L4Span is a prototype implementation for Low-Latency Low-Loss and Scalable (L4S) congestion signal architecture in the 5G network to achieve ultra-low sojourn time in the RLC buffer while maintaining a good capacity usage. L4Span is located above the SDAP layer to perform the ECN marking for 1) uplink ACK packet to short-circuit the RAN if possible (for TCP traffic) or 2) downlink packet to enable generic L4S marking mechanism (for UDP or QUIC traffic).

L4Span is published in ACM CoNEXT 2025: if you use this code, please cite our paper.

@article{10.1145/3768972,
author = {Wan, Haoran and Jamieson, Kyle},
title = {L4Span: Spanning Congestion Signaling over NextG Networks for Interactive Applications},
year = {2025},
issue_date = {December 2025},
publisher = {Association for Computing Machinery},
address = {New York, NY, USA},
volume = {3},
number = {CoNEXT4},
url = {https://doi.org/10.1145/3768972},
doi = {10.1145/3768972},
journal = {Proc. ACM Netw.},
month = nov,
articleno = {25},
numpages = {25},
keywords = {5G Network; Congestion Control; ECN Feedback; L4S Architecture}
}

L4Span is implemented on top of the srsRAN Project. Build instructions and user guides for srsRAN Project - srsRAN Project documentation.

Build Preparation

Dependencies

• Build tools:

  • cmake: https://cmake.org/

• Mandatory requirements:

  • libfftw: https://www.fftw.org/
  • libsctp: https://github.com/sctp/lksctp-tools
  • yaml-cpp: https://github.com/jbeder/yaml-cpp
  • mbedTLS: https://www.trustedfirmware.org/projects/mbed-tls/
  • googletest: https://github.com/google/googletest/

You can install the build tools and mandatory requirements with the commands below in Ubuntu:

Ubuntu 22.04

sudo apt-get install cmake make gcc g++ pkg-config libfftw3-dev libmbedtls-dev libsctp-dev libyaml-cpp-dev libgtest-dev

# uhd driver install
sudo add-apt-repository ppa:ettusresearch/uhd
sudo apt-get update
sudo apt-get install libuhd-dev uhd-host

Build and Run Instructions

First, clone the L4Span Project repository:

    git clone git@github.com:PrincetonUniversity/L4Span.git

Then build the code-base:

    cd L4Span
    mkdir build
    cd build
    cmake ../ 
    make -j $(nproc)
    make test -j $(nproc)

Start the Open-5GS as the 5G core.

And use the configuration file in config/l4span/cu.yml and config/l4span/du_rf_b200_tdd_n78_20mhz.yml as the CU and DU configuration files.

cp config/l4span/cu.yml build/app/cu/
cp config/l4span/du_rf_b200_tdd_n78_20mhz.yml build/app/du/
cd build/app/cu
sudo ./srscu -c cu.yml
# in another terminal window
cd build/app/du
sudo ./srsdu -c du_rf_b200_tdd_n78_20mhz.yml

Then, you can connect your phone into the RAN and test with the L4S-enabled congestion control scheme in the downlink direction with the L4S kernel. We tested with the Moto G 5G phones and Amarisoft UE Emulator.

File Structure

All our implementations of the L4Span functionalities are in the directory of lib/mark and include/mark, check these files to enable short-circuiting uplink packet marking or downlink packet marking. Please also refer to lib/cu_up/pdu_session_manager_impl.cpp on the creation of the L4Span entity.