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Showing content from https://github.com/multipath-tcp/mptcp_net-next/issues/75 below:

packet scheduler · Issue #75 · multipath-tcp/mptcp_net-next · GitHub

Extending MPTCP with BPF is clearly something we want.

It looks like extending the Upstream MPTCP kernel to allow taking some packet scheduling decisions with BPF will be needed and would be needed in priority to #74.

I think the implementation would be similar to what is done in the kernel with BPF TCP CC: the ability to write a congestion control protocol in BPF with BPF_STRUCT_OPS, see: https://linuxplumbersconf.org/event/7/contributions/687/

Or check these file:

• BPF "kernelspace": net/ipv4/bpf_tcp_ca.c
• BPF "userspace": tools/testing/selftests/bpf/progs/bpf_cubic.c

From what I saw, the kernel side is a bit tricky. Here, it looks like this solution with TCP CC is designed like that because adding a new TCP CC is done by adding a new TCP CC kernel module. For BPF TCP CC, this module can be controlled via BPF.

On our side with MPTCP, we currently don't have the ability to create other packet schedulers (or path managers).

• Maybe a first step would be to add the ability to select different packets schedulers implemented in the kernel.
• Or maybe we could have the current scheduler having this ability to be controlled via BPF. But in this case, can we easily have both: a single packet scheduler that can do the job with and without a BPF program controlling it?

Issues:

• Issues with BPF packet scheduler → Issues with BPF packet scheduler #336

Linked to #350:

• Ability to write data in dedicated socket structure: MPTCP and subflow levels → scheduler: area in the socket structures reserved for schedulers #342
• New callback to initiate optimisations → scheduler: new callback to initiate optimisations #344
• Ability to penalise some subflows (and remove that) → scheduler: "penalise" some subflows by sending less than their cwnd #345
• Ability to initiate opportunistic retransmissions → scheduler: implement a "opportunistic retransmission" #332
• Ability to (un)mark a subflow as "stale" → scheduler: (un)mark a subflow as "stale" #349
• Ability to change the behaviour depending on the backup flag
• (and start/stop probing if not only managed by the core → scheduler: frequently probe "stale" subflow with reinjected data #348)
• BPF selftests: use a dedicated netns for each test, see 02d6a05

Note: this task is currently in progress. The new BPF schedulers in our git repository are only there to validate the WIP development, and NOT recommended for production. If you are interested to participate in the development, you can test them by compiling the export branch, then loading them with bpftool, e.g.:

cd tools/testing/selftests/bpf/progs
clang -O2 -target -bpf -g -c mptcp_bpf_burst.c -o mptcp_bpf_burst.bpf.o  ## or another one, or "make"
bpftool struct_ops register mptcp_bpf_burst.bpf.o
sysctl net.mptcp.scheduler=bpf_burst

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