Multi-homed mobile hosts situated in physical proximity may spontaneously team up to run high-bandwidth applications by pooling their low wireless wide-area network (WWAN) bandwidths together for communication with a remote application server and utilizing their high-bandwidth wireless local-area network (WLAN) in ad-hoc mode for aggregation and distribution of application contents among the participating mobile hosts. In this paper, we first describe the need for such a mobile collaborative community, or a community, in which multi-homed mobile hosts exploit the diversity of WWAN connections to improve a user-perceived bandwidth and network utilization. Then, we show that existing one-to-one communication protocols like TCP suffer significant performance degradation due to frequent packet reordering and heterogeneity ofWWAN links in the community. To address the above TCP problem, we propose a proxybased inverse multiplexer, called PRISM, that enables TCP to efficiently utilize the community members’ WWAN connections. PRISM runs at a proxy’s network layer as a routing component and stripes each TCP flow over multiple WWAN links by exploiting the transport-layer feedback information. Moreover, it masks variety of adverse effects specific to eachWWAN link via intelligent ACK-control mechanism. Finally, PRISM includes a sender-side enhancement of congestion control, enabling TCP to respond correctly to dynamically-changing network states. We have evaluated the PRISM protocol using both experimentation and ns-2-based simulation. Our experimental evaluation has shown PRISM to improve TCP’s performance by up to 310% even with two collaborative mobile hosts. Our in-depth simulation study also shows that PRISM delivers a near-optimal aggregated bandwidth in the community formed by heterogeneous mobile hosts, and improves network utilization significantly.
The below figure depicts the architectural overview of PRISM and its operational environment. PRISM consists of a network-layer inverse multiplexer (PRISM-IMUX) at the proxy and a network-assisted congestion-control mechanism (TCP-PRISM) at the sender side. PRISM interacts with a mobile community that is composed of multihomed mobile hosts.
- PRISM-IMUX: PRISM-IMUX is the routing component in a proxy that handles both the forward (data) and backward (ACKs) traffic of a TCP connection using up-to-date wireless links state information. PRISM-IMUX captures data traffic from a sender in the proxy’s network layer, and decides the best WWAN link as a next-hop through the Adaptive Scheduler (ADAS). It also captures and controls ACK packets to shield the adverse effects of striping over multipleWWAN links via the Reverse Path Controller (RPC). Finally, PRISM-IMUX maintains aWWAN links’ state table, has a buffer that temporarily stores ACKs which need to be re-sequenced, and supports GRE for indirection.
- TCP-PRISM: TCP-PRISM is a new sender-side congestion-control mechanism that works with PRISM-IMUX to expedite loss recovery, thus improving network utilization. TCP-PRISM reduces the loss recovery time via using the negative ACK information shipped by RPC at the proxy to detect a packet loss. Also, it adjusts the congestion window size according to the congested link bandwidth only, thus preventing waste of uncongested links’ bandwidth.
- Mobile Community: A mobile community is formed voluntarily and incrementally. When a new mobile node wants to join an existing mobile community, it rst searches for communities nearby using the Service Location Protocol. After determining the community of most interest to itself, the mobile node/host joins the community and works as either a relay node or a receiver. The node receives packets from PRISM-IMUX via its WWAN link, and forwards packets to the receiver, through its WLAN interface in ad-hoc mode. Or, the node receives packets via multiple community members’ WWAN links, and sends ACKs to the sender through one of the WWAN links.
- Kang G. Shin
- Kyu-Han Kim
- Kyu-Han Kim and Kang G. Shin. PRISM: Improving the Performace of Inverse-Multiplexed TCP in Wireless Networks. IEEE/ACM Transactions on Mobile Computing. , 2007. PDF.
- Kyu-Han Kim and Kang G. Shin. Improving TCP Performance over Wireless Networks with Collaborative Multi-homed Mobile Hosts. Proceedings of the 3rd USENIX/ACM International Conference on Mobile Systems, Applications, and Services, Seattle, WA. , June, 2005. PDF.
- Handheld Router in collaboration with HP Labs