High Performance TCP Communication over Mobile IP Handoff

近年来，移动通信环境得到了广泛的推广。以第三代蜂窝网络为例，采用移动IP作为该网络的基本协议之一。在移动IP网络环境中，由于无线链路传输错误和/或终端移动过程中的切换导致丢包的情况时有发生，在这种环境下，通常还会使用TCP来访问WWW服务器和传输电子邮件。然而，由于上述丢包频繁地调用了TCP的拥塞控制功能，导致移动环境下的TCP通信吞吐量严重下降，目前已有很多研究针对这一问题，但大多集中在传输错误引起的丢包问题上。然而，在移动IP网络中，切换造成的分组丢失也会导致严重的吞吐量下降。在本研究中，我们提出了一种即使在移动IP切换导致丢包的情况下也能提高吞吐量的TCP通信过程，该过程允许外地代理在M-TCP过程中共同作为管理主机工作，并通过使通信节点处于持久模式来暂停TCP通信。我们的程序还允许外地代理在移动IP注册过程中进行协调，并允许归属代理通过发送窗口更新而不是移动节点来立即重新启动TCP通信。我们得出的结论是，我们的过程比传统的基于移动IP的TCP和基于移动IP的M-TCP都要好，即外地代理只支持原始的M-TCP功能，而不允许归属代理发送任何窗口更新，这是我们建议的一部分。

Recently, the mobile communication environment is widely spread. An example is the 3rd generation cellular network, and the Mobile IP is adopted as one of the basic protocols for such a network. In the Mobile IP network environment, it is often the case that packet losses are caused by the transmission errors in wireless links and/or the handoff during terminal movement.In such an environment, TCP is also commonly used to access WWW servers and to transfer e-mail. However, the packet losses are described above invoke the congestion control function of TCP frequently, and as the result, the throughput of TCP communication over mobile environment is degraded heavily.So far, there have been a lot of studies for this problem, but most of them focus mainly on the packet losses caused by transmission errors. However, in the Mobile IP networks, the packet losses caused by handoff also invoke serious throughput degradation. In this research, we propose a TCP communication procedure improving the throughput even when there are packet losses caused by the Mobile IP handoff.Our procedure allows foreign agents to collectively work as a supervisor host in M-TCP procedure and to suspend TCP communication by making a correspondent node in persist mode. Our procedure also allows foreign agents to coordinate during the Mobile IP registration procedure and a home agent to restart the TCP communication immediately by sending a window update instead of the mobile node.We have also shown the performance evaluation by using modified ns-2 simulator. We have concluded that our procedure outperforms both the conventional TCP over Mobile IP and the "M-TCP over Mobile IP" i.e. foreign agents support only the original M-TCP functions without allowing home agent to send any window update, which is a part of our proposal.