diff --git a/lib/mss/mss_parser.js b/lib/mss/mss_parser.js
index e8e4ab3e36..9e5c42d363 100644
--- a/lib/mss/mss_parser.js
+++ b/lib/mss/mss_parser.js
@@ -618,15 +618,20 @@ shaka.mss.MssParser = class {
       if (this.initSegmentDataByStreamId_.has(stream.id)) {
         initSegmentData = this.initSegmentDataByStreamId_.get(stream.id);
       } else {
-        const timescale = stream.mssPrivateData.timescale;
-        const duration = stream.mssPrivateData.duration;
-        let videoNalus = null;
+        let videoNalus = [];
         if (stream.type == ContentType.VIDEO) {
           const codecPrivateData = stream.mssPrivateData.codecPrivateData;
           videoNalus = codecPrivateData.split('00000001').slice(1);
         }
-        const mp4Generator = new shaka.util.Mp4Generator(
-            stream, timescale, duration, videoNalus);
+        /** @type {shaka.util.Mp4Generator.StreamInfo} */
+        const streamInfo = {
+          timescale: stream.mssPrivateData.timescale,
+          duration: stream.mssPrivateData.duration,
+          videoNalus: videoNalus,
+          data: null, // Data is not necessary for init segement.
+          stream: stream,
+        };
+        const mp4Generator = new shaka.util.Mp4Generator(streamInfo);
         initSegmentData = mp4Generator.initSegment();
         this.initSegmentDataByStreamId_.set(stream.id, initSegmentData);
       }
diff --git a/lib/util/mp4_generator.js b/lib/util/mp4_generator.js
index 5092284dae..69dc8c4b85 100644
--- a/lib/util/mp4_generator.js
+++ b/lib/util/mp4_generator.js
@@ -11,33 +11,43 @@ goog.require('shaka.util.ManifestParserUtils');
 goog.require('shaka.util.Uint8ArrayUtils');
 
 
-/**
- * @export
- */
 shaka.util.Mp4Generator = class {
   /**
-   * @param {shaka.extern.Stream} stream
-   * @param {number} timescale
-   * @param {?number} duration
-   * @param {?Array.<string>} videoNalus
+   * @param {shaka.util.Mp4Generator.StreamInfo} streamInfo
    */
-  constructor(stream, timescale, duration, videoNalus) {
+  constructor(streamInfo) {
     shaka.util.Mp4Generator.initStaticProperties_();
 
-    /** @private {shaka.extern.Stream} */
-    this.stream_ = stream;
+    /** @private {!shaka.extern.Stream} */
+    this.stream_ = streamInfo.stream;
 
     /** @private {number} */
-    this.timescale_ = timescale;
+    this.timescale_ = streamInfo.timescale;
 
     /** @private {number} */
-    this.duration_ = duration || 0xffffffff;
+    this.duration_ = streamInfo.duration;
     if (this.duration_ === Infinity) {
       this.duration_ = 0xffffffff;
     }
 
-    /** @private {Array.<string>} */
-    this.videoNalus_ = videoNalus || [];
+    /** @private {!Array.<string>} */
+    this.videoNalus_ = streamInfo.videoNalus;
+
+    /** @private {number} */
+    this.sequenceNumber_ = 0;
+
+    /** @private {number} */
+    this.baseMediaDecodeTime_ = 0;
+
+    /** @private {!Array.<shaka.util.Mp4Generator.Mp4Sample>} */
+    this.samples_ = [];
+
+    const data = streamInfo.data;
+    if (data) {
+      this.sequenceNumber_ = data.sequenceNumber;
+      this.baseMediaDecodeTime_ = data.baseMediaDecodeTime;
+      this.samples_ = data.samples;
+    }
   }
 
   /**
@@ -698,6 +708,179 @@ shaka.util.Mp4Generator = class {
     return Mp4Generator.box('tenc', bytes, defaultKeyId);
   }
 
+  /**
+   * Generate a Segment Data (MP4).
+   *
+   * @return {!Uint8Array}
+   */
+  segmentData() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    const movie = this.moof_();
+    const length = Mp4Generator.FTYP_.byteLength + movie.byteLength;
+    const result = new Uint8Array(length);
+    result.set(Mp4Generator.FTYP_);
+    result.set(movie, Mp4Generator.FTYP_.byteLength);
+    return result;
+  }
+
+  /**
+   * Generate a MOOF box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  moof_() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    return Mp4Generator.box('moof', this.mfhd_(), this.traf_());
+  }
+
+  /**
+   * Generate a MOOF box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  mfhd_() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    const bytes = new Uint8Array([
+      0x00, // version 0
+      0x00, 0x00, 0x00, // flags
+      ...this.breakNumberIntoBytes_(this.sequenceNumber_, 4),
+    ]);
+    return Mp4Generator.box('mfhd', bytes);
+  }
+
+  /**
+   * Generate a TRAF box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  traf_() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    const sampleDependencyTable = this.sdtp_();
+    const offset = sampleDependencyTable.length +
+          32 + // tfhd
+          20 + // tfdt
+          8 + // traf header
+          16 + // mfhd
+          8 + // moof header
+          8; // mdat header;
+    return Mp4Generator.box('traf', this.tfhd_(), this.tfdt_(),
+        this.trun_(offset), sampleDependencyTable);
+  }
+
+  /**
+   * Generate a SDTP box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  sdtp_() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    const bytes = new Uint8Array(4 + this.samples_.length);
+    // leave the full box header (4 bytes) all zero
+    // write the sample table
+    for (let i = 0; i < this.samples_.length; i++) {
+      const flags = this.samples_[i].flags;
+      bytes[i + 4] = (flags.dependsOn << 4) |
+          (flags.isDependedOn << 2) |
+          flags.hasRedundancy;
+    }
+    return Mp4Generator.box('sdtp', bytes);
+  }
+
+  /**
+   * Generate a TFHD box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  tfhd_() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    const id = this.stream_.id + 1;
+    const bytes = new Uint8Array([
+      0x00, // version 0
+      0x00, 0x00, 0x3a, // flags
+      ...this.breakNumberIntoBytes_(id, 4), // track_ID
+      0x00, 0x00, 0x00, 0x01, // sample_description_index
+      0x00, 0x00, 0x00, 0x00, // default_sample_duration
+      0x00, 0x00, 0x00, 0x00, // default_sample_size
+      0x00, 0x00, 0x00, 0x00,  // default_sample_flags
+    ]);
+    return Mp4Generator.box('tfhd', bytes);
+  }
+
+  /**
+   * Generate a TFDT box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  tfdt_() {
+    const Mp4Generator = shaka.util.Mp4Generator;
+    const upperWordBaseMediaDecodeTime =
+      Math.floor(this.baseMediaDecodeTime_ / (Mp4Generator.UINT32_MAX_ + 1));
+    const lowerWordBaseMediaDecodeTime =
+      Math.floor(this.baseMediaDecodeTime_ % (Mp4Generator.UINT32_MAX_ + 1));
+    const bytes = new Uint8Array([
+      0x01, // version 1
+      0x00, 0x00, 0x00, // flags
+      ...this.breakNumberIntoBytes_(upperWordBaseMediaDecodeTime, 4),
+      ...this.breakNumberIntoBytes_(lowerWordBaseMediaDecodeTime, 4),
+    ]);
+    return Mp4Generator.box('mfhd', bytes);
+  }
+
+  /**
+   * Generate a TRUN box
+   *
+   * @return {!Uint8Array}
+   * @private
+   */
+  trun_(offset) {
+    const ContentType = shaka.util.ManifestParserUtils.ContentType;
+    const Mp4Generator = shaka.util.Mp4Generator;
+
+    const samplesLength = this.samples_.length;
+    const byteslen = 12 + 16 * samplesLength;
+    const bytes = new Uint8Array(byteslen);
+    offset += 8 + byteslen;
+    const isVideo = this.stream_.type === ContentType.VIDEO;
+    bytes.set(
+        [
+        // version 1 for video with signed-int sample_composition_time_offset
+        isVideo ? 0x01 : 0x00,
+        0x00, 0x0f, 0x01, // flags
+        ...this.breakNumberIntoBytes_(samplesLength, 4), // sample_count
+        ...this.breakNumberIntoBytes_(offset, 4), // data_offset
+        ],
+        0,
+    );
+    for (let i = 0; i < samplesLength; i++) {
+      const sample = this.samples_[i];
+      const duration = this.breakNumberIntoBytes_(sample.duration, 4);
+      const size = this.breakNumberIntoBytes_(sample.size, 4);
+      const flags = sample.flags;
+      const cts = this.breakNumberIntoBytes_(sample.cts, 4);
+      bytes.set(
+          [
+            ...duration, // sample_duration
+            ...size, // sample_size
+            (flags.isLeading << 2) | flags.dependsOn,
+            (flags.isDependedOn << 6) | (flags.hasRedundancy << 4) |
+              flags.isNonSync,
+            flags.degradPrio & (0xf0 << 8),
+            flags.degradPrio & 0x0f, // sample_flags
+            ...cts, // sample_composition_time_offset
+          ],
+          12 + 16 * i,
+      );
+    }
+    return Mp4Generator.box('trun', bytes);
+  }
+
+
   /**
    * @param {number} number
    * @param {number} numBytes
@@ -940,3 +1123,79 @@ shaka.util.Mp4Generator.DREF_ = new Uint8Array([
  * @private {!Uint8Array}
  */
 shaka.util.Mp4Generator.DINF_ = new Uint8Array([]);
+
+/**
+ * @typedef {{
+ *    timescale: number,
+ *    duration: number,
+ *    videoNalus: !Array.<string>,
+ *    data: ?shaka.util.Mp4Generator.Data,
+ *    stream: !shaka.extern.Stream
+ * }}
+ *
+ * @property {number} timescale
+ *   The Stream's timescale.
+ * @property {number} duration
+ *   The Stream's duration.
+ * @property {!Array.<string>} videoNalus
+ *   The stream's video nalus.
+ * @property {?shaka.util.Mp4Generator.Data} data
+ *   The stream's data.
+ * @property {!shaka.extern.Stream} stream
+ *   The Stream.
+ */
+shaka.util.Mp4Generator.StreamInfo;
+
+/**
+ * @typedef {{
+ *    sequenceNumber: number,
+ *    baseMediaDecodeTime: number,
+ *    samples: !Array.<shaka.util.Mp4Generator.Mp4Sample>
+ * }}
+ *
+ * @property {number} sequenceNumber
+ *   The sequence number.
+ * @property {number} baseMediaDecodeTime
+ *   The base media decode time.
+ * @property {!Array.<shaka.util.Mp4Generator.Mp4Sample>} samples
+ *   The data samples.
+ */
+shaka.util.Mp4Generator.Data;
+
+/**
+ * @typedef {{
+ *    size: number,
+ *    duration: number,
+ *    cts: number,
+ *    flags: !shaka.util.Mp4Generator.Mp4SampleFlags
+ * }}
+ *
+ * @property {number} size
+ *   The sample size.
+ * @property {number} duration
+ *   The sample duration.
+ * @property {number} cts
+ *   The sample composition time.
+ * @property {!shaka.util.Mp4Generator.Mp4SampleFlags} flags
+ *   The sample flags.
+ */
+shaka.util.Mp4Generator.Mp4Sample;
+
+/**
+ * @typedef {{
+ *    isLeading: number,
+ *    isDependedOn: number,
+ *    hasRedundancy: number,
+ *    degradPrio: number,
+ *    dependsOn: number,
+ *    isNonSync: number
+ * }}
+ *
+ * @property {number} isLeading
+ * @property {number} isDependedOn
+ * @property {number} hasRedundancy
+ * @property {number} degradPrio
+ * @property {number} dependsOn
+ * @property {number} isNonSync
+ */
+shaka.util.Mp4Generator.Mp4SampleFlags;