Remove RTX codec if no primary

While adding transceivers from SetRemoteDescription,
the filtered codecs could filter out the primart codec
and leave the RTX codec in. Generating an answer with
that fails `SetRemoteDescription` on remote peer due
to an unrecognisable codec. Fix it by filtering out
RTX is primary is not there.

Author: boks1971

peerconnection.go

@@ -1170,6 +1170,20 @@ func (pc *PeerConnection) SetRemoteDescription(desc SessionDescription) error {
 							filteredCodecs = append(filteredCodecs, codec)
 						}
 					}
+
+					// remove RTX codecs if there is no corresponding primary codec
+					for i := len(filteredCodecs) - 1; i >= 0; i-- {
+						c := filteredCodecs[i]
+						if !strings.EqualFold(c.MimeType, MimeTypeRTX) {
+							continue
+						}
+
+						primaryPayload := findPrimaryPayloadTypeForRTX(c, filteredCodecs)
+						if primaryPayload == PayloadType(0) {
+							// no primary for RTX, remove the RTX
+							filteredCodecs = append(filteredCodecs[:i], filteredCodecs[i+1:]...)
+						}
+					}
 					_ = transceiver.SetCodecPreferences(filteredCodecs)
 				}
 

rtpcodec.go

@@ -5,6 +5,7 @@ package webrtc
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/pion/webrtc/v4/internal/fmtp"
@@ -155,6 +156,27 @@ func findRTXPayloadType(needle PayloadType, haystack []RTPCodecParameters) Paylo
 	return PayloadType(0)
 }
 
+// Given RTX CodecParameters find the primary CodecParameters if one exists.
+func findPrimaryPayloadTypeForRTX(needle RTPCodecParameters, haystack []RTPCodecParameters) PayloadType {
+	parts := strings.Split(needle.SDPFmtpLine, "=")
+	if len(parts) != 2 || parts[0] != "apt" {
+		return PayloadType(0)
+	}
+
+	primaryPayloadType, err := strconv.Atoi(parts[1])
+	if err != nil || primaryPayloadType < 0 || primaryPayloadType > 255 {
+		return PayloadType(0)
+	}
+
+	for _, c := range haystack {
+		if c.PayloadType == PayloadType(primaryPayloadType) {
+			return c.PayloadType
+		}
+	}
+
+	return PayloadType(0)
+}
+
 // For now, only FlexFEC is supported.
 func findFECPayloadType(haystack []RTPCodecParameters) PayloadType {
 	for _, c := range haystack {