivery paths, rather than FEC repair data alone | The receiver can use packets arriving over the protected paths for seamless protection. | It is a different protection architecture and depends on the paths and system design. |
These mechanisms should not be conflated. SMPTE describes ST 2022-7 as seamless protection using packet redundancy, while ST 2022-1 and ST 2022-5 provide FEC for signals carried by the ST 2022 suite.
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Where the standards fit
RFC 5109: generic FEC for RTP
RFC 5109 specifies a generic parity-based FEC payload format for media encapsulated in RTP. It allows different protection lengths and levels and supports unequal protection. It is an RTP payload format, not a universal FEC specification for every video-over-IP system. RFC 5109 obsoletes RFC 2733 and RFC 3009.
SMPTE ST 2022: FEC and other protection for carried signals
SMPTE’s ST 2022 overview describes a suite for carrying signals traditionally transported over serial interfaces across IP. In that suite, ST 2022-6 carries uncompressed video and ancillary data typical of SDI; ST 2022-1 and ST 2022-5 provide FEC; and ST 2022-7 provides seamless protection using packet redundancy.
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The cited ST 2022-5 document is the 2012 edition and addresses FEC for high-bit-rate real-time media over IP, including burst-loss protection and packet-size and MTU considerations. Check the applicable current edition before relying on edition-specific normative details.
RFC 6015: a legacy interleaved parity format
RFC 6015 defines a one-dimensional interleaved parity RTP payload format and explains its relationship to SMPTE 2022-1. The RFC warns that the format is intended for legacy applications where its limitations are known not to affect system components. Do not assume it is the universal or current choice for a new deployment.
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SMPTE ST 2110: professional managed-IP media context
SMPTE ST 2110 defines carriage, synchronization, and description of separate media essence streams over managed IP for professional production, playout, and related applications. It provides important context for modern IP production, but the cited sources do not establish one FEC method as mandatory across all ST 2110 systems. The multipart suite remains in active development, so check the applicable standards and product documentation for the system being built.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose and validate an FEC approach
There is no universally best protection setting. Compare the actual sender and receiver options against the network path and the stream’s playback deadline.
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- Loss pattern: Determine whether the path is expected to produce isolated losses, bursts, or both. Check whether the format’s grouping or interleaving can cover the pattern you need to handle.
- Latency budget: The receiver may need to wait for enough packets in a protection group before it can recover a loss. Ensure the repair window fits within the end-to-end deadline; a repair arriving after playback is not useful.
- Bandwidth headroom: Account for repair packets in addition to media and other traffic. Consider what happens under loss, and monitor total traffic so protection does not compound congestion.
- Packet size and MTU: Repair packets can be larger than source packets. Check their size against the path MTU and verify that they are not fragmented or dropped on the route.
- Interoperability: Confirm that the sender and receiver support the same payload format, signaling, and relevant standard revision. A standards label alone does not prove that a particular pair of products can exchange the intended protection data.
- Protection architecture: Decide whether the requirement is to add FEC repair data, use retransmission where timing allows, or protect delivery with redundant packet paths. These choices have different bandwidth, timing, and network-design implications.
Validate the configuration on the intended path and endpoints. Test representative isolated and burst-loss conditions, observe whether recovery completes inside the playback deadline, and check bandwidth use, packet sizing, and receiver error reporting. The cited specifications define mechanisms and constraints; they do not establish a universal recoverable-loss percentage or guarantee lossless delivery for every network.
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