Controlling the bandwidth used by WebRTC during calls

RTCClient.swift

//...

func sendOffer() {
  
  let peerConnection = getPeerConnection()
  let mediaConstraints = getMediaConstraints()
  
  peerConnection.offer(for: mediaConstraints) { [weak self] (sdp, error) in
            
            guard error == nil else {
                self?.delegate?.handleError(error: error)
                return
            }
            
            var modifiedSDP = sdp
                                               
            if lowDataModeEnabled {
              //If low data mode is enabled modify the SDP
              let sdpString = sdp!.sdp
              //For example purpose, I have set videoBitrate to 233 & audioBitrate to 100. You can change it as per your needs
              let modifiedSDPString = self?.setMediaBitrates(sdp: sdpString, videoBitrate: 233, audioBitrate: 100)
              //Create a new SDP using the modified SDP string
              modifiedSDP = RTCSessionDescription(type: .offer, sdp: modifiedSDPString!)
            }
                                               
            //Set the modified SDP as local description
            self?.peerConnection.setLocalDescription(modifiedSDP!, completionHandler: { (error) in
                
                guard error == nil else {
                    self?.delegate?.handleError(error: error)
                    return
                }
                
                //Send the modified SDP to other party using your signalling channel
                self?.sendLocalSessionDescriptionSignalling(sdp: modifiedSDP!)
            })
        }
}

func sendAnswer() {
  
  let peerConnection = getPeerConnection()
  let mediaConstraints = getMediaConstraints()
  
  peerConnection.answer(for: mediaConstraints) { [weak self] (sdp, error) in
            //Handle error if any
            guard error == nil else {
                self?.delegate?.handleError(error: error)
                return
            }
            
            var modifiedSDP = sdp
            if lowDataModeEnabled {
              //If low data mode is enabled modify the SDP
                let sdpString = sdp!.sdp
              //For example purpose, I have set videoBitrate to 233 & audioBitrate to 100. You can change it as per your needs
                let modifiedSDPString = self?.setMediaBitrates(sdp: sdpString, videoBitrate: 233, audioBitrate: 100)
              //Create a new SDP using the modified SDP string
                modifiedSDP = RTCSessionDescription(type: .answer, sdp: modifiedSDPString!)
            }
            
            //Set the modified SDP as local description
            self?.peerConnection.setLocalDescription(modifiedSDP!, completionHandler: { (error) in
                //Handle error if any
                guard error == nil else {
                    self?.delegate?.handleError(error: error)
                    return
                }
                                                                                       
                //Send the modified SDP to other party using your signalling channel
                self?.sendLocalSessionDescriptionSignalling(sdp: modifiedSDP!)
            })
        }
  
}

private func sendLocalSessionDescriptionSignalling(sdp: RTCSessionDescription) {
  // Send the sdp to other party using your siganlling channel
}

private func getMediaConstraints() -> RTCMediaConstraints {
  //Creates & returns media constraints for call
}

private func getPeerConnection() -> RTCPeerConnection {
  //Creates & returns an peer connection
}

func setMediaBitrates(sdp: String, videoBitrate: Int, audioBitrate: Int) -> String {
  return setMediaBitrate(sdp: setMediaBitrate(sdp: sdp, mediaType: "video", bitrate: videoBitrate), mediaType: "audio", bitrate: audioBitrate)
}

private func setMediaBitrate(sdp: String, mediaType: String, bitrate: Int) -> String {
  
  var lines = sdp.components(separatedBy: "\n")
  var line = -1
        
  for (index, lineString) in lines.enumerated() {
    if lineString.hasPrefix("m=\(mediaType)") {
      line = index
      break
    }
  }
        
  guard line != -1 else {
    //Couldn't find the m (media) line return the original sdp
    print("Couldn't find the m line in SDP so returning the original sdp")
    return sdp
  }
  
  // Go to next line i.e. line after m
  line += 1
        
  //Now skip i and c lines
  while (lines[line].hasPrefix("i=") || lines[line].hasPrefix("c=")) {
    line += 1
  }
  
  let newLine = "b=AS:\(bitrate)"
  //Check if we're on b (bitrate) line, if so replace it
  if lines[line].hasPrefix("b") {
    print("Replacing the b line of the SDP")
    lines[line] = newLine
  } else {
    //If there's no b line, add a new b line
    lines.insert(newLine, at: line)
  }
  
  let modifiedSDP = lines.joined(separator: "\n")
  return modifiedSDP
    
}

//...