rtt calculation handles time go backwards

webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc

Index: webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc
diff --git a/webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc b/webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc
index 0259b73ff4e12fd666fda20d4b986df85d9d22bd..0098beb94030d5b2429e0b582c0d1fc32d6bf38f 100644
--- a/webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtcp_receiver.cc
@@ -13,8 +13,6 @@
 #include <assert.h>
 #include <string.h>
 
-#include <algorithm>
-
 #include "webrtc/base/checks.h"
 #include "webrtc/base/logging.h"
 #include "webrtc/base/trace_event.h"
@@ -521,7 +519,7 @@ void RTCPReceiver::HandleReportBlock(
   }
 
   uint32_t send_time = rtcpPacket.ReportBlockItem.LastSR;
-  uint32_t rtt = 0;
+  int64_t rtt = 0;
 
   if (send_time > 0) {
     uint32_t delay = rtcpPacket.ReportBlockItem.DelayLastSR;
@@ -531,8 +529,7 @@ void RTCPReceiver::HandleReportBlock(
     // RTT in 1/(2^16) seconds.
     uint32_t rtt_ntp = receive_time - delay - send_time;
     // Convert to 1/1000 seconds (milliseconds).
-    uint32_t rtt_ms = CompactNtpIntervalToMs(rtt_ntp);
-    rtt = std::max<uint32_t>(rtt_ms, 1);
+    rtt = CompactNtpRttToMs(rtt_ntp);

[åsapersson, 2016/03/08 12:59:44]

I think this will now also result in RTT estimates for receive streams (earlier
prevented by _rtpRtcp.SendTimeOfSendReport > 0) and accounted for several times
in the CallStats class? Maybe add a check that this is not a receive_only
channel (and the opposite for XR rtt)?

[danilchap, 2016/03/08 15:02:02]

It shouldn't: send_time should be set to 0 to indicate there is no send time.
That is checked instead of previous _rtpRtcp.SendTimeOfSendReport > 0. Same is
for XR rtt.
Added comment for SR, fixed comment for XR.

[åsapersson, 2016/03/08 15:22:30]

The receive stream(s) has the same main_ssrc_ as the send stream. The received
RTCP packets are input to all streams, therefore i think receive streams also
will get an RTT estimate (which is picked up the the CallStats class).

     if (rtt > reportBlock->maxRTT) {
       // Store max RTT.
       reportBlock->maxRTT = rtt;
@@ -927,8 +924,7 @@ void RTCPReceiver::HandleXrDlrrReportBlockItem(
   uint32_t now = CompactNtp(NtpTime(*_clock));
 
   uint32_t rtt_ntp = now - delay_rr - send_time;
-  uint32_t rtt_ms = CompactNtpIntervalToMs(rtt_ntp);
-  xr_rr_rtt_ms_ = std::max<uint32_t>(rtt_ms, 1);
+  xr_rr_rtt_ms_ = CompactNtpRttToMs(rtt_ntp);
 
   rtcpPacketInformation.rtcpPacketTypeFlags |= kRtcpXrDlrrReportBlock;
 }