diff --git a/doc/APIchanges b/doc/APIchanges
index 5c7d21a5b7..9ca71b7f8b 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -15,6 +15,10 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-xx-xx - xxxxxxx / db7968b - lavf 57.40.100 / 57.7.0 - avio.h
+  Add AVIODataMarkerType, write_data_type, ignore_boundary_point and
+  avio_write_marker.
+
 2016-xx-xx - xxxxxxx / 0c4468d - lavu 55.26.100 / 55.12.0 - opt.h
   Add av_stereo3d_type_name() and av_stereo3d_from_name().
 
diff --git a/libavformat/avio.h b/libavformat/avio.h
index 5e86f7ecc4..b1ce1d1c72 100644
--- a/libavformat/avio.h
+++ b/libavformat/avio.h
@@ -96,6 +96,42 @@ typedef struct AVIODirContext {
     struct URLContext *url_context;
 } AVIODirContext;
 
+/**
+ * Different data types that can be returned via the AVIO
+ * write_data_type callback.
+ */
+enum AVIODataMarkerType {
+    /**
+     * Header data; this needs to be present for the stream to be decodeable.
+     */
+    AVIO_DATA_MARKER_HEADER,
+    /**
+     * A point in the output bytestream where a decoder can start decoding
+     * (i.e. a keyframe). A demuxer/decoder given the data flagged with
+     * AVIO_DATA_MARKER_HEADER, followed by any AVIO_DATA_MARKER_SYNC_POINT,
+     * should give decodeable results.
+     */
+    AVIO_DATA_MARKER_SYNC_POINT,
+    /**
+     * A point in the output bytestream where a demuxer can start parsing
+     * (for non self synchronizing bytestream formats). That is, any
+     * non-keyframe packet start point.
+     */
+    AVIO_DATA_MARKER_BOUNDARY_POINT,
+    /**
+     * This is any, unlabelled data. It can either be a muxer not marking
+     * any positions at all, it can be an actual boundary/sync point
+     * that the muxer chooses not to mark, or a later part of a packet/fragment
+     * that is cut into multiple write callbacks due to limited IO buffer size.
+     */
+    AVIO_DATA_MARKER_UNKNOWN,
+    /**
+     * Trailer data, which doesn't contain actual content, but only for
+     * finalizing the output file.
+     */
+    AVIO_DATA_MARKER_TRAILER
+};
+
 /**
  * Bytestream IO Context.
  * New fields can be added to the end with minor version bumps.
@@ -259,6 +295,24 @@ typedef struct AVIOContext {
      * ',' separated list of disallowed protocols.
      */
     const char *protocol_blacklist;
+
+    /**
+     * A callback that is used instead of write_packet.
+     */
+    int (*write_data_type)(void *opaque, uint8_t *buf, int buf_size,
+                           enum AVIODataMarkerType type, int64_t time);
+    /**
+     * If set, don't call write_data_type separately for AVIO_DATA_MARKER_BOUNDARY_POINT,
+     * but ignore them and treat them as AVIO_DATA_MARKER_UNKNOWN (to avoid needlessly
+     * small chunks of data returned from the callback).
+     */
+    int ignore_boundary_point;
+
+    /**
+     * Internal, not meant to be used from outside of AVIOContext.
+     */
+    enum AVIODataMarkerType current_type;
+    int64_t last_time;
 } AVIOContext;
 
 /**
@@ -411,6 +465,18 @@ int avio_put_str16le(AVIOContext *s, const char *str);
  */
 int avio_put_str16be(AVIOContext *s, const char *str);
 
+/**
+ * Mark the written bytestream as a specific type.
+ *
+ * Zero-length ranges are omitted from the output.
+ *
+ * @param time the stream time the current bytestream pos corresponds to
+ *             (in AV_TIME_BASE units), or AV_NOPTS_VALUE if unknown or not
+ *             applicable
+ * @param type the kind of data written starting at the current pos
+ */
+void avio_write_marker(AVIOContext *s, int64_t time, enum AVIODataMarkerType type);
+
 /**
  * ORing this as the "whence" parameter to a seek function causes it to
  * return the filesize without seeking anywhere. Supporting this is optional.
diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
index f02ae21dfd..95793c92cd 100644
--- a/libavformat/aviobuf.c
+++ b/libavformat/aviobuf.c
@@ -115,6 +115,11 @@ int ffio_init_context(AVIOContext *s,
     s->read_pause = NULL;
     s->read_seek  = NULL;
 
+    s->write_data_type       = NULL;
+    s->ignore_boundary_point = 0;
+    s->current_type          = AVIO_DATA_MARKER_UNKNOWN;
+    s->last_time             = AV_NOPTS_VALUE;
+
     return 0;
 }
 
@@ -137,12 +142,24 @@ AVIOContext *avio_alloc_context(
 
 static void writeout(AVIOContext *s, const uint8_t *data, int len)
 {
-    if (s->write_packet && !s->error) {
-        int ret = s->write_packet(s->opaque, (uint8_t *)data, len);
+    if (!s->error) {
+        int ret = 0;
+        if (s->write_data_type)
+            ret = s->write_data_type(s->opaque, (uint8_t *)data,
+                                     len,
+                                     s->current_type,
+                                     s->last_time);
+        else if (s->write_packet)
+            ret = s->write_packet(s->opaque, (uint8_t *)data, len);
         if (ret < 0) {
             s->error = ret;
         }
     }
+    if (s->current_type == AVIO_DATA_MARKER_SYNC_POINT ||
+        s->current_type == AVIO_DATA_MARKER_BOUNDARY_POINT) {
+        s->current_type = AVIO_DATA_MARKER_UNKNOWN;
+    }
+    s->last_time = AV_NOPTS_VALUE;
     s->writeout_count ++;
     s->pos += len;
 }
@@ -450,6 +467,37 @@ void avio_wb24(AVIOContext *s, unsigned int val)
     avio_w8(s, (uint8_t)val);
 }
 
+void avio_write_marker(AVIOContext *s, int64_t time, enum AVIODataMarkerType type)
+{
+    if (!s->write_data_type)
+        return;
+    // If ignoring boundary points, just treat it as unknown
+    if (type == AVIO_DATA_MARKER_BOUNDARY_POINT && s->ignore_boundary_point)
+        type = AVIO_DATA_MARKER_UNKNOWN;
+    // Avoid unnecessary flushes if we are already in non-header/trailer
+    // data and setting the type to unknown
+    if (type == AVIO_DATA_MARKER_UNKNOWN &&
+        (s->current_type != AVIO_DATA_MARKER_HEADER &&
+         s->current_type != AVIO_DATA_MARKER_TRAILER))
+        return;
+
+    switch (type) {
+    case AVIO_DATA_MARKER_HEADER:
+    case AVIO_DATA_MARKER_TRAILER:
+        // For header/trailer, ignore a new marker of the same type;
+        // consecutive header/trailer markers can be merged.
+        if (type == s->current_type)
+            return;
+        break;
+    }
+
+    // If we've reached here, we have a new, noteworthy marker.
+    // Flush the previous data and mark the start of the new data.
+    avio_flush(s);
+    s->current_type = type;
+    s->last_time = time;
+}
+
 /* Input stream */
 
 static void fill_buffer(AVIOContext *s)
diff --git a/libavformat/mux.c b/libavformat/mux.c
index 76db60752f..8488043006 100644
--- a/libavformat/mux.c
+++ b/libavformat/mux.c
@@ -475,6 +475,8 @@ static int init_pts(AVFormatContext *s)
 
 static int write_header_internal(AVFormatContext *s)
 {
+    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
+        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_HEADER);
     if (s->oformat->write_header) {
         int ret = s->oformat->write_header(s);
         if (ret >= 0 && s->pb && s->pb->error < 0)
@@ -486,6 +488,8 @@ static int write_header_internal(AVFormatContext *s)
             avio_flush(s->pb);
     }
     s->internal->header_written = 1;
+    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
+        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);
     return 0;
 }
 
@@ -1164,12 +1168,15 @@ int av_write_trailer(AVFormatContext *s)
     }
 
 fail:
-    if (s->internal->header_written && s->oformat->write_trailer)
+    if (s->internal->header_written && s->oformat->write_trailer) {
+        if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
+            avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_TRAILER);
         if (ret >= 0) {
         ret = s->oformat->write_trailer(s);
         } else {
             s->oformat->write_trailer(s);
         }
+    }
 
     if (s->oformat->deinit)
         s->oformat->deinit(s);
diff --git a/libavformat/version.h b/libavformat/version.h
index 0293b488c8..543afbb166 100644
--- a/libavformat/version.h
+++ b/libavformat/version.h
@@ -32,7 +32,7 @@
 // Major bumping may affect Ticket5467, 5421, 5451(compatibility with Chromium)
 // Also please add any ticket numbers that you belive might be affected here
 #define LIBAVFORMAT_VERSION_MAJOR  57
-#define LIBAVFORMAT_VERSION_MINOR  39
+#define LIBAVFORMAT_VERSION_MINOR  40
 #define LIBAVFORMAT_VERSION_MICRO 100
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \