Support removing b=AS bandwidth constraints.

webrtc/media/engine/webrtcvideoengine2.cc

 /*
  *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 697 matching lines @@
   // Handle RTP header extensions.
   std::vector<webrtc::RtpExtension> filtered_extensions = FilterRtpExtensions(
       params.extensions, webrtc::RtpExtension::IsSupportedForVideo, true);
   if (!send_rtp_extensions_ || (*send_rtp_extensions_ != filtered_extensions)) {
     changed_params->rtp_header_extensions =
         rtc::Optional<std::vector<webrtc::RtpExtension>>(filtered_extensions);
   }
 
   // Handle max bitrate.
   if (params.max_bandwidth_bps != send_params_.max_bandwidth_bps &&
-      params.max_bandwidth_bps >= 0) {
-    // 0 uncaps max bitrate (-1).
+      params.max_bandwidth_bps >= -1) {
+    // 0 or -1 uncaps max bitrate.
+    // TODO(pbos): Reconsider how 0 should be treated. It is not mentioned as a
+    // special value and might very well be used for stopping sending.
     changed_params->max_bandwidth_bps = rtc::Optional<int>(
         params.max_bandwidth_bps == 0 ? -1 : params.max_bandwidth_bps);
   }
 
   // Handle conference mode.
   if (params.conference_mode != send_params_.conference_mode) {
     changed_params->conference_mode =
         rtc::Optional<bool>(params.conference_mode);
   }
 
@@ skipping 23 matching lines @@
     const VideoCodecSettings& codec_settings = *changed_params.codec;
     send_codec_ = rtc::Optional<VideoCodecSettings>(codec_settings);
     LOG(LS_INFO) << "Using codec: " << codec_settings.codec.ToString();
   }
 
   if (changed_params.rtp_header_extensions) {
     send_rtp_extensions_ = changed_params.rtp_header_extensions;
   }
 
   if (changed_params.codec || changed_params.max_bandwidth_bps) {
+    if (params.max_bandwidth_bps == -1) {
+      // Unset the global max bitrate (max_bitrate_bps) if max_bandwidth_bps is
+      // -1, which corresponds to no "b=AS" attribute in SDP. Note that the
+      // global max bitrate may be set below in GetBitrateConfigForCodec, from
+      // the codec max bitrate.
+      // TODO(pbos): This should be reconsidered (codec max bitrate should
+      // probably not affect global call max bitrate).
+      bitrate_config_.max_bitrate_bps = -1;
+    }
     if (send_codec_) {
       // TODO(holmer): Changing the codec parameters shouldn't necessarily mean
       // that we change the min/max of bandwidth estimation. Reevaluate this.
       bitrate_config_ = GetBitrateConfigForCodec(send_codec_->codec);
       if (!changed_params.codec) {
         // If the codec isn't changing, set the start bitrate to -1 which means
         // "unchanged" so that BWE isn't affected.
         bitrate_config_.start_bitrate_bps = -1;
       }
     }
@@ skipping 1775 matching lines @@
         rtx_mapping[video_codecs[i].codec.id] !=
             ulpfec_config.red_payload_type) {
       video_codecs[i].rtx_payload_type = rtx_mapping[video_codecs[i].codec.id];
     }
   }
 
   return video_codecs;
 }
 
 }  // namespace cricket