netem_trace/model/

rwnd.rs

//! This module contains some predefined rwnd trace models.
//!
//! Enabled with feature `rwnd-model` or `model`.
//!
//! ## Predefined models
//!
//! - [`StaticRwnd`]: A trace model with a single rwnd decision.
//! - [`RepeatedRwndPattern`]: A trace model with a repeated rwnd pattern.
//!
//! ## Step semantics
//!
//! A step carries two independent fields, `set_rcv_buf` and `app_read_bytes`,
//! and between them they say what the receiver does for the step's duration.
//! See [`RwndDecision`] for the full table; in short, `set_rcv_buf` states a
//! buffer the application keeps drained (so the window stays at that value),
//! and `app_read_bytes` states an application that reads only so much (so the
//! window decays as the backlog grows).
//!
//! A step carrying **both** resizes the buffer first and takes the read against
//! the new size. The window is then `set_rcv_buf - unread`, not pinned at
//! `set_rcv_buf`: stating a read means the application is the bottleneck, so the
//! "application keeps up" half of a buffer-only step no longer applies. The
//! unread backlog carries across the resize untouched, so a resize to at or
//! below the current backlog advertises a zero window until the application
//! catches up.
//!
//! ## Examples
//!
//! An example to build model from configuration:
//!
//! ```
//! # use netem_trace::model::StaticRwndConfig;
//! # use netem_trace::{Duration, RwndTrace};
//! let mut static_rwnd = StaticRwndConfig::new()
//!     .set_rcv_buf(65536)
//!     .app_read(1024)
//!     .duration(Duration::from_secs(1))
//!     .build();
//! let (decision, duration) = static_rwnd.next_rwnd().unwrap();
//! assert_eq!(decision.set_rcv_buf, Some(65536));
//! assert_eq!(decision.app_read_bytes, Some(1024));
//! assert_eq!(duration, Duration::from_secs(1));
//! assert_eq!(static_rwnd.next_rwnd(), None);
//! ```
//!
//! The step above carries both fields, so it resizes the buffer to 64 KiB and
//! then has the application read 1 KiB from it. The window that follows is
//! `65536 - unread` rather than a window pinned at 65536 -- compare the
//! buffer-only step below, which does pin it:
//!
//! ```
//! # use netem_trace::model::StaticRwndConfig;
//! # use netem_trace::{Duration, RwndTrace};
//! // Buffer only: the window is held at 65536 and the application keeps up.
//! let mut pinned = StaticRwndConfig::new()
//!     .set_rcv_buf(65536)
//!     .duration(Duration::from_secs(1))
//!     .build();
//! let (decision, _) = pinned.next_rwnd().unwrap();
//! assert_eq!(decision.set_rcv_buf, Some(65536));
//! assert_eq!(decision.app_read_bytes, None);
//!
//! // Both: same buffer, but the application now reads only 1 KiB per step, so
//! // the window follows the backlog instead of staying at 65536.
//! let mut app_limited = StaticRwndConfig::new()
//!     .set_rcv_buf(65536)
//!     .app_read(1024)
//!     .duration(Duration::from_secs(1))
//!     .build();
//! let (decision, _) = app_limited.next_rwnd().unwrap();
//! assert_eq!(decision.set_rcv_buf, Some(65536));
//! assert_eq!(decision.app_read_bytes, Some(1024));
//!
//! // Shrinking the buffer under a standing backlog is stated the same way; the
//! // window saturates at zero until the application reads its way back under it.
//! let mut shrink = StaticRwndConfig::new()
//!     .set_rcv_buf(8192)
//!     .app_read(0)
//!     .duration(Duration::from_secs(1))
//!     .build();
//! let (decision, _) = shrink.next_rwnd().unwrap();
//! assert_eq!(decision.set_rcv_buf, Some(8192));
//! assert_eq!(decision.app_read_bytes, Some(0));
//! ```
//!
//! A more common use case is to build model from a configuration file (e.g. json file):
//!
//! ```
//! # use netem_trace::model::{StaticRwndConfig, RwndTraceConfig};
//! # use netem_trace::{Duration, RwndTrace};
//! # #[cfg(feature = "human")]
//! # let config_file_content = "{\"RepeatedRwndPatternConfig\":{\"pattern\":[{\"StaticRwndConfig\":{\"duration\":\"1s\",\"set_rcv_buf\":65536}},{\"StaticRwndConfig\":{\"duration\":\"1s\",\"app_read_bytes\":1024}}],\"count\":2}}";
//! // The content would be "{\"RepeatedRwndPatternConfig\":{\"pattern\":[{\"StaticRwndConfig\":{\"duration\":{\"secs\":1,\"nanos\":0},\"set_rcv_buf\":65536}},{\"StaticRwndConfig\":{\"duration\":{\"secs\":1,\"nanos\":0},\"app_read_bytes\":1024}}],\"count\":2}}"
//! // if the `human` feature is not enabled.
//! # #[cfg(not(feature = "human"))]
//! let config_file_content = "{\"RepeatedRwndPatternConfig\":{\"pattern\":[{\"StaticRwndConfig\":{\"duration\":{\"secs\":1,\"nanos\":0},\"set_rcv_buf\":65536}},{\"StaticRwndConfig\":{\"duration\":{\"secs\":1,\"nanos\":0},\"app_read_bytes\":1024}}],\"count\":2}}";
//! let des: Box<dyn RwndTraceConfig> = serde_json::from_str(config_file_content).unwrap();
//! let mut model = des.into_model();
//! let (decision, _) = model.next_rwnd().unwrap();
//! assert_eq!(decision.set_rcv_buf, Some(65536));
//! let (decision, _) = model.next_rwnd().unwrap();
//! assert_eq!(decision.app_read_bytes, Some(1024));
//! ```
use crate::{Duration, RwndDecision, RwndTrace};
use dyn_clone::DynClone;

/// This trait is used to convert a rwnd trace configuration into a rwnd trace model.
///
/// Since trace model is often configured with files and often has inner states which
/// is not suitable to be serialized/deserialized, this trait makes it possible to
/// separate the configuration part into a simple struct for serialization/deserialization, and
/// construct the model from the configuration.
#[cfg_attr(feature = "serde", typetag::serde)]
pub trait RwndTraceConfig: DynClone + Send {
    fn into_model(self: Box<Self>) -> Box<dyn RwndTrace>;
}

dyn_clone::clone_trait_object!(RwndTraceConfig);

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

/// The model of a static rwnd trace: a single decision valid for one duration.
///
/// ## Examples
///
/// ```
/// # use netem_trace::model::StaticRwndConfig;
/// # use netem_trace::{Duration, RwndTrace};
/// let mut static_rwnd = StaticRwndConfig::new()
///     .set_rcv_buf(65536)
///     .duration(Duration::from_secs(1))
///     .build();
/// let (decision, duration) = static_rwnd.next_rwnd().unwrap();
/// assert_eq!(decision.set_rcv_buf, Some(65536));
/// assert_eq!(decision.app_read_bytes, None);
/// assert_eq!(duration, Duration::from_secs(1));
/// assert_eq!(static_rwnd.next_rwnd(), None);
/// ```
#[derive(Debug, Clone)]
pub struct StaticRwnd {
    pub decision: RwndDecision,
    pub duration: Option<Duration>,
}

/// The configuration struct for [`StaticRwnd`].
///
/// The serialized JSON form is **flat**: a step looks like
/// `{"duration":"1s","set_rcv_buf":65536}` or
/// `{"duration":"1s","app_read_bytes":1024}`, and may carry both keys.
///
/// The two fields are independent -- there is no invariant to enforce, so
/// `Serialize`/`Deserialize` are derived. A step with neither field is valid and
/// carries the previous configuration forward.
///
/// Unknown keys are rejected rather than ignored. The schema dropped
/// `rwnd_remaining`, and serde's default of skipping what it does not recognise
/// would turn a trace written against the old schema into a run of steps that
/// state nothing -- a replay that looks healthy while reproducing no receiver at
/// all. Failing to deserialize names the offending field instead.
#[cfg_attr(
    feature = "serde",
    derive(Serialize, Deserialize),
    serde(default, deny_unknown_fields)
)]
#[derive(Debug, Clone, Default)]
pub struct StaticRwndConfig {
    #[cfg_attr(
        feature = "human",
        serde(with = "humantime_serde"),
        serde(skip_serializing_if = "Option::is_none")
    )]
    #[cfg_attr(
        all(feature = "serde", not(feature = "human")),
        serde(skip_serializing_if = "Option::is_none")
    )]
    pub duration: Option<Duration>,
    #[cfg_attr(feature = "serde", serde(skip_serializing_if = "Option::is_none"))]
    pub set_rcv_buf: Option<u64>,
    #[cfg_attr(feature = "serde", serde(skip_serializing_if = "Option::is_none"))]
    pub app_read_bytes: Option<u64>,
}

/// The model contains an array of rwnd trace models.
///
/// Combine multiple rwnd trace models into one rwnd pattern,
/// and repeat the pattern for `count` times.
///
/// If `count` is 0, the pattern will be repeated forever.
///
/// ## Examples
///
/// The most common use case is to read from a configuration file and
/// deserialize it into a [`RepeatedRwndPatternConfig`]:
///
/// ```
/// # use netem_trace::model::{StaticRwndConfig, RwndTraceConfig};
/// # use netem_trace::{Duration, RwndTrace};
/// # #[cfg(feature = "human")]
/// # let config_file_content = "{\"RepeatedRwndPatternConfig\":{\"pattern\":[{\"StaticRwndConfig\":{\"duration\":\"1s\",\"set_rcv_buf\":65536,\"app_read_bytes\":1024}}],\"count\":2}}";
/// # #[cfg(not(feature = "human"))]
/// let config_file_content = "{\"RepeatedRwndPatternConfig\":{\"pattern\":[{\"StaticRwndConfig\":{\"duration\":{\"secs\":1,\"nanos\":0},\"set_rcv_buf\":65536,\"app_read_bytes\":1024}}],\"count\":2}}";
/// let des: Box<dyn RwndTraceConfig> = serde_json::from_str(config_file_content).unwrap();
/// let mut model = des.into_model();
/// let (decision, _) = model.next_rwnd().unwrap();
/// assert_eq!(decision.set_rcv_buf, Some(65536));
/// assert_eq!(decision.app_read_bytes, Some(1024));
/// ```
pub struct RepeatedRwndPattern {
    pub pattern: Vec<Box<dyn RwndTraceConfig>>,
    pub count: usize,
    current_model: Option<Box<dyn RwndTrace>>,
    current_cycle: usize,
    current_pattern: usize,
}

/// The configuration struct for [`RepeatedRwndPattern`].
///
/// See [`RepeatedRwndPattern`] for more details.
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(default))]
#[derive(Default, Clone)]
pub struct RepeatedRwndPatternConfig {
    pub pattern: Vec<Box<dyn RwndTraceConfig>>,
    pub count: usize,
}

impl RwndTrace for StaticRwnd {
    fn next_rwnd(&mut self) -> Option<(RwndDecision, Duration)> {
        if let Some(duration) = self.duration.take() {
            if duration.is_zero() {
                None
            } else {
                Some((self.decision, duration))
            }
        } else {
            None
        }
    }
}

impl RwndTrace for RepeatedRwndPattern {
    fn next_rwnd(&mut self) -> Option<(RwndDecision, Duration)> {
        let pattern_len = self.pattern.len();
        // Allow at most pattern_len + 1 consecutive inner-None results before
        // giving up. The +1 covers a possibly-exhausted current_model at entry;
        // after that, each remaining slot is a fresh clone whose behaviour is
        // deterministic. If all pattern_len fresh clones return None, the
        // pattern will never produce a value regardless of count.
        let mut budget = pattern_len + 1;
        loop {
            if pattern_len == 0 || (self.count != 0 && self.current_cycle >= self.count) {
                return None;
            }
            if budget == 0 {
                return None;
            }
            if self.current_model.is_none() {
                self.current_model = Some(self.pattern[self.current_pattern].clone().into_model());
            }
            match self.current_model.as_mut().unwrap().next_rwnd() {
                Some(item) => return Some(item),
                None => {
                    self.current_model = None;
                    budget -= 1;
                    self.current_pattern += 1;
                    if self.current_pattern >= pattern_len {
                        self.current_pattern = 0;
                        self.current_cycle += 1;
                        if self.count != 0 && self.current_cycle >= self.count {
                            return None;
                        }
                    }
                }
            }
        }
    }
}

impl StaticRwndConfig {
    pub fn new() -> Self {
        Self {
            duration: None,
            set_rcv_buf: None,
            app_read_bytes: None,
        }
    }

    pub fn duration(mut self, duration: Duration) -> Self {
        self.duration = Some(duration);
        self
    }

    /// Size the receive buffer.
    ///
    /// On its own this also holds the advertised window at that value, with the
    /// application draining continuously. Combined with [`Self::app_read`] it
    /// only sets the size: the resize applies first, the read is taken against
    /// the new size, and the window then follows `set_rcv_buf - unread` rather
    /// than being pinned.
    pub fn set_rcv_buf(mut self, set_rcv_buf: u64) -> Self {
        self.set_rcv_buf = Some(set_rcv_buf);
        self
    }

    /// The application reads exactly this many bytes over the step, then stops.
    ///
    /// The window follows from what is left unread of the standing buffer, or
    /// of the buffer this step sets when combined with [`Self::set_rcv_buf`].
    /// The backlog carries across such a resize untouched, so the window
    /// saturates at zero if the new size is at or below it.
    pub fn app_read(mut self, bytes: u64) -> Self {
        self.app_read_bytes = Some(bytes);
        self
    }

    pub fn build(self) -> StaticRwnd {
        StaticRwnd {
            decision: RwndDecision {
                set_rcv_buf: self.set_rcv_buf,
                app_read_bytes: self.app_read_bytes,
            },
            duration: Some(self.duration.unwrap_or_else(|| Duration::from_secs(1))),
        }
    }
}

impl RepeatedRwndPatternConfig {
    pub fn new() -> Self {
        Self {
            pattern: vec![],
            count: 0,
        }
    }

    pub fn pattern(mut self, pattern: Vec<Box<dyn RwndTraceConfig>>) -> Self {
        self.pattern = pattern;
        self
    }

    pub fn count(mut self, count: usize) -> Self {
        self.count = count;
        self
    }

    pub fn build(self) -> RepeatedRwndPattern {
        RepeatedRwndPattern {
            pattern: self.pattern,
            count: self.count,
            current_model: None,
            current_cycle: 0,
            current_pattern: 0,
        }
    }
}

macro_rules! impl_rwnd_trace_config {
    ($name:ident) => {
        #[cfg_attr(feature = "serde", typetag::serde)]
        impl RwndTraceConfig for $name {
            fn into_model(self: Box<$name>) -> Box<dyn RwndTrace> {
                Box::new(self.build())
            }
        }
    };
}

impl_rwnd_trace_config!(StaticRwndConfig);
impl_rwnd_trace_config!(RepeatedRwndPatternConfig);

#[cfg(test)]
mod test {
    use super::*;
    use crate::RwndTrace;

    /// Both fields on one step: the buffer is resized and the read is taken
    /// against the new size, so the decision carries both rather than one
    /// overriding the other.
    #[test]
    fn test_static_rwnd_model_buffer_and_app_read() {
        let mut static_rwnd = StaticRwndConfig::new()
            .set_rcv_buf(65536)
            .app_read(1024)
            .duration(Duration::from_secs(1))
            .build();
        let (decision, duration) = static_rwnd.next_rwnd().unwrap();
        assert_eq!(decision.set_rcv_buf, Some(65536));
        assert_eq!(decision.app_read_bytes, Some(1024));
        assert_eq!(duration, Duration::from_secs(1));
        assert_eq!(static_rwnd.next_rwnd(), None);
    }

    /// A buffer on its own is a complete statement: the window sits there and
    /// the application keeps up. This is what used to need `rwnd_remaining`.
    #[test]
    fn test_static_rwnd_model_buffer_only() {
        let mut static_rwnd = StaticRwndConfig::new()
            .set_rcv_buf(32768)
            .duration(Duration::from_secs(2))
            .build();
        let (decision, duration) = static_rwnd.next_rwnd().unwrap();
        assert_eq!(decision.set_rcv_buf, Some(32768));
        assert_eq!(decision.app_read_bytes, None);
        assert_eq!(duration, Duration::from_secs(2));
        assert_eq!(static_rwnd.next_rwnd(), None);
    }

    /// A step may carry neither field, and then it simply holds whatever the
    /// previous step configured for its duration.
    #[test]
    fn test_static_rwnd_model_carries_forward() {
        let mut model = StaticRwndConfig::new()
            .duration(Duration::from_secs(1))
            .build();
        let (decision, duration) = model.next_rwnd().unwrap();
        assert_eq!(decision.set_rcv_buf, None);
        assert_eq!(decision.app_read_bytes, None);
        assert_eq!(duration, Duration::from_secs(1));
    }

    #[test]
    fn test_repeated_rwnd_pattern() {
        let pat = vec![
            Box::new(
                StaticRwndConfig::new()
                    .set_rcv_buf(65536)
                    .duration(Duration::from_secs(1)),
            ) as Box<dyn RwndTraceConfig>,
            Box::new(
                StaticRwndConfig::new()
                    .app_read(1024)
                    .duration(Duration::from_secs(1)),
            ) as Box<dyn RwndTraceConfig>,
        ];
        let mut model = RepeatedRwndPatternConfig::new()
            .pattern(pat)
            .count(2)
            .build();
        let next = model.next_rwnd().unwrap();
        assert_eq!(next.0.set_rcv_buf, Some(65536));
        assert_eq!(next.1, Duration::from_secs(1));
        assert_eq!(model.next_rwnd().unwrap().0.app_read_bytes, Some(1024));
        assert_eq!(model.next_rwnd().unwrap().0.set_rcv_buf, Some(65536));
        assert_eq!(model.next_rwnd().unwrap().0.app_read_bytes, Some(1024));
        assert_eq!(model.next_rwnd(), None);
    }

    #[test]
    #[cfg(feature = "serde")]
    fn test_serde_roundtrip_buffer_only() {
        let cfg = Box::new(
            StaticRwndConfig::new()
                .set_rcv_buf(65536)
                .duration(Duration::from_secs(1)),
        ) as Box<dyn RwndTraceConfig>;
        let ser_str = serde_json::to_string(&cfg).unwrap();
        #[cfg(feature = "human")]
        let expected = "{\"StaticRwndConfig\":{\"duration\":\"1s\",\"set_rcv_buf\":65536}}";
        #[cfg(not(feature = "human"))]
        let expected =
            "{\"StaticRwndConfig\":{\"duration\":{\"secs\":1,\"nanos\":0},\"set_rcv_buf\":65536}}";
        assert_eq!(ser_str, expected);

        let des: Box<dyn RwndTraceConfig> = serde_json::from_str(&ser_str).unwrap();
        let mut model = des.into_model();
        let (decision, duration) = model.next_rwnd().unwrap();
        assert_eq!(decision.set_rcv_buf, Some(65536));
        assert_eq!(decision.app_read_bytes, None);
        assert_eq!(duration, Duration::from_secs(1));
    }

    /// Both keys on one step is legal now, and round-trips.
    #[test]
    #[cfg(feature = "serde")]
    fn test_serde_roundtrip_both_fields() {
        let json = "{\"StaticRwndConfig\":{\"set_rcv_buf\":131072,\"app_read_bytes\":4096}}";
        let des: Box<dyn RwndTraceConfig> = serde_json::from_str(json).unwrap();
        let mut model = des.into_model();
        let (decision, _) = model.next_rwnd().unwrap();
        assert_eq!(decision.set_rcv_buf, Some(131072));
        assert_eq!(decision.app_read_bytes, Some(4096));
    }

    #[test]
    #[cfg(feature = "serde")]
    fn test_serde_omits_absent_fields() {
        let cfg = Box::new(StaticRwndConfig::new().app_read(0)) as Box<dyn RwndTraceConfig>;
        let ser_str = serde_json::to_string(&cfg).unwrap();
        assert!(!ser_str.contains("set_rcv_buf"), "got: {ser_str}");
        assert!(ser_str.contains("app_read_bytes"), "got: {ser_str}");
    }

    /// A trace written against the old schema names a field that no longer
    /// exists. Rejecting it is the point: silently ignoring `rwnd_remaining`
    /// would turn every window step into a no-op and replay a trace that says
    /// nothing, which looks like a healthy run producing wrong numbers.
    #[test]
    #[cfg(feature = "serde")]
    fn test_serde_rejects_the_old_rwnd_remaining_field() {
        let json = "{\"StaticRwndConfig\":{\"rwnd_remaining\":32768}}";
        let result: Result<Box<dyn RwndTraceConfig>, _> = serde_json::from_str(json);
        let err = result
            .err()
            .expect("a trace using the removed field should not deserialize")
            .to_string();
        assert!(err.contains("rwnd_remaining"), "got: {err}");
    }

    #[test]
    fn test_repeated_rwnd_pattern_all_zero_duration_terminates() {
        // All inner models have duration == 0 and return None immediately.
        // With count == 0 (infinite repeat) a recursive implementation would
        // spin forever; the loop-based one must return None promptly.
        let pat = vec![
            Box::new(
                StaticRwndConfig::new()
                    .app_read(1024)
                    .duration(Duration::ZERO),
            ) as Box<dyn RwndTraceConfig>,
            Box::new(
                StaticRwndConfig::new()
                    .set_rcv_buf(32768)
                    .duration(Duration::ZERO),
            ) as Box<dyn RwndTraceConfig>,
        ];
        let mut model = RepeatedRwndPatternConfig::new()
            .pattern(pat)
            .count(0) // infinite
            .build();
        assert_eq!(model.next_rwnd(), None);
    }
}