wgengine/magicsock: set UDP socket buffer sizes to 7MB

- At high data rates more buffer space is required in order to avoid
  packet loss during any cause of delay.
- On slower machines more buffer space is required in order to avoid
  packet loss while decryption & tun writing is underway.
- On higher latency network paths more buffer space is required in order
  to overcome BDP.
- On Linux set with SO_*BUFFORCE to bypass net.core.{r,w}mem_max.
- 7MB is the current default maximum on macOS 12.6
- Windows test is omitted, as Windows does not support getsockopt for
  these options.

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -69,6 +69,11 @@ const (
 	// _linux variant.
 	discoMagic1 = 0x5453f09f
 	discoMagic2 = 0x92ac
+
+	// UDP socket read/write buffer size (7MB). The value of 7MB is chosen as it
+	// is the max supported by a default configuration of macOS. Some platforms
+	// will silently clamp the value.
+	socketBufferSize = 7 << 20
 )
 
 // useDerpRoute reports whether magicsock should enable the DERP
@@ -2893,6 +2898,7 @@ func (c *Conn) bindSocket(ruc *RebindingUDPConn, network string, curPortFate cur
 			c.logf("magicsock: unable to bind %v port %d: %v", network, port, err)
 			continue
 		}
+		trySetSocketBuffer(pconn, c.logf)
 		// Success.
 		ruc.setConnLocked(pconn)
 		if network == "udp4" {
@@ -3948,6 +3954,20 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src netip
 	return
 }
 
+// portableTrySetSocketBuffer sets SO_SNDBUF and SO_RECVBUF on pconn to socketBufferSize,
+// logging an error if it occurs.
+func portableTrySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {
+	if c, ok := pconn.(*net.UDPConn); ok {
+		// Attempt to increase the buffer size, and allow failures.
+		if err := c.SetReadBuffer(socketBufferSize); err != nil {
+			logf("magicsock: failed to set UDP read buffer size to %d: %v", socketBufferSize, err)
+		}
+		if err := c.SetWriteBuffer(socketBufferSize); err != nil {
+			logf("magicsock: failed to set UDP write buffer size to %d: %v", socketBufferSize, err)
+		}
+	}
+}
+
 // addrLatency is an IPPort with an associated latency.
 type addrLatency struct {
 	netip.AddrPort