beaconing: fix diversity calculation (scionproto#4854)

Currently the beacon diversity calculation was wrong. We only considered
ingress interface in construction direction. However the first ASEntry
will always have a zero ingress interface. Furthermore with this
approach we did not really consider the last ingress interface, which
was the ingress interface into the AS, which is not yet represented by
an ASEntry (as the termination/extension only happens after selection).

By using the egress interfaces interfaces instead we solve both
problems.

Co-authored-by: Roman Sharkov <roman.scharkov@gmail.com>

Author: lukedirtwalker

control/beacon/beacon.go

@@ -60,5 +60,5 @@ func (b Beacon) String() string {
 }
 
 func link(entry seg.ASEntry) (addr.IA, uint16) {
-	return entry.Local, entry.HopEntry.HopField.ConsIngress
+	return entry.Local, entry.HopEntry.HopField.ConsEgress
 }

control/beacon/beacon_test.go

@@ -20,14 +20,19 @@ import (
 	"github.com/golang/mock/gomock"
 	"github.com/stretchr/testify/assert"
 
+	"github.com/scionproto/scion/control/beacon"
+	"github.com/scionproto/scion/pkg/addr"
 	"github.com/scionproto/scion/pkg/private/xtest/graph"
+	"github.com/scionproto/scion/pkg/segment"
 )
 
 // TestBeaconDiversity tests that diversity is calculated correctly.
 func TestBeaconDiversity(t *testing.T) {
 	var tests = []struct {
-		name      string
-		beacon    []uint16
+		name   string
+		beacon []uint16
+		// mBeacon allows to manually specify the other beacon.
+		mBeacon   beacon.Beacon
 		diversity int
 	}{
 		{
@@ -51,14 +56,47 @@ func TestBeaconDiversity(t *testing.T) {
 			},
 			diversity: 2,
 		},
+		{
+			name: "Last link distinct",
+			mBeacon: beacon.Beacon{
+				Segment: &segment.PathSegment{
+					ASEntries: []segment.ASEntry{
+						{
+							Local: addr.MustParseIA("1-ff00:0:130"),
+							Next:  addr.MustParseIA("1-ff00:0:110"),
+							HopEntry: segment.HopEntry{
+								HopField: segment.HopField{
+									ConsEgress: graph.If_130_A_110_X,
+								},
+							},
+						},
+						{
+							Local: addr.MustParseIA("1-ff00:0:110"),
+							Next:  addr.MustParseIA("2-ff00:0:210"),
+							HopEntry: segment.HopEntry{
+								HopField: segment.HopField{
+									ConsIngress: graph.If_110_X_130_A,
+									ConsEgress:  2321,
+								},
+							},
+						},
+					},
+				},
+				InIfID: 2123,
+			},
+			diversity: 1,
+		},
 	}
 	mctrl := gomock.NewController(t)
 
 	g := graph.NewDefaultGraph(mctrl)
 	bseg := testBeacon(g, tests[0].beacon...)
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
-			other := testBeacon(g, test.beacon...)
+			other := test.mBeacon
+			if len(test.beacon) > 0 {
+				other = testBeacon(g, test.beacon...)
+			}
 			diversity := bseg.Diversity(other)
 			assert.Equal(t, test.diversity, diversity)
 		})

control/beacon/store_test.go

@@ -274,10 +274,9 @@ func testCoreStoreSelection(t *testing.T,
 }
 
 func testBeacon(g *graph.Graph, desc ...uint16) beacon.Beacon {
-	pseg := testSegment(g, desc)
-	asEntry := pseg.ASEntries[pseg.MaxIdx()]
+	pseg := testSegment(g, desc[:len(desc)-1])
 	return beacon.Beacon{
-		InIfID:  asEntry.HopEntry.HopField.ConsIngress,
+		InIfID:  desc[len(desc)-1],
 		Segment: pseg,
 	}
 }