This is the SCE (Some Congestion Experienced) reference implementation, containing:

• new TCP CCAs (congestion control algorithms) with SCE support: reno-sce, dctcp-sce and cubic-sce
• SCE qdiscs: deltic, cake, cobalt, twin_codel_af, cnq_codel_af, cnq_cobalt, lfq_cobalt
• changes to TCP input for SCE to ESCE feedback

The CCAs are all working, but may need subtle tweaks.

The various qdiscs in development are experiments with how to improve the deployability of SCE in a small number of queues. While flow isolation and fairness is straightforward with FQ, mechanisms for achieving both without FQ are still under research. Possibilities include approximate fairness (see "af" qdiscs and CodelAF), DSCP, flow-aware queue selection, or queueing algorithms that do not adhere to strict FIFO semantics such as LFQ.

We welcome any problem reports as Issues on this repo.

Compile the Linux kernel as usual, making sure to include the new CCAs in the config (under Networking support > Networking options > TCP: advanced congestion control) and the SCE qdiscs (under Networking support > Networking options > QoS and/or fair queueing). Here's a quick overview of a typical way to do this:

sudo apt-get install build-essential libncurses-dev libssl-dev
git clone https://github.com/chromi/sce
cd sce
make menuconfig # configure kernel, include all TCP CCAs and Cake
make -j $(nproc)
make modules
sudo make modules_install
sudo make install
sudo reboot

New qdiscs and parameters are added which require changes to the tc binary. Below are the commands that may be used to compile and install the updated tc.

sudo apt-get install pkg-config bison flex libcap-dev libmnl-dev libelf-dev libdb-dev
git clone https://github.com/heistp/iproute2-sce
cd iproute2-sce
./configure
make
# use the resulting tc/tc binary when working with SCE qdiscs

By default, SCE is not enabled. The following sysctls may be used to control the operation of SCE and related functionality:

• net.ipv4.tcp_sce - set to 1 to feed back SCE to ESCE (for receivers)
• net.ipv4.tcp_ecn - set to 1 to enable ECN during connection initiation (for senders)
• net.ipv4.ip_forward - set to 1 to enable IP forwarding, e.g. on middleboxes running Cake
• net.ipv4.tcp_congestion_control - set to one of reno-sce, dctcp-sce or cubic-sce to default all TCP connections to use this CCA, although setting it explicitly for individual test flows may be the preferred method

To test SCE, a bottleneck and SCE marking are needed between the TCP sender and receiver. As an example, the Cake qdisc from this kernel may be used at the bottleneck. Cake's shaper is used to restrict bandwidth and create a bottleneck, then Cake can be configured to do SCE marking, which will mark IP packets with SCE as queue sojourn times increase. An example invocation of Cake is as follows:

tc qdisc add dev enp1s0 root cake besteffort bandwidth 50Mbit sce

Replace enp1s0 with your interface and 50Mbit with your bandwidth. On a middlebox with two interfaces, you may want Cake applied on both interfaces. The sce parameter tells Cake to do SCE marking. The following additional parameters may be used to control the operation of SCE:

• sce-single - maximally prefers SCE vs non-SCE fairness over early SCE signaling.
• sce-thresh # - where # is a number from 1-1024, with 1 being equivalent to sce, and 1024 being nearly equivalent to sce-single. This balances SCE vs non-SCE throughput fairness with earlier SCE signaling. Numbers around 20 may yield a reasonable balance for 50Mbit at 80ms for reno vs reno-sce competition, for example.

iperf3 may be used for simple TCP flow tests. Note that root is required to explicitly set the CCA:

sudo iperf3 -C reno-sce -c server

Flent or Antler may be used to run multi-flow tests with different CCAs, along with latency measurement flows. Flent is more mature, while Antler is still under development, but provides some new capabilities, like auto-installed test nodes, VBR UDP flows and CUE configuration.

A tool called scetrace has been implemented that uses libpcap to record per-flow SCE and related statisics from either pcap files or live captures. This is available here.

Test repositories that may be of interest:

• sce-l4s-bakeoff
• l4s-tests