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.

rtpcodec.go

@@ -5,6 +5,7 @@ package webrtc
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/pion/webrtc/v4/internal/fmtp"
@@ -155,6 +156,38 @@ func findRTXPayloadType(needle PayloadType, haystack []RTPCodecParameters) Paylo
 	return PayloadType(0)
 }
 
+// Given needle CodecParameters, returns if needle is RTX and
+// if primary codec corresponding to that needle is in the haystack of codecs.
+func primaryPayloadTypeForRTXExists(needle RTPCodecParameters, haystack []RTPCodecParameters) (
+	isRTX bool, primaryExists bool,
+) {
+	if !strings.EqualFold(needle.MimeType, MimeTypeRTX) {
+		return
+	}
+
+	isRTX = true
+	parsed := fmtp.Parse(needle.MimeType, needle.ClockRate, needle.Channels, needle.SDPFmtpLine)
+	aptPayload, ok := parsed.Parameter("apt")
+	if !ok {
+		return
+	}
+
+	primaryPayloadType, err := strconv.Atoi(aptPayload)
+	if err != nil || primaryPayloadType < 0 || primaryPayloadType > 255 {
+		return
+	}
+
+	for _, c := range haystack {
+		if c.PayloadType == PayloadType(primaryPayloadType) {
+			primaryExists = true
+
+			return
+		}
+	}
+
+	return
+}
+
 // For now, only FlexFEC is supported.
 func findFECPayloadType(haystack []RTPCodecParameters) PayloadType {
 	for _, c := range haystack {

rtpcodec_test.go

@@ -9,6 +9,199 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestFindPrimaryPayloadTypeForRTX(t *testing.T) {
+	for _, test := range []struct {
+		Name                string
+		Needle              RTPCodecParameters
+		Haystack            []RTPCodecParameters
+		ResultIsRTX         bool
+		ResultPrimaryExists bool
+	}{
+		{
+			Name: "not RTX",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeH264,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=2",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         false,
+			ResultPrimaryExists: false,
+		},
+		{
+			Name: "incorrect fmtp",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "incorrect-fmtp",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: false,
+		},
+		{
+			Name: "incomplete fmtp",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: false,
+		},
+		{
+			Name: "primary payload type outside range (negative)",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=-10",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: false,
+		},
+		{
+			Name: "primary payload type outside range (high positive)",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=1000",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: false,
+		},
+		{
+			Name: "non-matching needle",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=23",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: false,
+		},
+		{
+			Name: "matching needle",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=1",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: true,
+		},
+		{
+			Name: "matching fmtp is a substring",
+			Needle: RTPCodecParameters{
+				PayloadType: 2,
+				RTPCodecCapability: RTPCodecCapability{
+					MimeType:    MimeTypeRTX,
+					ClockRate:   90000,
+					SDPFmtpLine: "apt=1;rtx-time:2000",
+				},
+			},
+			Haystack: []RTPCodecParameters{
+				{
+					PayloadType: 1,
+					RTPCodecCapability: RTPCodecCapability{
+						MimeType:  MimeTypeH264,
+						ClockRate: 90000,
+					},
+				},
+			},
+			ResultIsRTX:         true,
+			ResultPrimaryExists: true,
+		},
+	} {
+		t.Run(test.Name, func(t *testing.T) {
+			isRTX, primaryExists := primaryPayloadTypeForRTXExists(test.Needle, test.Haystack)
+			assert.Equal(t, test.ResultIsRTX, isRTX)
+			assert.Equal(t, test.ResultPrimaryExists, primaryExists)
+		})
+	}
+}
+
 func TestFindFECPayloadType(t *testing.T) {
 	for _, test := range []struct {
 		Haystack          []RTPCodecParameters

rtptransceiver.go

@@ -8,6 +8,7 @@ package webrtc
 
 import (
 	"fmt"
+	"strings"
 	"sync"
 	"sync/atomic"
 
@@ -60,6 +61,19 @@ func (t *RTPTransceiver) SetCodecPreferences(codecs []RTPCodecParameters) error
 		}
 	}
 
+	// remove RTX codecs if there is no corresponding primary codec
+	for i := len(codecs) - 1; i >= 0; i-- {
+		c := codecs[i]
+		if !strings.EqualFold(c.MimeType, MimeTypeRTX) {
+			continue
+		}
+
+		if isRTX, primaryExists := primaryPayloadTypeForRTXExists(c, codecs); isRTX && !primaryExists {
+			// no primary for RTX, remove the RTX
+			codecs = append(codecs[:i], codecs[i+1:]...)
+		}
+	}
+
 	t.codecs = codecs
 
 	return nil