Consider the scenario wherein a mobile host (MH) is communicating with a fixed h
ID: 3599954 • Letter: C
Question
Consider the scenario wherein a mobile host (MH) is communicating with a fixed host (FH) through Mobility Support Router (MSR-1) using the Indirect TCP (I-TCP) protocol. The MSR sends to the MH the packets P1, P2, P3, P4 and P5 in that order. Packet P3 is dropped but packet P4 reaches the MH causing it to generate the first duplicate acknowledgement (DUPACK). Before this DUPACK packet arrives at the MSR it has already sent out packet P5. Write down the sequence of steps that will be performed by the MSR and MH to bring the system to a consistent state.
Explanation / Answer
Internetworking protocols for mobile hosts have hitherto treated host mobility as a routing problem to be handled entirely within the network (IP) layer. Such an approach however, ignores the distinctive features of wireless mobile computing. IP-based transport protocols thus suffer from poor performance when used for communication between a mobile host and hosts on the wired network. This is caused by frequent disruptions in network layer connectivity due to mobility and wireless losses. We describe indirect transport layer protocols for mobile hosts which can tackle mobility and wireless related performance problems without compromising backward compatibility with the transport protocols used over the wired network. Indirect protocols utilize the resources of Mobility Support Routers (MSRs) to provide transport layer communication between mobile hosts and those on the fixed network. We also present performance figures for I-TCP, an indirect transport protocol for mobile computers that is compatible with TCP, showing substantial improvement in throughput over regular TCP.