// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause

package magicsock

import (
	"bufio"
	"context"
	"encoding/binary"
	"errors"
	"fmt"
	iter "github.com/metacubex/tailscale/util/go120/iter"
	"github.com/metacubex/tailscale/util/go120/randv2"
	slices "github.com/metacubex/tailscale/util/go120/slices"
	"math"
	"net"
	"net/netip"
	"reflect"
	"runtime"
	"sync/atomic"
	"time"

	"github.com/metacubex/tailscale/disco"
	"github.com/metacubex/tailscale/ipn/ipnstate"
	"github.com/metacubex/tailscale/net/packet"
	"github.com/metacubex/tailscale/net/stun"
	"github.com/metacubex/tailscale/net/tstun"
	"github.com/metacubex/tailscale/syncs"
	"github.com/metacubex/tailscale/tailcfg"
	"github.com/metacubex/tailscale/tstime/mono"
	"github.com/metacubex/tailscale/types/key"
	"github.com/metacubex/tailscale/types/logger"
	"github.com/metacubex/tailscale/util/mak"
	"github.com/metacubex/tailscale/util/ringlog"
	"github.com/metacubex/tailscale/util/slicesx"
	"golang.org/x/net/ipv4"
	"golang.org/x/net/ipv6"
)

var mtuProbePingSizesV4 []int
var mtuProbePingSizesV6 []int

// discoKeyAdvertisementInterval tells how often a disco update via TSMP can
// happen. The update is triggered via enqueueCallMeMaybe, and thus it will
// only be sent if the magicsock is in a state to send out CallMeMaybe.
const discoKeyAdvertisementInterval = time.Second * 60

func init() {
	for _, m := range tstun.WireMTUsToProbe {
		mtuProbePingSizesV4 = append(mtuProbePingSizesV4, pktLenToPingSize(m, false))
		mtuProbePingSizesV6 = append(mtuProbePingSizesV6, pktLenToPingSize(m, true))
	}
}

// endpoint is a wireguard/conn.Endpoint. In wireguard-go and kernel WireGuard
// there is only one endpoint for a peer, but in Tailscale we distribute a
// number of possible endpoints for a peer which would include the all the
// likely addresses at which a peer may be reachable. This endpoint type holds
// the information required that when wireguard-go wants to send to a
// particular peer (essentially represented by this endpoint type), the send
// function can use the currently best known Tailscale endpoint to send packets
// to the peer.
type endpoint struct {
	// atomically accessed; declared first for alignment reasons
	lastRecvWG            mono.Time // last time there were incoming packets from this peer destined for wireguard-go (e.g. not disco)
	lastRecvUDPAny        mono.Time // last time there were incoming UDP packets from this peer of any kind
	numStopAndResetAtomic int64
	debugUpdates          *ringlog.RingLog[EndpointChange]

	// These fields are initialized once and never modified.
	c            *Conn
	nodeID       tailcfg.NodeID
	publicKey    key.NodePublic // peer public key (for WireGuard + DERP)
	publicKeyHex string         // cached output of publicKey.UntypedHexString
	fakeWGAddr   netip.AddrPort // the UDP address we tell wireguard-go we're using
	nodeAddr     netip.Addr     // the node's first tailscale address; used for logging & wireguard rate-limiting (Issue 6686)

	disco atomic.Pointer[endpointDisco] // if the peer supports disco, the key and short string

	// mu protects all following fields.
	mu syncs.Mutex // Lock ordering: Conn.mu, then endpoint.mu

	heartBeatTimer            *time.Timer    // nil when idle
	lastSendExt               mono.Time      // last time there were outgoing packets sent to this peer from an external trigger (e.g. wireguard-go or disco pingCLI)
	lastSendAny               mono.Time      // last time there were outgoing packets sent this peer from any trigger, internal or external to magicsock
	lastFullPing              mono.Time      // last time we pinged all disco or wireguard only endpoints
	lastUDPRelayPathDiscovery mono.Time      // last time we ran UDP relay path discovery
	lastDiscoKeyAdvertisement mono.Time      // last time we sent a TSMPDiscoAdvertisement or not to this endpoint
	derpAddr                  netip.AddrPort // fallback/bootstrap path, if non-zero (non-zero for well-behaved clients)

	bestAddr           addrQuality // best non-DERP path; zero if none; mutate via setBestAddrLocked()
	bestAddrAt         mono.Time   // time best address re-confirmed
	trustBestAddrUntil mono.Time   // time when bestAddr expires
	sentPing           map[stun.TxID]sentPing
	endpointState      map[netip.AddrPort]*endpointState // netip.AddrPort type for key (instead of [epAddr]) as [endpointState] is irrelevant for Geneve-encapsulated paths
	isCallMeMaybeEP    map[netip.AddrPort]bool

	// The following fields are related to the new "silent disco"
	// implementation that's a WIP as of 2022-10-20.
	// See #540 for background.
	heartbeatDisabled bool
	probeUDPLifetime  *probeUDPLifetime // UDP path lifetime probing; nil if disabled

	expired         bool // whether the node has expired
	isWireguardOnly bool // whether the endpoint is WireGuard only
	relayCapable    bool // whether the node is capable of speaking via a [github.com/metacubex/tailscale/net/udprelay.Server]
}

// udpRelayEndpointReady determines whether the given relay [addrQuality] should
// be installed as de.bestAddr. It is only called by [relayManager] once it has
// determined maybeBest is functional via [disco.Pong] reception.
func (de *endpoint) udpRelayEndpointReady(maybeBest addrQuality) {
	de.mu.Lock()
	defer de.mu.Unlock()
	now := mono.Now()
	curBestAddrTrusted := now.Before(de.trustBestAddrUntil)
	sameRelayServer := de.bestAddr.vni.IsSet() && maybeBest.relayServerDisco.Compare(de.bestAddr.relayServerDisco) == 0

	if !curBestAddrTrusted ||
		sameRelayServer ||
		betterAddr(maybeBest, de.bestAddr) {
		// We must set maybeBest as de.bestAddr if:
		//   1. de.bestAddr is untrusted. betterAddr does not consider
		//      time-based trust.
		//   2. maybeBest & de.bestAddr are on the same relay. If the maybeBest
		//      handshake happened to use a different source address/transport,
		//      the relay will drop packets from the 'old' de.bestAddr's.
		//   3. maybeBest is a 'betterAddr'.
		//
		// TODO(jwhited): add observability around !curBestAddrTrusted and sameRelayServer
		// TODO(jwhited): collapse path change logging with endpoint.handlePongConnLocked()
		de.c.logf("magicsock: disco: node %v %v now using %v mtu=%v", de.publicKey.ShortString(), de.discoShort(), maybeBest.epAddr, maybeBest.wireMTU)
		de.setBestAddrLocked(maybeBest)
		de.trustBestAddrUntil = now.Add(trustUDPAddrDuration)
	}
}

func (de *endpoint) setBestAddrLocked(v addrQuality) {
	if v.epAddr != de.bestAddr.epAddr {
		de.probeUDPLifetime.resetCycleEndpointLocked()

		// Reaching here, if we are using data from a cached netmap and we are
		// upgrading from an invalid (missing) address to a valid one, increment
		// the counter for peers established.
		if !de.bestAddr.ap.IsValid() && v.ap.IsValid() && de.c.usingCachedNetmap.Load() {
			metricCachedPeerContactDirect.Add(1)
		}
	}
	de.bestAddr = v
}

const (
	// udpLifetimeProbeCliffSlack is how much slack to use relative to a
	// ProbeUDPLifetimeConfig.Cliffs duration in order to account for RTT,
	// scheduling jitter, buffers, etc. If the cliff is 10s, we attempt to probe
	// after 10s - 2s (8s) amount of inactivity.
	udpLifetimeProbeCliffSlack = time.Second * 2
	// udpLifetimeProbeSchedulingTolerance is how much of a difference can be
	// tolerated between a UDP lifetime probe scheduling target and when it
	// actually fired. This must be some fraction of udpLifetimeProbeCliffSlack.
	udpLifetimeProbeSchedulingTolerance = udpLifetimeProbeCliffSlack / 8
)

// probeUDPLifetime represents the configuration and state tied to probing UDP
// path lifetime. A probe "cycle" involves pinging the UDP path at various
// timeout cliffs, which are pre-defined durations of interest commonly used by
// NATs/firewalls as default stateful session timeout values. Cliffs are probed
// in ascending order. A "cycle" completes when all cliffs have received a pong,
// or when a ping times out. Cycles may extend across endpoint session lifetimes
// if they are disrupted by user traffic.
type probeUDPLifetime struct {
	// All fields are guarded by endpoint.mu. probeUDPLifetime methods are for
	// convenience.

	// config holds the probing configuration.
	config ProbeUDPLifetimeConfig

	// timer is nil when idle. A non-nil timer indicates we intend to probe a
	// timeout cliff in the future.
	timer *time.Timer

	// bestAddr contains the endpoint.bestAddr.epAddr at the time a cycle was
	// scheduled to start. A probing cycle is 1:1 with the current
	// endpoint.bestAddr.epAddr in the interest of simplicity. When
	// endpoint.bestAddr.epAddr changes, any active probing cycle will reset.
	bestAddr epAddr
	// cycleStartedAt contains the time at which the first cliff
	// (ProbeUDPLifetimeConfig.Cliffs[0]) was pinged for the current/last cycle.
	cycleStartedAt time.Time
	// cycleActive is true if a probing cycle is active, otherwise false.
	cycleActive bool
	// currentCliff represents the index into ProbeUDPLifetimeConfig.Cliffs for
	// the cliff that we are waiting to ping, or waiting on a pong/timeout.
	currentCliff int
	// lastTxID is the ID for the last ping that was sent.
	lastTxID stun.TxID
}

func (p *probeUDPLifetime) currentCliffDurationEndpointLocked() time.Duration {
	if p == nil {
		return 0
	}
	return p.config.Cliffs[p.currentCliff]
}

// cycleCompleteMaxCliffEndpointLocked records the max cliff (as an index of
// ProbeUDPLifetimeConfig.Cliffs) a probing cycle reached, i.e. received a pong
// for. A value < 0 indicates no cliff was reached. It is a no-op if the active
// configuration does not equal defaultProbeUDPLifetimeConfig.
func (p *probeUDPLifetime) cycleCompleteMaxCliffEndpointLocked(cliff int) {
	if !p.config.Equals(defaultProbeUDPLifetimeConfig) {
		return
	}
	switch {
	case cliff < 0:
		metricUDPLifetimeCycleCompleteNoCliffReached.Add(1)
	case cliff == 0:
		metricUDPLifetimeCycleCompleteAt10sCliff.Add(1)
	case cliff == 1:
		metricUDPLifetimeCycleCompleteAt30sCliff.Add(1)
	case cliff == 2:
		metricUDPLifetimeCycleCompleteAt60sCliff.Add(1)
	}
}

// resetCycleEndpointLocked resets the state contained in p to reflect an
// inactive cycle.
func (p *probeUDPLifetime) resetCycleEndpointLocked() {
	if p == nil {
		return
	}
	if p.timer != nil {
		p.timer.Stop()
		p.timer = nil
	}
	p.cycleActive = false
	p.currentCliff = 0
	p.bestAddr = epAddr{}
}

// ProbeUDPLifetimeConfig represents the configuration for probing UDP path
// lifetime.
type ProbeUDPLifetimeConfig struct {
	// The timeout cliffs to probe. Values are in ascending order. Ascending
	// order is chosen over descending because we have limited opportunities to
	// probe. With a descending order we are stuck waiting for a new UDP
	// path/session if the first value times out. When that new path is
	// established is anyone's guess.
	Cliffs []time.Duration
	// CycleCanStartEvery represents the min duration between cycles starting
	// up.
	CycleCanStartEvery time.Duration
}

var (
	// defaultProbeUDPLifetimeConfig is the configuration that must be used
	// for UDP path lifetime probing until it can be wholly disseminated (not
	// just on/off) from upstream control components, and associated metrics
	// (metricUDPLifetime*) have lifetime management.
	//
	// TODO(#10928): support dynamic config via tailcfg.PeerCapMap.
	defaultProbeUDPLifetimeConfig = &ProbeUDPLifetimeConfig{
		Cliffs: []time.Duration{
			time.Second * 10,
			time.Second * 30,
			time.Second * 60,
		},
		CycleCanStartEvery: time.Hour * 24,
	}
)

// Equals returns true if b equals p, otherwise false. If both sides are nil,
// Equals returns true. If only one side is nil, Equals returns false.
func (p *ProbeUDPLifetimeConfig) Equals(b *ProbeUDPLifetimeConfig) bool {
	if p == b {
		return true
	}
	if (p == nil && b != nil) || (b == nil && p != nil) {
		return false
	}
	if !slices.Equal(p.Cliffs, b.Cliffs) {
		return false
	}
	if p.CycleCanStartEvery != b.CycleCanStartEvery {
		return false
	}
	return true
}

// Valid returns true if p is valid, otherwise false. p must be non-nil.
func (p *ProbeUDPLifetimeConfig) Valid() bool {
	if len(p.Cliffs) < 1 {
		// We need at least one cliff, otherwise there is nothing to probe.
		return false
	}
	if p.CycleCanStartEvery < 1 {
		// Probing must be constrained by a positive CycleCanStartEvery.
		return false
	}
	for i, c := range p.Cliffs {
		if c <= maxDuration(udpLifetimeProbeCliffSlack*2, heartbeatInterval) {
			// A timeout cliff less than or equal to twice
			// udpLifetimeProbeCliffSlack is invalid due to being effectively
			// zero when the cliff slack is subtracted from the cliff value at
			// scheduling time.
			//
			// A timeout cliff less or equal to the heartbeatInterval is also
			// invalid, as we may attempt to schedule on the tail end of the
			// last heartbeat tied to an active session.
			//
			// These values are constants, but max()'d in case they change in
			// the future.
			return false
		}
		if i == 0 {
			continue
		}
		if c <= p.Cliffs[i-1] {
			// Cliffs must be in ascending order.
			return false
		}
	}
	return true
}

// setProbeUDPLifetimeOn enables or disables probing of UDP path lifetime based
// on v. In the case of enablement defaultProbeUDPLifetimeConfig is used as the
// desired configuration.
func (de *endpoint) setProbeUDPLifetimeOn(v bool) {
	de.mu.Lock()
	if v {
		de.setProbeUDPLifetimeConfigLocked(defaultProbeUDPLifetimeConfig)
	} else {
		de.setProbeUDPLifetimeConfigLocked(nil)
	}
	de.mu.Unlock()
}

// setProbeUDPLifetimeConfigLocked sets the desired configuration for probing
// UDP path lifetime. Ownership of desired is passed to endpoint, it must not be
// mutated once this call is made. A nil value disables the feature. If desired
// is non-nil but desired.Valid() returns false this is a no-op.
func (de *endpoint) setProbeUDPLifetimeConfigLocked(desired *ProbeUDPLifetimeConfig) {
	if de.isWireguardOnly {
		return
	}
	if desired == nil {
		if de.probeUDPLifetime == nil {
			// noop, not currently configured or desired
			return
		}
		de.probeUDPLifetime.resetCycleEndpointLocked()
		de.probeUDPLifetime = nil
		return
	}
	if !desired.Valid() {
		return
	}
	if de.probeUDPLifetime != nil {
		if de.probeUDPLifetime.config.Equals(desired) {
			// noop, current config equals desired
			return
		}
		de.probeUDPLifetime.resetCycleEndpointLocked()
	} else {
		de.probeUDPLifetime = &probeUDPLifetime{}
	}
	p := de.probeUDPLifetime
	p.config = *desired
	p.resetCycleEndpointLocked()
}

// endpointDisco is the current disco key and short string for an endpoint. This
// structure is immutable.
type endpointDisco struct {
	key   key.DiscoPublic // for discovery messages.
	short string          // ShortString of discoKey.
}

type sentPing struct {
	to      epAddr
	at      mono.Time
	timer   *time.Timer // timeout timer
	purpose discoPingPurpose
	size    int                    // size of the disco message
	resCB   *pingResultAndCallback // or nil for internal use
}

// endpointState is some state and history for a specific endpoint of
// a endpoint. (The subject is the endpoint.endpointState
// map key)
type endpointState struct {
	// all fields guarded by endpoint.mu

	// lastPing is the last (outgoing) ping time.
	lastPing mono.Time

	// lastGotPing, if non-zero, means that this was an endpoint
	// that we learned about at runtime (from an incoming ping)
	// and that is not in the network map. If so, we keep the time
	// updated and use it to discard old candidates.
	lastGotPing time.Time

	// lastGotPingTxID contains the TxID for the last incoming ping. This is
	// used to de-dup incoming pings that we may see on both the raw disco
	// socket on Linux, and UDP socket. We cannot rely solely on the raw socket
	// disco handling due to https://github.com/tailscale/tailscale/issues/7078.
	lastGotPingTxID stun.TxID

	// callMeMaybeTime, if non-zero, is the time this endpoint
	// was advertised last via a call-me-maybe disco message.
	callMeMaybeTime time.Time

	recentPongs []pongReply // ring buffer up to pongHistoryCount entries
	recentPong  uint16      // index into recentPongs of most recent; older before, wrapped

	index int16 // index in nodecfg.Node.Endpoints; meaningless if lastGotPing non-zero
}

// clear removes all derived / probed state from an endpointState.
func (s *endpointState) clear() {
	*s = endpointState{
		index:       s.index,
		lastGotPing: s.lastGotPing,
	}
}

// pongHistoryCount is how many pongReply values we keep per endpointState
const pongHistoryCount = 64

type pongReply struct {
	latency time.Duration
	pongAt  mono.Time      // when we received the pong
	from    netip.AddrPort // the pong's src (usually same as endpoint map key)
	pongSrc netip.AddrPort // what they reported they heard
}

// EndpointChange is a structure containing information about changes made to a
// particular endpoint. This is not a stable interface and could change at any
// time.
type EndpointChange struct {
	When time.Time // when the change occurred
	What string    // what this change is
	From any       `json:",omitempty"` // information about the previous state
	To   any       `json:",omitempty"` // information about the new state
}

// shouldDeleteLocked reports whether we should delete this endpoint.
func (st *endpointState) shouldDeleteLocked() bool {
	switch {
	case !st.callMeMaybeTime.IsZero():
		return false
	case st.lastGotPing.IsZero():
		// This was an endpoint from the network map. Is it still in the network map?
		return st.index == indexSentinelDeleted
	default:
		// This was an endpoint discovered at runtime.
		return time.Since(st.lastGotPing) > sessionActiveTimeout
	}
}

// latencyLocked returns the most recent latency measurement, if any.
// endpoint.mu must be held.
func (st *endpointState) latencyLocked() (lat time.Duration, ok bool) {
	if len(st.recentPongs) == 0 {
		return 0, false
	}
	return st.recentPongs[st.recentPong].latency, true
}

// endpoint.mu must be held.
func (st *endpointState) addPongReplyLocked(r pongReply) {
	if n := len(st.recentPongs); n < pongHistoryCount {
		st.recentPong = uint16(n)
		st.recentPongs = append(st.recentPongs, r)
		return
	}
	i := st.recentPong + 1
	if i == pongHistoryCount {
		i = 0
	}
	st.recentPongs[i] = r
	st.recentPong = i
}

func (de *endpoint) deleteEndpointLocked(why string, ep netip.AddrPort) {
	de.debugUpdates.Add(EndpointChange{
		When: time.Now(),
		What: "deleteEndpointLocked-" + why,
		From: ep,
	})
	delete(de.endpointState, ep)
	asEpAddr := epAddr{ap: ep}
	if de.bestAddr.epAddr == asEpAddr {
		de.debugUpdates.Add(EndpointChange{
			When: time.Now(),
			What: "deleteEndpointLocked-bestAddr-" + why,
			From: de.bestAddr,
		})
		de.setBestAddrLocked(addrQuality{})
	}
}

// initFakeUDPAddr populates fakeWGAddr with a globally unique fake UDPAddr.
// The current implementation just uses the pointer value of de jammed into an IPv6
// address, but it could also be, say, a counter.
func (de *endpoint) initFakeUDPAddr() {
	var addr [16]byte
	addr[0] = 0xfd
	addr[1] = 0x00
	binary.BigEndian.PutUint64(addr[2:], uint64(reflect.ValueOf(de).Pointer()))
	de.fakeWGAddr = netip.AddrPortFrom(netip.AddrFrom16(addr).Unmap(), 12345)
}

// noteRecvActivity records receive activity on de, and invokes
// Conn.noteRecvActivity no more than once every 10s, returning true if it
// was called, otherwise false.
func (de *endpoint) noteRecvActivity(src epAddr, now mono.Time) bool {
	if de.isWireguardOnly {
		de.mu.Lock()
		de.bestAddr.ap = src.ap
		de.bestAddrAt = now
		de.trustBestAddrUntil = now.Add(5 * time.Second)
		de.mu.Unlock()
	} else {
		// TODO(jwhited): subject to change as part of silent disco effort.
		// Necessary when heartbeat is disabled for the endpoint, otherwise we
		// kick off discovery disco pings every trustUDPAddrDuration and mirror
		// to DERP.
		de.mu.Lock()
		if de.heartbeatDisabled && de.bestAddr.epAddr == src {
			de.trustBestAddrUntil = now.Add(trustUDPAddrDuration)
		}
		de.mu.Unlock()
	}

	elapsed := now.Sub(de.lastRecvWG.LoadAtomic())
	if elapsed > 10*time.Second {
		de.lastRecvWG.StoreAtomic(now)
		return true
	}
	return false
}

func (de *endpoint) discoShort() string {
	var short string
	if d := de.disco.Load(); d != nil {
		short = d.short
	}
	return short
}

// String exists purely so wireguard-go internals can log.Printf("%v")
// its internal conn.Endpoints and we don't end up with data races
// from fmt (via log) reading mutex fields and such.
func (de *endpoint) String() string {
	return fmt.Sprintf("magicsock.endpoint{%v, %v}", de.publicKey.ShortString(), de.discoShort())
}

func (de *endpoint) ClearSrc()           {}
func (de *endpoint) SrcToString() string { panic("unused") } // unused by wireguard-go
func (de *endpoint) SrcIP() netip.Addr   { panic("unused") } // unused by wireguard-go
func (de *endpoint) DstToString() string { return de.publicKeyHex }
func (de *endpoint) DstIP() netip.Addr   { return de.nodeAddr } // see tailscale/tailscale#6686
func (de *endpoint) DstToBytes() []byte  { return packIPPort(de.fakeWGAddr) }

// addrForSendLocked returns the address(es) that should be used for
// sending the next packet. Zero, one, or both of UDP address and DERP
// addr may be non-zero. If the endpoint is WireGuard only and does not have
// latency information, a bool is returned to indicate that the
// WireGuard latency discovery pings should be sent.
//
// de.mu must be held.
//
// TODO(val): Rewrite the addrFor*Locked() variations to share code.
func (de *endpoint) addrForSendLocked(now mono.Time) (udpAddr epAddr, derpAddr netip.AddrPort, sendWGPing bool) {
	udpAddr = de.bestAddr.epAddr

	if udpAddr.ap.IsValid() && !now.After(de.trustBestAddrUntil) {
		return udpAddr, netip.AddrPort{}, false
	}

	if de.isWireguardOnly {
		// If the endpoint is wireguard-only, we don't have a DERP
		// address to send to, so we have to send to the UDP address.
		udpAddr, shouldPing := de.addrForWireGuardSendLocked(now)
		return udpAddr, netip.AddrPort{}, shouldPing
	}

	// We had a bestAddr but it expired so send both to it
	// and DERP.
	return udpAddr, de.derpAddr, false
}

// addrForWireGuardSendLocked returns the address that should be used for
// sending the next packet. If a packet has never or not recently been sent to
// the endpoint, then a randomly selected address for the endpoint is returned,
// as well as a bool indicating that WireGuard discovery pings should be started.
// If the addresses have latency information available, then the address with the
// best latency is used.
//
// de.mu must be held.
func (de *endpoint) addrForWireGuardSendLocked(now mono.Time) (udpAddr epAddr, shouldPing bool) {
	if len(de.endpointState) == 0 {
		de.c.logf("magicsock: addrForSendWireguardLocked: [unexpected] no candidates available for endpoint")
		return udpAddr, false
	}

	// lowestLatency is a high duration initially, so we
	// can be sure we're going to have a duration lower than this
	// for the first latency retrieved.
	lowestLatency := time.Hour
	var oldestPing mono.Time
	for ipp, state := range de.endpointState {
		if oldestPing.IsZero() {
			oldestPing = state.lastPing
		} else if state.lastPing.Before(oldestPing) {
			oldestPing = state.lastPing
		}

		if latency, ok := state.latencyLocked(); ok {
			if latency < lowestLatency || latency == lowestLatency && ipp.Addr().Is6() {
				// If we have the same latency,IPv6 is prioritized.
				// TODO(catzkorn): Consider a small increase in latency to use
				// IPv6 in comparison to IPv4, when possible.
				lowestLatency = latency
				udpAddr.ap = ipp
			}
		}
	}
	needPing := len(de.endpointState) > 1 && now.Sub(oldestPing) > wireguardPingInterval

	if !udpAddr.ap.IsValid() {
		candidates := slicesx.MapKeys(de.endpointState)

		// Randomly select an address to use until we retrieve latency information
		// and give it a short trustBestAddrUntil time so we avoid flapping between
		// addresses while waiting on latency information to be populated.
		udpAddr.ap = candidates[rand.IntN(len(candidates))]
	}

	de.bestAddr.epAddr = epAddr{ap: udpAddr.ap}
	// Only extend trustBestAddrUntil by one second to avoid packet
	// reordering and/or CPU usage from random selection during the first
	// second. We should receive a response due to a WireGuard handshake in
	// less than one second in good cases, in which case this will be then
	// extended to 15 seconds.
	de.trustBestAddrUntil = now.Add(time.Second)
	return udpAddr, needPing
}

// addrForPingSizeLocked returns the address(es) that should be used for sending
// the next ping. It will only return addrs with a large enough path MTU to
// permit a ping payload of size bytes to be delivered (DERP is always one such
// addr as it is a TCP connection). If it returns a zero-value udpAddr, then we
// should continue probing the MTU of all paths to this endpoint. Zero, one, or
// both of the returned UDP address and DERP address may be non-zero.
//
// de.mu must be held.
func (de *endpoint) addrForPingSizeLocked(now mono.Time, size int) (udpAddr epAddr, derpAddr netip.AddrPort) {
	if size == 0 {
		udpAddr, derpAddr, _ = de.addrForSendLocked(now)
		return
	}

	udpAddr = de.bestAddr.epAddr
	pathMTU := de.bestAddr.wireMTU
	requestedMTU := pingSizeToPktLen(size, udpAddr)
	mtuOk := requestedMTU <= pathMTU

	if udpAddr.ap.IsValid() && mtuOk {
		if !now.After(de.trustBestAddrUntil) {
			return udpAddr, netip.AddrPort{}
		}
		// We had a bestAddr with large enough MTU but it expired, so
		// send both to it and DERP.
		return udpAddr, de.derpAddr
	}

	// The UDP address isn't valid or it doesn't have a path MTU big enough
	// for the packet. Return a zero-value udpAddr to signal that we should
	// keep probing the path MTU to all addresses for this endpoint, and a
	// valid DERP addr to signal that we should also send via DERP.
	return epAddr{}, de.derpAddr
}

// maybeProbeUDPLifetimeLocked returns an afterInactivityFor duration and true
// if de is a candidate for UDP path lifetime probing in the future, otherwise
// false.
func (de *endpoint) maybeProbeUDPLifetimeLocked() (afterInactivityFor time.Duration, maybe bool) {
	p := de.probeUDPLifetime
	if p == nil {
		return afterInactivityFor, false
	}
	if !de.bestAddr.ap.IsValid() {
		return afterInactivityFor, false
	}
	epDisco := de.disco.Load()
	if epDisco == nil {
		// peer does not support disco
		return afterInactivityFor, false
	}
	// We compare disco keys, which may have a shorter lifetime than node keys
	// since disco keys reset on startup. This has the desired side effect of
	// shuffling probing probability where the local node ends up with a large
	// key value lexicographically relative to the other nodes it tends to
	// communicate with. If de's disco key changes, the cycle will reset.
	if de.c.discoAtomic.Public().Compare(epDisco.key) >= 0 {
		// lower disco pub key node probes higher
		return afterInactivityFor, false
	}
	if !p.cycleActive && time.Since(p.cycleStartedAt) < p.config.CycleCanStartEvery {
		// This is conservative as it doesn't account for afterInactivityFor use
		// by the caller, potentially delaying the start of the next cycle. We
		// assume the cycle could start immediately following
		// maybeProbeUDPLifetimeLocked(), regardless of the value of
		// afterInactivityFor relative to latest packets in/out time.
		return afterInactivityFor, false
	}
	afterInactivityFor = p.currentCliffDurationEndpointLocked() - udpLifetimeProbeCliffSlack
	if afterInactivityFor < 0 {
		// shouldn't happen
		return afterInactivityFor, false
	}
	return afterInactivityFor, true
}

// heartbeatForLifetimeVia represents the scheduling source of
// endpoint.heartbeatForLifetime().
type heartbeatForLifetimeVia string

const (
	heartbeatForLifetimeViaSessionInactive heartbeatForLifetimeVia = "session-inactive"
	heartbeatForLifetimeViaPongRx          heartbeatForLifetimeVia = "pong-rx"
	heartbeatForLifetimeViaSelf            heartbeatForLifetimeVia = "self"
)

// scheduleHeartbeatForLifetimeLocked schedules de.heartbeatForLifetime to fire
// in the future (after). The caller must describe themselves in the via arg.
func (de *endpoint) scheduleHeartbeatForLifetimeLocked(after time.Duration, via heartbeatForLifetimeVia) {
	p := de.probeUDPLifetime
	if p == nil {
		return
	}
	de.c.dlogf("[v1] magicsock: disco: scheduling UDP lifetime probe for cliff=%v via=%v to %v (%v)",
		p.currentCliffDurationEndpointLocked(), via, de.publicKey.ShortString(), de.discoShort())
	p.bestAddr = de.bestAddr.epAddr
	p.timer = time.AfterFunc(after, de.heartbeatForLifetime)
	if via == heartbeatForLifetimeViaSelf {
		metricUDPLifetimeCliffsRescheduled.Add(1)
	} else {
		metricUDPLifetimeCliffsScheduled.Add(1)
	}
}

// heartbeatForLifetime sends a disco ping recorded locally with a purpose of
// pingHeartbeatForUDPLifetime to de if de.bestAddr has remained stable, and it
// has been inactive for a duration that is within the error bounds for current
// lifetime probing cliff. Alternatively it may reschedule itself into the
// future, which is one of three scheduling sources. The other scheduling
// sources are de.heartbeat() and de.probeUDPLifetimeCliffDoneLocked().
func (de *endpoint) heartbeatForLifetime() {
	de.mu.Lock()
	defer de.mu.Unlock()
	p := de.probeUDPLifetime
	if p == nil || p.timer == nil {
		// We raced with a code path trying to p.timer.Stop() us. Give up early
		// in the interest of simplicity. If p.timer.Stop() happened in
		// de.heartbeat() presumably because of recent packets in/out we *could*
		// still probe here, and it would be meaningful, but the time logic
		// below would reschedule as-is.
		return
	}
	p.timer = nil
	if !p.bestAddr.ap.IsValid() || de.bestAddr.epAddr != p.bestAddr {
		// best path changed
		p.resetCycleEndpointLocked()
		return
	}
	afterInactivityFor, ok := de.maybeProbeUDPLifetimeLocked()
	if !ok {
		p.resetCycleEndpointLocked()
		return
	}
	inactiveFor := mono.Now().Sub(maxMonoTime(de.lastRecvUDPAny.LoadAtomic(), de.lastSendAny))
	delta := afterInactivityFor - inactiveFor
	if delta.Abs() > udpLifetimeProbeSchedulingTolerance {
		if delta < 0 {
			// We missed our opportunity. We can resume this cliff at the tail
			// end of another session.
			metricUDPLifetimeCliffsMissed.Add(1)
			return
		} else {
			// We need to wait longer before sending a ping. This can happen for
			// a number of reasons, which are described in more detail in
			// de.heartbeat().
			de.scheduleHeartbeatForLifetimeLocked(delta, heartbeatForLifetimeViaSelf)
			return
		}
	}
	if p.currentCliff == 0 {
		p.cycleStartedAt = time.Now()
		p.cycleActive = true
	}
	de.c.dlogf("[v1] magicsock: disco: sending disco ping for UDP lifetime probe cliff=%v to %v (%v)",
		p.currentCliffDurationEndpointLocked(), de.publicKey.ShortString(), de.discoShort())
	de.startDiscoPingLocked(de.bestAddr.epAddr, mono.Now(), pingHeartbeatForUDPLifetime, 0, nil)
}

// heartbeat is called every heartbeatInterval to keep the best UDP path alive,
// kick off discovery of other paths, or schedule the probing of UDP path
// lifetime on the tail end of an active session.
func (de *endpoint) heartbeat() {
	de.mu.Lock()
	defer de.mu.Unlock()

	if de.probeUDPLifetime != nil && de.probeUDPLifetime.timer != nil {
		de.probeUDPLifetime.timer.Stop()
		de.probeUDPLifetime.timer = nil
	}
	de.heartBeatTimer = nil

	if de.heartbeatDisabled {
		// If control override to disable heartBeatTimer set, return early.
		return
	}

	if de.lastSendExt.IsZero() {
		// Shouldn't happen.
		return
	}

	now := mono.Now()
	if now.Sub(de.lastSendExt) > sessionActiveTimeout {
		// Session's idle. Stop heartbeating.
		de.c.dlogf("[v1] magicsock: disco: ending heartbeats for idle session to %v (%v)", de.publicKey.ShortString(), de.discoShort())
		if afterInactivityFor, ok := de.maybeProbeUDPLifetimeLocked(); ok {
			// This is the best place to best effort schedule a probe of UDP
			// path lifetime in the future as it loosely translates to "UDP path
			// is inactive".
			//
			// Note: wireguard-go schedules a WireGuard keepalive packet (by
			// default, not tied to persistent keepalive feature) 10 seconds in
			// the future after receiving an authenticated data packet. It's
			// typically only sent by one side based on how the WireGuard state
			// machine controls the timer. So, if we are on the receiving end of
			// that keepalive, de.lastSendExt won't move, assuming there is no
			// other user-generated traffic. This is one reason why we perform
			// a more granular check of the last packets in/out time, below, as
			// a WireGuard keepalive may have fallen somewhere within the
			// sessionActiveTimeout window. heartbeatForLifetime will also
			// perform a similar check, and reschedule as necessary.
			inactiveFor := now.Sub(maxMonoTime(de.lastSendAny, de.lastRecvUDPAny.LoadAtomic()))
			after := afterInactivityFor - inactiveFor
			if after < 0 {
				// shouldn't happen
				return
			}
			de.scheduleHeartbeatForLifetimeLocked(after, heartbeatForLifetimeViaSessionInactive)
		}
		return
	}

	udpAddr, _, _ := de.addrForSendLocked(now)
	if udpAddr.ap.IsValid() {
		// We have a preferred path. Ping that every 'heartbeatInterval'.
		de.startDiscoPingLocked(udpAddr, now, pingHeartbeat, 0, nil)
	}

	if de.wantFullPingLocked(now) {
		de.sendDiscoPingsLocked(now, true)
	}

	if de.wantUDPRelayPathDiscoveryLocked(now) {
		de.discoverUDPRelayPathsLocked(now)
	}

	de.heartBeatTimer = time.AfterFunc(heartbeatInterval, de.heartbeat)
}

// setHeartbeatDisabled sets heartbeatDisabled to the provided value.
func (de *endpoint) setHeartbeatDisabled(v bool) {
	de.mu.Lock()
	defer de.mu.Unlock()
	de.heartbeatDisabled = v
}

// discoverUDPRelayPathsLocked starts UDP relay path discovery.
func (de *endpoint) discoverUDPRelayPathsLocked(now mono.Time) {
	de.lastUDPRelayPathDiscovery = now
	lastBest := de.bestAddr
	lastBestIsTrusted := mono.Now().Before(de.trustBestAddrUntil)
	de.c.relayManager.startUDPRelayPathDiscoveryFor(de, lastBest, lastBestIsTrusted)
}

// wantUDPRelayPathDiscoveryLocked reports whether we should kick off UDP relay
// path discovery.
func (de *endpoint) wantUDPRelayPathDiscoveryLocked(now mono.Time) bool {
	if runtime.GOOS == "js" {
		return false
	}
	if !de.c.relayManager.hasPeerRelayServers.Load() {
		// Changes in this value between its access and a call to
		// [endpoint.discoverUDPRelayPathsLocked] are fine, we will eventually
		// do the "right" thing during future path discovery. The worst case is
		// we suppress path discovery for the current cycle, or we unnecessarily
		// call into [relayManager] and do some wasted work.
		return false
	}
	if !de.relayCapable {
		return false
	}
	if de.bestAddr.isDirect() && now.Before(de.trustBestAddrUntil) {
		return false
	}
	if !de.lastUDPRelayPathDiscovery.IsZero() && now.Sub(de.lastUDPRelayPathDiscovery) < discoverUDPRelayPathsInterval {
		return false
	}
	// TODO(jwhited): consider applying 'goodEnoughLatency' suppression here,
	//  but not until we have a strategy for triggering CallMeMaybeVia regularly
	//  and/or enabling inbound packets to act as a UDP relay path discovery
	//  trigger, otherwise clients without relay servers may fall off a UDP
	//  relay path and never come back. They are dependent on the remote side
	//  regularly TX'ing CallMeMaybeVia, which currently only happens as part
	//  of full UDP relay path discovery.
	if now.After(de.trustBestAddrUntil) {
		return true
	}
	if !de.lastUDPRelayPathDiscovery.IsZero() && now.Sub(de.lastUDPRelayPathDiscovery) >= upgradeUDPRelayInterval {
		return true
	}
	return false
}

// wantFullPingLocked reports whether we should ping to all our peers looking for
// a better path.
//
// de.mu must be held.
func (de *endpoint) wantFullPingLocked(now mono.Time) bool {
	if runtime.GOOS == "js" {
		return false
	}
	if !de.bestAddr.isDirect() || de.lastFullPing.IsZero() {
		return true
	}
	if now.After(de.trustBestAddrUntil) {
		return true
	}
	if de.bestAddr.latency <= goodEnoughLatency {
		return false
	}
	if now.Sub(de.lastFullPing) >= upgradeUDPDirectInterval {
		return true
	}
	return false
}

func (de *endpoint) noteTxActivityExtTriggerLocked(now mono.Time) {
	de.lastSendExt = now
	if de.heartBeatTimer == nil && !de.heartbeatDisabled {
		de.heartBeatTimer = time.AfterFunc(heartbeatInterval, de.heartbeat)
	}
}

// MaxDiscoPingSize is the largest useful ping message size that we
// can send - the maximum packet size minus the IPv4 and UDP headers.
var MaxDiscoPingSize = tstun.MaxPacketSize - 20 - 8

type pingResultAndCallback struct {
	taken atomic.Bool // first CompareAndSwamp from false to true takes ownership of res
	res   *ipnstate.PingResult
	cb    func(*ipnstate.PingResult)
}

func (p *pingResultAndCallback) reply() bool {
	return p != nil && p.taken.CompareAndSwap(false, true)
}

// discoPing starts a disco-level ping for the "tailscale ping" command (or other
// callers, such as c2n). res is value to call cb with, already partially
// filled. cb must be called at most once. Once called, ownership of res passes to cb.
func (de *endpoint) discoPing(res *ipnstate.PingResult, size int, cb func(*ipnstate.PingResult)) {
	de.mu.Lock()
	defer de.mu.Unlock()

	if de.expired {
		res.Err = errExpired.Error()
		cb(res)
		return
	}
	if size > MaxDiscoPingSize {
		res.Err = errPingTooBig.Error()
		cb(res)
		return
	}

	resCB := &pingResultAndCallback{res: res, cb: cb}

	now := mono.Now()
	udpAddr, derpAddr := de.addrForPingSizeLocked(now, size)

	if derpAddr.IsValid() {
		de.startDiscoPingLocked(epAddr{ap: derpAddr}, now, pingCLI, size, resCB)
	}

	switch {
	case udpAddr.ap.IsValid() && now.Before(de.trustBestAddrUntil):
		// We have a "trusted" direct OR peer relay address, ping it.
		de.startDiscoPingLocked(udpAddr, now, pingCLI, size, resCB)
		if !udpAddr.vni.IsSet() {
			// If the path is direct we do not want to fallthrough to pinging
			// all candidate direct paths, otherwise "tailscale ping" results to
			// a node on the local network can look like they're bouncing
			// between, say 10.0.0.0/8 and the peer's IPv6 address, both 1ms
			// away, and it's random who replies first. cb() is called with the
			// first reply, vs background path discovery that is subject to
			// betterAddr() comparison and hysteresis
			break
		}
		// If the trusted path is via a peer relay we want to fallthrough in
		// order to also try all candidate direct paths.
		fallthrough
	default:
		// Ping all candidate direct paths and start peer relay path discovery,
		// if appropriate. This work overlaps with what [de.heartbeat] will
		// periodically fire when it calls [de.sendDiscoPingsLocked] and
		// [de.discoveryUDPRelayPathsLocked], but a user-initiated [pingCLI] is
		// a "do it now" operation that should not be subject to
		// [heartbeatInterval] tick or [discoPingInterval] rate-limiting.
		for ep := range de.endpointState {
			de.startDiscoPingLocked(epAddr{ap: ep}, now, pingCLI, size, resCB)
		}
		if de.wantUDPRelayPathDiscoveryLocked(now) {
			de.discoverUDPRelayPathsLocked(now)
		}
	}
}

var (
	errExpired     = errors.New("peer's node key has expired")
	errNoUDPOrDERP = errors.New("no UDP or DERP addr")
	errPingTooBig  = errors.New("ping size too big")
)

func (de *endpoint) send(buffs [][]byte, offset int) error {
	de.mu.Lock()
	if de.expired {
		de.mu.Unlock()
		return errExpired
	}

	now := mono.Now()
	udpAddr, derpAddr, startWGPing := de.addrForSendLocked(now)

	if de.isWireguardOnly {
		if startWGPing {
			de.sendWireGuardOnlyPingsLocked(now)
		}
	} else if !udpAddr.isDirect() || now.After(de.trustBestAddrUntil) {
		de.sendDiscoPingsLocked(now, true)
		if de.wantUDPRelayPathDiscoveryLocked(now) {
			de.discoverUDPRelayPathsLocked(now)
		}
	}
	de.noteTxActivityExtTriggerLocked(now)
	de.lastSendAny = now
	de.mu.Unlock()

	if !udpAddr.ap.IsValid() && !derpAddr.IsValid() {
		// Make a last ditch effort to see if we have a DERP route for them. If
		// they contacted us over DERP and we don't know their UDP endpoints or
		// their DERP home, we can at least assume they're reachable over the
		// DERP they used to contact us.
		if rid := de.c.fallbackDERPRegionForPeer(de.publicKey); rid != 0 {
			derpAddr = netip.AddrPortFrom(tailcfg.DerpMagicIPAddr, uint16(rid))
		} else {
			return errNoUDPOrDERP
		}
	}
	var err error
	if udpAddr.ap.IsValid() {
		_, err = de.c.sendUDPBatch(udpAddr, buffs, offset)

		// If the error is known to indicate that the endpoint is no longer
		// usable, clear the endpoint statistics so that the next send will
		// re-evaluate the best endpoint.
		if err != nil && isBadEndpointErr(err) {
			de.noteBadEndpoint(udpAddr)
		}

		var txBytes int
		for _, b := range buffs {
			txBytes += len(b[offset:])
		}

		switch {
		case udpAddr.ap.Addr().Is4():
			if udpAddr.vni.IsSet() {
				de.c.metrics.outboundPacketsPeerRelayIPv4Total.Add(int64(len(buffs)))
				de.c.metrics.outboundBytesPeerRelayIPv4Total.Add(int64(txBytes))
			} else {
				de.c.metrics.outboundPacketsIPv4Total.Add(int64(len(buffs)))
				de.c.metrics.outboundBytesIPv4Total.Add(int64(txBytes))
			}
		case udpAddr.ap.Addr().Is6():
			if udpAddr.vni.IsSet() {
				de.c.metrics.outboundPacketsPeerRelayIPv6Total.Add(int64(len(buffs)))
				de.c.metrics.outboundBytesPeerRelayIPv6Total.Add(int64(txBytes))
			} else {
				de.c.metrics.outboundPacketsIPv6Total.Add(int64(len(buffs)))
				de.c.metrics.outboundBytesIPv6Total.Add(int64(txBytes))
			}
		}

		// TODO(raggi): needs updating for accuracy, as in error conditions we may have partial sends.
		if update := de.c.connCounter.Load(); err == nil && update != nil {
			update(0, netip.AddrPortFrom(de.nodeAddr, 0), udpAddr.ap, len(buffs), txBytes, false)
		}
	}
	if derpAddr.IsValid() {
		allOk := true
		var txBytes int
		for _, buff := range buffs {
			buff = buff[offset:]
			const isDisco = false
			const isGeneveEncap = false
			ok, _ := de.c.sendAddr(derpAddr, de.publicKey, buff, isDisco, isGeneveEncap)
			txBytes += len(buff)
			if !ok {
				allOk = false
			}
		}

		if update := de.c.connCounter.Load(); update != nil {
			update(0, netip.AddrPortFrom(de.nodeAddr, 0), derpAddr, len(buffs), txBytes, false)
		}
		if allOk {
			return nil
		}
	}
	return err
}

// probeUDPLifetimeCliffDoneLocked is called when a disco
// pingHeartbeatForUDPLifetime is being cleaned up. result contains the reason
// for the cleanup, txid contains the ping's txid.
// probeUDPLifetimeCliffDoneLocked may schedule another
// pingHeartbeatForUDPLifetime in the future if there is another cliff remaining
// for the current probing cycle.
func (de *endpoint) probeUDPLifetimeCliffDoneLocked(result discoPingResult, txid stun.TxID) {
	p := de.probeUDPLifetime
	if p == nil || !p.cycleActive || de.probeUDPLifetime.timer != nil || txid != p.lastTxID {
		// Probing may have been disabled while heartbeats were in flight. This
		// can also be a duplicate or late arriving result.
		return
	}
	metricUDPLifetimeCliffsCompleted.Add(1)
	if result != discoPongReceived || p.currentCliff >= len(p.config.Cliffs)-1 {
		maxCliffIndex := p.currentCliff
		if result != discoPongReceived {
			maxCliffIndex = p.currentCliff - 1
		}
		var maxCliffDuration time.Duration
		if maxCliffIndex >= 0 {
			maxCliffDuration = p.config.Cliffs[maxCliffIndex]
		}
		p.cycleCompleteMaxCliffEndpointLocked(maxCliffIndex)
		de.c.dlogf("[v1] magicsock: disco: UDP lifetime probe cycle completed max cliff=%v for %v (%v)",
			maxCliffDuration, de.publicKey.ShortString(), de.discoShort())
		metricUDPLifetimeCyclesCompleted.Add(1)
		p.resetCycleEndpointLocked()
	} else {
		p.currentCliff++
		if after, ok := de.maybeProbeUDPLifetimeLocked(); ok {
			de.scheduleHeartbeatForLifetimeLocked(after, heartbeatForLifetimeViaPongRx)
		}
	}
}

func (de *endpoint) discoPingTimeout(txid stun.TxID) {
	de.mu.Lock()
	defer de.mu.Unlock()
	sp, ok := de.sentPing[txid]
	if !ok {
		return
	}
	bestUntrusted := mono.Now().After(de.trustBestAddrUntil)
	if sp.to == de.bestAddr.epAddr && bestUntrusted {
		de.clearBestAddrLocked()
	}
	if debugDisco() || !de.bestAddr.ap.IsValid() || bestUntrusted {
		de.c.dlogf("[v1] magicsock: disco: timeout waiting for pong %x from %v (%v, %v)", txid[:6], sp.to, de.publicKey.ShortString(), de.discoShort())
	}
	de.removeSentDiscoPingLocked(txid, sp, discoPingTimedOut)
}

// forgetDiscoPing is called when a ping fails to send.
func (de *endpoint) forgetDiscoPing(txid stun.TxID) {
	de.mu.Lock()
	defer de.mu.Unlock()
	if sp, ok := de.sentPing[txid]; ok {
		de.removeSentDiscoPingLocked(txid, sp, discoPingFailed)
	}
}

// discoPingResult represents the result of an attempted disco ping send
// operation.
type discoPingResult int

const (
	discoPingResultUnknown discoPingResult = iota
	discoPingFailed
	discoPingTimedOut
	discoPongReceived
)

func (de *endpoint) removeSentDiscoPingLocked(txid stun.TxID, sp sentPing, result discoPingResult) {
	// Stop the timer for the case where sendPing failed to write to UDP.
	// In the case of a timer already having fired, this is a no-op:
	sp.timer.Stop()
	if sp.purpose == pingHeartbeatForUDPLifetime {
		de.probeUDPLifetimeCliffDoneLocked(result, txid)
	}
	delete(de.sentPing, txid)
}

// poly1305AuthenticatorSize is the size, in bytes, of a poly1305 authenticator.
// It's the same as golang.org/x/crypto/poly1305.TagSize, but that
// page is deprecated and we only need this one constant, so we copy it.
const poly1305AuthenticatorSize = 16

// discoPingSize is the size of a complete disco ping packet, without any padding.
const discoPingSize = len(disco.Magic) + key.DiscoPublicRawLen + disco.NonceLen +
	poly1305AuthenticatorSize + disco.MessageHeaderLen + disco.PingLen

// sendDiscoPing sends a ping with the provided txid to ep using de's discoKey. size
// is the desired disco message size, including all disco headers but excluding IP/UDP
// headers.
//
// The caller (startDiscoPingLocked) should've already recorded the ping in
// sentPing and set up the timer.
//
// The caller should use de.discoKey as the discoKey argument.
// It is passed in so that sendDiscoPing doesn't need to lock de.mu.
func (de *endpoint) sendDiscoPing(ep epAddr, discoKey key.DiscoPublic, txid stun.TxID, size int, logLevel discoLogLevel) {
	size = minInt(size, MaxDiscoPingSize)
	padding := maxInt(size-discoPingSize, 0)

	sent, _ := de.c.sendDiscoMessage(ep, de.publicKey, discoKey, &disco.Ping{
		TxID:    [12]byte(txid),
		NodeKey: de.c.publicKeyAtomic.Load(),
		Padding: padding,
	}, logLevel)
	if !sent {
		de.forgetDiscoPing(txid)
		return
	}

	if size != 0 {
		metricSentDiscoPeerMTUProbes.Add(1)
		metricSentDiscoPeerMTUProbeBytes.Add(int64(pingSizeToPktLen(size, ep)))
	}
}

// discoPingPurpose is the reason why a discovery ping message was sent.
type discoPingPurpose int

//go:generate go run github.com/metacubex/tailscale/cmd/addlicense -file discopingpurpose_string.go go run golang.org/x/tools/cmd/stringer -type=discoPingPurpose -trimprefix=ping
const (
	// pingDiscovery means that purpose of a ping was to see if a
	// path was valid.
	pingDiscovery discoPingPurpose = iota

	// pingHeartbeat means that purpose of a ping was whether a
	// peer was still there.
	pingHeartbeat

	// pingCLI means that the user is running "tailscale ping"
	// from the CLI. These types of pings can go over DERP.
	pingCLI

	// pingHeartbeatForUDPLifetime means that the purpose of a ping was to
	// discover whether the UDP path was still active through any and all
	// stateful middleboxes involved.
	pingHeartbeatForUDPLifetime
)

// startDiscoPingLocked sends a disco ping to ep in a separate goroutine. resCB,
// if non-nil, means that a caller external to the magicsock package internals
// is interested in the result (such as a CLI "tailscale ping" or a c2n ping
// request, etc)
func (de *endpoint) startDiscoPingLocked(ep epAddr, now mono.Time, purpose discoPingPurpose, size int, resCB *pingResultAndCallback) {
	if runtime.GOOS == "js" {
		return
	}
	if debugNeverDirectUDP() && !ep.vni.IsSet() && ep.ap.Addr() != tailcfg.DerpMagicIPAddr {
		return
	}
	epDisco := de.disco.Load()
	if epDisco == nil {
		return
	}
	if purpose != pingCLI &&
		!ep.vni.IsSet() { // de.endpointState is only relevant for direct/non-vni epAddr's
		st, ok := de.endpointState[ep.ap]
		if !ok {
			// Shouldn't happen. But don't ping an endpoint that's
			// not active for us.
			de.c.logf("magicsock: disco: [unexpected] attempt to ping no longer live endpoint %v", ep)
			return
		}
		st.lastPing = now
	}

	// If we are doing a discovery ping or a CLI ping with no specified size
	// to a non DERP address, then probe the MTU. Otherwise just send the
	// one specified ping.

	// Default to sending a single ping of the specified size
	sizes := []int{size}
	if de.c.PeerMTUEnabled() {
		isDerp := ep.ap.Addr() == tailcfg.DerpMagicIPAddr
		if !isDerp && ((purpose == pingDiscovery) || (purpose == pingCLI && size == 0)) {
			de.c.dlogf("[v1] magicsock: starting MTU probe")
			sizes = mtuProbePingSizesV4
			if ep.ap.Addr().Is6() {
				sizes = mtuProbePingSizesV6
			}
		}
	}

	logLevel := discoLog
	if purpose == pingHeartbeat {
		logLevel = discoVerboseLog
	}
	if purpose == pingCLI {
		de.noteTxActivityExtTriggerLocked(now)
	}
	de.lastSendAny = now
	for _, s := range sizes {
		txid := stun.NewTxID()
		de.sentPing[txid] = sentPing{
			to:      ep,
			at:      now,
			timer:   time.AfterFunc(pingTimeoutDuration, func() { de.discoPingTimeout(txid) }),
			purpose: purpose,
			resCB:   resCB,
			size:    s,
		}
		if purpose == pingHeartbeatForUDPLifetime && de.probeUDPLifetime != nil {
			de.probeUDPLifetime.lastTxID = txid
		}
		go de.sendDiscoPing(ep, epDisco.key, txid, s, logLevel)
	}

}

// sendDiscoPingsLocked starts pinging all of ep's endpoints.
func (de *endpoint) sendDiscoPingsLocked(now mono.Time, sendCallMeMaybe bool) {
	de.lastFullPing = now
	var sentAny bool
	for ep, st := range de.endpointState {
		if st.shouldDeleteLocked() {
			de.deleteEndpointLocked("sendPingsLocked", ep)
			continue
		}
		if runtime.GOOS == "js" {
			continue
		}
		if !st.lastPing.IsZero() && now.Sub(st.lastPing) < discoPingInterval {
			continue
		}

		firstPing := !sentAny
		sentAny = true

		if firstPing && sendCallMeMaybe {
			de.c.dlogf("[v1] magicsock: disco: send, starting discovery for %v (%v)", de.publicKey.ShortString(), de.discoShort())
		}

		de.startDiscoPingLocked(epAddr{ap: ep}, now, pingDiscovery, 0, nil)
	}
	derpAddr := de.derpAddr
	if sentAny && sendCallMeMaybe && derpAddr.IsValid() {
		// Have our magicsock.Conn figure out its STUN endpoint (if
		// it doesn't know already) and then send a CallMeMaybe
		// message to our peer via DERP informing them that we've
		// sent so our firewall ports are probably open and now
		// would be a good time for them to connect.
		go de.c.enqueueCallMeMaybe(derpAddr, de)
	}
}

// sendWireGuardOnlyPingsLocked evaluates all available addresses for
// a WireGuard only endpoint and initiates an ICMP ping for useable
// addresses.
func (de *endpoint) sendWireGuardOnlyPingsLocked(now mono.Time) {
	if runtime.GOOS == "js" {
		return
	}

	// Normally we only send pings at a low rate as the decision to start
	// sending a ping sets bestAddrAtUntil with a reasonable time to keep trying
	// that address, however, if that code changed we may want to be sure that
	// we don't ever send excessive pings to avoid impact to the client/user.
	if !now.After(de.lastFullPing.Add(10 * time.Second)) {
		return
	}
	de.lastFullPing = now

	for ipp := range de.endpointState {
		if ipp.Addr().Is4() && de.c.noV4.Load() {
			continue
		}
		if ipp.Addr().Is6() && de.c.noV6.Load() {
			continue
		}

		go de.sendWireGuardOnlyPing(ipp, now)
	}
}

// sendWireGuardOnlyPing sends a ICMP ping to a WireGuard only address to
// discover the latency.
func (de *endpoint) sendWireGuardOnlyPing(ipp netip.AddrPort, now mono.Time) {
	ctx, cancel := context.WithTimeout(de.c.connCtx, 5*time.Second)
	defer cancel()

	de.setLastPing(ipp, now)

	addr := &net.IPAddr{
		IP:   net.IP(ipp.Addr().AsSlice()),
		Zone: ipp.Addr().Zone(),
	}

	p := de.c.getPinger()
	if p == nil {
		de.c.logf("[v2] magicsock: sendWireGuardOnlyPingLocked: pinger is nil")
		return
	}

	latency, err := p.Send(ctx, addr, nil)
	if err != nil {
		de.c.logf("[v2] magicsock: sendWireGuardOnlyPingLocked: %s", err)
		return
	}

	de.mu.Lock()
	defer de.mu.Unlock()

	state, ok := de.endpointState[ipp]
	if !ok {
		return
	}
	state.addPongReplyLocked(pongReply{
		latency: latency,
		pongAt:  now,
		from:    ipp,
		pongSrc: netip.AddrPort{}, // We don't know this.
	})
}

// setLastPing sets lastPing on the endpointState to now.
func (de *endpoint) setLastPing(ipp netip.AddrPort, now mono.Time) {
	de.mu.Lock()
	defer de.mu.Unlock()
	state, ok := de.endpointState[ipp]
	if !ok {
		return
	}
	state.lastPing = now
}

// updateDiscoKey replaces the disco key for de. If the key is a zero value key,
// set the key to nil.
func (de *endpoint) updateDiscoKey(key key.DiscoPublic) {
	if key.IsZero() {
		de.disco.Store(nil)
	} else {
		de.disco.Store(&endpointDisco{
			key:   key,
			short: key.ShortString(),
		})
	}
}

// updateFromNode updates the endpoint based on a tailcfg.Node from a NetMap
// update.
func (de *endpoint) updateFromNode(n tailcfg.NodeView, heartbeatDisabled bool, probeUDPLifetimeEnabled bool) {
	if !n.Valid() {
		panic("nil node when updating endpoint")
	}
	de.mu.Lock()
	defer de.mu.Unlock()

	de.heartbeatDisabled = heartbeatDisabled
	if probeUDPLifetimeEnabled {
		de.setProbeUDPLifetimeConfigLocked(defaultProbeUDPLifetimeConfig)
	} else {
		de.setProbeUDPLifetimeConfigLocked(nil)
	}
	de.expired = n.Expired()

	epDisco := de.disco.Load()
	var discoKey key.DiscoPublic
	if epDisco != nil {
		discoKey = epDisco.key
	}

	if discoKey != n.DiscoKey() {
		de.c.logf("[v1] magicsock: disco: node %s changed from %s to %s", de.publicKey.ShortString(), discoKey, n.DiscoKey())
		key := n.DiscoKey()
		de.updateDiscoKey(key)
		de.debugUpdates.Add(EndpointChange{
			When: time.Now(),
			What: "updateFromNode-resetLocked",
		})
	}
	if n.HomeDERP() == 0 {
		if de.derpAddr.IsValid() {
			de.debugUpdates.Add(EndpointChange{
				When: time.Now(),
				What: "updateFromNode-remove-DERP",
				From: de.derpAddr,
			})
		}
		de.derpAddr = netip.AddrPort{}
	} else {
		newDerp := netip.AddrPortFrom(tailcfg.DerpMagicIPAddr, uint16(n.HomeDERP()))
		if de.derpAddr != newDerp {
			de.debugUpdates.Add(EndpointChange{
				When: time.Now(),
				What: "updateFromNode-DERP",
				From: de.derpAddr,
				To:   newDerp,
			})
		}
		de.derpAddr = newDerp
	}

	de.setEndpointsLocked(n.Endpoints())

	de.relayCapable = capVerIsRelayCapable(n.Cap())
}

func (de *endpoint) setEndpointsLocked(eps interface {
	All() iter.Seq2[int, netip.AddrPort]
}) {
	for _, st := range de.endpointState {
		st.index = indexSentinelDeleted // assume deleted until updated in next loop
	}

	var newIpps []netip.AddrPort
	eps.All()(func(i int, ipp netip.AddrPort) bool {
		if i > math.MaxInt16 {
			// Seems unlikely.
			return false
		}
		if !ipp.IsValid() {
			de.c.logf("magicsock: bogus netmap endpoint from %v", eps)
			return true
		}
		if st, ok := de.endpointState[ipp]; ok {
			st.index = int16(i)
		} else {
			de.endpointState[ipp] = &endpointState{index: int16(i)}
			newIpps = append(newIpps, ipp)
		}
		return true
	})
	if len(newIpps) > 0 {
		de.debugUpdates.Add(EndpointChange{
			When: time.Now(),
			What: "updateFromNode-new-Endpoints",
			To:   newIpps,
		})
	}

	// Now delete anything unless it's still in the network map or
	// was a recently discovered endpoint.
	for ep, st := range de.endpointState {
		if st.shouldDeleteLocked() {
			de.deleteEndpointLocked("updateFromNode", ep)
		}
	}
}

// addCandidateEndpoint adds ep as an endpoint to which we should send
// future pings. If there is an existing endpointState for ep, and forRxPingTxID
// matches the last received ping TxID, this function reports true, otherwise
// false.
//
// This is called once we've already verified that we got a valid
// discovery message from de via ep.
func (de *endpoint) addCandidateEndpoint(ep netip.AddrPort, forRxPingTxID stun.TxID) (duplicatePing bool) {
	de.mu.Lock()
	defer de.mu.Unlock()

	if st, ok := de.endpointState[ep]; ok {
		duplicatePing = forRxPingTxID == st.lastGotPingTxID
		if !duplicatePing {
			st.lastGotPingTxID = forRxPingTxID
		}
		if st.lastGotPing.IsZero() {
			// Already-known endpoint from the network map.
			return duplicatePing
		}
		st.lastGotPing = time.Now()
		return duplicatePing
	}

	// Newly discovered endpoint. Exciting!
	de.c.dlogf("[v1] magicsock: disco: adding %v as candidate endpoint for %v (%s)", ep, de.discoShort(), de.publicKey.ShortString())
	de.endpointState[ep] = &endpointState{
		lastGotPing:     time.Now(),
		lastGotPingTxID: forRxPingTxID,
	}

	// If for some reason this gets very large, do some cleanup.
	if size := len(de.endpointState); size > 100 {
		for ep, st := range de.endpointState {
			if st.shouldDeleteLocked() {
				de.deleteEndpointLocked("addCandidateEndpoint", ep)
			}
		}
		size2 := len(de.endpointState)
		de.c.dlogf("[v1] magicsock: disco: addCandidateEndpoint pruned %v (%s) candidate set from %v to %v entries", de.discoShort(), de.publicKey.ShortString(), size, size2)
	}
	return false
}

// clearBestAddrLocked clears the bestAddr and related fields such that future
// packets will re-evaluate the best address to send to next.
//
// de.mu must be held.
func (de *endpoint) clearBestAddrLocked() {
	de.setBestAddrLocked(addrQuality{})
	de.bestAddrAt = 0
	de.trustBestAddrUntil = 0
}

// noteBadEndpoint marks udpAddr as a bad endpoint that would need to be
// re-evaluated before future use, this should be called for example if a send
// to udpAddr fails due to a host unreachable error or similar.
func (de *endpoint) noteBadEndpoint(udpAddr epAddr) {
	de.mu.Lock()
	defer de.mu.Unlock()

	de.clearBestAddrLocked()

	if !udpAddr.vni.IsSet() {
		if st, ok := de.endpointState[udpAddr.ap]; ok {
			st.clear()
		}
	}
}

// noteConnectivityChange is called when connectivity changes enough
// that we should question our earlier assumptions about which paths
// work.
func (de *endpoint) noteConnectivityChange() {
	de.mu.Lock()
	defer de.mu.Unlock()

	de.clearBestAddrLocked()

	for k := range de.endpointState {
		de.endpointState[k].clear()
	}
}

// pingSizeToPktLen calculates the minimum path MTU that would permit
// a disco ping message of length size to reach its target at
// udpAddr. size is the length of the entire disco message including
// disco headers. If size is zero, assume it is the safe wire MTU.
func pingSizeToPktLen(size int, udpAddr epAddr) tstun.WireMTU {
	if size == 0 {
		return tstun.SafeWireMTU()
	}
	headerLen := ipv4.HeaderLen
	if udpAddr.ap.Addr().Is6() {
		headerLen = ipv6.HeaderLen
	}
	headerLen += 8 // UDP header length
	if udpAddr.vni.IsSet() {
		headerLen += packet.GeneveFixedHeaderLength
	}
	return tstun.WireMTU(size + headerLen)
}

// pktLenToPingSize calculates the ping payload size that would
// create a disco ping message whose on-the-wire length is exactly mtu
// bytes long. If mtu is zero or less than the minimum ping size, then
// no MTU probe is desired and return zero for an unpadded ping.
func pktLenToPingSize(mtu tstun.WireMTU, is6 bool) int {
	if mtu == 0 {
		return 0
	}
	headerLen := ipv4.HeaderLen
	if is6 {
		headerLen = ipv6.HeaderLen
	}
	headerLen += 8 // UDP header length
	if mtu < tstun.WireMTU(headerLen) {
		return 0
	}
	return int(mtu) - headerLen
}

// handlePongConnLocked handles a Pong message (a reply to an earlier ping).
// It should be called with the Conn.mu held.
//
// It reports whether m.TxID corresponds to a ping that this endpoint sent.
func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src epAddr) (knownTxID bool) {
	de.mu.Lock()
	defer de.mu.Unlock()

	isDerp := src.ap.Addr() == tailcfg.DerpMagicIPAddr

	sp, ok := de.sentPing[m.TxID]
	if !ok {
		// This is not a pong for a ping we sent.
		return false
	}
	knownTxID = true // for naked returns below
	de.removeSentDiscoPingLocked(m.TxID, sp, discoPongReceived)

	pktLen := int(pingSizeToPktLen(sp.size, src))
	if sp.size != 0 {
		m := getPeerMTUsProbedMetric(tstun.WireMTU(pktLen))
		m.Add(1)
		if metricMaxPeerMTUProbed.Value() < int64(pktLen) {
			metricMaxPeerMTUProbed.Set(int64(pktLen))
		}
	}

	now := mono.Now()
	latency := now.Sub(sp.at)

	if !isDerp && !src.vni.IsSet() {
		// Note: we check vni.isSet() as relay [epAddr]'s are not stored in
		// endpointState, they are either de.bestAddr or not.
		st, ok := de.endpointState[sp.to.ap]
		if !ok {
			// This is no longer an endpoint we care about.
			return
		}

		de.c.peerMap.setNodeKeyForEpAddr(src, de.publicKey)

		st.addPongReplyLocked(pongReply{
			latency: latency,
			pongAt:  now,
			from:    src.ap,
			pongSrc: m.Src,
		})
	}

	if sp.purpose != pingHeartbeat && sp.purpose != pingHeartbeatForUDPLifetime {
		de.c.dlogf("[v1] magicsock: disco: %v<-%v (%v, %v)  got pong tx=%x latency=%v pktlen=%v pong.src=%v%v", de.c.discoAtomic.Short(), de.discoShort(), de.publicKey.ShortString(), src, m.TxID[:6], latency.Round(time.Millisecond), pktLen, m.Src, logger.ArgWriter(func(bw *bufio.Writer) {
			if sp.to != src {
				fmt.Fprintf(bw, " ping.to=%v", sp.to)
			}
		}))
	}

	// Currently only CLI ping uses this callback.
	if sp.resCB.reply() {
		if sp.purpose == pingCLI {
			de.c.populateCLIPingResponseLocked(sp.resCB.res, latency, sp.to)
		}
		go sp.resCB.cb(sp.resCB.res)
	}

	// Promote this pong response to our current best address if it's lower latency.
	// TODO(bradfitz): decide how latency vs. preference order affects decision
	if !isDerp {
		thisPong := addrQuality{
			epAddr:  sp.to,
			latency: latency,
			wireMTU: pingSizeToPktLen(sp.size, sp.to),
		}
		bestUntrusted := now.After(de.trustBestAddrUntil)
		if betterAddr(thisPong, de.bestAddr) || bestUntrusted {
			de.c.logf("magicsock: disco: node %v %v now using %v mtu=%v tx=%x", de.publicKey.ShortString(), de.discoShort(), sp.to, thisPong.wireMTU, m.TxID[:6])
			de.debugUpdates.Add(EndpointChange{
				When: time.Now(),
				What: "handlePongConnLocked-bestAddr-update",
				From: de.bestAddr,
				To:   thisPong,
			})
			de.setBestAddrLocked(thisPong)
		}
		if de.bestAddr.epAddr == thisPong.epAddr {
			de.debugUpdates.Add(EndpointChange{
				When: time.Now(),
				What: "handlePongConnLocked-bestAddr-latency",
				From: de.bestAddr,
				To:   thisPong,
			})
			de.bestAddr.latency = latency
			de.bestAddrAt = now
			de.trustBestAddrUntil = now.Add(trustUDPAddrDuration)
		}
	}
	return
}

// epAddr is a [netip.AddrPort] with an optional Geneve header (RFC8926)
// [packet.VirtualNetworkID].
type epAddr struct {
	ap  netip.AddrPort          // if ap == tailcfg.DerpMagicIPAddr then vni is never set
	vni packet.VirtualNetworkID // vni.IsSet() indicates if this [epAddr] involves a Geneve header
}

// isDirect returns true if e.ap is valid and not tailcfg.DerpMagicIPAddr,
// and a VNI is not set.
func (e epAddr) isDirect() bool {
	return e.ap.IsValid() && e.ap.Addr() != tailcfg.DerpMagicIPAddr && !e.vni.IsSet()
}

func (e epAddr) String() string {
	if !e.vni.IsSet() {
		return e.ap.String()
	}
	return fmt.Sprintf("%v:vni:%d", e.ap.String(), e.vni.Get())
}

// addrQuality is an [epAddr], an optional [key.DiscoPublic] if a relay server
// is associated, a round-trip latency measurement, and path mtu.
type addrQuality struct {
	epAddr
	relayServerDisco key.DiscoPublic // only relevant if epAddr.vni.isSet(), otherwise zero value
	latency          time.Duration
	wireMTU          tstun.WireMTU
}

func (a addrQuality) String() string {
	// TODO(jwhited): consider including relayServerDisco
	return fmt.Sprintf("%v@%v+%v", a.epAddr, a.latency, a.wireMTU)
}

// betterAddr reports whether a is a better addr to use than b.
func betterAddr(a, b addrQuality) bool {
	if a.epAddr == b.epAddr {
		if a.wireMTU > b.wireMTU {
			// TODO(val): Think harder about the case of lower
			// latency and smaller or unknown MTU, and higher
			// latency but larger MTU. Probably in most cases the
			// largest MTU will also be the lowest latency but we
			// can't depend on that.
			return true
		}
		return false
	}
	if !b.ap.IsValid() {
		return true
	}
	if !a.ap.IsValid() {
		return false
	}

	// Geneve-encapsulated paths (UDP relay servers) are lower preference in
	// relation to non.
	if !a.vni.IsSet() && b.vni.IsSet() {
		return true
	}
	if a.vni.IsSet() && !b.vni.IsSet() {
		return false
	}

	// Each address starts with a set of points (from 0 to 100) that
	// represents how much faster they are than the highest-latency
	// endpoint. For example, if a has latency 200ms and b has latency
	// 190ms, then a starts with 0 points and b starts with 5 points since
	// it's 5% faster.
	var aPoints, bPoints int
	if a.latency > b.latency && a.latency > 0 {
		bPoints = int(100 - ((b.latency * 100) / a.latency))
	} else if b.latency > 0 {
		aPoints = int(100 - ((a.latency * 100) / b.latency))
	}

	// Prefer private IPs over public IPs as long as the latencies are
	// roughly equivalent, since it's less likely that a user will have to
	// pay for the bandwidth in a cloud environment.
	//
	// Additionally, prefer any loopback address strongly over non-loopback
	// addresses, and prefer link-local unicast addresses over other types
	// of private IP addresses since it's definitionally more likely that
	// they'll be on the same network segment than a general private IP.
	if a.ap.Addr().IsLoopback() {
		aPoints += 50
	} else if a.ap.Addr().IsLinkLocalUnicast() {
		aPoints += 30
	} else if a.ap.Addr().IsPrivate() {
		aPoints += 20
	}
	if b.ap.Addr().IsLoopback() {
		bPoints += 50
	} else if b.ap.Addr().IsLinkLocalUnicast() {
		bPoints += 30
	} else if b.ap.Addr().IsPrivate() {
		bPoints += 20
	}

	// Prefer IPv6 for being a bit more robust, as long as
	// the latencies are roughly equivalent.
	if a.ap.Addr().Is6() {
		aPoints += 10
	}
	if b.ap.Addr().Is6() {
		bPoints += 10
	}

	// Don't change anything if the latency improvement is less than 1%; we
	// want a bit of "stickiness" (a.k.a. hysteresis) to avoid flapping if
	// there's two roughly-equivalent endpoints.
	//
	// Points are essentially the percentage improvement of latency vs. the
	// slower endpoint; absent any boosts from private IPs, IPv6, etc., a
	// will be a better address than b by a fraction of 1% or less if
	// aPoints <= 1 and bPoints == 0.
	if aPoints <= 1 && bPoints == 0 {
		return false
	}

	return aPoints > bPoints
}

// handleCallMeMaybe handles a CallMeMaybe discovery message via
// DERP. The contract for use of this message is that the peer has
// already sent to us via UDP, so their stateful firewall should be
// open. Now we can Ping back and make it through.
func (de *endpoint) handleCallMeMaybe(m *disco.CallMeMaybe) {
	if runtime.GOOS == "js" {
		// Nothing to do on js/wasm if we can't send UDP packets anyway.
		return
	}
	de.mu.Lock()
	defer de.mu.Unlock()

	now := time.Now()
	for ep := range de.isCallMeMaybeEP {
		de.isCallMeMaybeEP[ep] = false // mark for deletion
	}
	var newEPs []netip.AddrPort
	for _, ep := range m.MyNumber {
		if ep.Addr().Is6() && ep.Addr().IsLinkLocalUnicast() {
			// We send these out, but ignore them for now.
			// TODO: teach the ping code to ping on all interfaces
			// for these.
			continue
		}
		mak.Set(&de.isCallMeMaybeEP, ep, true)
		if es, ok := de.endpointState[ep]; ok {
			es.callMeMaybeTime = now
		} else {
			de.endpointState[ep] = &endpointState{callMeMaybeTime: now}
			newEPs = append(newEPs, ep)
		}
	}
	if len(newEPs) > 0 {
		de.debugUpdates.Add(EndpointChange{
			When: time.Now(),
			What: "handleCallMeMaybe-new-endpoints",
			To:   newEPs,
		})

		de.c.dlogf("[v1] magicsock: disco: call-me-maybe from %v %v added new endpoints: %v",
			de.publicKey.ShortString(), de.discoShort(),
			logger.ArgWriter(func(w *bufio.Writer) {
				for i, ep := range newEPs {
					if i > 0 {
						w.WriteString(", ")
					}
					w.WriteString(ep.String())
				}
			}))
	}

	// Delete any prior CallMeMaybe endpoints that weren't included
	// in this message.
	for ep, want := range de.isCallMeMaybeEP {
		if !want {
			delete(de.isCallMeMaybeEP, ep)
			de.deleteEndpointLocked("handleCallMeMaybe", ep)
		}
	}

	// Zero out all the lastPing times to force sendPingsLocked to send new ones,
	// even if it's been less than 5 seconds ago.
	for _, st := range de.endpointState {
		st.lastPing = 0
	}
	monoNow := mono.Now()
	de.sendDiscoPingsLocked(monoNow, false)

	// This hook is required to trigger peer relay path discovery around
	// disco "tailscale ping" initiated by de. We may be configured with peer
	// relay servers that differ from de.
	//
	// The only other peer relay path discovery hook is in [endpoint.heartbeat],
	// which is kicked off around outbound WireGuard packet flow, or if you are
	// the "tailscale ping" initiator. Disco "tailscale ping" does not propagate
	// into wireguard-go.
	//
	// We choose not to hook this around disco ping reception since peer relay
	// path discovery can also trigger disco ping transmission, which *could*
	// lead to an infinite loop of peer relay path discovery between two peers,
	// absent intended triggers.
	if de.wantUDPRelayPathDiscoveryLocked(monoNow) {
		de.discoverUDPRelayPathsLocked(monoNow)
	}
}

func (de *endpoint) populatePeerStatus(ps *ipnstate.PeerStatus) {
	de.mu.Lock()
	defer de.mu.Unlock()

	ps.Relay = de.c.derpRegionCodeOfIDLocked(int(de.derpAddr.Port()))

	if de.lastSendExt.IsZero() {
		return
	}

	now := mono.Now()
	ps.LastWrite = de.lastSendExt.WallTime()
	ps.Active = now.Sub(de.lastSendExt) < sessionActiveTimeout

	if udpAddr, derpAddr, _ := de.addrForSendLocked(now); udpAddr.ap.IsValid() && !derpAddr.IsValid() {
		if udpAddr.vni.IsSet() {
			ps.PeerRelay = udpAddr.String()
		} else {
			ps.CurAddr = udpAddr.String()
		}
	}
}

// stopAndReset stops timers associated with de and resets its state back to zero.
// It's called when a discovery endpoint is no longer present in the
// NetworkMap, or when magicsock is transitioning from running to
// stopped state (via SetPrivateKey(zero))
func (de *endpoint) stopAndReset() {
	atomic.AddInt64(&de.numStopAndResetAtomic, 1)
	de.mu.Lock()
	defer de.mu.Unlock()

	if closing := de.c.closing.Load(); !closing {
		if de.isWireguardOnly {
			de.c.logf("[v1] magicsock: doing cleanup for wireguard key %s", de.publicKey.ShortString())
		} else {
			de.c.logf("[v1] magicsock: doing cleanup for discovery key %s", de.discoShort())
		}
	}

	de.debugUpdates.Add(EndpointChange{
		When: time.Now(),
		What: "stopAndReset-resetLocked",
	})
	de.resetLocked()
	if de.heartBeatTimer != nil {
		de.heartBeatTimer.Stop()
		de.heartBeatTimer = nil
	}
}

// resetLocked clears all the endpoint's p2p state, reverting it to a
// DERP-only endpoint. It does not stop the endpoint's heartbeat
// timer, if one is running.
func (de *endpoint) resetLocked() {
	de.lastSendExt = 0
	de.lastFullPing = 0
	de.clearBestAddrLocked()
	for _, es := range de.endpointState {
		es.lastPing = 0
	}
	if !de.isWireguardOnly {
		for txid, sp := range de.sentPing {
			de.removeSentDiscoPingLocked(txid, sp, discoPingResultUnknown)
		}
	}
	de.probeUDPLifetime.resetCycleEndpointLocked()
	de.c.relayManager.stopWork(de)
}

func (de *endpoint) numStopAndReset() int64 {
	return atomic.LoadInt64(&de.numStopAndResetAtomic)
}

// setDERPHome sets the provided regionID as home for de. Calls to setDERPHome
// must never run concurrent to [Conn.updateRelayServersSet], otherwise
// [candidatePeerRelay] DERP home changes may be missed from the perspective of
// [relayManager].
func (de *endpoint) setDERPHome(regionID uint16) {
	de.mu.Lock()
	defer de.mu.Unlock()
	de.derpAddr = netip.AddrPortFrom(tailcfg.DerpMagicIPAddr, uint16(regionID))
	if de.c.relayManager.hasPeerRelayServers.Load() {
		de.c.relayManager.handleDERPHomeChange(de.publicKey, regionID)
	}
}

func minInt(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func maxInt(a, b int) int {
	if a > b {
		return a
	}
	return b
}

func maxDuration(a, b time.Duration) time.Duration {
	if a > b {
		return a
	}
	return b
}

func maxMonoTime(a, b mono.Time) mono.Time {
	if a.After(b) {
		return a
	}
	return b
}
