From a09331a6038bb6191c7662680d8928940463a099 Mon Sep 17 00:00:00 2001
From: Andreas Pehrson <apehrson@mozilla.com>
Date: Fri, 12 May 2023 09:36:34 +0200
Subject: [PATCH] Don't write TransmissionOffset when capture time is not set
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RTX padding packets sent before media packets can legitimately have no
timestamps set (they are 0). Writing the TransmissionOffset extension
with capture time 0 will overflow once current time exceeds ~3 minutes.

Bug: webrtc:15172
Change-Id: I4dd1f341802d45016549b330f0e08cd3a00cfa19
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/305020
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40055}
---
 modules/rtp_rtcp/source/rtp_sender_egress.cc  |  5 +++--
 .../source/rtp_sender_egress_unittest.cc      | 20 +++++++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/modules/rtp_rtcp/source/rtp_sender_egress.cc b/modules/rtp_rtcp/source/rtp_sender_egress.cc
index 22da99b786..a4aa7482f1 100644
--- a/modules/rtp_rtcp/source/rtp_sender_egress.cc
+++ b/modules/rtp_rtcp/source/rtp_sender_egress.cc
@@ -208,8 +208,9 @@ void RtpSenderEgress::SendPacket(std::unique_ptr<RtpPacketToSend> packet,
   // In case of VideoTimingExtension, since it's present not in every packet,
   // data after rtp header may be corrupted if these packets are protected by
   // the FEC.
-  TimeDelta diff = now - packet->capture_time();
-  if (packet->HasExtension<TransmissionOffset>()) {
+  if (packet->HasExtension<TransmissionOffset>() &&
+      packet->capture_time() > Timestamp::Zero()) {
+    TimeDelta diff = now - packet->capture_time();
     packet->SetExtension<TransmissionOffset>(kTimestampTicksPerMs * diff.ms());
   }
   if (packet->HasExtension<AbsoluteSendTime>()) {
diff --git a/modules/rtp_rtcp/source/rtp_sender_egress_unittest.cc b/modules/rtp_rtcp/source/rtp_sender_egress_unittest.cc
index 75f18354ec..a31ee37c43 100644
--- a/modules/rtp_rtcp/source/rtp_sender_egress_unittest.cc
+++ b/modules/rtp_rtcp/source/rtp_sender_egress_unittest.cc
@@ -309,6 +309,26 @@ TEST_F(RtpSenderEgressTest,
   EXPECT_TRUE(transport_.last_packet()->options.included_in_allocation);
 }
 
+TEST_F(RtpSenderEgressTest,
+       DoesntWriteTransmissionOffsetOnRtxPaddingBeforeMedia) {
+  header_extensions_.RegisterByUri(kTransmissionOffsetExtensionId,
+                                   TransmissionOffset::Uri());
+
+  // Prior to sending media, timestamps are 0.
+  std::unique_ptr<RtpPacketToSend> padding =
+      BuildRtpPacket(/*marker_bit=*/true, /*capture_time_ms=*/0);
+  padding->set_packet_type(RtpPacketMediaType::kPadding);
+  padding->SetSsrc(kRtxSsrc);
+  EXPECT_TRUE(padding->HasExtension<TransmissionOffset>());
+
+  std::unique_ptr<RtpSenderEgress> sender = CreateRtpSenderEgress();
+  sender->SendPacket(std::move(padding), PacedPacketInfo());
+
+  absl::optional<int32_t> offset =
+      transport_.last_packet()->packet.GetExtension<TransmissionOffset>();
+  EXPECT_EQ(offset, 0);
+}
+
 TEST_F(RtpSenderEgressTest, OnSendSideDelayUpdated) {
   StrictMock<MockSendSideDelayObserver> send_side_delay_observer;
   RtpRtcpInterface::Configuration config = DefaultConfig();