The wide range of IoT applications creates a demand for various types of communication. While both types of communication, unreliable and reliable, are important, it is crucial to extend scalable congestion control in the Internet to cover IoT communication also. Constrained Application Protocol (CoAP), a web transfer protocol for constrained devices and networks proposed by Internet Engineering Task Force (IETF), has optional reliability based on retransmission timeout (RTO) together with exponential RTO backoff to implement a simple basic congestion control mechanism. While the CoAP basic congestion control is simple, it is relatively conservative and potentially inefficient. Hence, there is a need for a more efficient congestion control alternative and CoCoA has been proposed as an option. In this paper we experimentally evaluate the efficiency and scalability of alternative congestion control algorithms for CoAP, including CoCoA and two TCP-based mechanisms. Our results show that while all the alternatives are scalable, they are more aggressive than the default congestion control mechanism of CoAP resulting in more efficient operation particularly at higher congestion level(s).
Vertical hand-offs between different wireless access technologies have become more relevant after the recent introduction of multi-access mobile terminals with Wireless LAN (WLAN) and Wireless WAN (WWAN) technologies. While the IP mobility mechanisms are rather well known, the performance of TCP still has problems when moving between WLAN and WWAN accesses. First, with a high-latency WWAN link technology such as GPRS it takes several seconds before the TCP congestion window has reached the path capacity. Second, when the notification of the first packet loss arrives at the TCP sender, several packets have already been lost due to the slow-start overshoot and the TCP sender needs to retransmit a large number of the packets from the last transmission window. Third, after a vertical hand-off the path characteristics might have changed dramatically in which case the TCP congestion control state is not valid anymore. In this paper we investigate Quick-Start, a mechanism for avoiding the initial slow-start delay, in the context of wireless multi-access terminals. We also propose an enhancement to Quick-Start to alleviate the effects of slow-start overshoot and apply Quick-Start after a vertical hand-off to quickly learn the available capacity on the new end-to-end path. An explicit cross-layer hand-off notification is employed to trigger Quick-Start when the hand-off completes. We conduct simulations with different hand-off models, and our simulations yield promising results with Quick-Start.
In this paper we study the performance of TCP with a vertical handoff between access networks with widely varying link characteristics. TCP being an end-to-end protocol has performance problems as its behaviour depends on the end-to-end path properties which are likely to be affected by a vertical handoff. We propose a set of enhancements to the TCP sender algorithm that leverage on explicit cross-layer information regarding the changes in the access link delay and bandwidth. We carry out a systematic study to identify the problems of regular TCP and compare the performance of the regular TCP with the performance of the enhanced TCP algorithms in various handoff scenarios between access networks having different bandwidth and delay characteristics. Our experiments show that with cross-layer notifications TCP performance can be improved significantly in most vertical handoff scenarios.
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