[KA02] internal cleanup and refactor for SocketKeepalive

In previous change, the new SocketKeepalive API was exported.
But internally, old PacketKeepalive names and structures are
still used.

This change rename them properly for code consistency and also
refactor KeepalivePacketData to support different types of
KeepalivePacketData.

(clean cherry-pick from aosp/860394)

Bug: 114151147
Test: 1. atest FrameworksNetTests
      2. atest FrameworksWifiTests
      3. atest FrameworksTelephonyTests

Change-Id: Ia9917d12987e91e87e34ffb3f126e7bc7c9c187e

core/java/android/net/NattKeepalivePacketData.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright (C) 2018 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 android.net;
+
+import android.net.SocketKeepalive.InvalidPacketException;
+import android.net.util.IpUtils;
+import android.system.OsConstants;
+
+import java.net.Inet4Address;
+import java.net.InetAddress;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+
+/** @hide */
+public final class NattKeepalivePacketData extends KeepalivePacketData {
+
+    // This should only be constructed via static factory methods, such as
+    // nattKeepalivePacket
+    private NattKeepalivePacketData(InetAddress srcAddress, int srcPort,
+            InetAddress dstAddress, int dstPort, byte[] data) throws
+            InvalidPacketException {
+        super(srcAddress, srcPort, dstAddress, dstPort, data);
+    }
+
+    /**
+     * Factory method to create Nat-T keepalive packet structure.
+     */
+    public static NattKeepalivePacketData nattKeepalivePacket(
+            InetAddress srcAddress, int srcPort, InetAddress dstAddress, int dstPort)
+            throws InvalidPacketException {
+
+        if (!(srcAddress instanceof Inet4Address) || !(dstAddress instanceof Inet4Address)) {
+            throw new InvalidPacketException(SocketKeepalive.ERROR_INVALID_IP_ADDRESS);
+        }
+
+        if (dstPort != NattSocketKeepalive.NATT_PORT) {
+            throw new InvalidPacketException(SocketKeepalive.ERROR_INVALID_PORT);
+        }
+
+        int length = IPV4_HEADER_LENGTH + UDP_HEADER_LENGTH + 1;
+        ByteBuffer buf = ByteBuffer.allocate(length);
+        buf.order(ByteOrder.BIG_ENDIAN);
+        buf.putShort((short) 0x4500);             // IP version and TOS
+        buf.putShort((short) length);
+        buf.putInt(0);                            // ID, flags, offset
+        buf.put((byte) 64);                       // TTL
+        buf.put((byte) OsConstants.IPPROTO_UDP);
+        int ipChecksumOffset = buf.position();
+        buf.putShort((short) 0);                  // IP checksum
+        buf.put(srcAddress.getAddress());
+        buf.put(dstAddress.getAddress());
+        buf.putShort((short) srcPort);
+        buf.putShort((short) dstPort);
+        buf.putShort((short) (length - 20));      // UDP length
+        int udpChecksumOffset = buf.position();
+        buf.putShort((short) 0);                  // UDP checksum
+        buf.put((byte) 0xff);                     // NAT-T keepalive
+        buf.putShort(ipChecksumOffset, IpUtils.ipChecksum(buf, 0));
+        buf.putShort(udpChecksumOffset, IpUtils.udpChecksum(buf, 0, IPV4_HEADER_LENGTH));
+
+        return new NattKeepalivePacketData(srcAddress, srcPort, dstAddress, dstPort, buf.array());
+    }
+}