webrtc 有關 SDP 部分的解析流程分析

std::unique_ptr<SessionDescriptionInterface> JavaToNativeSessionDescription(
    JNIEnv* jni,
    const JavaRef<jobject>& j_sdp) {
  std::string std_type = JavaToStdString(
      jni, Java_SessionDescription_getTypeInCanonicalForm(jni, j_sdp));
  std::string std_description =
      JavaToStdString(jni, Java_SessionDescription_getDescription(jni, j_sdp));
  absl::optional<SdpType> sdp_type_maybe = SdpTypeFromString(std_type);
  if (!sdp_type_maybe) {
    RTC_LOG(LS_ERROR) << "Unexpected SDP type: " << std_type;
    return nullptr;
  }
  // 需要進一步分析這個函式
  return CreateSessionDescription(*sdp_type_maybe, std_description);
}

std::unique_ptr<SessionDescriptionInterface> CreateSessionDescription(
    SdpType type,
    const std::string& sdp) {
  return CreateSessionDescription(type, sdp, nullptr);
}

std::unique_ptr<SessionDescriptionInterface> CreateSessionDescription(
    SdpType type,
    const std::string& sdp,
    SdpParseError* error_out) {
  auto jsep_desc = std::make_unique<JsepSessionDescription>(type);
  if (type != SdpType::kRollback) {
    // 需要進一步分析這個函式
    if (!SdpDeserialize(sdp, jsep_desc.get(), error_out)) {
      return nullptr;
    }
  }
  return std::move(jsep_desc);
}

bool SdpDeserialize(const std::string& message,
                    JsepSessionDescription* jdesc,
                    SdpParseError* error) {
  std::string session_id;
  std::string session_version;
  TransportDescription session_td("", "");
  RtpHeaderExtensions session_extmaps;
  rtc::SocketAddress session_connection_addr;
  auto desc = std::make_unique<cricket::SessionDescription>();
  size_t current_pos = 0;

  // Session Description
  if (!ParseSessionDescription(message, &current_pos, &session_id,
                               &session_version, &session_td, &session_extmaps,
                               &session_connection_addr, desc.get(), error)) {
    return false;
  }

  // Media Description
  std::vector<std::unique_ptr<JsepIceCandidate>> candidates;
  if (!ParseMediaDescription(message, session_td, session_extmaps, &current_pos,
                             session_connection_addr, desc.get(), &candidates,
                             error)) {
    return false;
  }

  jdesc->Initialize(std::move(desc), session_id, session_version);

  for (const auto& candidate : candidates) {
    jdesc->AddCandidate(candidate.get());
  }
  return true;
}

bool ParseSessionDescription(const std::string& message,
                             size_t* pos,
                             std::string* session_id,
                             std::string* session_version,
                             TransportDescription* session_td,
                             RtpHeaderExtensions* session_extmaps,
                             rtc::SocketAddress* connection_addr,
                             cricket::SessionDescription* desc,
                             SdpParseError* error) {
  std::string line;

  desc->set_msid_supported(false);
  desc->set_extmap_allow_mixed(false);
  // RFC 4566
  // v=  (protocol version)
  if (!GetLineWithType(message, pos, &line, kLineTypeVersion)) {
    return ParseFailedExpectLine(message, *pos, kLineTypeVersion, std::string(),
                                 error);
  }
  // RFC 4566
  // o=<username> <sess-id> <sess-version> <nettype> <addrtype>
  // <unicast-address>
  if (!GetLineWithType(message, pos, &line, kLineTypeOrigin)) {
    return ParseFailedExpectLine(message, *pos, kLineTypeOrigin, std::string(),
                                 error);
  }
  std::vector<std::string> fields;
  rtc::split(line.substr(kLinePrefixLength), kSdpDelimiterSpaceChar, &fields);
  const size_t expected_fields = 6;
  if (fields.size() != expected_fields) {
    return ParseFailedExpectFieldNum(line, expected_fields, error);
  }
  *session_id = fields[1];
  *session_version = fields[2];

  // RFC 4566
  // s=  (session name)
  if (!GetLineWithType(message, pos, &line, kLineTypeSessionName)) {
    return ParseFailedExpectLine(message, *pos, kLineTypeSessionName,
                                 std::string(), error);
  }

  // absl::optional lines
  // Those are the optional lines, so shouldn't return false if not present.
  // RFC 4566
  // i=* (session information)
  GetLineWithType(message, pos, &line, kLineTypeSessionInfo);

  // RFC 4566
  // u=* (URI of description)
  GetLineWithType(message, pos, &line, kLineTypeSessionUri);

  // RFC 4566
  // e=* (email address)
  GetLineWithType(message, pos, &line, kLineTypeSessionEmail);

  // RFC 4566
  // p=* (phone number)
  GetLineWithType(message, pos, &line, kLineTypeSessionPhone);

  // RFC 4566
  // c=* (connection information -- not required if included in
  //      all media)
  if (GetLineWithType(message, pos, &line, kLineTypeConnection)) {
    if (!ParseConnectionData(line, connection_addr, error)) {
      return false;
    }
  }

  // RFC 4566
  // b=* (zero or more bandwidth information lines)
  while (GetLineWithType(message, pos, &line, kLineTypeSessionBandwidth)) {
    // By pass zero or more b lines.
  }

  // RFC 4566
  // One or more time descriptions ("t=" and "r=" lines; see below)
  // t=  (time the session is active)
  // r=* (zero or more repeat times)
  // Ensure there's at least one time description
  if (!GetLineWithType(message, pos, &line, kLineTypeTiming)) {
    return ParseFailedExpectLine(message, *pos, kLineTypeTiming, std::string(),
                                 error);
  }

  while (GetLineWithType(message, pos, &line, kLineTypeRepeatTimes)) {
    // By pass zero or more r lines.
  }

  // Go through the rest of the time descriptions
  while (GetLineWithType(message, pos, &line, kLineTypeTiming)) {
    while (GetLineWithType(message, pos, &line, kLineTypeRepeatTimes)) {
      // By pass zero or more r lines.
    }
  }

  // RFC 4566
  // z=* (time zone adjustments)
  GetLineWithType(message, pos, &line, kLineTypeTimeZone);

  // RFC 4566
  // k=* (encryption key)
  GetLineWithType(message, pos, &line, kLineTypeEncryptionKey);

  // RFC 4566
  // a=* (zero or more session attribute lines)
  while (GetLineWithType(message, pos, &line, kLineTypeAttributes)) {
    if (HasAttribute(line, kAttributeGroup)) {
      if (!ParseGroupAttribute(line, desc, error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeIceUfrag)) {
      if (!GetValue(line, kAttributeIceUfrag, &(session_td->ice_ufrag),
                    error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeIcePwd)) {
      if (!GetValue(line, kAttributeIcePwd, &(session_td->ice_pwd), error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeIceLite)) {
      session_td->ice_mode = cricket::ICEMODE_LITE;
    } else if (HasAttribute(line, kAttributeIceOption)) {
      if (!ParseIceOptions(line, &(session_td->transport_options), error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeFingerprint)) {
      if (session_td->identity_fingerprint.get()) {
        return ParseFailed(
            line,
            "Can't have multiple fingerprint attributes at the same level.",
            error);
      }
      std::unique_ptr<rtc::SSLFingerprint> fingerprint;
      if (!ParseFingerprintAttribute(line, &fingerprint, error)) {
        return false;
      }
      session_td->identity_fingerprint = std::move(fingerprint);
    } else if (HasAttribute(line, kAttributeSetup)) {
      if (!ParseDtlsSetup(line, &(session_td->connection_role), error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeMsidSemantics)) {
      std::string semantics;
      if (!GetValue(line, kAttributeMsidSemantics, &semantics, error)) {
        return false;
      }
      desc->set_msid_supported(
          CaseInsensitiveFind(semantics, kMediaStreamSemantic));
    } else if (HasAttribute(line, kAttributeExtmapAllowMixed)) {
      desc->set_extmap_allow_mixed(true);
    } else if (HasAttribute(line, kAttributeExtmap)) {
      RtpExtension extmap;
      if (!ParseExtmap(line, &extmap, error)) {
        return false;
      }
      session_extmaps->push_back(extmap);
    }
  }

  return true;
}

bool ParseMediaDescription(
    const std::string& message,
    const TransportDescription& session_td,
    const RtpHeaderExtensions& session_extmaps,
    size_t* pos,
    const rtc::SocketAddress& session_connection_addr,
    cricket::SessionDescription* desc,
    std::vector<std::unique_ptr<JsepIceCandidate>>* candidates,
    SdpParseError* error) {
  RTC_DCHECK(desc != NULL);
  std::string line;
  int mline_index = -1;
  int msid_signaling = 0;

  // Zero or more media descriptions
  // RFC 4566
  // m=<media> <port> <proto> <fmt>
  while (GetLineWithType(message, pos, &line, kLineTypeMedia)) {
    ++mline_index;

    std::vector<std::string> fields;
    rtc::split(line.substr(kLinePrefixLength), kSdpDelimiterSpaceChar, &fields);

    const size_t expected_min_fields = 4;
    if (fields.size() < expected_min_fields) {
      return ParseFailedExpectMinFieldNum(line, expected_min_fields, error);
    }
    bool port_rejected = false;
    // RFC 3264
    // To reject an offered stream, the port number in the corresponding stream
    // in the answer MUST be set to zero.
    if (fields[1] == kMediaPortRejected) {
      port_rejected = true;
    }

    int port = 0;
    if (!rtc::FromString<int>(fields[1], &port) || !IsValidPort(port)) {
      return ParseFailed(line, "The port number is invalid", error);
    }
    std::string protocol = fields[2];

    // <fmt>
    std::vector<int> payload_types;
    if (cricket::IsRtpProtocol(protocol)) {
      for (size_t j = 3; j < fields.size(); ++j) {
        // TODO(wu): Remove when below bug is fixed.
        // http://bugzilla.mozilla.org/show_bug.cgi?id=996329
        if (fields[j].empty() && j == fields.size() - 1) {
          continue;
        }

        int pl = 0;
        if (!GetPayloadTypeFromString(line, fields[j], &pl, error)) {
          return false;
        }
        payload_types.push_back(pl);
      }
    }

    // Make a temporary TransportDescription based on |session_td|.
    // Some of this gets overwritten by ParseContent.
    TransportDescription transport(
        session_td.transport_options, session_td.ice_ufrag, session_td.ice_pwd,
        session_td.ice_mode, session_td.connection_role,
        session_td.identity_fingerprint.get());

    std::unique_ptr<MediaContentDescription> content;
    std::string content_name;
    bool bundle_only = false;
    int section_msid_signaling = 0;
    if (HasAttribute(line, kMediaTypeVideo)) {
      content = ParseContentDescription<VideoContentDescription>(
          message, cricket::MEDIA_TYPE_VIDEO, mline_index, protocol,
          payload_types, pos, &content_name, &bundle_only,
          &section_msid_signaling, &transport, candidates, error);
    } else if (HasAttribute(line, kMediaTypeAudio)) {
      content = ParseContentDescription<AudioContentDescription>(
          message, cricket::MEDIA_TYPE_AUDIO, mline_index, protocol,
          payload_types, pos, &content_name, &bundle_only,
          &section_msid_signaling, &transport, candidates, error);
    } else if (HasAttribute(line, kMediaTypeData)) {
      if (cricket::IsDtlsSctp(protocol)) {
        // The draft-03 format is:
        // m=application <port> DTLS/SCTP <sctp-port>...
        // use_sctpmap should be false.
        // The draft-26 format is:
        // m=application <port> UDP/DTLS/SCTP webrtc-datachannel
        // use_sctpmap should be false.
        auto data_desc = std::make_unique<SctpDataContentDescription>();
        // Default max message size is 64K
        // according to draft-ietf-mmusic-sctp-sdp-26
        data_desc->set_max_message_size(kDefaultSctpMaxMessageSize);
        int p;
        if (rtc::FromString(fields[3], &p)) {
          data_desc->set_port(p);
        } else if (fields[3] == kDefaultSctpmapProtocol) {
          data_desc->set_use_sctpmap(false);
        }
        if (!ParseContent(message, cricket::MEDIA_TYPE_DATA, mline_index,
                          protocol, payload_types, pos, &content_name,
                          &bundle_only, &section_msid_signaling,
                          data_desc.get(), &transport, candidates, error)) {
          return false;
        }
        data_desc->set_protocol(protocol);
        content = std::move(data_desc);
      } else {
        // RTP
        std::unique_ptr<RtpDataContentDescription> data_desc =
            ParseContentDescription<RtpDataContentDescription>(
                message, cricket::MEDIA_TYPE_DATA, mline_index, protocol,
                payload_types, pos, &content_name, &bundle_only,
                &section_msid_signaling, &transport, candidates, error);
        content = std::move(data_desc);
      }
    } else {
      RTC_LOG(LS_WARNING) << "Unsupported media type: " << line;
      continue;
    }
    if (!content.get()) {
      // ParseContentDescription returns NULL if failed.
      return false;
    }

    msid_signaling |= section_msid_signaling;

    bool content_rejected = false;
    // A port of 0 is not interpreted as a rejected m= section when it's
    // used along with a=bundle-only.
    if (bundle_only) {
      if (!port_rejected) {
        // Usage of bundle-only with a nonzero port is unspecified. So just
        // ignore bundle-only if we see this.
        bundle_only = false;
        RTC_LOG(LS_WARNING)
            << "a=bundle-only attribute observed with a nonzero "
               "port; this usage is unspecified so the attribute is being "
               "ignored.";
      }
    } else {
      // If not using bundle-only, interpret port 0 in the normal way; the m=
      // section is being rejected.
      content_rejected = port_rejected;
    }

    if (cricket::IsRtpProtocol(protocol) && !content->as_sctp()) {
      content->set_protocol(protocol);
      // Set the extmap.
      if (!session_extmaps.empty() &&
          !content->rtp_header_extensions().empty()) {
        return ParseFailed("",
                           "The a=extmap MUST be either all session level or "
                           "all media level.",
                           error);
      }
      for (size_t i = 0; i < session_extmaps.size(); ++i) {
        content->AddRtpHeaderExtension(session_extmaps[i]);
      }
    } else if (content->as_sctp()) {
      // Do nothing, it's OK
    } else {
      RTC_LOG(LS_WARNING) << "Parse failed with unknown protocol " << protocol;
      return false;
    }

    // Use the session level connection address if the media level addresses are
    // not specified.
    rtc::SocketAddress address;
    address = content->connection_address().IsNil()
                  ? session_connection_addr
                  : content->connection_address();
    address.SetPort(port);
    content->set_connection_address(address);

    desc->AddContent(content_name,
                     cricket::IsDtlsSctp(protocol) ? MediaProtocolType::kSctp
                                                   : MediaProtocolType::kRtp,
                     content_rejected, bundle_only, std::move(content));
    // Create TransportInfo with the media level "ice-pwd" and "ice-ufrag".
    desc->AddTransportInfo(TransportInfo(content_name, transport));
  }

  desc->set_msid_signaling(msid_signaling);

  size_t end_of_message = message.size();
  if (mline_index == -1 && *pos != end_of_message) {
    ParseFailed(message, *pos, "Expects m line.", error);
    return false;
  }
  return true;
}