diff --git a/src/networking.c b/src/networking.c
index e7879384d54b70c5474b82fabb0c160cbcb5b350..071f5aafdf40ceb25a008a93485a95c0fa330346 100644
--- a/src/networking.c
+++ b/src/networking.c
@@ -88,6 +88,8 @@ redisClient *createClient(int fd) {
     c->authenticated = 0;
     c->replstate = REDIS_REPL_NONE;
     c->reploff = 0;
+    c->repl_ack_off = 0;
+    c->repl_ack_time = 0;
     c->slave_listening_port = 0;
     c->reply = listCreate();
     c->reply_bytes = 0;
diff --git a/src/redis.h b/src/redis.h
index 3002dd9733fc203d5ab4d029d163c1d0e6841b7a..f9a692ea67cee28d649dca57136abd1ad515dc03 100644
--- a/src/redis.h
+++ b/src/redis.h
@@ -450,6 +450,8 @@ typedef struct redisClient {
     long repldboff;         /* replication DB file offset */
     off_t repldbsize;       /* replication DB file size */
     long long reploff;      /* replication offset if this is our master */
+    long long repl_ack_off; /* replication ack offset, if this is a slave */
+    long long repl_ack_time;/* replication ack time, if this is a slave */
     char replrunid[REDIS_RUN_ID_SIZE+1]; /* master run id if this is a master */
     int slave_listening_port; /* As configured with: SLAVECONF listening-port */
     multiState mstate;      /* MULTI/EXEC state */
diff --git a/src/replication.c b/src/replication.c
index 6e89d873902fd419d294176eba561fd322c6e306..06cfe89aeeeb783bbe6b00feb3ca303bf293ff30 100644
--- a/src/replication.c
+++ b/src/replication.c
@@ -588,6 +588,19 @@ void replconfCommand(redisClient *c) {
                     &port,NULL) != REDIS_OK))
                 return;
             c->slave_listening_port = port;
+        } else if (!strcasecmp(c->argv[j]->ptr,"ack")) {
+            /* REPLCONF ACK is used by slave to inform the master the amount
+             * of replication stream that it processed so far. It is an
+             * internal only command that normal clients should never use. */
+            long long offset;
+
+            if (!(c->flags & REDIS_SLAVE)) return;
+            if ((getLongLongFromObject(c->argv[j+1], &offset) != REDIS_OK))
+                return;
+            if (offset > c->repl_ack_off)
+                c->repl_ack_off = offset;
+            c->repl_ack_time = server.unixtime;
+            /* Note: this command does not reply anything! */
         } else {
             addReplyErrorFormat(c,"Unrecognized REPLCONF option: %s",
                 (char*)c->argv[j]->ptr);