With the advent of small portable computers and the technological advances in wireless communications, mobile wireless computing is likely to become very popular in the near future. Wireless links are slower and less reliable compared to wired links and are prone to loss of signal due to noise and fading. Furthermore, host mobility can give rise to periods of disconnection from the fixed network. The use of existing network protocols, which were developed mainly for the high bandwidth and faster wired links, with mobile computers thus gives rise to unique performance problems arising from host mobility and due to the characteristics of wireless medium. Indirect protocols can isolate mobility and wireless related problems using mobility support routers (MSRs) as intermediaries, which also provide backward compatibility with fixed network protocols. We present the implementation and performance evaluation of I-TCP, which is an indirect transport layer protocol for mobile wireless environments. Throughput comparison with regular (BSD) TCP shows that I-TCP performs significantly better in a wide range of conditions related to wireless losses and host mobility. We also describe the implementation and performance of I-TCP handoffs.

This paper deals with multicasting in an internetwork with mobile hosts, particularly with regard to mobile-IP and distance vector multicast routing (DVMRP) protocols. When the source of a multicast datagram is a mobile host (MH), the datagram may not reach all group members to which the datagram is addressed, including other mobile hosts. When the source is a static host and the multicast group includes mobile hosts, a mobile group member may receive datagrams in one cell but not in another. Further when a MH enters a cell which contains no other member of the same group, the MH will experience a delay before it starts receiving datagrams addressed to that group. Mobility between campuses which result in a MH acquiring an additional unicast address, also has an effect on multicast routing. We propose enhancements to DVMRP executed at the mobility support routers (MSR) that ensure correct forwarding of multicast datagrams to and from mobile hosts. Our solutions do not require any change at hosts and routers unaware of mobility, i.e. the modifications are limited to MSRs and MHs. We also describe an implementation incorporating a subset of our proposals. Lastly, we show that alternate styles of multicasting or mobile networking, viz. link-state (MOSPF) and IETF proposal, will face similar problems and our proposed solutions are still valid in their context.

IP based solutions to accommodate mobile hosts within existing internetworks do not address the distinctive features of wireless mobile computing. IP-based transport protocols thus suffer from poor performance when a mobile host communicates with a host on the fixed network. This is caused by frequent disruptions in network layer connectivity due to i) mobility and ii) unreliable nature of the wireless link. We describe I-TCP, which is an indirect transport layer protocol for mobile hosts. I-TCP utilizes the resources of Mobility Support Routers (MSRs) to provide transport layer communication between mobile hosts and hosts on the fixed network. With I-TCP, the problems related to mobility and unreliability of wireless link are handled entirely within the wireless link; the TCP/IP software on the fixed hosts is not modified. Using I-TCP on our testbed, the throughput between a fixed host and a mobile host improved substantially in comparison to regular TCP.