Prefer matching labels and longest matching prefix

...when performing source address selection for IPv6.

These are defined in RFC 6724 section 5 rule 6 (prefer matching label)
and rule 8 (use longest matching prefix).

This change also considers ULA of global scope instead of its own scope,
as per RFC 6724 section 3.1:

   Also, note that ULAs are considered as global, not
   site-local, scope but are handled via the prefix policy table as
   discussed in Section 10.6.

Test: stack_test.TestIPv6SourceAddressSelectionScope

Startblock:
  has LGTM from peterjohnston
  and then
  add reviewer brunodalbo
PiperOrigin-RevId: 348580996

1 files changed, 40 insertions, 0 deletions

diff --git a/pkg/tcpip/tcpip_test.go b/pkg/tcpip/tcpip_test.go
index c461da137..9bd563c46 100644
--- a/pkg/tcpip/tcpip_test.go
+++ b/pkg/tcpip/tcpip_test.go
@@ -270,3 +270,43 @@ func TestAddressUnspecified(t *testing.T) {
 		})
 	}
 }
+
+func TestAddressMatchingPrefix(t *testing.T) {
+	tests := []struct {
+		addrA  Address
+		addrB  Address
+		prefix uint8
+	}{
+		{
+			addrA:  "\x01\x01",
+			addrB:  "\x01\x01",
+			prefix: 16,
+		},
+		{
+			addrA:  "\x01\x01",
+			addrB:  "\x01\x00",
+			prefix: 15,
+		},
+		{
+			addrA:  "\x01\x01",
+			addrB:  "\x81\x00",
+			prefix: 0,
+		},
+		{
+			addrA:  "\x01\x01",
+			addrB:  "\x01\x80",
+			prefix: 8,
+		},
+		{
+			addrA:  "\x01\x01",
+			addrB:  "\x02\x80",
+			prefix: 6,
+		},
+	}
+
+	for _, test := range tests {
+		if got := test.addrA.MatchingPrefix(test.addrB); got != test.prefix {
+			t.Errorf("got (%s).MatchingPrefix(%s) = %d, want = %d", test.addrA, test.addrB, got, test.prefix)
+		}
+	}
+}