Do not resolve routes immediately

When a response needs to be sent to an incoming packet, the stack should
consult its neighbour table to determine the remote address's link
address.

When an entry does not exist in the stack's neighbor table, the stack
should queue the packet while link resolution completes. See comments.

PiperOrigin-RevId: 336185457

2 files changed, 58 insertions, 5 deletions

diff --git a/pkg/tcpip/header/eth.go b/pkg/tcpip/header/eth.go
index eaface8cb..95ade0e5c 100644
--- a/pkg/tcpip/header/eth.go
+++ b/pkg/tcpip/header/eth.go
@@ -117,25 +117,31 @@ func (b Ethernet) Encode(e *EthernetFields) {
 	copy(b[dstMAC:][:EthernetAddressSize], e.DstAddr)
 }
 
-// IsValidUnicastEthernetAddress returns true if addr is a valid unicast
+// IsMulticastEthernetAddress returns true if the address is a multicast
+// ethernet address.
+func IsMulticastEthernetAddress(addr tcpip.LinkAddress) bool {
+	if len(addr) != EthernetAddressSize {
+		return false
+	}
+
+	return addr[unicastMulticastFlagByteIdx]&unicastMulticastFlagMask != 0
+}
+
+// IsValidUnicastEthernetAddress returns true if the address is a unicast
 // ethernet address.
 func IsValidUnicastEthernetAddress(addr tcpip.LinkAddress) bool {
-	// Must be of the right length.
 	if len(addr) != EthernetAddressSize {
 		return false
 	}
 
-	// Must not be unspecified.
 	if addr == unspecifiedEthernetAddress {
 		return false
 	}
 
-	// Must not be a multicast.
 	if addr[unicastMulticastFlagByteIdx]&unicastMulticastFlagMask != 0 {
 		return false
 	}
 
-	// addr is a valid unicast ethernet address.
 	return true
 }
 
diff --git a/pkg/tcpip/header/eth_test.go b/pkg/tcpip/header/eth_test.go
index 14413f2ce..3bc8b2b21 100644
--- a/pkg/tcpip/header/eth_test.go
+++ b/pkg/tcpip/header/eth_test.go
@@ -67,6 +67,53 @@ func TestIsValidUnicastEthernetAddress(t *testing.T) {
 	}
 }
 
+func TestIsMulticastEthernetAddress(t *testing.T) {
+	tests := []struct {
+		name     string
+		addr     tcpip.LinkAddress
+		expected bool
+	}{
+		{
+			"Nil",
+			tcpip.LinkAddress([]byte(nil)),
+			false,
+		},
+		{
+			"Empty",
+			tcpip.LinkAddress(""),
+			false,
+		},
+		{
+			"InvalidLength",
+			tcpip.LinkAddress("\x01\x02\x03"),
+			false,
+		},
+		{
+			"Unspecified",
+			unspecifiedEthernetAddress,
+			false,
+		},
+		{
+			"Multicast",
+			tcpip.LinkAddress("\x01\x02\x03\x04\x05\x06"),
+			true,
+		},
+		{
+			"Unicast",
+			tcpip.LinkAddress("\x02\x02\x03\x04\x05\x06"),
+			false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := IsMulticastEthernetAddress(test.addr); got != test.expected {
+				t.Fatalf("got IsMulticastEthernetAddress = %t, want = %t", got, test.expected)
+			}
+		})
+	}
+}
+
 func TestEthernetAddressFromMulticastIPv4Address(t *testing.T) {
 	tests := []struct {
 		name             string