ljm0/modules/sdp/RtxModifier.js

import { getLogger } from '@jitsi/logger';

import { MediaDirection } from '../../service/RTC/MediaDirection';
import { MediaType } from '../../service/RTC/MediaType';
import { SSRC_GROUP_SEMANTICS } from '../../service/RTC/StandardVideoQualitySettings';

import SDPUtil from './SDPUtil';
import { SdpTransformWrap, parseSecondarySSRC } from './SdpTransformUtil';

const logger = getLogger('modules/sdp/RtxModifier');

/**
 * Begin helper functions
 */
/**
 * Updates or inserts the appropriate rtx information for primarySsrc with
 *  the given rtxSsrc.  If no rtx ssrc for primarySsrc currently exists, it will
 *  add the appropriate ssrc and ssrc group lines.  If primarySsrc already has
 *  an rtx ssrc, the appropriate ssrc and group lines will be updated
 * @param {MLineWrap} mLine
 * @param {object} primarySsrcInfo the info (ssrc, msid & cname) for the
 *  primary ssrc
 * @param {number} rtxSsrc the rtx ssrc to associate with the primary ssrc
 */
function updateAssociatedRtxStream(mLine, primarySsrcInfo, rtxSsrc) {
    const primarySsrc = primarySsrcInfo.id;
    const primarySsrcMsid = primarySsrcInfo.msid;
    const primarySsrcCname = primarySsrcInfo.cname;

    const previousRtxSSRC = mLine.getRtxSSRC(primarySsrc);

    if (previousRtxSSRC === rtxSsrc) {
        return;
    }
    if (previousRtxSSRC) {
        // Stream already had an rtx ssrc that is different than the one given,
        //  remove all trace of the old one
        mLine.removeSSRC(previousRtxSSRC);
        mLine.removeGroupsWithSSRC(previousRtxSSRC);
    }
    mLine.addSSRCAttribute({
        id: rtxSsrc,
        attribute: 'cname',
        value: primarySsrcCname
    });
    primarySsrcMsid && mLine.addSSRCAttribute({
        id: rtxSsrc,
        attribute: 'msid',
        value: primarySsrcMsid
    });
    mLine.addSSRCGroup({
        semantics: SSRC_GROUP_SEMANTICS.FID,
        ssrcs: `${primarySsrc} ${rtxSsrc}`
    });
}

/**
 * End helper functions
 */

/**
 * Adds any missing RTX streams for video streams
 *  and makes sure that they remain consistent
 */
export default class RtxModifier {
    /**
     * Constructor
     */
    constructor() {
        /**
         * Map of video ssrc to corresponding RTX
         *  ssrc
         */
        this.correspondingRtxSsrcs = new Map();
    }

    /**
     * Clear the cached map of primary video ssrcs to
     *  their corresponding rtx ssrcs so that they will
     *  not be used for the next call to modifyRtxSsrcs
     */
    clearSsrcCache() {
        this.correspondingRtxSsrcs.clear();
    }

    /**
     * Explicitly set the primary video ssrc -> rtx ssrc
     *  mapping to be used in modifyRtxSsrcs
     * @param {Map} ssrcMapping a mapping of primary video
     *  ssrcs to their corresponding rtx ssrcs
     */
    setSsrcCache(ssrcMapping) {
        logger.debug('Setting ssrc cache to ', ssrcMapping);
        this.correspondingRtxSsrcs = ssrcMapping;
    }

    /**
     * Adds RTX ssrcs for any video ssrcs that don't already have them.  If the video ssrc has been seen before, and
     * already had an RTX ssrc generated, the same RTX ssrc will be used again.
     *
     * @param {string} sdpStr sdp in raw string format
     * @returns {string} The modified sdp in raw string format.
     */
    modifyRtxSsrcs(sdpStr) {
        let modified = false;
        const sdpTransformer = new SdpTransformWrap(sdpStr);
        const videoMLines = sdpTransformer.selectMedia(MediaType.VIDEO);

        if (!videoMLines?.length) {
            logger.debug(`No 'video' media found in the sdp: ${sdpStr}`);

            return sdpStr;
        }

        for (const videoMLine of videoMLines) {
            if (this.modifyRtxSsrcs2(videoMLine)) {
                modified = true;
            }
        }

        return modified ? sdpTransformer.toRawSDP() : sdpStr;
    }

    /**
     * Does the same thing as {@link modifyRtxSsrcs}, but takes the {@link MLineWrap} instance wrapping video media as
     * an argument.
     * @param {MLineWrap} videoMLine
     * @return {boolean} <tt>true</tt> if the SDP wrapped by {@link SdpTransformWrap} has been modified or
     * <tt>false</tt> otherwise.
     */
    modifyRtxSsrcs2(videoMLine) {
        if (videoMLine.direction === MediaDirection.RECVONLY) {
            return false;
        }
        if (videoMLine.getSSRCCount() < 1) {
            return false;
        }
        const primaryVideoSsrcs = videoMLine.getPrimaryVideoSSRCs();

        for (const ssrc of primaryVideoSsrcs) {
            const msid = videoMLine.getSSRCAttrValue(ssrc, 'msid');
            const cname = videoMLine.getSSRCAttrValue(ssrc, 'cname');
            let correspondingRtxSsrc = this.correspondingRtxSsrcs.get(ssrc);

            if (!correspondingRtxSsrc) {
                // If there's one in the sdp already for it, we'll just set
                //  that as the corresponding one
                const previousAssociatedRtxStream = videoMLine.getRtxSSRC(ssrc);

                if (previousAssociatedRtxStream) {
                    correspondingRtxSsrc = previousAssociatedRtxStream;
                } else {
                    correspondingRtxSsrc = SDPUtil.generateSsrc();
                }
                this.correspondingRtxSsrcs.set(ssrc, correspondingRtxSsrc);
            }
            updateAssociatedRtxStream(
                videoMLine,
                {
                    id: ssrc,
                    cname,
                    msid
                },
                correspondingRtxSsrc);
        }

        // FIXME we're not looking into much details whether the SDP has been
        // modified or not once the precondition requirements are met.
        return true;
    }

    /**
     * Strip all rtx streams from the given sdp.
     *
     * @param {string} sdpStr sdp in raw string format
     * @returns {string} sdp string with all rtx streams stripped
     */
    stripRtx(sdpStr) {
        const sdpTransformer = new SdpTransformWrap(sdpStr);
        const videoMLines = sdpTransformer.selectMedia(MediaType.VIDEO);

        if (!videoMLines?.length) {
            logger.debug(`No 'video' media found in the sdp: ${sdpStr}`);

            return sdpStr;
        }

        for (const videoMLine of videoMLines) {
            if (videoMLine.direction !== MediaDirection.RECVONLY
                && videoMLine.getSSRCCount()
                && videoMLine.containsAnySSRCGroups()) {
                const fidGroups = videoMLine.findGroups(SSRC_GROUP_SEMANTICS.FID);

                // Remove the fid groups from the mline
                videoMLine.removeGroupsBySemantics(SSRC_GROUP_SEMANTICS.FID);

                // Get the rtx ssrcs and remove them from the mline
                for (const fidGroup of fidGroups) {
                    const rtxSsrc = parseSecondarySSRC(fidGroup);

                    videoMLine.removeSSRC(rtxSsrc);
                }
            }
        }

        return sdpTransformer.toRawSDP();
    }
}