webrtc 有关 SDP 部分的解析流程分析

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一分钟快速搭建 rtmpd 服务器: https://blog.csdn.net/freeabc/article/details/102880984

软件下载地址: http://www.qiyicc.com/download/rtmpd.rar

github 地址：https://github.com/superconvert/smart_rtmpd

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webrtc 有关 SDP 部分的解析流程分析

在这个例子里，我们以 android 端为例进行说明整个流程，但整个流程都发生在 JNI 层，别的基本一样。

1. 

./sdk/android/src/jni/pc/session_description.cc

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;
  }
  // 需要进一步分析这个函数
  return CreateSessionDescription(*sdp_type_maybe, std_description);
}

需要进一步分析这个函数 CreateSessionDescription 

2.

./pc/jsep_session_description.cc

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) {
    // 需要进一步分析这个函数
    if (!SdpDeserialize(sdp, jsep_desc.get(), error_out)) {
      return nullptr;
    }
  }
  return std::move(jsep_desc);
}

需要进一步分析这个函数 SdpDeserialize

3.

./pc/webrtc_sdp.cc

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;
}

看上述代码，我们知道 SDP 解析分为这几个部分，会话层解析 ParseSessionDescription,  媒体层解析 ParseMediaDescription， 还有 Candidate 的解析（如果存在）

会话层解析

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;
}

媒体层解析

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.
        // https://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;
}

我们继续分析一下 ParseContentDescription 函数 ./pc/webrtc_sdp.cc

template <class C>
static std::unique_ptr<C> ParseContentDescription(
    const std::string& message,
    const cricket::MediaType media_type,
    int mline_index,
    const std::string& protocol,
    const std::vector<int>& payload_types,
    size_t* pos,
    std::string* content_name,
    bool* bundle_only,
    int* msid_signaling,
    TransportDescription* transport,
    std::vector<std::unique_ptr<JsepIceCandidate>>* candidates,
    webrtc::SdpParseError* error) {
  auto media_desc = std::make_unique<C>();
  if (!ParseContent(message, media_type, mline_index, protocol, payload_types,
                    pos, content_name, bundle_only, msid_signaling,
                    media_desc.get(), transport, candidates, error)) {
    return nullptr;
  }
  // Sort the codecs according to the m-line fmt list.
  std::unordered_map<int, int> payload_type_preferences;
  // "size + 1" so that the lowest preference payload type has a preference of
  // 1, which is greater than the default (0) for payload types not in the fmt
  // list.
  int preference = static_cast<int>(payload_types.size() + 1);
  for (int pt : payload_types) {
    payload_type_preferences[pt] = preference--;
  }
  std::vector<typename C::CodecType> codecs = media_desc->codecs();
  absl::c_sort(
      codecs, [&payload_type_preferences](const typename C::CodecType& a,
                                          const typename C::CodecType& b) {
        return payload_type_preferences[a.id] > payload_type_preferences[b.id];
      });
  media_desc->set_codecs(codecs);
  return media_desc;
}

 ParseContent 函数 ./pc/webrtc_sdp.cc

bool ParseContent(const std::string& message,
                  const cricket::MediaType media_type,
                  int mline_index,
                  const std::string& protocol,
                  const std::vector<int>& payload_types,
                  size_t* pos,
                  std::string* content_name,
                  bool* bundle_only,
                  int* msid_signaling,
                  MediaContentDescription* media_desc,
                  TransportDescription* transport,
                  std::vector<std::unique_ptr<JsepIceCandidate>>* candidates,
                  SdpParseError* error) {
  RTC_DCHECK(media_desc != NULL);
  RTC_DCHECK(content_name != NULL);
  RTC_DCHECK(transport != NULL);

  if (media_type == cricket::MEDIA_TYPE_AUDIO) {
    MaybeCreateStaticPayloadAudioCodecs(payload_types, media_desc->as_audio());
  }

  // The media level "ice-ufrag" and "ice-pwd".
  // The candidates before update the media level "ice-pwd" and "ice-ufrag".
  Candidates candidates_orig;
  std::string line;
  std::string mline_id;
  // Tracks created out of the ssrc attributes.
  StreamParamsVec tracks;
  SsrcInfoVec ssrc_infos;
  SsrcGroupVec ssrc_groups;
  std::string maxptime_as_string;
  std::string ptime_as_string;
  std::vector<std::string> stream_ids;
  std::string track_id;
  SdpSerializer deserializer;
  std::vector<RidDescription> rids;
  SimulcastDescription simulcast;

  // Loop until the next m line
  while (!IsLineType(message, kLineTypeMedia, *pos)) {
    if (!GetLine(message, pos, &line)) {
      if (*pos >= message.size()) {
        break;  // Done parsing
      } else {
        return ParseFailed(message, *pos, "Invalid SDP line.", error);
      }
    }

    // RFC 4566
    // b=* (zero or more bandwidth information lines)
    if (IsLineType(line, kLineTypeSessionBandwidth)) {
      std::string bandwidth;
      if (HasAttribute(line, kApplicationSpecificMaximum)) {
        if (!GetValue(line, kApplicationSpecificMaximum, &bandwidth, error)) {
          return false;
        } else {
          int b = 0;
          if (!GetValueFromString(line, bandwidth, &b, error)) {
            return false;
          }
          // TODO(deadbeef): Historically, applications may be setting a value
          // of -1 to mean "unset any previously set bandwidth limit", even
          // though ommitting the "b=AS" entirely will do just that. Once we've
          // transitioned applications to doing the right thing, it would be
          // better to treat this as a hard error instead of just ignoring it.
          if (b == -1) {
            RTC_LOG(LS_WARNING)
                << "Ignoring \"b=AS:-1\"; will be treated as \"no "
                   "bandwidth limit\".";
            continue;
          }
          if (b < 0) {
            return ParseFailed(line, "b=AS value can't be negative.", error);
          }
          // We should never use more than the default bandwidth for RTP-based
          // data channels. Don't allow SDP to set the bandwidth, because
          // that would give JS the opportunity to "break the Internet".
          // See: https://code.google.com/p/chromium/issues/detail?id=280726
          if (media_type == cricket::MEDIA_TYPE_DATA &&
              cricket::IsRtpProtocol(protocol) &&
              b > cricket::kDataMaxBandwidth / 1000) {
            rtc::StringBuilder description;
            description << "RTP-based data channels may not send more than "
                        << cricket::kDataMaxBandwidth / 1000 << "kbps.";
            return ParseFailed(line, description.str(), error);
          }
          // Prevent integer overflow.
          b = std::min(b, INT_MAX / 1000);
          media_desc->set_bandwidth(b * 1000);
        }
      }
      continue;
    }

    // Parse the media level connection data.
    if (IsLineType(line, kLineTypeConnection)) {
      rtc::SocketAddress addr;
      if (!ParseConnectionData(line, &addr, error)) {
        return false;
      }
      media_desc->set_connection_address(addr);
      continue;
    }

    if (!IsLineType(line, kLineTypeAttributes)) {
      // TODO(deadbeef): Handle other lines if needed.
      RTC_LOG(LS_INFO) << "Ignored line: " << line;
      continue;
    }

    // Handle attributes common to SCTP and RTP.
    if (HasAttribute(line, kAttributeMid)) {
      // RFC 3388
      // mid-attribute      = "a=mid:" identification-tag
      // identification-tag = token
      // Use the mid identification-tag as the content name.
      if (!GetValue(line, kAttributeMid, &mline_id, error)) {
        return false;
      }
      *content_name = mline_id;
    } else if (HasAttribute(line, kAttributeBundleOnly)) {
      *bundle_only = true;
    } else if (HasAttribute(line, kAttributeCandidate)) {
      Candidate candidate;
      if (!ParseCandidate(line, &candidate, error, false)) {
        return false;
      }
      // ParseCandidate will parse non-standard ufrag and password attributes,
      // since it's used for candidate trickling, but we only want to process
      // the "a=ice-ufrag"/"a=ice-pwd" values in a session description, so
      // strip them off at this point.
      candidate.set_username(std::string());
      candidate.set_password(std::string());
      candidates_orig.push_back(candidate);
    } else if (HasAttribute(line, kAttributeIceUfrag)) {
      if (!GetValue(line, kAttributeIceUfrag, &transport->ice_ufrag, error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeIcePwd)) {
      if (!GetValue(line, kAttributeIcePwd, &transport->ice_pwd, error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeIceOption)) {
      if (!ParseIceOptions(line, &transport->transport_options, error)) {
        return false;
      }
    } else if (HasAttribute(line, kOpaqueTransportParametersLine)) {
      transport->opaque_parameters = cricket::OpaqueTransportParameters();
      if (!ParseOpaqueTransportLine(
              line, &transport->opaque_parameters->protocol,
              &transport->opaque_parameters->parameters, error)) {
        return false;
      }
    } else if (HasAttribute(line, kAltProtocolLine)) {
      std::string alt_protocol;
      if (!ParseAltProtocolLine(line, &alt_protocol, error)) {
        return false;
      }
      media_desc->set_alt_protocol(alt_protocol);
    } else if (HasAttribute(line, kAttributeFmtp)) {
      if (!ParseFmtpAttributes(line, media_type, media_desc, error)) {
        return false;
      }
    } else if (HasAttribute(line, kAttributeFingerprint)) {
      std::unique_ptr<rtc::SSLFingerprint> fingerprint;
      if (!ParseFingerprintAttribute(line, &fingerprint, error)) {
        return false;
      }
      transport->identity_fingerprint = std::move(fingerprint);
    } else if (HasAttribute(line, kAttributeSetup)) {
      if (!ParseDtlsSetup(line, &(transport->connection_role), error)) {
        return false;
      }
    } else if (cricket::IsDtlsSctp(protocol) &&
               HasAttribute(line, kAttributeSctpPort)) {
      if (media_type != cricket::MEDIA_TYPE_DATA) {
        return ParseFailed(
            line, "sctp-port attribute found in non-data media description.",
            error);
      }
      if (media_desc->as_sctp()->use_sctpmap()) {
        return ParseFailed(
            line, "sctp-port attribute can't be used with sctpmap.", error);
      }
      int sctp_port;
      if (!ParseSctpPort(line, &sctp_port, error)) {
        return false;
      }
      media_desc->as_sctp()->set_port(sctp_port);
    } else if (cricket::IsDtlsSctp(protocol) &&
               HasAttribute(line, kAttributeMaxMessageSize)) {
      if (media_type != cricket::MEDIA_TYPE_DATA) {
        return ParseFailed(
            line,
            "max-message-size attribute found in non-data media description.",
            error);
      }
      int max_message_size;
      if (!ParseSctpMaxMessageSize(line, &max_message_size, error)) {
        return false;
      }
      media_desc->as_sctp()->set_max_message_size(max_message_size);
    } else if (cricket::IsRtpProtocol(protocol)) {
      //
      // RTP specific attrubtes
      //
      if (HasAttribute(line, kAttributeRtcpMux)) {
        media_desc->set_rtcp_mux(true);
      } else if (HasAttribute(line, kAttributeRtcpReducedSize)) {
        media_desc->set_rtcp_reduced_size(true);
      } else if (HasAttribute(line, kAttributeRtcpRemoteEstimate)) {
        media_desc->set_remote_estimate(true);
      } else if (HasAttribute(line, kAttributeSsrcGroup)) {
        if (!ParseSsrcGroupAttribute(line, &ssrc_groups, error)) {
          return false;
        }
      } else if (HasAttribute(line, kAttributeSsrc)) {
        if (!ParseSsrcAttribute(line, &ssrc_infos, msid_signaling, error)) {
          return false;
        }
      } else if (HasAttribute(line, kAttributeCrypto)) {
        if (!ParseCryptoAttribute(line, media_desc, error)) {
          return false;
        }
      } else if (HasAttribute(line, kAttributeRtpmap)) {
        if (!ParseRtpmapAttribute(line, media_type, payload_types, media_desc,
                                  error)) {
          return false;
        }
      } else if (HasAttribute(line, kCodecParamMaxPTime)) {
        if (!GetValue(line, kCodecParamMaxPTime, &maxptime_as_string, error)) {
          return false;
        }
      } else if (HasAttribute(line, kAttributePacketization)) {
        if (!ParsePacketizationAttribute(line, media_type, media_desc, error)) {
          return false;
        }
      } else if (HasAttribute(line, kAttributeRtcpFb)) {
        if (!ParseRtcpFbAttribute(line, media_type, media_desc, error)) {
          return false;
        }
      } else if (HasAttribute(line, kCodecParamPTime)) {
        if (!GetValue(line, kCodecParamPTime, &ptime_as_string, error)) {
          return false;
        }
      } else if (HasAttribute(line, kAttributeSendOnly)) {
        media_desc->set_direction(RtpTransceiverDirection::kSendOnly);
      } else if (HasAttribute(line, kAttributeRecvOnly)) {
        media_desc->set_direction(RtpTransceiverDirection::kRecvOnly);
      } else if (HasAttribute(line, kAttributeInactive)) {
        media_desc->set_direction(RtpTransceiverDirection::kInactive);
      } else if (HasAttribute(line, kAttributeSendRecv)) {
        media_desc->set_direction(RtpTransceiverDirection::kSendRecv);
      } else if (HasAttribute(line, kAttributeExtmapAllowMixed)) {
        media_desc->set_extmap_allow_mixed_enum(
            MediaContentDescription::kMedia);
      } else if (HasAttribute(line, kAttributeExtmap)) {
        RtpExtension extmap;
        if (!ParseExtmap(line, &extmap, error)) {
          return false;
        }
        media_desc->AddRtpHeaderExtension(extmap);
      } else if (HasAttribute(line, kAttributeXGoogleFlag)) {
        // Experimental attribute.  Conference mode activates more aggressive
        // AEC and NS settings.
        // TODO(deadbeef): expose API to set these directly.
        std::string flag_value;
        if (!GetValue(line, kAttributeXGoogleFlag, &flag_value, error)) {
          return false;
        }
        if (flag_value.compare(kValueConference) == 0)
          media_desc->set_conference_mode(true);
      } else if (HasAttribute(line, kAttributeMsid)) {
        if (!ParseMsidAttribute(line, &stream_ids, &track_id, error)) {
          return false;
        }
        *msid_signaling |= cricket::kMsidSignalingMediaSection;
      } else if (HasAttribute(line, kAttributeRid)) {
        const size_t kRidPrefixLength =
            kLinePrefixLength + arraysize(kAttributeRid);
        if (line.size() <= kRidPrefixLength) {
          RTC_LOG(LS_INFO) << "Ignoring empty RID attribute: " << line;
          continue;
        }
        RTCErrorOr<RidDescription> error_or_rid_description =
            deserializer.DeserializeRidDescription(
                line.substr(kRidPrefixLength));

        // Malformed a=rid lines are discarded.
        if (!error_or_rid_description.ok()) {
          RTC_LOG(LS_INFO) << "Ignoring malformed RID line: '" << line
                           << "'. Error: "
                           << error_or_rid_description.error().message();
          continue;
        }

        rids.push_back(error_or_rid_description.MoveValue());
      } else if (HasAttribute(line, kAttributeSimulcast)) {
        const size_t kSimulcastPrefixLength =
            kLinePrefixLength + arraysize(kAttributeSimulcast);
        if (line.size() <= kSimulcastPrefixLength) {
          return ParseFailed(line, "Simulcast attribute is empty.", error);
        }

        if (!simulcast.empty()) {
          return ParseFailed(line, "Multiple Simulcast attributes specified.",
                             error);
        }

        RTCErrorOr<SimulcastDescription> error_or_simulcast =
            deserializer.DeserializeSimulcastDescription(
                line.substr(kSimulcastPrefixLength));
        if (!error_or_simulcast.ok()) {
          return ParseFailed(line,
                             std::string("Malformed simulcast line: ") +
                                 error_or_simulcast.error().message(),
                             error);
        }

        simulcast = error_or_simulcast.value();
      } else {
        // Unrecognized attribute in RTP protocol.
        RTC_LOG(LS_INFO) << "Ignored line: " << line;
        continue;
      }
    } else {
      // Only parse lines that we are interested of.
      RTC_LOG(LS_INFO) << "Ignored line: " << line;
      continue;
    }
  }

  // Remove duplicate or inconsistent rids.
  RemoveInvalidRidDescriptions(payload_types, &rids);

  // If simulcast is specifed, split the rids into send and receive.
  // Rids that do not appear in simulcast attribute will be removed.
  // If it is not specified, we assume that all rids are for send layers.
  std::vector<RidDescription> send_rids;
  std::vector<RidDescription> receive_rids;
  if (!simulcast.empty()) {
    // Verify that the rids in simulcast match rids in sdp.
    RemoveInvalidRidsFromSimulcast(rids, &simulcast);

    // Use simulcast description to figure out Send / Receive RIDs.
    std::map<std::string, RidDescription> rid_map;
    for (const RidDescription& rid : rids) {
      rid_map[rid.rid] = rid;
    }

    for (const auto& layer : simulcast.send_layers().GetAllLayers()) {
      auto iter = rid_map.find(layer.rid);
      RTC_DCHECK(iter != rid_map.end());
      send_rids.push_back(iter->second);
    }

    for (const auto& layer : simulcast.receive_layers().GetAllLayers()) {
      auto iter = rid_map.find(layer.rid);
      RTC_DCHECK(iter != rid_map.end());
      receive_rids.push_back(iter->second);
    }

    media_desc->set_simulcast_description(simulcast);
  } else {
    send_rids = rids;
  }

  media_desc->set_receive_rids(receive_rids);

  // Create tracks from the |ssrc_infos|.
  // If the stream_id/track_id for all SSRCS are identical, one StreamParams
  // will be created in CreateTracksFromSsrcInfos, containing all the SSRCs from
  // the m= section.
  if (!ssrc_infos.empty()) {
    CreateTracksFromSsrcInfos(ssrc_infos, stream_ids, track_id, &tracks,
                              *msid_signaling);
  } else if (media_type != cricket::MEDIA_TYPE_DATA &&
             (*msid_signaling & cricket::kMsidSignalingMediaSection)) {
    // If the stream_ids/track_id was signaled but SSRCs were unsignaled we
    // still create a track. This isn't done for data media types because
    // StreamParams aren't used for SCTP streams, and RTP data channels don't
    // support unsignaled SSRCs.
    CreateTrackWithNoSsrcs(stream_ids, track_id, send_rids, &tracks);
  }

  // Add the ssrc group to the track.
  for (const SsrcGroup& ssrc_group : ssrc_groups) {
    if (ssrc_group.ssrcs.empty()) {
      continue;
    }
    uint32_t ssrc = ssrc_group.ssrcs.front();
    for (StreamParams& track : tracks) {
      if (track.has_ssrc(ssrc)) {
        track.ssrc_groups.push_back(ssrc_group);
      }
    }
  }

  // Add the new tracks to the |media_desc|.
  for (StreamParams& track : tracks) {
    media_desc->AddStream(track);
  }

  if (media_type == cricket::MEDIA_TYPE_AUDIO) {
    AudioContentDescription* audio_desc = media_desc->as_audio();
    UpdateFromWildcardCodecs(audio_desc);

    // Verify audio codec ensures that no audio codec has been populated with
    // only fmtp.
    if (!VerifyAudioCodecs(audio_desc)) {
      return ParseFailed("Failed to parse audio codecs correctly.", error);
    }
    AddAudioAttribute(kCodecParamMaxPTime, maxptime_as_string, audio_desc);
    AddAudioAttribute(kCodecParamPTime, ptime_as_string, audio_desc);
  }

  if (media_type == cricket::MEDIA_TYPE_VIDEO) {
    VideoContentDescription* video_desc = media_desc->as_video();
    UpdateFromWildcardCodecs(video_desc);
    // Verify video codec ensures that no video codec has been populated with
    // only rtcp-fb.
    if (!VerifyVideoCodecs(video_desc)) {
      return ParseFailed("Failed to parse video codecs correctly.", error);
    }
  }

  // RFC 5245
  // Update the candidates with the media level "ice-pwd" and "ice-ufrag".
  for (Candidate& candidate : candidates_orig) {
    RTC_DCHECK(candidate.username().empty() ||
               candidate.username() == transport->ice_ufrag);
    candidate.set_username(transport->ice_ufrag);
    RTC_DCHECK(candidate.password().empty());
    candidate.set_password(transport->ice_pwd);
    candidates->push_back(
        std::make_unique<JsepIceCandidate>(mline_id, mline_index, candidate));
  }

  return true;
}

 

 经过上述流程我们知道底层的 SDP 对象其实就是一个 JsepSessionDescription 对象，而 JsepSessionDescription  又包含这个对象 SessionDescription 作为属性，其实核心数据都在 SessionDescription

底层对象 PeerConnection 的属性

./pc/peer_connection.h

  std::unique_ptr<SessionDescriptionInterface> current_local_description_
  std::unique_ptr<SessionDescriptionInterface> pending_local_description_
  std::unique_ptr<SessionDescriptionInterface> current_remote_description_
  std::unique_ptr<SessionDescriptionInterface> pending_remote_description_

其实也就是 JsepSessionDescription 对象，有关 SDP 的具体数据就是 SessionDescription

我们所有的 SDP 信息都在  current_local_description_, current_remote_description_ 而它的 contents_ 包含了每一个 SDP 的 m 段，代表一路音频或视频的具体信息。