1.简介

data stall检测机制就我现在的理解看来是Android 在网络校验成功后，对网络的一种持续监控措施，一旦发现当前网络断网，则通知ConnectivityService，进行相应的处理措施。

2.流程分析

2.1 tcp health 获取

NetworkMonitor.java

    private class ValidatedState extends State {@Overridepublic void enter() {maybeLogEvaluationResult(networkEventType(validationStage(), EvaluationResult.VALIDATED));// If the user has accepted partial connectivity and HTTPS probing is disabled, then// mark the network as validated and partial so that settings can keep informing the// user that the connection is limited.int result = NETWORK_VALIDATION_RESULT_VALID;if (!mUseHttps && mAcceptPartialConnectivity) {result |= NETWORK_VALIDATION_RESULT_PARTIAL;}mEvaluationState.reportEvaluationResult(result, null /* redirectUrl */);mValidations++;initSocketTrackingIfRequired();// start periodical polling.sendTcpPollingEvent();maybeStopCollectionAndSendMetrics();}

看下网络在校验通过后干了啥

        private void initSocketTrackingIfRequired() {if (!isValidationRequired()) return;final TcpSocketTracker tst = getTcpSocketTracker();if (tst != null) {tst.pollSocketsInfo();}}/*** Request to send a SockDiag Netlink request. Receive and parse the returned message. This* function is not thread-safe and should only be called from only one thread.** @Return if this polling request executes successfully or not.*/public boolean pollSocketsInfo() {if (!mDependencies.isTcpInfoParsingSupported()) return false;FileDescriptor fd = null;try {final long time = SystemClock.elapsedRealtime();fd = mDependencies.connectToKernel();final TcpStat stat = new TcpStat();for (final int family : ADDRESS_FAMILIES) {mDependencies.sendPollingRequest(fd, mSockDiagMsg.get(family));// Messages are composed with the following format. Stop parsing when receiving// message with nlmsg_type NLMSG_DONE.// +------------------+---------------+--------------+--------+// | Netlink Header   | Family Header | Attributes   | rtattr |// | struct nlmsghdr  | struct rtmsg  | struct rtattr|  data  |// +------------------+---------------+--------------+--------+//               :           :               :// +------------------+---------------+--------------+--------+// | Netlink Header   | Family Header | Attributes   | rtattr |// | struct nlmsghdr  | struct rtmsg  | struct rtattr|  data  |// +------------------+---------------+--------------+--------+final ByteBuffer bytes = mDependencies.recvMessage(fd);try {while (enoughBytesRemainForValidNlMsg(bytes)) {final StructNlMsgHdr nlmsghdr = StructNlMsgHdr.parse(bytes);if (nlmsghdr == null) {Log.e(TAG, "Badly formatted data.");break;}final int nlmsgLen = nlmsghdr.nlmsg_len;log("pollSocketsInfo: nlmsghdr=" + nlmsghdr + ", limit=" + bytes.limit());// End of the message. Stop parsing.if (nlmsghdr.nlmsg_type == NLMSG_DONE) break;if (nlmsghdr.nlmsg_type != SOCK_DIAG_BY_FAMILY) {Log.e(TAG, "Expect to get family " + family+ " SOCK_DIAG_BY_FAMILY message but get "+ nlmsghdr.nlmsg_type);break;}if (isValidInetDiagMsgSize(nlmsgLen)) {// Get the socket cookie value. Composed by two Integers value.// Corresponds to inet_diag_sockid in// &lt;linux_src&gt;/include/uapi/linux/inet_diag.hbytes.position(bytes.position() + IDIAG_COOKIE_OFFSET);// It's stored in native with 2 int. Parse it as long for convenience.final long cookie = bytes.getLong();// Skip the rest part of StructInetDiagMsg.bytes.position(bytes.position()+ StructInetDiagMsg.STRUCT_SIZE - IDIAG_COOKIE_OFFSET- Long.BYTES);final SocketInfo info = parseSockInfo(bytes, family, nlmsgLen, time);// Update TcpStats based on previous and current socket info.stat.accumulate(calculateLatestPacketsStat(info, mSocketInfos.get(cookie)));mSocketInfos.put(cookie, info);}}} catch (IllegalArgumentException | BufferUnderflowException e) {Log.wtf(TAG, "Unexpected socket info parsing, family " + family+ " buffer:" + bytes + " "+ Base64.getEncoder().encodeToString(bytes.array()), e);}}// Calculate mLatestReceiveCount, mSentSinceLastRecv and mLatestPacketFailPercentage.mSentSinceLastRecv = (stat.receivedCount == 0)? (mSentSinceLastRecv + stat.sentCount) : 0;mLatestReceivedCount = stat.receivedCount;mLatestPacketFailPercentage = ((stat.sentCount != 0)? ((stat.retransmitCount + stat.lostCount) * 100 / stat.sentCount) : 0);// Remove out-of-date socket info.cleanupSocketInfo(time);return true;} catch (ErrnoException | SocketException | InterruptedIOException e) {Log.e(TAG, "Fail to get TCP info via netlink.", e);} finally {NetworkStackUtils.closeSocketQuietly(fd);}return false;}

获取当前最新的发包数+失败率+收包数

    // Number of packets sent since the last received packetprivate int mSentSinceLastRecv;// The latest fail rate calculated by the latest tcp info.private int mLatestPacketFailPercentage;// Number of packets received in the latest polling cycle.private int mLatestReceivedCount;

上面的获取收发包情况的逻辑会在如下消息处理中循环往复，间隔为20s

    @VisibleForTestingvoid sendTcpPollingEvent() {if (isValidationRequired()) {sendMessageDelayed(EVENT_POLL_TCPINFO, getTcpPollingInterval());}}case EVENT_POLL_TCPINFO:final TcpSocketTracker tst = getTcpSocketTracker();if (tst == null) break;// Transit if retrieve socket info is succeeded and suspected as a stall.if (tst.pollSocketsInfo() && evaluateDataStall()) {transitionTo(mEvaluatingState);} else {sendTcpPollingEvent();}break;

2.2 tcp health 判定

        boolean evaluateDataStall() {if (isDataStall()) {validationLog("Suspecting data stall, reevaluate");return true;}return false;}@VisibleForTestingprotected boolean isDataStall() {if (!isValidationRequired()) {return false;}Boolean result = null;final StringJoiner msg = (DBG || VDBG_STALL) ? new StringJoiner(", ") : null;// Reevaluation will generate traffic. Thus, set a minimal reevaluation timer to limit the// possible traffic cost in metered network.if (!mNetworkCapabilities.hasCapability(NET_CAPABILITY_NOT_METERED)&& (SystemClock.elapsedRealtime() - getLastProbeTime()< mDataStallMinEvaluateTime)) {return false;}// Check TCP signal. Suspect it may be a data stall if :// 1. TCP connection fail rate(lost+retrans) is higher than threshold.// 2. Accumulate enough packets count.final TcpSocketTracker tst = getTcpSocketTracker();if (dataStallEvaluateTypeEnabled(DATA_STALL_EVALUATION_TYPE_TCP) && tst != null) {if (tst.getLatestReceivedCount() > 0) {result = false;} else if (tst.isDataStallSuspected()) {result = true;mDataStallTypeToCollect = DATA_STALL_EVALUATION_TYPE_TCP;final DataStallReportParcelable p = new DataStallReportParcelable();p.detectionMethod = DETECTION_METHOD_TCP_METRICS;p.timestampMillis = SystemClock.elapsedRealtime();p.tcpPacketFailRate = tst.getLatestPacketFailPercentage();p.tcpMetricsCollectionPeriodMillis = getTcpPollingInterval();notifyDataStallSuspected(p);}if (DBG || VDBG_STALL) {msg.add("tcp packets received=" + tst.getLatestReceivedCount()).add("latest tcp fail rate=" + tst.getLatestPacketFailPercentage());}}// Check dns signal. Suspect it may be a data stall if both :// 1. The number of consecutive DNS query timeouts >= mConsecutiveDnsTimeoutThreshold.// 2. Those consecutive DNS queries happened in the last mValidDataStallDnsTimeThreshold ms.final DnsStallDetector dsd = getDnsStallDetector();if ((result == null) && (dsd != null)&& dataStallEvaluateTypeEnabled(DATA_STALL_EVALUATION_TYPE_DNS)) {if (dsd.isDataStallSuspected(mConsecutiveDnsTimeoutThreshold,mDataStallValidDnsTimeThreshold)) {result = true;mDataStallTypeToCollect = DATA_STALL_EVALUATION_TYPE_DNS;logNetworkEvent(NetworkEvent.NETWORK_CONSECUTIVE_DNS_TIMEOUT_FOUND);final DataStallReportParcelable p = new DataStallReportParcelable();p.detectionMethod = DETECTION_METHOD_DNS_EVENTS;p.timestampMillis = SystemClock.elapsedRealtime();p.dnsConsecutiveTimeouts = mDnsStallDetector.getConsecutiveTimeoutCount();notifyDataStallSuspected(p);}if (DBG || VDBG_STALL) {msg.add("consecutive dns timeout count=" + dsd.getConsecutiveTimeoutCount());}}// log only data stall suspected.if ((DBG && Boolean.TRUE.equals(result)) || VDBG_STALL) {log("isDataStall: result=" + result + ", " + msg);}return (result == null) ? false : result;}

首先是统计当前tcp是否能收到包，若能收到则认为网络正常，其次判定收发包失败率是否大于80%

    /*** Default tcp packets fail rate to suspect as a data stall.** Calculated by ((# of packets lost)+(# of packets retrans))/(# of packets sent)*100. Ideally,* the percentage should be 100%. However, the ongoing packets may not be considered as neither* lost or retrans yet. It will cause the percentage lower.*/public static final int DEFAULT_TCP_PACKETS_FAIL_PERCENTAGE = 80;

再接着会判定dns的情况，若30min中连续5次dns失败则认为是断网

    // Default configuration values for data stall detection.public static final int DEFAULT_CONSECUTIVE_DNS_TIMEOUT_THRESHOLD = 5;public static final int DEFAULT_DATA_STALL_VALID_DNS_TIME_THRESHOLD_MS = 30 * 60 * 1000;

3.后续处理

NetworkMonitor发现当前网络没往后会重新进行网络校验，后续通知给CS

另外发现断网的即刻就会回调通知CS告知与当前网络绑定的各应用，该网络断网了

    private void handleDataStallSuspected(@NonNull NetworkAgentInfo nai, long timestampMillis, int detectionMethod,@NonNull PersistableBundle extras) {final NetworkCapabilities networkCapabilities =getNetworkCapabilitiesWithoutUids(nai.networkCapabilities);final DataStallReport report =new DataStallReport(nai.network,timestampMillis,detectionMethod,nai.linkProperties,networkCapabilities,extras);final List<IConnectivityDiagnosticsCallback> results =getMatchingPermissionedCallbacks(nai);for (final IConnectivityDiagnosticsCallback cb : results) {try {cb.onDataStallSuspected(report);} catch (RemoteException ex) {loge("Error invoking onDataStallSuspected", ex);}}}

4.总结

网络连接成功后有个叫做data stall的检测机制来持续检测网络可达性，判定标准为是否可以正常收包或者包失败率大于80%或者在30min内dns连续失败5次，即判定断网，通报给ConnectivityService。

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