net/tcp: add Rust implementation of BIC

Reimplement the Binary Increase Congestion (BIC) control algorithm in
Rust. BIC is one of the smallest CCAs in the kernel and this mainly
serves as a minimal example for a real-world algorithm.

Author: Valentin Obst

net/ipv4/Kconfig

@@ -510,6 +510,15 @@ config TCP_CONG_BIC
 	  increase provides TCP friendliness.
 	  See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/
 
+config TCP_CONG_BIC_RUST
+	tristate "Binary Increase Congestion (BIC) control (Rust rewrite)"
+	depends on RUST_TCP_ABSTRACTIONS
+	help
+	  Rust rewrite of the original implementation of Binary Increase
+	  Congestion (BIC) control.
+
+	  If unsure, say N.
+
 config TCP_CONG_CUBIC
 	tristate "CUBIC TCP"
 	default y
@@ -705,6 +714,9 @@ choice
 	config DEFAULT_BIC
 		bool "Bic" if TCP_CONG_BIC=y
 
+	config DEFAULT_BIC_RUST
+		bool "Bic (Rust)" if TCP_CONG_BIC_RUST=y
+
 	config DEFAULT_CUBIC
 		bool "Cubic" if TCP_CONG_CUBIC=y
 
@@ -746,6 +758,7 @@ config TCP_CONG_CUBIC
 config DEFAULT_TCP_CONG
 	string
 	default "bic" if DEFAULT_BIC
+	default "bic_rust" if DEFAULT_BIC_RUST
 	default "cubic" if DEFAULT_CUBIC
 	default "htcp" if DEFAULT_HTCP
 	default "hybla" if DEFAULT_HYBLA

net/ipv4/Makefile

@@ -46,6 +46,7 @@ obj-$(CONFIG_INET_UDP_DIAG) += udp_diag.o
 obj-$(CONFIG_INET_RAW_DIAG) += raw_diag.o
 obj-$(CONFIG_TCP_CONG_BBR) += tcp_bbr.o
 obj-$(CONFIG_TCP_CONG_BIC) += tcp_bic.o
+obj-$(CONFIG_TCP_CONG_BIC_RUST) += tcp_bic_rust.o
 obj-$(CONFIG_TCP_CONG_CDG) += tcp_cdg.o
 obj-$(CONFIG_TCP_CONG_CUBIC) += tcp_cubic.o
 obj-$(CONFIG_TCP_CONG_DCTCP) += tcp_dctcp.o

net/ipv4/tcp_bic_rust.rs

@@ -0,0 +1,300 @@
+//! SPDX-License-Identifier: GPL-2.0
+//!
+//! Binary Increase Congestion control (BIC). Based on:
+//!     Binary Increase Congestion Control (BIC) for Fast Long-Distance
+//!     Networks - Lisong Xu, Khaled Harfoush, and Injong Rhee
+//!     IEEE INFOCOM 2004, Hong Kong, China, 2004, pp. 2514-2524 vol.4
+//!     doi: 10.1109/INFCOM.2004.1354672
+//!     Link: https://doi.org/10.1109/INFCOM.2004.1354672
+//!     Link: https://web.archive.org/web/20160417213452/http://netsrv.csc.ncsu.edu/export/bitcp.pdf
+
+use core::cmp::{max, min};
+use core::num::NonZeroU32;
+use kernel::net::tcp::cong;
+use kernel::prelude::*;
+use kernel::time;
+use kernel::{c_str, module_cca};
+
+const ACK_RATIO_SHIFT: u32 = 4;
+
+// TODO: Convert to module parameters once they are available.
+/// Value of ssthresh for new connections.
+const INITIAL_SSTHRESH: Option<u32> = None;
+/// If cwnd is larger than this threshold, BIC engages; otherwise normal TCP
+/// increase/decrease will be performed.
+// NOTE: cwnd is expressed in units of full-sized segments.
+const LOW_WINDOW: u32 = 14;
+/// In binary search, go to point: `cwnd + (W_max - cwnd) / BICTCP_B`.
+// SAFETY: This will panic at compile time when passing zero.
+// TODO: Convert to `new::(x).unwrap()` once 'const_option' is stabilised.
+const BICTCP_B: NonZeroU32 = unsafe { NonZeroU32::new_unchecked(4) };
+/// The maximum increment, i.e., `S_max`. This is used during additive increase.
+/// After crossing `W_max`, slow start is performed until passing
+/// `MAX_INCREMENT * (BICTCP_B - 1)`.
+// SAFETY: This will panic at compile time when passing zero.
+const MAX_INCREMENT: NonZeroU32 = unsafe { NonZeroU32::new_unchecked(16) };
+/// The number of RTT it takes to get from `W_max - BICTCP_B` to `W_max` (and
+/// from `W_max` to `W_max + BICTCP_B`). This is not part of the original paper
+/// and results in a slow additive increase across `W_max`.
+const SMOOTH_PART: u32 = 20;
+/// Enable or disable fast convergence.
+const FAST_CONVERGENCE: bool = true;
+/// Factor for multiplicative decrease. In fast retransmit we have:
+/// `cwnd = cwnd * BETA/BETA_SCALE`
+/// and if fast convergence is active:
+/// `W_max = cwnd * (1 + BETA/BETA_SCALE)/2`
+/// instead of `W_max = cwnd`.
+const BETA: u32 = 819;
+/// Used to calculate beta in [0, 1] with integer arithmetics.
+// SAFETY: This will panic at compile time when passing zero.
+const BETA_SCALE: NonZeroU32 = unsafe { NonZeroU32::new_unchecked(1024) };
+/// The minimum amount of time that has to pass between two updates of the cwnd.
+const MIN_UPDATE_INTERVAL: time::Nsecs = 31250000;
+
+module_cca! {
+    type: Bic,
+    name: "tcp_bic_rust",
+    author: "Rust for Linux Contributors",
+    description: "Binary Increase Congestion (BIC) control algorithm, Rust implementation",
+    license: "GPL v2",
+}
+
+struct Bic {}
+
+#[vtable]
+impl cong::Algorithm for Bic {
+    type Data = BicState;
+
+    const NAME: &'static CStr = c_str!("bic_rust");
+
+    fn pkts_acked(sk: &mut cong::Sock<'_, Self>, sample: &cong::AckSample) {
+        if let Ok(cong::State::Open) = sk.inet_csk().ca_state() {
+            let ca = sk.inet_csk_ca_mut();
+
+            // This is supposed to wrap.
+            ca.delayed_ack = ca.delayed_ack.wrapping_add(
+                sample
+                    .pkts_acked()
+                    .wrapping_sub(ca.delayed_ack >> ACK_RATIO_SHIFT),
+            );
+        }
+    }
+
+    fn ssthresh(sk: &mut cong::Sock<'_, Self>) -> u32 {
+        let cwnd = sk.tcp_sk().snd_cwnd();
+        let ca = sk.inet_csk_ca_mut();
+
+        pr_info!(
+            // TODO: remove
+            "Enter fast retransmit: time {}, start {}",
+            time::ktime_get_boot_fast_ns(),
+            ca.start_time
+        );
+
+        // Epoch has ended.
+        ca.epoch_start = 0;
+        ca.last_max_cwnd = if cwnd < ca.last_max_cwnd && FAST_CONVERGENCE {
+            (cwnd * (BETA_SCALE.get() + BETA)) / (2 * BETA_SCALE.get())
+        } else {
+            cwnd
+        };
+
+        if cwnd <= LOW_WINDOW {
+            max(cwnd >> 1, 2)
+        } else {
+            max((cwnd * BETA) / BETA_SCALE, 2)
+        }
+    }
+
+    fn cong_avoid(sk: &mut cong::Sock<'_, Self>, _ack: u32, mut acked: u32) {
+        if !sk.tcp_is_cwnd_limited() {
+            return;
+        }
+
+        let tp = sk.tcp_sk_mut();
+
+        if tp.in_slow_start() {
+            acked = tp.slow_start(acked);
+            if acked == 0 {
+                pr_info!(
+                    // TODO: remove
+                    "New cwnd {}, time {}, ssthresh {}, start {}, ss 1",
+                    sk.tcp_sk().snd_cwnd(),
+                    time::ktime_get_boot_fast_ns(),
+                    sk.tcp_sk().snd_ssthresh(),
+                    sk.inet_csk_ca().start_time
+                );
+                return;
+            }
+        }
+
+        let cwnd = tp.snd_cwnd();
+        let cnt = sk.inet_csk_ca_mut().update(cwnd);
+        sk.tcp_sk_mut().cong_avoid_ai(cnt, acked);
+
+        pr_info!(
+            // TODO: remove
+            "New cwnd {}, time {}, ssthresh {}, start {}, ss 0",
+            sk.tcp_sk().snd_cwnd(),
+            time::ktime_get_boot_fast_ns(),
+            sk.tcp_sk().snd_ssthresh(),
+            sk.inet_csk_ca().start_time
+        );
+    }
+
+    fn set_state(sk: &mut cong::Sock<'_, Self>, new_state: cong::State) {
+        if matches!(new_state, cong::State::Loss) {
+            pr_info!(
+                // TODO: remove
+                "Retransmission timeout fired: time {}, start {}",
+                time::ktime_get_boot_fast_ns(),
+                sk.inet_csk_ca().start_time
+            );
+            sk.inet_csk_ca_mut().reset()
+        }
+    }
+
+    fn undo_cwnd(sk: &mut cong::Sock<'_, Self>) -> u32 {
+        pr_info!(
+            // TODO: remove
+            "Undo cwnd reduction: time {}, start {}",
+            time::ktime_get_boot_fast_ns(),
+            sk.inet_csk_ca().start_time
+        );
+
+        cong::reno::undo_cwnd(sk)
+    }
+
+    fn init(sk: &mut cong::Sock<'_, Self>) {
+        if let Some(ssthresh) = INITIAL_SSTHRESH {
+            sk.tcp_sk_mut().set_snd_ssthresh(ssthresh);
+        }
+
+        // TODO: remove
+        pr_info!("Socket created: start {}", sk.inet_csk_ca().start_time);
+    }
+
+    // TODO: remove
+    fn release(sk: &mut cong::Sock<'_, Self>) {
+        pr_info!(
+            "Socket destroyed: start {}, end {}",
+            sk.inet_csk_ca().start_time,
+            time::ktime_get_boot_fast_ns()
+        );
+    }
+}
+
+/// Internal state of each instance of the algorithm.
+struct BicState {
+    /// During congestion avoidance, cwnd is increased at most every `cnt`
+    /// acknowledged packets, i.e., the average increase per acknowledged packet
+    /// is proportional to `1 / cnt`.
+    // NOTE: The C impl initialises this to zero. It then ensures that zero is
+    // never passed to `cong_avoid_ai`, which could divide by it. Make it
+    // explicit in the types that zero is not a valid value.
+    cnt: NonZeroU32,
+    /// Last maximum `snd_cwnd`, i.e, `W_max`.
+    last_max_cwnd: u32,
+    /// The last `snd_cwnd`.
+    last_cwnd: u32,
+    /// Time when `last_cwnd` was updated.
+    last_time: time::Nsecs,
+    /// Records the beginning of an epoch.
+    epoch_start: time::Nsecs,
+    /// Estimates the ratio of `packets/ACK << 4`. This allows us to adjust cwnd
+    /// per packet when a receiver is sending a single ACK for multiple received
+    /// packets.
+    delayed_ack: u32,
+    /// Time when algorithm was initialised.
+    // TODO: remove
+    start_time: time::Nsecs,
+}
+
+impl Default for BicState {
+    fn default() -> Self {
+        Self {
+            // NOTE: Initializing this to 1 deviates from the C code. It does
+            // not change the behavior.
+            cnt: NonZeroU32::MIN,
+            last_max_cwnd: 0,
+            last_cwnd: 0,
+            last_time: 0,
+            epoch_start: 0,
+            delayed_ack: 2 << ACK_RATIO_SHIFT,
+            // TODO: remove
+            start_time: time::ktime_get_boot_fast_ns(),
+        }
+    }
+}
+
+impl BicState {
+    /// Compute congestion window to use. Returns the new `cnt`.
+    ///
+    /// This governs the behavior of the algorithm during congestion avoidance.
+    fn update(&mut self, cwnd: u32) -> NonZeroU32 {
+        let timestamp = time::ktime_get_boot_fast_ns();
+
+        // Do nothing if we are invoked too frequently.
+        if self.last_cwnd == cwnd && (timestamp - self.last_time) <= MIN_UPDATE_INTERVAL {
+            return self.cnt;
+        }
+
+        self.last_cwnd = cwnd;
+        self.last_time = timestamp;
+
+        // Record the beginning of an epoch.
+        if self.epoch_start == 0 {
+            self.epoch_start = timestamp;
+        }
+
+        // Start off like normal TCP.
+        if cwnd <= LOW_WINDOW {
+            self.cnt = NonZeroU32::new(cwnd).unwrap_or(NonZeroU32::MIN);
+            return self.cnt;
+        }
+
+        let mut new_cnt = if cwnd < self.last_max_cwnd {
+            // binary increase
+            let dist: u32 = (self.last_max_cwnd - cwnd) / BICTCP_B;
+
+            if dist > MAX_INCREMENT.get() {
+                cwnd / MAX_INCREMENT // additive increase
+            } else if dist <= 1 {
+                (cwnd * SMOOTH_PART) / BICTCP_B // careful additive increase
+            } else {
+                cwnd / dist // binary search
+            }
+        } else {
+            if cwnd < self.last_max_cwnd + BICTCP_B.get() {
+                (cwnd * SMOOTH_PART) / BICTCP_B // careful additive increase
+            } else if cwnd < self.last_max_cwnd + MAX_INCREMENT.get() * (BICTCP_B.get() - 1) {
+                (cwnd * (BICTCP_B.get() - 1)) / (cwnd - self.last_max_cwnd) // slow start
+            } else {
+                cwnd / MAX_INCREMENT // linear increase
+            }
+        };
+
+        // If in initial slow start or link utilization is very low.
+        if self.last_max_cwnd == 0 {
+            new_cnt = min(new_cnt, 20);
+        }
+
+        // Account for estimated packets/ACK to ensure that we increase per
+        // packet.
+        new_cnt = (new_cnt << ACK_RATIO_SHIFT) / self.delayed_ack;
+
+        self.cnt = NonZeroU32::new(new_cnt).unwrap_or(NonZeroU32::MIN);
+
+        self.cnt
+    }
+
+    fn reset(&mut self) {
+        // TODO: remove
+        let tmp = self.start_time;
+
+        *self = Self::default();
+
+        // TODO: remove
+        self.start_time = tmp;
+    }
+}