sctp: implement SNAP / sctp-init

examples/warp/main.go

@@ -12,7 +12,7 @@
 	"fmt"
 	"net/http"
 
 	"github.com/pion/webrtc/v4"
 )
 
 func main() {
@@ -36,7 +36,7 @@
 	}
 }
 
 func setupOfferHandler(pc **webrtc.PeerConnection) {
 	http.HandleFunc("/offer", func(responseWriter http.ResponseWriter, r *http.Request) {
 		var offer webrtc.SessionDescription
 		if err := json.NewDecoder(r.Body).Decode(&offer); err != nil {
@@ -45,8 +45,13 @@
 			return
 		}
 
+		// Enable SNAP.
+		s := webrtc.SettingEngine{}
+		s.EnableSctpSnap(true)
+		api := webrtc.NewAPI(webrtc.WithSettingEngine(s))
+
 		var err error
-		*pc, err = webrtc.NewPeerConnection(webrtc.Configuration{
+		*pc, err = api.NewPeerConnection(webrtc.Configuration{
 			BundlePolicy: webrtc.BundlePolicyMaxBundle,
 		})
 		if err != nil {
@@ -66,7 +71,7 @@
 	})
 }
 
 func setupICECandidateHandler(pc *webrtc.PeerConnection) {
 	pc.OnICECandidate(func(c *webrtc.ICECandidate) {
 		if c != nil {
 			fmt.Printf("🌐 New ICE candidate: %s\n", c.Address)

peerconnection.go

@@ -1615,10 +1615,11 @@ func (pc *PeerConnection) startRTPSenders(currentTransceivers []*RTPTransceiver)
 }
 
 // Start SCTP subsystem.
-func (pc *PeerConnection) startSCTP(maxMessageSize uint32) {
+func (pc *PeerConnection) startSCTP(maxMessageSize uint32, remoteSctpInit []byte) {
 	// Start sctp
 	if err := pc.sctpTransport.Start(SCTPCapabilities{
 		MaxMessageSize: maxMessageSize,
+		SctpInit:       remoteSctpInit,
 	}); err != nil {
 		pc.log.Warnf("Failed to start SCTP: %s", err)
 		if err = pc.sctpTransport.Stop(); err != nil {
@@ -2791,7 +2792,8 @@ func (pc *PeerConnection) startRTP(
 
 	pc.startRTPReceivers(remoteDesc, currentTransceivers)
 	if d := haveDataChannel(remoteDesc); d != nil {
-		pc.startSCTP(getMaxMessageSize(d))
+		remoteSctpInit, _ := getSctpInit(d)
+		pc.startSCTP(getMaxMessageSize(d), remoteSctpInit)
 	}
 }
 
@@ -2824,6 +2826,11 @@ func (pc *PeerConnection) generateUnmatchedSDP(
 
 	// Needed for pc.sctpTransport.dataChannelsRequested
 	pc.sctpTransport.lock.Lock()
+
+	var localSctpInit []byte
+	if pc.sctpTransport.dataChannelsRequested != 0 && pc.api.settingEngine.sctp.enableSnap {
+		localSctpInit = pc.sctpTransport.GetSctpInit()
+	}
 	defer pc.sctpTransport.lock.Unlock()
 
 	if isPlanB { //nolint:nestif
@@ -2860,7 +2867,7 @@ func (pc *PeerConnection) generateUnmatchedSDP(
 		}
 
 		if pc.sctpTransport.dataChannelsRequested != 0 {
-			mediaSections = append(mediaSections, mediaSection{id: strconv.Itoa(len(mediaSections)), data: true})
+			mediaSections = append(mediaSections, mediaSection{id: strconv.Itoa(len(mediaSections)), data: true, sctpInit: localSctpInit})
 		}
 	}
 
@@ -2889,7 +2896,7 @@ func (pc *PeerConnection) generateUnmatchedSDP(
 }
 
 // generateMatchedSDP generates a SDP and takes the remote state into account
-// this is used everytime we have a RemoteDescription
+// This is used everytime we have a RemoteDescription
 //
 //nolint:gocognit,gocyclo,cyclop
 func (pc *PeerConnection) generateMatchedSDP(
@@ -2929,14 +2936,22 @@ func (pc *PeerConnection) generateMatchedSDP(
 
 	mediaSections := []mediaSection{}
 	alreadyHaveApplicationMediaSection := false
+	var localSctpInit []byte
 	for _, media := range remoteDescription.parsed.MediaDescriptions {
 		midValue := getMidValue(media)
 		if midValue == "" {
 			return nil, errPeerConnRemoteDescriptionWithoutMidValue
 		}
 
 		if media.MediaName.Media == mediaSectionApplication {
-			mediaSections = append(mediaSections, mediaSection{id: midValue, data: true})
+			init, _ := getSctpInit(media)
+			if init != nil && pc.api.settingEngine.sctp.enableSnap {
+				pc.sctpTransport.lock.Lock()
+				localSctpInit = pc.sctpTransport.GetSctpInit()
+				pc.sctpTransport.lock.Unlock()
+			}
+
+			mediaSections = append(mediaSections, mediaSection{id: midValue, data: true, sctpInit: localSctpInit})
 			alreadyHaveApplicationMediaSection = true
 
 			continue
@@ -3023,7 +3038,7 @@ func (pc *PeerConnection) generateMatchedSDP(
 			if detectedPlanB {
 				mediaSections = append(mediaSections, mediaSection{id: "data", data: true})
 			} else {
-				mediaSections = append(mediaSections, mediaSection{id: strconv.Itoa(len(mediaSections)), data: true})
+				mediaSections = append(mediaSections, mediaSection{id: strconv.Itoa(len(mediaSections)), data: true, sctpInit: localSctpInit})
 			}
 		}
 	} else if remoteDescription != nil {

peerconnection_test.go

@@ -807,6 +807,23 @@
 	closePairNow(t, pcOffer, pcAnswer)
 }
 
+func TestSctpSnap(t *testing.T) {
+	s := SettingEngine{}
+	s.EnableSnap(true)
+	api := NewAPI(WithSettingEngine(s))
+
+	offer, err := api.NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+	answer, err := api.NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+
+	peerConnectionsConnected := untilConnectionState(PeerConnectionStateConnected, offer, answer)
+	assert.NoError(t, signalPair(offer, answer))
+	peerConnectionsConnected.Wait()
+
+	closePairNow(t, offer, answer)
+}
+
 func TestICETrickleCapabilityString(t *testing.T) {
 	tests := []struct {
 		value    ICETrickleCapability

sctpcapabilities.go

@@ -6,4 +6,5 @@ package webrtc
 // SCTPCapabilities indicates the capabilities of the SCTPTransport.
 type SCTPCapabilities struct {
 	MaxMessageSize uint32 `json:"maxMessageSize"`
+	SctpInit       []byte `json:"sctpInit"`
 }

sctptransport.go

@@ -53,6 +53,8 @@
 	dataChannelsRequested uint32
 	dataChannelsAccepted  uint32
 
+	localSctpInit []byte
+
 	api *API
 	log logging.LeveledLogger
 }
@@ -107,6 +109,7 @@
 	if maxMessageSize == 0 {
 		maxMessageSize = sctpMaxMessageSizeUnsetValue
 	}
+	remoteSctpInit := capabilities.SctpInit
 
 	dtlsTransport := r.Transport()
 	if dtlsTransport == nil || dtlsTransport.conn == nil {
@@ -119,11 +122,14 @@
 		LoggerFactory:        r.api.settingEngine.LoggerFactory,
 		RTOMax:               float64(r.api.settingEngine.sctp.rtoMax) / float64(time.Millisecond),
 		BlockWrite:           r.api.settingEngine.detach.DataChannels && r.api.settingEngine.dataChannelBlockWrite,
-		MaxMessageSize:       maxMessageSize,
 		MTU:                  outboundMTU,
 		MinCwnd:              r.api.settingEngine.sctp.minCwnd,
 		FastRtxWnd:           r.api.settingEngine.sctp.fastRtxWnd,
 		CwndCAStep:           r.api.settingEngine.sctp.cwndCAStep,
+	}, sctp.SctpParameters{
+		MaxMessageSize: maxMessageSize,
+		LocalSctpInit:  r.localSctpInit,
+		RemoteSctpInit: remoteSctpInit,
 	})
 	if err != nil {
 		return err
@@ -456,3 +462,15 @@
 
 	return r.sctpAssociation.BufferedAmount()
 }
+
+// The caller should hold the lock.
+func (r *SCTPTransport) GetSctpInit() []byte {
+	if len(r.localSctpInit) == 0 {
+		r.localSctpInit, _ = sctp.GenerateOutOfBandToken(sctp.Config{
+			MaxReceiveBufferSize: r.api.settingEngine.sctp.maxReceiveBufferSize,
+			EnableZeroChecksum:   r.api.settingEngine.sctp.enableZeroChecksum,
+		})
+	}
+
+	return r.localSctpInit
+}

sdp.go

@@ -7,6 +7,7 @@
 package webrtc
 
 import (
+	"encoding/base64"
 	"errors"
 	"fmt"
 	"net/url"
@@ -369,6 +370,7 @@ func addDataMediaSection(
 	dtlsRole sdp.ConnectionRole,
 	iceGatheringState ICEGatheringState,
 	sctpMaxMessageSize uint32,
+	sctpInit []byte,
 ) error {
 	media := (&sdp.MediaDescription{
 		MediaName: sdp.MediaName{
@@ -388,10 +390,14 @@ func addDataMediaSection(
 		WithValueAttribute(sdp.AttrKeyConnectionSetup, dtlsRole.String()).
 		WithValueAttribute(sdp.AttrKeyMID, midValue).
 		WithPropertyAttribute(RTPTransceiverDirectionSendrecv.String()).
+		// TODO: do not hardcode this.
 		WithPropertyAttribute("sctp-port:5000").
 		WithValueAttribute("max-message-size", fmt.Sprintf("%d", sctpMaxMessageSize)).
 		WithICECredentials(iceParams.UsernameFragment, iceParams.Password)
 
+	if len(sctpInit) != 0 {
+		media = media.WithValueAttribute("sctp-init", base64.StdEncoding.EncodeToString(sctpInit))
+	}
 	for _, f := range dtlsFingerprints {
 		media = media.WithFingerprint(f.Algorithm, strings.ToUpper(f.Value))
 	}
@@ -669,6 +675,7 @@ type mediaSection struct {
 	id              string
 	transceivers    []*RTPTransceiver
 	data            bool
+	sctpInit        []byte
 	matchExtensions map[string]int
 	rids            []*simulcastRid
 }
@@ -742,6 +749,7 @@ func populateSDP(
 				connectionRole,
 				iceGatheringState,
 				sctpMaxMessageSize,
+				section.sctpInit,
 			); err != nil {
 				return nil, err
 			}
@@ -1210,3 +1218,20 @@ func getMaxMessageSize(desc *sdp.MediaDescription) uint32 {
 
 	return 0
 }
+
+func getSctpInit(desc *sdp.MediaDescription) ([]byte, error) {
+	var err error
+	var decoded []byte
+	for _, a := range desc.Attributes {
+		if strings.TrimSpace(a.Key) == "sctp-init" {
+			decoded, err = base64.StdEncoding.DecodeString(a.Value)
+			if err == nil {
+				return decoded, nil
+			}
+
+			return nil, err
+		}
+	}
+
+	return nil, nil
+}

settingengine.go

@@ -91,6 +91,7 @@ type SettingEngine struct {
 		minCwnd              uint32
 		fastRtxWnd           uint32
 		cwndCAStep           uint32
+		enableSnap           bool
 	}
 	sdpMediaLevelFingerprints                 bool
 	answeringDTLSRole                         DTLSRole
@@ -590,6 +591,11 @@ func (e *SettingEngine) EnableSCTPZeroChecksum(isEnabled bool) {
 	e.sctp.enableZeroChecksum = isEnabled
 }
 
+// EnableSctpSnap enables the use of the SCTP SNAP connect optimization.
+func (e *SettingEngine) EnableSctpSnap(isEnabled bool) {
+	e.sctp.enableSnap = isEnabled
+}
+
 // SetSCTPMaxMessageSize sets the largest message we are willing to accept.
 // Leave this 0 for the default max message size.
 func (e *SettingEngine) SetSCTPMaxMessageSize(maxMessageSize uint32) {