TCP HandOff: A Practical TCP Enhancement for Heterogeneous Mobile Environments

In recent years, many different kinds of wireless access networks have been deployed for the Internet and have become inseparable parts of the Internet. But TCP, the most widely used transport protocol of the Internet, was designed for stationery hosts. In particular, TCP faces severe challenges when user moves around in these networks and handoff occurs frequently. In this paper, TCP handoff (TCP-HO), a practical end-to-end mechanism, is proposed for improving TCP performance in heterogeneous mobile environments. TCP-HO assumes that a mobile host is able to detect the completion of handoff immediately and has a coarse estimation of new wireless link's bandwidth. When a mobile host detects handoff completion, it will immediately notify the server through two duplicate ACKs, whose TCP option also carries the bandwidth of new wireless link. After receiving this notification, the server begins to transmit immediately and keeps updating ssthresh according to the bandwidth from mobile host and its new RTT samples. This updating will be stopped after four RTT samples or after congestion is detected. TCP-HO has been implemented in FreeBSD 5.4. Experimental results show that TCP-HO does improve TCP performance without adversely affecting cross traffic in a heterogeneous mobile environment.