TCP Congestion Control Performance Issues in Non-Standalone 5G NR Networks
Lukas Prause, Mark Akselrod · 2023
The ongoing deployment of 5G networks and the heterogeneity of carrier aggregation in 4G networks have resulted in vast differences of cellular network performance across different locations. Especially for mobile users, such fluctuation in performance based on location can result in a highly degraded user experience. The problem is compounded by the fact that popular TCP congestion control variants, such as CUBIC and BBR, can be slow to adjust their congestion window when network conditions change rapidly.In this paper, we analyze the extent to which the network conditions can change over brief periods of time for mobile users by performing repeated drive tests on a route with a diverse network coverage. We show that due to differences in the availability of different 4G and 5G carriers, the total carrier bandwidth available to a user can change multiple times in a period of a few seconds, which results in major changes of achievable throughput. As TCP can be slow in adjusting its congestion window, this can result in either low bandwidth utilization or high delays due to bufferbloat. We propose that for a more efficient bandwidth utilization, the information about total carrier bandwidth could be used in TCP congestion control.