The WebRTC support in Asterisk has evolved and improved over time (in particular with Asterisk 15) but has not yet fully ventured into the user experience area. The two most important areas of this are the handling of lost or out of order packets and bandwidth management.
Lost or out of order packets are important because they have a critical impact on video streams. The video decoder can stall and freeze if a constant stream of video frames is not provided. This means that if a single RTP packet is lost the video stream may be lost until a full frame is received (which by default we restrict requests for these to 1 ever 5 seconds) or the packet is received out of order.
Bandwidth management is important because it can lead to video stutters, stalls, and congestion.
This page documents the way that two solutions to this can be implemented to help the experience under both Chrome and Firefox. While the browsers support other options they only have a subset in common.
RTP Packet Retransmission
Supported by: Chrome and Firefox (but not using RFC)
Firefox support tracked at: https://bugzilla.mozilla.org/show_bug.cgi?id=1164187
RTP packet retransmission allows a client to request retransmission of an RTP packet if they determine that it has been lost. It is up to their own logic to determine when they request this to be done. The request is done using a NACK RTCP feedback message. If the packet is in the history of the sender it is then resent. In the case of Chrome is is sent using RFC4588 (if negotiated) which encapsulates the retransmitted packet and sends it on a separate stream. In the case of Firefox the packet is resent as-is. To support this the res_rtp_asterisk will need to store a history of sent packets. This history will use a newly created API called data buffer:
Data Buffer API
A data buffer acts as a ring buffer of data. It is given a fixed number of data packets to store (which may be dynamically changed). Given a number it will store a data packet at that position relative to the others. Given a number it will retrieve the given data packet if it is present. This is purposely a storage of arbitrary things so it can be used not just for RTP packets but also Asterisk frames in the future if needed. The given number when putting a data packet in must be within the data buffer size range.
To simplify configuration no new configuration options will be added to support RTP packet retransmission. The existing "webrtc" option will enable it using the underlying RTP engine API. Currently RTP packet retransmission is only supported in WebRTC clients for video stream types. We will mirror this and only enable it on the video stream types as well. We will enable support on both receiving and sending.
The res_pjsip_sdp_rtp module will need to place rtx into the SDP if configured on the RTP instance. This can use the existing codec method that is also used for telephone-event.
The module will also need to parse the "a=ssrc-group:FID 229172185 2616243815" attribute line which contains the SSRC used for RTX retransmission. We will also need to place our own in there with the respective SSRCs.
The RTP engine API will need to have two extended properties added:
- Enable RTP packet retransmission support for packets we are receiving
- Enable RTP packet retransmission support for packets we are sending
This will be passed through to the underlying RTP engine to enable or configure as it needs.
The RTP engine API also needs to have two API calls added:
Finally the "rtx" codec will need to be added as a valid option and enabled if RTP packet retransmission is enabled on the RTP instance.
The res_rtp_asterisk module will act on the extended properties that have been added to the RTP engine API.
If RTP packet retransmission support for packets we are receiving is enabled:
- Allocate a data buffer and use it to store incoming packets based on RTP sequence number
- If a retransmitted packet is received de-encapsulate it (if RTX is negotiated) and place it into the correct location in the data buffer
- Place minor delay to allow reordering of packets (and retransmission) to occur
- Take newest data payload from data buffer
- If a sufficient (to be defined) gap is detected in stream of RTP packets send NACK to request retransmission of missing packets
- Use RTCP information to adjust the data buffer size (nice to have)
If RTP packet retransmission support for packets we are sending is enabled:
- Allocate a data buffer and use it to store outgoing packets based on RTP sequence number
- If a NACK request is received get data payload from data buffer if possible and encapsulate it (if RTX is negotiated) and send on appropriate SSRC
- Use RTCP information to adjust the data buffer size (nice to have)
Bandwidth Estimation and Control (RTCP Feedback)
Specs: goog-remb, transport-cc, tmmbr
Supported by: Chrome (goog-remb, transport-cc), Firefox (goog-remb, tmmbr but disabled)
Chrome support for tmmbr at: https://bugs.chromium.org/p/webrtc/issues/detail?id=7103
Firefox support for tmmbr preffing on at: https://bugzilla.mozilla.org/show_bug.cgi?id=1270230
This wiki page focuses solely on goog-remb. This is because this is common across all WebRTC clients with the only other option being transport-cc that is currently only supported by Chrome.
The chan_pjsip module will not have any new configuration options added to it. Instead if the "webrtc" option is enabled we will place the "goog-remb" attribute into the SDP for each configure media format. It is not necessary to perform actual negotiation of the attribute. The remote WebRTC client should support goog-remb and if it does not it will discard any RTCP feedback messages we generate with it. The channel driver will also set the goog-remb extended property on any created video RTP instances if the "webrtc" option is enabled. Just like in the case of RTP packet retransmission the clients only enable goog-remb on video streams.
A new extended property should be added to enable support for goog-remb. This will be set by a channel driver that wants to enable support for it (such as chan_pjsip).
If the goog-remb extended property is not set then the below should not occur.
If an RTCP feedback message containing REMB Is received perform the following:
- Place the REMB packet into an AST_FRAME_RTCP frame.
- Set the stream number on the AST_FRAME_RTCP frame to correspond to the stream the REMB packet is in regards to.
- Modify the REMB packet to have a zero SSRC for both SSRCs.
- Return the AST_FRAME_RTCP frame from res_rtp_asterisk.
If an RTCP feedback message containing REMB is provided to ast_rtp_instance_write:
- Update the REMB packet to contain the correct SSRCs.
- Send the REMB packet in an RTCP feedback message on the correct stream.
The abs-send-time specification (which is small) should be implemented according to the goog-remb draft.
If an RTP packet is sent:
- Add abs-send-time information to the packet
If an AST_FRAME_RTCP frame is received from a channel:
- Examine its stream identifier
- Update the frame to the correct stream of the other party
- Write the frame to the other party