SRS RTC NACK源码分析

SRS作为接收方，对端作为发送方

生成Nack RTCP包的流程


SrsRtcConnection::SrsRtcConnection(SrsRtcServer* s, const SrsContextId& cid)
{req = NULL;cid_ = cid;hijacker_ = NULL;sendonly_skt = NULL;server_ = s;transport_ = new SrsSecurityTransport(this);cache_iov_ = new iovec();cache_iov_->iov_base = new char[kRtpPacketSize];cache_iov_->iov_len = kRtpPacketSize;cache_buffer_ = new SrsBuffer((char*)cache_iov_->iov_base, kRtpPacketSize);state_ = INIT;last_stun_time = 0;session_timeout = 0;disposing_ = false;twcc_id_ = 0;nn_simulate_player_nack_drop = 0;pp_address_change = new SrsErrorPithyPrint();pli_epp = new SrsErrorPithyPrint();nack_enabled_ = false;timer_nack_ = new SrsRtcConnectionNackTimer(this);_srs_rtc_manager->subscribe(this);
}

在SrsRtcConnection构造函数中，创建nack的定时器：timer_nack_ = new SrsRtcConnectionNackTimer(this)。nack定时器用于定时发送nack包。

SrsRtcConnectionNackTimer::SrsRtcConnectionNackTimer(SrsRtcConnection* p) : p_(p)
{_srs_hybrid->timer20ms()->subscribe(this);
}

RTC连接的Nack定时器注册到服务的全局定时器_srs_hybrid中，使用的是20毫秒定时器。定时器的回调函数如下：

srs_error_t SrsRtcConnectionNackTimer::on_timer(srs_utime_t interval)
{srs_error_t err = srs_success;//判断是否启用NACKif (!p_->nack_enabled_) {return err;}//全局的统计++_srs_pps_conn->sugar;// If circuit-breaker is enabled, disable nack.//如果断路器启用并且过载，则禁用Nack。断路器用于保护系统不过载。if (_srs_circuit_breaker->hybrid_critical_water_level()) {++_srs_pps_snack4->sugar;return err;}//查找所有的rtc推流，检测是否需要发送Nack包。std::map<std::string, SrsRtcPublishStream*>::iterator it;for (it = p_->publishers_.begin(); it != p_->publishers_.end(); it++) {SrsRtcPublishStream* publisher = it->second;if ((err = publisher->check_send_nacks()) != srs_success) {srs_warn("ignore nack err %s", srs_error_desc(err).c_str());srs_freep(err);}}return err;
}

对于每个RTC推流，都拥有音频接收轨道SrsRtcAudioRecvTrack和视频接收轨道SrsRtcVideoRecvTrack。分别去请求发送视频轨道和音频轨道的NACK信息


srs_error_t SrsRtcPublishStream::check_send_nacks()
{srs_error_t err = srs_success;if (!nack_enabled_) {return err;}for (int i = 0; i < (int)video_tracks_.size(); ++i) {SrsRtcVideoRecvTrack* track = video_tracks_.at(i);if ((err = track->check_send_nacks()) != srs_success) {return srs_error_wrap(err, "video track=%s", track->get_track_id().c_str());}}for (int i = 0; i < (int)audio_tracks_.size(); ++i) {SrsRtcAudioRecvTrack* track = audio_tracks_.at(i);if ((err = track->check_send_nacks()) != srs_success) {return srs_error_wrap(err, "audio track=%s", track->get_track_id().c_str());}}return err;
}

对于视频轨道：调用父类的do_check_send_nacks函数，可以获取超时的nack包数量，如果有超时的RTP包，可以去请求PLI，即请求视频关键帧数据（未实现）。

srs_error_t SrsRtcVideoRecvTrack::check_send_nacks()
{srs_error_t err = srs_success;++_srs_pps_svnack->sugar;uint32_t timeout_nacks = 0;if ((err = do_check_send_nacks(timeout_nacks)) != srs_success) {return srs_error_wrap(err, "video");}// If NACK timeout, start PLI if not requesting.if (timeout_nacks == 0) {return err;}srs_trace2(TAG_MAYBE, "RTC: NACK timeout=%u, request PLI, track=%s, ssrc=%u", timeout_nacks,track_desc_->id_.c_str(), track_desc_->ssrc_);return err;
}

对于音频轨道：直接调用父类的do_check_send_nacks函数即可。

srs_error_t SrsRtcAudioRecvTrack::check_send_nacks()
{srs_error_t err = srs_success;++_srs_pps_sanack->sugar;uint32_t timeout_nacks = 0;if ((err = do_check_send_nacks(timeout_nacks)) != srs_success) {return srs_error_wrap(err, "audio");}return err;
}

父类的do_check_send_nacks直接调用所在会话的check_send_nacks。

srs_error_t SrsRtcRecvTrack::do_check_send_nacks(uint32_t& timeout_nacks)
{srs_error_t err = srs_success;uint32_t sent_nacks = 0;session_->check_send_nacks(nack_receiver_, track_desc_->ssrc_, sent_nacks, timeout_nacks);return err;
}

void SrsRtcConnection::check_send_nacks(SrsRtpNackForReceiver* nack, uint32_t ssrc, uint32_t& sent_nacks, uint32_t& timeout_nacks)
{//全局统计数据++_srs_pps_snack->sugar;//创建RTCP Nack对象SrsRtcpNack rtcpNack(ssrc);//设置nack对象的ssrc，获取并设置需要反馈的丢包数据rtcpNack.set_media_ssrc(ssrc);nack->get_nack_seqs(rtcpNack, timeout_nacks);if(rtcpNack.empty()){return;}++_srs_pps_snack2->sugar;++_srs_pps_srtcps->sugar;char buf[kRtcpPacketSize];SrsBuffer stream(buf, sizeof(buf));// 对Nack对象进行封装为RTCP包的操作// TODO: FIXME: Check error.rtcpNack.encode(&stream);//加密RTCP包// TODO: FIXME: Check error.int nb_protected_buf = stream.pos();transport_->protect_rtcp(stream.data(), &nb_protected_buf);//将加密后的RTCP数据发送出去。// TODO: FIXME: Check error.sendonly_skt->sendto(stream.data(), nb_protected_buf, 0);
}

获取需要请求NACK的丢包数据


void SrsRtpNackForReceiver::get_nack_seqs(SrsRtcpNack& seqs, uint32_t& timeout_nacks)
{// If circuit-breaker is enabled, disable nack.if (_srs_circuit_breaker->hybrid_high_water_level()) {queue_.clear();++_srs_pps_snack4->sugar;return;}srs_utime_t now = srs_get_system_time();srs_utime_t interval = now - pre_check_time_;if (interval < opts_.nack_check_interval) {return;}pre_check_time_ = now;std::map<uint16_t, SrsRtpNackInfo>::iterator iter = queue_.begin();while (iter != queue_.end()) {const uint16_t& seq = iter->first;SrsRtpNackInfo& nack_info = iter->second;int alive_time = now - nack_info.generate_time_;if (alive_time > opts_.max_alive_time || nack_info.req_nack_count_ > opts_.max_count) {++timeout_nacks;rtp_->notify_drop_seq(seq);queue_.erase(iter++);continue;}// TODO:Statistics unorder packet.if (now - nack_info.generate_time_ < opts_.first_nack_interval) {break;}srs_utime_t nack_interval = srs_max(opts_.min_nack_interval, opts_.nack_interval / 3);if(opts_.nack_interval < 50 * SRS_UTIME_MILLISECONDS){nack_interval = srs_max(opts_.min_nack_interval, opts_.nack_interval);}if (now - nack_info.pre_req_nack_time_ >= nack_interval ) {++nack_info.req_nack_count_;nack_info.pre_req_nack_time_ = now;seqs.add_lost_sn(seq);}++iter;}
}

1、首先判断当前系统的负载水位。
2、获取当前时间。后面的多个操作都依赖时间差值比较。
3、如果上次nack的检测时间和当前时间差值小于配置的最小检测间隔，则直接返回。否则设置上次检测时间为当前时间，供下一轮比较使用。
4、遍历丢包的queue_，queue_是一个map。
5、获取nack_info已经在queue_中的生存时间alive_time，如果alive_time大于配置的最大生存时间；或者该nack_info请求重传的次数已经大于配置的最大重传次数，就会从queue_中删除并通知SrsRtpRingBuffer清除该nack_info对应的序列号。这是为了避免重传过早的数据导致增加延迟。
6、如果nack_info生成时间和当前时间差值小于配置的最小首次请求nack间隔，直接跳过。这是为了避免请求重传刚刚发现的丢包数据，可能并不是真正的丢包，只是还在网络链路传输中。
7、计算nack_interval，和rtt相关。在update_rtt中会重新计算opts_.nack_interval
8、当前时间减去上次请求nack的时间超过nack_interval时间差的丢包数据才会加入到SrsRtcpNack对象seqs中，请求重传。

接收数据，判断是否加入到nack_receiver_中

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解密后的rtp数据

SrsRtcServer::on_udp_packet

session->on_rtp

publisher->on_rtp

on_rtp_plaintext

do_on_rtp_plaintext

audio_track->on_nack

video_track->on_nack

SrsRtcRecvTrack::on_nack

可以看到SRS对接收到到每个RTP包都会进入到SrsRtcRecvTrack::on_nack的处理逻辑中，接下来我们分析下该函数的具体逻辑。


srs_error_t SrsRtcRecvTrack::on_nack(SrsRtpPacket** ppkt)
{srs_error_t err = srs_success;SrsRtpPacket* pkt = *ppkt;uint16_t seq = pkt->header.get_sequence();SrsRtpNackInfo* nack_info = nack_receiver_->find(seq);if (nack_info) {// seq had been received.nack_receiver_->remove(seq);return err;}// insert check nack listuint16_t nack_first = 0, nack_last = 0;if (!rtp_queue_->update(seq, nack_first, nack_last)) {srs_warn("NACK: too old seq %u, range [%u, %u]", seq, rtp_queue_->begin, rtp_queue_->end);}if (srs_rtp_seq_distance(nack_first, nack_last) > 0) {srs_trace("NACK: update seq=%u, nack range [%u, %u]", seq, nack_first, nack_last);nack_receiver_->insert(nack_first, nack_last);nack_receiver_->check_queue_size();}// insert into video_queue and audio_queue// We directly use the pkt, never copy it, so we should set the pkt to NULL.if (nack_no_copy_) {rtp_queue_->set(seq, pkt);*ppkt = NULL;} else {rtp_queue_->set(seq, pkt->copy());}return err;
}

1、判断该序号到pkt是否已经加入到nack_receiver_中；

2、根据当前接收到的包的序号更新rtp_queue_中的起始和终止位置；

3、如果nack_last大于nack_first，将nack_first到nack_last之间的所有包都当作未接收到的包；

4、将当前接收到的RTP包加入到已经接收到的RtpRingBuffer中。