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device: add priority message transmission around session establishment

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Add SetPriorityMessageOnEstablishmentFunc, which registers a
PeerPriorityMessageFunc callback invoked when a peer's session keypair
is established or re-keyed for forward data transmission. The bytes it
returns are transmitted to the peer as a transport message.

The message is "priority" in two senses: it bypasses the staged packet
queue entirely, so it cannot be evicted by TUN-sourced packets, and it
is enqueued ahead of the keepalive/staged packets that follow keypair
establishment.

Updates tailscale/tailscale#20081

Signed-off-by: Jordan Whited <jordan@tailscale.com>
This commit is contained in:
Jordan Whited 2026-07-07 14:55:53 -07:00 • committed by 世界
parent 15b912c1c0
commit 7a66fbee4a
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3 changed files with 100 additions and 1 deletions
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35
device/device.go
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@ -64,7 +64,8 @@ type Device struct {
lookupFunc PeerLookupFunc // or nil if unused
}
peerStateFn atomic.Pointer[PeerSessionStateFunc] // observes peer session state changes, nil if unset
peerStateFn atomic.Pointer[PeerSessionStateFunc] // observes peer session state changes, nil if unset
priorityMsgFn atomic.Pointer[PeerPriorityMessageFunc] // returns a priority message to be sent around session establishment, nil if unset
rate struct {
underLoadUntil atomic.Int64
@ -553,6 +554,38 @@ func (device *Device) SetSessionStateFunc(f PeerSessionStateFunc) {
device.peerStateFn.Store(&f)
}
// MaxPriorityMessageContentSize is the maximum size of a message returned by a
// [PeerPriorityMessageFunc]. It's a power of 2 that leaves significant space
// when accounting for all WireGuard overhead and encapsulating network protocol
// headers. Future adjustments to this value should consider all these overheads
// and any [conn.Bind] implementation limitations.
const MaxPriorityMessageContentSize = 512
// PeerPriorityMessageFunc is called when a peer's WireGuard session keypair is
// established (or re-keyed) for forward data transmission.
//
// The returned message is transmitted to the peer in priority fashion. Priority
// means it cannot be evicted from the staged packet queue by non-priority
// (read from [tun.Device]) packets. It avoids the staged queue altogether.
//
// The callback must be cheap and must not call back into [Device]. A zero length
// message or a message whose length exceeds [MaxPriorityMessageContentSize] will
// be silently dropped. Message should start with an IPv4 or IPv6 header as it
// is subject to allowed IPs lookup on the receiver, same as any other transport
// message.
type PeerPriorityMessageFunc func(peer NoisePublicKey) (msg []byte)
// SetPriorityMessageOnEstablishmentFunc sets a function to be used for sending
// a priority message around session establishment. See [PeerPriorityMessageFunc]
// docs for more details. A nil value clears any previously set value.
func (device *Device) SetPriorityMessageOnEstablishmentFunc(f PeerPriorityMessageFunc) {
if f == nil {
device.priorityMsgFn.Store(nil)
return
}
device.priorityMsgFn.Store(&f)
}
func (device *Device) Close() {
device.state.Lock()
defer device.state.Unlock()