UDP-Lite

UDP-Lite (Lightweight User Datagram Protocol, sometimes UDP Lite) is a that allows a potentially damaged data payload to be delivered to an application rather than being discarded by the receiving station. This is useful as it allows decisions about the integrity of the data to be made in the application layer (application or the codec), where the significance of the bits is understood. UDP-Lite is described in RFC 3828.

UDP-Lite is based on (UDP), but unlike UDP, where either all or none of a packet is protected by a checksum, UDP-Lite allows for partial checksums that only covers part of a datagram (an arbitrary count of octets at the beginning of the packet), and will therefore deliver packets that have been partially corrupted. It is designed for multimedia protocols, such as Voice over IP (VoIP) or streamed video, in which receiving a packet with a damaged payload is better than receiving no packet at all. For conventional UDP and (TCP), a single bit in error will cause a "bad" checksum, meaning that the whole packet must be discarded: in this way, bit errors are "promoted" to entire packet errors even where the damage to the data is trivial. For computing the checksum UDP-Lite uses the same checksum algorithm used for UDP (and TCP).

Modern multimedia codecs, like G.718 and Adaptive Multi-Rate (AMR) for audio and H.264 and MPEG-4 for video, have resilience features already built into the syntax and structure of the stream. This allows the codec to (a) detect errors in the stream and (b) potentially correct, or at least conceal, the error during playback. These codecs are ideal partners for UDP-Lite, since they are designed to work with a damaged data stream, and it is better for these codecs to receive perhaps 200 bytes where a few bits are damaged rather than have to conceal the loss of an entire packet that was discarded due to a bad checksum. The application layer understands the significance of the data, where the transport only sees UDP packets. This means that error protection can be added if necessary at a higher layer, for example with a forward error correction scheme. The application is the best place to decide which parts of the stream are most sensitive to error and protect them accordingly, rather than have a single "brute force" checksum that covers everything equally. An example of this can be seen in research by Hammer et al. where UDP-Lite is coupled with the AMR codec to give improved speech quality in lossy network conditions.

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