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      1 /*
      2  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
      3  *
      4  *  Use of this source code is governed by a BSD-style license
      5  *  that can be found in the LICENSE file in the root of the source
      6  *  tree. An additional intellectual property rights grant can be found
      7  *  in the file PATENTS.  All contributing project authors may
      8  *  be found in the AUTHORS file in the root of the source tree.
      9  */
     10 
     11 #include "webrtc/system_wrappers/interface/rtp_to_ntp.h"
     12 
     13 #include "webrtc/system_wrappers/interface/clock.h"
     14 
     15 #include <assert.h>
     16 
     17 namespace webrtc {
     18 
     19 RtcpMeasurement::RtcpMeasurement()
     20     : ntp_secs(0), ntp_frac(0), rtp_timestamp(0) {}
     21 
     22 RtcpMeasurement::RtcpMeasurement(uint32_t ntp_secs, uint32_t ntp_frac,
     23                                  uint32_t timestamp)
     24     : ntp_secs(ntp_secs), ntp_frac(ntp_frac), rtp_timestamp(timestamp) {}
     25 
     26 // Calculates the RTP timestamp frequency from two pairs of NTP and RTP
     27 // timestamps.
     28 bool CalculateFrequency(
     29     int64_t rtcp_ntp_ms1,
     30     uint32_t rtp_timestamp1,
     31     int64_t rtcp_ntp_ms2,
     32     uint32_t rtp_timestamp2,
     33     double* frequency_khz) {
     34   if (rtcp_ntp_ms1 <= rtcp_ntp_ms2) {
     35     return false;
     36   }
     37   *frequency_khz = static_cast<double>(rtp_timestamp1 - rtp_timestamp2) /
     38       static_cast<double>(rtcp_ntp_ms1 - rtcp_ntp_ms2);
     39   return true;
     40 }
     41 
     42 // Detects if there has been a wraparound between |old_timestamp| and
     43 // |new_timestamp|, and compensates by adding 2^32 if that is the case.
     44 bool CompensateForWrapAround(uint32_t new_timestamp,
     45                              uint32_t old_timestamp,
     46                              int64_t* compensated_timestamp) {
     47   assert(compensated_timestamp);
     48   int64_t wraps = CheckForWrapArounds(new_timestamp, old_timestamp);
     49   if (wraps < 0) {
     50     // Reordering, don't use this packet.
     51     return false;
     52   }
     53   *compensated_timestamp = new_timestamp + (wraps << 32);
     54   return true;
     55 }
     56 
     57 bool UpdateRtcpList(uint32_t ntp_secs,
     58                     uint32_t ntp_frac,
     59                     uint32_t rtp_timestamp,
     60                     RtcpList* rtcp_list,
     61                     bool* new_rtcp_sr) {
     62   *new_rtcp_sr = false;
     63   if (ntp_secs == 0 && ntp_frac == 0) {
     64     return false;
     65   }
     66 
     67   RtcpMeasurement measurement;
     68   measurement.ntp_secs = ntp_secs;
     69   measurement.ntp_frac = ntp_frac;
     70   measurement.rtp_timestamp = rtp_timestamp;
     71 
     72   for (RtcpList::iterator it = rtcp_list->begin();
     73        it != rtcp_list->end(); ++it) {
     74     if (measurement.ntp_secs == (*it).ntp_secs &&
     75         measurement.ntp_frac == (*it).ntp_frac) {
     76       // This RTCP has already been added to the list.
     77       return true;
     78     }
     79   }
     80 
     81   // We need two RTCP SR reports to map between RTP and NTP. More than two will
     82   // not improve the mapping.
     83   if (rtcp_list->size() == 2) {
     84     rtcp_list->pop_back();
     85   }
     86   rtcp_list->push_front(measurement);
     87   *new_rtcp_sr = true;
     88   return true;
     89 }
     90 
     91 // Converts |rtp_timestamp| to the NTP time base using the NTP and RTP timestamp
     92 // pairs in |rtcp|. The converted timestamp is returned in
     93 // |rtp_timestamp_in_ms|. This function compensates for wrap arounds in RTP
     94 // timestamps and returns false if it can't do the conversion due to reordering.
     95 bool RtpToNtpMs(int64_t rtp_timestamp,
     96                 const RtcpList& rtcp,
     97                 int64_t* rtp_timestamp_in_ms) {
     98   assert(rtcp.size() == 2);
     99   int64_t rtcp_ntp_ms_new = Clock::NtpToMs(rtcp.front().ntp_secs,
    100                                            rtcp.front().ntp_frac);
    101   int64_t rtcp_ntp_ms_old = Clock::NtpToMs(rtcp.back().ntp_secs,
    102                                            rtcp.back().ntp_frac);
    103   int64_t rtcp_timestamp_new = rtcp.front().rtp_timestamp;
    104   int64_t rtcp_timestamp_old = rtcp.back().rtp_timestamp;
    105   if (!CompensateForWrapAround(rtcp_timestamp_new,
    106                                rtcp_timestamp_old,
    107                                &rtcp_timestamp_new)) {
    108     return false;
    109   }
    110   double freq_khz;
    111   if (!CalculateFrequency(rtcp_ntp_ms_new,
    112                           rtcp_timestamp_new,
    113                           rtcp_ntp_ms_old,
    114                           rtcp_timestamp_old,
    115                           &freq_khz)) {
    116     return false;
    117   }
    118   double offset = rtcp_timestamp_new - freq_khz * rtcp_ntp_ms_new;
    119   int64_t rtp_timestamp_unwrapped;
    120   if (!CompensateForWrapAround(rtp_timestamp, rtcp_timestamp_old,
    121                                &rtp_timestamp_unwrapped)) {
    122     return false;
    123   }
    124   double rtp_timestamp_ntp_ms = (static_cast<double>(rtp_timestamp_unwrapped) -
    125       offset) / freq_khz + 0.5f;
    126   if (rtp_timestamp_ntp_ms < 0) {
    127     return false;
    128   }
    129   *rtp_timestamp_in_ms = rtp_timestamp_ntp_ms;
    130   return true;
    131 }
    132 
    133 int CheckForWrapArounds(uint32_t new_timestamp, uint32_t old_timestamp) {
    134   if (new_timestamp < old_timestamp) {
    135     // This difference should be less than -2^31 if we have had a wrap around
    136     // (e.g. |new_timestamp| = 1, |rtcp_rtp_timestamp| = 2^32 - 1). Since it is
    137     // cast to a int32_t, it should be positive.
    138     if (static_cast<int32_t>(new_timestamp - old_timestamp) > 0) {
    139       // Forward wrap around.
    140       return 1;
    141     }
    142   } else if (static_cast<int32_t>(old_timestamp - new_timestamp) > 0) {
    143     // This difference should be less than -2^31 if we have had a backward wrap
    144     // around. Since it is cast to a int32_t, it should be positive.
    145     return -1;
    146   }
    147   return 0;
    148 }
    149 
    150 }  // namespace webrtc
    151