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New types of networks are emerging for the purpose of transmitting large amounts of scientific data among research institutions quickly and reliably. These exotic networks are characterized by being high-bandwidth, high- latency, and free from congestion. In this environment, TCP ceases to be an appropriate protocol for reliable bulk data transfer because it fails to saturate link throughput. Of the new protocols designed to take advantage of these networks, a subclass has emerged using UDP for data transfer and TCP for control. These high-speed variants of reliable UDP, however, tend to underperform on all but high-end systems due to constraints of the CPU, network, and hard disk. It is therefore necessary to build a high-speed protocol adaptive to the performance of each system. This paper develops such a protocol, Performance Adaptive UDP (henceforth PA-UDP), which aims to dynamically and autonomously maximize performance under different systems. A mathematical model and related algorithms are proposed to describe the theoretical basis behind effective buffer and CPU management. Based on this model, we implemented a prototype under Linux and the experimental results demonstrate that PA-UDP outperforms an existing high-speed protocol on commodity hardware in terms of throughput and packet loss. PA- UDP is efficient not only for high-speed research networks but also for reliable high-performance bulk data transfer over dedicated local area networks where congestion and fairness are typically not a concern.