diff --git a/call/rtp_video_sender.cc b/call/rtp_video_sender.cc
index fbfdc09..73e356d 100644
--- a/call/rtp_video_sender.cc
+++ b/call/rtp_video_sender.cc
@@ -177,7 +177,7 @@
     bool enable_flexfec = flexfec_sender != nullptr &&
                           std::find(flexfec_protected_ssrcs.begin(),
                                     flexfec_protected_ssrcs.end(),
-                                    configuration.local_media_ssrc) !=
+                                    *configuration.local_media_ssrc) !=
                               flexfec_protected_ssrcs.end();
     configuration.flexfec_sender = enable_flexfec ? flexfec_sender : nullptr;
     auto playout_delay_oracle = std::make_unique<PlayoutDelayOracle>();
diff --git a/modules/rtp_rtcp/include/rtp_rtcp.h b/modules/rtp_rtcp/include/rtp_rtcp.h
index a046f64..69ca8f8 100644
--- a/modules/rtp_rtcp/include/rtp_rtcp.h
+++ b/modules/rtp_rtcp/include/rtp_rtcp.h
@@ -122,7 +122,7 @@
 
     // SSRCs for media and retransmission, respectively.
     // FlexFec SSRC is fetched from |flexfec_sender|.
-    uint32_t local_media_ssrc;
+    absl::optional<uint32_t> local_media_ssrc;
     absl::optional<uint32_t> rtx_send_ssrc;
 
    private:
@@ -200,6 +200,10 @@
   // Returns SSRC.
   uint32_t SSRC() const override = 0;
 
+  // Sets SSRC, default is a random number.
+  // TODO(bugs.webrtc.org/10774): Remove.
+  virtual void SetSSRC(uint32_t ssrc) = 0;
+
   // Sets the value for sending in the RID (and Repaired) RTP header extension.
   // RIDs are used to identify an RTP stream if SSRCs are not negotiated.
   // If the RID and Repaired RID extensions are not registered, the RID will
@@ -223,6 +227,11 @@
   // a combination of values of the enumerator RtxMode.
   virtual int RtxSendStatus() const = 0;
 
+  // Sets the SSRC to use when sending RTX packets. This doesn't enable RTX,
+  // only the SSRC is set.
+  // TODO(bugs.webrtc.org/10774): Remove.
+  virtual void SetRtxSsrc(uint32_t ssrc) = 0;
+
   // Sets the payload type to use when sending RTX packets. Note that this
   // doesn't enable RTX, only the payload type is set.
   virtual void SetRtxSendPayloadType(int payload_type,
diff --git a/modules/rtp_rtcp/source/nack_rtx_unittest.cc b/modules/rtp_rtcp/source/nack_rtx_unittest.cc
index 17601dd..a75fd6e 100644
--- a/modules/rtp_rtcp/source/nack_rtx_unittest.cc
+++ b/modules/rtp_rtcp/source/nack_rtx_unittest.cc
@@ -134,7 +134,6 @@
     configuration.outgoing_transport = &transport_;
     configuration.retransmission_rate_limiter = &retransmission_rate_limiter_;
     configuration.local_media_ssrc = kTestSsrc;
-    configuration.rtx_send_ssrc = kTestRtxSsrc;
     rtp_rtcp_module_ = RtpRtcp::Create(configuration);
     FieldTrialBasedConfig field_trials;
     RTPSenderVideo::Config video_config;
@@ -201,6 +200,7 @@
     rtx_receiver_ = transport_.stream_receiver_controller_.CreateReceiver(
         kTestRtxSsrc, &rtx_stream_);
     rtp_rtcp_module_->SetRtxSendStatus(rtx_method);
+    rtp_rtcp_module_->SetRtxSsrc(kTestRtxSsrc);
     transport_.DropEveryNthPacket(loss);
     uint32_t timestamp = 3000;
     uint16_t nack_list[kVideoNackListSize];
diff --git a/modules/rtp_rtcp/source/rtcp_receiver.cc b/modules/rtp_rtcp/source/rtcp_receiver.cc
index 6b64473..f06fd1c 100644
--- a/modules/rtp_rtcp/source/rtcp_receiver.cc
+++ b/modules/rtp_rtcp/source/rtcp_receiver.cc
@@ -65,18 +65,6 @@
 
 constexpr int32_t kDefaultVideoReportInterval = 1000;
 constexpr int32_t kDefaultAudioReportInterval = 5000;
-
-std::set<uint32_t> GetRegisteredSsrcs(const RtpRtcp::Configuration& config) {
-  std::set<uint32_t> ssrcs;
-  ssrcs.insert(config.local_media_ssrc);
-  if (config.rtx_send_ssrc) {
-    ssrcs.insert(*config.rtx_send_ssrc);
-  }
-  if (config.flexfec_sender) {
-    ssrcs.insert(config.flexfec_sender->ssrc());
-  }
-  return ssrcs;
-}
 }  // namespace
 
 struct RTCPReceiver::PacketInformation {
@@ -138,8 +126,6 @@
     : clock_(config.clock),
       receiver_only_(config.receiver_only),
       rtp_rtcp_(owner),
-      main_ssrc_(config.local_media_ssrc),
-      registered_ssrcs_(GetRegisteredSsrcs(config)),
       rtcp_bandwidth_observer_(config.bandwidth_callback),
       rtcp_intra_frame_observer_(config.intra_frame_callback),
       rtcp_loss_notification_observer_(config.rtcp_loss_notification_observer),
@@ -151,6 +137,7 @@
                               : (config.audio ? kDefaultAudioReportInterval
                                               : kDefaultVideoReportInterval)),
       // TODO(bugs.webrtc.org/10774): Remove fallback.
+      main_ssrc_(config.local_media_ssrc.value_or(0)),
       remote_ssrc_(0),
       remote_sender_rtp_time_(0),
       xr_rrtr_status_(false),
@@ -165,6 +152,15 @@
       num_skipped_packets_(0),
       last_skipped_packets_warning_ms_(clock_->TimeInMilliseconds()) {
   RTC_DCHECK(owner);
+  if (config.local_media_ssrc) {
+    registered_ssrcs_.insert(*config.local_media_ssrc);
+  }
+  if (config.rtx_send_ssrc) {
+    registered_ssrcs_.insert(*config.rtx_send_ssrc);
+  }
+  if (config.flexfec_sender) {
+    registered_ssrcs_.insert(config.flexfec_sender->ssrc());
+  }
 }
 
 RTCPReceiver::~RTCPReceiver() {}
@@ -198,6 +194,13 @@
   return remote_ssrc_;
 }
 
+void RTCPReceiver::SetSsrcs(uint32_t main_ssrc,
+                            const std::set<uint32_t>& registered_ssrcs) {
+  rtc::CritScope lock(&rtcp_receiver_lock_);
+  main_ssrc_ = main_ssrc;
+  registered_ssrcs_ = registered_ssrcs;
+}
+
 int32_t RTCPReceiver::RTT(uint32_t remote_ssrc,
                           int64_t* last_rtt_ms,
                           int64_t* avg_rtt_ms,
diff --git a/modules/rtp_rtcp/source/rtcp_receiver.h b/modules/rtp_rtcp/source/rtcp_receiver.h
index 5b92d55..3056711 100644
--- a/modules/rtp_rtcp/source/rtcp_receiver.h
+++ b/modules/rtp_rtcp/source/rtcp_receiver.h
@@ -59,6 +59,7 @@
 
   int64_t LastReceivedReportBlockMs() const;
 
+  void SetSsrcs(uint32_t main_ssrc, const std::set<uint32_t>& registered_ssrcs);
   void SetRemoteSSRC(uint32_t ssrc);
   uint32_t RemoteSSRC() const;
 
@@ -214,8 +215,6 @@
   Clock* const clock_;
   const bool receiver_only_;
   ModuleRtpRtcp* const rtp_rtcp_;
-  const uint32_t main_ssrc_;
-  const std::set<uint32_t> registered_ssrcs_;
 
   rtc::CriticalSection feedbacks_lock_;
   RtcpBandwidthObserver* const rtcp_bandwidth_observer_;
@@ -227,7 +226,9 @@
   const int report_interval_ms_;
 
   rtc::CriticalSection rtcp_receiver_lock_;
+  uint32_t main_ssrc_ RTC_GUARDED_BY(rtcp_receiver_lock_);
   uint32_t remote_ssrc_ RTC_GUARDED_BY(rtcp_receiver_lock_);
+  std::set<uint32_t> registered_ssrcs_ RTC_GUARDED_BY(rtcp_receiver_lock_);
 
   // Received sender report.
   NtpTime remote_sender_ntp_time_ RTC_GUARDED_BY(rtcp_receiver_lock_);
diff --git a/modules/rtp_rtcp/source/rtcp_sender.cc b/modules/rtp_rtcp/source/rtcp_sender.cc
index fba9b45..15325d1 100644
--- a/modules/rtp_rtcp/source/rtcp_sender.cc
+++ b/modules/rtp_rtcp/source/rtcp_sender.cc
@@ -150,7 +150,6 @@
 
 RTCPSender::RTCPSender(const RtpRtcp::Configuration& config)
     : audio_(config.audio),
-      ssrc_(config.local_media_ssrc),
       clock_(config.clock),
       random_(clock_->TimeInMicroseconds()),
       method_(RtcpMode::kOff),
@@ -165,6 +164,7 @@
       timestamp_offset_(0),
       last_rtp_timestamp_(0),
       last_frame_capture_time_ms_(-1),
+      ssrc_(config.local_media_ssrc.value_or(0)),
       remote_ssrc_(0),
       receive_statistics_(config.receive_statistics),
 
@@ -331,6 +331,23 @@
   rtp_clock_rates_khz_[payload_type] = rtp_clock_rate_hz / 1000;
 }
 
+uint32_t RTCPSender::SSRC() const {
+  rtc::CritScope lock(&critical_section_rtcp_sender_);
+  return ssrc_;
+}
+
+void RTCPSender::SetSSRC(uint32_t ssrc) {
+  rtc::CritScope lock(&critical_section_rtcp_sender_);
+
+  if (ssrc_ != 0 && ssrc != ssrc_) {
+    // not first SetSSRC, probably due to a collision
+    // schedule a new RTCP report
+    // make sure that we send a RTP packet
+    next_time_to_send_rtcp_ = clock_->TimeInMilliseconds() + 100;
+  }
+  ssrc_ = ssrc;
+}
+
 void RTCPSender::SetRemoteSSRC(uint32_t ssrc) {
   rtc::CritScope lock(&critical_section_rtcp_sender_);
   remote_ssrc_ = ssrc;
diff --git a/modules/rtp_rtcp/source/rtcp_sender.h b/modules/rtp_rtcp/source/rtcp_sender.h
index 97b4b70..6deee87 100644
--- a/modules/rtp_rtcp/source/rtcp_sender.h
+++ b/modules/rtp_rtcp/source/rtcp_sender.h
@@ -85,7 +85,9 @@
 
   void SetRtpClockRate(int8_t payload_type, int rtp_clock_rate_hz);
 
-  uint32_t SSRC() const { return ssrc_; }
+  uint32_t SSRC() const;
+
+  void SetSSRC(uint32_t ssrc);
 
   void SetRemoteSSRC(uint32_t ssrc);
 
@@ -185,7 +187,6 @@
 
  private:
   const bool audio_;
-  const uint32_t ssrc_;
   Clock* const clock_;
   Random random_ RTC_GUARDED_BY(critical_section_rtcp_sender_);
   RtcpMode method_ RTC_GUARDED_BY(critical_section_rtcp_sender_);
@@ -204,6 +205,7 @@
   uint32_t last_rtp_timestamp_ RTC_GUARDED_BY(critical_section_rtcp_sender_);
   int64_t last_frame_capture_time_ms_
       RTC_GUARDED_BY(critical_section_rtcp_sender_);
+  uint32_t ssrc_ RTC_GUARDED_BY(critical_section_rtcp_sender_);
   // SSRC that we receive on our RTP channel
   uint32_t remote_ssrc_ RTC_GUARDED_BY(critical_section_rtcp_sender_);
   std::string cname_ RTC_GUARDED_BY(critical_section_rtcp_sender_);
diff --git a/modules/rtp_rtcp/source/rtcp_sender_unittest.cc b/modules/rtp_rtcp/source/rtcp_sender_unittest.cc
index c732a35..c3f3920 100644
--- a/modules/rtp_rtcp/source/rtcp_sender_unittest.cc
+++ b/modules/rtp_rtcp/source/rtcp_sender_unittest.cc
@@ -825,6 +825,31 @@
   EXPECT_FALSE(rtcp_sender_->TimeToSendRTCPReport(false));
 }
 
+TEST_F(RtcpSenderTest, DoesntSchedulesInitialReportOnFirstSetSsrc) {
+  // Set up without first SSRC not set at construction.
+  RtpRtcp::Configuration configuration = GetDefaultConfig();
+  configuration.local_media_ssrc = absl::nullopt;
+
+  rtcp_sender_.reset(new RTCPSender(configuration));
+  rtcp_sender_->SetRemoteSSRC(kRemoteSsrc);
+  rtcp_sender_->SetTimestampOffset(kStartRtpTimestamp);
+  rtcp_sender_->SetLastRtpTime(kRtpTimestamp, clock_.TimeInMilliseconds(),
+                               /*payload_type=*/0);
+  rtcp_sender_->SetRTCPStatus(RtcpMode::kReducedSize);
+
+  // Set SSRC for the first time. New report should not be scheduled.
+  rtcp_sender_->SetSSRC(kSenderSsrc);
+  clock_.AdvanceTimeMilliseconds(100);
+  EXPECT_FALSE(rtcp_sender_->TimeToSendRTCPReport(false));
+}
+
+TEST_F(RtcpSenderTest, SchedulesReportOnSsrcChange) {
+  rtcp_sender_->SetRTCPStatus(RtcpMode::kReducedSize);
+  rtcp_sender_->SetSSRC(kSenderSsrc + 1);
+  clock_.AdvanceTimeMilliseconds(100);
+  EXPECT_TRUE(rtcp_sender_->TimeToSendRTCPReport(false));
+}
+
 TEST_F(RtcpSenderTest, SendsCombinedRtcpPacket) {
   rtcp_sender_->SetRTCPStatus(RtcpMode::kReducedSize);
 
diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl.cc b/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
index 7938396..7d8e338 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
@@ -175,6 +175,10 @@
   return rtp_sender_ ? rtp_sender_->RtxStatus() : kRtxOff;
 }
 
+void ModuleRtpRtcpImpl::SetRtxSsrc(uint32_t ssrc) {
+  rtp_sender_->SetRtxSsrc(ssrc);
+}
+
 void ModuleRtpRtcpImpl::SetRtxSendPayloadType(int payload_type,
                                               int associated_payload_type) {
   rtp_sender_->SetRtxPayloadType(payload_type, associated_payload_type);
@@ -236,6 +240,18 @@
   return rtp_sender_->GetRtxRtpState();
 }
 
+uint32_t ModuleRtpRtcpImpl::SSRC() const {
+  return rtcp_sender_.SSRC();
+}
+
+void ModuleRtpRtcpImpl::SetSSRC(const uint32_t ssrc) {
+  if (rtp_sender_) {
+    rtp_sender_->SetSSRC(ssrc);
+  }
+  rtcp_sender_.SetSSRC(ssrc);
+  SetRtcpReceiverSsrcs(ssrc);
+}
+
 void ModuleRtpRtcpImpl::SetRid(const std::string& rid) {
   if (rtp_sender_) {
     rtp_sender_->SetRid(rid);
@@ -290,6 +306,11 @@
     if (rtcp_sender_.SetSendingStatus(GetFeedbackState(), sending) != 0) {
       RTC_LOG(LS_WARNING) << "Failed to send RTCP BYE";
     }
+    if (sending && rtp_sender_) {
+      // Update Rtcp receiver config, to track Rtx config changes from
+      // the SetRtxStatus and SetRtxSsrc methods.
+      SetRtcpReceiverSsrcs(rtp_sender_->SSRC());
+    }
   }
   return 0;
 }
@@ -734,6 +755,17 @@
   return rtcp_receiver_.BoundingSet(tmmbr_owner);
 }
 
+void ModuleRtpRtcpImpl::SetRtcpReceiverSsrcs(uint32_t main_ssrc) {
+  std::set<uint32_t> ssrcs;
+  ssrcs.insert(main_ssrc);
+  if (RtxSendStatus() != kRtxOff)
+    ssrcs.insert(rtp_sender_->RtxSsrc());
+  absl::optional<uint32_t> flexfec_ssrc = FlexfecSsrc();
+  if (flexfec_ssrc)
+    ssrcs.insert(*flexfec_ssrc);
+  rtcp_receiver_.SetSsrcs(main_ssrc, ssrcs);
+}
+
 void ModuleRtpRtcpImpl::set_rtt_ms(int64_t rtt_ms) {
   rtc::CritScope cs(&critical_section_rtt_);
   rtt_ms_ = rtt_ms;
diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl.h b/modules/rtp_rtcp/source/rtp_rtcp_impl.h
index 312f9d6..9ec481c 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl.h
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl.h
@@ -94,7 +94,10 @@
   RtpState GetRtpState() const override;
   RtpState GetRtxState() const override;
 
-  uint32_t SSRC() const override { return rtcp_sender_.SSRC(); }
+  uint32_t SSRC() const override;
+
+  // Configure SSRC, default is a random number.
+  void SetSSRC(uint32_t ssrc) override;
 
   void SetRid(const std::string& rid) override;
 
@@ -107,6 +110,8 @@
   void SetRtxSendStatus(int mode) override;
   int RtxSendStatus() const override;
 
+  void SetRtxSsrc(uint32_t ssrc) override;
+
   void SetRtxSendPayloadType(int payload_type,
                              int associated_payload_type) override;
 
@@ -297,6 +302,7 @@
  private:
   FRIEND_TEST_ALL_PREFIXES(RtpRtcpImplTest, Rtt);
   FRIEND_TEST_ALL_PREFIXES(RtpRtcpImplTest, RttForReceiverOnly);
+  void SetRtcpReceiverSsrcs(uint32_t main_ssrc);
 
   void set_rtt_ms(int64_t rtt_ms);
   int64_t rtt_ms() const;
diff --git a/modules/rtp_rtcp/source/rtp_sender.cc b/modules/rtp_rtcp/source/rtp_sender.cc
index 5aa707f..c88e0e2 100644
--- a/modules/rtp_rtcp/source/rtp_sender.cc
+++ b/modules/rtp_rtcp/source/rtp_sender.cc
@@ -124,8 +124,6 @@
     : clock_(config.clock),
       random_(clock_->TimeInMicroseconds()),
       audio_configured_(config.audio),
-      ssrc_(config.local_media_ssrc),
-      rtx_ssrc_(config.rtx_send_ssrc),
       flexfec_ssrc_(config.flexfec_sender
                         ? absl::make_optional(config.flexfec_sender->ssrc())
                         : absl::nullopt),
@@ -156,6 +154,7 @@
       bitrate_callback_(config.send_bitrate_observer),
       // RTP variables
       sequence_number_forced_(false),
+      ssrc_(config.local_media_ssrc),
       ssrc_has_acked_(false),
       rtx_ssrc_has_acked_(false),
       last_rtp_timestamp_(0),
@@ -165,6 +164,7 @@
       last_packet_marker_bit_(false),
       csrcs_(),
       rtx_(kRtxOff),
+      ssrc_rtx_(config.rtx_send_ssrc),
       rtp_overhead_bytes_per_packet_(0),
       supports_bwe_extension_(false),
       retransmission_rate_limiter_(config.retransmission_rate_limiter),
@@ -267,6 +267,17 @@
   return rtx_;
 }
 
+void RTPSender::SetRtxSsrc(uint32_t ssrc) {
+  rtc::CritScope lock(&send_critsect_);
+  ssrc_rtx_.emplace(ssrc);
+}
+
+uint32_t RTPSender::RtxSsrc() const {
+  rtc::CritScope lock(&send_critsect_);
+  RTC_DCHECK(ssrc_rtx_);
+  return *ssrc_rtx_;
+}
+
 void RTPSender::SetRtxPayloadType(int payload_type,
                                   int associated_payload_type) {
   rtc::CritScope lock(&send_critsect_);
@@ -417,7 +428,7 @@
       case RtpPacketToSend::Type::kPadding:
         // Both padding and retransmission must be on either the media or the
         // RTX stream.
-        if (packet_ssrc == rtx_ssrc_) {
+        if (packet_ssrc == ssrc_rtx_) {
           is_rtx = true;
         } else if (packet_ssrc != ssrc_) {
           return false;
@@ -610,7 +621,7 @@
       }
 
       RTC_DCHECK(ssrc_);
-      padding_packet->SetSsrc(ssrc_);
+      padding_packet->SetSsrc(*ssrc_);
       padding_packet->SetPayloadType(last_payload_type_);
       padding_packet->SetSequenceNumber(sequence_number_++);
     } else {
@@ -634,8 +645,8 @@
         padding_packet->set_capture_time_ms(padding_packet->capture_time_ms() +
                                             (now_ms - last_timestamp_time_ms_));
       }
-      RTC_DCHECK(rtx_ssrc_);
-      padding_packet->SetSsrc(*rtx_ssrc_);
+      RTC_DCHECK(ssrc_rtx_);
+      padding_packet->SetSsrc(*ssrc_rtx_);
       padding_packet->SetSequenceNumber(sequence_number_rtx_++);
       padding_packet->SetPayloadType(rtx_payload_type_map_.begin()->second);
     }
@@ -791,10 +802,17 @@
   if (!bitrate_callback_)
     return;
   int64_t now_ms = clock_->TimeInMilliseconds();
+  uint32_t ssrc;
+  {
+    rtc::CritScope lock(&send_critsect_);
+    if (!ssrc_)
+      return;
+    ssrc = *ssrc_;
+  }
 
   rtc::CritScope lock(&statistics_crit_);
   bitrate_callback_->Notify(total_bitrate_sent_.Rate(now_ms).value_or(0),
-                            nack_bitrate_sent_.Rate(now_ms).value_or(0), ssrc_);
+                            nack_bitrate_sent_.Rate(now_ms).value_or(0), ssrc);
 }
 
 size_t RTPSender::RtpHeaderLength() const {
@@ -832,7 +850,7 @@
   auto packet = std::make_unique<RtpPacketToSend>(
       &rtp_header_extension_map_, max_packet_size_ + kExtraCapacity);
   RTC_DCHECK(ssrc_);
-  packet->SetSsrc(ssrc_);
+  packet->SetSsrc(*ssrc_);
   packet->SetCsrcs(csrcs_);
   // Reserve extensions, if registered, RtpSender set in SendToNetwork.
   packet->ReserveExtension<AbsoluteSendTime>();
@@ -905,6 +923,30 @@
   return timestamp_offset_;
 }
 
+void RTPSender::SetSSRC(uint32_t ssrc) {
+  {
+    rtc::CritScope lock(&send_critsect_);
+    if (ssrc_ == ssrc) {
+      return;  // Since it's the same SSRC, don't reset anything.
+    }
+
+    ssrc_.emplace(ssrc);
+    if (!sequence_number_forced_) {
+      sequence_number_ = random_.Rand(1, kMaxInitRtpSeqNumber);
+    }
+  }
+
+  // Clear RTP packet history, since any packets there belong to the old SSRC
+  // and they may conflict with packets from the new one.
+  packet_history_.Clear();
+}
+
+uint32_t RTPSender::SSRC() const {
+  rtc::CritScope lock(&send_critsect_);
+  RTC_DCHECK(ssrc_);
+  return *ssrc_;
+}
+
 void RTPSender::SetRid(const std::string& rid) {
   // RID is used in simulcast scenario when multiple layers share the same mid.
   rtc::CritScope lock(&send_critsect_);
@@ -919,6 +961,10 @@
   mid_ = mid;
 }
 
+absl::optional<uint32_t> RTPSender::FlexfecSsrc() const {
+  return flexfec_ssrc_;
+}
+
 void RTPSender::SetCsrcs(const std::vector<uint32_t>& csrcs) {
   RTC_DCHECK_LE(csrcs.size(), kRtpCsrcSize);
   rtc::CritScope lock(&send_critsect_);
@@ -1006,7 +1052,7 @@
     if (!sending_media_)
       return nullptr;
 
-    RTC_DCHECK(rtx_ssrc_);
+    RTC_DCHECK(ssrc_rtx_);
 
     // Replace payload type.
     auto kv = rtx_payload_type_map_.find(packet.PayloadType());
@@ -1022,7 +1068,7 @@
     rtx_packet->SetSequenceNumber(sequence_number_rtx_++);
 
     // Replace SSRC.
-    rtx_packet->SetSsrc(*rtx_ssrc_);
+    rtx_packet->SetSsrc(*ssrc_rtx_);
 
     CopyHeaderAndExtensionsToRtxPacket(packet, rtx_packet.get());
 
diff --git a/modules/rtp_rtcp/source/rtp_sender.h b/modules/rtp_rtcp/source/rtp_sender.h
index 9194d44..d0a8396 100644
--- a/modules/rtp_rtcp/source/rtp_sender.h
+++ b/modules/rtp_rtcp/source/rtp_sender.h
@@ -67,6 +67,9 @@
   uint32_t TimestampOffset() const;
   void SetTimestampOffset(uint32_t timestamp);
 
+  // TODO(bugs.webrtc.org/10774): Remove.
+  void SetSSRC(uint32_t ssrc);
+
   void SetRid(const std::string& rid);
 
   void SetMid(const std::string& mid);
@@ -113,10 +116,10 @@
   // RTX.
   void SetRtxStatus(int mode);
   int RtxStatus() const;
-  uint32_t RtxSsrc() const {
-    RTC_DCHECK(rtx_ssrc_);
-    return *rtx_ssrc_;
-  }
+  uint32_t RtxSsrc() const;
+
+  // TODO(bugs.webrtc.org/10774): Remove.
+  void SetRtxSsrc(uint32_t ssrc);
 
   void SetRtxPayloadType(int payload_type, int associated_payload_type);
 
@@ -140,9 +143,9 @@
   // Including RTP headers.
   size_t MaxRtpPacketSize() const;
 
-  uint32_t SSRC() const { return ssrc_; }
+  uint32_t SSRC() const;
 
-  absl::optional<uint32_t> FlexfecSsrc() const { return flexfec_ssrc_; }
+  absl::optional<uint32_t> FlexfecSsrc() const;
 
   // Sends packet to |transport_| or to the pacer, depending on configuration.
   // TODO(bugs.webrtc.org/XXX): Remove in favor of EnqueuePackets().
@@ -222,8 +225,6 @@
 
   const bool audio_configured_;
 
-  const uint32_t ssrc_;
-  const absl::optional<uint32_t> rtx_ssrc_;
   const absl::optional<uint32_t> flexfec_ssrc_;
 
   const std::unique_ptr<NonPacedPacketSender> non_paced_packet_sender_;
@@ -267,6 +268,9 @@
   bool sequence_number_forced_ RTC_GUARDED_BY(send_critsect_);
   uint16_t sequence_number_ RTC_GUARDED_BY(send_critsect_);
   uint16_t sequence_number_rtx_ RTC_GUARDED_BY(send_critsect_);
+  // Must be explicitly set by the application, use of absl::optional
+  // only to keep track of correct use.
+  absl::optional<uint32_t> ssrc_ RTC_GUARDED_BY(send_critsect_);
   // RID value to send in the RID or RepairedRID header extension.
   std::string rid_ RTC_GUARDED_BY(send_critsect_);
   // MID value to send in the MID header extension.
@@ -282,6 +286,7 @@
   bool last_packet_marker_bit_ RTC_GUARDED_BY(send_critsect_);
   std::vector<uint32_t> csrcs_ RTC_GUARDED_BY(send_critsect_);
   int rtx_ RTC_GUARDED_BY(send_critsect_);
+  absl::optional<uint32_t> ssrc_rtx_ RTC_GUARDED_BY(send_critsect_);
   // Mapping rtx_payload_type_map_[associated] = rtx.
   std::map<int8_t, int8_t> rtx_payload_type_map_ RTC_GUARDED_BY(send_critsect_);
   size_t rtp_overhead_bytes_per_packet_ RTC_GUARDED_BY(send_critsect_);
diff --git a/modules/rtp_rtcp/source/rtp_sender_unittest.cc b/modules/rtp_rtcp/source/rtp_sender_unittest.cc
index 0b2d48e..da7ba4f 100644
--- a/modules/rtp_rtcp/source/rtp_sender_unittest.cc
+++ b/modules/rtp_rtcp/source/rtp_sender_unittest.cc
@@ -2562,6 +2562,34 @@
   EXPECT_EQ(*transmission_time_extension, 2 * kOffsetMs * kTimestampTicksPerMs);
 }
 
+TEST_P(RtpSenderTestWithoutPacer, ClearHistoryOnSsrcChange) {
+  const int64_t kRtt = 10;
+
+  rtp_sender_->SetSendingMediaStatus(true);
+  rtp_sender_->SetRtxStatus(kRtxRetransmitted | kRtxRedundantPayloads);
+  rtp_sender_->SetRtxPayloadType(kRtxPayload, kPayload);
+  rtp_sender_->SetStorePacketsStatus(true, 10);
+  rtp_sender_->SetRtt(kRtt);
+
+  // Send a packet and record its sequence numbers.
+  SendGenericPacket();
+  ASSERT_EQ(1u, transport_.sent_packets_.size());
+  const uint16_t packet_seqence_number =
+      transport_.sent_packets_.back().SequenceNumber();
+
+  // Advance time and make sure it can be retransmitted, even if we try to set
+  // the ssrc the what it already is.
+  rtp_sender_->SetSSRC(kSsrc);
+  fake_clock_.AdvanceTimeMilliseconds(kRtt);
+  EXPECT_GT(rtp_sender_->ReSendPacket(packet_seqence_number), 0);
+
+  // Change the SSRC, then move the time and try to retransmit again. The old
+  // packet should now be gone.
+  rtp_sender_->SetSSRC(kSsrc + 1);
+  fake_clock_.AdvanceTimeMilliseconds(kRtt);
+  EXPECT_EQ(rtp_sender_->ReSendPacket(packet_seqence_number), 0);
+}
+
 TEST_P(RtpSenderTestWithoutPacer, ClearHistoryOnSequenceNumberCange) {
   const int64_t kRtt = 10;
 
diff --git a/video/video_send_stream_tests.cc b/video/video_send_stream_tests.cc
index d769bfe..0e4c114 100644
--- a/video/video_send_stream_tests.cc
+++ b/video/video_send_stream_tests.cc
@@ -933,7 +933,6 @@
         config.clock = Clock::GetRealTimeClock();
         config.outgoing_transport = transport_adapter_.get();
         config.rtcp_report_interval_ms = kRtcpIntervalMs;
-        config.local_media_ssrc = kReceiverLocalVideoSsrc;
         RTCPSender rtcp_sender(config);
 
         rtcp_sender.SetRTCPStatus(RtcpMode::kReducedSize);
@@ -1150,7 +1149,6 @@
         config.receive_statistics = &lossy_receive_stats;
         config.outgoing_transport = transport_adapter_.get();
         config.rtcp_report_interval_ms = kRtcpIntervalMs;
-        config.local_media_ssrc = kVideoSendSsrcs[0];
         RTCPSender rtcp_sender(config);
 
         rtcp_sender.SetRTCPStatus(RtcpMode::kReducedSize);
@@ -1402,7 +1400,6 @@
       config.receive_statistics = &receive_stats;
       config.outgoing_transport = transport_adapter_.get();
       config.rtcp_report_interval_ms = kRtcpIntervalMs;
-      config.local_media_ssrc = kVideoSendSsrcs[0];
       RTCPSender rtcp_sender(config);
 
       rtcp_sender.SetRTCPStatus(RtcpMode::kReducedSize);