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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/include/rtp_to_ntp.h"
12 
13 #include "webrtc/system_wrappers/include/clock.h"
14 
15 #include <assert.h>
16 
17 namespace webrtc {
18 
RtcpMeasurement()19 RtcpMeasurement::RtcpMeasurement()
20     : ntp_secs(0), ntp_frac(0), rtp_timestamp(0) {}
21 
RtcpMeasurement(uint32_t ntp_secs,uint32_t ntp_frac,uint32_t timestamp)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.
CalculateFrequency(int64_t rtcp_ntp_ms1,uint32_t rtp_timestamp1,int64_t rtcp_ntp_ms2,uint32_t rtp_timestamp2,double * frequency_khz)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.
CompensateForWrapAround(uint32_t new_timestamp,uint32_t old_timestamp,int64_t * compensated_timestamp)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 
UpdateRtcpList(uint32_t ntp_secs,uint32_t ntp_frac,uint32_t rtp_timestamp,RtcpList * rtcp_list,bool * new_rtcp_sr)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.
RtpToNtpMs(int64_t rtp_timestamp,const RtcpList & rtcp,int64_t * rtp_timestamp_in_ms)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 
CheckForWrapArounds(uint32_t new_timestamp,uint32_t old_timestamp)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