为什么不在 WebRTC TURN 客户端中只使用频道数据？

Question

在WebRTC TURN客户端中，两个过程（创建权限和频道绑定请求）如下：

(1) 在

TurnEntry

构造函数中创建权限： https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/p2p/base/turn_port.cc;l=1757

TurnEntry::TurnEntry(TurnPort* port, Connection* conn, int channel_id)
    : port_(port),
      channel_id_(channel_id),
      ext_addr_(conn->remote_candidate().address()),
      state_(STATE_UNBOUND),
      connections_({conn}) {
  // Creating permission for `ext_addr_`.
  SendCreatePermissionRequest(0);
}

(2) 稍后发送频道绑定请求。关键词是

SendChannelBindRequest

： https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/p2p/base/turn_port.cc;l=1817

int TurnEntry::Send(const void* data,
                    size_t size,
                    bool payload,
                    const rtc::PacketOptions& options) {
  rtc::ByteBufferWriter buf;
  if (state_ != STATE_BOUND ||
      !port_->TurnCustomizerAllowChannelData(data, size, payload)) {
    // If we haven't bound the channel yet, we have to use a Send Indication.
    // The turn_customizer_ can also make us use Send Indication.
    TurnMessage msg(TURN_SEND_INDICATION);
    msg.AddAttribute(std::make_unique<StunXorAddressAttribute>(
        STUN_ATTR_XOR_PEER_ADDRESS, ext_addr_));
    msg.AddAttribute(
        std::make_unique<StunByteStringAttribute>(STUN_ATTR_DATA, data, size));

    port_->TurnCustomizerMaybeModifyOutgoingStunMessage(&msg);

    const bool success = msg.Write(&buf);
    RTC_DCHECK(success);

    // If we're sending real data, request a channel bind that we can use later.
    if (state_ == STATE_UNBOUND && payload) {
      SendChannelBindRequest(0);
      state_ = STATE_BINDING;
    }
  } else {
    // If the channel is bound, we can send the data as a Channel Message.
    buf.WriteUInt16(channel_id_);
    buf.WriteUInt16(static_cast<uint16_t>(size));
    buf.WriteBytes(reinterpret_cast<const char*>(data), size);
  }
  rtc::PacketOptions modified_options(options);
  modified_options.info_signaled_after_sent.turn_overhead_bytes =
      buf.Length() - size;
  return port_->Send(buf.Data(), buf.Length(), modified_options);
}

根据 RFC 8656 (https://datatracker.ietf.org/doc/html/rfc8656)，通道数据的开销较小（仅 4B），而指示有 36B 的开销。

为什么不只使用频道数据？我认为

SendCreatePermissionRequest

可以在

SendChannelBindRequest

构造函数中替换为

TurnEntry

.