Server Reflexive Candidate，是通过主机候选地址向STUN服务器发送STUN请求获得的网络地址。实际上就是终端的网络包经过一重或多重NAT穿透之后，由STUN服务器观察到的经过NAT转换之后的地址。

解析StunBindingResponse过程

UDPPort::OnReadPacket

如果之前有向stun 服务器发送过请求，则serveraddresses会有记录，可以直接查找得到，这里得到的是reponse消息。UDPPort::OnResolveResult中插入到serveraddresses。

StunBindingRequest::OnResponse

H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\p2p\base\stun_port.cc

class StunBindingRequest : public StunRequest { 
void OnResponse(StunMessage* response) override {
    const StunAddressAttribute* addr_attr =
        response->GetAddress(STUN_ATTR_MAPPED_ADDRESS);
    if (!addr_attr) {
      RTC_LOG(LS_ERROR) << "Binding response missing mapped address.";
    } else if (addr_attr->family() != STUN_ADDRESS_IPV4 &&
               addr_attr->family() != STUN_ADDRESS_IPV6) {
      RTC_LOG(LS_ERROR) << "Binding address has bad family";
    } else {
      rtc::SocketAddress addr(addr_attr->ipaddr(), addr_attr->port());
      port_->OnStunBindingRequestSucceeded(this->Elapsed(), server_addr_, addr);
    }
    // The keep-alive requests will be stopped after its lifetime has passed.
    if (WithinLifetime(rtc::TimeMillis())) {
      port_->requests_.SendDelayed(
          new StunBindingRequest(port_, server_addr_, start_time_),
          port_->stun_keepalive_delay());
    }
  }
}

UDPPort::OnStunBindingRequestSucceeded

void UDPPort::OnStunBindingRequestSucceeded(
    int rtt_ms,
    const rtc::SocketAddress& stun_server_addr,
    const rtc::SocketAddress& stun_reflected_addr) {
  RTC_DCHECK(stats_.stun_binding_responses_received <
             stats_.stun_binding_requests_sent);
  stats_.stun_binding_responses_received++;
  stats_.stun_binding_rtt_ms_total += rtt_ms;
  stats_.stun_binding_rtt_ms_squared_total += rtt_ms * rtt_ms;
  if (bind_request_succeeded_servers_.find(stun_server_addr) !=
      bind_request_succeeded_servers_.end()) {
    return;
  }
  bind_request_succeeded_servers_.insert(stun_server_addr);
  // If socket is shared and |stun_reflected_addr| is equal to local socket
  // address, or if the same address has been added by another STUN server,
  // then discarding the stun address.
  // For STUN, related address is the local socket address.
  if ((!SharedSocket() || stun_reflected_addr != socket_->GetLocalAddress()) &&
      !HasCandidateWithAddress(stun_reflected_addr)) {
    rtc::SocketAddress related_address = socket_->GetLocalAddress();
    // If we can't stamp the related address correctly, empty it to avoid leak.
    if (!MaybeSetDefaultLocalAddress(&related_address)) {
      related_address =
          rtc::EmptySocketAddressWithFamily(related_address.family());
    }
    rtc::StringBuilder url;
    url << "stun:" << stun_server_addr.ipaddr().ToString() << ":"
        << stun_server_addr.port();
    AddAddress(stun_reflected_addr, socket_->GetLocalAddress(), related_address,
               UDP_PROTOCOL_NAME, "", "", STUN_PORT_TYPE,
               ICE_TYPE_PREFERENCE_SRFLX, 0, url.str(), false);
  }
  MaybeSetPortCompleteOrError();
}