TcpAckThinning

An attempt to understand impact of ACK thinning on TCP using Linux Kernel 5.3.0.

Following are the primary changes:

Kernel changes: A new sockops called BPF_SOCK_OPS_ACK_THRESH_INIT is added which can be controlled using BPF_CGROUP_SOCK_OPS.
BPF bytecode: Sample bpf code (samples/bpf/ack_thinning_*.c) to use the above BPF option to change the threshold.
Tools: Sample tools for monitoring.

Setup Kernel

Step 1: Install the updated kernel.

make
make install
make modules_install
make headers_install

Reboot and boot into the new kernel

Step 2: Compile ack thinning bpf kernel bytecode

# Open the ack_thinning_kern.c and modify the threshold that you want to set for sending ACK
cd samples/bpf
make
# ack_thinning_kern.o should be created.

Step 3: Load the bpf

Follow tcp_bpf.readme

Using Tools

bpftrace

We use bpftrace to track the tcp_send_ack() and tcp_recv_data_event() calls. We also verify the data/acks counts from the packet capture and ensure that they match. Further we use the bpftrace to track from which higher level apis the send-ack call was initiated. Check tools/ack_check.bt has the details.