ECN Implementations in the NS Simulator
Sally Floyd, Kevin Fall · 1998
This document illustrates the validation test for ECN (Explicit Congestion Notification) in the NS simulator [NS95]. Figure 1 shows a single simulation with two-way traffic, showing the use of the CE (Congestion Experienced) bit in the IP packet header, and the CWR (Congestion Window Reduction) and ECN-Echo bits in the TCP header. We refer to a packet with the CE bit marked as a CE packet. Similarly, a CWR packet denotes a packet carrying a CWR notification, and an ECN-Echo ACK packet denotes an ACK packet with the ECN-Echo bit set. Figures 2 and 3 show that multiple CE packets in a window of data are considered as a single instance of congestion. Figures 4, 5, 6, and 7 show that interspersed dropped packets and CE packets are treated as a single instance of congestion. While the behavior in these simulations is specific to Tahoe TCP, similar simulations in the test suite show that the same property holds for the Reno, NewReno, and Sack TCP implementations. Figures 8 and 9 are a separate simulation scenario showing that interspersed dropped packets and CE packets are treated as a single instance of congestion. Figures 10, 11, and 12 show that interspersed dropped packets and CE packets are treated as a single instance of congestion even when the packet drops are sufficiently severe to result in a retransmit timeout. Figure 13 shows that retransmitted packets with the CE bit set are interpreted as a new instance of congestion. 2 Running the tests in NS