## 5 Experimental Evaluation
### Evaluation and Benchmarking of AQM Algorithms
#### 5.1.5 L4S  
In experiments (20)-(23), the internal parameters of L4S were configured with target = 15 ms, tupdate = 15 ms, alpha = 0.125, beta = 1.25, max-burst = 150 ms, max-ccrth = 0.1, quantum = 1514 bytes, limit = 10,240 packets, and flows = 6 queues.  
**Scenario 1 (Bandwidth = 10 Mbps, Delay = 20 ms)**: We employed two cases: (i) Case 1 is with enabled ECN; the results are shown in Figure 20, and (ii) Case 2: ECN is disabled, and the results are in Figure 21.  
L4S is based on FQ-PIE, so it behaves quite similarly, dropping packets instead of marking them when they exceed the ECN threshold. Upon receiving duplicate ACKS, the NewReno algorithm transitions from the slow-start phase to the congestion avoidance phase. Upon discovering retransmission timeout due to packet loss or high packet delay, it sets _cvnd_ to the maximum segment size of a TCP(MSS), and the'ssh_thrsh's halved and then enters the congestion avoidance phase. In the case of ECN, it sets the _cvnd_ threshold to the current _cvnd_ and enters the congestion avoidance phase. We can see this process in Figure 20b, and in Figure 21b, we can observe L4S going into fast-recovery and then into congestion avoidance immediately.  
L4S segregates incoming packets into separate L4S and classic queues, prioritizing L4S packets, so we observe a noticeably reduced overall RTT when ECN is enabled, as depicted in Figure 20c, compared to in Figure 21c where ECN is disabled. If ECN is enabled, all data-carrying packets in a TCP transmission are marked with ECN capability. Still, other packets, like acknowledgment (ACK) packets sent by the server, do not have ECN enabled in their packet header and are thus segregated into separate classic queues. Its prioritization scheme allows for more buffer capacity and prioritization for L4S packets.  
Figure 19: Case 2: FQ-PIE (ECN disabled)—Scenario 2 with Bandwidth = 1 Mbps, Delay = 20 ms. (a) Throughput; (b) Congestion Window; (c) Smoothed TCP RTT.  
Figure 18: Case 1: FQ-PIE (ECN enabled)—Scenario 2 with Bandwidth = 1 Mbps, Delay = 20 ms. (a) Throughput; (b) Congestion Window; (c) Smoothed TCP RTT.  
**Scenario 2 (Bandwidth = 1 Mbps, Delay = 20 ms)**: We evaluated the performance of L4S in two cases: Case 1 with enabled ECN and Case 2 with disabled ECN for comparison. Figures 22 and 23 present the results for the two cases.  
Similarly to FQ-PIE, L4S exhibits consistent behavior in both low- and high-bandwidth environments. In a low-bandwidth environment, reduced bandwidth capacity induces greater _cund_ fluctuation due to higher packet loss, as illustrated in Figures 22b and 23b. This higher _cund_ variation adversely affects throughput and RTT, inducing elevated levels of unfairness when bandwidth is low compared to when bandwidth is high, as depicted in Figures 22c and 23c.  
L4S in low-bandwidth environment demonstrates notably more exaggerated RTT when ECN is disabled. This increase in RTT is a direct consequence of lower available bandwidth, resulting in diminished throughput and performance of _cund_ due to higher packet loss and congestion.  
Figure 21: Case 2: L4S (ECN disabled)—Scenario 1 with Bandwidth = 10 Mbps, Delay = 20 ms. (a) Throughput; (b) Congestion Window; (c) Smoothed TCP RTT.  
Figure 22: Case 1: L4S (ECN enabled)—Scenario 2 with Bandwidth = 1 Mbps, Delay = 20 ms. (a) Throughput; (b) Congestion Window; (c) Smoothed TCP RTT.