Definitions
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RTT(Round-Trip Time): 往返时延。在计算机网络中它是一个重要的性能指标，表示从发送端发送数据开始，到发送端收到来自接收端的确认（接收端收到数据后便立即发送确认），总共经历的时延。
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一般认为单向时延=传输时延t1+传播时延t2+排队时延t3 t1是数据从进入节点到传输媒体所需要的时间，通常等于数据块长度/信道带宽 t2是信号在信道中需要传播一定距离而花费的时间，等于信道长度/传播速率（光纤中电磁波的传播速率约为210^5 km/s，铜缆中2.310^5 km/s） t3可笼统归纳为随机噪声，由途径的每一跳设备及收发两端负荷情况及吞吐排队情况决定(包含互联网设备和传输设备时延) 综上：时延并无标准值只有经验值。某运营商规定网内路由器间时延1000公里之内<=40ms,2000公里之内<=60ms,3000公里之内<=80ms
RTT Calculation
大写S代表发送时间，R代表接收时间，D代表延时 S(a0) A------------------------------------------------------ > B R( b0)
S(a0) D(b0) A<--------------------------------------------------------B D(b0) = S(b0) - R(b0)
1,A先发一个数据包，在本地记录下发送时间 S(a0),并把 S(a0)放在数据包中发给B 2,B收到数据包，在本地记录下数据接收时间R( b0),并把数据包发回A,发送时把本地处理延时时间 D(b0)也记录到数据包中，D(b0) = S(b0) - R(b0) 3,A收到B的回应后，记录到数据接收时间R(a0) 4, rtt = R(a0) - S(a0) - D0
WebRtc RTT Calculation
1.名词解释
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LSR: 最近一次SR包的NTP时间戳(remote_sender_ntp_time_)；LSR由NTP秒(second)低16位和毫秒(fraction)高16位组合而成;
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DLSR: 最近一次收到SR包到打包Report Block包的间隔.
SR报文格式：
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ header |V=2|P| RC | PT=SR=200 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ sender | NTP timestamp, most significant word | info +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NTP timestamp, least significant word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sender's packet count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sender's octet count | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ report | SSRC_1 (SSRC of first source) | block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 | fraction lost | cumulative number of packets lost | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | extended highest sequence number received | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | interarrival jitter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | last SR (LSR) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | delay since last SR (DLSR) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
RR报文格式：
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ header |V=2|P| RC | PT=RR=201 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of packet sender | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ report | SSRC_1 (SSRC of first source) | block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 | fraction lost | cumulative number of packets lost | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | extended highest sequence number received | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | interarrival jitter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | last SR (LSR) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | delay since last SR (DLSR) | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ report | SSRC_2 (SSRC of second source) | block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2 : ... : +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | profile-specific extensions | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2. 探测流程
探测流程:
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发送端构造和发送SR包,携带发送时间戳
LSR; -
接收端接收到最新的SR之后,使用
last_received_sr_ntp_字段记录当前ntp时间戳; -
接收端构造RR包,设置DLSR字段为
当前ntp时间戳 - last_received_sr_ntp_,之后发出RR包; -
发送端在接收到RR包之后,记录RR包到达时间
now_ntp; -
计算rtt:
now_ntp - LSR - DLSR
SR和RR包的数量并不需要完全相同,它们之间并不是一一对应的关系,而是相互独立发送的,各自按照自己的发送节奏发送数据. 即使SR或者RR丢失了一部分,只要发送端接收到过RR,它总能计算出rtt,因为发送端只需要一次RR包中的LSR和DLSR字段就能够算出一次rtt.
3. 更新流程
由上一步得到的rtt值会被传递到CallStats中进行定时更新操作(CallStats::Process), 时间间隔为1s(kUpdateIntervalMs). 一次处理流程CallStats::Process为:
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RemoveOldReports移除1.5s之前的旧的rtt数据; -
计算最近1.5s之内的rtt平均值和最大值;
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如果最大值和平均值都是非负数,那么便认为rtt合法;
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以平均rtt值和之前的旧的值做一个加权(3:7)作为最终的rtt计算值,通知所有观察者.
4. 相关代码
`RTCPSender::BuildSR` `RTCPSender::BuildRR` `RTCPSender::SendCompoundRTCP` `RTCPReceiver::HandleReceiverReport` `RTCPReceiver::HandleReportBlock
获取LSR
bool ModuleRtpRtcpImpl::LastReceivedNTP(
uint32_t* rtcp_arrival_time_secs, // When we got the last report.
uint32_t* rtcp_arrival_time_frac,
uint32_t* remote_sr) const {
// Remote SR: NTP inside the last received (mid 16 bits from sec and frac).
uint32_t ntp_secs = 0;
uint32_t ntp_frac = 0;
if (!rtcp_receiver_.NTP(&ntp_secs,
&ntp_frac,
rtcp_arrival_time_secs,
rtcp_arrival_time_frac,
NULL)) {
return false;
}
//remote_sr 就是 last SR (LSR) LSR由NTP秒(second)低16位和毫秒(fraction)高16位组合而成;
*remote_sr =
((ntp_secs & 0x0000ffff) << 16) + ((ntp_frac & 0xffff0000) >> 16);
return true;
}
计算DLSR
bool RTCPSender::AddReportBlock(const FeedbackState& feedback_state,
uint32_t ssrc,
StreamStatistician* statistician) {
...
...
...
// Delay since last received report.
if ((feedback_state.last_rr_ntp_secs != 0) ||
(feedback_state.last_rr_ntp_frac != 0)) {
// Get the 16 lowest bits of seconds and the 16 highest bits of fractions.
uint32_t now = CompactNtp(ntp);
//接收到sr的的时间 last_rr_ntp_secs last_rr_ntp_frac
uint32_t receiveTime = feedback_state.last_rr_ntp_secs & 0x0000FFFF;
receiveTime <<= 16;
receiveTime += (feedback_state.last_rr_ntp_frac & 0xffff0000) >> 16;
block->SetDelayLastSr(now - receiveTime);
}
return true;
}
计算RTT
void RTCPReceiver::HandleReportBlock(const ReportBlock& report_block,
PacketInformation* packet_information,
uint32_t remote_ssrc) {
...
...
...
int64_t rtt_ms = 0;
//LSR
uint32_t send_time_ntp = report_block.last_sr();
// RFC3550, section 6.4.1, LSR field discription states:
// If no SR has been received yet, the field is set to zero.
// Receiver rtp_rtcp module is not expected to calculate rtt using
// Sender Reports even if it accidentally can.
if (!receiver_only_ && send_time_ntp != 0) {
//DLSR
uint32_t delay_ntp = report_block.delay_since_last_sr();
// Local NTP time.
uint32_t receive_time_ntp = CompactNtp(clock_->CurrentNtpTime());
//计算RTT
// RTT in 1/(2^16) seconds.
uint32_t rtt_ntp = receive_time_ntp - delay_ntp - send_time_ntp;
// Convert to 1/1000 seconds (milliseconds).
rtt_ms = CompactNtpRttToMs(rtt_ntp);
if (rtt_ms > report_block_info->max_rtt_ms)
report_block_info->max_rtt_ms = rtt_ms;
if (report_block_info->num_rtts == 0 ||
rtt_ms < report_block_info->min_rtt_ms)
report_block_info->min_rtt_ms = rtt_ms;
report_block_info->last_rtt_ms = rtt_ms;
report_block_info->sum_rtt_ms += rtt_ms;
++report_block_info->num_rtts;
}
...
...
...
}
更新回调
void CallStats::Process() {
RTC_DCHECK_RUN_ON(&process_thread_checker_);
int64_t now = clock_->TimeInMilliseconds();
last_process_time_ = now;
// |avg_rtt_ms_| is allowed to be read on the process thread since that's the
// only thread that modifies the value.
int64_t avg_rtt_ms = avg_rtt_ms_;
RemoveOldReports(now, &reports_);
max_rtt_ms_ = GetMaxRttMs(reports_);
avg_rtt_ms = GetNewAvgRttMs(reports_, avg_rtt_ms);
{
rtc::CritScope lock(&avg_rtt_ms_lock_);
avg_rtt_ms_ = avg_rtt_ms;
}
// If there is a valid rtt, update all observers with the max rtt.
if (max_rtt_ms_ >= 0) {
RTC_DCHECK_GE(avg_rtt_ms, 0);
for (CallStatsObserver* observer : observers_)
observer->OnRttUpdate(avg_rtt_ms, max_rtt_ms_);
// Sum for Histogram of average RTT reported over the entire call.
sum_avg_rtt_ms_ += avg_rtt_ms;
++num_avg_rtt_;
}
}
int64_t GetNewAvgRttMs(const std::list<CallStats::RttTime>& reports,
int64_t prev_avg_rtt) {
if (reports.empty())
return -1; // Reset (invalid average).
int64_t cur_rtt_ms = GetAvgRttMs(reports);
if (prev_avg_rtt == -1)
return cur_rtt_ms; // New initial average value.
// Weight factor to apply to the average rtt.
// We weigh the old average at 70% against the new average (30%).
constexpr const float kWeightFactor = 0.3f;
return prev_avg_rtt * (1.0f - kWeightFactor) + cur_rtt_ms * kWeightFactor;
}
References
[http://www.voidcn.com/article/p-dkzjvoaj-bpd.html] http://www.voidcn.com/article/p-dkzjvoaj-bpd.html
[https://www.cnblogs.com/xl2432/p/13424448.html] https://www.cnblogs.com/xl2432/p/13424448.html