On the Interplay of Congestion and OSNR Degradation for TCP over Optical Networks

Abstract—This work investigates transmission control protocol (TCP) performance over optical networks, highlighting the interplay of physical layer effects and congestion caused by buffer overflow at intermediate nodes on different congestion control mechanisms. In optical networks packet dropping will happen due to optical signal-to noise ratio (OSNR) degradation due to amplified spontaneous emission (ASE) and nonlinear effects (NLI) in fiber affecting differently congestion control algorithms (CCAs) employed by TCP. This study also aims to evaluate the bandwidth competition behavior of TCP CCAs in congestion (i.e, buffer overflows) and discard due to bit error rate caused by OSNR degradation. Experiments were conducted using Mininet-Optical, along with Linux tc and iperf3, to emulate real world conditions. The degradation of OSNR level to 19.45, 19.15 and 18.95 dB, causing approximately 0.2, 0.5 and 0.8% of packet losses, seems to have a positive impact on fairness by avoiding self-synchronization and slow convergence of competing CCAs-based flows. This may indicate that stringent signal quality requirements can be relaxed in forthcoming IPoDWDM consolidation of optical and packet networks.

Index Terms — Congestion Control Algorithms, Physical Layer Impairments, Optical Networks.

Fig 1 - Setup: Setup created for flow testing with competition between 10 TCP flows.

Results

Fig. 2 - Average throughput and Jain Fairness Index by TCP congestion control lossless algorithm, gOSNR ≥ 20 dB

Fig. 3 - Retransmission and Throughput average per Queue Variation by TCP congestion control lossless algorithm, gOSNR ≥ 20 dB

Fig. 4 - Average throughput and Jain Fairness Index by TCP congestion control algorithm with 0.2% losses - gOSNR ≈ 19.45 dB

Fig. 5 - Retransmission and Throughput average per Queue Variation by TCP congestion control algorithm with 0.2% losses - gOSNR ≈ 19.45 dB

Fig. 6 - Average throughput and Jain Fairness Index by TCP congestion control algorithm with 0.5% losses - gOSNR ≈ 19.15 dBs

Fig. 7 - Average throughput and Jain Fairness Index by TCP congestion control algorithm with 0.8% losses - gOSNR ≈ 18.95 dB

Fig. 8 - Average throughput and Jain Fairness Index by TCP congestion control algorithm with 0.8% losses - gOSNR ≈ 18.95 dB

Fig. 9 - Retransmission and Throughput average per Queue Variation by TCP congestion control algorithm with 0.8% losses - gOSNR ≈ 18.95 dB

All results can be reproduced by running the scripts available in the "scripts" directory