Add support for sending abs-capture-time RTP extension

src/handlers/Chrome74.ts

@@ -364,7 +364,33 @@ export class Chrome74 extends HandlerInterface {
 		});
 		let offer = await this._pc.createOffer();
 		let localSdpObject = sdpTransform.parse(offer.sdp);
-		let offerMediaObject;
+		let offerMediaObject = localSdpObject.media[mediaSectionIdx.idx];
+
+		// May force abs-capture-time RTP extension negotiation.
+		{
+			const exten = sdpCommonUtils.addRtpExtensionToMediaObject(
+				{
+					mediaObject : offerMediaObject,
+					uri         : 'http://www.webrtc.org/experiments/rtp-hdrext/abs-capture-time'
+				});
+
+			if (exten)
+			{
+				offer = { type: 'offer', sdp: sdpTransform.write(localSdpObject) };
+
+				sdpCommonUtils.addRtpExtensionToRtpParameters(
+					{
+						rtpParameters : sendingRtpParameters,
+						extension     : exten
+					});
+
+				sdpCommonUtils.addRtpExtensionToRtpParameters(
+					{
+						rtpParameters : sendingRemoteRtpParameters,
+						extension     : exten
+					});
+			}
+		}
 
 		if (!this._transportReady) {
 			await this.setupTransport({

src/handlers/ReactNativeUnifiedPlan.ts

@@ -377,7 +377,33 @@ export class ReactNativeUnifiedPlan extends HandlerInterface {
 
 		let offer = await this._pc.createOffer();
 		let localSdpObject = sdpTransform.parse(offer.sdp);
-		let offerMediaObject;
+		let offerMediaObject = localSdpObject.media[mediaSectionIdx.idx];
+
+		// May force abs-capture-time RTP extension negotiation.
+		{
+			const exten = sdpCommonUtils.addRtpExtensionToMediaObject(
+				{
+					mediaObject : offerMediaObject,
+					uri         : 'http://www.webrtc.org/experiments/rtp-hdrext/abs-capture-time'
+				});
+
+			if (exten)
+			{
+				offer = { type: 'offer', sdp: sdpTransform.write(localSdpObject) };
+
+				sdpCommonUtils.addRtpExtensionToRtpParameters(
+					{
+						rtpParameters : sendingRtpParameters,
+						extension     : exten
+					});
+
+				sdpCommonUtils.addRtpExtensionToRtpParameters(
+					{
+						rtpParameters : sendingRemoteRtpParameters,
+						extension     : exten
+					});
+			}
+		}
 
 		if (!this._transportReady) {
 			await this.setupTransport({

src/handlers/sdp/commonUtils.ts

@@ -4,8 +4,10 @@ import {
 	RtpCapabilities,
 	RtpCodecCapability,
 	RtpHeaderExtension,
+	RtpHeaderExtensionParameters,
 	RtpParameters,
 	RtcpFeedback,
+	RtpHeaderExtensionUri,
 } from '../../RtpParameters';
 
 /**
@@ -285,3 +287,88 @@ export function applyCodecParameters({
 		}
 	}
 }
+
+/**
+ * Adds the given RTP extension to the given SDP media object and returns a
+ * RtpHeaderExtensionParameters object.
+ * If the extension is already present, this function doesn't add anything and
+ * doesn't return anything.
+ */
+export function addRtpExtensionToMediaObject(
+	{ mediaObject, uri }:
+	{ mediaObject: any; uri: RtpHeaderExtensionUri }
+): RtpHeaderExtensionParameters | undefined
+{
+	if (!Array.isArray(mediaObject.ext))
+	{
+		mediaObject.ext = [];
+	}
+
+	let id = 1;
+
+	for (const exten of mediaObject.ext)
+	{
+		// If extension uri is already present, don't do anything.
+		if (exten.uri === uri)
+		{
+			id = 0;
+
+			break;
+		}
+
+		if (exten.value >= id)
+			id = exten.value + 1;
+	}
+
+	if (id > 0)
+	{
+		mediaObject.ext.push({ value: id, uri });
+
+		// NOTE: No support for encrypt/parameters fields.
+		return { uri,	id };
+	}
+}
+
+/**
+ * Adds the given RTP extension to the given RTP parameters.
+ * If the extension is already present (with same id), this function doesn't
+ * add anything. If the extension is present with a different id, then existing
+ * one is removed and the new one is added.
+ */
+export function addRtpExtensionToRtpParameters(
+	{
+		rtpParameters,
+		extension
+	}:
+	{
+		rtpParameters: RtpParameters;
+		extension: RtpHeaderExtensionParameters;
+	}
+): void
+{
+	if (!Array.isArray(rtpParameters.headerExtensions))
+	{
+		rtpParameters.headerExtensions = [];
+	}
+
+	let replaced = false;
+
+	for (const exten of rtpParameters.headerExtensions)
+	{
+		if (exten.uri === extension.uri && exten.id === extension.id)
+		{
+			return;
+		}
+		else if (exten.uri === extension.uri && exten.id !== extension.id)
+		{
+			exten.id = extension.id;
+
+			replaced = true;
+		}
+	}
+
+	if (!replaced)
+	{
+		rtpParameters.headerExtensions.push(extension);
+	}
+}