sdp.js

/* eslint-env node */
'use strict';

// SDP helpers.
const SDPUtils = {};

// Generate an alphanumeric identifier for cname or mids.
// TODO: use UUIDs instead? https://gist.github.com/jed/982883
SDPUtils.generateIdentifier = function() {
  return Math.random().toString(36).substr(2, 10);
};

// The RTCP CNAME used by all peerconnections from the same JS.
SDPUtils.localCName = SDPUtils.generateIdentifier();

// Splits SDP into lines, dealing with both CRLF and LF.
SDPUtils.splitLines = function(blob) {
  return blob.trim().split('\n').map(line => line.trim());
};
// Splits SDP into sessionpart and mediasections. Ensures CRLF.
SDPUtils.splitSections = function(blob) {
  const parts = blob.split('\nm=');
  return parts.map((part, index) => (index > 0 ?
    'm=' + part : part).trim() + '\r\n');
};

// Returns the session description.
SDPUtils.getDescription = function(blob) {
  const sections = SDPUtils.splitSections(blob);
  return sections && sections[0];
};

// Returns the individual media sections.
SDPUtils.getMediaSections = function(blob) {
  const sections = SDPUtils.splitSections(blob);
  sections.shift();
  return sections;
};

// Returns lines that start with a certain prefix.
SDPUtils.matchPrefix = function(blob, prefix) {
  return SDPUtils.splitLines(blob).filter(line => line.indexOf(prefix) === 0);
};

// Parses an ICE candidate line. Sample input:
// candidate:702786350 2 udp 41819902 8.8.8.8 60769 typ relay raddr 8.8.8.8
// rport 55996"
// Input can be prefixed with a=.
SDPUtils.parseCandidate = function(line) {
  let parts;
  // Parse both variants.
  if (line.indexOf('a=candidate:') === 0) {
    parts = line.substring(12).split(' ');
  } else {
    parts = line.substring(10).split(' ');
  }

  const candidate = {
    foundation: parts[0],
    component: {1: 'rtp', 2: 'rtcp'}[parts[1]] || parts[1],
    protocol: parts[2].toLowerCase(),
    priority: parseInt(parts[3], 10),
    ip: parts[4],
    address: parts[4], // address is an alias for ip.
    port: parseInt(parts[5], 10),
    // skip parts[6] == 'typ'
    type: parts[7],
  };

  for (let i = 8; i < parts.length; i += 2) {
    switch (parts[i]) {
      case 'raddr':
        candidate.relatedAddress = parts[i + 1];
        break;
      case 'rport':
        candidate.relatedPort = parseInt(parts[i + 1], 10);
        break;
      case 'tcptype':
        candidate.tcpType = parts[i + 1];
        break;
      case 'ufrag':
        candidate.ufrag = parts[i + 1]; // for backward compatibility.
        candidate.usernameFragment = parts[i + 1];
        break;
      default: // extension handling, in particular ufrag. Don't overwrite.
        if (candidate[parts[i]] === undefined) {
          candidate[parts[i]] = parts[i + 1];
        }
        break;
    }
  }
  return candidate;
};

// Translates a candidate object into SDP candidate attribute.
// This does not include the a= prefix!
SDPUtils.writeCandidate = function(candidate) {
  const sdp = [];
  sdp.push(candidate.foundation);

  const component = candidate.component;
  if (component === 'rtp') {
    sdp.push(1);
  } else if (component === 'rtcp') {
    sdp.push(2);
  } else {
    sdp.push(component);
  }
  sdp.push(candidate.protocol.toUpperCase());
  sdp.push(candidate.priority);
  sdp.push(candidate.address || candidate.ip);
  sdp.push(candidate.port);

  const type = candidate.type;
  sdp.push('typ');
  sdp.push(type);
  if (type !== 'host' && candidate.relatedAddress &&
      candidate.relatedPort) {
    sdp.push('raddr');
    sdp.push(candidate.relatedAddress);
    sdp.push('rport');
    sdp.push(candidate.relatedPort);
  }
  if (candidate.tcpType && candidate.protocol.toLowerCase() === 'tcp') {
    sdp.push('tcptype');
    sdp.push(candidate.tcpType);
  }
  if (candidate.usernameFragment || candidate.ufrag) {
    sdp.push('ufrag');
    sdp.push(candidate.usernameFragment || candidate.ufrag);
  }
  return 'candidate:' + sdp.join(' ');
};

// Parses an ice-options line, returns an array of option tags.
// Sample input:
// a=ice-options:foo bar
SDPUtils.parseIceOptions = function(line) {
  return line.substr(14).split(' ');
};

// Parses a rtpmap line, returns RTCRtpCoddecParameters. Sample input:
// a=rtpmap:111 opus/48000/2
SDPUtils.parseRtpMap = function(line) {
  let parts = line.substr(9).split(' ');
  const parsed = {
    payloadType: parseInt(parts.shift(), 10), // was: id
  };

  parts = parts[0].split('/');

  parsed.name = parts[0];
  parsed.clockRate = parseInt(parts[1], 10); // was: clockrate
  parsed.channels = parts.length === 3 ? parseInt(parts[2], 10) : 1;
  // legacy alias, got renamed back to channels in ORTC.
  parsed.numChannels = parsed.channels;
  return parsed;
};

// Generates a rtpmap line from RTCRtpCodecCapability or
// RTCRtpCodecParameters.
SDPUtils.writeRtpMap = function(codec) {
  let pt = codec.payloadType;
  if (codec.preferredPayloadType !== undefined) {
    pt = codec.preferredPayloadType;
  }
  const channels = codec.channels || codec.numChannels || 1;
  return 'a=rtpmap:' + pt + ' ' + codec.name + '/' + codec.clockRate +
      (channels !== 1 ? '/' + channels : '') + '\r\n';
};

// Parses a extmap line (headerextension from RFC 5285). Sample input:
// a=extmap:2 urn:ietf:params:rtp-hdrext:toffset
// a=extmap:2/sendonly urn:ietf:params:rtp-hdrext:toffset
SDPUtils.parseExtmap = function(line) {
  const parts = line.substr(9).split(' ');
  return {
    id: parseInt(parts[0], 10),
    direction: parts[0].indexOf('/') > 0 ? parts[0].split('/')[1] : 'sendrecv',
    uri: parts[1],
  };
};

// Generates an extmap line from RTCRtpHeaderExtensionParameters or
// RTCRtpHeaderExtension.
SDPUtils.writeExtmap = function(headerExtension) {
  return 'a=extmap:' + (headerExtension.id || headerExtension.preferredId) +
      (headerExtension.direction && headerExtension.direction !== 'sendrecv'
        ? '/' + headerExtension.direction
        : '') +
      ' ' + headerExtension.uri + '\r\n';
};

// Parses a fmtp line, returns dictionary. Sample input:
// a=fmtp:96 vbr=on;cng=on
// Also deals with vbr=on; cng=on
SDPUtils.parseFmtp = function(line) {
  const parsed = {};
  let kv;
  const parts = line.substr(line.indexOf(' ') + 1).split(';');
  for (let j = 0; j < parts.length; j++) {
    kv = parts[j].trim().split('=');
    parsed[kv[0].trim()] = kv[1];
  }
  return parsed;
};

// Generates a fmtp line from RTCRtpCodecCapability or RTCRtpCodecParameters.
SDPUtils.writeFmtp = function(codec) {
  let line = '';
  let pt = codec.payloadType;
  if (codec.preferredPayloadType !== undefined) {
    pt = codec.preferredPayloadType;
  }
  if (codec.parameters && Object.keys(codec.parameters).length) {
    const params = [];
    Object.keys(codec.parameters).forEach(param => {
      if (codec.parameters[param]) {
        params.push(param + '=' + codec.parameters[param]);
      } else {
        params.push(param);
      }
    });
    line += 'a=fmtp:' + pt + ' ' + params.join(';') + '\r\n';
  }
  return line;
};

// Parses a rtcp-fb line, returns RTCPRtcpFeedback object. Sample input:
// a=rtcp-fb:98 nack rpsi
SDPUtils.parseRtcpFb = function(line) {
  const parts = line.substr(line.indexOf(' ') + 1).split(' ');
  return {
    type: parts.shift(),
    parameter: parts.join(' '),
  };
};

// Generate a=rtcp-fb lines from RTCRtpCodecCapability or RTCRtpCodecParameters.
SDPUtils.writeRtcpFb = function(codec) {
  let lines = '';
  let pt = codec.payloadType;
  if (codec.preferredPayloadType !== undefined) {
    pt = codec.preferredPayloadType;
  }
  if (codec.rtcpFeedback && codec.rtcpFeedback.length) {
    // FIXME: special handling for trr-int?
    codec.rtcpFeedback.forEach(fb => {
      lines += 'a=rtcp-fb:' + pt + ' ' + fb.type +
      (fb.parameter && fb.parameter.length ? ' ' + fb.parameter : '') +
          '\r\n';
    });
  }
  return lines;
};

// Parses a RFC 5576 ssrc media attribute. Sample input:
// a=ssrc:3735928559 cname:something
SDPUtils.parseSsrcMedia = function(line) {
  const sp = line.indexOf(' ');
  const parts = {
    ssrc: parseInt(line.substr(7, sp - 7), 10),
  };
  const colon = line.indexOf(':', sp);
  if (colon > -1) {
    parts.attribute = line.substr(sp + 1, colon - sp - 1);
    parts.value = line.substr(colon + 1);
  } else {
    parts.attribute = line.substr(sp + 1);
  }
  return parts;
};

// Parse a ssrc-group line (see RFC 5576). Sample input:
// a=ssrc-group:semantics 12 34
SDPUtils.parseSsrcGroup = function(line) {
  const parts = line.substr(13).split(' ');
  return {
    semantics: parts.shift(),
    ssrcs: parts.map(ssrc => parseInt(ssrc, 10)),
  };
};

// Extracts the MID (RFC 5888) from a media section.
// Returns the MID or undefined if no mid line was found.
SDPUtils.getMid = function(mediaSection) {
  const mid = SDPUtils.matchPrefix(mediaSection, 'a=mid:')[0];
  if (mid) {
    return mid.substr(6);
  }
};

// Parses a fingerprint line for DTLS-SRTP.
SDPUtils.parseFingerprint = function(line) {
  const parts = line.substr(14).split(' ');
  return {
    algorithm: parts[0].toLowerCase(), // algorithm is case-sensitive in Edge.
    value: parts[1].toUpperCase(), // the definition is upper-case in RFC 4572.
  };
};

// Extracts DTLS parameters from SDP media section or sessionpart.
// FIXME: for consistency with other functions this should only
//   get the fingerprint line as input. See also getIceParameters.
SDPUtils.getDtlsParameters = function(mediaSection, sessionpart) {
  const lines = SDPUtils.matchPrefix(mediaSection + sessionpart,
    'a=fingerprint:');
  // Note: a=setup line is ignored since we use the 'auto' role in Edge.
  return {
    role: 'auto',
    fingerprints: lines.map(SDPUtils.parseFingerprint),
  };
};

// Serializes DTLS parameters to SDP.
SDPUtils.writeDtlsParameters = function(params, setupType) {
  let sdp = 'a=setup:' + setupType + '\r\n';
  params.fingerprints.forEach(fp => {
    sdp += 'a=fingerprint:' + fp.algorithm + ' ' + fp.value + '\r\n';
  });
  return sdp;
};

// Parses a=crypto lines into
//   https://rawgit.com/aboba/edgertc/master/msortc-rs4.html#dictionary-rtcsrtpsdesparameters-members
SDPUtils.parseCryptoLine = function(line) {
  const parts = line.substr(9).split(' ');
  return {
    tag: parseInt(parts[0], 10),
    cryptoSuite: parts[1],
    keyParams: parts[2],
    sessionParams: parts.slice(3),
  };
};

SDPUtils.writeCryptoLine = function(parameters) {
  return 'a=crypto:' + parameters.tag + ' ' +
    parameters.cryptoSuite + ' ' +
    (typeof parameters.keyParams === 'object'
      ? SDPUtils.writeCryptoKeyParams(parameters.keyParams)
      : parameters.keyParams) +
    (parameters.sessionParams ? ' ' + parameters.sessionParams.join(' ') : '') +
    '\r\n';
};

// Parses the crypto key parameters into
//   https://rawgit.com/aboba/edgertc/master/msortc-rs4.html#rtcsrtpkeyparam*
SDPUtils.parseCryptoKeyParams = function(keyParams) {
  if (keyParams.indexOf('inline:') !== 0) {
    return null;
  }
  const parts = keyParams.substr(7).split('|');
  return {
    keyMethod: 'inline',
    keySalt: parts[0],
    lifeTime: parts[1],
    mkiValue: parts[2] ? parts[2].split(':')[0] : undefined,
    mkiLength: parts[2] ? parts[2].split(':')[1] : undefined,
  };
};

SDPUtils.writeCryptoKeyParams = function(keyParams) {
  return keyParams.keyMethod + ':'
    + keyParams.keySalt +
    (keyParams.lifeTime ? '|' + keyParams.lifeTime : '') +
    (keyParams.mkiValue && keyParams.mkiLength
      ? '|' + keyParams.mkiValue + ':' + keyParams.mkiLength
      : '');
};

// Extracts all SDES parameters.
SDPUtils.getCryptoParameters = function(mediaSection, sessionpart) {
  const lines = SDPUtils.matchPrefix(mediaSection + sessionpart,
    'a=crypto:');
  return lines.map(SDPUtils.parseCryptoLine);
};

// Parses ICE information from SDP media section or sessionpart.
// FIXME: for consistency with other functions this should only
//   get the ice-ufrag and ice-pwd lines as input.
SDPUtils.getIceParameters = function(mediaSection, sessionpart) {
  const ufrag = SDPUtils.matchPrefix(mediaSection + sessionpart,
    'a=ice-ufrag:')[0];
  const pwd = SDPUtils.matchPrefix(mediaSection + sessionpart,
    'a=ice-pwd:')[0];
  if (!(ufrag && pwd)) {
    return null;
  }
  return {
    usernameFragment: ufrag.substr(12),
    password: pwd.substr(10),
  };
};

// Serializes ICE parameters to SDP.
SDPUtils.writeIceParameters = function(params) {
  let sdp = 'a=ice-ufrag:' + params.usernameFragment + '\r\n' +
      'a=ice-pwd:' + params.password + '\r\n';
  if (params.iceLite) {
    sdp += 'a=ice-lite\r\n';
  }
  return sdp;
};

// Parses the SDP media section and returns RTCRtpParameters.
SDPUtils.parseRtpParameters = function(mediaSection) {
  const description = {
    codecs: [],
    headerExtensions: [],
    fecMechanisms: [],
    rtcp: [],
  };
  const lines = SDPUtils.splitLines(mediaSection);
  const mline = lines[0].split(' ');
  for (let i = 3; i < mline.length; i++) { // find all codecs from mline[3..]
    const pt = mline[i];
    const rtpmapline = SDPUtils.matchPrefix(
      mediaSection, 'a=rtpmap:' + pt + ' ')[0];
    if (rtpmapline) {
      const codec = SDPUtils.parseRtpMap(rtpmapline);
      const fmtps = SDPUtils.matchPrefix(
        mediaSection, 'a=fmtp:' + pt + ' ');
      // Only the first a=fmtp:<pt> is considered.
      codec.parameters = fmtps.length ? SDPUtils.parseFmtp(fmtps[0]) : {};
      codec.rtcpFeedback = SDPUtils.matchPrefix(
        mediaSection, 'a=rtcp-fb:' + pt + ' ')
        .map(SDPUtils.parseRtcpFb);
      description.codecs.push(codec);
      // parse FEC mechanisms from rtpmap lines.
      switch (codec.name.toUpperCase()) {
        case 'RED':
        case 'ULPFEC':
          description.fecMechanisms.push(codec.name.toUpperCase());
          break;
        default: // only RED and ULPFEC are recognized as FEC mechanisms.
          break;
      }
    }
  }
  SDPUtils.matchPrefix(mediaSection, 'a=extmap:').forEach(line => {
    description.headerExtensions.push(SDPUtils.parseExtmap(line));
  });
  // FIXME: parse rtcp.
  return description;
};

// Generates parts of the SDP media section describing the capabilities /
// parameters.
SDPUtils.writeRtpDescription = function(kind, caps) {
  let sdp = '';

  // Build the mline.
  sdp += 'm=' + kind + ' ';
  sdp += caps.codecs.length > 0 ? '9' : '0'; // reject if no codecs.
  sdp += ' UDP/TLS/RTP/SAVPF ';
  sdp += caps.codecs.map(codec => {
    if (codec.preferredPayloadType !== undefined) {
      return codec.preferredPayloadType;
    }
    return codec.payloadType;
  }).join(' ') + '\r\n';

  sdp += 'c=IN IP4 0.0.0.0\r\n';
  sdp += 'a=rtcp:9 IN IP4 0.0.0.0\r\n';

  // Add a=rtpmap lines for each codec. Also fmtp and rtcp-fb.
  caps.codecs.forEach(codec => {
    sdp += SDPUtils.writeRtpMap(codec);
    sdp += SDPUtils.writeFmtp(codec);
    sdp += SDPUtils.writeRtcpFb(codec);
  });
  let maxptime = 0;
  caps.codecs.forEach(codec => {
    if (codec.maxptime > maxptime) {
      maxptime = codec.maxptime;
    }
  });
  if (maxptime > 0) {
    sdp += 'a=maxptime:' + maxptime + '\r\n';
  }

  if (caps.headerExtensions) {
    caps.headerExtensions.forEach(extension => {
      sdp += SDPUtils.writeExtmap(extension);
    });
  }
  // FIXME: write fecMechanisms.
  return sdp;
};

// Parses the SDP media section and returns an array of
// RTCRtpEncodingParameters.
SDPUtils.parseRtpEncodingParameters = function(mediaSection) {
  const encodingParameters = [];
  const description = SDPUtils.parseRtpParameters(mediaSection);
  const hasRed = description.fecMechanisms.indexOf('RED') !== -1;
  const hasUlpfec = description.fecMechanisms.indexOf('ULPFEC') !== -1;

  // filter a=ssrc:... cname:, ignore PlanB-msid
  const ssrcs = SDPUtils.matchPrefix(mediaSection, 'a=ssrc:')
    .map(line => SDPUtils.parseSsrcMedia(line))
    .filter(parts => parts.attribute === 'cname');
  const primarySsrc = ssrcs.length > 0 && ssrcs[0].ssrc;
  let secondarySsrc;

  const flows = SDPUtils.matchPrefix(mediaSection, 'a=ssrc-group:FID')
    .map(line => {
      const parts = line.substr(17).split(' ');
      return parts.map(part => parseInt(part, 10));
    });
  if (flows.length > 0 && flows[0].length > 1 && flows[0][0] === primarySsrc) {
    secondarySsrc = flows[0][1];
  }

  description.codecs.forEach(codec => {
    if (codec.name.toUpperCase() === 'RTX' && codec.parameters.apt) {
      let encParam = {
        ssrc: primarySsrc,
        codecPayloadType: parseInt(codec.parameters.apt, 10),
      };
      if (primarySsrc && secondarySsrc) {
        encParam.rtx = {ssrc: secondarySsrc};
      }
      encodingParameters.push(encParam);
      if (hasRed) {
        encParam = JSON.parse(JSON.stringify(encParam));
        encParam.fec = {
          ssrc: primarySsrc,
          mechanism: hasUlpfec ? 'red+ulpfec' : 'red',
        };
        encodingParameters.push(encParam);
      }
    }
  });
  if (encodingParameters.length === 0 && primarySsrc) {
    encodingParameters.push({
      ssrc: primarySsrc,
    });
  }

  // we support both b=AS and b=TIAS but interpret AS as TIAS.
  let bandwidth = SDPUtils.matchPrefix(mediaSection, 'b=');
  if (bandwidth.length) {
    if (bandwidth[0].indexOf('b=TIAS:') === 0) {
      bandwidth = parseInt(bandwidth[0].substr(7), 10);
    } else if (bandwidth[0].indexOf('b=AS:') === 0) {
      // use formula from JSEP to convert b=AS to TIAS value.
      bandwidth = parseInt(bandwidth[0].substr(5), 10) * 1000 * 0.95
          - (50 * 40 * 8);
    } else {
      bandwidth = undefined;
    }
    encodingParameters.forEach(params => {
      params.maxBitrate = bandwidth;
    });
  }
  return encodingParameters;
};

// parses http://draft.ortc.org/#rtcrtcpparameters*
SDPUtils.parseRtcpParameters = function(mediaSection) {
  const rtcpParameters = {};

  // Gets the first SSRC. Note that with RTX there might be multiple
  // SSRCs.
  const remoteSsrc = SDPUtils.matchPrefix(mediaSection, 'a=ssrc:')
    .map(line => SDPUtils.parseSsrcMedia(line))
    .filter(obj => obj.attribute === 'cname')[0];
  if (remoteSsrc) {
    rtcpParameters.cname = remoteSsrc.value;
    rtcpParameters.ssrc = remoteSsrc.ssrc;
  }

  // Edge uses the compound attribute instead of reducedSize
  // compound is !reducedSize
  const rsize = SDPUtils.matchPrefix(mediaSection, 'a=rtcp-rsize');
  rtcpParameters.reducedSize = rsize.length > 0;
  rtcpParameters.compound = rsize.length === 0;

  // parses the rtcp-mux attrіbute.
  // Note that Edge does not support unmuxed RTCP.
  const mux = SDPUtils.matchPrefix(mediaSection, 'a=rtcp-mux');
  rtcpParameters.mux = mux.length > 0;

  return rtcpParameters;
};

SDPUtils.writeRtcpParameters = function(rtcpParameters) {
  let sdp = '';
  if (rtcpParameters.reducedSize) {
    sdp += 'a=rtcp-rsize\r\n';
  }
  if (rtcpParameters.mux) {
    sdp += 'a=rtcp-mux\r\n';
  }
  if (rtcpParameters.ssrc !== undefined && rtcpParameters.cname) {
    sdp += 'a=ssrc:' + rtcpParameters.ssrc +
      ' cname:' + rtcpParameters.cname + '\r\n';
  }
  return sdp;
};


// parses either a=msid: or a=ssrc:... msid lines and returns
// the id of the MediaStream and MediaStreamTrack.
SDPUtils.parseMsid = function(mediaSection) {
  let parts;
  const spec = SDPUtils.matchPrefix(mediaSection, 'a=msid:');
  if (spec.length === 1) {
    parts = spec[0].substr(7).split(' ');
    return {stream: parts[0], track: parts[1]};
  }
  const planB = SDPUtils.matchPrefix(mediaSection, 'a=ssrc:')
    .map(line => SDPUtils.parseSsrcMedia(line))
    .filter(msidParts => msidParts.attribute === 'msid');
  if (planB.length > 0) {
    parts = planB[0].value.split(' ');
    return {stream: parts[0], track: parts[1]};
  }
};

// SCTP
// parses draft-ietf-mmusic-sctp-sdp-26 first and falls back
// to draft-ietf-mmusic-sctp-sdp-05
SDPUtils.parseSctpDescription = function(mediaSection) {
  const mline = SDPUtils.parseMLine(mediaSection);
  const maxSizeLine = SDPUtils.matchPrefix(mediaSection, 'a=max-message-size:');
  let maxMessageSize;
  if (maxSizeLine.length > 0) {
    maxMessageSize = parseInt(maxSizeLine[0].substr(19), 10);
  }
  if (isNaN(maxMessageSize)) {
    maxMessageSize = 65536;
  }
  const sctpPort = SDPUtils.matchPrefix(mediaSection, 'a=sctp-port:');
  if (sctpPort.length > 0) {
    return {
      port: parseInt(sctpPort[0].substr(12), 10),
      protocol: mline.fmt,
      maxMessageSize,
    };
  }
  const sctpMapLines = SDPUtils.matchPrefix(mediaSection, 'a=sctpmap:');
  if (sctpMapLines.length > 0) {
    const parts = sctpMapLines[0]
      .substr(10)
      .split(' ');
    return {
      port: parseInt(parts[0], 10),
      protocol: parts[1],
      maxMessageSize,
    };
  }
};

// SCTP
// outputs the draft-ietf-mmusic-sctp-sdp-26 version that all browsers
// support by now receiving in this format, unless we originally parsed
// as the draft-ietf-mmusic-sctp-sdp-05 format (indicated by the m-line
// protocol of DTLS/SCTP -- without UDP/ or TCP/)
SDPUtils.writeSctpDescription = function(media, sctp) {
  let output = [];
  if (media.protocol !== 'DTLS/SCTP') {
    output = [
      'm=' + media.kind + ' 9 ' + media.protocol + ' ' + sctp.protocol + '\r\n',
      'c=IN IP4 0.0.0.0\r\n',
      'a=sctp-port:' + sctp.port + '\r\n',
    ];
  } else {
    output = [
      'm=' + media.kind + ' 9 ' + media.protocol + ' ' + sctp.port + '\r\n',
      'c=IN IP4 0.0.0.0\r\n',
      'a=sctpmap:' + sctp.port + ' ' + sctp.protocol + ' 65535\r\n',
    ];
  }
  if (sctp.maxMessageSize !== undefined) {
    output.push('a=max-message-size:' + sctp.maxMessageSize + '\r\n');
  }
  return output.join('');
};

// Generate a session ID for SDP.
// https://tools.ietf.org/html/draft-ietf-rtcweb-jsep-20#section-5.2.1
// recommends using a cryptographically random +ve 64-bit value
// but right now this should be acceptable and within the right range
SDPUtils.generateSessionId = function() {
  return Math.random().toString().substr(2, 21);
};

// Write boiler plate for start of SDP
// sessId argument is optional - if not supplied it will
// be generated randomly
// sessVersion is optional and defaults to 2
// sessUser is optional and defaults to 'thisisadapterortc'
SDPUtils.writeSessionBoilerplate = function(sessId, sessVer, sessUser) {
  let sessionId;
  const version = sessVer !== undefined ? sessVer : 2;
  if (sessId) {
    sessionId = sessId;
  } else {
    sessionId = SDPUtils.generateSessionId();
  }
  const user = sessUser || 'thisisadapterortc';
  // FIXME: sess-id should be an NTP timestamp.
  return 'v=0\r\n' +
      'o=' + user + ' ' + sessionId + ' ' + version +
        ' IN IP4 127.0.0.1\r\n' +
      's=-\r\n' +
      't=0 0\r\n';
};

// Gets the direction from the mediaSection or the sessionpart.
SDPUtils.getDirection = function(mediaSection, sessionpart) {
  // Look for sendrecv, sendonly, recvonly, inactive, default to sendrecv.
  const lines = SDPUtils.splitLines(mediaSection);
  for (let i = 0; i < lines.length; i++) {
    switch (lines[i]) {
      case 'a=sendrecv':
      case 'a=sendonly':
      case 'a=recvonly':
      case 'a=inactive':
        return lines[i].substr(2);
      default:
        // FIXME: What should happen here?
    }
  }
  if (sessionpart) {
    return SDPUtils.getDirection(sessionpart);
  }
  return 'sendrecv';
};

SDPUtils.getKind = function(mediaSection) {
  const lines = SDPUtils.splitLines(mediaSection);
  const mline = lines[0].split(' ');
  return mline[0].substr(2);
};

SDPUtils.isRejected = function(mediaSection) {
  return mediaSection.split(' ', 2)[1] === '0';
};

SDPUtils.parseMLine = function(mediaSection) {
  const lines = SDPUtils.splitLines(mediaSection);
  const parts = lines[0].substr(2).split(' ');
  return {
    kind: parts[0],
    port: parseInt(parts[1], 10),
    protocol: parts[2],
    fmt: parts.slice(3).join(' '),
  };
};

SDPUtils.parseOLine = function(mediaSection) {
  const line = SDPUtils.matchPrefix(mediaSection, 'o=')[0];
  const parts = line.substr(2).split(' ');
  return {
    username: parts[0],
    sessionId: parts[1],
    sessionVersion: parseInt(parts[2], 10),
    netType: parts[3],
    addressType: parts[4],
    address: parts[5],
  };
};

// a very naive interpretation of a valid SDP.
SDPUtils.isValidSDP = function(blob) {
  if (typeof blob !== 'string' || blob.length === 0) {
    return false;
  }
  const lines = SDPUtils.splitLines(blob);
  for (let i = 0; i < lines.length; i++) {
    if (lines[i].length < 2 || lines[i].charAt(1) !== '=') {
      return false;
    }
    // TODO: check the modifier a bit more.
  }
  return true;
};

// Expose public methods.
if (typeof module === 'object') {
  module.exports = SDPUtils;
}