dcSIM Simulator

A discrete-event network simulator for evaluating datacenter transport protocols, with a focus on the dcSIM algorithm (referred to as SNS in the codebase).

Publication

Simulate Before Sending: Rethinking Transport in Datacenter Networks Dan Straussman (Technion), Isaac Keslassy (Technion and UC Berkeley), Alexander Shpiner, Liran Liss (Nvidia).

Presented at the NINeS Conference, February 2026.

• Paper
• Presentation (Video)

Overview

dcSIM is a simulation-based scheduling protocol for datacenter networks. The core idea is to send lightweight simulation (SIM) packets ahead of data to probe network path congestion, then schedule actual data transmission based on observed conditions.

dcSIM (SNS) Algorithm

1. SIM Phase — The sender transmits small header-only simulation packets that traverse the network path to the receiver, probing congestion and queuing delays.
2. SIM_ACK Phase — The receiver replies with a SIM_ACK packet (acting as a bandwidth reservation) back to the sender.
3. DATA Phase — Upon receiving the SIM_ACK, the sender schedules the actual data packet at a calculated time (sim_send_time + worst_case_RTT), ensuring data arrives when the probed path is available.
4. DATA_ACK Phase — The receiver acknowledges received data.

Supported Protocols

ID	Protocol	Description
117	SNS (dcSIM)	Simulation-based scheduling
116	PIM	Multi-round distributed matching
115	RUF	Receiver-driven scheduling
114	Fastpass	Centralized scheduling
112	Capability	ExpressPass-like credit-based
113	Magic	Ideal (oracle) scheduling
2	pFabric	Priority-based flow scheduling
42	Vanilla TCP	Standard TCP
43	DCTCP	Data Center TCP
120	Ideal	Ideal baseline with arbiter

Building

Prerequisites

• GCC (version 6 recommended)
• GNU Autotools (autoconf, automake, aclocal)

Compile

aclocal
automake --add-missing
autoconf
./configure
make

Note: If your GCC version is greater than 6, you may encounter compilation issues. Use GCC 6 explicitly:

CC='gcc-6' CXX='g++-6' ./configure
CC='gcc-6' CXX='g++-6' make

This produces two binaries:

• simulator — Optimized build (-O3)
• simdebug — Debug build (-O0)

Usage

./simulator <exp_type> <conf_file>

• exp_type:
  • 1 — Run full simulation
  • 2 — Generate flows only (no simulation)
• conf_file — Path to a configuration file (key-value format)

Configuration File Format

Plain text, one parameter per line (key: value). Lines starting with # or // are comments.

Core Parameters

Parameter	Description	Example
topology	LeafSpine or FatTree	LeafSpine
num_hosts	Number of hosts	144
bandwidth	Link bandwidth (bps)	10000000000
propagation_delay	Link propagation delay (s)	0.0000002
queue_size	Switch queue size (bytes)	36864
flow_type	Protocol ID (see table above)	117
queue_type	1=DropTail, 2=pFabric, 5=DCTCP, 6=SNS	6
load	Network load factor	0.8
num_flow	Number of flows to simulate	100000
flow_trace	CDF or flow trace file path	trace.txt

dcSIM (SNS) Specific Parameters

Parameter	Description
big_mtu	MTU size: 0 = 1500B, 1 = 4KB, 2 = 9KB (jumbo)
synced_mode	1 = synchronized scheduling, 0 = immediate
clock_drift	Host clock drift for realistic timing
worst_rrt_factor	Worst-case RTT multiplier
debug_sns	Enable SNS debug output

Project Structure

coresim/       Core simulator engine (events, topology, hosts, packets)
ext/           Protocol implementations (dcSIM/SNS, PIM, RUF, pFabric, TCP, etc.)
run/           Experiment runner, flow generators, configuration parsing

Acknowledgments

This simulator is built upon the dcPIM simulator.