Research On CoAP Adaptive Congestion Control Mechanism

The development of the Internet of Things?IoT?relies on communication protocols,and traditional communication protocols have problems such as large head overhead and complex communication modes for constrained devices.The Constrained Application Protocol?CoAP?is a RESTful application layer protocol developed by the Internet Engineering Task Force?IETF?for constrained devices in IoT.Because CoAP is built upon UDP,the congestion control problem needs to be resolved within the protocol.The naked CoAP congestion control mechanism is Stop-and-wait ARQ retransmission mechanism based on Binary Exponential Backoff algorithm.However,the fact that the transmission parameter NSTART is set to 1 and remains unchanged during the communication limits the full use of network throughput potential.CoAP Simple Congestion Control/Advanced?CoCoA?,an IETF draft,provides CoAP with an extended congestion control mechanism that dynamically adjusts Retransmission Timeout?RTO?based on Round-Trip Time?RTT?.However,CoCoA's setting a fixed threshold for it's Variable Backoff Factor?VBF?mechanism makes it somewhat degraded to the naked CoAP congestion control mechanism.In order to solve the above problems,this paper proposes a VBF threshold adaptive adjustment scheme based on RTO_strong,which effectively avoids degradation of RTO estimator effect in CoCoA and can compensate for the overlarge RTO_overall problem caused by CoCoA's weak estimator ambiguity.At the same time,this paper also proposes a scheme that can adaptively adjust NSTART according to the state of the network to achieve a TCP-like sending window,and analyzes the working principle of the sending window through the mathematical modeling of the transmission rate.This scheme can make full use of the throughput potential of the network to improve transmission efficiency.The experimental results show that the proposed scheme can improve the throughput and rate of successful transaction by enhancing the adaptive characteristics of the CoAP congestion control mechanism.It has certain value for the theoretical research and application implementation of the CoAP congestion control mechanism.