diff --git a/trunk/configure b/trunk/configure
index 5fb52bbc3..d3872e08e 100755
--- a/trunk/configure
+++ b/trunk/configure
@@ -386,7 +386,7 @@ if [[ $SRS_UTEST == YES ]]; then
     MODULE_FILES+=("srs_utest_ai01" "srs_utest_ai02" "srs_utest_ai03" "srs_utest_ai04" "srs_utest_ai05"
         "srs_utest_ai06" "srs_utest_ai07" "srs_utest_ai08" "srs_utest_ai09" "srs_utest_ai10" "srs_utest_ai11"
         "srs_utest_ai12" "srs_utest_ai13" "srs_utest_ai14" "srs_utest_ai15" "srs_utest_ai16" "srs_utest_ai17"
-        "srs_utest_ai18" "srs_utest_ai19" "srs_utest_ai20" "srs_utest_workflow_forward")
+        "srs_utest_ai18" "srs_utest_ai19" "srs_utest_ai20" "srs_utest_ai24" "srs_utest_workflow_forward")
     if [[ $SRS_GB28181 == YES ]]; then
         MODULE_FILES+=("srs_utest_manual_gb28181" "srs_utest_ai23")
     fi
diff --git a/trunk/src/app/srs_app_rtc_source.cpp b/trunk/src/app/srs_app_rtc_source.cpp
index 6ba964878..1b5cdc7c2 100644
--- a/trunk/src/app/srs_app_rtc_source.cpp
+++ b/trunk/src/app/srs_app_rtc_source.cpp
@@ -1820,8 +1820,9 @@ srs_error_t SrsRtcFrameBuilder::on_rtp(SrsRtpPacket *pkt)
         return err;
     }
 
-    // Have no received any sender report, can't calculate avsync_time,
-    // discard it to avoid timestamp problem in live source
+    // Check if avsync_time is valid (> 0).
+    // NOTE: This check should NEVER fail unless SDP has no sample rate, in which case packets are discarded
+    // to avoid timestamp problems in live source.
     const SrsRtpHeader &h = pkt->header_;
     if (pkt->get_avsync_time() <= 0) {
         if (sync_state_ < 0) {
@@ -3105,7 +3106,17 @@ SrsRtcRecvTrack::SrsRtcRecvTrack(ISrsRtcPacketReceiver *receiver, SrsRtcTrackDes
 
     last_sender_report_rtp_time_ = 0;
     last_sender_report_rtp_time1_ = 0;
+
+    // Initialize rate from SDP sample rate
+    // rate_ is RTP units per millisecond (e.g., 90 for video 90kHz, 48 for audio 48kHz)
+    // This allows immediate A/V sync before receiving 2 RTCP SR packets
+    // Will be updated to precise rate after receiving 2nd SR
     rate_ = 0.0;
+    if (track_desc_->media_) {
+        rate_ = static_cast<double>(track_desc_->media_->sample_) / 1000.0;
+        srs_trace("RTC: Init %s track, ssrc=%u, rate from SDP=%.0f (RTP units per ms, will be updated after 2nd SR)",
+                 track_desc_->type_.c_str(), track_desc_->ssrc_, rate_);
+    }
 
     last_sender_report_sys_time_ = 0;
 }
@@ -3163,8 +3174,14 @@ void SrsRtcRecvTrack::update_send_report_time(const SrsNtp &ntp, uint32_t rtp_ti
         double rtp_time_elpased = static_cast<double>(last_sender_report_rtp_time_) - static_cast<double>(last_sender_report_rtp_time1_);
         double rate = round(rtp_time_elpased / sys_time_elapsed);
 
-        // TODO: FIXME: use the sample rate from sdp.
         if (rate > 0) {
+            if (rate_ != rate) {
+                srs_warn("RTC: SR update %s, ssrc=%u, ntp_ms=%u->%u (delta=%.0fms), rtp_time=%u->%u (delta=%.0f), rate %.0f->%.0f",
+                        track_desc_->type_.c_str(), track_desc_->ssrc_,
+                        last_sender_report_ntp1_.system_ms_, last_sender_report_ntp_.system_ms_, sys_time_elapsed,
+                        (uint32_t)last_sender_report_rtp_time1_, (uint32_t)last_sender_report_rtp_time_, rtp_time_elpased,
+                        rate_, rate);
+                }
             rate_ = rate;
         }
     }
diff --git a/trunk/src/utest/srs_utest_ai24.cpp b/trunk/src/utest/srs_utest_ai24.cpp
new file mode 100644
index 000000000..615fb9e46
--- /dev/null
+++ b/trunk/src/utest/srs_utest_ai24.cpp
@@ -0,0 +1,302 @@
+//
+// Copyright (c) 2013-2025 The SRS Authors
+//
+// SPDX-License-Identifier: MIT
+//
+#include <srs_utest_ai_rtc_avsync.hpp>
+
+#include <srs_app_rtc_source.hpp>
+#include <srs_kernel_error.hpp>
+#include <srs_kernel_rtc_rtcp.hpp>
+#include <srs_protocol_sdp.hpp>
+
+using namespace std;
+
+// Mock class to access protected members of SrsRtcRecvTrack
+class MockSrsRtcRecvTrackForAVSync : public SrsRtcRecvTrack
+{
+SRS_DECLARE_PRIVATE:
+    static SrsRtcTrackDescription* create_track_desc(const string& type, uint32_t ssrc, int sample_rate)
+    {
+        SrsRtcTrackDescription* desc = new SrsRtcTrackDescription();
+        desc->type_ = type;
+        desc->id_ = "test_track";
+        desc->ssrc_ = ssrc;
+        desc->is_active_ = true;
+        
+        // Create media description with sample rate
+        desc->media_ = new SrsAudioPayload();
+        desc->media_->sample_ = sample_rate;
+        
+        return desc;
+    }
+
+public:
+    MockSrsRtcRecvTrackForAVSync(const string& type, uint32_t ssrc, int sample_rate, bool is_audio)
+        : SrsRtcRecvTrack(NULL, create_track_desc(type, ssrc, sample_rate), is_audio)
+    {
+    }
+
+    // Expose protected methods for testing
+    double get_rate() const { return rate_; }
+    
+    void set_rate(double rate) { rate_ = rate; }
+    
+    int64_t test_cal_avsync_time(uint32_t rtp_time) {
+        return cal_avsync_time(rtp_time);
+    }
+    
+    void test_update_send_report_time(const SrsNtp& ntp, uint32_t rtp_time) {
+        update_send_report_time(ntp, rtp_time);
+    }
+
+    // Implement pure virtual methods
+    virtual srs_error_t on_rtp(SrsSharedPtr<SrsRtcSource>& source, SrsRtpPacket* pkt) {
+        return srs_success;
+    }
+    
+    virtual srs_error_t check_send_nacks() {
+        return srs_success;
+    }
+};
+
+// Test: Rate initialization from SDP for audio track (48kHz)
+VOID TEST(RtcAVSyncTest, AudioRateInitFromSDP)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // Rate should be initialized to 48 (48000 Hz / 1000 = 48 RTP units per ms)
+    EXPECT_DOUBLE_EQ(48.0, track.get_rate());
+}
+
+// Test: Rate initialization from SDP for video track (90kHz)
+VOID TEST(RtcAVSyncTest, VideoRateInitFromSDP)
+{
+    MockSrsRtcRecvTrackForAVSync track("video", 67890, 90000, false);
+    
+    // Rate should be initialized to 90 (90000 Hz / 1000 = 90 RTP units per ms)
+    EXPECT_DOUBLE_EQ(90.0, track.get_rate());
+}
+
+// Test: cal_avsync_time with SDP rate (before receiving SR)
+VOID TEST(RtcAVSyncTest, CalAVSyncTimeWithSDPRate)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // Simulate first SR received
+    SrsNtp ntp;
+    ntp.system_ms_ = 1000;  // 1000 ms
+    uint32_t rtp_time = 48000;  // 48000 RTP units
+    track.test_update_send_report_time(ntp, rtp_time);
+    
+    // Calculate avsync time for a later RTP packet
+    // RTP time: 48000 + 4800 = 52800 (100ms later at 48kHz)
+    // Expected avsync_time: 1000 + (52800 - 48000) / 48 = 1000 + 100 = 1100 ms
+    int64_t avsync_time = track.test_cal_avsync_time(52800);
+    EXPECT_EQ(1100, avsync_time);
+}
+
+// Test: cal_avsync_time returns -1 when rate is 0
+VOID TEST(RtcAVSyncTest, CalAVSyncTimeWithZeroRate)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // Manually set rate to 0
+    track.set_rate(0.0);
+    
+    // Should return -1 when rate is too small
+    int64_t avsync_time = track.test_cal_avsync_time(1000);
+    EXPECT_EQ(-1, avsync_time);
+}
+
+// Test: Rate update after receiving 2nd SR (audio)
+VOID TEST(RtcAVSyncTest, AudioRateUpdateAfter2ndSR)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // Initial rate from SDP
+    EXPECT_DOUBLE_EQ(48.0, track.get_rate());
+    
+    // First SR
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 1000;
+    uint32_t rtp_time1 = 48000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Rate should still be 48 (from SDP)
+    EXPECT_DOUBLE_EQ(48.0, track.get_rate());
+    
+    // Second SR (20ms later, RTP increased by 960)
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 1020;  // 20ms later
+    uint32_t rtp_time2 = 48960;  // 960 RTP units later (48 * 20)
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Rate should be updated to calculated value: 960 / 20 = 48
+    EXPECT_DOUBLE_EQ(48.0, track.get_rate());
+}
+
+// Test: Rate update after receiving 2nd SR (video)
+VOID TEST(RtcAVSyncTest, VideoRateUpdateAfter2ndSR)
+{
+    MockSrsRtcRecvTrackForAVSync track("video", 67890, 90000, false);
+    
+    // Initial rate from SDP
+    EXPECT_DOUBLE_EQ(90.0, track.get_rate());
+    
+    // First SR
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 2000;
+    uint32_t rtp_time1 = 180000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Rate should still be 90 (from SDP)
+    EXPECT_DOUBLE_EQ(90.0, track.get_rate());
+    
+    // Second SR (100ms later, RTP increased by 9000)
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 2100;  // 100ms later
+    uint32_t rtp_time2 = 189000;  // 9000 RTP units later (90 * 100)
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Rate should be updated to calculated value: 9000 / 100 = 90
+    EXPECT_DOUBLE_EQ(90.0, track.get_rate());
+}
+
+// Test: Rate calculation with clock drift (slightly off from SDP)
+VOID TEST(RtcAVSyncTest, RateUpdateWithClockDrift)
+{
+    MockSrsRtcRecvTrackForAVSync track("video", 67890, 90000, false);
+    
+    // Initial rate from SDP
+    EXPECT_DOUBLE_EQ(90.0, track.get_rate());
+    
+    // First SR
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 1000;
+    uint32_t rtp_time1 = 90000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Second SR with slight clock drift
+    // Expected: 100ms -> 9000 RTP units
+    // Actual: 100ms -> 9010 RTP units (slight drift)
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 1100;
+    uint32_t rtp_time2 = 99010;  // Slightly more than expected
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Rate should be updated to: round(9010 / 100) = 90
+    EXPECT_DOUBLE_EQ(90.0, track.get_rate());
+}
+
+// Test: Rate calculation with larger time interval
+VOID TEST(RtcAVSyncTest, RateUpdateWithLargeInterval)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // First SR
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 5000;
+    uint32_t rtp_time1 = 240000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Second SR (1000ms later)
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 6000;
+    uint32_t rtp_time2 = 288000;  // 48000 RTP units later (48 * 1000)
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Rate should be: 48000 / 1000 = 48
+    EXPECT_DOUBLE_EQ(48.0, track.get_rate());
+}
+
+// Test: cal_avsync_time with precise rate after 2nd SR
+VOID TEST(RtcAVSyncTest, CalAVSyncTimeAfter2ndSR)
+{
+    MockSrsRtcRecvTrackForAVSync track("video", 67890, 90000, false);
+    
+    // First SR
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 1000;
+    uint32_t rtp_time1 = 90000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Second SR
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 1100;
+    uint32_t rtp_time2 = 99000;
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Now calculate avsync time for a packet
+    // RTP time: 99000 + 4500 = 103500 (50ms later at 90kHz)
+    // Expected: 1100 + (103500 - 99000) / 90 = 1100 + 50 = 1150 ms
+    int64_t avsync_time = track.test_cal_avsync_time(103500);
+    EXPECT_EQ(1150, avsync_time);
+}
+
+// Test: Immediate A/V sync availability (issue #4418 fix)
+VOID TEST(RtcAVSyncTest, ImmediateAVSyncAvailability)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // Before any SR, rate should be available from SDP
+    EXPECT_DOUBLE_EQ(48.0, track.get_rate());
+    
+    // First SR received
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 1000;
+    uint32_t rtp_time1 = 48000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Should be able to calculate avsync_time immediately (not -1)
+    int64_t avsync_time = track.test_cal_avsync_time(48480);  // 10ms later
+    EXPECT_GT(avsync_time, 0);  // Should be > 0, not -1
+    EXPECT_EQ(1010, avsync_time);  // Should be 1000 + 10 = 1010
+}
+
+// Test: RTP timestamp wraparound handling
+VOID TEST(RtcAVSyncTest, RTPTimestampWraparound)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // First SR near wraparound
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 1000;
+    uint32_t rtp_time1 = 0xFFFFF000;  // Near max uint32_t
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    // Second SR after wraparound
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 1020;  // 20ms later
+    uint32_t rtp_time2 = 0x000003C0;  // Wrapped around, 960 units after wraparound
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Note: Current implementation may not handle wraparound correctly
+    // This test documents the current behavior
+    // Rate calculation: (0x000003C0 - 0xFFFFF000) will underflow
+    // This is a known limitation that may need fixing in the future
+}
+
+// Test: Zero time elapsed between SRs (edge case)
+VOID TEST(RtcAVSyncTest, ZeroTimeElapsedBetweenSRs)
+{
+    MockSrsRtcRecvTrackForAVSync track("audio", 12345, 48000, true);
+    
+    // First SR
+    SrsNtp ntp1;
+    ntp1.system_ms_ = 1000;
+    uint32_t rtp_time1 = 48000;
+    track.test_update_send_report_time(ntp1, rtp_time1);
+    
+    double rate_before = track.get_rate();
+    
+    // Second SR with same timestamp (0ms elapsed)
+    SrsNtp ntp2;
+    ntp2.system_ms_ = 1000;  // Same time
+    uint32_t rtp_time2 = 48000;  // Same RTP time
+    track.test_update_send_report_time(ntp2, rtp_time2);
+    
+    // Rate should remain unchanged (SDP rate)
+    EXPECT_DOUBLE_EQ(rate_before, track.get_rate());
+}
+
diff --git a/trunk/src/utest/srs_utest_ai24.hpp b/trunk/src/utest/srs_utest_ai24.hpp
new file mode 100644
index 000000000..b1854ca3a
--- /dev/null
+++ b/trunk/src/utest/srs_utest_ai24.hpp
@@ -0,0 +1,12 @@
+//
+// Copyright (c) 2013-2025 The SRS Authors
+//
+// SPDX-License-Identifier: MIT
+//
+#ifndef SRS_UTEST_AI_RTC_AVSYNC_HPP
+#define SRS_UTEST_AI_RTC_AVSYNC_HPP
+
+#include <srs_utest.hpp>
+
+#endif
+