stream/tcp: be more liberal in last_ack

Don't set even if seq is before next_seq, as this could still be
a valid packet that was sent before the state was reached.

src/stream-tcp.c

@@ -4023,14 +4023,6 @@ static int StreamTcpPacketStateLastAck(ThreadVars *tv, Packet *p,
                 retransmission = 1;
             }
 
-            if (TCP_GET_SEQ(p) != ssn->client.next_seq && TCP_GET_SEQ(p) != ssn->client.next_seq + 1) {
-                SCLogDebug("ssn %p: -> SEQ mismatch, packet SEQ %" PRIu32 ""
-                        " != %" PRIu32 " from stream", ssn,
-                        TCP_GET_SEQ(p), ssn->client.next_seq);
-                StreamTcpSetEvent(p, STREAM_LASTACK_ACK_WRONG_SEQ);
-                return -1;
-            }
-
             if (StreamTcpValidateAck(ssn, &ssn->server, p) == -1) {
                 SCLogDebug("ssn %p: rejecting because of invalid ack value", ssn);
                 StreamTcpSetEvent(p, STREAM_LASTACK_INVALID_ACK);
@@ -4038,9 +4030,19 @@ static int StreamTcpPacketStateLastAck(ThreadVars *tv, Packet *p,
             }
 
             if (!retransmission) {
-                StreamTcpPacketSetState(p, ssn, TCP_CLOSED);
-                SCLogDebug("ssn %p: state changed to TCP_CLOSED", ssn);
+                if (SEQ_LT(TCP_GET_SEQ(p), ssn->client.next_seq)) {
+                    SCLogDebug("ssn %p: not updating state as packet is before next_seq", ssn);
+                } else if (TCP_GET_SEQ(p) != ssn->client.next_seq && TCP_GET_SEQ(p) != ssn->client.next_seq + 1) {
+                    SCLogDebug("ssn %p: -> SEQ mismatch, packet SEQ %" PRIu32 ""
+                            " != %" PRIu32 " from stream", ssn,
+                            TCP_GET_SEQ(p), ssn->client.next_seq);
+                    StreamTcpSetEvent(p, STREAM_LASTACK_ACK_WRONG_SEQ);
+                    return -1;
+                } else {
+                    StreamTcpPacketSetState(p, ssn, TCP_CLOSED);
+                    SCLogDebug("ssn %p: state changed to TCP_CLOSED", ssn);
 
+                }
                 ssn->server.window = TCP_GET_WINDOW(p) << ssn->server.wscale;
             }