summaryrefslogtreecommitdiffstats
path: root/third_party/rust/neqo-transport/src/path.rs
blob: 0e4c82b1ca41522f834a458a9449332d102b8bc4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![allow(clippy::module_name_repetitions)]

use std::{
    cell::RefCell,
    fmt::{self, Display},
    mem,
    net::{IpAddr, SocketAddr},
    rc::Rc,
    time::{Duration, Instant},
};

use neqo_common::{hex, qdebug, qinfo, qlog::NeqoQlog, qtrace, Datagram, Encoder, IpTos};
use neqo_crypto::random;

use crate::{
    ackrate::{AckRate, PeerAckDelay},
    cc::CongestionControlAlgorithm,
    cid::{ConnectionId, ConnectionIdRef, ConnectionIdStore, RemoteConnectionIdEntry},
    ecn::{EcnCount, EcnInfo},
    frame::{FRAME_TYPE_PATH_CHALLENGE, FRAME_TYPE_PATH_RESPONSE, FRAME_TYPE_RETIRE_CONNECTION_ID},
    packet::PacketBuilder,
    recovery::RecoveryToken,
    rtt::RttEstimate,
    sender::PacketSender,
    stats::FrameStats,
    tracking::{PacketNumberSpace, SentPacket},
    Stats,
};

/// This is the MTU that we assume when using IPv6.
/// We use this size for Initial packets, so we don't need to worry about probing for support.
/// If the path doesn't support this MTU, we will assume that it doesn't support QUIC.
///
/// This is a multiple of 16 greater than the largest possible short header (1 + 20 + 4).
pub const PATH_MTU_V6: usize = 1337;
/// The path MTU for IPv4 can be 20 bytes larger than for v6.
pub const PATH_MTU_V4: usize = PATH_MTU_V6 + 20;
/// The number of times that a path will be probed before it is considered failed.
const MAX_PATH_PROBES: usize = 3;
/// The maximum number of paths that `Paths` will track.
const MAX_PATHS: usize = 15;

pub type PathRef = Rc<RefCell<Path>>;

/// A collection for network paths.
/// This holds a collection of paths that have been used for sending or
/// receiving, plus an additional "temporary" path that is held only while
/// processing a packet.
/// This structure limits its storage and will forget about paths if it
/// is exposed to too many paths.
#[derive(Debug, Default)]
pub struct Paths {
    /// All of the paths.  All of these paths will be permanent.
    #[allow(unknown_lints)] // available with Rust v1.75
    #[allow(clippy::struct_field_names)]
    paths: Vec<PathRef>,
    /// This is the primary path.  This will only be `None` initially, so
    /// care needs to be taken regarding that only during the handshake.
    /// This path will also be in `paths`.
    primary: Option<PathRef>,

    /// The path that we would prefer to migrate to.
    migration_target: Option<PathRef>,

    /// Connection IDs that need to be retired.
    to_retire: Vec<u64>,

    /// `QLog` handler.
    qlog: NeqoQlog,
}

impl Paths {
    /// Find the path for the given addresses.
    /// This might be a temporary path.
    pub fn find_path(
        &self,
        local: SocketAddr,
        remote: SocketAddr,
        cc: CongestionControlAlgorithm,
        pacing: bool,
        now: Instant,
    ) -> PathRef {
        self.paths
            .iter()
            .find_map(|p| {
                if p.borrow().received_on(local, remote, false) {
                    Some(Rc::clone(p))
                } else {
                    None
                }
            })
            .unwrap_or_else(|| {
                let mut p = Path::temporary(local, remote, cc, pacing, self.qlog.clone(), now);
                if let Some(primary) = self.primary.as_ref() {
                    p.prime_rtt(primary.borrow().rtt());
                }
                Rc::new(RefCell::new(p))
            })
    }

    /// Find the path, but allow for rebinding.  That matches the pair of addresses
    /// to paths that match the remote address only based on IP addres, not port.
    /// We use this when the other side migrates to skip address validation and
    /// creating a new path.
    pub fn find_path_with_rebinding(
        &self,
        local: SocketAddr,
        remote: SocketAddr,
        cc: CongestionControlAlgorithm,
        pacing: bool,
        now: Instant,
    ) -> PathRef {
        self.paths
            .iter()
            .find_map(|p| {
                if p.borrow().received_on(local, remote, false) {
                    Some(Rc::clone(p))
                } else {
                    None
                }
            })
            .or_else(|| {
                self.paths.iter().find_map(|p| {
                    if p.borrow().received_on(local, remote, true) {
                        Some(Rc::clone(p))
                    } else {
                        None
                    }
                })
            })
            .unwrap_or_else(|| {
                Rc::new(RefCell::new(Path::temporary(
                    local,
                    remote,
                    cc,
                    pacing,
                    self.qlog.clone(),
                    now,
                )))
            })
    }

    /// Get a reference to the primary path, if one exists.
    pub fn primary(&self) -> Option<PathRef> {
        self.primary.clone()
    }

    /// Returns true if the path is not permanent.
    pub fn is_temporary(&self, path: &PathRef) -> bool {
        // Ask the path first, which is simpler.
        path.borrow().is_temporary() || !self.paths.iter().any(|p| Rc::ptr_eq(p, path))
    }

    fn retire(to_retire: &mut Vec<u64>, retired: &PathRef) {
        let seqno = retired
            .borrow()
            .remote_cid
            .as_ref()
            .unwrap()
            .sequence_number();
        to_retire.push(seqno);
    }

    /// Adopt a temporary path as permanent.
    /// The first path that is made permanent is made primary.
    pub fn make_permanent(
        &mut self,
        path: &PathRef,
        local_cid: Option<ConnectionId>,
        remote_cid: RemoteConnectionIdEntry,
    ) {
        debug_assert!(self.is_temporary(path));

        // Make sure not to track too many paths.
        // This protects index 0, which contains the primary path.
        if self.paths.len() >= MAX_PATHS {
            debug_assert_eq!(self.paths.len(), MAX_PATHS);
            let removed = self.paths.remove(1);
            Self::retire(&mut self.to_retire, &removed);
            if self
                .migration_target
                .as_ref()
                .map_or(false, |target| Rc::ptr_eq(target, &removed))
            {
                qinfo!(
                    [path.borrow()],
                    "The migration target path had to be removed"
                );
                self.migration_target = None;
            }
            debug_assert_eq!(Rc::strong_count(&removed), 1);
        }

        qdebug!([path.borrow()], "Make permanent");
        path.borrow_mut().make_permanent(local_cid, remote_cid);
        self.paths.push(Rc::clone(path));
        if self.primary.is_none() {
            assert!(self.select_primary(path).is_none());
        }
    }

    /// Select a path as the primary.  Returns the old primary path.
    /// Using the old path is only necessary if this change in path is a reaction
    /// to a migration from a peer, in which case the old path needs to be probed.
    #[must_use]
    fn select_primary(&mut self, path: &PathRef) -> Option<PathRef> {
        qdebug!([path.borrow()], "set as primary path");
        let old_path = self.primary.replace(Rc::clone(path)).map(|old| {
            old.borrow_mut().set_primary(false);
            old
        });

        // Swap the primary path into slot 0, so that it is protected from eviction.
        let idx = self
            .paths
            .iter()
            .enumerate()
            .find_map(|(i, p)| if Rc::ptr_eq(p, path) { Some(i) } else { None })
            .expect("migration target should be permanent");
        self.paths.swap(0, idx);

        path.borrow_mut().set_primary(true);
        old_path
    }

    /// Migrate to the identified path.  If `force` is true, the path
    /// is forcibly marked as valid and the path is used immediately.
    /// Otherwise, migration will occur after probing succeeds.
    /// The path is always probed and will be abandoned if probing fails.
    /// Returns `true` if the path was migrated.
    pub fn migrate(&mut self, path: &PathRef, force: bool, now: Instant) -> bool {
        debug_assert!(!self.is_temporary(path));
        let baseline = self.primary().map_or_else(
            || EcnInfo::default().baseline(),
            |p| p.borrow().ecn_info.baseline(),
        );
        path.borrow_mut().set_ecn_baseline(baseline);
        if force || path.borrow().is_valid() {
            path.borrow_mut().set_valid(now);
            mem::drop(self.select_primary(path));
            self.migration_target = None;
        } else {
            self.migration_target = Some(Rc::clone(path));
        }
        path.borrow_mut().probe();
        self.migration_target.is_none()
    }

    /// Process elapsed time for active paths.
    /// Returns an true if there are viable paths remaining after tidying up.
    ///
    /// TODO(mt) - the paths should own the RTT estimator, so they can find the PTO
    /// for themselves.
    pub fn process_timeout(&mut self, now: Instant, pto: Duration) -> bool {
        let to_retire = &mut self.to_retire;
        let mut primary_failed = false;
        self.paths.retain(|p| {
            if p.borrow_mut().process_timeout(now, pto) {
                true
            } else {
                qdebug!([p.borrow()], "Retiring path");
                if p.borrow().is_primary() {
                    primary_failed = true;
                }
                Self::retire(to_retire, p);
                false
            }
        });

        if primary_failed {
            self.primary = None;
            // Find a valid path to fall back to.
            if let Some(fallback) = self
                .paths
                .iter()
                .rev() // More recent paths are toward the end.
                .find(|p| p.borrow().is_valid())
            {
                // Need a clone as `fallback` is borrowed from `self`.
                let path = Rc::clone(fallback);
                qinfo!([path.borrow()], "Failing over after primary path failed");
                mem::drop(self.select_primary(&path));
                true
            } else {
                false
            }
        } else {
            true
        }
    }

    /// Get when the next call to `process_timeout()` should be scheduled.
    pub fn next_timeout(&self, pto: Duration) -> Option<Instant> {
        self.paths
            .iter()
            .filter_map(|p| p.borrow().next_timeout(pto))
            .min()
    }

    /// Set the identified path to be primary.
    /// This panics if `make_permanent` hasn't been called.
    pub fn handle_migration(&mut self, path: &PathRef, remote: SocketAddr, now: Instant) {
        // The update here needs to match the checks in `Path::received_on`.
        // Here, we update the remote port number to match the source port on the
        // datagram that was received.  This ensures that we send subsequent
        // packets back to the right place.
        path.borrow_mut().update_port(remote.port());

        if path.borrow().is_primary() {
            // Update when the path was last regarded as valid.
            path.borrow_mut().update(now);
            return;
        }

        if let Some(old_path) = self.select_primary(path) {
            // Need to probe the old path if the peer migrates.
            old_path.borrow_mut().probe();
            // TODO(mt) - suppress probing if the path was valid within 3PTO.
        }
    }

    /// Select a path to send on.  This will select the first path that has
    /// probes to send, then fall back to the primary path.
    pub fn select_path(&self) -> Option<PathRef> {
        self.paths
            .iter()
            .find_map(|p| {
                if p.borrow().has_probe() {
                    Some(Rc::clone(p))
                } else {
                    None
                }
            })
            .or_else(|| self.primary.clone())
    }

    /// A `PATH_RESPONSE` was received.
    /// Returns `true` if migration occurred.
    #[must_use]
    pub fn path_response(&mut self, response: [u8; 8], now: Instant) -> bool {
        // TODO(mt) consider recording an RTT measurement here as we don't train
        // RTT for non-primary paths.
        for p in &self.paths {
            if p.borrow_mut().path_response(response, now) {
                // The response was accepted.  If this path is one we intend
                // to migrate to, then migrate.
                if self
                    .migration_target
                    .as_ref()
                    .map_or(false, |target| Rc::ptr_eq(target, p))
                {
                    let primary = self.migration_target.take();
                    mem::drop(self.select_primary(&primary.unwrap()));
                    return true;
                }
                break;
            }
        }
        false
    }

    /// Retire all of the connection IDs prior to the indicated sequence number.
    /// Keep active paths if possible by pulling new connection IDs from the provided store.
    /// One slightly non-obvious consequence of this is that if migration is being attempted
    /// and the new path cannot obtain a new connection ID, the migration attempt will fail.
    pub fn retire_cids(&mut self, retire_prior: u64, store: &mut ConnectionIdStore<[u8; 16]>) {
        let to_retire = &mut self.to_retire;
        let migration_target = &mut self.migration_target;

        // First, tell the store to release any connection IDs that are too old.
        let mut retired = store.retire_prior_to(retire_prior);
        to_retire.append(&mut retired);

        self.paths.retain(|p| {
            let current = p.borrow().remote_cid.as_ref().unwrap().sequence_number();
            if current < retire_prior {
                to_retire.push(current);
                let new_cid = store.next();
                let has_replacement = new_cid.is_some();
                // There must be a connection ID available for the primary path as we
                // keep that path at the first index.
                debug_assert!(!p.borrow().is_primary() || has_replacement);
                p.borrow_mut().remote_cid = new_cid;
                if !has_replacement
                    && migration_target
                        .as_ref()
                        .map_or(false, |target| Rc::ptr_eq(target, p))
                {
                    qinfo!(
                        [p.borrow()],
                        "NEW_CONNECTION_ID with Retire Prior To forced migration to fail"
                    );
                    *migration_target = None;
                }
                has_replacement
            } else {
                true
            }
        });
    }

    /// Write out any `RETIRE_CONNECTION_ID` frames that are outstanding.
    pub fn write_frames(
        &mut self,
        builder: &mut PacketBuilder,
        tokens: &mut Vec<RecoveryToken>,
        stats: &mut FrameStats,
    ) {
        while let Some(seqno) = self.to_retire.pop() {
            if builder.remaining() < 1 + Encoder::varint_len(seqno) {
                self.to_retire.push(seqno);
                break;
            }
            builder.encode_varint(FRAME_TYPE_RETIRE_CONNECTION_ID);
            builder.encode_varint(seqno);
            tokens.push(RecoveryToken::RetireConnectionId(seqno));
            stats.retire_connection_id += 1;
        }

        if let Some(path) = self.primary() {
            // Write out any ACK_FREQUENCY frames.
            path.borrow_mut().write_cc_frames(builder, tokens, stats);
        }
    }

    pub fn lost_retire_cid(&mut self, lost: u64) {
        self.to_retire.push(lost);
    }

    pub fn acked_retire_cid(&mut self, acked: u64) {
        self.to_retire.retain(|&seqno| seqno != acked);
    }

    pub fn lost_ack_frequency(&mut self, lost: &AckRate) {
        if let Some(path) = self.primary() {
            path.borrow_mut().lost_ack_frequency(lost);
        }
    }

    pub fn acked_ack_frequency(&mut self, acked: &AckRate) {
        if let Some(path) = self.primary() {
            path.borrow_mut().acked_ack_frequency(acked);
        }
    }

    /// Get an estimate of the RTT on the primary path.
    #[cfg(test)]
    pub fn rtt(&self) -> Duration {
        // Rather than have this fail when there is no active path,
        // make a new RTT esimate and interrogate that.
        // That is more expensive, but it should be rare and breaking encapsulation
        // is worse, especially as this is only used in tests.
        self.primary()
            .map_or(RttEstimate::default().estimate(), |p| {
                p.borrow().rtt().estimate()
            })
    }

    pub fn set_qlog(&mut self, qlog: NeqoQlog) {
        for p in &mut self.paths {
            p.borrow_mut().set_qlog(qlog.clone());
        }
        self.qlog = qlog;
    }
}

/// The state of a path with respect to address validation.
#[derive(Debug)]
enum ProbeState {
    /// The path was last valid at the indicated time.
    Valid,
    /// The path was previously valid, but a new probe is needed.
    ProbeNeeded { probe_count: usize },
    /// The path hasn't been validated, but a probe has been sent.
    Probing {
        /// The number of probes that have been sent.
        probe_count: usize,
        /// The probe that was last sent.
        data: [u8; 8],
        /// Whether the probe was sent in a datagram padded to the path MTU.
        mtu: bool,
        /// When the probe was sent.
        sent: Instant,
    },
    /// Validation failed the last time it was attempted.
    Failed,
}

impl ProbeState {
    /// Determine whether the current state requires probing.
    fn probe_needed(&self) -> bool {
        matches!(self, Self::ProbeNeeded { .. })
    }
}

/// A network path.
///
/// Paths are used a little bit strangely by connections:
/// they need to encapsulate all the state for a path (which
/// is normal), but that information is not propagated to the
/// `Paths` instance that holds them.  This is because the packet
/// processing where changes occur can't hold a reference to the
/// `Paths` instance that owns the `Path`.  Any changes to the
/// path are communicated to `Paths` afterwards.
#[derive(Debug)]
pub struct Path {
    /// A local socket address.
    local: SocketAddr,
    /// A remote socket address.
    remote: SocketAddr,
    /// The connection IDs that we use when sending on this path.
    /// This is only needed during the handshake.
    local_cid: Option<ConnectionId>,
    /// The current connection ID that we are using and its details.
    remote_cid: Option<RemoteConnectionIdEntry>,

    /// Whether this is the primary path.
    primary: bool,
    /// Whether the current path is considered valid.
    state: ProbeState,
    /// For a path that is not validated, this is `None`.  For a validated
    /// path, the time that the path was last valid.
    validated: Option<Instant>,
    /// A path challenge was received and `PATH_RESPONSE` has not been sent.
    challenge: Option<[u8; 8]>,

    /// The round trip time estimate for this path.
    rtt: RttEstimate,
    /// A packet sender for the path, which includes congestion control and a pacer.
    sender: PacketSender,
    /// The IP TTL to use for outgoing packets on this path.
    ttl: u8,

    /// The number of bytes received on this path.
    /// Note that this value might saturate on a long-lived connection,
    /// but we only use it before the path is validated.
    received_bytes: usize,
    /// The number of bytes sent on this path.
    sent_bytes: usize,
    /// The ECN-related state for this path (see RFC9000, Section 13.4 and Appendix A.4)
    ecn_info: EcnInfo,
    /// For logging of events.
    qlog: NeqoQlog,
}

impl Path {
    /// Create a path from addresses and a remote connection ID.
    /// This is used for migration and for new datagrams.
    pub fn temporary(
        local: SocketAddr,
        remote: SocketAddr,
        cc: CongestionControlAlgorithm,
        pacing: bool,
        qlog: NeqoQlog,
        now: Instant,
    ) -> Self {
        let mut sender = PacketSender::new(cc, pacing, Self::mtu_by_addr(remote.ip()), now);
        sender.set_qlog(qlog.clone());
        Self {
            local,
            remote,
            local_cid: None,
            remote_cid: None,
            primary: false,
            state: ProbeState::ProbeNeeded { probe_count: 0 },
            validated: None,
            challenge: None,
            rtt: RttEstimate::default(),
            sender,
            ttl: 64, // This is the default TTL on many OSes.
            received_bytes: 0,
            sent_bytes: 0,
            ecn_info: EcnInfo::default(),
            qlog,
        }
    }

    pub fn set_ecn_baseline(&mut self, baseline: EcnCount) {
        self.ecn_info.set_baseline(baseline);
    }

    /// Return the DSCP/ECN marking to use for outgoing packets on this path.
    pub fn tos(&self) -> IpTos {
        self.ecn_info.ecn_mark().into()
    }

    /// Whether this path is the primary or current path for the connection.
    pub fn is_primary(&self) -> bool {
        self.primary
    }

    /// Whether this path is a temporary one.
    pub fn is_temporary(&self) -> bool {
        self.remote_cid.is_none()
    }

    /// By adding a remote connection ID, we make the path permanent
    /// and one that we will later send packets on.
    /// If `local_cid` is `None`, the existing value will be kept.
    pub(crate) fn make_permanent(
        &mut self,
        local_cid: Option<ConnectionId>,
        remote_cid: RemoteConnectionIdEntry,
    ) {
        if self.local_cid.is_none() {
            self.local_cid = local_cid;
        }
        self.remote_cid.replace(remote_cid);
    }

    /// Determine if this path was the one that the provided datagram was received on.
    /// This uses the full local socket address, but ignores the port number on the peer
    /// if `flexible` is true, allowing for NAT rebinding that retains the same IP.
    fn received_on(&self, local: SocketAddr, remote: SocketAddr, flexible: bool) -> bool {
        self.local == local
            && self.remote.ip() == remote.ip()
            && (flexible || self.remote.port() == remote.port())
    }

    /// Update the remote port number.  Any flexibility we allow in `received_on`
    /// need to be adjusted at this point.
    fn update_port(&mut self, port: u16) {
        self.remote.set_port(port);
    }

    /// Set whether this path is primary.
    pub(crate) fn set_primary(&mut self, primary: bool) {
        qtrace!([self], "Make primary {}", primary);
        debug_assert!(self.remote_cid.is_some());
        self.primary = primary;
        if !primary {
            self.sender.discard_in_flight();
        }
    }

    /// Set the current path as valid.  This updates the time that the path was
    /// last validated and cancels any path validation.
    pub fn set_valid(&mut self, now: Instant) {
        qdebug!([self], "Path validated {:?}", now);
        self.state = ProbeState::Valid;
        self.validated = Some(now);
    }

    /// Update the last use of this path, if it is valid.
    /// This will keep the path active slightly longer.
    pub fn update(&mut self, now: Instant) {
        if self.validated.is_some() {
            self.validated = Some(now);
        }
    }

    fn mtu_by_addr(addr: IpAddr) -> usize {
        match addr {
            IpAddr::V4(_) => PATH_MTU_V4,
            IpAddr::V6(_) => PATH_MTU_V6,
        }
    }

    /// Get the path MTU.  This is currently fixed based on IP version.
    pub fn mtu(&self) -> usize {
        Self::mtu_by_addr(self.remote.ip())
    }

    /// Get the first local connection ID.
    /// Only do this for the primary path during the handshake.
    pub fn local_cid(&self) -> &ConnectionId {
        self.local_cid.as_ref().unwrap()
    }

    /// Set the remote connection ID based on the peer's choice.
    /// This is only valid during the handshake.
    pub fn set_remote_cid(&mut self, cid: ConnectionIdRef) {
        self.remote_cid
            .as_mut()
            .unwrap()
            .update_cid(ConnectionId::from(cid));
    }

    /// Access the remote connection ID.
    pub fn remote_cid(&self) -> &ConnectionId {
        self.remote_cid.as_ref().unwrap().connection_id()
    }

    /// Set the stateless reset token for the connection ID that is currently in use.
    /// Panics if the sequence number is non-zero as this is only necessary during
    /// the handshake; all other connection IDs are initialized with a token.
    pub fn set_reset_token(&mut self, token: [u8; 16]) {
        self.remote_cid
            .as_mut()
            .unwrap()
            .set_stateless_reset_token(token);
    }

    /// Determine if the provided token is a stateless reset token.
    pub fn is_stateless_reset(&self, token: &[u8; 16]) -> bool {
        self.remote_cid
            .as_ref()
            .map_or(false, |rcid| rcid.is_stateless_reset(token))
    }

    /// Make a datagram.
    pub fn datagram<V: Into<Vec<u8>>>(&mut self, payload: V) -> Datagram {
        self.ecn_info.on_packet_sent();
        Datagram::new(self.local, self.remote, self.tos(), Some(self.ttl), payload)
    }

    /// Get local address as `SocketAddr`
    pub fn local_address(&self) -> SocketAddr {
        self.local
    }

    /// Get remote address as `SocketAddr`
    pub fn remote_address(&self) -> SocketAddr {
        self.remote
    }

    /// Whether the path has been validated.
    pub fn is_valid(&self) -> bool {
        self.validated.is_some()
    }

    /// Handle a `PATH_RESPONSE` frame. Returns true if the response was accepted.
    pub fn path_response(&mut self, response: [u8; 8], now: Instant) -> bool {
        if let ProbeState::Probing { data, mtu, .. } = &mut self.state {
            if response == *data {
                let need_full_probe = !*mtu;
                self.set_valid(now);
                if need_full_probe {
                    qdebug!([self], "Sub-MTU probe successful, reset probe count");
                    self.probe();
                }
                true
            } else {
                false
            }
        } else {
            false
        }
    }

    /// The path has been challenged.  This generates a response.
    /// This only generates a single response at a time.
    pub fn challenged(&mut self, challenge: [u8; 8]) {
        self.challenge = Some(challenge.to_owned());
    }

    /// At the next opportunity, send a probe.
    /// If the probe count has been exhausted already, marks the path as failed.
    fn probe(&mut self) {
        let probe_count = match &self.state {
            ProbeState::Probing { probe_count, .. } => *probe_count + 1,
            ProbeState::ProbeNeeded { probe_count, .. } => *probe_count,
            _ => 0,
        };
        self.state = if probe_count >= MAX_PATH_PROBES {
            qinfo!([self], "Probing failed");
            ProbeState::Failed
        } else {
            qdebug!([self], "Initiating probe");
            ProbeState::ProbeNeeded { probe_count }
        };
    }

    /// Returns true if this path have any probing frames to send.
    pub fn has_probe(&self) -> bool {
        self.challenge.is_some() || self.state.probe_needed()
    }

    pub fn write_frames(
        &mut self,
        builder: &mut PacketBuilder,
        stats: &mut FrameStats,
        mtu: bool, // Whether the packet we're writing into will be a full MTU.
        now: Instant,
    ) -> bool {
        if builder.remaining() < 9 {
            return false;
        }

        // Send PATH_RESPONSE.
        let resp_sent = if let Some(challenge) = self.challenge.take() {
            qtrace!([self], "Responding to path challenge {}", hex(challenge));
            builder.encode_varint(FRAME_TYPE_PATH_RESPONSE);
            builder.encode(&challenge[..]);

            // These frames are not retransmitted in the usual fashion.
            // There is no token, therefore we need to count `all` specially.
            stats.path_response += 1;
            stats.all += 1;

            if builder.remaining() < 9 {
                return true;
            }
            true
        } else {
            false
        };

        // Send PATH_CHALLENGE.
        if let ProbeState::ProbeNeeded { probe_count } = self.state {
            qtrace!([self], "Initiating path challenge {}", probe_count);
            let data = random::<8>();
            builder.encode_varint(FRAME_TYPE_PATH_CHALLENGE);
            builder.encode(&data);

            // As above, no recovery token.
            stats.path_challenge += 1;
            stats.all += 1;

            self.state = ProbeState::Probing {
                probe_count,
                data,
                mtu,
                sent: now,
            };
            true
        } else {
            resp_sent
        }
    }

    /// Write `ACK_FREQUENCY` frames.
    pub fn write_cc_frames(
        &mut self,
        builder: &mut PacketBuilder,
        tokens: &mut Vec<RecoveryToken>,
        stats: &mut FrameStats,
    ) {
        self.rtt.write_frames(builder, tokens, stats);
    }

    pub fn lost_ack_frequency(&mut self, lost: &AckRate) {
        self.rtt.frame_lost(lost);
    }

    pub fn acked_ack_frequency(&mut self, acked: &AckRate) {
        self.rtt.frame_acked(acked);
    }

    /// Process a timer for this path.
    /// This returns true if the path is viable and can be kept alive.
    pub fn process_timeout(&mut self, now: Instant, pto: Duration) -> bool {
        if let ProbeState::Probing { sent, .. } = &self.state {
            if now >= *sent + pto {
                self.probe();
            }
        }
        if let ProbeState::Failed = self.state {
            // Retire failed paths immediately.
            false
        } else if self.primary {
            // Keep valid primary paths otherwise.
            true
        } else if let ProbeState::Valid = self.state {
            // Retire validated, non-primary paths.
            // Allow more than `MAX_PATH_PROBES` times the PTO so that an old
            // path remains around until after a previous path fails.
            let count = u32::try_from(MAX_PATH_PROBES + 1).unwrap();
            self.validated.unwrap() + (pto * count) > now
        } else {
            // Keep paths that are being actively probed.
            true
        }
    }

    /// Return the next time that this path needs servicing.
    /// This only considers retransmissions of probes, not cleanup of the path.
    /// If there is no other activity, then there is no real need to schedule a
    /// timer to cleanup old paths.
    pub fn next_timeout(&self, pto: Duration) -> Option<Instant> {
        if let ProbeState::Probing { sent, .. } = &self.state {
            Some(*sent + pto)
        } else {
            None
        }
    }

    /// Get the RTT estimator for this path.
    pub fn rtt(&self) -> &RttEstimate {
        &self.rtt
    }

    /// Mutably borrow the RTT estimator for this path.
    pub fn rtt_mut(&mut self) -> &mut RttEstimate {
        &mut self.rtt
    }

    /// Read-only access to the owned sender.
    pub fn sender(&self) -> &PacketSender {
        &self.sender
    }

    /// Pass on RTT configuration: the maximum acknowledgment delay of the peer,
    /// and maybe the minimum delay.
    pub fn set_ack_delay(
        &mut self,
        max_ack_delay: Duration,
        min_ack_delay: Option<Duration>,
        ack_ratio: u8,
    ) {
        let ack_delay = min_ack_delay.map_or_else(
            || PeerAckDelay::fixed(max_ack_delay),
            |m| {
                PeerAckDelay::flexible(
                    max_ack_delay,
                    m,
                    ack_ratio,
                    self.sender.cwnd(),
                    self.mtu(),
                    self.rtt.estimate(),
                )
            },
        );
        self.rtt.set_ack_delay(ack_delay);
    }

    /// Initialize the RTT for the path based on an existing estimate.
    pub fn prime_rtt(&mut self, rtt: &RttEstimate) {
        self.rtt.prime_rtt(rtt);
    }

    /// Record received bytes for the path.
    pub fn add_received(&mut self, count: usize) {
        self.received_bytes = self.received_bytes.saturating_add(count);
    }

    /// Record sent bytes for the path.
    pub fn add_sent(&mut self, count: usize) {
        self.sent_bytes = self.sent_bytes.saturating_add(count);
    }

    /// Record a packet as having been sent on this path.
    pub fn packet_sent(&mut self, sent: &mut SentPacket) {
        if !self.is_primary() {
            sent.clear_primary_path();
        }
        self.sender.on_packet_sent(sent, self.rtt.estimate());
    }

    /// Discard a packet that previously might have been in-flight.
    pub fn discard_packet(&mut self, sent: &SentPacket, now: Instant, stats: &mut Stats) {
        if self.rtt.first_sample_time().is_none() {
            // When discarding a packet there might not be a good RTT estimate.
            // But discards only occur after receiving something, so that means
            // that there is some RTT information, which is better than nothing.
            // Two cases: 1. at the client when handling a Retry and
            // 2. at the server when disposing the Initial packet number space.
            qinfo!(
                [self],
                "discarding a packet without an RTT estimate; guessing RTT={:?}",
                now - sent.time_sent
            );
            stats.rtt_init_guess = true;
            self.rtt.update(
                &mut self.qlog,
                now - sent.time_sent,
                Duration::new(0, 0),
                false,
                now,
            );
        }

        self.sender.discard(sent);
    }

    /// Record packets as acknowledged with the sender.
    pub fn on_packets_acked(
        &mut self,
        acked_pkts: &[SentPacket],
        ack_ecn: Option<EcnCount>,
        now: Instant,
    ) {
        debug_assert!(self.is_primary());

        let ecn_ce_received = self.ecn_info.on_packets_acked(acked_pkts, ack_ecn);
        if ecn_ce_received {
            let cwnd_reduced = self
                .sender
                .on_ecn_ce_received(acked_pkts.first().expect("must be there"));
            if cwnd_reduced {
                self.rtt.update_ack_delay(self.sender.cwnd(), self.mtu());
            }
        }

        self.sender.on_packets_acked(acked_pkts, &self.rtt, now);
    }

    /// Record packets as lost with the sender.
    pub fn on_packets_lost(
        &mut self,
        prev_largest_acked_sent: Option<Instant>,
        space: PacketNumberSpace,
        lost_packets: &[SentPacket],
    ) {
        debug_assert!(self.is_primary());
        let cwnd_reduced = self.sender.on_packets_lost(
            self.rtt.first_sample_time(),
            prev_largest_acked_sent,
            self.rtt.pto(space), // Important: the base PTO, not adjusted.
            lost_packets,
        );
        if cwnd_reduced {
            self.rtt.update_ack_delay(self.sender.cwnd(), self.mtu());
        }
    }

    /// Get the number of bytes that can be written to this path.
    pub fn amplification_limit(&self) -> usize {
        if matches!(self.state, ProbeState::Failed) {
            0
        } else if self.is_valid() {
            usize::MAX
        } else {
            self.received_bytes
                .checked_mul(3)
                .map_or(usize::MAX, |limit| {
                    let budget = if limit == 0 {
                        // If we have received absolutely nothing thus far, then this endpoint
                        // is the one initiating communication on this path.  Allow enough space for
                        // probing.
                        self.mtu() * 5
                    } else {
                        limit
                    };
                    budget.saturating_sub(self.sent_bytes)
                })
        }
    }

    /// Update the `NeqoQLog` instance.
    pub fn set_qlog(&mut self, qlog: NeqoQlog) {
        self.sender.set_qlog(qlog);
    }
}

impl Display for Path {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        if self.is_primary() {
            write!(f, "pri-")?; // primary
        }
        if !self.is_valid() {
            write!(f, "unv-")?; // unvalidated
        }
        write!(f, "path")?;
        if let Some(entry) = self.remote_cid.as_ref() {
            write!(f, ":{}", entry.connection_id())?;
        }
        write!(f, " {}->{}", self.local, self.remote)?;
        Ok(())
    }
}