[feat] modify the implementation of rtx packet

src/rtp/rtp_packet/rtp_packet.h

@@ -239,6 +239,12 @@ class RtpPacket {
   }
 
  public:
+  struct Extension {
+    uint8_t id;
+    uint8_t len;
+    std::vector<uint8_t> data;
+  };
+
   // Get Header
   uint32_t Version() const { return version_; }
   bool HasPadding() const { return has_padding_; }
@@ -252,6 +258,8 @@ class RtpPacket {
   uint32_t Ssrc() const { return ssrc_; }
   std::vector<uint32_t> Csrcs() const { return csrcs_; };
   uint16_t ExtensionProfile() const { return extension_profile_; }
+  uint16_t ExtensionLen() const { return extension_len_; }
+  std::vector<Extension> Extensions() const { return extensions_; }
 
   uint32_t GetAbsoluteSendTimestamp(uint32_t *abs_send_time) const {
     if (!extensions_.empty()) {
@@ -305,11 +313,6 @@ class RtpPacket {
   // Extension header
   uint16_t extension_profile_ = 0;
   uint16_t extension_len_ = 0;
-  struct Extension {
-    uint8_t id;
-    uint8_t len;
-    std::vector<uint8_t> data;
-  };
   std::vector<Extension> extensions_;
 
   // Payload

src/rtp/rtp_packet/rtp_packet_to_send.cpp

@@ -34,4 +34,97 @@ void RtpPacketToSend::set_packet_type(RtpPacketMediaType type) {
   packet_type_ = type;
 }
 
+void AddAbsSendTimeExtension(std::vector<uint8_t>& rtp_packet_frame) {
+  uint16_t extension_profile = 0xBEDE;  // One-byte header extension
+  uint8_t sub_extension_id = 3;         // ID for Absolute Send Time
+  uint8_t sub_extension_length =
+      2;  // Length of the extension data in bytes minus 1
+
+  uint32_t abs_send_time =
+      std::chrono::duration_cast<std::chrono::microseconds>(
+          std::chrono::system_clock::now().time_since_epoch())
+          .count();
+  abs_send_time &= 0x00FFFFFF;  // Absolute Send Time is 24 bits
+
+  // Add extension profile
+  rtp_packet_frame.push_back((extension_profile >> 8) & 0xFF);
+  rtp_packet_frame.push_back(extension_profile & 0xFF);
+
+  // Add extension length (in 32-bit words, minus one)
+  rtp_packet_frame.push_back(
+      0x00);  // Placeholder for length, will be updated later
+  rtp_packet_frame.push_back(0x01);  // One 32-bit word
+
+  // Add Absolute Send Time extension
+  rtp_packet_frame.push_back((sub_extension_id << 4) | sub_extension_length);
+  rtp_packet_frame.push_back((abs_send_time >> 16) & 0xFF);
+  rtp_packet_frame.push_back((abs_send_time >> 8) & 0xFF);
+  rtp_packet_frame.push_back(abs_send_time & 0xFF);
+}
+
+bool RtpPacketToSend::BuildRtxPacket() {
+  if (!retransmitted_sequence_number_.has_value()) {
+    return false;
+  }
+
+  uint8_t version = Version();
+  uint8_t has_padding = HasPadding();
+  uint8_t has_extension = HasExtension();
+  uint8_t csrc_count = Csrcs().size();
+  bool marker = Marker();
+  uint8_t payload_type = PayloadType();
+  uint16_t sequence_number = SequenceNumber();
+  uint32_t ssrc = Ssrc();
+  std::vector<uint32_t> csrcs = Csrcs();
+
+  uint32_t timestamp = std::chrono::duration_cast<std::chrono::microseconds>(
+                           std::chrono::system_clock::now().time_since_epoch())
+                           .count();
+
+  if (!csrc_count) {
+  }
+
+  SetPayloadType(rtp::PAYLOAD_TYPE::RTX);
+
+  rtp_packet_frame_.clear();
+  rtp_packet_frame_.push_back((version << 6) | (has_padding << 5) |
+                              (has_extension << 4) | csrc_count);
+  rtp_packet_frame_.push_back((marker << 7) | payload_type);
+  rtp_packet_frame_.push_back((sequence_number >> 8) & 0xFF);
+  rtp_packet_frame_.push_back(sequence_number & 0xFF);
+  rtp_packet_frame_.push_back((timestamp >> 24) & 0xFF);
+  rtp_packet_frame_.push_back((timestamp >> 16) & 0xFF);
+  rtp_packet_frame_.push_back((timestamp >> 8) & 0xFF);
+  rtp_packet_frame_.push_back(timestamp & 0xFF);
+  rtp_packet_frame_.push_back((ssrc >> 24) & 0xFF);
+  rtp_packet_frame_.push_back((ssrc >> 16) & 0xFF);
+  rtp_packet_frame_.push_back((ssrc >> 8) & 0xFF);
+  rtp_packet_frame_.push_back(ssrc & 0xFF);
+
+  for (uint32_t index = 0; index < csrc_count && !csrcs.empty(); index++) {
+    rtp_packet_frame_.push_back((csrcs[index] >> 24) & 0xFF);
+    rtp_packet_frame_.push_back((csrcs[index] >> 16) & 0xFF);
+    rtp_packet_frame_.push_back((csrcs[index] >> 8) & 0xFF);
+    rtp_packet_frame_.push_back(csrcs[index] & 0xFF);
+  }
+
+  if (has_extension) {
+    std::vector<Extension> extensions = Extensions();
+    AddAbsSendTimeExtension(rtp_packet_frame_);
+    uint32_t abs_send_time = 0;
+    LOG_WARN("abs {}", GetAbsoluteSendTimestamp(&abs_send_time));
+  }
+
+  rtp_packet_frame_.push_back((retransmitted_sequence_number_.value() >> 8) &
+                              0xFF);
+  rtp_packet_frame_.push_back(retransmitted_sequence_number_.value() & 0xFF);
+
+  rtp_packet_frame_.insert(rtp_packet_frame_.end(), Payload(),
+                           Payload() + PayloadSize());
+
+  Build(rtp_packet_frame_.data(), rtp_packet_frame_.size());
+
+  return true;
+}
+
 }  // namespace webrtc

src/rtp/rtp_packet/rtp_packet_to_send.h

@@ -44,6 +44,8 @@ class RtpPacketToSend : public ::RtpPacket {
 
   ~RtpPacketToSend();
 
+  bool BuildRtxPacket();
+
   // Time in local time base as close as it can to frame capture time.
   webrtc::Timestamp capture_time() const { return capture_time_; }
   void set_capture_time(webrtc::Timestamp time) { capture_time_ = time; }
@@ -169,6 +171,9 @@ class RtpPacketToSend : public ::RtpPacket {
   bool is_red_ = false;
   bool send_as_ect1_ = false;
   std::optional<TimeDelta> time_in_send_queue_;
+
+ private:
+  std::vector<uint8_t> rtp_packet_frame_;
 };
 
 }  // namespace webrtc

src/rtp/rtp_packet/rtx_packet.h

@@ -0,0 +1,32 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2025-03-31
+ * Copyright (c) 2025 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _RTX_PACKET_H_
+#define _RTX_PACKET_H_
+
+#include "rtp_packet.h"
+#include "rtp_packet_to_send.h"
+
+// 0                   1                   2                   3
+// 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// |                         RTP Header                            |
+// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// |            OSN                |                               |
+// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
+// |                  Original RTP Packet Payload                  |
+// |                                                               |
+//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+class RtxPacket {
+ public:
+  RtxPacket();
+  ~RtxPacket();
+
+  void Build(RtpPacket& rtp_packet, uint32_t origin_ssrc, uint16_t seq_no);
+};
+
+#endif