Outline

Bandwidth Estimation

       Delay Based Control       Loss Based Control
Error Concealment
       RTP  Retransmission(NACK)       FEC       PLI/SLI/RPSI
Packet Arrival Model
Group of Packets, merge together the packets arrived  in a burst.Using Abstract Send Time – RTP Head Extension.T(i) – T(i-1)  means  the difference of the departure time of packet group i and i-1. t(i) – t(i-1)  means the difference of the arrival time of the packet group i and i-1The difference of delay between the packet group i and i-1 is defined as d(i) = t(i) – t(i-1) – (T(i) – T(i-1))Model the d(i) as stochastic PRocess w:   d(i) = w(i) = m(i) + v(i),  m(i) is the mean of stochastic process w,   v(i) is the noise. 
Arrival Time Filter
Model the m(i+1) = m(i) + u(i),  u(i) is the Gaussion statistic with the zero mean and variance q(i), q(i) is RECOMMANDED equal to 10^-3.d(i) = m(i) + v(i),  v(i) is zero mean white noise var_v = E{v(i)^2}.z(i) = d(i) – m_hat(i), where m_hat(i) is the estimation of m(i).m_hat(i) = m_hat(i-1)  + z(i) * k(i)k(i) = (e(i-1) + q(i))/(var_v_hat(i) + e(i-1) + q(i))e(i) = (1-k(i))*(e(i-1)+q(i))var_v_hat(i) = max(alpha*var_v_hat(i-1) + (1-alpha)*z(i)^2, 1)alpha = (1-chi)^(30/1000*f_max),   where f_max = max{1/(T(j) – T(j-1)} for j in i-K+1, … , i,  chi is the filter coefficient, typically choose as a number in [0.001, 0.1].
Over-use Detector
The output of  Arrival Time Filter is the delay estimation m(i).m(i) is compared with the threshold del_var_th(i), An estimate above the threshold is considered as an indication of over-use, but over-use will be signaled only if over-use has been detected for at least overuse_time_th milliseconds, however, if m(i) less than m(i-1), overuse will not be signaled.   Similarly, the underuse is detected when m(i) less than -del_var_th(i).  If neigher overuse or underuse is detected,  the detector will be in normal state. del_var_th(i) = del_var_th(i) + (t(i)-t(i-1))*K(i)*(|m(i)| - del_var_th(i-1)),  where del_var_th(is) is RECOMMANDED to clamp to the range [6, 600].
Rate Control
Delay Based Estimation      R_hat_delay(i) = 1/T * sum(L(j)) for j from 1 to N(i)      A_hat_delay(i) = A_hat_delay(i-1) + max(1000, alpha*expected_packet_size_bits) if under-use       A_hat_delay(i) = beta*R_hat_delay(i),   beta is RECOMMANDED to 0.85 if over-useLoss Based Estimation      Do nothing if loss between 2% ~ 10%      R_hat_loss(i) = R_hat(i-1)*(1 – 0.5*lossRate) if loss is more than 10%      R_hat_lss(i) = R_hat(i-1)*(1+0.08) if loss is less than 2% Combined Rate Control      R_hat = min(R_hat_loss, A_hat_delay)
REMB Format
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|V=2|P| FMT=15 | PT=206 | length |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SSRC of packet sender |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SSRC of media source |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Unique identifier ’R’ ’E’ ’M’ ’B’ |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Num SSRC | BR Exp | BR Mantissa |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| SSRC feedback |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+FMT = 15,   application Layer Message.  PT = 206,  Payload Specific Feedback Message
Retransmission and FEC Combined Error Concealment
FEC – Forward Error Correction      RFC 5109 XOR FEC,  [m, 1] Pattern,   1 redundant packet for 1 packets group.RTP Retransmission      Retrasmission plays key roles in lossy network as our test.       How to build NACK feedback,   dependent on Video Jitter Buffer Status.       Sender/Receiver Delay Control,   lots of accuracy control here.  PLI/SLI/RPSI     To re-sync decoder with encoder. 
Reference:
Draft-ietf-rmcat-gcc-02Draft-ietf-rtcweb-rtp-usage-26Draft-alverstrand-rmcat-remb-03RFC 3550  RTPRFC 4584  Extended RTP ProfileRFC 5104  Codec Control Message in the RTP AVPFOfficial Site:  www.webrtc.org
From: http://blog.csdn.net/volvet/article/details/53177643