netstack3_base/tcp/
seqnum.rs

1// Copyright 2021 The Fuchsia Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5//! TCP sequence numbers and operations on them.
6
7use core::convert::TryFrom as _;
8use core::num::TryFromIntError;
9use core::ops;
10
11use explicit::ResultExt as _;
12
13/// Sequence number of a transferred TCP segment.
14///
15/// Per https://tools.ietf.org/html/rfc793#section-3.3:
16///   This space ranges from 0 to 2**32 - 1. Since the space is finite, all
17///   arithmetic dealing with sequence numbers must be performed modulo 2**32.
18///   This unsigned arithmetic preserves the relationship of sequence numbers
19///   as they cycle from 2**32 - 1 to 0 again.  There are some subtleties to
20///   computer modulo arithmetic, so great care should be taken in programming
21///   the comparison of such values.
22///
23/// For any sequence number, there are 2**31 numbers after it and 2**31 - 1
24/// numbers before it.
25#[derive(Debug, PartialEq, Eq, Clone, Copy)]
26pub struct SeqNum(u32);
27
28impl ops::Add<i32> for SeqNum {
29    type Output = SeqNum;
30
31    fn add(self, rhs: i32) -> Self::Output {
32        let Self(lhs) = self;
33        Self(lhs.wrapping_add_signed(rhs))
34    }
35}
36
37impl ops::Sub<i32> for SeqNum {
38    type Output = SeqNum;
39
40    fn sub(self, rhs: i32) -> Self::Output {
41        let Self(lhs) = self;
42        Self(lhs.wrapping_add_signed(rhs.wrapping_neg()))
43    }
44}
45
46impl ops::Add<u32> for SeqNum {
47    type Output = SeqNum;
48
49    fn add(self, rhs: u32) -> Self::Output {
50        let Self(lhs) = self;
51        Self(lhs.wrapping_add(rhs))
52    }
53}
54
55impl ops::Sub<u32> for SeqNum {
56    type Output = SeqNum;
57
58    fn sub(self, rhs: u32) -> Self::Output {
59        let Self(lhs) = self;
60        Self(lhs.wrapping_sub(rhs))
61    }
62}
63
64impl ops::Sub<WindowSize> for SeqNum {
65    type Output = SeqNum;
66
67    fn sub(self, WindowSize(wnd): WindowSize) -> Self::Output {
68        // The conversion from u32 to i32 will never truncate because the
69        // maximum window size is less than 2^30, which will comfortably fit
70        // into an i32.
71        self - i32::try_from(wnd).unwrap()
72    }
73}
74
75impl ops::Add<usize> for SeqNum {
76    type Output = SeqNum;
77
78    fn add(self, rhs: usize) -> Self::Output {
79        // The following `as` coercion is sound because:
80        // 1. if `u32` is wider than `usize`, the unsigned extension will
81        //    result in the same number.
82        // 2. if `usize` is wider than `u32`, then `rhs` can be written as
83        //    `A * 2 ^ 32 + B`. Because of the wrapping nature of sequnce
84        //    numbers, the effect of adding `rhs` is the same as adding `B`
85        //    which is the number after the truncation, i.e., `rhs as u32`.
86        self + (rhs as u32)
87    }
88}
89
90impl ops::Sub for SeqNum {
91    // `i32` is more intuitive than `u32`, since subtraction may yield negative
92    // values.
93    type Output = i32;
94
95    fn sub(self, rhs: Self) -> Self::Output {
96        let Self(lhs) = self;
97        let Self(rhs) = rhs;
98        // The following `as` coercion is sound because:
99        // Rust uses 2's complement for signed integers [1], meaning when cast
100        // to an `i32, an `u32` >= 1<<32 becomes negative and an `u32` < 1<<32
101        // becomes positive. `wrapping_sub` ensures that if `rhs` is a `SeqNum`
102        // after `lhs`, the result will wrap into the `u32` space > 1<<32.
103        // Recall that `SeqNums` are only valid for a `WindowSize` < 1<<31; this
104        // prevents the difference of `wrapping_sub` from being so large that it
105        // wraps into the `u32` space < 1<<32.
106        // [1]: https://doc.rust-lang.org/reference/types/numeric.html
107        lhs.wrapping_sub(rhs) as i32
108    }
109}
110
111impl From<u32> for SeqNum {
112    fn from(x: u32) -> Self {
113        Self::new(x)
114    }
115}
116
117impl From<SeqNum> for u32 {
118    fn from(x: SeqNum) -> Self {
119        let SeqNum(x) = x;
120        x
121    }
122}
123
124impl SeqNum {
125    /// Creates a new sequence number.
126    pub const fn new(x: u32) -> Self {
127        Self(x)
128    }
129}
130
131impl SeqNum {
132    /// A predicate for whether a sequence number is before the other.
133    ///
134    /// Please refer to [`SeqNum`] for the defined order.
135    pub fn before(self, other: SeqNum) -> bool {
136        self - other < 0
137    }
138
139    /// A predicate for whether a sequence number is equal to or before the
140    /// other.
141    ///
142    /// Please refer to [`SeqNum`] for the defined order.
143    pub fn before_or_eq(self, other: SeqNum) -> bool {
144        self - other <= 0
145    }
146
147    /// A predicate for whether a sequence number is after the other.
148    ///
149    /// Please refer to [`SeqNum`] for the defined order.
150    pub fn after(self, other: SeqNum) -> bool {
151        self - other > 0
152    }
153
154    /// A predicate for whether a sequence number is equal to or after the
155    /// other.
156    ///
157    /// Please refer to [`SeqNum`] for the defined order.
158    pub fn after_or_eq(self, other: SeqNum) -> bool {
159        self - other >= 0
160    }
161
162    /// Returns the earliest sequence number between `self` and `other`.
163    ///
164    /// This is equivalent to [`Ord::min`], but keeps within the temporal
165    /// instead of numeric semantics.
166    pub fn earliest(self, other: SeqNum) -> SeqNum {
167        if self.before(other) {
168            self
169        } else {
170            other
171        }
172    }
173
174    /// Returns the latest sequence number between `self` and `other`.
175    ///
176    /// This is equivalent to [`Ord::max`], but keeps within the temporal
177    /// instead of numeric semantics.
178    pub fn latest(self, other: SeqNum) -> SeqNum {
179        if self.after(other) {
180            self
181        } else {
182            other
183        }
184    }
185}
186
187/// A witness type for TCP window size.
188///
189/// Per [RFC 7323 Section 2.3]:
190/// > ..., the above constraints imply that two times the maximum window size
191/// > must be less than 2^31, or
192/// >                    max window < 2^30
193///
194/// [RFC 7323 Section 2.3]: https://tools.ietf.org/html/rfc7323#section-2.3
195#[derive(Debug, PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
196pub struct WindowSize(u32);
197
198impl WindowSize {
199    /// The largest possible window size.
200    pub const MAX: WindowSize = WindowSize(1 << 30 - 1);
201    /// The smallest possible window size.
202    pub const ZERO: WindowSize = WindowSize(0);
203
204    /// The Netstack3 default window size.
205    // TODO(https://github.com/rust-lang/rust/issues/67441): put this constant
206    // in the state module once `Option::unwrap` is stable.
207    pub const DEFAULT: WindowSize = WindowSize(65535);
208
209    /// Create a new `WindowSize` from the provided `u32`.
210    ///
211    /// If the provided window size is out of range, then `None` is returned.
212    pub const fn from_u32(wnd: u32) -> Option<Self> {
213        let WindowSize(max) = Self::MAX;
214        if wnd > max {
215            None
216        } else {
217            Some(Self(wnd))
218        }
219    }
220
221    /// Add a `u32` to this WindowSize, saturating at [`WindowSize::MAX`].
222    pub fn saturating_add(self, rhs: u32) -> Self {
223        Self::from_u32(u32::from(self).saturating_add(rhs)).unwrap_or(Self::MAX)
224    }
225
226    /// Create a new [`WindowSize`], returning `None` if the argument is out of range.
227    pub fn new(wnd: usize) -> Option<Self> {
228        u32::try_from(wnd).ok_checked::<TryFromIntError>().and_then(WindowSize::from_u32)
229    }
230
231    /// Subtract `diff` from `self`, returning `None` if the result would be negative.
232    pub fn checked_sub(self, diff: usize) -> Option<Self> {
233        // The call to Self::new will never return None.
234        //
235        // If diff is larger than self, the checked_sub will return None. Otherwise the result must
236        // be less than Self::MAX, since the value of self before subtraction must be less than or
237        // equal to Self::MAX.
238        usize::from(self).checked_sub(diff).and_then(Self::new)
239    }
240
241    /// Subtract `diff` from `self` returning [`WindowSize::ZERO`] if the result
242    /// would be negative.
243    pub fn saturating_sub(self, diff: usize) -> Self {
244        self.checked_sub(diff).unwrap_or(WindowSize::ZERO)
245    }
246
247    /// The window scale that needs to be advertised during the handshake.
248    pub fn scale(self) -> WindowScale {
249        let WindowSize(size) = self;
250        let effective_bits = u8::try_from(32 - u32::leading_zeros(size)).unwrap();
251        let scale = WindowScale(effective_bits.saturating_sub(16));
252        scale
253    }
254}
255
256impl ops::Add<WindowSize> for SeqNum {
257    type Output = SeqNum;
258
259    fn add(self, WindowSize(wnd): WindowSize) -> Self::Output {
260        self + wnd
261    }
262}
263
264impl From<WindowSize> for u32 {
265    fn from(WindowSize(wnd): WindowSize) -> Self {
266        wnd
267    }
268}
269
270#[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
271impl From<WindowSize> for usize {
272    fn from(WindowSize(wnd): WindowSize) -> Self {
273        wnd as usize
274    }
275}
276
277#[derive(Debug, PartialEq, Eq, Clone, Copy, Default)]
278/// This type is a witness for a valid window scale exponent value.
279///
280/// Per RFC 7323 Section 2.2, the restriction is as follows:
281///   The maximum scale exponent is limited to 14 for a maximum permissible
282///   receive window size of 1 GiB (2^(14+16)).
283pub struct WindowScale(u8);
284
285impl WindowScale {
286    /// The largest possible [`WindowScale`].
287    pub const MAX: WindowScale = WindowScale(14);
288    /// The smallest possible [`WindowScale`].
289    pub const ZERO: WindowScale = WindowScale(0);
290
291    /// Creates a new `WindowScale`.
292    ///
293    /// Returns `None` if the input exceeds the maximum possible value.
294    pub fn new(ws: u8) -> Option<Self> {
295        (ws <= Self::MAX.get()).then_some(WindowScale(ws))
296    }
297
298    /// Returns the inner value.
299    pub fn get(&self) -> u8 {
300        let Self(ws) = self;
301        *ws
302    }
303}
304
305#[derive(Debug, PartialEq, Eq, Clone, Copy)]
306/// Window size that is used in the window field of a TCP segment.
307///
308/// For connections with window scaling enabled, the receiver has to scale this
309/// value back to get the real window size advertised by the peer.
310pub struct UnscaledWindowSize(u16);
311
312impl ops::Shl<WindowScale> for UnscaledWindowSize {
313    type Output = WindowSize;
314
315    fn shl(self, WindowScale(scale): WindowScale) -> Self::Output {
316        let UnscaledWindowSize(size) = self;
317        // `scale` is guaranteed to be <= 14, so the result must fit in a u32.
318        WindowSize::from_u32(u32::from(size) << scale).unwrap()
319    }
320}
321
322impl ops::Shr<WindowScale> for WindowSize {
323    type Output = UnscaledWindowSize;
324
325    fn shr(self, WindowScale(scale): WindowScale) -> Self::Output {
326        let WindowSize(size) = self;
327        UnscaledWindowSize(u16::try_from(size >> scale).unwrap_or(u16::MAX))
328    }
329}
330
331impl From<u16> for UnscaledWindowSize {
332    fn from(value: u16) -> Self {
333        Self(value)
334    }
335}
336
337impl From<UnscaledWindowSize> for u16 {
338    fn from(UnscaledWindowSize(value): UnscaledWindowSize) -> Self {
339        value
340    }
341}
342
343#[cfg(feature = "testutils")]
344mod testutils {
345    use super::*;
346
347    impl UnscaledWindowSize {
348        /// Create a new UnscaledWindowSize.
349        ///
350        /// Panics if `size` is not in range.
351        pub fn from_usize(size: usize) -> Self {
352            UnscaledWindowSize::from(u16::try_from(size).unwrap())
353        }
354
355        /// Create a new UnscaledWindowSize.
356        ///
357        /// Panics if `size` is not in range.
358        pub fn from_u32(size: u32) -> Self {
359            UnscaledWindowSize::from(u16::try_from(size).unwrap())
360        }
361    }
362}
363
364#[cfg(test)]
365mod tests {
366    use alloc::format;
367
368    use proptest::arbitrary::any;
369    use proptest::strategy::{Just, Strategy};
370    use proptest::test_runner::Config;
371    use proptest::{prop_assert, prop_assert_eq, proptest};
372    use proptest_support::failed_seeds_no_std;
373    use test_case::test_case;
374
375    use super::super::segment::MAX_PAYLOAD_AND_CONTROL_LEN;
376    use super::*;
377
378    fn arb_seqnum() -> impl Strategy<Value = SeqNum> {
379        any::<u32>().prop_map(SeqNum::from)
380    }
381
382    // Generates a triple (a, b, c) s.t. a < b < a + 2^30 && b < c < a + 2^30.
383    // This triple is used to verify that transitivity holds.
384    fn arb_seqnum_trans_tripple() -> impl Strategy<Value = (SeqNum, SeqNum, SeqNum)> {
385        arb_seqnum().prop_flat_map(|a| {
386            (1..=MAX_PAYLOAD_AND_CONTROL_LEN).prop_flat_map(move |diff_a_b| {
387                let b = a + diff_a_b;
388                (1..=MAX_PAYLOAD_AND_CONTROL_LEN - diff_a_b).prop_flat_map(move |diff_b_c| {
389                    let c = b + diff_b_c;
390                    (Just(a), Just(b), Just(c))
391                })
392            })
393        })
394    }
395
396    #[test_case(WindowSize::new(1).unwrap() => (UnscaledWindowSize::from(1), WindowScale::default()))]
397    #[test_case(WindowSize::new(65535).unwrap() => (UnscaledWindowSize::from(65535), WindowScale::default()))]
398    #[test_case(WindowSize::new(65536).unwrap() => (UnscaledWindowSize::from(32768), WindowScale::new(1).unwrap()))]
399    #[test_case(WindowSize::new(65537).unwrap() => (UnscaledWindowSize::from(32768), WindowScale::new(1).unwrap()))]
400    fn window_scale(size: WindowSize) -> (UnscaledWindowSize, WindowScale) {
401        let scale = size.scale();
402        (size >> scale, scale)
403    }
404
405    proptest! {
406        #![proptest_config(Config {
407            // Add all failed seeds here.
408            failure_persistence: failed_seeds_no_std!(),
409            ..Config::default()
410        })]
411
412        #[test]
413        fn seqnum_ord_is_reflexive(a in arb_seqnum()) {
414            prop_assert_eq!(a, a)
415        }
416
417        #[test]
418        fn seqnum_ord_is_total(a in arb_seqnum(), b in arb_seqnum()) {
419            if a == b {
420                prop_assert!(!a.before(b) && !b.before(a))
421            } else {
422                prop_assert!(a.before(b) ^ b.before(a))
423            }
424        }
425
426        #[test]
427        fn seqnum_ord_is_transitive((a, b, c) in arb_seqnum_trans_tripple()) {
428            prop_assert!(a.before(b) && b.before(c) && a.before(c));
429        }
430
431        #[test]
432        fn seqnum_add_positive_greater(a in arb_seqnum(), b in 1..=i32::MAX) {
433            prop_assert!(a.before(a + b))
434        }
435
436        #[test]
437        fn seqnum_add_negative_smaller(a in arb_seqnum(), b in i32::MIN..=-1) {
438            prop_assert!(a.after(a + b))
439        }
440
441        #[test]
442        fn seqnum_sub_positive_smaller(a in arb_seqnum(), b in 1..=i32::MAX) {
443            prop_assert!(a.after(a - b))
444        }
445
446        #[test]
447        fn seqnum_sub_negative_greater(a in arb_seqnum(), b in i32::MIN..=-1) {
448            prop_assert!(a.before(a - b))
449        }
450
451        #[test]
452        fn seqnum_zero_identity(a in arb_seqnum()) {
453            prop_assert_eq!(a, a + 0)
454        }
455
456        #[test]
457        fn seqnum_before_after_inverse(a in arb_seqnum(), b in arb_seqnum()) {
458            prop_assert_eq!(a.after(b), b.before(a))
459        }
460
461        #[test]
462        fn seqnum_wraps_around_at_max_length(a in arb_seqnum()) {
463            prop_assert!(a.before(a + MAX_PAYLOAD_AND_CONTROL_LEN));
464            prop_assert!(a.after(a + MAX_PAYLOAD_AND_CONTROL_LEN + 1));
465        }
466
467        #[test]
468        fn window_size_less_than_or_eq_to_max(wnd in 0..=WindowSize::MAX.0) {
469            prop_assert_eq!(WindowSize::from_u32(wnd), Some(WindowSize(wnd)));
470        }
471
472        #[test]
473        fn window_size_greater_than_max(wnd in WindowSize::MAX.0+1..=u32::MAX) {
474            prop_assert_eq!(WindowSize::from_u32(wnd), None);
475        }
476    }
477}
netstack3_base/tcp/seqnum.rs

netstack3_base/tcp/
seqnum.rs
