UTCP: Unordered transmission control protocol (TCP) for high throughput bulk data transfer

Abstract

Transmission Control Protocol (TCP), which offers a reliable, in-order packet delivery service, is at the heart of many Internet protocols including File Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP). It is a known phenomenon that in-order nature of TCP causes what is known as a head-of-line blocking problem. That is, when the first packet within a TCP sender window is lost, the window will not move forward until the packet is retransmitted and successfully recovered at the receiver. This causes the channel to stay idle until the lost packet is recovered, which decreases the overall throughput. In this paper we propose adding an unordered packet delivery service interface to TCP (called Unordered transmission control protocol UTCP), that would remedy this problem. To implement this interface the sender views the active window to consist of multiple independent logical channels and pumps data in from any idle channel. Thus the loss of the first packet within the sender window does not cause the channel to stay idle. A receiver selectively acknowledges each packet and delivers the packets in arrival order to the applications. Applications using UTCP interface would be required to perform packet reordering at the application layer. We describe how UTCP can be implemented with simple changes to an existing TCP implementation, compare UTCP to various TCP implementations using the popular "ns" simulator, and show that the performance of bulk data transfer applications improve up to 350% compared to Reno TCP and up to 25% compared to New Reno TCP