前言

createOffer之前，必须生成证书。。

代码分析

Conductor::ConnectToPeer
->
if (InitializePeerConnection()) {
    peer_id_ = peer_id;
    peer_connection_->CreateOffer(
        this, webrtc::PeerConnectionInterface::RTCOfferAnswerOptions());
  } else {
    main_wnd_->MessageBox("Error", "Failed to initialize PeerConnection", true);
  }
->
PeerConnection::CreateOffer
->
SdpOfferAnswerHandler::CreateOffer(
    CreateSessionDescriptionObserver* observer,
    const PeerConnectionInterface::RTCOfferAnswerOptions& options)
->
SdpOfferAnswerHandler::DoCreateOffer
->
WebRtcSessionDescriptionFactory::CreateOffer
->
WebRtcSessionDescriptionFactory::InternalCreateOffer

SdpOfferAnswerHandler::DoCreateOffer

void SdpOfferAnswerHandler::DoCreateOffer(
    const PeerConnectionInterface::RTCOfferAnswerOptions& options,
    rtc::scoped_refptr<CreateSessionDescriptionObserver> observer) {
    ******
  获取会话选项参数，并创建offer
  cricket::MediaSessionOptions session_options;
  GetOptionsForOffer(options, &session_options);
  webrtc_session_desc_factory_->CreateOffer(observer, options, session_options);
}

WebRtcSessionDescriptionFactory::CreateOffer

void WebRtcSessionDescriptionFactory::CreateOffer(
    CreateSessionDescriptionObserver* observer,
    const PeerConnectionInterface::RTCOfferAnswerOptions& options,
    const cricket::MediaSessionOptions& session_options) {
******
    // 创建申请，然后设置observer回调
  CreateSessionDescriptionRequest request(
      CreateSessionDescriptionRequest::kOffer, observer, session_options);
  if (certificate_request_state_ == CERTIFICATE_WAITING) {
    create_session_description_requests_.push(request);
  } else {
    RTC_DCHECK(certificate_request_state_ == CERTIFICATE_SUCCEEDED ||
               certificate_request_state_ == CERTIFICATE_NOT_NEEDED);
    InternalCreateOffer(request);
  }
}

工作线程创建好证书后，就会调用WebRtcCertificateGeneratorCallback::OnSuccess ，后续给信令线程继续操作

WebRtcCertificateGeneratorCallback::OnSuccess

void WebRtcCertificateGeneratorCallback::OnSuccess(
    const rtc::scoped_refptr<rtc::RTCCertificate>& certificate) {
  SignalCertificateReady(certificate);
}

后面是在void RTCCertificateGenerator::GenerateCertificateAsync调用的OnSuccess

void RTCCertificateGenerator::GenerateCertificateAsync(
    const KeyParams& key_params,
    const absl::optional<uint64_t>& expires_ms,
    const scoped_refptr<RTCCertificateGeneratorCallback>& callback) {
  RTC_DCHECK(signaling_thread_->IsCurrent());
  RTC_DCHECK(callback);
  // Create a new |RTCCertificateGenerationTask| for this generation request. It
  // is reference counted and referenced by the message data, ensuring it lives
  // until the task has completed (independent of |RTCCertificateGenerator|).
  worker_thread_->PostTask(RTC_FROM_HERE, [key_params, expires_ms,
                                           signaling_thread = signaling_thread_,
                                           cb = callback]() {
    scoped_refptr<RTCCertificate> certificate =
        RTCCertificateGenerator::GenerateCertificate(key_params, expires_ms);
    signaling_thread->PostTask(
        RTC_FROM_HERE, [cert = std::move(certificate), cb = std::move(cb)]() {
          cert ? cb->OnSuccess(cert) : cb->OnFailure();
        });
  });

WebRtcCertificateGeneratorCallback::OnSuccess函数里面的SignalCertificateReady(certificate);函数实际上就是调用WebRtcSessionDescriptionFactory::SetCertificate 。

WebRtcSessionDescriptionFactory::SetCertificate

void WebRtcSessionDescriptionFactory::SetCertificate(
    const rtc::scoped_refptr<rtc::RTCCertificate>& certificate) {
  RTC_DCHECK(certificate);
  RTC_LOG(LS_VERBOSE) << "Setting new certificate.";
  certificate_request_state_ = CERTIFICATE_SUCCEEDED;
  on_certificate_ready_(certificate);
  transport_desc_factory_.set_certificate(certificate);
  transport_desc_factory_.set_secure(cricket::SEC_ENABLED);
    // 判断请求是否为空
  while (!create_session_description_requests_.empty()) {
    if (create_session_description_requests_.front().type ==
        CreateSessionDescriptionRequest::kOffer) {
      InternalCreateOffer(create_session_description_requests_.front());
    } else {
      InternalCreateAnswer(create_session_description_requests_.front());
    }
    create_session_description_requests_.pop();
  }
}

当前创建的是offer，所以进去InternalCreateOffer

WebRtcSessionDescriptionFactory::InternalCreateOffer

void WebRtcSessionDescriptionFactory::InternalCreateOffer(
    CreateSessionDescriptionRequest request) {
    // 如果是本地
  if (sdp_info_->local_description()) {
    // If the needs-ice-restart flag is set as described by JSEP, we should
    // generate an offer with a new ufrag/password to trigger an ICE restart.
    for (cricket::MediaDescriptionOptions& options :
         request.options.media_description_options) {
      if (sdp_info_->NeedsIceRestart(options.mid)) {
        options.transport_options.ice_restart = true;
      }
    }
  }
    // 创建offer
  std::unique_ptr<cricket::SessionDescription> desc =
      session_desc_factory_.CreateOffer(
          request.options, sdp_info_->local_description()
                               ? sdp_info_->local_description()->description()
                               : nullptr);
  if (!desc) {
    PostCreateSessionDescriptionFailed(request.observer,
                                       "Failed to initialize the offer.");
    return;
  }

MediaSessionDescriptionFactory::CreateOffer

std::unique_ptr<SessionDescription> MediaSessionDescriptionFactory::CreateOffer(
   *****
  AudioCodecs offer_audio_codecs;
  VideoCodecs offer_video_codecs;
  RtpDataCodecs offer_rtp_data_codecs;
    // 获取编码信息
  GetCodecsForOffer(
      current_active_contents, &offer_audio_codecs, &offer_video_codecs,
      session_options.data_channel_type == DataChannelType::DCT_SCTP
          ? nullptr
          : &offer_rtp_data_codecs);
  if (!session_options.vad_enabled) {
    // If application doesn't want CN codecs in offer.
    StripCNCodecs(&offer_audio_codecs);
  }
    //获取rtp扩展头
  AudioVideoRtpHeaderExtensions extensions_with_ids =
      GetOfferedRtpHeaderExtensionsWithIds(
          current_active_contents, session_options.offer_extmap_allow_mixed,
          session_options.media_description_options);
  auto offer = std::make_unique<SessionDescription>();
  // Iterate through the media description options, matching with existing media
  // descriptions in |current_description|.
  size_t msection_index = 0;
    // 循环处理属性选项
  for (const MediaDescriptionOptions& media_description_options :
       session_options.media_description_options) {
    const ContentInfo* current_content = nullptr;
    if (current_description &&
        msection_index < current_description->contents().size()) {
      current_content = &current_description->contents()[msection_index];
      // Media type must match unless this media section is being recycled.
      RTC_DCHECK(current_content->name != media_description_options.mid ||
                 IsMediaContentOfType(current_content,
                                      media_description_options.type));
    }
    switch (media_description_options.type) {
      case MEDIA_TYPE_AUDIO:
            // 添加音频媒体数据
        if (!AddAudioContentForOffer(media_description_options, session_options,
                                     current_content, current_description,
                                     extensions_with_ids.audio,
                                     offer_audio_codecs, &current_streams,
                                     offer.get(), &ice_credentials)) {
***
        break;
      case MEDIA_TYPE_VIDEO:
        if (!AddVideoContentForOffer(media_description_options, session_options,
                                     current_content, current_description,
                                     extensions_with_ids.video,
                                     offer_video_codecs, &current_streams,
                                     offer.get(), &ice_credentials)) {
***
      case MEDIA_TYPE_DATA:
        if (!AddDataContentForOffer(media_description_options, session_options,
                                    current_content, current_description,
                                    offer_rtp_data_codecs, &current_streams,
                                    offer.get(), &ice_credentials)) {
***
      case MEDIA_TYPE_UNSUPPORTED:
        if (!AddUnsupportedContentForOffer(
                media_description_options, session_options, current_content,
                current_description, offer.get(), &ice_credentials)) {
***
    }
    ++msection_index;
  }
  // Bundle the contents together, if we've been asked to do so, and update any
  // parameters that need to be tweaked for BUNDLE.
  if (session_options.bundle_enabled) {
    ContentGroup offer_bundle(GROUP_TYPE_BUNDLE);
    for (const ContentInfo& content : offer->contents()) {
      if (content.rejected) {
        continue;
      }
      // TODO(deadbeef): There are conditions that make bundling two media
      // descriptions together illegal. For example, they use the same payload
      // type to represent different codecs, or same IDs for different header
      // extensions. We need to detect this and not try to bundle those media
      // descriptions together.
      offer_bundle.AddContentName(content.name);
    }
    if (!offer_bundle.content_names().empty()) {
      offer->AddGroup(offer_bundle);
      if (!UpdateTransportInfoForBundle(offer_bundle, offer.get())) {
        RTC_LOG(LS_ERROR)
            << "CreateOffer failed to UpdateTransportInfoForBundle.";
        return nullptr;
      }
      if (!UpdateCryptoParamsForBundle(offer_bundle, offer.get())) {
        RTC_LOG(LS_ERROR)
            << "CreateOffer failed to UpdateCryptoParamsForBundle.";
        return nullptr;
      }
    }
  }
  // The following determines how to signal MSIDs to ensure compatibility with
  // older endpoints (in particular, older Plan B endpoints).
  if (is_unified_plan_) {
    // Be conservative and signal using both a=msid and a=ssrc lines. Unified
    // Plan answerers will look at a=msid and Plan B answerers will look at the
    // a=ssrc MSID line.
    offer->set_msid_signaling(cricket::kMsidSignalingMediaSection |
                              cricket::kMsidSignalingSsrcAttribute);
  } else {
    // Plan B always signals MSID using a=ssrc lines.
    offer->set_msid_signaling(cricket::kMsidSignalingSsrcAttribute);
  }
  offer->set_extmap_allow_mixed(session_options.offer_extmap_allow_mixed);
  return offer;
}

以音频为例

AddAudioContentForOffer

bool MediaSessionDescriptionFactory::AddAudioContentForOffer(
    const MediaDescriptionOptions& media_description_options,
    const MediaSessionOptions& session_options,
    const ContentInfo* current_content,
    const SessionDescription* current_description,
    const RtpHeaderExtensions& audio_rtp_extensions,
    const AudioCodecs& audio_codecs,
    StreamParamsVec* current_streams,
    SessionDescription* desc,
    IceCredentialsIterator* ice_credentials) const {
 *****
  if (!CreateMediaContentOffer(media_description_options, session_options,
                               filtered_codecs, sdes_policy,
                               GetCryptos(current_content), crypto_suites,
                               audio_rtp_extensions, ssrc_generator_,
                               current_streams, audio.get())) {
    return false;
  }
    // 设置媒体协议
  bool secure_transport = (transport_desc_factory_->secure() != SEC_DISABLED);
  SetMediaProtocol(secure_transport, audio.get());
  audio->set_direction(media_description_options.direction);
    // 添加内容
  desc->AddContent(media_description_options.mid, MediaProtocolType::kRtp,
                   media_description_options.stopped, std::move(audio));
  if (!AddTransportOffer(media_description_options.mid,
                         media_description_options.transport_options,
                         current_description, desc, ice_credentials)) {
    return false;
  }
  return true;
}

CreateMediaContentOffer

template <class C>
static bool CreateMediaContentOffer(
    const MediaDescriptionOptions& media_description_options,
    const MediaSessionOptions& session_options,
    const std::vector<C>& codecs,
    const SecurePolicy& secure_policy,
    const CryptoParamsVec* current_cryptos,
    const std::vector<std::string>& crypto_suites,
    const RtpHeaderExtensions& rtp_extensions,
    UniqueRandomIdGenerator* ssrc_generator,
    StreamParamsVec* current_streams,
    MediaContentDescriptionImpl<C>* offer) {
  offer->AddCodecs(codecs);
  if (!AddStreamParams(media_description_options.sender_options,
                       session_options.rtcp_cname, ssrc_generator,
                       current_streams, offer)) {
    return false;
  }
  return CreateContentOffer(media_description_options, session_options,
                            secure_policy, current_cryptos, crypto_suites,
                            rtp_extensions, ssrc_generator, current_streams,
                            offer);
}

AddStreamParams-》
CreateStreamParamsForNewSenderWithSsrcs 每一路流ssrc的创建都调用它

CreateContentOffer

// Create a media content to be offered for the given |sender_options|,
// according to the given options.rtcp_mux, session_options.is_muc, codecs,
// secure_transport, crypto, and current_streams. If we don't currently have
// crypto (in current_cryptos) and it is enabled (in secure_policy), crypto is
// created (according to crypto_suites). The created content is added to the
// offer.
static bool CreateContentOffer(
    const MediaDescriptionOptions& media_description_options,
    const MediaSessionOptions& session_options,
    const SecurePolicy& secure_policy,
    const CryptoParamsVec* current_cryptos,
    const std::vector<std::string>& crypto_suites,
    const RtpHeaderExtensions& rtp_extensions,
    UniqueRandomIdGenerator* ssrc_generator,
    StreamParamsVec* current_streams,
    MediaContentDescription* offer) {
  offer->set_rtcp_mux(session_options.rtcp_mux_enabled);
  if (offer->type() == cricket::MEDIA_TYPE_VIDEO) {
    offer->set_rtcp_reduced_size(true);
  }
  // Build the vector of header extensions with directions for this
  // media_description's options.
  RtpHeaderExtensions extensions;
  for (auto extension_with_id : rtp_extensions) {
    for (const auto& extension : media_description_options.header_extensions) {
      if (extension_with_id.uri == extension.uri) {
        // TODO(crbug.com/1051821): Configure the extension direction from
        // the information in the media_description_options extension
        // capability.
        extensions.push_back(extension_with_id);
      }
    }
  }
  offer->set_rtp_header_extensions(extensions);
  AddSimulcastToMediaDescription(media_description_options, offer);
    // sdes 安全策略，我们用的是dtls，暂时不会进来这里
  if (secure_policy != SEC_DISABLED) {
    if (current_cryptos) {
      AddMediaCryptos(*current_cryptos, offer);
    }
    if (offer->cryptos().empty()) {
      if (!CreateMediaCryptos(crypto_suites, offer)) {
        return false;
      }
    }
  }
  if (secure_policy == SEC_REQUIRED && offer->cryptos().empty()) {
    return false;
  }
  return true;
}

offer->set_rtp_header_extensions(extensions); 设置拓展头

当该函数返回时，已经创建好offer了，可以查看

打开第一个标签的sendstreams 属性，可以看到是音频标签。

还可以看codecs

SetMediaProtocol

static void SetMediaProtocol(bool secure_transport,
                             MediaContentDescription* desc) {
  if (!desc->cryptos().empty()) //sdes协议
    desc->set_protocol(kMediaProtocolSavpf);
  else if (secure_transport)  //dtls协议
    desc->set_protocol(kMediaProtocolDtlsSavpf);
  else
    desc->set_protocol(kMediaProtocolAvpf);
}