A Cross Layer Design for Improving Trivial File Transfer Protocol in Mobile Transparent Computing

In Mobile Transparent Computing, client loads operating systems, applications and user data from server through wireless network. TFTP(Trivial File Transfer Protocol) is implemented to transfer data between client and server in wired LAN(Local Area Network) and is known for simplicity. However, because of the difference between wired and wireless LAN, TFTP is useless in WLAN(Wireless Local Area Network). TFTP uses a lock-step algorithm. That means after sending a data packet, the sender has to wait for an acknowledgement from the receiver before sending the next data packet. So that the transfer rate is limited by the RTT (round-trip time). The WLAN usually has a long latency, in other words, the RTT of WLAN is much higher than LAN, which means that the server is waiting for the arrival of acknowledgement during the most of time during the data transmission process, and wireless channel is idle because there is only one packet being transferred. The TFTP transfer rate is extremely low. To solve this problem, ``transfer window´´ scheme is introduced into TFTP. Meanwhile, in Mobile Transparent Computing, the transmission medium is 802.11a/b/g, and they all provide multi-rate capabilities, which means the bit rate of the wireless channel is changing dynamically, and consequently, the throughout of the channel is changing dynamically. Therefore, constant window size is not suitable for the TFTP in Mobile Transparent Computing. In this paper, we propose an improvement of TFTP named DTFTP(Dynamic Trivial File Transfer Protocol). DTFTP is based on a cross-layer design, in which the window size changes reasonably along with the bit rate change. The throughout increases about 54 percent when we use DTFTP to transfer data in Mobile Transparent Computing.