# On Cross-Layer Interactions for Congestion Control in the Internet
## 6 Conclusions  
In this paper, an extensive evaluation of cross-layer interactions between commonly used TCP flavors, including New Reno, Cubic, Compound, and recent Active Queue Management strategies, was conducted to answer fundamental questions:  
1. To what extent an AQM influences the end-to-end congestion control mechanism? 2. What are the effects of network parameters, such as RTT variation and congestion level, on the combined TCP and AQM performance? 3. Can cross-layer interaction between TCP and AQM overcome the challenges of network heterogeneity and variability? 4. Is there one universal TCP-AQM combination for all cases? Re (1) The simulation results revealed that in combination with DT, none of the evaluated TCP provides fair bandwidth sharing, especially when the network load increases. Moreover, when all TCPs are used simultaneously without any AQM TCP Compound, which is a delay-based algorithm, there is a decrease in performance when competing with TCP flows from the loss-based group. On the other hand, TCP Cubic is the most aggressive congestion control mechanism, utilizing over 50% of available bandwidth. Applying PIE or CoDel significantly improves the inter- and intra-fairness provided by the evaluated congestion control mechanisms. Re (2) Varied RTT significantly worsens the performance of all TCP-AQM combinations in terms of achievable fairness among competing flows. Moreover, with the growing network load, the fairness decreases even more, the drop ratio is higher, and providing a low and stable queue becomes very challenging. Re (3) DT always leads to a persistently full buffer, which results in long delays and high delay variations; furthermore, the higher network load, the fewer flows can transmit data packets efficiently. Applying any AQM noticeably improves the RTT-fairness; though, the aggressiveness of the congestion control protocol cannot be eliminated. In most cases, PIE and CoDel allow maintaining relatively short and stable queues, which reduces the experienced delay and delay variations, and improves the inter- and intra-fairness among competing flows. Re (4) No combination would result in the highest fairness, low drop ratio, and low and stable queue simultaneously, regardless of the congestion level or RTT variability. In general, CoDel, especially in combination with C-TCP, maintains the shortest queues and provides satisfactory fairness, however only in an uncongested network. PIE is the only  
Figure 9: The queue length of DT, PIE and CoDel in combination with mix TCP flows in the Access Link scenario (**a**) in uncongested network, (**b**) during mild congestion, (**c**) during heavy congestion. AQM that, with all TCPs, in all load scenarios provides similar results in terms of queue length and stability, reasonable fairness, however at the cost of a slightly higher drop ratio. PIE and CoDel configuration parameters were supposed to be auto-tuned and insensitive to varied round-trip delays, link rates, and traffic loads. Unfortunately, there are some operational ranges for which both AQMs work as desired and provide low and stable queuing delay. Outside those ranges some additional configuration is beneficial. Especially, the target delay should be reduced, according to the network speed, for both AQMs. PIE and TCP Cubic seem to represent the most universal combination that works well in both the high-speed network with low delays and the access link connecting an institution to an ISP. PIE is insensitive to traffic load, and TCP Cubic deals better with varied RTT in terms of fairness provided. CoDel with C-TCP works well in the network with similar RTTs and low traffic load. C-TCP is a good choice in an uncongested network when we cannot deploy the AQM, because it provides the highest fairness and prohibits filling the entire buffer. The results obtained in this paper are contradictory to the results presented in [16]. However, it is not surprising, taking into account the configuration of the simulation environment.