srtp: fix decrypt errors

Before this patch when the the sender is restarted, we get the last sequence number from media_encoder that is refreshed on each frame only. But the sequence number continue to be incremented on each RTP packet sent, this result in rollover counter (ROC) desynchronisation in srtp context because the largest sequence number sent is now superior to sequence number that is supposed to occur only when sequence number overflow 65535.

With the patch when the sender is restarted, we keep the last sequence number from socket_pair (RTP level)

Change-Id: I531e3d0a073c251c78bbf9f0ffc702aafaf6ccc8

src/media/socket_pair.cpp

@@ -520,9 +520,9 @@ SocketPair::readCallback(uint8_t* buf, int buf_size)
                 rtpDelayCallback_(gradient, deltaT);
 
         auto err = ff_srtp_decrypt(&srtpContext_->srtp_in, buf, &len);
-        if(packetLossCallback_ and (buf[2] << 8 | buf[3]) != lastSeqNum_+1)
+        if(packetLossCallback_ and (buf[2] << 8 | buf[3]) != lastSeqNumIn_+1)
             packetLossCallback_();
-        lastSeqNum_ = buf[2] << 8 | buf[3];
+        lastSeqNumIn_ = buf[2] << 8 | buf[3];
         if (err < 0)
             JAMI_WARN("decrypt error %d", err);
     }
@@ -574,6 +574,9 @@ SocketPair::writeData(uint8_t* buf, int buf_size)
 int
 SocketPair::writeCallback(uint8_t* buf, int buf_size)
 {
+    if (noWrite_)
+        return 0;
+
     int ret;
     bool isRTCP = RTP_PT_IS_RTCP(buf[1]);
     unsigned int ts_LSB, ts_MSB;
@@ -701,4 +704,13 @@ SocketPair::parse_RTP_ext(uint8_t* buf, float* abs)
     return true;
 }
 
+uint16_t
+SocketPair::lastSeqValOut()
+{
+    if (srtpContext_)
+        return srtpContext_->srtp_out.seq_largest;
+    JAMI_ERR("SRTP context not found.");
+    return 0;
+}
+
 } // namespace jami

src/media/socket_pair.h

@@ -175,6 +175,8 @@ class SocketPair {
 
         int writeData(uint8_t* buf, int buf_size);
 
+        uint16_t lastSeqValOut();
+
     private:
         NON_COPYABLE(SocketPair);
         using clock = std::chrono::steady_clock;
@@ -222,7 +224,7 @@ class SocketPair {
         std::list<double> histoLatency_;
 
         time_point lastRR_time;
-        uint16_t lastSeqNum_ {0};
+        uint16_t lastSeqNumIn_ {0};
         float lastSendTS_ {0.0f};
         time_point lastReceiveTS_ {};
         time_point arrival_TS {};

src/media/video/video_rtp_session.cpp

@@ -133,13 +133,14 @@ void VideoRtpSession::startSender()
         send_.linkableHW = conference_ == nullptr;
         send_.bitrate = videoBitrateInfo_.videoBitrateCurrent;
 
-        if (sender_)
-            initSeqVal_ = sender_->getLastSeqValue() + 10; // Skip a few sequences to make nvenc happy on a sender restart
+        if (socketPair_)
+            initSeqVal_ = socketPair_->lastSeqValOut();
+
         try {
             sender_.reset();
             socketPair_->stopSendOp(false);
             sender_.reset(new VideoSender(getRemoteRtpUri(), localVideoParams_,
-                                          send_, *socketPair_, initSeqVal_, mtu_));
+                                          send_, *socketPair_, initSeqVal_+1, mtu_));
             if (changeOrientationCallback_)
                 sender_->setChangeOrientationCallback(changeOrientationCallback_);
             if (socketPair_)