feat(sdp): mirror media direction in answer

Answer now reflects the offer's direction per RFC 3264
(sendonly↔recvonly, inactive/sendrecv unchanged) instead of
always emitting sendrecv. AnswerMedia takes the direction as
a parameter.

Author: OxSever

sdp/offer.go

@@ -186,7 +186,7 @@ func OfferMedia(rtpListenerPort int, encrypted Encryption) (MediaDesc, *sdp.Medi
 }
 
 // AnswerMedia creates a new SDP media description for an answer.
-func AnswerMedia(rtpListenerPort int, audio *AudioConfig, crypt *srtp.Profile) *sdp.MediaDescription {
+func AnswerMedia(rtpListenerPort int, audio *AudioConfig, crypt *srtp.Profile, dir sdp.Direction) *sdp.MediaDescription {
 	// Static compiler check for frame duration hardcoded below.
 	var _ = [1]struct{}{}[20*time.Millisecond-rtp.DefFrameDur]
 
@@ -210,7 +210,7 @@ func AnswerMedia(rtpListenerPort int, audio *AudioConfig, crypt *srtp.Profile) *
 	}
 	attrs = append(attrs, []sdp.Attribute{
 		{Key: "ptime", Value: "20"},
-		{Key: "sendrecv"},
+		{Key: dir.String()},
 	}...)
 	return &sdp.MediaDescription{
 		MediaName: sdp.MediaName{
@@ -223,6 +223,21 @@ func AnswerMedia(rtpListenerPort int, audio *AudioConfig, crypt *srtp.Profile) *
 	}
 }
 
+func mirror(d sdp.Direction) sdp.Direction {
+	switch d {
+	case sdp.DirectionSendRecv:
+		return sdp.DirectionSendRecv
+	case sdp.DirectionSendOnly:
+		return sdp.DirectionRecvOnly
+	case sdp.DirectionRecvOnly:
+		return sdp.DirectionSendOnly
+	case sdp.DirectionInactive:
+		return sdp.DirectionInactive
+	default:
+		return sdp.DirectionSendRecv
+	}
+}
+
 type Description struct {
 	SDP  sdp.SessionDescription
 	Addr netip.AddrPort
@@ -306,7 +321,8 @@ func (d *Offer) Answer(publicIp netip.Addr, rtpListenerPort int, enc Encryption)
 		return nil, nil, ErrNoCommonCrypto
 	}
 
-	mediaDesc := AnswerMedia(rtpListenerPort, audio, sprof)
+	dir := mirror(d.MediaDesc.Direction)
+	mediaDesc := AnswerMedia(rtpListenerPort, audio, sprof, dir)
 	answer := sdp.SessionDescription{
 		Version: 0,
 		Origin: sdp.Origin{
@@ -402,7 +418,7 @@ func buildLocalSDP(sessionID uint64, local netip.AddrPort, audio *AudioConfig, s
 	}
 	addrStr := local.Addr().String()
 	portVal := int(local.Port())
-	mediaDesc := AnswerMedia(portVal, audio, sprof)
+	mediaDesc := AnswerMedia(portVal, audio, sprof, sdp.DirectionSendRecv)
 	s := &sdp.SessionDescription{
 		Version: 0,
 		Origin: sdp.Origin{

sdp/offer_test.go

@@ -879,3 +879,51 @@ a=inactive
 		})
 	}
 }
+
+func TestAnswerMirrorsDirection(t *testing.T) {
+	g := media.GlobalCodecs()
+
+	const offer = `v=0
+o=- 1 1 IN IP4 203.0.113.5
+s=LiveKit
+c=IN IP4 203.0.113.5
+t=0 0
+m=audio 10000 RTP/AVP 0
+a=rtpmap:0 PCMU/8000
+`
+
+	tests := []struct {
+		name string
+		dir  string        // direction in the offer
+		want sdp.Direction // expected mirrored direction in the answer
+	}{
+		{name: "sendonly mirrors to recvonly", dir: "a=sendonly\n", want: sdp.DirectionRecvOnly},
+		{name: "recvonly mirrors to sendonly", dir: "a=recvonly\n", want: sdp.DirectionSendOnly},
+		{name: "inactive stays inactive", dir: "a=inactive\n", want: sdp.DirectionInactive},
+		{name: "sendrecv stays sendrecv", dir: "a=sendrecv\n", want: sdp.DirectionSendRecv},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			o, err := ParseOfferWith(g, []byte(offer+test.dir))
+			require.NoError(t, err)
+
+			answer, _, err := o.Answer(netip.MustParseAddr("203.0.113.9"), 20000, EncryptionNone)
+			require.NoError(t, err)
+			
+			require.Equal(t, test.want, answerDirection(t, answer.SDP.MediaDescriptions[0]))
+		})
+	}
+}
+
+// answerDirection returns the direction attribute set on a media description.
+func answerDirection(t *testing.T, m *sdp.MediaDescription) sdp.Direction {
+	t.Helper()
+	for _, a := range m.Attributes {
+		if dir, err := sdp.NewDirection(a.Key); err == nil {
+			return dir
+		}
+	}
+	t.Fatal("no direction attribute in answer")
+	return 0
+}