diff --git a/net/colo-compare.c b/net/colo-compare.c
index bcc1beb..9596c8b 100644
--- a/net/colo-compare.c
+++ b/net/colo-compare.c
@@ -33,8 +33,12 @@
 #define COLO_COMPARE(obj) \
     OBJECT_CHECK(CompareState, (obj), TYPE_COLO_COMPARE)
 
+#define COMPARE_READ_LEN_MAX NET_BUFSIZE
 #define MAX_QUEUE_SIZE 1024
 
+/* TODO: Should be configurable */
+#define REGULAR_PACKET_CHECK_MS 3000
+
 /*
   + CompareState ++
   |               |
@@ -76,6 +80,11 @@ typedef struct CompareState {
     GQueue conn_list;
     /* hashtable to save connection */
     GHashTable *connection_track_table;
+    /* compare thread, a thread for each NIC */
+    QemuThread thread;
+    /* Timer used on the primary to find packets that are never matched */
+    QEMUTimer *timer;
+    QemuMutex timer_check_lock;
 } CompareState;
 
 typedef struct CompareClass {
@@ -148,6 +157,118 @@ static int packet_enqueue(CompareState *s, int mode)
     return 0;
 }
 
+/*
+ * The IP packets sent by primary and secondary
+ * will be compared in here
+ * TODO support ip fragment, Out-Of-Order
+ * return:    0  means packet same
+ *            > 0 || < 0 means packet different
+ */
+static int colo_packet_compare(Packet *ppkt, Packet *spkt)
+{
+    trace_colo_compare_ip_info(ppkt->size, inet_ntoa(ppkt->ip->ip_src),
+                               inet_ntoa(ppkt->ip->ip_dst), spkt->size,
+                               inet_ntoa(spkt->ip->ip_src),
+                               inet_ntoa(spkt->ip->ip_dst));
+
+    if (ppkt->size == spkt->size) {
+        return memcmp(ppkt->data, spkt->data, spkt->size);
+    } else {
+        return -1;
+    }
+}
+
+static int colo_packet_compare_all(Packet *spkt, Packet *ppkt)
+{
+    trace_colo_compare_main("compare all");
+    return colo_packet_compare(ppkt, spkt);
+}
+
+static int colo_old_packet_check_one(Packet *pkt, int64_t *check_time)
+{
+    int64_t now = qemu_clock_get_ms(QEMU_CLOCK_HOST);
+
+    if ((now - pkt->creation_ms) > (*check_time)) {
+        trace_colo_old_packet_check_found(pkt->creation_ms);
+        return 0;
+    } else {
+        return 1;
+    }
+}
+
+static void colo_old_packet_check_one_conn(void *opaque,
+                                           void *user_data)
+{
+    Connection *conn = opaque;
+    GList *result = NULL;
+    int64_t check_time = REGULAR_PACKET_CHECK_MS;
+
+    result = g_queue_find_custom(&conn->primary_list,
+                                 &check_time,
+                                 (GCompareFunc)colo_old_packet_check_one);
+
+    if (result) {
+        /* do checkpoint will flush old packet */
+        /* TODO: colo_notify_checkpoint();*/
+    }
+}
+
+/*
+ * Look for old packets that the secondary hasn't matched,
+ * if we have some then we have to checkpoint to wake
+ * the secondary up.
+ */
+static void colo_old_packet_check(void *opaque)
+{
+    CompareState *s = opaque;
+
+    g_queue_foreach(&s->conn_list, colo_old_packet_check_one_conn, NULL);
+}
+
+/*
+ * Called from the compare thread on the primary
+ * for compare connection
+ */
+static void colo_compare_connection(void *opaque, void *user_data)
+{
+    CompareState *s = user_data;
+    Connection *conn = opaque;
+    Packet *pkt = NULL;
+    GList *result = NULL;
+    int ret;
+
+    while (!g_queue_is_empty(&conn->primary_list) &&
+           !g_queue_is_empty(&conn->secondary_list)) {
+        qemu_mutex_lock(&s->timer_check_lock);
+        pkt = g_queue_pop_tail(&conn->primary_list);
+        qemu_mutex_unlock(&s->timer_check_lock);
+        result = g_queue_find_custom(&conn->secondary_list,
+                              pkt, (GCompareFunc)colo_packet_compare_all);
+
+        if (result) {
+            ret = compare_chr_send(s->chr_out, pkt->data, pkt->size);
+            if (ret < 0) {
+                error_report("colo_send_primary_packet failed");
+            }
+            trace_colo_compare_main("packet same and release packet");
+            g_queue_remove(&conn->secondary_list, result->data);
+            packet_destroy(pkt, NULL);
+        } else {
+            /*
+             * If one packet arrive late, the secondary_list or
+             * primary_list will be empty, so we can't compare it
+             * until next comparison.
+             */
+            trace_colo_compare_main("packet different");
+            qemu_mutex_lock(&s->timer_check_lock);
+            g_queue_push_tail(&conn->primary_list, pkt);
+            qemu_mutex_unlock(&s->timer_check_lock);
+            /* TODO: colo_notify_checkpoint();*/
+            break;
+        }
+    }
+}
+
 static int compare_chr_send(CharDriverState *out,
                             const uint8_t *buf,
                             uint32_t size)
@@ -175,6 +296,65 @@ err:
     return ret < 0 ? ret : -EIO;
 }
 
+static int compare_chr_can_read(void *opaque)
+{
+    return COMPARE_READ_LEN_MAX;
+}
+
+/*
+ * Called from the main thread on the primary for packets
+ * arriving over the socket from the primary.
+ */
+static void compare_pri_chr_in(void *opaque, const uint8_t *buf, int size)
+{
+    CompareState *s = COLO_COMPARE(opaque);
+    int ret;
+
+    ret = net_fill_rstate(&s->pri_rs, buf, size);
+    if (ret == -1) {
+        qemu_chr_add_handlers(s->chr_pri_in, NULL, NULL, NULL, NULL);
+        error_report("colo-compare primary_in error");
+    }
+}
+
+/*
+ * Called from the main thread on the primary for packets
+ * arriving over the socket from the secondary.
+ */
+static void compare_sec_chr_in(void *opaque, const uint8_t *buf, int size)
+{
+    CompareState *s = COLO_COMPARE(opaque);
+    int ret;
+
+    ret = net_fill_rstate(&s->sec_rs, buf, size);
+    if (ret == -1) {
+        qemu_chr_add_handlers(s->chr_sec_in, NULL, NULL, NULL, NULL);
+        error_report("colo-compare secondary_in error");
+    }
+}
+
+static void *colo_compare_thread(void *opaque)
+{
+    GMainContext *worker_context;
+    GMainLoop *compare_loop;
+    CompareState *s = opaque;
+
+    worker_context = g_main_context_new();
+
+    qemu_chr_add_handlers_full(s->chr_pri_in, compare_chr_can_read,
+                          compare_pri_chr_in, NULL, s, worker_context);
+    qemu_chr_add_handlers_full(s->chr_sec_in, compare_chr_can_read,
+                          compare_sec_chr_in, NULL, s, worker_context);
+
+    compare_loop = g_main_loop_new(worker_context, FALSE);
+
+    g_main_loop_run(compare_loop);
+
+    g_main_loop_unref(compare_loop);
+    g_main_context_unref(worker_context);
+    return NULL;
+}
+
 static char *compare_get_pri_indev(Object *obj, Error **errp)
 {
     CompareState *s = COLO_COMPARE(obj);
@@ -227,6 +407,9 @@ static void compare_pri_rs_finalize(SocketReadState *pri_rs)
     if (packet_enqueue(s, PRIMARY_IN)) {
         trace_colo_compare_main("primary: unsupported packet in");
         compare_chr_send(s->chr_out, pri_rs->buf, pri_rs->packet_len);
+    } else {
+        /* compare connection */
+        g_queue_foreach(&s->conn_list, colo_compare_connection, s);
     }
 }
 
@@ -236,6 +419,9 @@ static void compare_sec_rs_finalize(SocketReadState *sec_rs)
 
     if (packet_enqueue(s, SECONDARY_IN)) {
         trace_colo_compare_main("secondary: unsupported packet in");
+    } else {
+        /* compare connection */
+        g_queue_foreach(&s->conn_list, colo_compare_connection, s);
     }
 }
 
@@ -294,12 +480,34 @@ static int find_and_check_chardev(CharDriverState **chr,
 }
 
 /*
+ * Check old packet regularly so it can watch for any packets
+ * that the secondary hasn't produced equivalents of.
+ */
+static void check_old_packet_regular(void *opaque)
+{
+    CompareState *s = opaque;
+
+    timer_mod(s->timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
+              REGULAR_PACKET_CHECK_MS);
+    /* if have old packet we will notify checkpoint */
+    /*
+     * TODO: Make timer handler run in compare thread
+     * like qemu_chr_add_handlers_full.
+     */
+    qemu_mutex_lock(&s->timer_check_lock);
+    colo_old_packet_check(s);
+    qemu_mutex_unlock(&s->timer_check_lock);
+}
+
+/*
  * Called from the main thread on the primary
  * to setup colo-compare.
  */
 static void colo_compare_complete(UserCreatable *uc, Error **errp)
 {
     CompareState *s = COLO_COMPARE(uc);
+    char thread_name[64];
+    static int compare_id;
 
     if (!s->pri_indev || !s->sec_indev || !s->outdev) {
         error_setg(errp, "colo compare needs 'primary_in' ,"
@@ -335,12 +543,25 @@ static void colo_compare_complete(UserCreatable *uc, Error **errp)
     net_socket_rs_init(&s->sec_rs, compare_sec_rs_finalize);
 
     g_queue_init(&s->conn_list);
+    qemu_mutex_init(&s->timer_check_lock);
 
     s->connection_track_table = g_hash_table_new_full(connection_key_hash,
                                                       connection_key_equal,
                                                       g_free,
                                                       connection_destroy);
 
+    sprintf(thread_name, "colo-compare %d", compare_id);
+    qemu_thread_create(&s->thread, thread_name,
+                       colo_compare_thread, s,
+                       QEMU_THREAD_JOINABLE);
+    compare_id++;
+
+    /* A regular timer to kick any packets that the secondary doesn't match */
+    s->timer = timer_new_ms(QEMU_CLOCK_VIRTUAL, /* Only when guest runs */
+                            check_old_packet_regular, s);
+    timer_mod(s->timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) +
+                        REGULAR_PACKET_CHECK_MS);
+
     return;
 }
 
@@ -382,6 +603,18 @@ static void colo_compare_finalize(Object *obj)
 
     g_queue_free(&s->conn_list);
 
+    if (qemu_thread_is_self(&s->thread)) {
+        /* compare connection */
+        g_queue_foreach(&s->conn_list, colo_compare_connection, s);
+        qemu_thread_join(&s->thread);
+    }
+
+    if (s->timer) {
+        timer_del(s->timer);
+    }
+
+    qemu_mutex_destroy(&s->timer_check_lock);
+
     g_free(s->pri_indev);
     g_free(s->sec_indev);
     g_free(s->outdev);
diff --git a/net/colo.c b/net/colo.c
index 40b3b5e..94f5992 100644
--- a/net/colo.c
+++ b/net/colo.c
@@ -143,6 +143,7 @@ Packet *packet_new(const void *data, int size)
 
     pkt->data = g_memdup(data, size);
     pkt->size = size;
+    pkt->creation_ms = qemu_clock_get_ms(QEMU_CLOCK_HOST);
 
     return pkt;
 }
diff --git a/net/colo.h b/net/colo.h
index c511bcd..9a7d5e0 100644
--- a/net/colo.h
+++ b/net/colo.h
@@ -17,6 +17,7 @@
 
 #include "slirp/slirp.h"
 #include "qemu/jhash.h"
+#include "qemu/timer.h"
 
 #define HASHTABLE_MAX_SIZE 16384
 
@@ -40,6 +41,8 @@ typedef struct Packet {
     };
     uint8_t *transport_header;
     int size;
+    /* Time of packet creation, in wall clock ms */
+    int64_t creation_ms;
 } Packet;
 
 typedef struct ConnectionKey {
diff --git a/trace-events b/trace-events
index 9b4186f..81de82c 100644
--- a/trace-events
+++ b/trace-events
@@ -144,6 +144,8 @@ colo_proxy_main(const char *chr) ": %s"
 
 # net/colo-compare.c
 colo_compare_main(const char *chr) ": %s"
+colo_compare_ip_info(int psize, const char *sta, const char *stb, int ssize, const char *stc, const char *std) "ppkt size = %d, ip_src = %s, ip_dst = %s, spkt size = %d, ip_src = %s, ip_dst = %s"
+colo_old_packet_check_found(int64_t old_time) "%" PRId64
 
 ### Guest events, keep at bottom
 
