The Transmission Control Protocol (TCP), used in the Internet, was not developed for a mobile, wireless environment. One reason why TCP might encounter problems in such an environment is rapid changes of the link characteristics. These rapid changes can occur due to handover between two subnetworks (macro handover), e.g., when a mobile node switches between different access networks. A possible and realistic handover scenario could be when a mobile node is roaming between a high bandwidth local area network (LAN) with limited coverage and a low bandwidth radio link with wide area coverage. The goals of this thesis were to set up a realistic environment for measurements of the handover performance of TCP, and to observe TCP behavior when the link characteristics suddenly change. Further objectives were to analyze the results and propose solutions for improving the performance. The mobility management in the measurement setup is handled by Mobile IP. Handovers are performed between a wireless LAN (WaveLAN) and a PPP link over a GSM circuit switched data connection. Our investigation shows that several spurious TCP timeouts occur after handover from the fast link to the slow link, triggering unnecessary retransmissions and hence resulting in TCP performance degradation. To avoid unnecessary retransmissions, we suggest a resetting of the retransmission timeout value (RTO) at the moment of handover. In the case of handover from the low bandwidth link (PPP) to the high bandwidth link (WaveLAN), our measurements show that queued packets in the link layer buffer continue to flow over the PPP link even after the handover. The high bandwidth available after the switch is thus poorly utilized before the buffer of the low bandwidth link has been emptied. The IP sending process should delay putting packets in the queue of a slow link, thus avoiding large link layer queues and enabling utilization of the high bandwidth link faster. This could be achieved by flow control between the IP layer and the link layer. After the packets have started flowing over the WaveLAN, the RTO value is unnecessarily high, which could result in extensive delays in the case of packet losses. To alleviate the problems we recommend resetting the RTO value or modifying the algorithm for calculating the RTO value to faster adapt to sudden and significant decrease of the round-trip time (RTT) in the case of handover. In both handover scenarios mentioned above we have found that a small window size is favorable to mitigate the negative effects due to the rapid changes of the link characteristics. The use of Active Queue Management to avoid large window sizes would be an interesting approach for future investigations. Another interesting approach could be to have flow control between the IP layer and the link layer to avoid a large link layer queue when the handover from [one link to another occurs.]
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:kth-95138 |
| Date | January 1999 |
| Creators | Ronquist, Mattias |
| Publisher | KTH, Teleinformatik |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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