Radio Link Protocol

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Radio Link Protocol (RLP) is an automatic repeat request (ARQ) protocol used over a wireless (typically cellular) air interface. Most wireless air interfaces are tuned to provide 1% packet loss, which is a tolerable loss rate for modern Vocoders. An RLP detects packet losses and performs retransmissions to bring packet loss down to .01%, which is suitable for TCP/IP applications. RLP also implements stream fragmentation and reassembly, and sometimes, in-order delivery. Newer forms of RLP also provide framing and compression, while older forms of RLP rely upon a higher-layer PPP protocols to provide these functions.

An RLP transport never knows how big a packet the air interface will provide. Instead, the air interface scheduler determines the packet size, and calls upon RLP to form a packet on-demand for transmission. Most other wireless fragmentation and framing protocols, such as those of 802.11b and TCP/IP, used fixed fragment sizes. These protocols are not as flexible as RLP, and can sometimes needlessly block transmissions during a deep fade in a wireless environment.

An RLP protocol can be ACK-based or NAK-based. Because the reverse link is very expensive on most cellular networks, most RLP's are NAK-based, meaning that the sender assumes that the transmission got through, and the receiver only NAKs when an out-of-order segment is received. When the transmit pipeline goes idle, a NAK-based RLP must eventually retransmit the last segment a second time to reach .01% packet loss rate. This duplicate transmission is typically controlled by a "flush timer" set to expire 300-500 milliseconds after transmission has completed.

The concept of an RLP protocol was invented by Phil Karn in 1990.

Cellular networks such as GSM and CDMA use different variations of RLP. The January 2006 IEEE 802.20 specification uses one of the newest forms of RLP.