diff --git a/service/jni/com_android_server_connectivity_ClatCoordinator.cpp b/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
index 9603139..eeaad18 100644
--- a/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
+++ b/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#define LOG_TAG "jniClatCoordinator"
 
 #include <arpa/inet.h>
 #include <errno.h>
@@ -22,6 +23,8 @@
 #include <log/log.h>
 #include <nativehelper/JNIHelp.h>
 #include <net/if.h>
+#include <spawn.h>
+#include <sys/wait.h>
 #include <string>
 
 #include <netjniutils/netjniutils.h>
@@ -33,9 +36,16 @@
 // Sync from system/netd/include/netid_client.h
 #define MARK_UNSET 0u
 
+// Sync from system/netd/server/NetdConstants.h
+#define __INT_STRLEN(i) sizeof(#i)
+#define _INT_STRLEN(i) __INT_STRLEN(i)
+#define INT32_STRLEN _INT_STRLEN(INT32_MIN)
+
 #define DEVICEPREFIX "v4-"
 
 namespace android {
+static const char* kClatdPath = "/apex/com.android.tethering/bin/for-system/clatd";
+
 static void throwIOException(JNIEnv* env, const char* msg, int error) {
     jniThrowExceptionFmt(env, "java/io/IOException", "%s: %s", msg, strerror(error));
 }
@@ -282,8 +292,7 @@
     return 0;
 }
 
-// TODO: fork clatd and rename to .._startClatd.
-static jint com_android_server_connectivity_ClatCoordinator_maybeStartBpf(
+static jint com_android_server_connectivity_ClatCoordinator_startClatd(
         JNIEnv* env, jobject clazz, jobject tunJavaFd, jobject readSockJavaFd,
         jobject writeSockJavaFd, jstring iface, jstring pfx96, jstring v4, jstring v6) {
     ScopedUtfChars ifaceStr(env, iface);
@@ -291,7 +300,121 @@
     ScopedUtfChars v4Str(env, v4);
     ScopedUtfChars v6Str(env, v6);
 
-    // Start BPF if any
+    int tunFd = netjniutils::GetNativeFileDescriptor(env, tunJavaFd);
+    if (tunFd < 0) {
+        jniThrowExceptionFmt(env, "java/io/IOException", "Invalid tun file descriptor");
+        return -1;
+    }
+
+    int readSock = netjniutils::GetNativeFileDescriptor(env, readSockJavaFd);
+    if (readSock < 0) {
+        jniThrowExceptionFmt(env, "java/io/IOException", "Invalid read socket");
+        return -1;
+    }
+
+    int writeSock = netjniutils::GetNativeFileDescriptor(env, writeSockJavaFd);
+    if (writeSock < 0) {
+        jniThrowExceptionFmt(env, "java/io/IOException", "Invalid write socket");
+        return -1;
+    }
+
+    // 1. create a throwaway socket to reserve a file descriptor number
+    int passedTunFd = socket(AF_INET6, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+    if (passedTunFd == -1) {
+        throwIOException(env, "socket(ipv6/udp) for tun fd failed", errno);
+        return -1;
+    }
+    int passedSockRead = socket(AF_INET6, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+    if (passedSockRead == -1) {
+        throwIOException(env, "socket(ipv6/udp) for read socket failed", errno);
+        return -1;
+    }
+    int passedSockWrite = socket(AF_INET6, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+    if (passedSockWrite == -1) {
+        throwIOException(env, "socket(ipv6/udp) for write socket failed", errno);
+        return -1;
+    }
+
+    // these are the FD we'll pass to clatd on the cli, so need it as a string
+    char passedTunFdStr[INT32_STRLEN];
+    char passedSockReadStr[INT32_STRLEN];
+    char passedSockWriteStr[INT32_STRLEN];
+    snprintf(passedTunFdStr, sizeof(passedTunFdStr), "%d", passedTunFd);
+    snprintf(passedSockReadStr, sizeof(passedSockReadStr), "%d", passedSockRead);
+    snprintf(passedSockWriteStr, sizeof(passedSockWriteStr), "%d", passedSockWrite);
+
+    // 2. we're going to use this as argv[0] to clatd to make ps output more useful
+    std::string progname("clatd-");
+    progname += ifaceStr.c_str();
+
+    // clang-format off
+    const char* args[] = {progname.c_str(),
+                          "-i", ifaceStr.c_str(),
+                          "-p", pfx96Str.c_str(),
+                          "-4", v4Str.c_str(),
+                          "-6", v6Str.c_str(),
+                          "-t", passedTunFdStr,
+                          "-r", passedSockReadStr,
+                          "-w", passedSockWriteStr,
+                          nullptr};
+    // clang-format on
+
+    // 3. register vfork requirement
+    posix_spawnattr_t attr;
+    if (int ret = posix_spawnattr_init(&attr)) {
+        throwIOException(env, "posix_spawnattr_init failed", ret);
+        return -1;
+    }
+
+    // TODO: use android::base::ScopeGuard.
+    if (int ret = posix_spawnattr_setflags(&attr, POSIX_SPAWN_USEVFORK)) {
+        posix_spawnattr_destroy(&attr);
+        throwIOException(env, "posix_spawnattr_setflags failed", ret);
+        return -1;
+    }
+
+    // 4. register dup2() action: this is what 'clears' the CLOEXEC flag
+    // on the tun fd that we want the child clatd process to inherit
+    // (this will happen after the vfork, and before the execve)
+    posix_spawn_file_actions_t fa;
+    if (int ret = posix_spawn_file_actions_init(&fa)) {
+        posix_spawnattr_destroy(&attr);
+        throwIOException(env, "posix_spawn_file_actions_init failed", ret);
+        return -1;
+    }
+
+    if (int ret = posix_spawn_file_actions_adddup2(&fa, tunFd, passedTunFd)) {
+        posix_spawnattr_destroy(&attr);
+        posix_spawn_file_actions_destroy(&fa);
+        throwIOException(env, "posix_spawn_file_actions_adddup2 for tun fd failed", ret);
+        return -1;
+    }
+    if (int ret = posix_spawn_file_actions_adddup2(&fa, readSock, passedSockRead)) {
+        posix_spawnattr_destroy(&attr);
+        posix_spawn_file_actions_destroy(&fa);
+        throwIOException(env, "posix_spawn_file_actions_adddup2 for read socket failed", ret);
+        return -1;
+    }
+    if (int ret = posix_spawn_file_actions_adddup2(&fa, writeSock, passedSockWrite)) {
+        posix_spawnattr_destroy(&attr);
+        posix_spawn_file_actions_destroy(&fa);
+        throwIOException(env, "posix_spawn_file_actions_adddup2 for write socket failed", ret);
+        return -1;
+    }
+
+    // 5. actually perform vfork/dup2/execve
+    pid_t pid;
+    if (int ret = posix_spawn(&pid, kClatdPath, &fa, &attr, (char* const*)args, nullptr)) {
+        posix_spawnattr_destroy(&attr);
+        posix_spawn_file_actions_destroy(&fa);
+        throwIOException(env, "posix_spawn failed", ret);
+        return -1;
+    }
+
+    posix_spawnattr_destroy(&attr);
+    posix_spawn_file_actions_destroy(&fa);
+
+    // 5. Start BPF if any
     if (!net::clat::initMaps()) {
         net::clat::ClatdTracker tracker = {};
         if (!initTracker(ifaceStr.c_str(), pfx96Str.c_str(), v4Str.c_str(), v6Str.c_str(),
@@ -300,19 +423,23 @@
         }
     }
 
-    return 0; // TODO: return forked clatd pid.
+    return pid;
 }
 
-// TODO: stop clatd and rename to .._stopClatd.
-static void com_android_server_connectivity_ClatCoordinator_maybeStopBpf(JNIEnv* env, jobject clazz,
-                                                                       jstring iface, jstring pfx96,
-                                                                       jstring v4, jstring v6,
-                                                                       jint pid /* unused */) {
+static void com_android_server_connectivity_ClatCoordinator_stopClatd(JNIEnv* env, jobject clazz,
+                                                                      jstring iface, jstring pfx96,
+                                                                      jstring v4, jstring v6,
+                                                                      jint pid) {
     ScopedUtfChars ifaceStr(env, iface);
     ScopedUtfChars pfx96Str(env, pfx96);
     ScopedUtfChars v4Str(env, v4);
     ScopedUtfChars v6Str(env, v6);
 
+    if (pid <= 0) {
+        jniThrowExceptionFmt(env, "java/io/IOException", "Invalid pid");
+        return;
+    }
+
     if (!net::clat::initMaps()) {
         net::clat::ClatdTracker tracker = {};
         if (!initTracker(ifaceStr.c_str(), pfx96Str.c_str(), v4Str.c_str(), v6Str.c_str(),
@@ -320,6 +447,9 @@
             net::clat::maybeStopBpf(tracker);
         }
     }
+
+    kill(pid, SIGTERM);
+    waitpid(pid, nullptr, 0);  // Should we block in JNI?
 }
 
 /*
@@ -344,13 +474,13 @@
          (void*)com_android_server_connectivity_ClatCoordinator_addAnycastSetsockopt},
         {"native_configurePacketSocket", "(Ljava/io/FileDescriptor;Ljava/lang/String;I)V",
          (void*)com_android_server_connectivity_ClatCoordinator_configurePacketSocket},
-        {"native_maybeStartBpf",
+        {"native_startClatd",
          "(Ljava/io/FileDescriptor;Ljava/io/FileDescriptor;Ljava/io/FileDescriptor;Ljava/lang/"
          "String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)I",
-         (void*)com_android_server_connectivity_ClatCoordinator_maybeStartBpf},
-        {"native_maybeStopBpf",
+         (void*)com_android_server_connectivity_ClatCoordinator_startClatd},
+        {"native_stopClatd",
          "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;I)V",
-          (void*)com_android_server_connectivity_ClatCoordinator_maybeStopBpf},
+         (void*)com_android_server_connectivity_ClatCoordinator_stopClatd},
 };
 
 int register_android_server_connectivity_ClatCoordinator(JNIEnv* env) {
diff --git a/service/src/com/android/server/connectivity/ClatCoordinator.java b/service/src/com/android/server/connectivity/ClatCoordinator.java
index 5a5e24a..7616457 100644
--- a/service/src/com/android/server/connectivity/ClatCoordinator.java
+++ b/service/src/com/android/server/connectivity/ClatCoordinator.java
@@ -170,20 +170,20 @@
         }
 
         /**
-         * Maybe start bpf.
+         * Start clatd.
          */
-        public int maybeStartBpf(@NonNull FileDescriptor tunfd, @NonNull FileDescriptor readsock6,
+        public int startClatd(@NonNull FileDescriptor tunfd, @NonNull FileDescriptor readsock6,
                 @NonNull FileDescriptor writesock6, @NonNull String iface, @NonNull String pfx96,
                 @NonNull String v4, @NonNull String v6) throws IOException {
-            return native_maybeStartBpf(tunfd, readsock6, writesock6, iface, pfx96, v4, v6);
+            return native_startClatd(tunfd, readsock6, writesock6, iface, pfx96, v4, v6);
         }
 
         /**
-         * Maybe stop bpf.
+         * Stop clatd.
          */
-        public void maybeStopBpf(String iface, String pfx96, String v4, String v6, int pid)
+        public void stopClatd(String iface, String pfx96, String v4, String v6, int pid)
                 throws IOException {
-            native_maybeStopBpf(iface, pfx96, v4, v6, pid);
+            native_stopClatd(iface, pfx96, v4, v6, pid);
         }
     }
 
@@ -219,6 +219,9 @@
     public String clatStart(final String iface, final int netId,
             @NonNull final IpPrefix nat64Prefix)
             throws IOException {
+        if (mIface != null || mPid != INVALID_PID) {
+            throw new IOException("Clatd has started on " + mIface + " (pid " + mPid + ")");
+        }
         if (nat64Prefix.getPrefixLength() != 96) {
             throw new IOException("Prefix must be 96 bits long: " + nat64Prefix);
         }
@@ -254,6 +257,7 @@
         try {
             mNetd.interfaceSetEnableIPv6(tunIface, false /* enabled */);
         } catch (RemoteException | ServiceSpecificException e) {
+            tunFd.close();
             Log.e(TAG, "Disable IPv6 on " + tunIface + " failed: " + e);
         }
 
@@ -270,6 +274,7 @@
         try {
             mNetd.interfaceSetMtu(tunIface, mtu);
         } catch (RemoteException | ServiceSpecificException e) {
+            tunFd.close();
             throw new IOException("Set MTU " + mtu + " on " + tunIface + " failed: " + e);
         }
         final InterfaceConfigurationParcel ifConfig = new InterfaceConfigurationParcel();
@@ -281,6 +286,7 @@
         try {
             mNetd.interfaceSetCfg(ifConfig);
         } catch (RemoteException | ServiceSpecificException e) {
+            tunFd.close();
             throw new IOException("Setting IPv4 address to " + ifConfig.ipv4Addr + "/"
                     + ifConfig.prefixLength + " failed on " + ifConfig.ifName + ": " + e);
         }
@@ -290,11 +296,12 @@
         final ParcelFileDescriptor readSock6;
         try {
             // Use a JNI call to get native file descriptor instead of Os.socket() because we would
-            // like to use ParcelFileDescriptor to close file descriptor automatically. But ctor
+            // like to use ParcelFileDescriptor to manage file descriptor. But ctor
             // ParcelFileDescriptor(FileDescriptor fd) is a @hide function. Need to use native file
             // descriptor to initialize ParcelFileDescriptor object instead.
             readSock6 = mDeps.adoptFd(mDeps.openPacketSocket());
         } catch (IOException e) {
+            tunFd.close();
             throw new IOException("Open packet socket failed: " + e);
         }
 
@@ -305,11 +312,16 @@
             // reason why we use jniOpenPacketSocket6().
             writeSock6 = mDeps.adoptFd(mDeps.openRawSocket6(fwmark));
         } catch (IOException e) {
+            tunFd.close();
+            readSock6.close();
             throw new IOException("Open raw socket failed: " + e);
         }
 
         final int ifaceIndex = mDeps.getInterfaceIndex(iface);
         if (ifaceIndex == INVALID_IFINDEX) {
+            tunFd.close();
+            readSock6.close();
+            writeSock6.close();
             throw new IOException("Fail to get interface index for interface " + iface);
         }
 
@@ -317,6 +329,9 @@
         try {
             mDeps.addAnycastSetsockopt(writeSock6.getFileDescriptor(), v6, ifaceIndex);
         } catch (IOException e) {
+            tunFd.close();
+            readSock6.close();
+            writeSock6.close();
             throw new IOException("add anycast sockopt failed: " + e);
         }
 
@@ -324,31 +339,41 @@
         try {
             mDeps.configurePacketSocket(readSock6.getFileDescriptor(), v6, ifaceIndex);
         } catch (IOException e) {
+            tunFd.close();
+            readSock6.close();
+            writeSock6.close();
             throw new IOException("configure packet socket failed: " + e);
         }
 
-        // [5] Maybe start bpf.
+        // [5] Start clatd.
         try {
-            mDeps.maybeStartBpf(tunFd.getFileDescriptor(), readSock6.getFileDescriptor(),
+            mPid = mDeps.startClatd(tunFd.getFileDescriptor(), readSock6.getFileDescriptor(),
                     writeSock6.getFileDescriptor(), iface, pfx96, v4, v6);
             mIface = iface;
             mNat64Prefix = pfx96;
             mXlatLocalAddress4 = v4;
             mXlatLocalAddress6 = v6;
         } catch (IOException e) {
-            throw new IOException("Error start bpf on " + iface + ": " + e);
+            throw new IOException("Error start clatd on " + iface + ": " + e);
+        } finally {
+            tunFd.close();
+            readSock6.close();
+            writeSock6.close();
         }
 
-        // TODO: start clatd and returns local xlat464 v6 address.
-        return null;
+        return v6;
     }
 
     /**
      * Stop clatd
      */
     public void clatStop() throws IOException {
-        mDeps.maybeStopBpf(mIface, mNat64Prefix, mXlatLocalAddress4, mXlatLocalAddress6,
-                mPid /* unused */);
+        if (mPid == INVALID_PID) {
+            throw new IOException("Clatd has not started");
+        }
+        Log.i(TAG, "Stopping clatd pid=" + mPid + " on " + mIface);
+
+        mDeps.stopClatd(mIface, mNat64Prefix, mXlatLocalAddress4, mXlatLocalAddress6, mPid);
         // TODO: remove setIptablesDropRule
 
         Log.i(TAG, "clatd on " + mIface + " stopped");
@@ -357,6 +382,7 @@
         mNat64Prefix = null;
         mXlatLocalAddress4 = null;
         mXlatLocalAddress6 = null;
+        mPid = INVALID_PID;
     }
 
     private static native String native_selectIpv4Address(String v4addr, int prefixlen)
@@ -372,9 +398,9 @@
             int ifindex) throws IOException;
     private static native void native_configurePacketSocket(FileDescriptor sock, String v6,
             int ifindex) throws IOException;
-    private static native int native_maybeStartBpf(FileDescriptor tunfd, FileDescriptor readsock6,
+    private static native int native_startClatd(FileDescriptor tunfd, FileDescriptor readsock6,
             FileDescriptor writesock6, String iface, String pfx96, String v4, String v6)
             throws IOException;
-    private static native void native_maybeStopBpf(String iface, String pfx96, String v4,
-            String v6, int pid) throws IOException;
+    private static native void native_stopClatd(String iface, String pfx96, String v4, String v6,
+            int pid) throws IOException;
 }
diff --git a/tests/unit/java/com/android/server/connectivity/ClatCoordinatorTest.java b/tests/unit/java/com/android/server/connectivity/ClatCoordinatorTest.java
new file mode 100644
index 0000000..bdcc21b
--- /dev/null
+++ b/tests/unit/java/com/android/server/connectivity/ClatCoordinatorTest.java
@@ -0,0 +1,385 @@
+/*
+ * Copyright (C) 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.connectivity;
+
+import static android.net.INetd.IF_STATE_UP;
+
+import static com.android.net.module.util.NetworkStackConstants.ETHER_MTU;
+import static com.android.server.connectivity.ClatCoordinator.CLAT_MAX_MTU;
+import static com.android.server.connectivity.ClatCoordinator.INIT_V4ADDR_PREFIX_LEN;
+import static com.android.server.connectivity.ClatCoordinator.INIT_V4ADDR_STRING;
+import static com.android.testutils.MiscAsserts.assertThrows;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+import static org.mockito.Mockito.anyInt;
+import static org.mockito.Mockito.anyString;
+import static org.mockito.Mockito.argThat;
+import static org.mockito.Mockito.clearInvocations;
+import static org.mockito.Mockito.eq;
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.never;
+
+import android.annotation.NonNull;
+import android.net.INetd;
+import android.net.IpPrefix;
+import android.os.Build;
+import android.os.ParcelFileDescriptor;
+
+import androidx.test.filters.SmallTest;
+
+import com.android.testutils.DevSdkIgnoreRule;
+import com.android.testutils.DevSdkIgnoreRunner;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.InOrder;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+import org.mockito.Spy;
+
+import java.io.FileDescriptor;
+import java.io.IOException;
+import java.util.Objects;
+
+@RunWith(DevSdkIgnoreRunner.class)
+@SmallTest
+@DevSdkIgnoreRule.IgnoreUpTo(Build.VERSION_CODES.R)
+public class ClatCoordinatorTest {
+    private static final String BASE_IFACE = "test0";
+    private static final String STACKED_IFACE = "v4-test0";
+    private static final int BASE_IFINDEX = 1000;
+
+    private static final IpPrefix NAT64_IP_PREFIX = new IpPrefix("64:ff9b::/96");
+    private static final String NAT64_PREFIX_STRING = "64:ff9b::";
+    private static final int GOOGLE_DNS_4 = 0x08080808;  // 8.8.8.8
+    private static final int NETID = 42;
+
+    // The test fwmark means: PERMISSION_SYSTEM (0x2), protectedFromVpn: true,
+    // explicitlySelected: true, netid: 42. For bit field structure definition, see union Fwmark in
+    // system/netd/include/Fwmark.h
+    private static final int MARK = 0xb002a;
+
+    private static final String XLAT_LOCAL_IPV4ADDR_STRING = "192.0.0.46";
+    private static final String XLAT_LOCAL_IPV6ADDR_STRING = "2001:db8:0:b11::464";
+    private static final int CLATD_PID = 10483;
+
+    private static final int TUN_FD = 534;
+    private static final int RAW_SOCK_FD = 535;
+    private static final int PACKET_SOCK_FD = 536;
+    private static final ParcelFileDescriptor TUN_PFD = new ParcelFileDescriptor(
+            new FileDescriptor());
+    private static final ParcelFileDescriptor RAW_SOCK_PFD = new ParcelFileDescriptor(
+            new FileDescriptor());
+    private static final ParcelFileDescriptor PACKET_SOCK_PFD = new ParcelFileDescriptor(
+            new FileDescriptor());
+
+    @Mock private INetd mNetd;
+    @Spy private TestDependencies mDeps = new TestDependencies();
+
+    /**
+      * The dependency injection class is used to mock the JNI functions and system functions
+      * for clatd coordinator control plane. Note that any testing used JNI functions need to
+      * be overridden to avoid calling native methods.
+      */
+    protected class TestDependencies extends ClatCoordinator.Dependencies {
+        /**
+          * Get netd.
+          */
+        @Override
+        public INetd getNetd() {
+            return mNetd;
+        }
+
+        /**
+         * @see ParcelFileDescriptor#adoptFd(int).
+         */
+        @Override
+        public ParcelFileDescriptor adoptFd(int fd) {
+            switch (fd) {
+                case TUN_FD:
+                    return TUN_PFD;
+                case RAW_SOCK_FD:
+                    return RAW_SOCK_PFD;
+                case PACKET_SOCK_FD:
+                    return PACKET_SOCK_PFD;
+                default:
+                    fail("unsupported arg: " + fd);
+                    return null;
+            }
+        }
+
+        /**
+         * Get interface index for a given interface.
+         */
+        @Override
+        public int getInterfaceIndex(String ifName) {
+            if (BASE_IFACE.equals(ifName)) {
+                return BASE_IFINDEX;
+            }
+            fail("unsupported arg: " + ifName);
+            return -1;
+        }
+
+        /**
+         * Create tun interface for a given interface name.
+         */
+        @Override
+        public int createTunInterface(@NonNull String tuniface) throws IOException {
+            if (STACKED_IFACE.equals(tuniface)) {
+                return TUN_FD;
+            }
+            fail("unsupported arg: " + tuniface);
+            return -1;
+        }
+
+        /**
+         * Pick an IPv4 address for clat.
+         */
+        @Override
+        public String selectIpv4Address(@NonNull String v4addr, int prefixlen)
+                throws IOException {
+            if (INIT_V4ADDR_STRING.equals(v4addr) && INIT_V4ADDR_PREFIX_LEN == prefixlen) {
+                return XLAT_LOCAL_IPV4ADDR_STRING;
+            }
+            fail("unsupported args: " + v4addr + ", " + prefixlen);
+            return null;
+        }
+
+        /**
+         * Generate a checksum-neutral IID.
+         */
+        @Override
+        public String generateIpv6Address(@NonNull String iface, @NonNull String v4,
+                @NonNull String prefix64) throws IOException {
+            if (BASE_IFACE.equals(iface) && XLAT_LOCAL_IPV4ADDR_STRING.equals(v4)
+                    && NAT64_PREFIX_STRING.equals(prefix64)) {
+                return XLAT_LOCAL_IPV6ADDR_STRING;
+            }
+            fail("unsupported args: " + iface + ", " + v4 + ", " + prefix64);
+            return null;
+        }
+
+        /**
+         * Detect MTU.
+         */
+        @Override
+        public int detectMtu(@NonNull String platSubnet, int platSuffix, int mark)
+                throws IOException {
+            if (NAT64_PREFIX_STRING.equals(platSubnet) && GOOGLE_DNS_4 == platSuffix
+                    && MARK == mark) {
+                return ETHER_MTU;
+            }
+            fail("unsupported args: " + platSubnet + ", " + platSuffix + ", " + mark);
+            return -1;
+        }
+
+        /**
+         * Open IPv6 raw socket and set SO_MARK.
+         */
+        @Override
+        public int openRawSocket6(int mark) throws IOException {
+            if (mark == MARK) {
+                return RAW_SOCK_FD;
+            }
+            fail("unsupported arg: " + mark);
+            return -1;
+        }
+
+        /**
+         * Open packet socket.
+         */
+        @Override
+        public int openPacketSocket() throws IOException {
+            // assume that open socket always successfully because there is no argument to check.
+            return PACKET_SOCK_FD;
+        }
+
+        /**
+         * Add anycast setsockopt.
+         */
+        @Override
+        public void addAnycastSetsockopt(@NonNull FileDescriptor sock, String v6, int ifindex)
+                throws IOException {
+            if (Objects.equals(RAW_SOCK_PFD.getFileDescriptor(), sock)
+                    && XLAT_LOCAL_IPV6ADDR_STRING.equals(v6)
+                    && BASE_IFINDEX == ifindex) return;
+            fail("unsupported args: " + sock + ", " + v6 + ", " + ifindex);
+        }
+
+        /**
+         * Configure packet socket.
+         */
+        @Override
+        public void configurePacketSocket(@NonNull FileDescriptor sock, String v6, int ifindex)
+                throws IOException {
+            if (Objects.equals(PACKET_SOCK_PFD.getFileDescriptor(), sock)
+                    && XLAT_LOCAL_IPV6ADDR_STRING.equals(v6)
+                    && BASE_IFINDEX == ifindex) return;
+            fail("unsupported args: " + sock + ", " + v6 + ", " + ifindex);
+        }
+
+        /**
+         * Start clatd.
+         */
+        @Override
+        public int startClatd(@NonNull FileDescriptor tunfd, @NonNull FileDescriptor readsock6,
+                @NonNull FileDescriptor writesock6, @NonNull String iface, @NonNull String pfx96,
+                @NonNull String v4, @NonNull String v6) throws IOException {
+            if (Objects.equals(TUN_PFD.getFileDescriptor(), tunfd)
+                    && Objects.equals(PACKET_SOCK_PFD.getFileDescriptor(), readsock6)
+                    && Objects.equals(RAW_SOCK_PFD.getFileDescriptor(), writesock6)
+                    && BASE_IFACE.equals(iface)
+                    && NAT64_PREFIX_STRING.equals(pfx96)
+                    && XLAT_LOCAL_IPV4ADDR_STRING.equals(v4)
+                    && XLAT_LOCAL_IPV6ADDR_STRING.equals(v6)) {
+                return CLATD_PID;
+            }
+            fail("unsupported args: " + tunfd + ", " + readsock6 + ", " + writesock6 + ", "
+                    + ", " + iface + ", " + v4 + ", " + v6);
+            return -1;
+        }
+
+        /**
+         * Stop clatd.
+         */
+        public void stopClatd(@NonNull String iface, @NonNull String pfx96, @NonNull String v4,
+                @NonNull String v6, int pid) throws IOException {
+            if (pid == -1) {
+                fail("unsupported arg: " + pid);
+            }
+        }
+    };
+
+    @NonNull
+    private ClatCoordinator makeClatCoordinator() throws Exception {
+        final ClatCoordinator coordinator = new ClatCoordinator(mDeps);
+        return coordinator;
+    }
+
+    @Before
+    public void setUp() throws Exception {
+        MockitoAnnotations.initMocks(this);
+    }
+
+    private boolean assertContainsFlag(String[] flags, String match) {
+        for (String flag : flags) {
+            if (flag.equals(match)) return true;
+        }
+        fail("Missing flag: " + match);
+        return false;
+    }
+
+    @Test
+    public void testStartStopClatd() throws Exception {
+        final ClatCoordinator coordinator = makeClatCoordinator();
+        final InOrder inOrder = inOrder(mNetd, mDeps);
+        clearInvocations(mNetd, mDeps);
+
+        // [1] Start clatd.
+        final String addr6For464xlat = coordinator.clatStart(BASE_IFACE, NETID, NAT64_IP_PREFIX);
+        assertEquals(XLAT_LOCAL_IPV6ADDR_STRING, addr6For464xlat);
+
+        // Pick an IPv4 address.
+        inOrder.verify(mDeps).selectIpv4Address(eq(INIT_V4ADDR_STRING),
+                eq(INIT_V4ADDR_PREFIX_LEN));
+
+        // Generate a checksum-neutral IID.
+        inOrder.verify(mDeps).generateIpv6Address(eq(BASE_IFACE),
+                eq(XLAT_LOCAL_IPV4ADDR_STRING), eq(NAT64_PREFIX_STRING));
+
+        // Open, configure and bring up the tun interface.
+        inOrder.verify(mDeps).createTunInterface(eq(STACKED_IFACE));
+        inOrder.verify(mDeps).adoptFd(eq(TUN_FD));
+        inOrder.verify(mNetd).interfaceSetEnableIPv6(eq(STACKED_IFACE), eq(false /* enable */));
+        inOrder.verify(mDeps).detectMtu(eq(NAT64_PREFIX_STRING), eq(GOOGLE_DNS_4), eq(MARK));
+        inOrder.verify(mNetd).interfaceSetMtu(eq(STACKED_IFACE),
+                eq(1472 /* ETHER_MTU(1500) - MTU_DELTA(28) */));
+        inOrder.verify(mNetd).interfaceSetCfg(argThat(cfg ->
+                STACKED_IFACE.equals(cfg.ifName)
+                && XLAT_LOCAL_IPV4ADDR_STRING.equals(cfg.ipv4Addr)
+                && (32 == cfg.prefixLength)
+                && "".equals(cfg.hwAddr)
+                && assertContainsFlag(cfg.flags, IF_STATE_UP)));
+
+        // Open and configure 464xlat read/write sockets.
+        inOrder.verify(mDeps).openPacketSocket();
+        inOrder.verify(mDeps).adoptFd(eq(PACKET_SOCK_FD));
+        inOrder.verify(mDeps).openRawSocket6(eq(MARK));
+        inOrder.verify(mDeps).adoptFd(eq(RAW_SOCK_FD));
+        inOrder.verify(mDeps).getInterfaceIndex(eq(BASE_IFACE));
+        inOrder.verify(mDeps).addAnycastSetsockopt(
+                argThat(fd -> Objects.equals(RAW_SOCK_PFD.getFileDescriptor(), fd)),
+                eq(XLAT_LOCAL_IPV6ADDR_STRING), eq(BASE_IFINDEX));
+        inOrder.verify(mDeps).configurePacketSocket(
+                argThat(fd -> Objects.equals(PACKET_SOCK_PFD.getFileDescriptor(), fd)),
+                eq(XLAT_LOCAL_IPV6ADDR_STRING), eq(BASE_IFINDEX));
+
+        // Start clatd.
+        inOrder.verify(mDeps).startClatd(
+                argThat(fd -> Objects.equals(TUN_PFD.getFileDescriptor(), fd)),
+                argThat(fd -> Objects.equals(PACKET_SOCK_PFD.getFileDescriptor(), fd)),
+                argThat(fd -> Objects.equals(RAW_SOCK_PFD.getFileDescriptor(), fd)),
+                eq(BASE_IFACE), eq(NAT64_PREFIX_STRING),
+                eq(XLAT_LOCAL_IPV4ADDR_STRING), eq(XLAT_LOCAL_IPV6ADDR_STRING));
+        inOrder.verifyNoMoreInteractions();
+
+        // [2] Start clatd again failed.
+        assertThrows("java.io.IOException: Clatd has started on test0 (pid 10483)",
+                IOException.class,
+                () -> coordinator.clatStart(BASE_IFACE, NETID, NAT64_IP_PREFIX));
+
+        // [3] Expect clatd to stop successfully.
+        coordinator.clatStop();
+        inOrder.verify(mDeps).stopClatd(eq(BASE_IFACE), eq(NAT64_PREFIX_STRING),
+                eq(XLAT_LOCAL_IPV4ADDR_STRING), eq(XLAT_LOCAL_IPV6ADDR_STRING), eq(CLATD_PID));
+        inOrder.verifyNoMoreInteractions();
+
+        // [4] Expect an IO exception while stopping a clatd that doesn't exist.
+        assertThrows("java.io.IOException: Clatd has not started", IOException.class,
+                () -> coordinator.clatStop());
+        inOrder.verify(mDeps, never()).stopClatd(anyString(), anyString(), anyString(),
+                anyString(), anyInt());
+        inOrder.verifyNoMoreInteractions();
+    }
+
+    @Test
+    public void testGetFwmark() throws Exception {
+        assertEquals(0xb0064, ClatCoordinator.getFwmark(100));
+        assertEquals(0xb03e8, ClatCoordinator.getFwmark(1000));
+        assertEquals(0xb2710, ClatCoordinator.getFwmark(10000));
+        assertEquals(0xbffff, ClatCoordinator.getFwmark(65535));
+    }
+
+    @Test
+    public void testAdjustMtu() throws Exception {
+        // Expected mtu is that IPV6_MIN_MTU(1280) minus MTU_DELTA(28).
+        assertEquals(1252, ClatCoordinator.adjustMtu(-1 /* detect mtu failed */));
+        assertEquals(1252, ClatCoordinator.adjustMtu(500));
+        assertEquals(1252, ClatCoordinator.adjustMtu(1000));
+        assertEquals(1252, ClatCoordinator.adjustMtu(1280));
+
+        // Expected mtu is that the detected mtu minus MTU_DELTA(28).
+        assertEquals(1372, ClatCoordinator.adjustMtu(1400));
+        assertEquals(1472, ClatCoordinator.adjustMtu(ETHER_MTU));
+        assertEquals(65508, ClatCoordinator.adjustMtu(CLAT_MAX_MTU));
+
+        // Expected mtu is that CLAT_MAX_MTU(65536) minus MTU_DELTA(28).
+        assertEquals(65508, ClatCoordinator.adjustMtu(CLAT_MAX_MTU + 1 /* over maximum mtu */));
+    }
+}