diff --git a/staticlibs/device/com/android/net/module/util/async/OsAccess.java b/staticlibs/device/com/android/net/module/util/async/OsAccess.java
new file mode 100644
index 0000000..df0ded2
--- /dev/null
+++ b/staticlibs/device/com/android/net/module/util/async/OsAccess.java
@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2023 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.net.module.util.async;
+
+import android.os.ParcelFileDescriptor;
+import android.system.StructPollfd;
+
+import java.io.FileDescriptor;
+import java.io.IOException;
+
+/**
+ * Provides access to all relevant OS functions..
+ *
+ * @hide
+ */
+public abstract class OsAccess {
+    /** Closes the given file, suppressing IO exceptions. */
+    public abstract void close(ParcelFileDescriptor fd);
+
+    /** Returns file name for debugging purposes. */
+    public abstract String getFileDebugName(ParcelFileDescriptor fd);
+
+    /** Returns inner FileDescriptor instance. */
+    public abstract FileDescriptor getInnerFileDescriptor(ParcelFileDescriptor fd);
+
+    /**
+     * Reads available data from the given non-blocking file descriptor.
+     *
+     * Returns zero if there's no data to read at this moment.
+     * Returns -1 if the file has reached its end or the input stream has been closed.
+     * Otherwise returns the number of bytes read.
+     */
+    public abstract int read(FileDescriptor fd, byte[] buffer, int pos, int len)
+            throws IOException;
+
+    /**
+     * Writes data into the given non-blocking file descriptor.
+     *
+     * Returns zero if there's no buffer space to write to at this moment.
+     * Otherwise returns the number of bytes written.
+     */
+    public abstract int write(FileDescriptor fd, byte[] buffer, int pos, int len)
+            throws IOException;
+
+    public abstract long monotonicTimeMillis();
+    public abstract void setNonBlocking(FileDescriptor fd) throws IOException;
+    public abstract ParcelFileDescriptor[] pipe() throws IOException;
+
+    public abstract int poll(StructPollfd[] fds, int timeoutMs) throws IOException;
+    public abstract short getPollInMask();
+    public abstract short getPollOutMask();
+}
diff --git a/staticlibs/testutils/Android.bp b/staticlibs/testutils/Android.bp
index c9b3d07..bcf89b3 100644
--- a/staticlibs/testutils/Android.bp
+++ b/staticlibs/testutils/Android.bp
@@ -36,6 +36,7 @@
         "libnanohttpd",
         "net-tests-utils-host-device-common",
         "net-utils-device-common",
+        "net-utils-device-common-async",
         "net-utils-device-common-netlink",
         "modules-utils-build_system",
     ],
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/async/FakeOsAccess.java b/staticlibs/testutils/devicetests/com/android/testutils/async/FakeOsAccess.java
new file mode 100644
index 0000000..1b8e26b
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/async/FakeOsAccess.java
@@ -0,0 +1,568 @@
+/*
+ * Copyright (C) 2023 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.testutils;
+
+import android.os.ParcelFileDescriptor;
+import android.system.StructPollfd;
+import android.util.Log;
+
+import com.android.net.module.util.async.CircularByteBuffer;
+import com.android.net.module.util.async.OsAccess;
+
+import java.io.FileDescriptor;
+import java.io.InterruptedIOException;
+import java.io.IOException;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Method;
+import java.lang.reflect.Field;
+import java.util.HashMap;
+import java.util.concurrent.TimeUnit;
+
+public class FakeOsAccess extends OsAccess {
+    public static final boolean ENABLE_FINE_DEBUG = true;
+
+    public static final int DEFAULT_FILE_DATA_QUEUE_SIZE = 8 * 1024;
+
+    private enum FileType { PAIR, PIPE }
+
+    // Common poll() constants:
+    private static final short POLLIN  = 0x0001;
+    private static final short POLLOUT = 0x0004;
+    private static final short POLLERR = 0x0008;
+    private static final short POLLHUP = 0x0010;
+
+    private static final Constructor<FileDescriptor> FD_CONSTRUCTOR;
+    private static final Field FD_FIELD_DESCRIPTOR;
+    private static final Field PFD_FIELD_DESCRIPTOR;
+    private static final Field PFD_FIELD_GUARD;
+    private static final Method CLOSE_GUARD_METHOD_CLOSE;
+
+    private final int mReadQueueSize = DEFAULT_FILE_DATA_QUEUE_SIZE;
+    private final int mWriteQueueSize = DEFAULT_FILE_DATA_QUEUE_SIZE;
+    private final HashMap<Integer, File> mFiles = new HashMap<>();
+    private final byte[] mTmpBuffer = new byte[1024];
+    private final long mStartTime;
+    private final String mLogTag;
+    private int mFileNumberGen = 3;
+    private boolean mHasRateLimitedData;
+
+    public FakeOsAccess(String logTag) {
+        mLogTag = logTag;
+        mStartTime = monotonicTimeMillis();
+    }
+
+    @Override
+    public long monotonicTimeMillis() {
+        return System.nanoTime() / 1000000;
+    }
+
+    @Override
+    public FileDescriptor getInnerFileDescriptor(ParcelFileDescriptor fd) {
+        try {
+            return (FileDescriptor) PFD_FIELD_DESCRIPTOR.get(fd);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    @Override
+    public void close(ParcelFileDescriptor fd) {
+        if (fd != null) {
+            close(getInnerFileDescriptor(fd));
+
+            try {
+                // Reduce CloseGuard warnings.
+                Object guard = PFD_FIELD_GUARD.get(fd);
+                CLOSE_GUARD_METHOD_CLOSE.invoke(guard);
+            } catch (Exception e) {
+                throw new RuntimeException(e);
+            }
+        }
+    }
+
+    public synchronized void close(FileDescriptor fd) {
+        if (fd != null) {
+            File file = getFileOrNull(fd);
+            if (file != null) {
+                file.decreaseRefCount();
+                mFiles.remove(getFileDescriptorNumber(fd));
+                setFileDescriptorNumber(fd, -1);
+                notifyAll();
+            }
+        }
+    }
+
+    private File getFile(String func, FileDescriptor fd) throws IOException {
+        File file = getFileOrNull(fd);
+        if (file == null) {
+            throw newIOException(func, "Unknown file descriptor: " + getFileDebugName(fd));
+        }
+        return file;
+    }
+
+    private File getFileOrNull(FileDescriptor fd) {
+        return mFiles.get(getFileDescriptorNumber(fd));
+    }
+
+    @Override
+    public String getFileDebugName(ParcelFileDescriptor fd) {
+        return (fd != null ? getFileDebugName(getInnerFileDescriptor(fd)) : "null");
+    }
+
+    public String getFileDebugName(FileDescriptor fd) {
+        if (fd == null) {
+            return "null";
+        }
+
+        final int fdNumber = getFileDescriptorNumber(fd);
+        File file = mFiles.get(fdNumber);
+
+        StringBuilder sb = new StringBuilder();
+        if (file != null) {
+            if (file.name != null) {
+                sb.append(file.name);
+                sb.append("/");
+            }
+            sb.append(file.type);
+            sb.append("/");
+        } else {
+            sb.append("BADFD/");
+        }
+        sb.append(fdNumber);
+        return sb.toString();
+    }
+
+    public synchronized void setFileName(FileDescriptor fd, String name) {
+        File file = getFileOrNull(fd);
+        if (file != null) {
+            file.name = name;
+        }
+    }
+
+    @Override
+    public synchronized void setNonBlocking(FileDescriptor fd) throws IOException {
+        File file = getFile("fcntl", fd);
+        file.isBlocking = false;
+    }
+
+    @Override
+    public synchronized int read(FileDescriptor fd, byte[] buffer, int pos, int len)
+            throws IOException {
+        checkBoundaries("read", buffer, pos, len);
+
+        File file = getFile("read", fd);
+        if (file.readQueue == null) {
+            throw newIOException("read", "File not readable");
+        }
+        file.checkNonBlocking("read");
+
+        if (len == 0) {
+            return 0;
+        }
+
+        final int availSize = file.readQueue.size();
+        if (availSize == 0) {
+            if (file.isEndOfStream) {
+                // Java convention uses -1 to indicate end of stream.
+                return -1;
+            }
+            return 0;  // EAGAIN
+        }
+
+        final int readCount = Math.min(len, availSize);
+        file.readQueue.readBytes(buffer, pos, readCount);
+        maybeTransferData(file);
+        return readCount;
+    }
+
+    @Override
+    public synchronized int write(FileDescriptor fd, byte[] buffer, int pos, int len)
+            throws IOException {
+        checkBoundaries("write", buffer, pos, len);
+
+        File file = getFile("write", fd);
+        if (file.writeQueue == null) {
+            throw newIOException("read", "File not writable");
+        }
+        if (file.type == FileType.PIPE && file.sink.openCount == 0) {
+            throw newIOException("write", "The other end of pipe is closed");
+        }
+        file.checkNonBlocking("write");
+
+        if (len == 0) {
+            return 0;
+        }
+
+        final int originalFreeSize = file.writeQueue.freeSize();
+        if (originalFreeSize == 0) {
+            return 0;  // EAGAIN
+        }
+
+        final int writeCount = Math.min(len, originalFreeSize);
+        file.writeQueue.writeBytes(buffer, pos, writeCount);
+        maybeTransferData(file);
+
+        if (file.writeQueue.freeSize() < originalFreeSize) {
+            final int additionalQueuedCount = originalFreeSize - file.writeQueue.freeSize();
+            Log.i(mLogTag, logStr("Delaying transfer of " + additionalQueuedCount
+                + " bytes, queued=" + file.writeQueue.size() + ", type=" + file.type
+                + ", src_red=" + file.outboundLimiter + ", dst_red=" + file.sink.inboundLimiter));
+        }
+
+        return writeCount;
+    }
+
+    private void maybeTransferData(File file) {
+        boolean hasChanges = copyFileBuffers(file, file.sink);
+        hasChanges = copyFileBuffers(file.source, file) || hasChanges;
+
+        if (hasChanges) {
+            // TODO(b/245971639): Avoid notifying if no-one is polling.
+            notifyAll();
+        }
+    }
+
+    private boolean copyFileBuffers(File src, File dst) {
+        if (src.writeQueue == null || dst.readQueue == null) {
+            return false;
+        }
+
+        final int originalCopyCount = Math.min(mTmpBuffer.length,
+            Math.min(src.writeQueue.size(), dst.readQueue.freeSize()));
+
+        final int allowedCopyCount = RateLimiter.limit(
+            src.outboundLimiter, dst.inboundLimiter, originalCopyCount);
+
+        if (allowedCopyCount < originalCopyCount) {
+            if (ENABLE_FINE_DEBUG) {
+                Log.i(mLogTag, logStr("Delaying transfer of "
+                    + (originalCopyCount - allowedCopyCount) + " bytes, original="
+                    + originalCopyCount + ", allowed=" + allowedCopyCount
+                    + ", type=" + src.type));
+            }
+            if (originalCopyCount > 0) {
+                mHasRateLimitedData = true;
+            }
+            if (allowedCopyCount == 0) {
+                return false;
+            }
+        }
+
+        boolean hasChanges = false;
+        if (allowedCopyCount > 0) {
+            if (dst.readQueue.size() == 0 || src.writeQueue.freeSize() == 0) {
+                hasChanges = true;  // Read queue had no data, or write queue was full.
+            }
+            src.writeQueue.readBytes(mTmpBuffer, 0, allowedCopyCount);
+            dst.readQueue.writeBytes(mTmpBuffer, 0, allowedCopyCount);
+        }
+
+        if (!dst.isEndOfStream && src.openCount == 0
+                && src.writeQueue.size() == 0 && dst.readQueue.size() == 0) {
+            dst.isEndOfStream = true;
+            hasChanges = true;
+        }
+
+        return hasChanges;
+    }
+
+    public void clearInboundRateLimit(FileDescriptor fd) {
+        setInboundRateLimit(fd, Integer.MAX_VALUE);
+    }
+
+    public void clearOutboundRateLimit(FileDescriptor fd) {
+        setOutboundRateLimit(fd, Integer.MAX_VALUE);
+    }
+
+    public synchronized void setInboundRateLimit(FileDescriptor fd, int bytesPerSecond) {
+        File file = getFileOrNull(fd);
+        if (file != null) {
+            file.inboundLimiter.setBytesPerSecond(bytesPerSecond);
+            maybeTransferData(file);
+        }
+    }
+
+    public synchronized void setOutboundRateLimit(FileDescriptor fd, int bytesPerSecond) {
+        File file = getFileOrNull(fd);
+        if (file != null) {
+            file.outboundLimiter.setBytesPerSecond(bytesPerSecond);
+            maybeTransferData(file);
+        }
+    }
+
+    public synchronized ParcelFileDescriptor[] socketpair() throws IOException {
+        int fdNumber1 = getNextFd("socketpair");
+        int fdNumber2 = getNextFd("socketpair");
+
+        File file1 = new File(FileType.PAIR, mReadQueueSize, mWriteQueueSize);
+        File file2 = new File(FileType.PAIR, mReadQueueSize, mWriteQueueSize);
+
+        return registerFilePair(fdNumber1, file1, fdNumber2, file2);
+    }
+
+    @Override
+    public synchronized ParcelFileDescriptor[] pipe() throws IOException {
+        int fdNumber1 = getNextFd("pipe");
+        int fdNumber2 = getNextFd("pipe");
+
+        File file1 = new File(FileType.PIPE, mReadQueueSize, 0);
+        File file2 = new File(FileType.PIPE, 0, mWriteQueueSize);
+
+        return registerFilePair(fdNumber1, file1, fdNumber2, file2);
+    }
+
+    private ParcelFileDescriptor[] registerFilePair(
+            int fdNumber1, File file1, int fdNumber2, File file2) {
+        file1.sink = file2;
+        file1.source = file2;
+        file2.sink = file1;
+        file2.source = file1;
+
+        mFiles.put(fdNumber1, file1);
+        mFiles.put(fdNumber2, file2);
+        return new ParcelFileDescriptor[] {
+            newParcelFileDescriptor(fdNumber1), newParcelFileDescriptor(fdNumber2)};
+    }
+
+    @Override
+    public short getPollInMask() {
+        return POLLIN;
+    }
+
+    @Override
+    public short getPollOutMask() {
+        return POLLOUT;
+    }
+
+    @Override
+    public synchronized int poll(StructPollfd[] fds, int timeoutMs) throws IOException {
+        if (timeoutMs < 0) {
+            timeoutMs = (int) TimeUnit.HOURS.toMillis(1);  // Make "infinite" equal to 1 hour.
+        }
+
+        if (fds == null || fds.length > 1000) {
+            throw newIOException("poll", "Invalid fds param");
+        }
+        for (StructPollfd pollFd : fds) {
+            getFile("poll", pollFd.fd);
+        }
+
+        int waitCallCount = 0;
+        final long deadline = monotonicTimeMillis() + timeoutMs;
+        while (true) {
+            if (mHasRateLimitedData) {
+                mHasRateLimitedData = false;
+                for (File file : mFiles.values()) {
+                    if (file.inboundLimiter.getLastRequestReduction() != 0) {
+                        copyFileBuffers(file.source, file);
+                    }
+                    if (file.outboundLimiter.getLastRequestReduction() != 0) {
+                        copyFileBuffers(file, file.sink);
+                    }
+                }
+            }
+
+            final int readyCount = calculateReadyCount(fds);
+            if (readyCount > 0) {
+                if (ENABLE_FINE_DEBUG) {
+                    Log.v(mLogTag, logStr("Poll returns " + readyCount
+                            + " after " + waitCallCount + " wait calls"));
+                }
+                return readyCount;
+            }
+
+            long remainingTimeoutMs = deadline - monotonicTimeMillis();
+            if (remainingTimeoutMs <= 0) {
+                if (ENABLE_FINE_DEBUG) {
+                    Log.v(mLogTag, logStr("Poll timeout " + timeoutMs
+                            + "ms after " + waitCallCount + " wait calls"));
+                }
+                return 0;
+            }
+
+            if (mHasRateLimitedData) {
+                remainingTimeoutMs = Math.min(RateLimiter.BUCKET_DURATION_MS, remainingTimeoutMs);
+            }
+
+            try {
+                wait(remainingTimeoutMs);
+            } catch (InterruptedException e) {
+                // Ignore and retry
+            }
+            waitCallCount++;
+        }
+    }
+
+    private int calculateReadyCount(StructPollfd[] fds) {
+        int fdCount = 0;
+        for (StructPollfd pollFd : fds) {
+            pollFd.revents = 0;
+
+            File file = getFileOrNull(pollFd.fd);
+            if (file == null) {
+                Log.w(mLogTag, logStr("Ignoring FD concurrently closed by a buggy app: "
+                        + getFileDebugName(pollFd.fd)));
+                continue;
+            }
+
+            if (ENABLE_FINE_DEBUG) {
+                Log.v(mLogTag, logStr("calculateReadyCount fd=" + getFileDebugName(pollFd.fd)
+                        + ", events=" + pollFd.events + ", eof=" + file.isEndOfStream
+                        + ", r=" + (file.readQueue != null ? file.readQueue.size() : -1)
+                        + ", w=" + (file.writeQueue != null ? file.writeQueue.freeSize() : -1)));
+            }
+
+            if ((pollFd.events & POLLIN) != 0) {
+                if (file.readQueue != null && file.readQueue.size() != 0) {
+                    pollFd.revents |= POLLIN;
+                }
+                if (file.isEndOfStream) {
+                    pollFd.revents |= POLLHUP;
+                }
+            }
+
+            if ((pollFd.events & POLLOUT) != 0) {
+                if (file.type == FileType.PIPE && file.sink.openCount == 0) {
+                    pollFd.revents |= POLLERR;
+                }
+                if (file.writeQueue != null && file.writeQueue.freeSize() != 0) {
+                    pollFd.revents |= POLLOUT;
+                }
+            }
+
+            if (pollFd.revents != 0) {
+                fdCount++;
+            }
+        }
+        return fdCount;
+    }
+
+    private int getNextFd(String func) throws IOException {
+        if (mFileNumberGen > 100000) {
+            throw newIOException(func, "Too many files open");
+        }
+
+        return mFileNumberGen++;
+    }
+
+    private static IOException newIOException(String func, String message) {
+        return new IOException(message + ", func=" + func);
+    }
+
+    public static void checkBoundaries(String func, byte[] buffer, int pos, int len)
+            throws IOException {
+        if (((buffer.length | pos | len) < 0 || pos > buffer.length - len)) {
+            throw newIOException(func, "Invalid array bounds");
+        }
+    }
+
+    private ParcelFileDescriptor newParcelFileDescriptor(int fdNumber) {
+        try {
+            return new ParcelFileDescriptor(newFileDescriptor(fdNumber));
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    private FileDescriptor newFileDescriptor(int fdNumber) {
+        try {
+            return FD_CONSTRUCTOR.newInstance(Integer.valueOf(fdNumber));
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    public int getFileDescriptorNumber(FileDescriptor fd) {
+        try {
+            return (Integer) FD_FIELD_DESCRIPTOR.get(fd);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    private void setFileDescriptorNumber(FileDescriptor fd, int fdNumber) {
+        try {
+            FD_FIELD_DESCRIPTOR.set(fd, Integer.valueOf(fdNumber));
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    private String logStr(String message) {
+        return "[FakeOs " + (monotonicTimeMillis() - mStartTime) + "] " + message;
+    }
+
+    private class File {
+        final FileType type;
+        final CircularByteBuffer readQueue;
+        final CircularByteBuffer writeQueue;
+        final RateLimiter inboundLimiter = new RateLimiter(FakeOsAccess.this, Integer.MAX_VALUE);
+        final RateLimiter outboundLimiter = new RateLimiter(FakeOsAccess.this, Integer.MAX_VALUE);
+        String name;
+        int openCount = 1;
+        boolean isBlocking = true;
+        File sink;
+        File source;
+        boolean isEndOfStream;
+
+        File(FileType type, int readQueueSize, int writeQueueSize) {
+            this.type = type;
+            readQueue = (readQueueSize > 0 ? new CircularByteBuffer(readQueueSize) : null);
+            writeQueue = (writeQueueSize > 0 ? new CircularByteBuffer(writeQueueSize) : null);
+        }
+
+        void decreaseRefCount() {
+            if (openCount <= 0) {
+                throw new IllegalStateException();
+            }
+            openCount--;
+        }
+
+        void checkNonBlocking(String func) throws IOException {
+            if (isBlocking) {
+                throw newIOException(func, "File in blocking mode");
+            }
+        }
+    }
+
+    static {
+        try {
+            FD_CONSTRUCTOR = FileDescriptor.class.getDeclaredConstructor(int.class);
+            FD_CONSTRUCTOR.setAccessible(true);
+
+            Field descriptorIntField;
+            try {
+                descriptorIntField = FileDescriptor.class.getDeclaredField("descriptor");
+            } catch (NoSuchFieldException e) {
+                descriptorIntField = FileDescriptor.class.getDeclaredField("fd");
+            }
+            FD_FIELD_DESCRIPTOR = descriptorIntField;
+            FD_FIELD_DESCRIPTOR.setAccessible(true);
+
+            PFD_FIELD_DESCRIPTOR = ParcelFileDescriptor.class.getDeclaredField("mFd");
+            PFD_FIELD_DESCRIPTOR.setAccessible(true);
+
+            PFD_FIELD_GUARD = ParcelFileDescriptor.class.getDeclaredField("mGuard");
+            PFD_FIELD_GUARD.setAccessible(true);
+
+            CLOSE_GUARD_METHOD_CLOSE = Class.forName("dalvik.system.CloseGuard")
+                .getDeclaredMethod("close");
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+}
diff --git a/staticlibs/testutils/devicetests/com/android/testutils/async/RateLimiter.java b/staticlibs/testutils/devicetests/com/android/testutils/async/RateLimiter.java
new file mode 100644
index 0000000..137873d
--- /dev/null
+++ b/staticlibs/testutils/devicetests/com/android/testutils/async/RateLimiter.java
@@ -0,0 +1,131 @@
+/*
+ * Copyright (C) 2023 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.testutils;
+
+import com.android.net.module.util.async.OsAccess;
+
+import java.util.Arrays;
+
+/**
+ * Limits the number of bytes processed to the given maximum of bytes per second.
+ *
+ * The limiter tracks the total for the past second, along with sums for each 10ms
+ * in the past second, allowing the total to be adjusted as the time passes.
+ */
+public final class RateLimiter {
+    private static final int PERIOD_DURATION_MS = 1000;
+    private static final int BUCKET_COUNT = 100;
+
+    public static final int BUCKET_DURATION_MS = PERIOD_DURATION_MS / BUCKET_COUNT;
+
+    private final OsAccess mOsAccess;
+    private final int[] mStatBuckets = new int[BUCKET_COUNT];
+    private int mMaxPerPeriodBytes;
+    private int mMaxPerBucketBytes;
+    private int mRecordedPeriodBytes;
+    private long mLastLimitTimestamp;
+    private int mLastRequestReduction;
+
+    public RateLimiter(OsAccess osAccess, int bytesPerSecond) {
+        mOsAccess = osAccess;
+        setBytesPerSecond(bytesPerSecond);
+        clear();
+    }
+
+    public int getBytesPerSecond() {
+        return mMaxPerPeriodBytes;
+    }
+
+    public void setBytesPerSecond(int bytesPerSecond) {
+        mMaxPerPeriodBytes = bytesPerSecond;
+        mMaxPerBucketBytes = Math.max(1, (mMaxPerPeriodBytes / BUCKET_COUNT) * 2);
+    }
+
+    public void clear() {
+        mLastLimitTimestamp = mOsAccess.monotonicTimeMillis();
+        mRecordedPeriodBytes = 0;
+        Arrays.fill(mStatBuckets, 0);
+    }
+
+    public static int limit(RateLimiter limiter1, RateLimiter limiter2, int requestedBytes) {
+        final long now = limiter1.mOsAccess.monotonicTimeMillis();
+        final int allowedCount = Math.min(limiter1.calculateLimit(now, requestedBytes),
+            limiter2.calculateLimit(now, requestedBytes));
+        limiter1.recordBytes(now, requestedBytes, allowedCount);
+        limiter2.recordBytes(now, requestedBytes, allowedCount);
+        return allowedCount;
+    }
+
+    public int limit(int requestedBytes) {
+        final long now = mOsAccess.monotonicTimeMillis();
+        final int allowedCount = calculateLimit(now, requestedBytes);
+        recordBytes(now, requestedBytes, allowedCount);
+        return allowedCount;
+    }
+
+    public int getLastRequestReduction() {
+        return mLastRequestReduction;
+    }
+
+    public boolean acceptAllOrNone(int requestedBytes) {
+        final long now = mOsAccess.monotonicTimeMillis();
+        final int allowedCount = calculateLimit(now, requestedBytes);
+        if (allowedCount < requestedBytes) {
+            return false;
+        }
+        recordBytes(now, requestedBytes, allowedCount);
+        return true;
+    }
+
+    private int calculateLimit(long now, int requestedBytes) {
+        // First remove all stale bucket data and adjust the total.
+        final long currentBucketAbsIdx = now / BUCKET_DURATION_MS;
+        final long staleCutoffIdx = currentBucketAbsIdx - BUCKET_COUNT;
+        for (long i = mLastLimitTimestamp / BUCKET_DURATION_MS; i < staleCutoffIdx; i++) {
+            final int idx = (int) (i % BUCKET_COUNT);
+            mRecordedPeriodBytes -= mStatBuckets[idx];
+            mStatBuckets[idx] = 0;
+        }
+
+        final int bucketIdx = (int) (currentBucketAbsIdx % BUCKET_COUNT);
+        final int maxAllowed = Math.min(mMaxPerPeriodBytes - mRecordedPeriodBytes,
+            Math.min(mMaxPerBucketBytes - mStatBuckets[bucketIdx], requestedBytes));
+        return Math.max(0, maxAllowed);
+    }
+
+    private void recordBytes(long now, int requestedBytes, int actualBytes) {
+        mStatBuckets[(int) ((now / BUCKET_DURATION_MS) % BUCKET_COUNT)] += actualBytes;
+        mRecordedPeriodBytes += actualBytes;
+        mLastRequestReduction = requestedBytes - actualBytes;
+        mLastLimitTimestamp = now;
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        sb.append("{max=");
+        sb.append(mMaxPerPeriodBytes);
+        sb.append(",max_bucket=");
+        sb.append(mMaxPerBucketBytes);
+        sb.append(",total=");
+        sb.append(mRecordedPeriodBytes);
+        sb.append(",last_red=");
+        sb.append(mLastRequestReduction);
+        sb.append('}');
+        return sb.toString();
+    }
+}