TCP ROCCET :rocket: (RTT Oriented CUBIC Congestion control ExTension)

TCP ROCCET is a new TCP congestion control algorithm suited for current cellular 5G NR beyond networks. It extends the kernel default congestion control CUBIC and improves its performance, and additionally solves unwanted side effects of CUBIC’s implementation. ROCCET uses its own Slow Start, called LAUNCH, where loss is not considered as a congestion event. The congestion avoidance phase, called ORBITER, uses CUBIC’s window growth function and adds, based on RTT and ACK rate, congestion events.

NOTE: A paper for TCP ROCCET is currently under review.

The oldest Linux kernel version we tested with ROCCET is 6.1.

Setup

• Debian: sudo apt install linux-headers-generic
• Fedora: sudo dnf install kernel-devel (sudo reboot)

Build

make

Loading & Unloading the Module

1. Insert into Kernel: sudo insmod tcp_roccet.ko In case you get the “Invalid module Format” error, it can help reinstalling the kernel-headers.

2. Use the Algorithm:

• Either via globally loading it: sudo sysctl net.ipv4.tcp_congestion_control=roccet
• Or via using it in specific measurements: sudo iperf3 -c <Server-IP> -C roccet

3. Unload the Module: sudo rmmod tcp_roccet

Debugging (Using kprobe)

In order to debug the tcp_roccet congestion control algorithm, there exists a Kprobe module (roccet_kprobe.c). Using this it is possible to inspect events generated by the algorithm.

In order to use the Kprobe module the following steps are necessary:

1. Specify the event to inspect For this head into the tcp_roccet.c source code and find the function the Kprobe should attach to.

For Example:

__bpf_kfunc static void roccettcp_cong_avoid(struct sock *sk, u32 ack, u32 acked);

Specify this function name in the field symbol_name of the kprobe struct in roccet_kprobe.c

Obtain the arguments in the kprobe using the regs field. (See the System V ABI for Linux under https://en.wikipedia.org/wiki/X86_calling_conventions#List_of_x86_calling_conventions)

2. Build Kprobe module Use the provided Makefile to build the roccet_kprobe.c module for the local machine. (make)

For this the adequate linux-headers are required

• Debian: sudo apt install linux-headers-generic
• Fedora: sudo dnf install kernel-devel

Required files for this are:

• Makefile
• roccet_kprobe.c
• tcp_roccet.h
• tcp_roccet.c (Optional; Need to remove corresponding entry in Makefile if missing)

3. Load Kprobe module To then load the Module use sudo insmod roccet_kprobe.ko If you are seeing the error “Unknown symbol in module” you need to first load the roccet algorithm.

4. See the trace output To see the trace output use sudo cat /sys/kernel/tracing/trace_pipe

5. Unload Kprobe module To remove the module again use sudo rmmod roccet_kprobe

See more info at https://docs.kernel.org/trace/kprobes.html and https://www.kernel.org/doc/Documentation/trace/kprobetrace.rst

Further Info

• On TCP-CC Ops:
  • https://www.yonch.com/tech/linux-tcp-congestion-control-internals
  • https://docs.ebpf.io/linux/program-type/BPF_PROG_TYPE_STRUCT_OPS/tcp_congestion_ops/

Setup Development Environment

For specific Kernel:

1. Download linux source (the version you want to develop for)
2. Create Config via make defconfig
3. Compile kernel via make
4. Generate clangd Config via python scripts/clang-tools/gen_compile_commands.py
5. Copy compile_commands.json to development directory