Refactor - now wireguard based. (#7)

peer/control/ping.go

@@ -0,0 +1,76 @@
+// Package control implements the VPN-internal peer control protocol.
+// Peers exchange Ping packets over UDP on the VPN control port to maintain
+// liveness and discover external endpoints for direct connection attempts.
+package control
+
+import (
+	"encoding/binary"
+	"fmt"
+	"net/netip"
+)
+
+const (
+	version = 1
+	Size    = 51 // 1 version + 8 PingTS + 6 SrcV4 + 18 SrcV6 + 18 Dst
+)
+
+// Ping is the single control packet type exchanged between VPN peers.
+//
+// In each peer pair, the peer with the lower VPN IP is the client: it sets
+// PingTS and sends pings on a timer. The server echoes PingTS back in its
+// response, allowing the client to compute RTT = now - PingTS.
+//
+// Both client and server populate SrcV4, SrcV6, and Dst on every packet so
+// endpoint information flows in both directions.
+//
+// Dst is the recipient's external endpoint as observed by the sender from the
+// WireGuard handshake source. Zero if the sender has not observed a handshake
+// from the recipient.
+type Ping struct {
+	PingTS int64          // Client ping send time in nanoseconds.
+	SrcV4  netip.AddrPort // Sender's discovered IPv4 address and port.
+	SrcV6  netip.AddrPort // Sender's discovered IPv6 address and port.
+	Dst    netip.AddrPort
+}
+
+// Marshal encodes p into buf (which must be at least Size bytes) and returns
+// buf[:Size]. Taking the buffer lets callers reuse one across sends; every
+// field is written unconditionally so a reused buffer needs no pre-zeroing.
+func (p Ping) Marshal(buf []byte) []byte {
+	buf[0] = version
+	binary.BigEndian.PutUint64(buf[1:9], uint64(p.PingTS))
+	if p.SrcV4.IsValid() {
+		a4 := p.SrcV4.Addr().As4()
+		copy(buf[9:13], a4[:])
+		binary.BigEndian.PutUint16(buf[13:15], p.SrcV4.Port())
+	} else {
+		clear(buf[9:15])
+	}
+	a16 := p.SrcV6.Addr().As16()
+	copy(buf[15:31], a16[:])
+	binary.BigEndian.PutUint16(buf[31:33], p.SrcV6.Port())
+	a16 = p.Dst.Addr().As16()
+	copy(buf[33:49], a16[:])
+	binary.BigEndian.PutUint16(buf[49:51], p.Dst.Port())
+	return buf[:Size]
+}
+
+// Unmarshal decodes a Ping from a fixed-size 51-byte array.
+func Unmarshal(buf [Size]byte) (Ping, error) {
+	if buf[0] != version {
+		return Ping{}, fmt.Errorf("unknown ping version %d", buf[0])
+	}
+	p := Ping{
+		PingTS: int64(binary.BigEndian.Uint64(buf[1:9])),
+	}
+	if addr := netip.AddrFrom4([4]byte(buf[9:13])); !addr.IsUnspecified() {
+		p.SrcV4 = netip.AddrPortFrom(addr, binary.BigEndian.Uint16(buf[13:15]))
+	}
+	if addr := netip.AddrFrom16([16]byte(buf[15:31])); !addr.IsUnspecified() {
+		p.SrcV6 = netip.AddrPortFrom(addr, binary.BigEndian.Uint16(buf[31:33]))
+	}
+	if addr := netip.AddrFrom16([16]byte(buf[33:49])).Unmap(); !addr.IsUnspecified() {
+		p.Dst = netip.AddrPortFrom(addr, binary.BigEndian.Uint16(buf[49:51]))
+	}
+	return p, nil
+}