diff --git a/Tethering/apishim/31/com/android/networkstack/tethering/apishim/api31/BpfCoordinatorShimImpl.java b/Tethering/apishim/31/com/android/networkstack/tethering/apishim/api31/BpfCoordinatorShimImpl.java
index 611c828..f537e90 100644
--- a/Tethering/apishim/31/com/android/networkstack/tethering/apishim/api31/BpfCoordinatorShimImpl.java
+++ b/Tethering/apishim/31/com/android/networkstack/tethering/apishim/api31/BpfCoordinatorShimImpl.java
@@ -371,6 +371,7 @@
         } catch (IllegalStateException e) {
             // Silent if the rule already exists. Note that the errno EEXIST was rethrown as
             // IllegalStateException. See BpfMap#insertEntry.
+            return false;
         }
         return true;
     }
diff --git a/Tethering/apishim/common/com/android/networkstack/tethering/apishim/common/BpfCoordinatorShim.java b/Tethering/apishim/common/com/android/networkstack/tethering/apishim/common/BpfCoordinatorShim.java
index 08ab9ca..3c2ce0f 100644
--- a/Tethering/apishim/common/com/android/networkstack/tethering/apishim/common/BpfCoordinatorShim.java
+++ b/Tethering/apishim/common/com/android/networkstack/tethering/apishim/common/BpfCoordinatorShim.java
@@ -141,6 +141,11 @@
 
     /**
      * Adds a tethering IPv4 offload rule to appropriate BPF map.
+     *
+     * @param downstream true if downstream, false if upstream.
+     * @param key the key to add.
+     * @param value the value to add.
+     * @return true iff the map was modified, false if the key exists or there was an error.
      */
     public abstract boolean tetherOffloadRuleAdd(boolean downstream, @NonNull Tether4Key key,
             @NonNull Tether4Value value);
diff --git a/Tethering/src/com/android/networkstack/tethering/BpfCoordinator.java b/Tethering/src/com/android/networkstack/tethering/BpfCoordinator.java
index f8a9251..97ef380 100644
--- a/Tethering/src/com/android/networkstack/tethering/BpfCoordinator.java
+++ b/Tethering/src/com/android/networkstack/tethering/BpfCoordinator.java
@@ -130,8 +130,14 @@
     static final int CONNTRACK_TIMEOUT_UPDATE_INTERVAL_MS = 60_000;
     @VisibleForTesting
     static final int CONNTRACK_TIMEOUT_UPDATE_SLACK_MS = 20_000;
+
+    // Default timeouts sync from /proc/sys/net/netfilter/nf_conntrack_*
+    // See also kernel document nf_conntrack-sysctl.txt.
     @VisibleForTesting
     static final int NF_CONNTRACK_TCP_TIMEOUT_ESTABLISHED = 432_000;
+    static final int NF_CONNTRACK_TCP_TIMEOUT_UNACKNOWLEDGED = 300;
+    // The default value is 120 for 5.10 and that thus the periodicity of the updates of 60s is
+    // low enough to support all ACK kernels.
     @VisibleForTesting
     static final int NF_CONNTRACK_UDP_TIMEOUT_STREAM = 180;
 
@@ -1566,6 +1572,18 @@
             final Tether4Key downstream4Key = makeTetherDownstream4Key(e, tetherClient,
                     upstreamIndex);
 
+            // Using the timeout to distinguish tcp state is not a decent way. Need to fix.
+            // The received IPCTNL_MSG_CT_NEW must pass ConntrackMonitor#isEstablishedNatSession
+            // which checks CTA_STATUS. It implies that this entry has at least reached tcp
+            // state "established". For safety, treat any timeout which is equal or larger than 300
+            // seconds (UNACKNOWLEDGED, ESTABLISHED, ..) to be "established".
+            // TODO: parse tcp state in conntrack monitor.
+            final boolean isTcpEstablished =
+                    e.msgType == (NetlinkConstants.NFNL_SUBSYS_CTNETLINK << 8
+                    | NetlinkConstants.IPCTNL_MSG_CT_NEW)
+                    && e.tupleOrig.protoNum == OsConstants.IPPROTO_TCP
+                    && (e.timeoutSec >= NF_CONNTRACK_TCP_TIMEOUT_UNACKNOWLEDGED);
+
             if (e.msgType == (NetlinkConstants.NFNL_SUBSYS_CTNETLINK << 8
                     | NetlinkConstants.IPCTNL_MSG_CT_DELETE)) {
                 final boolean deletedUpstream = mBpfCoordinatorShim.tetherOffloadRuleRemove(
@@ -1592,11 +1610,41 @@
             final Tether4Value upstream4Value = makeTetherUpstream4Value(e, upstreamIndex);
             final Tether4Value downstream4Value = makeTetherDownstream4Value(e, tetherClient,
                     upstreamIndex);
-
             maybeAddDevMap(upstreamIndex, tetherClient.downstreamIfindex);
             maybeSetLimit(upstreamIndex);
-            mBpfCoordinatorShim.tetherOffloadRuleAdd(UPSTREAM, upstream4Key, upstream4Value);
-            mBpfCoordinatorShim.tetherOffloadRuleAdd(DOWNSTREAM, downstream4Key, downstream4Value);
+
+            final boolean upstreamAdded = mBpfCoordinatorShim.tetherOffloadRuleAdd(UPSTREAM,
+                    upstream4Key, upstream4Value);
+            final boolean downstreamAdded = mBpfCoordinatorShim.tetherOffloadRuleAdd(DOWNSTREAM,
+                    downstream4Key, downstream4Value);
+
+            if (upstreamAdded != downstreamAdded) {
+                mLog.e("The bidirectional rules should be added or not added concurrently ("
+                        + "upstream: " + upstreamAdded
+                        + ", downstream: " + downstreamAdded + "). "
+                        + "Remove the added rules.");
+                if (upstreamAdded) {
+                    mBpfCoordinatorShim.tetherOffloadRuleRemove(UPSTREAM, upstream4Key);
+                }
+                if (downstreamAdded) {
+                    mBpfCoordinatorShim.tetherOffloadRuleRemove(DOWNSTREAM, downstream4Key);
+                }
+                return;
+            }
+
+            // Update TCP timeout iif it is first time we're adding the rules. Needed because a
+            // payload data packet may have gone through non-offload path, before we added offload
+            // rules, and that this may result in in-kernel conntrack state being in ESTABLISHED
+            // but pending ACK (ie. UNACKED) state. But the in-kernel conntrack might never see the
+            // ACK because we just added offload rules. As such after adding the rules we need to
+            // force the timeout back to the normal ESTABLISHED timeout of 5 days. Note that
+            // updating the timeout will trigger another netlink event with the updated timeout.
+            // TODO: Remove this once the tcp state is parsed.
+            if (isTcpEstablished && upstreamAdded && downstreamAdded) {
+                updateConntrackTimeout((byte) upstream4Key.l4proto,
+                        parseIPv4Address(upstream4Key.src4), (short) upstream4Key.srcPort,
+                        parseIPv4Address(upstream4Key.dst4), (short) upstream4Key.dstPort);
+            }
         }
     }
 
@@ -1879,6 +1927,7 @@
         // - proc/sys/net/netfilter/nf_conntrack_tcp_timeout_established
         // - proc/sys/net/netfilter/nf_conntrack_udp_timeout_stream
         // See kernel document nf_conntrack-sysctl.txt.
+        // TODO: we should account for the fact that lastUsed is in the past and not exactly now.
         final int timeoutSec = (proto == OsConstants.IPPROTO_TCP)
                 ? NF_CONNTRACK_TCP_TIMEOUT_ESTABLISHED
                 : NF_CONNTRACK_UDP_TIMEOUT_STREAM;
diff --git a/Tethering/tests/unit/src/com/android/networkstack/tethering/BpfCoordinatorTest.java b/Tethering/tests/unit/src/com/android/networkstack/tethering/BpfCoordinatorTest.java
index bcdedc7..32d7e5f 100644
--- a/Tethering/tests/unit/src/com/android/networkstack/tethering/BpfCoordinatorTest.java
+++ b/Tethering/tests/unit/src/com/android/networkstack/tethering/BpfCoordinatorTest.java
@@ -42,6 +42,7 @@
 import static com.android.dx.mockito.inline.extended.ExtendedMockito.staticMockMarker;
 import static com.android.networkstack.tethering.BpfCoordinator.CONNTRACK_TIMEOUT_UPDATE_INTERVAL_MS;
 import static com.android.networkstack.tethering.BpfCoordinator.CONNTRACK_TIMEOUT_UPDATE_SLACK_MS;
+import static com.android.networkstack.tethering.BpfCoordinator.NF_CONNTRACK_TCP_TIMEOUT_ESTABLISHED;
 import static com.android.networkstack.tethering.BpfCoordinator.NF_CONNTRACK_UDP_TIMEOUT_STREAM;
 import static com.android.networkstack.tethering.BpfCoordinator.StatsType;
 import static com.android.networkstack.tethering.BpfCoordinator.StatsType.STATS_PER_IFACE;
@@ -1369,8 +1370,14 @@
         }
 
         final int status = (msgType == IPCTNL_MSG_CT_NEW) ? ESTABLISHED_MASK : DYING_MASK;
-        final int timeoutSec = (msgType == IPCTNL_MSG_CT_NEW) ? 100 /* nonzero, new */
-                : 0 /* unused, delete */;
+        int timeoutSec;
+        if (msgType == IPCTNL_MSG_CT_NEW) {
+            timeoutSec = (proto == IPPROTO_TCP)
+                    ? NF_CONNTRACK_TCP_TIMEOUT_ESTABLISHED
+                    : NF_CONNTRACK_UDP_TIMEOUT_STREAM;
+        } else {
+            timeoutSec = 0; /* unused, CT_DELETE */
+        }
         return new ConntrackEvent(
                 (short) (NetlinkConstants.NFNL_SUBSYS_CTNETLINK << 8 | msgType),
                 new Tuple(new TupleIpv4(PRIVATE_ADDR, REMOTE_ADDR),