Ensure the same codec is used if peer reorders the SDP. (#336)

Author: dennwc

pkg/media/sdp/offer.go

@@ -252,7 +252,7 @@ func NewOffer(publicIp netip.Addr, rtpListenerPort int, encrypted Encryption) (*
 }
 
 func (d *Offer) Answer(publicIp netip.Addr, rtpListenerPort int, enc Encryption) (*Answer, *MediaConfig, error) {
-	audio, err := SelectAudio(d.MediaDesc)
+	audio, err := SelectAudio(d.MediaDesc, false)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -321,7 +321,7 @@ func (d *Offer) Answer(publicIp netip.Addr, rtpListenerPort int, enc Encryption)
 }
 
 func (d *Answer) Apply(offer *Offer, enc Encryption) (*MediaConfig, error) {
-	audio, err := SelectAudio(d.MediaDesc)
+	audio, err := SelectAudio(d.MediaDesc, true)
 	if err != nil {
 		return nil, err
 	}
@@ -473,7 +473,7 @@ type AudioConfig struct {
 	DTMFType byte
 }
 
-func SelectAudio(desc MediaDesc) (*AudioConfig, error) {
+func SelectAudio(desc MediaDesc, answer bool) (*AudioConfig, error) {
 	var (
 		priority   int
 		audioCodec rtp.AudioCodec
@@ -489,6 +489,9 @@ func SelectAudio(desc MediaDesc) (*AudioConfig, error) {
 			audioCodec = codec
 			priority = codec.Info().Priority
 		}
+		if answer {
+			break
+		}
 	}
 	if audioCodec == nil {
 		return nil, ErrNoCommonMedia

pkg/media/sdp/offer_test.go

@@ -127,11 +127,11 @@ func TestSDPMediaAnswer(t *testing.T) {
 			name: "default",
 			offer: sdp.MediaDescription{
 				MediaName: sdp.MediaName{
-					Formats: []string{"0", "9", "8", "101"},
+					Formats: []string{"9", "0", "8", "101"},
 				},
 				Attributes: []sdp.Attribute{
-					{Key: "rtpmap", Value: "0 PCMU/8000"},
 					{Key: "rtpmap", Value: "9 G722/8000"},
+					{Key: "rtpmap", Value: "0 PCMU/8000"},
 					{Key: "rtpmap", Value: "101 telephone-event/8000"},
 				},
 			},
@@ -145,11 +145,11 @@ func TestSDPMediaAnswer(t *testing.T) {
 			name: "lowercase",
 			offer: sdp.MediaDescription{
 				MediaName: sdp.MediaName{
-					Formats: []string{"0", "9", "101"},
+					Formats: []string{"9", "0", "101"},
 				},
 				Attributes: []sdp.Attribute{
-					{Key: "rtpmap", Value: "0 pcmu/8000"},
 					{Key: "rtpmap", Value: "9 g722/8000"},
+					{Key: "rtpmap", Value: "0 pcmu/8000"},
 					{Key: "rtpmap", Value: "101 telephone-event/8000"},
 				},
 			},
@@ -163,11 +163,11 @@ func TestSDPMediaAnswer(t *testing.T) {
 			name: "no dtmf",
 			offer: sdp.MediaDescription{
 				MediaName: sdp.MediaName{
-					Formats: []string{"0", "9"},
+					Formats: []string{"9", "0"},
 				},
 				Attributes: []sdp.Attribute{
-					{Key: "rtpmap", Value: "0 PCMU/8000"},
 					{Key: "rtpmap", Value: "9 G722/8000"},
+					{Key: "rtpmap", Value: "0 PCMU/8000"},
 				},
 			},
 			exp: &AudioConfig{
@@ -179,11 +179,11 @@ func TestSDPMediaAnswer(t *testing.T) {
 			name: "custom dtmf",
 			offer: sdp.MediaDescription{
 				MediaName: sdp.MediaName{
-					Formats: []string{"0", "9", "103"},
+					Formats: []string{"9", "0", "103"},
 				},
 				Attributes: []sdp.Attribute{
-					{Key: "rtpmap", Value: "0 PCMU/8000"},
 					{Key: "rtpmap", Value: "9 G722/8000"},
+					{Key: "rtpmap", Value: "0 PCMU/8000"},
 					{Key: "rtpmap", Value: "103 telephone-event/8000"},
 				},
 			},
@@ -270,13 +270,45 @@ func TestSDPMediaAnswer(t *testing.T) {
 				DTMFType: 101,
 			},
 		},
+		{
+			name: "changed order",
+			offer: sdp.MediaDescription{
+				MediaName: sdp.MediaName{
+					Formats: []string{"0", "9"},
+				},
+				Attributes: []sdp.Attribute{
+					{Key: "rtpmap", Value: "0 PCMU/8000"},
+					{Key: "rtpmap", Value: "9 G722/8000"},
+				},
+			},
+			exp: &AudioConfig{
+				Codec: getCodec(g711.ULawSDPName),
+				Type:  0,
+			},
+		},
+		{
+			name: "changed order g711",
+			offer: sdp.MediaDescription{
+				MediaName: sdp.MediaName{
+					Formats: []string{"8", "0"},
+				},
+				Attributes: []sdp.Attribute{
+					{Key: "rtpmap", Value: "8 PCMA/8000"},
+					{Key: "rtpmap", Value: "0 PCMU/8000"},
+				},
+			},
+			exp: &AudioConfig{
+				Codec: getCodec(g711.ALawSDPName),
+				Type:  8,
+			},
+		},
 	}
 	for _, c := range cases {
 		c := c
 		t.Run(c.name, func(t *testing.T) {
 			m, err := ParseMedia(&c.offer)
 			require.NoError(t, err)
-			got, err := SelectAudio(*m)
+			got, err := SelectAudio(*m, true)
 			if c.exp == nil {
 				require.Error(t, err)
 				return