An analytic model is presented for a partially reliable transport protocol based on retransmissions. The model illustrates tradeoffs between two QoS parameters (delay and throughput), and various levels of reliability. The model predicts that the use of a reliable transport service when an application only needs a partially reliable one causes considerable throughput decreases and delay increases in lossy networks. On the other hand, over lossy networks, an unreliable transport service is unable to respect an application's loss tolerance. In lossy environments, partially reliable transport service avoids the extra cost of a reliable transport service, and, simultaneously, guarantees the minimal reliability that an application requires. A retransmission-based partially reliable transport service can be provided through either sender-based or receiver-based loss detection and recovery. Results show that both techniques provide almost identical reliability and delay. However, a sender-based approach provides better throughput than a receiver-based approach at high acknowledge loss rates.

Many applications such as video and audio can tolerate loss. When the network layer provides a best-effort service such as on the Internet, the loss rate of the underlying network service may be higher than an application's tolerance for loss. This paper analytically studies retransmission-based partially reliable transport (layer protocol) service. Results show that a partially reliable transport service provides increasingly higher throughput and lower delay than a reliable transport service as an application's loss tolerance increases and as the underlying network service gets more lossy. Also, to some degree, a partially reliable transport service eases the negative effects of ack losses on throughput. Three cost functions associated with the reliability level that a system can support are introduced. These cost functions help demonstrate the penalty when a transport service does not support the ideal reliability level for an application. Results show that the use of a reliable transport service when an application only needs a partially reliable transport service can cause considerable throughput drops and delay increases in lossy networks. On the other hand, at high loss rates, an unreliable transport service is unable to respect an application's loss tolerance. Thus, in lossy environments, a partially reliable transport service is necessary to avoid the extra cost of a reliable transport service, and, at the same time, to guarantee the minimal reliability that an application requires.