Simulcast: getRids orders layers by a=rid line order, not the a=simulcast preference list (RFC 8853 §5.2) #3471

diff --git a/sdp.go b/sdp.go
index 1525f049..96d1d1b0 100644
--- a/sdp.go
+++ b/sdp.go
@@ -6,9 +6,11 @@
 package webrtc
 
 import (
+	"cmp"
 	"encoding/base64"
 	"errors"
 	"fmt"
+	"math"
 	"net/url"
 	"regexp"
 	"slices"
@@ -298,10 +300,17 @@ func getRids(media *sdp.MediaDescription) []*simulcastRid { // nolint:cyclop
 		if space := strings.Index(simulcastAttr, " "); space > 0 {
 			simulcastAttr = simulcastAttr[space+1:]
 		}
+		// RFC 8853 §5.2: the a=simulcast send list "suggests a proposed order of
+		// preference, in decreasing order"; the order of the a=rid lines is not
+		// significant. Record that order and sort the rids by it below so a
+		// generated answer mirrors the offered preference instead of the
+		// incidental a=rid line order.
+		simulcastOrder := map[string]int{}
 		ridStates := strings.SplitSeq(simulcastAttr, ";")
 		for ridState := range ridStates {
+			ridID := ridState
 			if len(ridState) > 0 && ridState[:1] == "~" {
-				ridID := ridState[1:]
+				ridID = ridState[1:]
 				for _, rid := range rids {
 					if rid.id == ridID {
 						rid.paused = true
@@ -310,7 +319,20 @@ func getRids(media *sdp.MediaDescription) []*simulcastRid { // nolint:cyclop
 					}
 				}
 			}
+			if _, ok := simulcastOrder[ridID]; !ok {
+				simulcastOrder[ridID] = len(simulcastOrder)
+			}
 		}
+		orderOf := func(id string) int {
+			if idx, ok := simulcastOrder[id]; ok {
+				return idx
+			}
+
+			return math.MaxInt
+		}
+		slices.SortStableFunc(rids, func(a, b *simulcastRid) int {
+			return cmp.Compare(orderOf(a.id), orderOf(b.id))
+		})
 	}
 
 	return rids
diff --git a/sdp_test.go b/sdp_test.go
index 540ecde7..4798bfc3 100644
--- a/sdp_test.go
+++ b/sdp_test.go
@@ -1306,6 +1306,33 @@ func TestGetRIDs(t *testing.T) {
 	}
 }
 
+// TestGetRIDsOrderFollowsSimulcast verifies that getRids orders the simulcast
+// streams by the a=simulcast attribute, not by the order of the a=rid lines.
+// RFC 8853 §5.2: the a=simulcast send list "suggests a proposed order of
+// preference, in decreasing order"; the order of the a=rid lines is not
+// significant. Here the a=rid lines are listed in the opposite order to
+// a=simulcast:send, so the result must follow the attribute (f, h, q).
+func TestGetRIDsOrderFollowsSimulcast(t *testing.T) {
+	media := &sdp.MediaDescription{
+		MediaName: sdp.MediaName{
+			Media: "video",
+		},
+		Attributes: []sdp.Attribute{
+			{Key: sdpAttributeRid, Value: "q send"},
+			{Key: sdpAttributeRid, Value: "h send"},
+			{Key: sdpAttributeRid, Value: "f send"},
+			{Key: sdpAttributeSimulcast, Value: "send f;h;q"},
+		},
+	}
+
+	got := make([]string, 0, 3)
+	for _, rid := range getRids(media) {
+		got = append(got, rid.id)
+	}
+
+	assert.Equal(t, []string{"f", "h", "q"}, got)
+}
+
 // TestGetRIDs_EmptySimulcastTokens verifies that getRids does not panic when the