tstest/natlab: add latency & loss simulation

A simple implementation of latency and loss simulation, applied to
writes to the ethernet interface of the NIC. The latency implementation
could be optimized substantially later if necessary.

Updates #13355
Signed-off-by: James Tucker <james@tailscale.com>

tstest/natlab/vnet/conf.go

@@ -10,6 +10,7 @@ import (
 	"net/netip"
 	"os"
 	"slices"
+	"time"
 
 	"github.com/google/gopacket/layers"
 	"github.com/google/gopacket/pcapgo"
@@ -279,10 +280,28 @@ type Network struct {
 
 	svcs set.Set[NetworkService]
 
+	latency  time.Duration // latency applied to interface writes
+	lossRate float64       // chance of packet loss (0.0 to 1.0)
+
 	// ...
 	err error // carried error
 }
 
+// SetLatency sets the simulated network latency for this network.
+func (n *Network) SetLatency(d time.Duration) {
+	n.latency = d
+}
+
+// SetPacketLoss sets the packet loss rate for this network 0.0 (no loss) to 1.0 (total loss).
+func (n *Network) SetPacketLoss(rate float64) {
+	if rate < 0 {
+		rate = 0
+	} else if rate > 1 {
+		rate = 1
+	}
+	n.lossRate = rate
+}
+
 // SetBlackholedIPv4 sets whether the network should blackhole all IPv4 traffic
 // out to the Internet. (DHCP etc continues to work on the LAN.)
 func (n *Network) SetBlackholedIPv4(v bool) {
@@ -361,6 +380,8 @@ func (s *Server) initFromConfig(c *Config) error {
 			wanIP4:     conf.wanIP4,
 			lanIP4:     conf.lanIP4,
 			breakWAN4:  conf.breakWAN4,
+			latency:    conf.latency,
+			lossRate:   conf.lossRate,
 			nodesByIP4: map[netip.Addr]*node{},
 			nodesByMAC: map[MAC]*node{},
 			logf:       logger.WithPrefix(s.logf, fmt.Sprintf("[net-%v] ", conf.mac)),

tstest/natlab/vnet/conf_test.go

@@ -3,7 +3,10 @@
 
 package vnet
 
-import "testing"
+import (
+	"testing"
+	"time"
+)
 
 func TestConfig(t *testing.T) {
 	tests := []struct {
@@ -18,6 +21,16 @@ func TestConfig(t *testing.T) {
 				c.AddNode(c.AddNetwork("2.2.2.2", "10.2.0.1/16", HardNAT))
 			},
 		},
+		{
+			name: "latency-and-loss",
+			setup: func(c *Config) {
+				n1 := c.AddNetwork("2.1.1.1", "192.168.1.1/24", EasyNAT, NATPMP)
+				n1.SetLatency(time.Second)
+				n1.SetPacketLoss(0.1)
+				c.AddNode(n1)
+				c.AddNode(c.AddNetwork("2.2.2.2", "10.2.0.1/16", HardNAT))
+			},
+		},
 		{
 			name: "indirect",
 			setup: func(c *Config) {

tstest/natlab/vnet/vnet.go

@@ -515,6 +515,8 @@ type network struct {
 	wanIP4         netip.Addr           // router's LAN IPv4, if any
 	lanIP4         netip.Prefix         // router's LAN IP + CIDR (e.g. 192.168.2.1/24)
 	breakWAN4      bool                 // break WAN IPv4 connectivity
+	latency        time.Duration        // latency applied to interface writes
+	lossRate       float64              // probability of dropping a packet (0.0 to 1.0)
 	nodesByIP4     map[netip.Addr]*node // by LAN IPv4
 	nodesByMAC     map[MAC]*node
 	logf           func(format string, args ...any)
@@ -977,7 +979,7 @@ func (n *network) writeEth(res []byte) bool {
 		for mac, nw := range n.writers.All() {
 			if mac != srcMAC {
 				num++
-				nw.write(res)
+				n.conditionedWrite(nw, res)
 			}
 		}
 		return num > 0
@@ -987,7 +989,7 @@ func (n *network) writeEth(res []byte) bool {
 		return false
 	}
 	if nw, ok := n.writers.Load(dstMAC); ok {
-		nw.write(res)
+		n.conditionedWrite(nw, res)
 		return true
 	}
 
@@ -1000,6 +1002,23 @@ func (n *network) writeEth(res []byte) bool {
 	return false
 }
 
+func (n *network) conditionedWrite(nw networkWriter, packet []byte) {
+	if n.lossRate > 0 && rand.Float64() < n.lossRate {
+		// packet lost
+		return
+	}
+	if n.latency > 0 {
+		// copy the packet as there's no guarantee packet is owned long enough.
+		// TODO(raggi): this could be optimized substantially if necessary,
+		// a pool of buffers and a cheaper delay mechanism are both obvious improvements.
+		var pkt = make([]byte, len(packet))
+		copy(pkt, packet)
+		time.AfterFunc(n.latency, func() { nw.write(pkt) })
+	} else {
+		nw.write(packet)
+	}
+}
+
 var (
 	macAllNodes   = MAC{0: 0x33, 1: 0x33, 5: 0x01}
 	macAllRouters = MAC{0: 0x33, 1: 0x33, 5: 0x02}