scim

README

Scim Congestion Control Simulator

Scim is a discrete event simulator for testing ECN-based congestion control, with a focus on high-fidelity signaling.

Scim was written for educational use, and for quickly trying new ideas, such as Multiplicative Decrease Scaling and Extended Slow Start with Pacing. It does not aim to precisely simulate real-world networks, but to show that congestion control algorithms (CCAs) and Active Queue Management (AQM) algorithms behave as expected in an idealized environment.

Installation

Scim can run on any Go-supported platform, but xplot is required to view the plots.

1. Install Go (1.22 or later required).
2. mkdir -p ~/go/src/github.com/heistp && cd "$_"
3. git clone https://github.com/heistp/scim
4. Install xplot.

Note: with GCC 14, I had to build xplot from xplot-0.90.7.1.tar.gz as follows:

CFLAGS="-Wno-error=implicit-int -Wno-error=implicit-function-declaration" ./configure
make

Running and Configuring

Change to the scim directory. Either use the ./run script (requires bash), or use go build manually and run ./scim.

Scim is configured via the file config.go, so the program must be recompiled each time the config is changed. That's what the run script does.

Sample Plot

In Figure 1, we see Reno vs Reno-SCE at two different RTTs in a 100 Mbps bottleneck, converging to equal cwnd, with sojourn time minimized by the DelTiM AQM.

Figure 1: Reno (80 ms, in white) vs Reno-SCE (20 ms, in green) at 100 Mbps with the DelTiM AQM

Features

General:

• Capacity seeking flows
• Per-flow path RTTs
• Flow scheduling
• Bottleneck rate changes
• Pacing
• Delayed ACKs
• Pluggable slow-start and CCAs
• SCE signaling

CCAs:

• Reno and Reno-SCE
• CUBIC and CUBIC-SCE
• Scalable and Scalable-SCE
• Maslo (experimental)

AQMs:

• DelTiC
• DelTiM (Delay Time Minimization, a DelTiC variant)
• Brick wall (CE threshold)
• Ramp (CE / SCE marking based on linear sojourn time ramp)

Slow-start algorithms:

• Standard (RFC5681)
• Extended Slow Start with Pacing
• HyStart++

Plots:

• In-flight bytes
• Cwnd
• Pacing
• TCP sequence
• Queue sojourn time and length
• AQM marking proportion or frequency
• Throughput

Scim does not handle packet loss, only ECN signaling.

It is configured with a list of Handlers (see main.go), which form a ring. Messages between Handlers are sent round-robin style, and events are processed linearly in time, so that any two simulator runs always produce the same results. Each Handler runs in a separate goroutine, but they are synchronized by the simulator (see sim.go) so the result is deterministic.

Scim's emphasis is on robustness rather than performance. I see it process around 70k packets/sec on a Ryzen 5 4500U, and 140k packets/sec on a Ryzen 9 5900HX. YMMV.